Flux cored arc welding, or FCAW, evolved from the gas metal arc welding, or GMAW process to improve arc action, metal transfer, weld metal properties, and weld appearance. The heat is provided by an arc between a continuously fed tubular electrode wire and the workpiece. The major difference is that FCAW utilizes an electrode very different from the solid electrode used in GMAW. In fact, it is closer to the electrodes used in shielded metal arc welding, or SMAW or stick welding, except the flux is on the inside of a flexible electrode instead of on the outside of a very stiff electrode.
The flux-cored electrode is a fabricated electrode and, as the name implies, flux material is deposited into its core. The flux-cored electrode begins as a flat metal strip that is formed first into a "U" shape. Flux and alloying elements are deposited into the "U" and then the shape is closed into a tubular configuration by a series of forming rolls. Shielding is obtained by the flux contained within the tubular electrode wire, or by the flux and the addition of a shielding gas.
This course is designed to give you a basic understanding of the FCAW process and equipment along with the key variables that affect the quality of welds, such as electrode selection, polarity and amperage, arc length, travel speed, and electrode angles. It also covers core competencies, such as setting up welding equipment, preparing weld materials, fitting up weld materials, welding carbon steel plates, and repairing welds. It will also provide you with an understanding of the safety precautions for FCAW and an awareness of the importance of safety in welding.
Always refer to the manufacturers manuals for specific operating and maintenance instructions.
When you have completed this course, you will be able to:
- Describe the process of flux cored arc welding.
- Describe the principles of operation used for flux cored arc welding.
- Describe the equipment associated with flux cored arc welding.
- Describe the setup, operation and shut down of flux cored arc welding equipment.
- Identify the classification and selection of flux-cored electrodes flux-cored electrodes used for flux cored arc welding.
- Identify the welding applications for flux cored arc welding.
- Describe the welding metallurgy of flux cored arc welding.
- Identify weld and joint designs used for flux cored arc welding.
- Describe the welding procedure variables associated with flux cored arc welding.
- Identify welding procedure schedules used for flux cored arc welding.
- Describe pre-weld preparations for flux cored arc welding.
- Identify defects and problems associated with flux cored arc welding.
- Describe post-weld procedures for flux cored arc welding.
- State the welder training and qualifications associated with flux cored arc welding.
- Describe the welding safety associated with flux cored arc welding.
|1.0.0 Introduction to the Process||9.0.0 Welding Procedure Variables|
Flux cored arc welding (FCAW) is an arc welding process in which the heat for welding is produced by an arc between a continuously fed tubular electrode wire and the work. Shielding is obtained by a flux contained within the tubular electrode wire or by the flux and an externally supplied shielding gas (Figure 1).
Figure 1 FCAW self shielded and external gas shielded electrodes.
Flux cored arc welding is similar to gas metal arc welding in many ways, but the flux-cored wires used for this process give it different characteristics. Flux cored arc welding is widely used for welding ferrous metals and is particularly good for applications where high deposition rates are desirable. Also, at high welding currents, the arc is smooth and more manageable when compared to using large diameter gas metal arc welding electrodes with carbon dioxide. With FCAW, the arc and weld pool are clearly visible to the welder, and a slag coating is left on the surface of the weld bead, which must be removed. Since the filler metal transfers across the arc, some spatter is created and some smoke produced.
As in GMAW, FCAW depends on a gas shield to protect the weld zone from detrimental atmospheric contamination. However, with FCAW, there are two primary ways this is accomplished:
- The gas is applied from an external source, in which case the electrode is referred to as a gas shielded flux-cored electrode.
- The gas is generated from the decomposition of gas-forming ingredients contained in the electrode's core. In this instance, the electrode is known as a self-shielding flux-cored electrode.
In addition to the gas shield, the flux-cored electrode produces a slag covering for further protection of the weld metal as it cools, which must be manually removed with a wire brush or chipping hammer.
The main advantage of the self-shielding method is that its operation is somewhat simplified because of the absence of external shielding equipment. Although self-shielding electrodes have been developed for welding low-alloy and stainless steels, they are most widely used on mild steels. The self-shielding method generally uses a long electrical stickout (distance between the contact tube and the end of the unmelted electrode, commonly from one to four inches). Electrical resistance is increased with the long extension, preheating the electrode before it is fed into the arc. This preheating enables the electrode to burn off at a faster rate and increases deposition. The preheating also decreases the heat available for melting the base metal, resulting in a more shallow penetration than the gas shielded process.
A major drawback of the self-shielded process is the metallurgical quality of the deposited weld metal. In addition to gaining its shielding ability from gas-forming ingredients in the core, the self-shielded electrode contains a high level of deoxidizing and denitrifying alloys, primarily aluminum, in its core. Although the aluminum performs well in neutralizing the effects of oxygen and nitrogen in the arc zone, its presence in the weld metal will reduce ductility and impact strength at low temperatures. For this reason, the self-shielding method is usually restricted to less critical applications.
The self-shielding electrodes are more suitable for welding in drafty locations than the gas-shielded types. Since the molten filler metal is on the outside of the flux, the gases formed by the decomposing flux are not totally relied upon to shield the arc from the atmosphere. To compensate, the deoxidizing and denitrifying elements in the flux further help to neutralize the effects of nitrogen and oxygen present in the weld zone.
The gas-shielded flux-cored electrode has a major advantage over the self-shielded flux-cored electrode, which is, the protective envelope formed by the auxiliary gas shield around the molten puddle. This envelope effectively excludes the atmosphere without the need for core ingredients, such as aluminum. Because of this more thorough shielding, the weld metallurgy is cleaner, which makes this process suitable for welding not only mild steels, but also low-alloy steels in a wide range of strength and impact levels.
The gas-shielded method uses a shorter electrical stickout than the self-shielded process. (Refer to Figure 1 again) Extensions from 1/2" to 3/4" are common on all diameters, and 3/4" to 1-1/2" on larger diameters. Higher welding currents are also used with this process, enabling high deposition rates. The auxiliary shielding helps to reduce the arc energy into a columnar pattern. The combination of high currents and the action of the shielding gas contributes to the deep penetration inherent with this process. Both spray and globular transfer are utilized with the gas-shielded process.
Although flux cored arc welding may be applied semi-automatically, by machine, or automatically, the process is usually applied semi-automatically. In semi-automatic welding, the wire feeder feeds the electrode wire and the power source maintains the arc length. The welder manipulates the welding gun and adjusts the welding parameters. FCAW is also used in machine welding where, in addition to feeding the wire and maintaining the arc length, the machinery also provides the joint travel. The welding operator continuously monitors the welding and makes adjustments in the welding parameters. Automatic welding is used in high production applications. In automatic welding, the welding operator only starts the operation.
Flux cored arc welding has many advantages for a wide variety of applications. It often competes with shielded metal arc welding, gas metal arc welding, and submerged arc welding (SAW) for many applications. Some of the advantages of this process are:
- It has a high deposition rate and faster travel speeds.
- Using small diameter electrode wires, welding can be done in all positions.
- Some flux-cored wires do not need an external supply of shielding gas, which simplifies the equipment.
- The electrode wire is fed continuously so there is very little time spent on changing electrodes.
- Deposits a higher percentage of the filler metal when compared to shielded metal arc welding.
- Obtains better penetration than shielded metal arc welding.
- To Table of Contents -
Flux cored arc welding uses the heat of an electric arc between a consumable, tubular electrode and the part to be welded. Electric current passing through an ionized gas produces an electric arc. The gas atoms and molecules are broken up and ionized by losing electrons and leaving a positive charge. The positive gas ions then flow from the positive pole to the negative pole and the electrons flow from the negative pole to the positive pole. The electrons carry about 95% of the heat and the rest is carried by the positive ions. The heat of the arc melts the electrode and the surface of the base metal.
One of two methods shields the molten weld metal, heated weld zone, and electrode. The first method is by the decomposition of the flux core of the electrode. The second method is by a combination of an externally supplied shielding gas and the decomposition of the flux core of the electrode wire. The flux core has essentially the same purpose as the coating on an electrode for shielded metal arc welding. The molten electrode filler metal transfers across the arc and into the molten weld puddle, and a slag forms on top of the weld bead that can be removed after welding.
The arc is struck by starting the wire feed which causes the electrode wire to touch the workpiece and initiate the arc. Arc travel is usually not started until a weld puddle is formed. The welding gun then moves along the weld joint manually or mechanically so that the edges of the weld joint are joined. The weld metal then solidifies behind the arc, completing the welding process. A large amount of flux is contained in the core of a self-shielding wire as compared to a gas-shielded wire. This is needed to provide adequate shielding and because of this, a thicker slag coating is formed. In these wires, deoxidizing and denitrifying elements are needed in the filler metal and flux core because some nitrogen is introduced from the atmosphere.
The FCAW process may be operated on both constant voltage and constant current power sources. A welding power source can be classified by its volt-ampere characteristics as a constant voltage (also called constant potential) or constant current (also called variable voltage) type, although there are some machines that can produce both characteristics. Constant voltage power sources are preferred for a majority of FCAW applications.
In the constant voltage arc system, the voltage delivered to the arc is maintained at a relatively constant level that gives a flat or nearly flat volt-ampere curve, as shown in Figure 2. This type of power source is widely used for the processes that require a continuously fed wire electrode. In this system, the arc length is controlled by setting the voltage level on the power source and the welding current is controlled by setting the wire feed speed.
Figure 2 Constant voltage system volt-ampere curve.
As Figure 2 shows, a slight change in the arc length (voltage level) will produce a large change in the welding current.
Most power sources have a fixed slope built in for a certain type of flux cored arc welding. Some constant voltage welding machines are equipped with a slope control used to change the slope of the volt-ampere curve.
Figure 3 shows different slopes obtained from one power source. The slope has the effect of limiting the amount of short-circuiting current the power supply can deliver. This is the current available from the power source on the short-circuit between the electrode wire and the work. This is not as important in FCAW as it was in GMAW because short-circuiting metal transfer is not encountered except with alloy cored, low flux content wires.
Figure 3 Different slopes from a constant voltage motor generator power source.
A slope control is not required, but may be desirable, when welding with small diameter, alloy cored, low flux content electrodes at low current levels. The short-circuit current determines the amount of pinch force available on the electrode. The pinch forces cause the molten electrode droplet to separate from the solid electrode. The flatter the slope of the volt-ampere curve, the higher the short-circuit and the pinch force. The steeper the slope, the lower the short-circuit and pinch force. The pinch force is important with these electrodes because it affects the way the droplet detaches from the tip of the electrode wire. When a high short-circuit and a flat slope cause pinch force, excessive spatter is created. When a very low short-circuit current and pinch force are caused by a steep slope, the electrode wire tends to freeze in the weld puddle or pile up on the work piece. When the proper amount of short-circuit current is used, it creates very little spatter.
The inductance of the power supply also has an effect on the arc stability. When the load on the power supply changes, the current takes time to find its new level. The rate of current change is determined by the inductance of the power supply. Increasing the inductance will reduce the rate of current rise. The rate of the welding current rise increases with the current that is also affected by the inductance in the circuit. Increased arc time or inductance produces a flatter and smoother weld bead as well as a more fluid weld puddle. Too much inductance will cause more difficult arc starting.
The constant current arc system provides a nearly constant welding current to the arc, which gives a drooping volt-ampere characteristic, as shown in Figure 4. This arc system is used with the SMAW and GTAW processes. A dial on the machine sets the welding current and the welding voltage is controlled by the arc length held by the welder.
Figure 4 Volt-ampere curve for a constant current arc system.
This system is necessary for manual welding because the welder cannot hold a constant arc length, which causes only small variations in the welding current. When flux cored arc welding is done with a constant current system, a special voltage-sensing wire feeder is used to maintain a constant arc length.
For any power source, the voltage drop across the welding arc is directly dependent on the arc length. An increase in the arc length results in a corresponding increase in the arc voltage and a decrease in the arc length results in a corresponding decrease in the arc voltage.
Another important relationship exists between the welding current and the melt off-rate of the electrode. With low current, the electrode melts off slower and the metal is deposited slower. This relationship between welding current and wire feed speed is definite, based on the wire size, shielding gas type and type of electrode. A faster wire feed speed will give a higher welding current.
In the constant voltage system, instead of regulating the wire to maintain a constant arc length, the wire is fed into the arc at a fixed speed and the power source is designed to melt off the wire at the same speed. The self-regulating characteristic of a constant voltage power source comes about by the ability of this type of power source to adjust its welding current in order to maintain a fixed voltage across the arc.
With the constant current arc system, the welder changes the wire feed speed as the gun is moved toward or away from the weld puddle. Since the welding current remains the same, the burn-off rate of the wire is unable to compensate for the variations in the wire feed speed, which allows stubbing or burning back of the wire into the contact tip to occur. To lessen this problem, a special voltage-sensing wire feeder is used, which regulates the wire feed speed to maintain a constant voltage across the arc.
The constant voltage system is preferred for most applications, particularly for small diameter wire. With smaller diameter electrodes, the voltage-sensing system is often unable to react fast enough to feed at the required burn-off rate, resulting in a higher instance of burnback into the contact tip of the gun.
Figure 5 shows a comparison of the volt-ampere curves for the two arc systems. This shows that for these particular curves, when a normal arc length is used, the current and voltage levels are the same for both the constant current and constant voltage systems. For a long arc length, there is a slight drop in the welding current for the constant current machine and large drop in the current for a constant voltage machine. For constant voltage power sources, the volt-ampere curve shows that when the arc length shortens slightly, a large increase in welding current occurs. This results in an increased burn-off rate, which brings the arc length back to the desired level. Under this system, changes in the wire feed speed, caused by the welder, are compensated for electrically by the power source.
Figure 5 Volt-ampere curves.
Metal transfer, from consumable electrodes across an arc, has been classified into three general modes of transfer: spray transfer, globular transfer, and short-circuiting transfer. The metal transfer of most flux-cored electrodes resembles a fine globular transfer. Only the alloy-cored, low flux content wires can produce a short-circuiting metal transfer similar to GMAW.
On flux-cored electrodes, the molten droplets build up around the periphery or outer metal sheath of the electrode. By contrast, the droplets on solid wires tend to form across the entire cross section at the end of the wire. A droplet forms on the cored wire, is transferred, and then a droplet is formed at another location on the metal sheath. The core material appears to transfer independently to the surface of the weld puddle. Figure 6 shows the metal transfer in flux=cored arc welding.
Figure 6 Metal transfer in FCAW.
At low currents, the droplets tend to be larger than at higher current levels. If the welding current using a 3/32 in. (2.4 mm) electrode wire is increased from 350 to 550 amps, the metal transfer characteristics will change. Transfer is much more frequent and the droplets become smaller as the current is increased. At 550 amperes, some of the metal may transfer by the spray mode, although the globular mode prevails. There is no indication that higher currents cause a transition to a spray mode of transfer, unless an argon-oxygen shielding gas mixture is used.
The larger droplets at the lower currents cause a certain amount of "splashing action" when they enter the weld puddle. This action decreases with the smaller droplet size. This explains why there is less visible spatter. The arc appears smoother to the operator, and the deposition efficiency is higher when a wire is used with a high current density rather than at the low end of its current range.
|Test Your Knowledge
1. What does the welding process leave on the surface of the weld bead that must be removed?
2. What is pinch force?
- To Table of Contents -
The equipment used for FCAW is very similar to that used for GMAW. The basic arc welding equipment consists of a power source, controls, wire feeder, welding gun, and welding cables. A major difference between the gas-shielded electrodes and self - shielded electrodes is that the gas shielded wires also require a gas shielding system. This may also have an effect on the type of welding gun used. Fume extractors are often used with this process. For machine and automatic welding, several items, such as seam followers and motion devices, are added to the basic equipment. A diagram of the equipment for semiautomatic FCAW is shown in Figure 7.
Figure 7 Equipment for flux cored arc welding.
The power source (welding machine) provides the electric power of the proper voltage and amperage to maintain a welding arc. Most power sources operate on 230 or 460 volt input power, but machines that operate on 200 or 575 volt input are available as options. Power sources may operate on either single-phase or three-phase input with a frequency of 50 to 60 Hz.
Duty cycle is defined as the ratio of arc time to total time. Most power sources used for FCAW have a duty cycle of 100%, which indicates that they can be used to weld continuously. However, some machines have a duty cycle of 60%. For a welding machine, a 10 minute time period is used. Thus, for a 60% duty cycle machine, the welding load would be applied continuously for 6 minutes and would be off for 4 minutes. Most industrial type, constant current machines are rated at 60% duty cycle. The formula for determining the duty cycle of a welding machine for a given load current is:
|% Duty Cycle =||(Rated Current)2||x Rated Duty Cycle|
For example, if a welding machine is rated at a 60% duty cycle at 300 amperes, the duty cycle of the machine when operated at 350 amperes would be.:
|% Duty Cycle =||(300)2||x 60 = 44%|
In general, these lower duty cycle machines are the constant current type, which are used in plants where the same machines are also used for SMAW and gas tungsten arc welding. Some of the smaller constant voltage welding machines have a 60% duty cycle.
FCAW uses direct current, which can be connected in one of two ways: electrode positive (reverse polarity) or electrode negative (straight polarity). The electrically charged particles flow between the tip of the electrode and the work as shown in Figure 8.
Figure 8 Particle flow for DCEP and DCEN.
Flux-cored electrode wires are designed to operate on either DCEP or DCEN. The wires designed for use with an external gas shielding system are generally designed for use with DCEP, while some self-shielding flux-cored wires are used with DCEP and others are used with DCEN. Electrode positive current gives better penetration into the weld joint. Electrode negative current gives lighter penetration, and is used for welding thinner metal or where there is poor fit-up. The weld created by DCEN is wider and shallower than the weld produced by DCEP
The power sources generally recommended for flux cored arc welding are direct current constant voltage types. Both rotating (generator) and static (single- or three-phase transformer-rectifiers) are used. Any of these types of machines are available to produce constant current or constant voltage output, or both. The same power sources used with GMAW are used with FCAW, but FCAW generally uses higher welding currents, which sometimes requires a larger power source. It is important to use a power source capable of producing the maximum current level required for an application.
Generator welding machines used for this process can be powered by an electric motor for shop use, or an internal combustion engine for field applications. Gasoline or diesel engine-driven welding machines have either liquid or air-cooled engines and many of them provide auxiliary power for emergency lighting, power tools, etc. Many of the engine-driven generators used for FCAW in the field are combination constant current-constant voltage types. These types are popular for applications where both SMAW and FCAW can be accomplished using the same power source. Figure 9 shows an engine-driven generator machine used for flux cored arc welding.
Figure 9 Gas powered welder/generator.
The motor-driven generator welding machines are gradually being replaced by transformer-rectifier welding machines. Motor-driven generators produce a very stable arc, but they are noisier, more expensive, consume more power and require more maintenance than transformer-rectifier machines. They can, however, function without being sourced by an electrical power supply and, in fact, can produce the auxiliary electricity during power outages.
An alternator welding machine is an electric generator made to produce AC power. This power source has a rotating assembly. These machines are also called rotating or revolving field machines.
Transformer-rectifiers are the most widely used welding machines for FCAW. . Adding a rectifier to a basic transformer circuit is a method of supplying direct current to the arc without using a rotating generator.. A rectifier is an electrical device which changes alternating current into direct current. These machines are more efficient electrically than motor-generator welding machines and they provide quieter operation. There are two basic types of transformer-rectifier welding machines: those that operate on single-phase input power and those that operate on three-phase input power.
The single-phase transformer-rectifier machines provide DC current to the arc and a constant current volt-ampere characteristic, but are not as popular as three-phase transformer-rectifier welding machines for FCAW. When using a constant current power source, a special variable speed or voltage-sensing wire feeder must be used to maintain a uniform current level. A limitation of the single-phase system is that the power required by the single-phase input power may create an unbalance of the power supply lines which is objectionable to most power companies. These machines normally have a duty cycle of 60%.
The most widely used type of power source for this process is the three-phase transformer-rectifier. These machines produce DC current for the arc, and for FCAW, most have a constant voltage volt-ampere characteristic. When using these constant voltage machines, a constant-speed wire feeder is used. This type of wire feeder maintains a constant wire feed speed with slight changes in welding current. The three-phase input power gives these machines a more stable arc than single-phase input power and avoids the line unbalance that occurs with the single-phase machines.
Many of these machines also use solid state controls for the welding. A 650 amp solid state controlled power source is shown in Figure 10. This machine will produce the flattest volt-ampere curve of the different constant voltage power sources. Most three-phase transformer-rectifier power sources are rated at a 100% duty cycle.
Figure 10 Three-phase, 650 amp solid state power source.
The controls for this process are located on the front of the welding machine, on the welding gun, and on the wire feeder or a control box.
The welding machine controls for a constant voltage machine include an on-off switch, a voltage control, and often a switch to select the polarity of direct current. The voltage control can be a single knob, or it can have a tap switch for setting the voltage range and a fine-voltage control knob.
Other controls are sometimes present, such as a switch for selecting constant current (CC) or constant voltage (CV) output on combination machines, or a switch for a remote control. On constant current welding machines, there is an on-off switch, a current level control knob, and sometimes a knob or switch for selecting the polarity of direct current.
The trigger or switch on the welding gun is a remote control used by the welder in semiautomatic welding to stop and start the welding current, wire feed, and shielding gas flow. For semiautomatic welding, a wire feed speed control is normally part of, or close by, the wire feeder assembly. The wire feed speed sets the welding current level on a constant voltage machine. For machine or automatic welding, a separate control box is often used to control the wire feed speed. A control box for semiautomatic or automatic welding is shown in Figure 11. There may also be switches to turn the control on and off on the wire feeder control box, and gradually feed the wire up and down.
Figure 11 Programmable control unit.
Other controls for this process are used for special applications, especially when a programmable power source is used. An example is a timer for spot welding. Controls that produce a digital readout are popular because it is easier for concise control.
The wire feed motor provides the power for driving the electrode through the cable and gun to the work. There are several different wire feeding systems available. The selection of the best type of system depends on the application. Most FCAW wire feed systems are the constant speed type, which are used with constant voltage power sources. This means the wire feed speed is set before welding. The wire feed speed controls the amount of welding current. Variable speed or voltage-sensing wire feeders are used with constant current power sources. With a variable speed wire feeder, a voltage-sensing circuit maintains the desired arc length by varying the wire feed speed. Variations in the arc length increase or decrease the wire feed speed.
A wire feeder consists of an electrical motor connected to a gear box containing drive rolls. The gear box and wire feed motor shown in Figure 12 have four feed rolls in the gear box. While many systems have only two, in a four-roll system, the lower two rolls drive the wire.
Figure 12 Wire feed assembly.
Because of their structure, flux-cored wires can be easily flattened. The type of drive roll used is based on the size of the tubular wire being fed. The three basic types of drive rolls are the U groove, V knurled, and U cogged, as shown in Figure 13. U groove drive rolls are only used on small diameter wires. These can be used because small diameter tubular wires are less easily flattened. V knurled drive rolls are most commonly used for wire sizes 1/16 in. (1.6 mm) and greater. These drive rolls are lightly knurled to prevent slipping of the wire. The U cogged drive rolls are used for large diameter flux-cored wires. A groove is cut into both rolls. Different gear ratios are used, depending on the wire feed speed required. Table 1 shows the wire feed speeds that can be obtained from different gear ratios.
Figure 13 Drive roll types and applications.
Table 1 Wire feed speeds obtained from different gear ratios.
Wire feed systems may be the pull, push, or push-pull type, depending on the method of application and the distance between the welding gun and the coil or spool of wire. Pull type wire feeders have the drive rolls attached to the welding gun. Most machine and automatic welding stations use this type of system, but pull type wire feeders are rarely used in semiautomatic welding. Pull wire feeders have the advantage for welding small diameter aluminum and soft nonferrous metals with GMAW because it reduces wire feeding problems, but, since most flux-cored wires are steel, this is not an advantage for FCAW.
The push type system with the drive rolls mounted near the coil or spool of wire is the most commonly used wire feed method for semiautomatic welding (Figure 14). The wire is pulled from the coil or spool and then pushed into a flexible conduit and through the gun. The relatively large diameter wires used in FCAW are well suited to this type of system. The length of the conduit can be up to about 12 feet (3.7 m). Another advantage of this push type system is that the wire feed mechanism is not attached to the gun, which reduces the weight and makes the gun easier to handle.
Figure 14 Semi-automatic, solid state control wire feeder.
Some wire feed systems contain a two-gun, two wire feeder arrangement connected to a single control box, which is connected to a single power source. Both wire feeders may be set up, and there is a switch on the control to automatically select which of the two systems will be used.
One advantage to this system is that the second wire feeder and gun can provide backup in case of breakdown, gun maintenance, or electrode change. Another advantage is that two different electrodes for different applications can be set up. For example, a GMAW electrode and gun can be set up on one schedule for welding a root pass, and a second schedule can be set up with a flux-cored wire to weld the rest of the joint with FCAWs faster deposition. This eliminates the need for two power sources or the need to change the electrode wire and gun. The liner is made of flexible metal and is available in sizes compatible with the electrode size. The liner guides the electrode wire from the wire feeder drive rolls through the cable assembly and prevents interruptions in the travel.
Heavy-duty welding guns are normally used because of the large size electrode wires typically used and the corresponding high welding current levels required. Because of the intense heat created by this process, heat shields are attached to the gun in front of the trigger to protect the welder's hand.
Both air-cooled and water-cooled guns are used for FCAW. Air-cooled guns are cooled primarily by the surrounding air, but when a shielding gas is used, this will have an additional cooling effect.
A water-cooled gun is similar to an air-cooled gun, except that ducts to permit the water to circulate around the contact tube and nozzle have been added. Water-cooled guns permit more efficient cooling of the gun. Water-cooled guns are preferred for many applications using 500 amperes and recommended for use with welding currents greater than 600 amperes. Welding guns are rated at the maximum current capacity for continuous operation.
Some self-shielded electrode wires require a specific minimum electrode extension to develop proper shielding, so welding guns for these electrodes have guide tubes with an insulated extension guide. This guide supports the electrode and insures a minimum electrode extension, as shown in Figure 17.
Figure 17 Insulated extension guide.
Machine and automatic welding guns use the same basic design principles and features as the semiautomatic welding guns. These guns often have very high current-carrying capacities and may also be air cooled or water-cooled. Large diameter wires up to 1/8 in. (3.2 mm) are commonly used with high amperages. Machine welding guns must be heavy duty because of the high amperages and duty cycles required, and the welding gun is mounted directly below the wire feeder. Figure 18 shows a machine welding head for FCAW.
If a gas-shielded wire is to be used, the gas can be supplied by a nozzle that is concentric around the electrode or by a side delivery tube, as is shown in Figure 18. The side shielding permits the welding gun to be used in deep, narrow grooves and reduces spatter buildup problems in the nozzle. Side shielding is only recommended for welding using carbon dioxide. A concentric nozzle is preferred when using argon-carbon dioxide and argon-oxygen mixtures, and a concentric nozzle provides better shielding and is sometimes recommended for CO2 at high current levels when a large weld puddle exists.
Figure 18 Automatic welding head.
Fume extractors are often used to help reduce the smoke levels produced by flux-cored electrodes. This reduces air pollution and gives better visibility. Welding guns can be equipped with a fume extractor that consists of an exhaust nozzle that encircles the gun nozzle, as shown in Figure 19. The nozzle is connected to a filter and an exhaust pump. The fume extraction nozzle should be located at a distance far enough from the arc to draw in the rising fumes without disturbing the shielding gas flow.
Figure 19 Fume extractor nozzle.
The major advantage of this fume extraction system is that it is always close to the point of welding. A portable fume exhaust fan cannot be positioned as close to the arc, and requires repositioning for every change in welding position.
The major disadvantage of the fume extractor is that it makes the gun bulkier and more difficult to manipulate. Fume extractors are generally not necessary in a welding booth that is well ventilated.
The shielding gas equipment used for gas-shielded flux-cored wires consists of a gas supply hose, a gas regulator, control valves, and supply hose to the welding gun.
The shielding gases are supplied in liquid form when they are in storage tanks with vaporizers or in a gas form in high-pressure cylinders. An exception is carbon dioxide. When put in high-pressure cylinders, it exists in both the liquid and gas forms. The bulk storage tank system is used when there are large numbers of welding stations using the same type of shielding gas in large quantities. For applications where there are large numbers of welding stations but relatively low gas usage, a manifold system is often used. This consists of several high pressure cylinders connected to a manifold, which then feeds a single line to the welding stations. Individual high-pressure cylinders are used when the amount of gas usage is low, when there are few welding stations, or when portability is required.
The purpose of a gas flow regulator is to reduce the pressure from the gas supply source and maintain a constant delivery pressure. The gas flowmeter is then used to control the flow of gas from the regulator to the welding gun. A valve at the flowmeter outlet adjusts the gas flow rate. The flowmeter is often attached to the regulator, as shown in Figure 20. Regulators and flowmeters are designated for use with specific shielding gases and should only be used with the gas for which they were designed.
Figure 20 Flowmeter and regulator for carbon dioxide.
The hoses are normally connected to solenoid valves on the wire feeder to turn the gas flow on and off with the welding current. A hose is used to connect the flowmeter to the welding gun, and is usually part of the welding gun assembly.
The welding cables and connectors connect the power source to the welding gun and to the work. These cables are normally made of copper or aluminum with copper being the most common. The cable consists of hundreds of wires enclosed in an insulated casing of natural or synthetic rubber. The cable connecting the power source to the welding gun is called the electrode lead. In semiautomatic welding, this cable is often part of the cable assembly, which also includes the shielding gas hose and the conduit the electrode wire feeds through. For machine or automatic welding, the electrode lead is normally separate.
The cable connecting the work to the power source is called the work lead. Work leads are usually connected to the work by pincher clamps or a bolt. The size of the welding cables used depends on the output capacity of the welding machine, the duty cycle of the machine, and the distance between the welding machine and the work. Cable sizes range from the smallest at American Wire Gauge (AWG) No.8 to AWG No. 4/0 with amperage ratings of 75 amperes on up. Table 2 shows recommended cable sizes for use with different welding currents and cable lengths; too small a cable may become too hot during welding.
Table 2 Recommended cable sizes for different welding currents and cable lengths.
For machine and automatic welding, several items, such as seam followers, water circulators, and motion devices, are added to the basic equipment
When a water-cooled gun is used, a water supply must be included in the system. This can be supplied by a water circulator or directly from a hose connection to a water tap. The water is carried to the welding gun through hoses that may or may not go through a valve in the welding machine. A typical water circulator is shown in Figure 21.
Figure 21 Water circulator.
Motion devices are used for machine and automatic welding. These motion devices can be used to move the welding head, workpiece, or gun, depending on the type and size of work and the preference of the user.
Motor-driven carriages that run on tracks or directly on the workpiece are commonly used. Carriages can be used for straight line, contour, vertical, or horizontal welding. Side beam carriages are supported on the vertical face of a flat track and can be used for straight line welding. Multiple electrode welding heads can be used to obtain higher deposition rates.
Welding head manipulators may be used for longitudinal welds and, in conjunction with a rotary weld positioner, for circumferential welds. Available in many boom sizes, they can also be used for semiautomatic welding with mounted welding heads.
Oscillators are optional equipment used to oscillate the gun for surfacing, vertical-up welding, and other welding operations that require a wide bead. Oscillators can either be mechanical or electromagnetic devices.
Accessory equipment for FCAW consists of items for cleaning the weld bead and cutting the electrode wire. Because of the slag coating formed, chipping hammers and wire brushes are usually required to remove the slag. A grinder is often used for final cleaning and for removing spatter. A pair of wire cutters or pliers is used to cut the end of the electrode wire between stops and starts.
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It is necessary for a welder to be able to set up, weld, and secure the equipment that will be used. The following is a brief overview on what materials you will need and what to look for when you are welding, followed by a short description on how to secure the welding machine.
The FCAW process could be a dangerous process if you do not protect yourself from the heat, radiation, and spatter. You must wear a leather coat, gloves, safety glasses, and a welding helmet.
Normally, a number 11 or 12 filter lens is required to protect your eyes from the intense arc created by this welding process. You should also be equipped with a wire brush, wire cutters, pliers, and chipping hammer.
You will need to select the proper electrode according to the base metal you will be welding. You can obtain the proper electrode type and diameter using the AWS classifications.
You may also be using a shielding gas, depending on which electrode wire you are using. Welding-grade carbon dioxide or a mixture of carbon dioxide and argon are normally used.
Now that you have your electrode wire, you need to know how to install it on the welding machine.
Small diameter flux-cored electrode wires are generally spooled in the manner as solid wires used for GMAW, and can be loaded in the same manner.
Large-diameter electrode wires are usually much stiffer. Rather than being stored on spools, the large-diameter flux-cored electrode wires are rolled into coils. These wires have a surprising amount of tension and can cause serious injury if they are allowed to unwind suddenly or uncontrollably.
When removing the wire, four equally spaced bands should be used in order to completely secure the wire and prevent the coil from distorting in shape while handling.
Cut the wire between the coil and the wire feeder, and then loosen the hold down brackets, to remove the secured coil.
The wire feed rollers should then be removed from the wire feeder before mounting the new coil.
With the coil removed, advance the wire feeder until the cutoff end of the wire is released from the drive rollers. Remove the wire with a pair of pliers.
Every time a coil or spool is used or changed, the liner should be cleaned or replaced if damaged. To clean the liner, first remove the two set screws, then remove the gun from the wire feeder and pull the liner from the cable. Use a compressed air supply to purge any contaminants from the liner. Replace in the same manner.
Before adding a new coil, the contact tube and nozzle should be removed from the welding gun and examined for evidence of excessive wear damage. Replace these parts if necessary.
With the coil in place on the feeder, slip the end of the electrode through the wire feeder guides. Manually advance the wire through the wire feed guides, replace the fee rolls, then clip the bands as the wire is advanced through the system.
Some self-shielded electrode wires require a higher preheat to help decompose the flux and provide shielding gas. The welding gun for these wires was designed to maintain as much as 2 1/2 inches of stickout. The contact tube is recessed as much as 1 1/2 inches, and an insert, which acts as an insulator, is placed in the nozzle to protect the preheated wire. The length of the insert controls the amount that the contact tube is recessed into the nozzle.
Gas-shielded wires require a gas nozzle. The electrode stickout is generally between three-fourths and 1 1/2 inches.
Welding guns may be cooled by either air or water, depending on the application. When welding currents over 500 amps are used, water-cooled guns are necessary.
Due to the large amounts of smoke given off by the flux-cored process, a smoke exhaust system can be fitted to the gun, or even manufactured as part of the gun.
High current densities and production welding may require that a heat shield be attached to the gun to protect the hand from the intense heat.
Welding gun maintenance is not complicated. Periodically, the gun should be cleaned to remove spatter and dirt from inside the nozzle.
The flux-cored electrode wire is easily flattened during feeding. To prevent this from happening, the feed rollers must match the size of the wire being used.
Of the types of feed rolls available, the knurled V-groove is generally used with largediameter electrodes, from one sixteenth to one eighth in diameter.
Medium diameter electrodes should be used with groove geared drive rolls. Normally, groove gear rolls can handle either solid or tubular wire from .045-to 7/64-inch in diameter.
Small-diameter electrodes require a concave roller with a smooth face to prevent the wire from flattening.
In most cases, the drive rollers are mounted in pairs, with two pair being a typical feeding system. The electrode wire is pushed from the wire feeder to the gun.
The voltage is adjusted by turning the voltage control knob to the desired range.
To adjust the gas flow rate, stand to one side as a safety measure, open the cylinder valve of the shielding gas, and check the regulator dial to assure there is sufficient pressure. Press the button on the wire feeder, and at the same time, adjust the flowmeter.
If the wire feeder is not equipped with a purge button, set the wire feed control to zero, press the gun trigger, and then set the flowmeter for the desired gas flow rate.
Select the correct current and polarity. Direct current electrode positive is usually used for gas-shielded wires. Direct current electrode positive or negative may be used for self-shielded wires as appropriate to the work material.
To adjust the amperage setting when using a constant voltage power source, it will be necessary to start the arc by pressing the gun trigger, and then tune the wire feed speed control until the current is within the desired range. Since the current will register on the ammeter only during welding, it may be necessary to ask someone to watch the meter while you maintain the arc.
Flux-cored wires are sensitive to changes in voltage; it is important that the electrode stickout remain in the recommended range (Figure 22).
Figure 22 Different effects of voltage and current on a weld.
Allowing the stickout to increase reduces the amperage, while reducing the stickout will cause the amperage to increase. Since penetration is greatly influenced by welding current, you can use stickout to a limited degree to control penetration without interrupting the arc to adjust the welding machine.
The flux core of the electrode will cover the weld with a glass-like slag, which must be chipped and brushed from the weld before inspecting. Always wear eye protection when performing any welding operation.
Shut down the welding equipment:
- Close the shielding gas cylinder valve.
- Purge the shielding gas cylinder lines. Some welding machines are equipped with a purge button. On other equipment, it may be necessary to set the wire feed to zero and press the gun trigger.
- Adjust the flowmeter to zero.
- Turn off the power source.
- Cleanup your work area
- To Table of Contents -
FCAW electrodes provide the filler metal to the weld puddle and shielding for the arc, but a shielding gas is required for some electrode types. The purpose of the shielding gas is to provide protection to the arc and molten weld puddle from the atmosphere. The chemical composition of the electrode wire and flux core in combination with the shielding gas will determine the weld metal composition and mechanical properties of the weld.
The primary purpose of a shielding gas in FCAW, as in any gas-shielded arc welding process, is to protect the arc and weld puddle from the contaminating effects of the atmosphere. If allowed to be exposed to the molten weld metal, the nitrogen and oxygen of the atmosphere can cause porosity and brittleness.
In SMAW, protection is accomplished by placing an outer coating on the electrode, which produces a gaseous shield as the coating disintegrates in the welding arc. In FCAW, the same effect is accomplished by decomposition of the electrode core, or by a combination of this and surrounding the arc area with a shielding gas supplied from an external source.
A shielding gas displaces air in the arc area. Welding is then accomplished under a blanket of shielding gas, and since the molten weld metal is exposed only to the shielding gas, the atmosphere does not contaminate it.
Oxygen, which makes up 21% of air, is a highly reactive element that, at high temperatures, combines readily with other elements in metals, and specifically in steels, to form undesirable oxides and gases. Oxygen combines with the iron in steels to form compounds that can lead to inclusions in the weld metal and lower its mechanical properties. On heating, free oxygen in the molten metal combines with the carbon of the steel to form carbon monoxide. If gas is trapped in the weld metal as it cools, it collects in pockets and causes pores in the weld deposit.
Nitrogen, which makes up 78% of air, causes the most serious problems when welding steel. When steel is molten, it can take a relatively large amount of nitrogen into solution. At room temperature, the solubility of nitrogen in steel is very low. Therefore, in cooling, nitrogen precipitates or comes out of the steel as nitrites. These nitrites cause high yield strength, tensile strength, hardness, and a pronounced decrease in the ductility and impact resistance of the steel. The loss of ductility due to the presence of iron nitrites often leads to cracking of the weld metal. Excessive amounts of nitrogen can also lead to extensive porosity in the weld deposit.
Hydrogen may come from water in the atmosphere or from moisture on surfaces welded and is harmful to welds. Hydrogen is also present in oils, paints, and some protective coverings. Even very small amounts of hydrogen in the atmosphere can produce an erratic arc. Of more importance is the effect that hydrogen has on the properties of the weld deposit. As in the case of nitrogen, steel can hold a relatively large amount of hydrogen when it is molten but, upon cooling, it has a low solubility for hydrogen. As the metal starts to solidify, it rejects the hydrogen. The hydrogen entrapped in the solidifying metal collects at small discontinuities and causes pressure stresses to occur. This pressure can lead to minute cracks in the weld metal, which can later develop into larger cracks. Hydrogen also causes defects known as "fish eyes" and underbead cracks. Underbead cracking is caused by excessive hydrogen that collects in the heat-affected zone.
Inert and active gases may be used for FCAW. Active gases, such as carbon dioxide, argon-oxygen mixtures, and argon-carbon dioxide mixtures are used for almost all applications, with carbon dioxide being the most common. Active gases are not chemically inert and can form compounds with the metals. Since almost all flux cored arc welding is done on ferrous metals, this is not a problem.
The choice of the proper shielding gas for a specific application is based on:
- Type of metal to be welded
- Arc characteristics and metal transfer
- Cost of the gas
- Mechanical property requirements
- Penetration and weld bead shape
Carbon dioxide is manufactured from fuel gases that are given off by the burning of natural gas, fuel oil, or coke. It is also obtained as a by-product of calcining operation in limekilns, from the manufacturing of ammonia, and from the fermentation of alcohol. The carbon dioxide given off by the manufacturing of ammonia and the fermentation of alcohol is almost 100% pure. Carbon dioxide is made available to the user in either cylinder or bulk containers, with the cylinder being more common. With the bulk system, carbon dioxide is usually drawn off as a liquid and heated to the gas state before going to the welding gun. The bulk system is normally only used when supplying a large number of welding stations. In the cylinder, the carbon dioxide is in both a liquid and a vapor form, with the liquid carbon dioxide occupying approximately two thirds of the space in the cylinder, as shown in Figure 23. By weight, this is approximately 90% of the content of the cylinder. Above the liquid, it exists as a vapor gas. As carbon dioxide is drawn from the cylinder, it is replaced with carbon dioxide that vaporizes from the liquid in the cylinder; therefore, the overall pressure will be indicated by the pressure gauge.
Figure 23 Carbon dioxide gas cylinder.
When the pressure in the cylinder has dropped to 200 psi (1.4 MPa) the cylinder should be replaced. A positive pressure should always be left in the cylinder in order to prevent moisture and other contaminants from backing up into the cylinder. The normal discharge rate of the CO2 cylinder is about 10 to 50 cubic feet per hour (4.7 to 24 liters per minute). However, a maximum discharge rate of 25 cfh (12 L/min.) is recommended when welding using a single cylinder. As the vapor pressure drops from cylinder pressure to discharge pressure through the regulator, it absorbs a great deal of heat. If flow rates are set too high, this absorption of heat can lead to freezing of the CO2 regulator and flow meter, which interrupts the shielding gas flow. When flow rates higher than 25 cfh (12 L/min.) are required, normal practice is to manifold two CO2 cylinders in parallel, or to place a heater between the cylinder and gas regulator, pressure regulator, and flow meter. Figure 24 shows a manifold system used for connecting several cylinders together. Excessive flow rates can also result in drawing liquid from the cylinder.
Figure 24 Manifold system for CO2.
Carbon dioxide is the most widely used shielding gas for FCAW. Most active gases cannot be used for shielding, but carbon dioxide provides several advantages for use in welding steel, such as deep penetration, low cost, and it promotes a globular transfer. The carbon dioxide shielding gas breaks down into components, such as carbon monoxide and oxygen. Because carbon dioxide is an oxidizing gas, deoxidizing elements are added to the core of the electrode wire to remove oxygen. The oxides formed by the deoxidizing elements float to the surface of the weld and become part of the slag covering. Some of the carbon dioxide gas will break down to carbon and oxygen. If the carbon content of the weld pool is below about .05%, carbon dioxide shielding will tend to increase the carbon content of the weld metal. Carbon, which can reduce the corrosion resistance of some stainless steels, is a problem for critical corrosion applications. Extra carbon can also reduce the toughness and ductility of some low-alloy steels. If the carbon content in the weld metal is greater than about .10%, carbon dioxide shielding will tend to reduce the carbon content. This loss of carbon can be attributed to the formation of carbon monoxide, which can be trapped in the weld as porosity deoxidizing elements in the flux core, reducing the effects of carbon monoxide formation.
Argon and carbon dioxide are sometimes mixed for use with FCAW. A high percentage of argon gas in the mixture tends to promote a higher deposition efficiency due to creating less spatter. This mixture also creates less oxidation and lower fumes. The most commonly used argon-carbon dioxide mixture contains 75% argon and 25% carbon dioxide. This gas mixture produces a fine globular metal transfer that approaches a spray. It also reduces the amount of oxidation that occurs, compared to pure carbon dioxide. The weld deposited in an argon-carbon dioxide shield generally has higher tensile and yield strengths. Argon-carbon dioxide mixtures are often used for out-of-position welding, achieving better arc characteristics and welder appeal. This mixture also improves arc transfer on smaller diameters. Argon/CO2 is often used on low-alloy steels and stainless steels.
Electrodes designed for use with CO2 may cause an excessive build-up of manganese, silicon, and other deoxidizing elements if they are used with shielding gas mixtures containing a high percentage of argon, and this will have an effect on the mechanical properties of the weld.
Argon-oxygen mixtures containing 1 or 2% oxygen are used for some applications. Argon-oxygen mixtures tend to promote a spray transfer that reduces the amount of spatter. A major application of these mixtures is in welding stainless steels where carbon dioxide can cause corrosion problems.
The electrodes for FCAW consist of a metal sheath surrounding a core of fluxing and/or alloying compounds, as shown in Figure 25. The core of carbon steel and low-alloy electrodes contains primarily fluxing compounds. Some of the low-alloy steel electrode cores contain high amounts of alloying compounds with a low flux content. Most low-alloy steel electrodes require gas shielding.
Figure 25 Cross section of a fluxcored wire.
The sheath comprises approximately 75 to 90% of the weight of the electrode. Self-shielded electrodes contain more fluxing compounds than gas shielded electrodes. The compounds contained in the electrode perform essentially the same functions as the coating of a covered electrode used in shielded metal arc welding. These functions are:
- To form a slag coating that floats on the surface of the weld metal and protects it during solidification
- To provide deoxidizer and scavengers which help purify and produce solid weld metal
- To provide arc stabilizers which produce a smooth welding arc and keep spatter to a minimum
- To add alloying elements to the weld metal which will increase the strength and improve other properties in the weld metal
- To provide shielding gas, as gas-shielded wires require an external supply of shielding gas to supplement that produced by the core of the electrode
The manufacture of a flux-cored electrode is an extremely technical and precise operation requiring specially designed machinery. Figure 26 shows a simplified version of the apparatus for producing tubular type cored electrodes on continuous production. A thin, narrow, flat, low-carbon steel strip passes through forming rolls, which form the strip into a U-shaped cross-section. This U-shaped steel passes through a special filling device where a measured amount of the specially formulated granular core material is added. The flux-filled U-shaped strip then flows through special closing rolls which form it into a tube and tightly compress the core materials. This tube is then pulled through draw dies to reduce its diameter and further compress the core materials. Drawing tightly seals the sheath and additionally secures the core materials inside the tube under compression, thus avoiding discontinuities in the flux. The electrode may or may not be baked during, or between, drawing operations. This depends on the type of electrode and the type of elements and compounds enclosed in the sheath.
Figure 26 Making a flux-cored wire.
Additional drawing operations are performed on the wire to produce various electrode diameters. Flux-cored electrode wires are commonly available in sizes ranging from .035- to 5/32-inch.
The finished electrode is wound into a continuous coil, spool, reel, or drum. These are available as 10 lb., 15 lb., or 50 lb. spools, 60 lb. (27 kg) coils, 250 or 500 lb. (113-225 kg) reels, or a 600 lb. drum. Electrode wires are generally wrapped in plastic to prevent moisture pick-up.
The American Welding Society (AWS) devised the classification system used for tubular wire electrodes throughout industry in the United States. There are several different specifications covering flux cored arc welding electrodes for steels as shown in Table 3.
Table 3 Specifications covering flux-cored electrodes.
Carbon and low-alloy steels are classified on the basis of the following items:
- Mechanical properties of the weld metal
- Position of welding
- Chemical composition of the weld metal
- Type of welding current 5. Whether or not CO2 shielding gas is used
An example of a carbon-steel electrode classification is E70T-4 where:
- The "E" indicates an electrode.
- The second digit indicates the minimum tensile strength in units of 10,000 psi (69 Mpa). Table 4 shows the mechanical property requirements for carbon steel electrodes.
- The third digit indicates the welding position. A "0" indicates flat and horizontal positions only, and a "1" indicates all positions.
- The "T" stands for a tubular (flux-cored) wire classification.
- The suffix "4" gives the performance and usability capabilities as shown in Table 5.
Table 5 Performance and usability characteristics of carbon steel flux-cored electrodes (AWS A5.20)
When a "G" classification is used, no specific performance requirements are indicated. This classification is intended for electrodes not covered by another classification. The chemical composition requirements of the deposited weld metal for carbon steel electrodes are shown in Table 6.
Table 6 Chemical composition requirements of carbon-steel flux-cored electrodes (AWS A5.20).
Table 7 shows the mechanical properties requirements of low-alloy flux-cored electrodes. Single-pass electrodes do not have chemical composition requirements because checking the chemistry of undiluted weld metal does not give the true results of normal single-pass weld chemistry.
Table 7 Mechanical property requirements of low-alloy flux-cored electrodes (AWS A5.29).
The classification of low-alloy steel electrodes is similar to the classification of carbonsteel electrodes. An example of a low-alloy steel classification is ES1T1-Ni2 where:
- The "E" indicates an electrode.
- The second digit indicates the minimum tensile strength in units of 10,000 psi (69 Mpa). The mechanical property requirements for low-alloy steel electrodes are shown in Table 8.
- The third digit indicates the welding position capabilities of the electrode. A "0" indicates flat and horizontal positions only, and a "1" indicates all positions.
- The "T" stands for a tubular (flux-cored) wire classification.
- The fifth digit describes the usability and performance characteristics of the electrode. These digits are the same as used in carbon steel electrode classification but only EXXT1-X, EXXT4-X, EXXT5-X and EXXTS-X are used with low-alloy steel flux-cored electrode classifications.
- The suffix tells the chemical composition of the deposited weld metal as shown in Table 9.
The classification system for stainless steel electrodes is based on the chemical composition of the weld metal and the type of shielding to be used during welding. An example of a stainless steel electrode classification is E30ST-1 where:
1. The "E" indicates an electrode.
2. The digits between the "E" and the "T" indicate the chemical composition of the weld as shown in Table 10.
3. The 'T' stands for a tubular (flux-cored) wire classification.
4. The suffix indicates the type of shielding to be used as shown in Table 11.
Table 8 Impact requirements for low-alloy flux-cored
Table 9 Chemical composition requirements for low-alloy flux-cored electrodes
Table 10 Undiluted weld
metal composition requirements for stainless steel electrodes
Table 11 Performance and usability characteristics for stainless steel fluxcored electrodes.
|AWS Classification||External Shielding Gas||Welding Polarity|
|EXXXT-4||75-80% Argon/remainder CO2||DCEP|
|EXXXT-G||Not Specified||Not Specified|
The selection of the proper electrode for an application is based on the type of metal to be welded and the specific chemical and mechanical properties required of the joint. Identification of the base metal is required to select an electrode. If the type of metal is not known, tests must be made based on visual, magnetic, chisel, flame, fracture, spark, or chemistry tests.
The selection of the proper filler metal for a specific job application is quite involved but may be based on the following factors.
- Base Metal Strength Properties This is done by choosing an electrode wire to match the tensile or yield strength of a metal. This is usually one of the most important criteria for selecting a filler metal to be used on low-carbon and many low-alloy steels.
- Base Metal Composition The chemical composition of the metal to be welded should be known. Closely matching the filler and base metal com positions is important when corrosion resistance and creep resistance are needed. The filler metals for welding stainless steels and alloy steels are usually chosen based on matching chemical compositions.
- Welding Position Flux-cored electrodes are designed to be used in specific positions. Wire diameter is the major factor limiting the position in which an electrode can be used. All-position electrodes are available only in the smaller sizes. Flat and horizontal-position-only electrodes may have very similar compositions but are available in all sizes or the larger sizes that cannot be easily used for vertical and overhead welding. Electrodes should be selected to match the welding position.
- Welding Current Flux-cored electrodes are designed to operate on either direct current electrode negative or direct current electrode positive. Electrodes operating on DCEN generally give lighter penetration and higher deposition rates. Electrodes operating on DCEP generally provide deeper penetration.
- Joint Design and Fit-up Electrodes should be chosen according to their penetration characteristics. Gas-shielded flux-cored wires produce deeper penetration than self-shielding wires. This can have an effect on the joint design used.
- Thickness and Shape of Base Metal Weldments may include thick sections or complex shapes that may require maximum ductility to avoid weld cracking. Electrodes that give the best ductility should be used for these applications.
- Service Conditions and/or Specifications For weldments subject to severe conditions, such as low temperature, high temperature, or shock loading, an electrode that matches the ductility and impact strength of the steel should be selected.
- Production Efficiency and Job Conditions Large-diameter electrodes should be used, if possible, to give higher deposition rates.
Flux-cored electrodes for carbon and low-alloy steels are each designed for specific applications based on the composition of the flux core of the wire. Each suffix used indicates a general grouping of electrodes that have similar flux components and usability characteristics.
T-I electrodes are single- or multiple-pass electrodes. They operate on DCEP and require gas shielding. They produce a flat to slightly convex weld bead with a moderate slag coating. T-I electrodes produce a fine globular transfer and low spatter levels. Welds produced with T-1 electrodes have good mechanical properties.
T-2 electrodes operate on DCEP and also require gas shielding. These electrodes are similar to T-I types, but are designed to weld over rust and scale. They are for singlepass welding only because of their high silicon and manganese contents.
T-3 electrodes are self-shielding wires using DCEP for single-pass welding operations. These electrodes produce a fine globular transfer, and are designed for welding sheet metal at high welding speeds.
T-4 electrodes are self-shielding wires using DCEP for single- or multiple-pass operation. These electrodes produce a globular metal transfer and light penetration for joints with poor fit-up. Desulfurizing elements are contained in the flux core to help prevent weld cracking.
T-5 electrodes can be used to weld higher carbon steels, or for joining low-alloy steels to carbon steels because of cleaner welds and lower hydrogen levels.
T-6 electrodes are self-shielded electrodes for single- or multiple-pass welding using DCEP. A fine globular transfer and deep penetration characterize these electrodes. The slag coating has good deep-groove removal characteristics and produces good low temperature impact properties.
T-7 electrodes are self-shielded electrodes that operate on DCEN for single- or multiplepass welding. The larger sizes of this type of electrode are designed to produce high deposition rates. The smaller sizes are used for all-position welding. The slag coating de-sulfurizes the weld metal to a very low level that helps prevent cracking.
T-8 electrodes are self-shielding electrodes for single- or multiple- pass welding that operate on DCEN. The slag system is designed to allow all-position welding. The slag also desulfurizes the weld metal and produces good low temperature impact properties.
T-10 electrodes are self-shielded, single-pass electrodes that operate on DCEN. These electrodes are used for making welds in the flat and horizontal positions at high travel speeds.
T-11 electrodes are self-shielded electrodes that operate on DCEN for single- and multiple-pass welding. These are general-purpose electrodes for all-position welding at moderate travel speeds. They produce a fine globular transfer.
T-G electrodes are for multiple-pass welding not covered by another classification.
T-GS electrodes are single-pass electrodes not covered by another classification. The operating conditions and characteristics are not defined for the T-G and the T-GS electrodes.
Flux cored arc welding electrodes must conform to specifications, or be approved by code-making organizations for many FCAW applications. Some of the code-making organizations that issue specifications or approvals are the American Welding Society (AWS), the American Bureau of Shipping (ABS), and other state and federal highway and military organizations.
The American Welding Society provides specifications for flux-cored wire electrodes. Electrodes must meet specific requirements in order to conform to a particular electrode classification.
Many code-making organizations, such as the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API), recognize and use the AWS specifications.
Some of the code-making organizations, such as the American Bureau of Shipping (ABS) and the military, must directly approve the electrodes before they can be used for welding on a project covered by that code. These organizations send inspectors to witness the welding and testing, as well as to approve the classification of the flux-cored electrodes.
To conform to the AWS specifications for carbon- and low-alloy steel filler metals, the electrodes must produce a weld deposit that meets the specific mechanical and chemical requirements. For stainless steel filler metal, the electrodes must produce a weld deposit with a specific chemical composition. The requirements will vary depending on the class of the electrodes.
|Test Your Knowledge
3. For what wire size is the knurled V-groove drive rolls most commonly used?
4. Electrodes are designed to be used in specific positions.
- To Table of Contents -
Flux cored arc welding has gained popularity for a wide variety of applications. FCAW has replaced SMAW for some applications. One of the major advantages of the process is the high deposition rates obtained when compared to the manual arc welding processes. FCAW deposition rates are also generally higher than those obtained from gas metal arc welding. Because FCAW is a semiautomatic process, higher productivity can be obtained compared to SMAW. This process also lends itself easily to machine and automatic welding. Because of the versatility of FCAW, it has obtained wide application in shop fabrication, maintenance, and field erection work.
Each of the two variations of FCAW has their advantages, but the areas of application of the two variations often overlap. The method of welding used depends on the joint design, fit-up, availability of electrodes, and mechanical property requirements of the welded joints.
The self-shielding electrode wire variation can often be used for applications that can be done by SMAW. This is especially true when welding in locations where compressed gas cylinders are difficult to handle.
Gas-shielded flux-cored wires are used for many applications that compete with GMAW. There are many different applications possible but the most common ones are discussed below.
FCAW is the welding process of choice in a number of civilian industries because it is versatile, has high deposition rates, and is user friendly.
One of the most important applications of FCAW is in the structural fabrication industry. This industry uses a wide variety of low-carbon and low-alloy steels in many different thicknesses. Welding is done in the shop and in the field, and FCAW is readily adaptable to both types of wires. The major advantages of this process in the structural industry are the high deposition rates, high production rates, deep penetrating characteristics, and the adaptability of the process for field erection welding. Because a large percentage of the welds made in structural work are fillets, FCAW is widely used for making large single-pass fillet welds. Many of these welds would require multiple passes using GMAW and SMAW.
Gas-shielded flux-cored wires have replaced SMAW and GMAW for many shop applications. FCAW is widely used for welding the thicker structural members where the higher deposition rates provide more advantage. Figure 27 shows welding a bridge girder using a gas-shielded flux-cored wire in the flat position. Out-of-position welding is done using the smaller diameter wires.
Figure 27 Flux cored arc welding of structures.
For field welding, the self-shielding flux-cored wires are commonly used. These flux-cored wires are preferred over the gas-shielded types because a supply of shielding gas is not required, which makes the equipment more portable.
Another advantage of the self-shielding electrodes for field construction is that they can be used in windier conditions. This is because the decomposition of the flux core that provides the shielding is less sensitive to wind than an external gas shielding supply.
Figure 11-28 shows FCAW being used. Note the welders hand shield in place to protect from the higher heat created by the FCAW process.
Figure 28 Self-shielded flux cored welding.
Another application of self-shielding electrodes is for welding galvanized steel roof decking. Single-pass electrodes using DCEN are preferred for most applications because of the lighter penetration produced, which reduces the chances of burning through the decking.
FCAW is used in the shipbuilding industry because of the wide variety of low-carbon and low-alloy steels and metal thicknesses being welded. Because this process can be used in the vertical and overhead positions, it is used in places where submerged arc welding (SAW) cannot be used. The process is also useful for vertical welding on metal thicknesses too thin for electroslag welding to be economical. Most FCAW is done semi-automatically but some automatic welding applications are used. Figure 29 shows an automatic welding system welding a cargo hoist control unit.
Figure 29 Automatic welding system. *
FCAW is used to some extent in the industrial piping industry. This process is used for welding pipe in both the shop and the field for steam generating plants, refineries, distilleries, and chemical processing plants. FCAW competes with submerged arc welding, SMAW, and GMAW.
This process may be used to deposit all passes or it may be used to deposit the fill and cover passes over a root pass welded by another process. Flat roll welding (I G position) is often used for both semiautomatic and automatic welding applications. This allows higher welding currents with larger diameter wires and requires fewer weld passes. Roll welding is often used, especially on large-diameter piping. Copper backing strips are sometimes used to allow higher current levels and insure full penetration to the root of the joint. When welding fixed position pipe, smaller diameter electrodes are used. These electrodes operate at lower current levels and require more passes. In these positions, the root pass is often welded using GMAW and sometimes GTAW. In horizontal fixed (5 G position) welding, the root pass by FCAW is done using a downhill technique. The remaining passes are then welded using an uphill technique.
FCAW is used for welding both carbon-steel and alloy-steel pipe. A major application of the process is for welding chromium-molybdenum steel pipe. This is the major type of alloy steel used for pipe. Flux-cored electrodes are preferred over the solid wire when matching chemical compositions. This is because porosity is hard to avoid. In addition, with the solid wire electrodes the operating characteristics of solid wires are not as good, which makes them more difficult to use. Most of the electrodes used for FCAW pipe are gas-shielded because of the better penetration and the generally better mechanical properties produced.
FCAW is used extensively in the railroad industry. Other processes, such as SMAW, GMAW and SAW, are also widely used, so the choice of the welding process is based on the weld size, joint accessibility, joint length and welding position. The longest welds on the heavier metal thicknesses in the flat position are generally welded using SAW. FCAW is usually used on the heavier metal thicknesses where SAW is not practical. Examples would be for joints in other positions, shorter joints, and where accessibility is more limited. FCAW is preferred over SMAW and GMAW for many uses because of the higher deposition rates obtained. Many different components on the engines and the rail cars are commonly welded. Figure 30 shows FCAW of a seam on a rail car.
Figure 30 FCAW of a railroad car.
FCAW has gained popularity for use in the automobile and truck manufacturing industries. This process is used because of the high production rates that can be obtained. Both the self-shielding and the gas-shielded electrode wires have been used. The gas-shielded wires are generally used when deeper penetration is required. FCAW is also popular because it can be easily automated. Components such as frames, truck wheels, trailers, and axle housings are common applications. FCAW is more popular for trucks because of the larger thicknesses of metal generally used.
An example of FCAW is shown in Figure 31 where a truck trailer chassis is being welded. This part had previously been a casting that was made into a weldment. Because of the relatively thick plate being welded, FCAW is more economical on this application than GMAW. Another advantage
Figure 31 FCAW of a truck frame.
of this application is that the depth of some bevels has been reduced and some bevels have been eliminated because of the deep penetrating characteristics of the process. The use of FCAW has increased over GMAW for many frame welding applications because joint fit-up is less important, better appearing weld beads can be produced, and FCAW has better welder appeal. Many flux-cored electrodes have been developed for welding over some rust and scale, which reduces the metal preparation time.
A special application of FCAW is for welding catalytic converters. These are made of type 409 stainless steel that is welded with an equivalent filler metal using gas shielding.
The heavy equipment manufacturing industry includes mining, agricultural, and earth moving equipment, as well as other items such as forklift trucks and armored vehicles. FCAW is popular in these industries because of the high deposition rates obtained. Fillet welds are often encountered in these industries, and large single-pass fillet welds can often be welded by FCAW, which eliminates interpass cleaning time and increases productivity.
The mining equipment manufacturing industry also is a major user of FCAW for welding a wide variety of low-carbon and low-alloy steels.
The FCAW process is very useful for maintenance and surfacing operations. Maintenance operations range from repairing and modifying plant and building facilities to repairing pipe, production equipment, and castings. Surfacing and salvaging operations include the repair of mis-machined parts, foundry defects, accommodating engineering changes, rebuilding worn parts (especially shafting and rollers), and overlaying parts with special materials. Reclamation includes the disassembly and rewelding of defective items manufactured in the factory and in the field. It has been used for maintaining and repairing items too expensive to repair with oxyacetylene welding and other arc welding processes. Self-shielding flux-cored electrodes are popular for field repairs and maintenance because the equipment is more portable.
A metal overlay can be used to extend the usable life of new parts that lack some of the wear-resistant qualities required for certain applications. Overlays are used mostly to replace metal that has been worn away by abrasion, corrosion, and impacts. An overlay provides toughness and resistance to corrosion, abrasion, and wear at the exact location on the part where it is needed most. The primary reason for weld overlaying parts is to prepare them for certain applications and to extend their service life. FCAW is widely used because of its characteristic high deposition rate and good weld bead appearance.
Flux cored arc spot welding (FCASW) is a variation of the process where a fusion weld is made through one sheet into an adjacent sheet of a lap joint while the welding gun is held stationary. The equipment used for arc spot welding is the same as for normal welding, except that it requires a timer and a special gun nozzle. FCASW is used on low-carbon and low-alloy steels and is generally preferred for welding thicker sheet metal and thin plate sections. This is because of the greater penetrating capability of the process as compared to gas tungsten (GTASW) or gas metal arc spot welding (GMASW). The FCASW process also provides a wider penetration spot weld at the interface between the plates to be joined. This produces a larger diameter spot weld **47 with greater strength. FCASW is identical to GMASW except that a flux-cored electrode wire is used. Carbon dioxide shielding is generally used but argon-CO2 mixtures are sometimes used to reduce the amount of penetration. When welding thinner metals, a backup bar is used under the sheet metal.
The advantages of FCASW over resistance spot welding are:
The main disadvantage of arc spot welding is the consistency of weld size and strength is not as good.
Either the gas-shielded or self-shielding fluxcored electrodes may be used. The weld is made by depressing the trigger that starts the shielding gas, if used, and, after a preflow interval, starts the arc and the wire feed. The arc melts through the top sheet of the lap joint and fuses into the bottom sheet. When the preset weld time is finished, the arc and wire feed are stopped, followed by the gas flow, if used. FCASW is shown in Figure 32. This process is used for making welds in metal ranging from 16 gauge (1.5 mm) to 1/4-in. (6.4 mm) in thickness. Metals of the same or different thicknesses can be made. If dissimilar thicknesses are being welded, the thinner member should always be placed on top. The length of the spot weld cycle affects the penetration and the amount of reinforcement on the surface of the weld bead. FCASW generally produces larger, stronger weld nuggets on the same metal thicknesses as compared to GMASW. The rest of the welding variables affect the weld in the same way as normal weld.
Figure 32 FCASW.
- To Table of Contents -
Welding metallurgy concerns the chemical, physical, and atomic properties and structures of metals, and the principles by which metals are combined to form alloys.
The properties of a weld include the chemical composition, mechanical strength, ductility, toughness, and the microstructure. These items will relate to the weldability of the metal. The weldability of a metal is the quality obtained and the ease of welding for the intended service conditions. The types of materials used affect the chemical, physical, and mechanical properties of the weld. The mechanical properties and microstructure are determined by the heat input as well as the chemical composition and physical properties of the weld.
The chemical composition of the base and filler metal has a great influence on the weldability of a metal, and this property has an influence on the preheating and postheating used, as well as the welding parameters.
For welding stainless steels, the chemical composition of the weld is often the most important property. The chemical composition of the weld must match the composition of the base metal when corrosion resistance, thermal and electrical conductivity, and appearance are major considerations. The chemical composition can also affect the high and low temperature strength, as well as the microstructure and mechanical properties of the weld. Preheating reduces the cooling rate of the weld after welding to prevent cracking. The amount of preheat needed depends on the type of metal being welded, the metal thickness, and the amount of joint restraint.
Steels with higher carbon contents need higher preheat than steels with lower carbon equivalents. Table 12 shows typical preheat values for different metals welded by FCAW.
Table 12 Preheats for various metals.
The actual preheat needed may depend on several other factors, such as the thickness of the base metal, the amount of joint restraint, and whether or not low-hydrogen types of electrodes are used. This chart is intended as general information; the specifications of the job should be checked for the specific preheat temperature used.
Another major factor that determines the amount of preheat needed is the thickness of the base metal. Thicker base metals usually need higher preheat temperatures than thinner base metals. Thick metal draws the heat away from the welding zone more quickly because there is a large mass of metal to absorb the heat. This would cause a quicker cooling of the weld if the same preheat temperature was used, as on thinner base metals.
The third major factor for determining the amount of preheat needed is the amount of joint restraint. Joint restraint is the resistance of a joint configuration to moving or relieving the stresses due to welding during the heating and cooling of the weld zone. Where there is high resistance to moving or high joint restraint, large amounts of internal stresses build up. Higher preheat temperatures are needed as the amount of joint restraint increases. Slower cooling rates reduce the amount of internal stresses that are building up as the weld cools.
The mechanical properties that are most important in the weld are the tensile strength, yield strength, elongation, reduction of area, and impact strength. The first two are measures of the strength of the material, the next two are a measure of the ductility, and the last is a measure of the impact toughness. These properties are often important in FCAW steels designed to give maximum strength, ductility, and toughness.
FCAW can produce good properties in the weld- and heat-affected zone. The slag coating in FCAW slows the cooling rate of the weld metal, which reduces the tendency to become brittle.
FCAW produces a higher heat input, which will also tend to produce a slower cooling rate. A disadvantage of the higher heat input is that distortion is more of a problem than with GMAW. The mechanical properties of the weld will vary, depending on whether a self-shielded or gas-shielded flux-cored wire is used. Some self-shielded electrodes contain high amounts of deoxidizers, which may produce weld metal with relatively low impact toughness. Most of the gas-shielded flux-cored wires produce welds that have better impact toughness.
The yield strength, ultimate tensile strength, elongation, and reduction of area are all measured from a .505 in. (12.8 mm) diameter machined tensile testing bar. The metal is tested by pulling it in a tensile testing machine. Figure 33 shows a tensile bar before and after testing. The yield strength of the metal is the stress at which the material is pulled beyond the point where it will return to its original length. The ultimate tensile strength is the maximum stress or load that can be carried by the metal without breaking. This is also measured in psi (MPa). Elongation is a measure of ductility that is also measured on the tensile bar. Two points are marked on the bar 2 in. (51 mm) apart before testing. After testing, the distance between the two points is measured again and the percent of change in the distance between them, or percent of elongation, is measured.
Figure 33 Tensile strength testing bars.
Reduction of area is another method of measuring ductility. The original diameter of the testing bar is .20 sq in (128 sq mm). During the testing, the diameter of the bar reduces as it elongates. When the bar finally breaks, the diameter of the bar at the breaking point is measured, which is then used to determine the area. The percent reduction of this cross-sectional area is called the reduction of area.
Impact tests are used to measure the toughness of a metal. The toughness of a metal is the ability of a metal to absorb mechanical energy by deforming before breaking. The Charpy V-notch test is the most commonly used method of making impact toughness tests. Figure 34 shows some typical Charpy V-notch test bars. Bars with V-notches are put in a machine where they are struck by a hammer attached to the end of a pendulum. The energy that it takes to break these bars is known as the impact strength and it is measured in foot-pounds (joules, also called newton-meters).
Figure 34 Charpy V-notch bars.
Figure 35 shows a cross section of a weld bead showing the weld metal zone, the heat-affected zone, and the base metal zone -- the three basic microstructural areas within a weldment. The weld metal zone is where the metal was molten during welding. The heat-affected zone is the area where the heat from welding has an effect on the microstructure of the base metal. The base metal zone is the area that was not affected by the welding.
Figure 35 Cross section of weld bead showing in the three areas.
The extent of change of the microstructure is dependent on four factors:
The weld metal zone, which is the area heated above about 2800°F (1540°C) and melted, generally has the coarsest grain structure of the three areas. Generally, a fairly fine grain size is produced on cooling in most metals. Large grain size is undesirable because it gives poor weld toughness and cracking resistance. The filler metal starts to solidify at the edges of the weld puddle. The grains that form at the edge are called dendrites and they grow toward the center of the weld into the area that is still molten (Figure 36).
Figure 36 Solidification pattern of the weld.
These dendrites give the weld metal its characteristic columnar grain structure. The grains that form in the weld zone are similar to the grains that form in castings. Deoxidizers and scavengers are often added to filler metal to help refine the grain size in the weld. The greater the heat input to the weld and the longer that it is held at high temperature, the larger the grain size. A faster cooling rate will produce a smaller grain size than a slower cooling rate. Preheating will give larger grain sizes, but is often necessary to prevent the formation of a hard, brittle microstructure.
The heat-affected zone is an area of change in the microstructure of the base metal. The area that is closest to the weld metal usually undergoes grain growth. Other parts of the heat-affected zone will go through grain refinement. Other areas may be annealed and considerably softened. Because of the changes due to the heat input and cooling rate, areas of the heat-affected zone can become embrittled and become the source of cracking. A large heat input during welding will cause a larger heat-affected zone. This is often not desirable, so the welding parameters used can help influence the size of the heat-affected zone.
FCAW is commonly used to weld most steels and stainless steels. This process also welds some nickel alloys. Most nonferrous metals are not welded by this process because of the high heat input and because suitable electrode wires have not been developed.
FCAW is widely used for welding steels. In general, steel is classified according to the carbon content, such as low-carbon, mild, medium-carbon, and high-carbon steels. In addition, steel is also classified according to the alloys used. For the purpose of discussion in this course, the different steels will be grouped according to their welding characteristics.
When welding steel, the carbon and other alloy content influences the hardness and hardenability of the weld metal, which in turn influences the amount of preheat needed. The two terms, hardness and hardenability, are not the same. The maximum hardness of a steel is primarily a function of the amount of carbon in the steel. Hardenability is a measure of how easily a martensite structure is formed when the steel is quenched. Martensite is the phase or metallurgical structure in steel where the maximum hardness of the steel can be obtained. Steels with low hardenability must have very high cooling rates after welding to form martensite, where steels with high hardenability will form martensite even when they are slow cooled. The hardenability will determine to what extent a steel will harden during welding. The carbon equivalent formula is one of the best methods of determining the weldability of steels. This value is determined by the amounts of the alloying elements used. There are several different formulas used. One of the most popular is as follows:
|Carbon Equivalent = %C +||%Cr||+||%Mn||+||%Mo||+||%Ni||+||%Cu|
Steels with lower carbon equivalents generally are more readily weldable and require fewer precautions, such as the use of preheat and postheat. Steels with higher carbon equivalents are generally more difficult to weld. When welding some of the steels, it is more important to match the mechanical properties than the chemical composition of the filler metal to the base metal. Often, filler metal with a lower carbon content than the base metal is used because the weld metal absorbs carbon from the base metal. This is done to minimize the tendency for weld cracking.
Low-carbon and mild steels are those that have low carbon contents and are the most readily weldable. This group of steels is the most widely used in industrial fabrication. This group also includes the high strength structural steels.
Low-carbon steels have carbon contents up to .14%. Mild steel has carbon contents ranging from .15 to .29%. For many applications, preheating is not required except on thick sections, highly restrained joints, or where codes require preheating. Other precautions, such as interpass temperature control and postheating, are sometimes used. With thicker sections and highly restrained joints, preheating, interpass temperature control, and post-heating are usually required to prevent cracking. When welding these steels, electrodes of the E70-T class are used with carbon dioxide. Self-shielding wires are also widely used. The filler metal should be chosen so that it matches the tensile strength of the base metal. When welding rimmed steels, which have silicon contents less than .05%, filler metal with sufficient amounts of deoxidizers must be chosen to prevent porosity. This precaution is not necessary for welding steels containing more than .05% silicon.
The high-strength structural steels are steels whose yield strength falls between 45,000 psi (310 MPa) and 70,000 psi (485 MPa) and their carbon content is generally below .25%. These steels have relatively small amounts of alloying elements. Some common examples of these steels are the ASTM designations of A242, A441, A572, A588, A553, and A537.
Some low-carbon and mild steel electrodes are designed for welding over some rust and mill scale. The flux core helps to reduce the bad effects of rust and mill scale but some reduction in weld quality may occur. These FCAW electrodes are preferred for many applications because cleaning of the base metal is less important. For applications where the maximum mechanical properties are not as important as higher deposition rates and travel speeds, high welding currents can be used.
The low-alloy steels discussed here will be those steels that are low-carbon and have alloy additions less than 5%. This includes the quenched and tempered steels, heat-treated low-alloy steels, and the low-nickel-alloy steels. Elements such as nickel, chromium, manganese, and molybdenum are the main alloying elements used. These steels have a higher hardenability than mild steels and that is the principal complication in welding. Low-alloy steels have good weldability but are not as good as the mild steels. This higher hardenability permits martensite to form at lower cooling rates. As the alloy content and the carbon content increases, the hardenability also increases.
In general, the weldability of the steel decreases as the hardenability increases. One of the best methods for determining the weldability of a low-alloy steel is the use of the carbon equivalent formula. Steels that have carbon equivalents below about .40% usually do not require the use of preheating and postheating in the welding procedure and generally have the best weldability. Steels with carbon equivalents higher than .40% require more precautions for welding. Generally, the higher the carbon equivalent, the more difficult the steel is to weld.
The selection of electrodes for welding steels is usually based on the strength and mechanical properties desired of the weld, rather than matching chemical compositions. Low-alloy steels are often welded using the gas-shielded EXXT-1 and EXXT-5 electrodes. These wires produce good, low temperature toughness and are preferred for most applications. EXXT-4 and EXXT-8 self-shielded wires often contain nickel for good strength and aluminum as a deoxidizer to help give good mechanical properties. In other cases, such as for welding low nickel steels, the electrode wires are chosen to match the chemical composition of the base metal.
The quenched and tempered heat-treated steels have yield strengths ranging from 50,000 psi (345 MPa) to very high yield strengths and have carbon contents ranging up to .25%. Some common examples of these types of steel are the ASTM designations A533 Grade B, A514, A517, A543, and A553. The .25% carbon limit is used to provide fairly good weldability. These steels provide high tensile and yield strength along with **54 good ductility, notch toughness, corrosion resistance, fatigue strength, and weldability. The presence of hydrogen is always bad in steel, but it is even more critical in these types of steels compared to mild steels. Preheat is generally not used on thinner sections, but it is used on thicker or highly restrained sections. Postweld heat treatment is usually not used because the flux cored arc welds made in these have a good toughness. The steels are generally used in the welded or stress-relieved conditions.
The nickel alloy steels included in these low-alloy steel groups are those with less than 5% nickel contents. The 2-1/4% and 3-1/2% nickel steels are usually welded with electrodes that have the same general chemical compositions as the base metal. Preheating is required with highly restrained joints. Most self-shielding wires for lowalloy steels have been developed for welding the low nickel steels.
The heat treatable steels are the medium- and high-carbon steels and medium-carbon steels that have been alloyed. This group includes quenched and tempered steels after welding, normalized or annealed steels, and medium- and high-carbon steels. These steels are more difficult to weld than other types of steels already mentioned in this course. The most important factor for selecting the type of electrode to be used is matching the chemical compositions of the base metal and the filler metal.
Medium-carbon steels are those that have carbon contents ranging from .30% to .59% and high-carbon steels have carbon contents ranging from .60% to about 1.0%. When medium- and high-carbon steels are welded, precautions should be included in the welding procedure because of the hardness that can occur in the weld joint. As the carbon content increases up to .60%, the hardness of the fully hardened structure (or martensite) increases to a maximum value. When the carbon content is above .60%, the hardness of the fully hardened structure does not increase, so these steels can be welded using about the same welding procedures as the medium-carbon steels. Martensite, which is the phase that steel is in at its fullest hardness, is harder and more brittle in a high-carbon steel than it is in low-carbon steel. A high-carbon, martensitic structure can have a tendency to crack in the weld metal and heat-affected zone during cooling.
Welding procedures that lower the hardness of the heat-affected zone and the weld metal reduce the cracking tendency. This can be done by using a procedure that requires a lower carbon content in the filler metal, and by slowing the cooling rate.
The procedure includes preheating, interpass temperature control, and postheating. The procedures used for welding medium-carbon steels can be simpler than the one just mentioned, but that depends on the specific applications. Medium-carbon steels can be welded with the E70T-E90T classifications. High-carbon steels should be welded with the E80T-E120T, using the electrode of the proper tensile strength to match the tensile strength of the base metal. Generally, very high-carbon steels are not used in welded production work. These steels are usually only welded in repair work. Mild steel electrodes may also be used, but the deposited weld metal absorbs carbon from the base metal and thus loses a considerable amount of ductility. Stainless steel electrodes of the austenitic type are sometimes used, but the fusion zone may still be hard and brittle. A preheat and/or postheat will help reduce the brittle structure.
The quenched and tempered steels, after welding, have carbon contents ranging from about .25% to .45%, which distinguishes them from the steels that are quenched and tempered before welding. These steels also have small additions of alloying elements. Some common examples of these steels are the AISI designations 4130, 4140, and 4340. Because of the higher carbon contents, the steels in this group can be heat treated to extremely high levels of strength and hardness. Some of these steels have enough alloy content to give them high hardenability. Because of this combination of carbon and alloy content, the steels must be preheated before welding. The weldability of these steels is also influenced by the purity of the steels. High amounts of sulfur and phosphorous in the steel increase the sensitivity to cracking and reduce the ductility. FCAW is often used for welding these steels. Filler metal of the same chemical composition as the base metal is required to obtain the maximum strength. The composition of the weld metal is usually similar to that of the base metal.
The low chromium-molybdenum steels in this section are those with alloy contents of about 6% or less. These steels are in the low-carbon range, generally up to .15%, and are readily weldable. The chromium- and molybdenum-alloying elements provide these steels with good oxidation resistance and high temperature strength. The chromium is mainly responsible for the high oxidation resistance and the molybdenum is mainly responsible for the high temperature strength.
The higher chrome-moly steels contain about 6-10% chromium and .5-1% molybdenum. These steels are limited to a maximum carbon content of about .10% to limit the hardness because these steels are very sensitive to air hardening. For welding these steels, preheating, interpass temperature control, slow cooling, and postweld heat treatment are required to make a weld with good mechanical properties. These steels generally do not require preheating except when welding thick sections or highly restrained joints. Postheating is usually not required on chromium molybdenum steels that contain less than 2-1/4% Cr and 1% Mo.
FCAW is one of the most common methods of welding the chromium-molybdenum steels. The steels with less than 6% chromium are welded with a carbon dioxide or argon-carbon dioxide mixture. For the steels with 6% chromium or more, argon with small additions of carbon dioxide is often used. The filler metal is chosen to match the chemical composition of the base metal as closely as possible to give good corrosion resistance.
Free machining steels are steels that have additions of sulfur, phosphorous, selenium, or lead in them to make these steels easier to machine. Except for the high sulfur, lead, selenium, or phosphorous, these steels have chemical compositions similar to mild, lowalloy, and stainless steels. The addition of these elements makes these steels difficult to weld. The reason for this is that the elements- lead, phosphorous, selenium and sulfurhave melting points that are much lower than the melting point of the steel. As the weld solidifies, these elements retain liquid much longer than the steel so that they coat the grain boundaries, which cause hot cracking in the weld. Hot cracking is cracking that occurs before the weld has had a chance to cool. Because of this hot cracking problem, free machining steels cannot be welded easily. High manganese filler metal and low base metal dilution will help give the best results possible.
FCAW can weld most types of stainless steels. The types that are very difficult to weld are types such as 303,416,416 Se, 430 F, and 430 FSe, which have high sulfur and selenium contents, and Type 440, which has a high carbon content. The element that distinguishes stainless steels from the other types of steel is the chromium. Steels that **56 have chromium contents greater than 11 % are considered stainless steels. The high chromium content gives them very good corrosion and oxidation resistance. The three major groups of stainless steels that are welded are the austenitic, martensitic, and ferritic types.
The austenitic types of stainless steels are generally the easiest to weld. In addition to the high chromium content of about 16-26%, these types have high nickel contents ranging from 6-22%. These steels are designated by the AISI as the 300 series. The 200 series, which has high manganese contents to replace some of the nickel, is also austenitic. Nickel and manganese are strong austenite formers and maintain an austenitic structure at all temperatures. This structure gives these steels good toughness and ductility but also makes them non-hardenable. A major problem when welding these types of steels is carbide precipitation or sensitization, which only occurs in the austenitic structure. This occurs when the temperature of the steel is between approximately 1000°-1600° F (540°-870° C) and can greatly reduce the resistance to corrosion. There are several methods for preventing this problem:
Martensitic stainless steels are not as easy to weld as the austenitic stainless steels. These stainless steels have approximately 11-18% chromium, (the major alloying element), and are designated by the AISI as the 400 series. Some examples are 403,410, 420, and 440. These types of stainless steels are heat treatable because they generally contain higher carbon contents and a martensitic structure. Stainless steels with higher carbon contents are more susceptible to cracking and some, such as Type 440, have carbon contents so high that they are often considered unweldable. A stainless steel with a carbon content greater than .10% will often need preheating. Preheating is usually done in the range of from 400-600° F (205-315° C) to avoid cracking. For steels containing carbon contents greater than .20%, a postweld heat treatment, such as annealing, is often required to improve the toughness of the weld produced.
Ferritic stainless steels are also more difficult to weld than austenitic stainless steels because they produce welds having lower toughness than the base metal. These stainless steels form a ferritic grain structure and are also designated by the AISI as the 400 series. Some examples are Types 405, 430, 442, and 446. These types are generally less corrosion resistant than austenitic stainless steel. To avoid a brittle structure in the weld, preheating and postheating are often required. Typical preheat temperatures range from 300-500° F (150-260° C). Annealing is often used after heat treatment welding to increase the toughness of the weld.
The FCAW process can produce stainless steel weld deposits with a quality similar to those produced by GMAW. Lower current levels may be desirable for welding stainless steel compared to welding mild steel because of the higher thermal expansion, lower thermal conductivity, and lower melting point of stainless steel. The lower thermal conductivity and higher thermal expansion cause more distortion and warpage for a given heat input.
Carbon dioxide, argon-carbon dioxide, and argon-oxygen mixtures are used. Carbon dioxide causes a loss of silicon and manganese and an increase in carbon in the lowcarbon stainless steels. The use of carbon dioxide or EXXT-1 electrodes is restricted for welding many of the stainless steels, especially austenitic grades, because the corrosion resistance may be reduced due to carbon added to the weld by gas. When good corrosion resistance is required, argon-carbon dioxide or argon-oxygen mixtures are used. The argon-oxygen mixtures containing 1 or 2% oxygen are used to improve the arc stability and weld puddle wetting, as well as to eliminate carbon pickup from the shielding gas. When the self-shielding EXXXT-3 electrodes are used, there is greater pickup of nitrogen from the atmosphere into the weld metal. Nitrogen is an austenite stabilizer and when the weld absorbs excessive nitrogen, there is a greater chance for micro-cracking to occur. The welding position and arc length have a large influence on this problem. An excessive arc length will usually cause excessive nitrogen pickup in the weld. For this reason, procedures for out-of-position welding with self-shielding wires should be carefully controlled to produce a sound weld deposit.
The filler metal used for welding stainless steel is generally chosen to match the chemical composition of the base metal. In the 200-series austenitic stainless steels, 300-series austenitic filler metal is usually used due to a lack of availability of 200-series filler metal. This weld joint will generally be weaker than the surrounding base metal. 300-series filler metal is used on 300-series base metal. The Type 410 and 420 electrodes are the only martensitic stainless steel types recognized by the AWS. This limitation is often the reason why austenitic stainless steel filler metal is used for welding martensitic stainless steel. Austenitic filler metal provides a weld with lower strength but higher toughness and eliminates the need for preheating and postheating. For welding ferritic stainless steels, both ferritic and austenitic filler metal may be used. Ferritic filler metal is used when higher strength and an annealing postheat are required. Austenitic filler metal is used when higher ductility is required. Table 13 shows filler metal selection for stainless steels.
Table 13 Filler metal selection for welding stainless steel.
|Test Your Knowledge
5. What primary property determines the maximum hardness of steel?
6. What type of stainless steel is generally the easiest to weld?
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Like other welding processes, the weld joint designs used in FCAW are determined by the design of the weldment, metallurgical considerations, and codes or specifications. Another factor to consider is the method of FCAW to be used. A properly selected joint design should allow the highest quality weld to be made at the lowest possible cost. A weld joint consists of a specific weld being made in a specific joint. A joint is defined as the junction of members which are to be, or have been, joined. Figure 37 shows the five basic joint classifications
Figure 37 Types of joints.
Each of the different types of joints can be joined by many different types of welds. Figure 38 shows the most common types of welds made.
Figure 38 Types of welds.
The type of weld made is governed by the joint configuration. Each of the different types of welds has its own specific advantages. The nomenclature used for the various parts of groove and fillet welds is given in Figure 39.
Figure 39 Weld nomenclature.
There are several factors that influence the joint design to be used:
The edge and joint preparation are important because they affect both the quality and cost of welding. The cost items to be considered are the amount of filler metal required, the method of joint preparation, the amount of labor required, and the quality level required. Joints that are more difficult to weld will often have more repair work necessary than those that are easier to weld. This can lead to significant increases in cost, since repair welding sometimes requires more time and expense than the original weld. All of the five basic joint types are applicable to FCAW, although the butt and T-joints are the most widely used. Lap joints have the advantage of not requiring much preparation other than squaring off the edges and making sure the members are in close contact. Edge joints are widely used on thin metal. Corner joints generally use similar edge preparations to those used on T-joints.
Many of the joint designs used for FCAW are similar to those used in GMAW or SMAW. FCAW has some characteristics that may affect the joint design. The joint should be designed so the welder has good access to the joint and is properly able to manipulate the electrode. Joints must be located so an adequate distance between the joint and nozzle of the welding gun is created. The proper distance will vary depending on the type of flux-cored electrode being used.
The joint design as well as the welding procedure will vary, depending on whether the welding is done using gas-shielded or self-shielded electrodes. Both methods of FCAW achieve deeper penetration than SMAW. This permits the use of narrower grooves with
smaller groove angles, larger root faces, and narrower root openings. Differences also exist between the two FCAW methods because of the deeper penetration that is produced by the gas-shielded electrode wires. Figure 40 shows a comparison of a flat position, V-groove weld on a backing strip for each of the two methods. The joint design for the self-shielding wire requires a larger root opening to allow better access to the root of the joint.
Figure 40 Comparison between gas-shielded and self-shielded wire joint designs for the flat position.
The joint design for the gas-shielded wire does not need such a wide root opening because complete penetration is easier to obtain. This weld would be less expensive to make using the gas-shielded electrode because less filler metal is required. This difference in joint design usually only applies when a backing strip is used. For joints not requiring a backing strip, gas-shielded and self-shielded wires use the same joint designs.
The FCAW process is used to weld steel, some stainless steels, and some nickels. The influence of the type of metal on the joint design is based primarily on the physical properties of the metal to be welded and whether or not the metal has an oxide coating. For example, stainless steels have a lower thermal conductivity than carbon steels. This causes the heat from welding to remain in the weld zone longer, which enables slightly greater thicknesses of stainless steels to be welded using a square groove joint design. Stainless steels also have an oxide coating that tends to reduce the depth of fusion of the weld. Consequently, stainless steels normally use larger groove angles and root openings than carbon steels. This allows the welder to direct the arc on the base metal surfaces to obtain complete fusion.
The strength required of a weld joint is a major factor governing weld joint design. Weld joints may be either full or partial penetration, depending on the strength required of the joint. Full or complete penetrating welds are those that have weld metal through the full cross section of the joint. Partial penetrating welds are those where weld metal only extends partially through the joint thickness. Welds that are subject to cyclic, impact, or dynamic loading require complete penetration. This is even more important for applications that require low temperature service. Partial penetration welds may be adequate for joints where loading is static only. This type of joint is easier to prepare and requires less filler metal than full penetration joints. Fillet welds of the same leg size made by this process are stronger than those made by SMAW. This is because of the deeper penetration obtained from FCAW, as shown in Figure 41. For some applications, the size of the weld can be reduced which decreases the amount of filler metal required. This can reduce the total cost also.
Figure 41 Comparison between the penetrating characteristics of SMAW and FCAW.
The root opening and root face used will affect the amount of penetration obtained. A root opening is used to allow good access to the root of the joint and is usually used in full penetrating weld joints. A root opening is usually not used in partial penetration weld joints because access to the root is not necessary and parts are easier to fit together without a root opening. The size of the root face is also affected. A larger root face is used more for partial penetration welds than for complete penetration welds because less penetration is required. Because of the deep penetrating characteristics of the FCAW process, larger root faces are used compared to SMAW and GMAW, which use short circuiting metal transfer. This is to prevent burning through the back of the joint being welded, which can be a problem in FCAW because of the high welding currents used. When compared to SMAW, smaller groove angles are used because the flux-cored wire is smaller than a covered electrode and operates with a higher current density. Because of the smaller electrode, access to the root of the joint is better.
FCAW may be used in all welding positions based on the size and type of electrode wire used. A diagram of the welding position capabilities is shown in Figure 42.
Figure 42 Welding test positions.
Welding positions are classified by a set of numbers and letters. The four basic welding positions are designated by the numbers 1 for flat, 2 for horizontal, 3 for vertical, and 4 for overhead. A G designation indicates a groove weld and an F designation indicates a fillet weld. The 5G and 6G positions are used in pipe welding. The large diameter wires, which are over 1/16 in. (1.6 mm) in diameter, are limited to the flat and horizontal positions only because the weld puddle becomes too large to control. The smaller diameter electrodes, which are 1/16 in. (1.6 mm) and less, can generally be used easily in all positions.
The joint configuration will vary depending on the position of welding. One example of this is wider groove angles needed for vertical position welding. This is done to provide enough room to manipulate the electrode wire in the joint. Weaving of the electrode is usually required in vertical position welding to prevent excessive reinforcement or dropping the weld metal out of the puddle. Joint designs for overhead welding are generally the same as for flat position welding. Joints that are welded in the horizontal position often have an unsymmetrical joint configuration. This usually consists of a groove angle that has a horizontal lower groove face, as shown in Figure 43. The upper groove face is raised accordingly to provide a groove angle large enough to provide good access. The horizontal lower groove face is used as a shelf to support the molten weld metal. This joint configuration is less expensive to prepare because the bevel is only made in one plate.
Figure 43 V-groove joint in the horizontal position.
The thickness of the base metal has a large influence on the joint preparation required to produce the best quality weld joint. FCAW is used to weld thicknesses down to 18 gauge (1.2 mm), but the process is also suitable for welding thick metal. Because of this, wide varieties of joint designs are used. The most common groove preparations used on butt joints are the square-, V-, J-, U- bevel-, and combination-grooves. The square-, J-, bevel-, and combination-groove preparations are also used on tee joints. The different preparations are used on different thicknesses to make it possible to get complete or adequate penetration.
Square-groove welds are used on the thinnest metal thicknesses. The square-groove joint design is the easiest to prepare and requires the least filler metal. Thicknesses up to 3/8-in. (9.5 mm) thick can be welded with full penetration from both sides. This is thicker than the square-groove joints that can be welded with full penetration by SMAW or GTAW because of the hotter arc and deeper penetration produced by this process. Root openings are used to allow complete penetration through the joint. Many squaregroove welds are made in one pass. A backing strip may be used so the root can be opened enough to provide better accessibility and insure adequate penetration.
V-grooves for butt joints and bevel-grooves for tee joints are commonly used for thicker metal up to about 3/4-in. (19.1 mm). These joints are more difficult to prepare and require more filler metal than square-groove welds. The included angle for a V-groove is usually up to 75° with smaller groove angles, such as 45° or 60°, being more commonly used. The smaller groove angles become even more economical as the thickness of the metal increases. The wider groove angles are used to provide better accessibility to the root of the joint. Because of the deeper penetrating characteristics of this process, single V-groove or single bevel-groove welds are often welded with little or no root opening. Larger root faces and smaller groove angles are often used compared to those used for SMAW and GTAW. This helps to minimize the amount of distortion and reduce the amount of filler metal required. For complete penetration welds, root faces usually are close to 1/8-in. (3.2 mm).
U- and J-grooves are generally used on thicknesses greater than 5/8-in. (14.3 mm). These joint preparations are the most difficult and expensive to prepare but the radius at the root of the joint allows better access to the root of the joint. Another advantage is that smaller groove angles may be used compared to those used in V-grooves. On thicker metal, this reduces the amount of filler metal required, and on very thick metals, the savings become very substantial.
The accessibility of the weld joint is another important factor in determining the weld joint design. Welds can be made from one or both sides of the weld joint. Single V-, J-, U-, bevel-, and combination grooves are used when accessibility is from one side only and on thinner metal. Double V-, J-, U-, bevel-, and combination grooves are used on thicker metal where the joint can be welded from both sides. Double-groove welds have **65 three major advantages over single-groove welds where accessibility is only from one side. The first is that distortion is more easily controlled through alternate weld bead sequencing. Weld beads are alternated from one side to the other to keep the distortion from building up in the one direction. The weld roots are nearer the center of the plate.
A second advantage is that less filler metal is required to fill a double groove joint than a single-groove joint. This tends to make double-groove welds more economical on metal 1-in. (25 mm) thick or greater.
The third advantage is that complete penetration can be more easily insured. The root of the first pass on the plate can be gouged or chipped out before the root pass on the second side is welded, to make sure there is complete fusion at the root. The disadvantages of joints welded from both sides are that more joint preparation is required and gouging or chipping is usually required to remove the root of the first pass. The amount of savings in the filler metal needed for a double-groove weld may more than compensate for the extra joint preparation costs; both of these add to the labor time required. Welding on both sides of a square-groove weld joint provides fuller penetration in thicker metal than metal welded from one side only. This would also save joint preparation time.
When backing strips are used, joints are accessible from one side only. Backing strips allow better access to the root of the joint and support the molten weld metal. These strips are available in two forms, which are fusible or non-fusible. Fusible backing strips are made of the metal being welded and remain part of the weldment after welding. These may be cut or machined off. Non-fusible backing strips are made of copper, carbon, flux, or ceramic backing in tape or composite form. These forms of backing do not become part of the weld. Backing strips on square-groove joints make a full penetration weld from one side easier. For this application, using a backing strip is more expensive because of the cost of a backing strip and the larger amount of filler metal required. However, on V-groove joints, the backing strip allows wider root openings and removes the need for a root face, which reduces the groove preparation costs. Another advantage is that because the root may be opened up, the groove angle may be reduced, which will reduce the amount of filler metal required in thicker metal. These effects are shown in Figure 44, where single V-groove joints are shown with and without a backing strip.
Figure 44 Single V-groove joints with and without backing strip in the same thickness metal.
As discussed earlier in this course, the use of a backing strip will have an effect on the joint designs used for gas-shielded and self-shielded electrodes. The deeper penetrating characteristics of the gas-shielded electrode allow the joint designs to be adjusted to take advantage of this.
The details of a joint, which include both the geometry and the required dimensions, are called the joint design. Just what type of joint design is best suited for a particular job depends on many factors. Although welded joints are designed primarily to meet strength and safety requirements, there are other factors that must be considered. A few of these factors are as follows:
Another consideration that must be made is the ratio of the strength of the joint compared to the strength of the base metal. This ratio is called joint efficiency. An efficient joint is one that is just as strong as the base metal. Normally, the joint design is determined by a designer or engineer and is included in the project plans and specifications. Even so, understanding the joint design for a weld enables you to produce better welds.
Earlier in this course, we discussed the five basic types of welded jointsbutt, corner, tee, lap, and edge.
Just keep in mind that there are many different variations of the basic joint welds. The weld joint designs shown in Figures 45 through 56 are those typically used for FCAW. All of the partial penetration weld joint designs covered may be welded using either the self-shielded or gas-shielded electrode wires. The joint dimensions will vary for full penetration welds using backing strips, depending on which method of FCAW is being used. The joint designs that should be used only by the gas-shielded method are indicated on these joints. All other full penetration welds may be made by either of the two methods. Ranges are given on many of the joint dimensions to account for varying fit-up and types of electrode wires. The thickness ranges given are those typically recommended for use with the joint designs. Minimum effective throat thicknesses are commonly used for partial penetration welds. Recommended minimum effective throat sizes are given in Table 14.
Table 14 Effective throat thickness for partial joint penetration groove welds.
Figure 45 Welding symbols
Figure 46 Welding symbols (cont.).
Figure 47 Application of arrow and other side convention.
Figure 48 Applications of break in arrow of welding symbol.
Figure 49 Combinations of weld symbols.
Figure 51 Specification of location and extent of fillet welds.
Figure 56 Applications of melt-through symbol.
The types of welds, joints, and welding positions used in FCAW are very similar to those used in GMAW, with the exception of overhead welding. Manual overhead welding is rarely used in FCAW because the filler metal is so fluid due to the powdered core.
Welding can be done in any position, but it is much simpler when done in the flat position. In this position, the work is less tiring, welding speed is faster, the molten puddle is not as likely to run, and better penetration can be achieved. Whenever possible, try to position the work so you can weld in the flat position. In the flat position, the face of the weld is approximately horizontal.
Butt joints are the primary type of joints used in the flat position of welding; however, flat-position welding can be made on just about any type of joint, providing you can rotate the section you are welding on to the appropriate position. Techniques that are useful in making butt joints in the flat position, with and without the use of backing strips, are described below.
Butt joints without backing strips A butt joint is used to join two plates having surfaces in about the same plane. Several forms of butt joints are shown in Figure 57. Plates up to 1/8-inch thick can be welded in one pass with no special edge preparation.
Figure 57 Butt joints in the flat position.
Plates from 1/8- to 3/16 -inch thick also can be welded with no special edge preparation by welding on both sides of the joint. Tack welds should be used to keep the plates aligned for welding. The electrode motion is the same as that used in making a bead weld
In welding 1/4-inch plate or heavier, you should prepare the edges of the plates by beveling or by J-, U-, or V-grooving, whichever is the most applicable. You should use single or double bevels or grooves when the specifications and/or the plate thickness require it. The first bead is deposited to seal the space between the two plates and to weld the root of the joint. This bead or layer of weld metal must be thoroughly cleaned to remove all slag and dirt before the second layer of metal is deposited.
In making multi pass welds, as shown in Figure 58, the second, third, and fourth layers of weld metal are made with a weaving motion of the electrode. Clean each layer of metal before laying additional beads. You may use one of the weaving motions shown in Figure 59, depending upon the type of joint and size of electrode. In the weaving motion, oscillate or move the electrode uniformly from side to side, with a slight hesitation at the end of each oscillation. Incline the electrode 5 to 15 degrees in the direction of welding as in bead welding. When the weaving motion is not done properly, undercutting could occur at the joint, as shown in Figure 60. Excessive welding speed also can cause undercutting and poor fusion at the edges of the weld bead.
Figure 58 Butt welds with multipass beads.
Figure 59 Weave motions used in FCAW.
Figure 60 Undercutting in butt joint welds.
Butt joints with backing strips Welding 3/16-inch plate or thicker requires backing strips to ensure complete fusion in the weld root pass and to provide better control of the arc and the weld metal. Prepare the edges of the plates in the same manner as required for welding without backing strips. For plates up to 3/8-inch thick, the backing strips should be approximately 1-inch wide and 3/16-inch thick. For plates more than 1/2-inch thick, the backing strips should be 1 1/2 inches wide and 1/4-inch thick. Tack-weld the backing strip to the base of the joint, as shown in Figure 61. The backing strip acts as a cushion for the root pass. Complete the joint by welding additional layers of metal. After you complete the joint, the backing strip may be washed off or cut away with a cutting torch. When specified, place a seal bead along the root of the joint.
Figure 61 Use of back strips in welding butt joints.
Bear in mind that many times it will not always be possible to use a backing strip; therefore, the welder must be able to run the root pass and get good penetration without the formation of icicles.
You will discover that it is impossible to weld all pieces in the flat position. Often the work must be done in the horizontal position. The horizontal position has two basic forms, depending upon whether it is used with a groove weld or a fillet weld. In a groove weld, the axis of the weld lies in a relative horizontal plane and the face of the weld is in a vertical plane (Figure 62). In a fillet weld, the welding is performed on the upper side of a relatively horizontal surface and against an approximately vertical plane (Figure 63).
Figure 62 Horizontal groove weld.
Figure 63 Horizontal fillet weld.
Inexperienced welders usually find the horizontal position of arc welding difficult, at least until they have developed a fair degree of skill in applying the proper technique. The primary difficulty is that in this position, you have no shoulder of previously deposited weld metal to hold the molten metal.
In horizontal welding, position the electrode so that it points upward at a 5- to 10-degree angle in conjunction with a 20-degree travel angle (Figure 64). Use a narrow weaving motion in laying the bead. This weaving motion distributes the heat evenly, reducing the tendency of the molten puddle to sag. You should use the shortest arc length possible, and when the force of the arc undercuts the plate at the top of the bead, lower the electrode holder a little to increase the upward angle. As you move in and out of the crater, pause slightly each time you return. This keeps the crater small and the bead has fewer tendencies to sag.
Figure 64 Horizontal welding angles.
Horizontal-position welding can be used on most types of joints. The most common types of joints it is used on are tee joints, lap joints, and butt joints.
Tee joints When you make tee joints in the horizontal position, the two plates are at right angles to each other in the form of an inverted T. The edge of the vertical plate may be tack-welded to the surface of the horizontal plate, as shown in Figure 65.
Figure 65 Tack-weld to hold the tee joint elements in place.
Figure 66 Position of electrode on a fillet weld.
A fillet weld is used in making the tee joint, and a short arc is necessary to provide good fusion at the root and along the legs of the weld (Figure 66, View A). Hold the electrode at an angle of 45 degrees to the two plate surfaces (Figure 66, View B) with an incline of approximately 15 degrees in the direction of welding.
When practical, weld light plates with a fillet weld in one pass with little or no weaving of the electrode. Welding of heavier plates may require two or more passes in which the second pass or layer is made with a semicircular weaving motion, as shown in Figure 67.
Figure 67 Weave motion for multipass fillet weld.
To ensure good fusion and the prevention of undercutting, you should make a slight pause at the end of each weave or oscillation. For fillet-welded tee joints on 1/2-inch plate or heavier, deposit stringer beads in the sequence shown in Figure 68.
Figure 68 Order of string beads for tee joint on heavy plate
Figure 69 Intermittent fillet welds.
Chain-intermittent or staggered-intermittent fillet welds, as shown in Figure 69, are used on long tee joints. Fillet welds of these types are for joints where high weld strength is not required; however, the short welds are arranged so the finished joint is equal in strength to that of a joint that has a fillet weld along the entire length of one side. Intermittent welds also have the advantage of reduced warpage and distortion.
Lap joints When you make a lap joint, two overlapping plates are tack-welded in place (Figure 70), and a fillet weld is deposited along the joint.
Figure 70 Tack welding a lap joint.
Figure 71 Position of electrode on a lap joint.
The procedure for making this fillet weld is similar to that used for making fillet welds in tee joints. You should hold the electrode so it forms an angle of about 30 degrees from the vertical and is inclined 15 degrees in the direction of welding. The position of the electrode in relation to the plates is shown in Figure 71. The weaving motion is the same as that used for tee joints, except that the pause at the edge of the top plate is long enough to ensure good fusion without undercut. Lap joints on 1/2-inch plate or heavier are made by depositing a sequence of stringer beads, as shown in Figure 71.
In making lap joints on plates of different thickness, you should hold the electrode so that it forms an angle of between 20 and 30 degrees from the vertical (Figure 72). Be careful not to overheat or undercut the thinner plate edge.
Figure 72 Lap joints on plates of different thickness.
Figure 73 Horizontal butt joint.
Butt joints Most butt joints designed for horizontal welding have the beveled plate positioned on the top. The plate that is not beveled is on the bottom and the flat edge of this plate provides a shelf for the molten metal so that it does not run out of the joint (Figure 73). Often, both edges are beveled to form a 60-degree included angle. When this type of joint is used, more skill is required because you do not have the retaining shelf to hold the molten puddle.
The number of passes required for a joint depends on the diameter of the electrode and the thickness of the metal. When multiple passes are required (Figure 74), place the first bead deep in the root of the joint. The electrode holder should be inclined about 5 degrees downward. Clean and remove all slag before applying each following bead. The second bead should be placed with the electrode holder held about 10 degrees upward. For the third pass, hold the electrode holder 10 to 15 degrees downward from the horizontal. Use a slight weaving motion and ensure that each bead penetrates the base metal.
Figure 74 Multiple passes.
A vertical weld is defined as a weld that is applied to a vertical surface or one that is inclined 45 degrees or less (Figure 75). Erecting structures, such as buildings, pontoons, tanks, and pipelines, require welding in this position. Welding on a vertical surface is much more difficult than welding in the flat or horizontal position due to the force of gravity. Gravity pulls the molten metal down. To counteract this force, you should use fast-freeze or fill-freeze electrodes.
Figure 75 Vertical weld plate positions.
Vertical welding is done in either an upward or downward position. The terms used for the direction of welding are vertical up or vertical down. Vertical down welding is suited for welding light gauge metal because the penetration is shallow and diminishes the possibility of burning through the metal. Furthermore, vertical down welding is faster, which is very important in production work.
Movement In vertical arc welding, the current settings should be less than those used for the same electrode in the flat position. Another difference is that the current used for welding upward on a vertical plate is slightly higher than the current used for welding downward on the same plate.
To produce good welds, you must maintain the proper angle between the electrode and the base metal. In welding upward, you should hold the electrode at 90 degrees to the vertical, as shown in Figure 76, View A. When weaving is necessary, oscillate the electrode, as shown in Figure 76, View B. In vertical down welding, incline the outer end of the electrode downward about 15 degrees from the horizontal while keeping the arc pointing upward toward the deposited molten metal (Figure 76, View C). When vertical down welding requires a weave bead, you should oscillate the electrode, as shown in Figure 76, View D.
Vertical welding is used on most types of joints. The types of joints you will most often use it on are tee joints, lap joints, and butt joints.
Hold the electrode at 90 degrees to the plates or not more than 15 degrees off the horizontal for proper molten metal control when making fillet welds in either tee or lap joints in the vertical position. Keep the arc short to obtain good fusion and penetration.
Tee joints To weld tee joints in the vertical position, start the joint at the bottom and weld upward. Move the electrode in a triangular weaving motion, as shown in Figure 77, View A. A slight pause in the weave at the points indicated improves the sidewall penetration and provides good fusion at the root of the joint.
When the weld metal overheats, you should quickly shift the electrode away from the crater without breaking the arc, as shown in Figure 77, View B. This permits the molten metal to solidify without running downward. Return the electrode immediately to the crater of the weld in order to maintain the desired size of the weld.
Figure 77 Fillet welds in the vertical position.
When more than one pass is necessary to make a tee weld, you may use either of the weaving motions shown in Figure 77, Views C and D. A slight pause at the end of the weave will ensure fusion without undercutting the edges of the plates.
Lap joints To make welds on lap joints in the vertical position, you should move the electrode in a triangular weaving motion, as shown in Figure 77, View E. Use the same procedure as outlined above for the tee joint, except direct the electrode more toward the vertical plate marked G. Hold the arc short and pause slightly at the surface of plate G. Try not to undercut either of the plates or to allow the molten metal to overlap at the edges of the weave.
Lap joints on heavier plate may require more than one bead. If it does, clean the initial bead thoroughly and place all subsequent beads, as shown in Figure 77, View F. The precautions to ensure good fusion and uniform weld deposits that were previously outlined for tee joints also apply to lap joints.
Butt joints Prepare the plates used in vertical welding identically to those prepared for welding in the flat position. To obtain good fusion and penetration with no undercutting, you should hold a short arc and the motion of the arc should be carefully controlled. Butt joints on beveled plates 1/4-inch thick can be welded in one pass by using a triangular weave motion, as shown in Figure 78, View A.
Welds made on 1/2-inch plate or heavier should be done in several passes, as shown in Figure 78, View B. Deposit the last pass with a semicircular weaving motion and a slight whip-up and pause of the electrode at the edge of the bead. This produces a good cover pass with no undercutting. Welds made on plates with a backup strip should be done in the same manner.
Figure 78 Butt joint welding in the vertical position
Overhead welding is the most difficult position in welding. Not only do you have to contend with the force of gravity, but the majority of the time you also have to assume an awkward stance. Nevertheless, with practice it is possible to make welds equal to those made in the other positions.
To retain complete control of the molten puddle, use a very short arc and reduce the amperage as recommended. As in the vertical position of welding, gravity causes the molten metal to drop or sag from the plate. When too long an arc is held, the transfer of metal from the electrode to the base metal becomes increasingly difficult and the chances of large globules of molten metal dropping from the electrode increase. When you routinely shorten and lengthen the arc, the dropping of molten metal can be prevented; however, you will defeat your purpose should you carry too large a pool of molten metal in the weld. One of the problems encountered in overhead welding is the weight of the cable. To reduce arm and wrist fatigue, drape the cable over your shoulder when welding in the standing position. When sitting, place the cable over your knee. With experience, cable placement will become second nature.
Because of the possibility of falling molten metal, use a protective garment that has a tight fitting collar that buttons or zips up to the neck. Roll down your sleeves and wear a cap and appropriate shoes.
Techniques used in making bead welds, butt joints, and fillet welds in the overhead position are discussed in the following paragraphs.
Bead welds For bead welds, the work angle of the electrode is 90 degrees to the base metal (Figure 79, View A). The travel angle should be 10 to 15 degrees in the direction of welding (Figure 79, View B).
Weave beads can be made by using the motion shown in Figure 79, View C. A rather rapid motion is necessary at the end of each semicircular weave to control the molten metal deposit. Avoid excessive weaving because this can cause overheating of the weld deposit and the formation of a large, uncontrollable pool.
Figure 79 Position of electrode and weave motion in the overhead position.
Butt Joint Prepare the plates for overhead butt welding in the same manner as required for the flat position. The best results are obtained when backing strips are used; however, you must remember that you will not always be able to use a backing strip. When you bevel the plates with a featheredge and do not use a backing strip, the weld will repeatedly burn through unless extreme care is taken by the operator.
For overhead butt welding, bead welds are preferred over weave welds. Clean each bead and chip out the rough areas before placing the next pass. The electrode position and the order of deposition of the weld beads when welding on 1/4- or 1/2-inch plate are shown in Figure 80, views B and C. Make the first pass with the electrode held at 90 degrees to the plate, as shown in Figure 80, View A. When you use an electrode that is too large, you cannot hold a short arc in the root area. This results in insufficient root penetration and inferior joints.
Figure 80 Multipass butt joint in the overhead position.
Fillet welds In making fillet welds in either tee or lap joints in the overhead position, maintain a short arc and refrain from weaving of the electrode. Hold the electrode at approximately 30 degrees to the vertical plate and move it uniformly in the direction of welding, as shown in Figure 80, View B. Control the arc motion to secure good penetration in the root of the weld and good fusion with the sidewalls of the vertical and horizontal plates. When the molten metal becomes too fluid and tends to sag, whip the electrode quickly away from the crater and ahead of the weld to lengthen the arc and allow the metal to solidify. Immediately return the electrode to the crater and continue welding.
Overhead fillet welds for either tee or lap joints on heavy plate require several passes or beads to complete the joint. One example of an order of bead deposition is shown in Figure 81, View A. The root pass is a string bead made with no weaving motion of the electrode. Tilt the electrode about 15 degrees in the direction of welding, as shown in Figure 81, View C, and with a slight circular motion make the second, third, and fourth pass. This motion of the electrode permits greater control and better distribution of the weld metal. Remove all slag and oxides from the surface of each pass by chipping or wire brushing before applying additional beads to the joint.
Figure 81 Fillet welding in the overhead position.
Welding is the simplest and easiest way to join sections of pipe. The need for complicated joint designs and special threading equipment is eliminated. Welded pipe has less flow restriction compared to mechanical connections and the overall installation costs are less. The most popular method for welding pipe is the shielded metal arc process; however, gas shielded arc methods (TIG, MIG & FCAW) have made big inroads as a result of new advances in welding technology.
Pipe welding has become recognized as a profession in itself. Even though many of the skills are comparable to other types of welding, pipe welders develop skills that are unique only to pipe welding. Because of the hazardous materials that most pipelines carry, pipe welders are required to pass specific tests before they can be certified.
In the following paragraphs, pipe welding positions, pipe welding procedures, definitions, and related information are discussed.
You may recall that there are four positions used in pipe welding. They are known as the horizontal rolled position (1G), the horizontal fixed position (5G), pipe inclined fixed (6G), and the vertical position (2G). Remember: these terms refer to the position of the pipe and not to the weld.
Welds that you cannot make in a single pass should be made in interlocked, multiple layers, not less than one layer for each 1/8- inch of pipe thickness. Deposit each layer with a weaving or oscillating motion. To prevent entrapping slag in the weld metal, you should clean each layer thoroughly before depositing the next layer.
Butt joints are commonly used between pipes and between pipes and welded fittings. They are also used for butt welding of flanges and welding stubs. In making a butt joint, place two pieces of pipe end to end, align them, and then weld them. (See Figure 82).
Figure 82 Butt joints and socket fitting joints.
When the wall thickness of the pipe is 3/4-inch or less, you can use either the single V or single U type of butt joint; however, when the wall thickness is more than 3/4-inch, only the single U type should be used. Fillet welds are used for welding slip-on and threaded flanges to pipe. Depending on the flange and type of service, fillet welds may be required on both sides of the flange or in combination with a bevel weld (Figure 83). Single-fillet welds are also used in welding screw or socket couplings to pipe (Figure 83). Sometimes flanges require alignment. Figure 84 shows one type of flange square and its use in vertical and horizontal alignment.
Figure 83 Flange connections.
Figure 84 Flange alignment.
Another form of fillet weld used in pipe fitting is a seal weld. A seal weld is used primarily to obtain tightness and prevent leakage. Seal welds should not be considered as adding strength to the joint.
You must carefully prepare pipe joints for welding if you want good results. Clean the weld edges or surfaces of all loose scale, slag, rust, paint, oil, and other foreign matter. Ensure that the joint surfaces are smooth and uniform. Remove the slag from flame-cut edges; however, it is not necessary to remove the temper color.
When you prepare joints for welding, remember that bevels must be cut accurately. Bevels can be made by machining, grinding, or using a gas cutting torch. In fieldwork, the welding operator usually must make the bevel cuts with a gas torch. When you are beveling, cut away as little metal as possible to allow for complete fusion and penetration. Proper beveling reduces the amount of filler metal required, which in turn reduces time and expense. In addition, it also means less strain in the weld and a better job of design and welding.
Align the piping before welding and maintain it in alignment during the welding operation. The maximum alignment tolerance is 20 percent of the pipe thickness. To ensure proper initial alignment, you should use clamps or jigs as holding devices. A piece of angle iron makes a good jig for a small-diameter pipe (Figure 85), while a section of channel or I-beam is more suitable for larger diameter pipe.
Figure 85 Angle iron jig.
When welding material solidly, you may use tack welds to hold it in place temporarily. Tack welding is one of the most important steps in pipe welding or any other type of welding. The number of tack welds required depends upon the diameter of the pipe. For 1/2-inch pipe, you need two tacks. Place them directly opposite each other. As a rule, four tacks are adequate for standard size of pipe. The size of a tack weld is determined by the wall thickness of the pipe. Be sure that a tack weld is not more than twice the pipe thickness in length or two-thirds of the pipe thickness in depth. Tack welds should be the same quality as the final weld. Ensure that the tack welds have good fusion and are thoroughly cleaned before proceeding with the weld.
In addition to tack welds, spacers sometimes are required to maintain proper joint alignment. Spacers are accurately machined pieces of metal that conform to the dimensions of the joint design used. Spacers are sometimes referred to as chill rings or backing rings, and they serve a number of purposes. For example, they provide a means for maintaining the specified root opening, provide a convenient location for tack welds, and aid in the pipe alignment. In addition, spacers can prevent weld spatter and the formation of slag or icicles inside the pipe.
Select the electrode that is best suited for the position and type of welding to be done. For the root pass of a multilayer weld, you need an electrode large enough, yet not exceeding 3/16-inch, that ensures complete fusion and penetration without undercutting and slag inclusions.
Make certain the welding current is within the range recommended by the manufacturers of the welding machines and electrodes.
Do not assign a welder to a job under any of the following conditions listed below unless the welder and the work area are properly protected: When the atmospheric temperature is less than 0°F When the surfaces are wet When rain or snow is falling, or moisture is condensing on the weld surfaces During periods of high wind, unless using self-shielded electrodes At temperatures between 0°F and 32°F, within 3 inches of the joint, heat the weld area with a torch to a temperature warm to the hand before beginning to weld.
|Test Your Knowledge
7. How many basic types of weld joints are there?
8. Which type of weld is used for welding slip-on and threaded flanges to pipe?
The welding procedure variables are those that control the welding process and the quality of the welds that are produced. When all of the variables are in proper balance, the result will be a smooth running arc and a quality weld deposit. You need to understand the effect of each variable on the different properties or characteristics of the weld to increase the probability of producing the required weld properties. You should recognize that some welding variables are more easily applied as controls of a welding process. There are three major types of welding variables used for welding. These are the fixed or preselected, primary adjustable, and the secondary adjustable variables.
The preselected or fixed variables are those that can only be changed in large steps or intervals and are therefore unfavorable as controls. For the FCAW process, these variables are set according to the type of material being welded, the thickness of the material, welding position, deposition rate required, and mechanical properties required. These variables cannot be changed once the welding starts.
The primary adjustable variables are the major variables used to control the welding process once the fixed variables have been selected. The primary variables control the formation of the weld bead by affecting the bead width, bead height, penetration, arc stability, and weld soundness. The primary welding variables are welding current, arc voltage, and travel speed. These can be easily adjusted and measured so they can be used effectively to control the welding process. Specific values can be assigned to the primary adjustable variables and these values can be accurately reset time after time.
The secondary adjustable variables can also be changed continuously over a wide range of values. However, they are sometimes difficult to measure accurately. It is not easy to use them as controls since, for the most part, they cannot be assigned exact values. This is especially true in semiautomatic welding operations. Although difficult to measure, these variables should be controlled within the range for proper operation. Secondary adjustable variables are such things as electrode extension or stickout, work and travel angles.
The different variables affect the characteristics of the weld, such as the penetration of the weld, bead height, bead width, and the deposition rate. The penetration of the weld is defined as the greatest depth below the surface of the base metal that the weld metal reaches. The bead height or reinforcement is the height of the weld metal above the surface of the base metal. The deposition rate is the weight of the metal that is deposited per unit of time. The definitions of bead height, bead width, and penetration are shown in Figure 86.
Figure 86 Bead height, bead width, and penetration.
The welding variables are discussed with particular attention to the three major characteristics of penetration, deposition rate, and bead shape. Table 15 is a chart showing the effects of welding variables on the three major characteristics.
Table 15 Recommended welding variable adjustment for FCAW.
Fixed variables include electrode size and type, welding current type, and polarity.
The type of electrode wire will have an effect on the welding characteristics of this process. The flux cores of the electrodes contain different components that affect bead shape, penetration, deposition rate, and the operating characteristics. Because of this, a wide variety of operating characteristics exist, which are similar to those found with the various covered electrodes used in SMAW. Some self-shielded flux-cored electrodes have been developed to operate on DCEN. These electrodes produce relatively light penetration, and are used for many sheet metal welding and weld surfacing operations. Self-shielded electrodes that operate on DCEP produce deeper penetration. Gas-shielded electrode wires operate on DCEP and provide the deepest penetration due to the gas shielding addition to the flux core.
Many FCAW electrodes are designed to produce a stable arc and high deposition rates at the higher current levels. Figures 87 and 88 show some deposition rate comparisons between several types of flux-cored electrodes.
Figure 87 Deposition rate vs. current for externally shielded FCAW electrode wire
Figure 88 Deposition rate vs. current for self-shielded FCAW electrode wire.
Each electrode wire diameter of a given type has a usable welding current range. Larger diameter electrode wires use higher welding currents to produce higher deposition rates and deeper penetration. The rate at which the electrode melts is based on the welding current density and the components in the flux. If two electrode wires of the same type, but different diameters, are operated at the same current level, the smaller electrode will give a higher deposition rate because the current density is higher.
Figures 87 and 88 also show the deposition rates produced by different electrode diameters. The amount of penetration is also based on the current density. A smaller electrode will produce deeper penetration than a larger electrode at the same current setting, but the weld bead will be wider when using the larger electrode wire. The choice of the optimum electrode size to be used is based on the thickness of the metal to be welded, the amount of penetration required, the position of welding, the deposition rate desired, the bead profile desired, and the cost of the electrode wires. A smaller diameter electrode is more costly on a weight basis, although for out-of-position welding, the smaller diameter electrodes are the only ones that can be used. For each application, an optimum electrode size can be used to produce minimum welding costs.
Primary variables include welding current, travel speed, and welding voltage.
The amount of welding current has the greatest effect on the deposition rate, weld bead size and shape, and the weld penetration. Welding current is proportional to the wire feed speed for a given electrode type, shielding gas type and pressure, and amount of electrode extension. In a constant voltage system, the welding current is controlled by the knob on the wire feeder control, which sets the wire feed speed. The welding current increases with the wire feed speed.
As shown in Figures 87 and 88, the deposition rate of the process increases as the welding current increases. The lower part of the curve is flatter than the upper part because at higher current levels, the melting rate of the electrode increases at a faster rate as the current increases. This can be attributed to resistance heating of the electrode extension beyond the contact tube. When all of the other variables are held constant, increasing the welding current will increase the electrode deposition rate, increase penetration, and increase the size of the weld bead. Figure 89 shows the effect of welding current.
Figure 89 Effect of welding current on bead formation.
An excessive welding current level will create a large, deeppenetrating weld bead that causes excessive convexity and can burn through the bottom of the joint. Insufficient welding current produces large globular transfer and excessive spatter in addition to poor penetration and excessive piling up of the weld metal. With self-shielding electrodes, insufficient current can cause porosity and pickup too much nitrogen from the atmosphere. The nitrogen causes a harder weld that has poorer ductility. Figures 90, 91, and 92 show the effects of welding current on the penetration, bead height, and bead width.
Figure 90 Effect of travel speed, arc volts, and welding current on penetration.
Figure 91 Effect of travel speed, arc volts, and welding current on bead height.
Figure 92 Effect of travel speed, arc volts, and welding current on bead width.
The welding voltage is determined by the distance between the tip of the electrode and the work. In a constant voltage system, a voltage control knob on the front of the power source adjusts the welding voltage. The power source maintains a given voltage that maintains a certain arc length. In a constant current system, the voltage-sensing wire feeder controls the voltage. The voltage-sensing wire feeder regulates the wire feed speed to maintain the arc length that produces the preselected arc voltage. For a given welding current, a certain voltage will provide the smoothest welding arc. The arc voltage required for an application is dependent on the electrode size, type of shielding gas, position of welding, type of joint, and base metal thickness. When the other welding variables are held constant and the welding voltage is increased, the weld bead becomes wider and flatter. The effect of varying the arc voltage on a gas-shielded electrode is shown in Figure 93.
The penetration will increase up to an optimum voltage level and then begin to decrease, as shown in Figure 90. A higher voltage is often used to bridge a gap because of the decreased penetration obtained. An excessive voltage or arc length will result in excessive amounts of spatter and irregularly shaped weld beads. When using self-shielded electrodes, an excessive arc length can also cause nitrogen pickup, which causes porosity in low-carbon steel weld metal. With the self-shielded stainless steel electrodes, nitrogen absorption can cause cracking. With all types of electrodes, undercutting can also be produced. A decrease in the arc length results in a narrower weld bead with a greater convexity and deeper penetration. An arc voltage that is too low will cause a narrow convex weld bead with excessive spatter and reduced penetration. Figures 91 and 11-92 show the effects of the welding voltage on bead height and bead width.
Figure 93 Effects of arc voltage on the weld bead.
The travel speed influences the weld penetration and the shape of the weld deposit. In semiautomatic welding, this is controlled by the welder and will vary somewhat, depending on the welder. In machine and automatic welding, as shown in Figure 90, the penetration is at a maximum with a certain travel speed. Increasing or decreasing the travel speed from this point will reduce the amount of penetration. When the travel speed is decreased, the amount of filler metal deposited per unit of length increases, which creates a large, shallow weld puddle. Weld metal tends to get slightly ahead of the arc, which reduces the penetration and produces a wide weld bead. Reducing the travel speed will increase the bead height, as is shown in Figure 91, and the bead width, as shown in Figure 92. Travel speeds that are too slow can result in overheating the weld metal because of the excessive heat input, which creates a very large heat affected zone. It can also cause excessive piling up of the weld metal, which has a rough appearance and may trap slag. As the travel speed is increased, the heat input into the base metal is reduced, which decreases the melting of the base metal, limits penetration, and the bead height and the bead width are also reduced. An excessive travel speed will result in an irregular, ropy weld bead that may have undercutting along the edges. Figure 94 shows the effects of travel speed on the shape of the weld bead.
Figure 94 Effects of travel speed on the weld bead.
The effects of the primary welding variables are summarized in Figure 95 for gas-shielded flux-cored electrodes and in Figure 96 for self-shielded flux-cored electrodes.
Figure 95 Externally shielded flux cored arc good and bad welds.
Figure 96 Self-shielded flux cored arc good and bad welds.
Secondary variables include work and travel angles of the electrode.
The electrode extension, sometimes referred to as the stickout, is the distance between the tip of the contact tube and the tip of the electrode as shown in Figure 97.
Figure 97 Electrode extension or stickout.
The length of electrode that extends beyond the contact tube is resistance heated in proportion to its length. The amount of resistance heating that occurs affects the electrode deposition rate and the amount of penetration, as well as weld quality and arc stability, by varying the welding current. Increasing the electrode extension reduces the welding current, as shown in Figure 98.
Figure 98 Effect of electrode extension on welding current.
In semiautomatic welding, the electrode extension can be varied by the welder to compensate for joint variation without interrupting the welding operation. Electrode extension provides a good control during welding to change the amount of penetration obtained. In FCAW, the electrode extension is a variable that must be held in balance with the shielding conditions and the related welding variables. As the electrode extension is increased, the amount of preheating of the wire is increased. For gas-shielded flux-cored electrodes, an electrode extension ranging from Ύ- to 1-1/2-in. (19- 38 mm) is normally recommended.
Because the shielding comes from the core of self-shielded electrodes alone, a longer electrode extension is generally recommended to take advantage of the extra preheating effect needed to activate the shielding components in the electrode core. Welding guns for self-shielded electrodes often have nozzles where the contact tube is set inside far enough to ensure a minimum electrode extension. Electrode extensions ranging from Ύ- to 3-1/2-in. (19-89 mm) are commonly used. This will vary depending on the type of electrode wire so the manufacturer's data should be consulted for each electrode. An electrode extension that is too long will produce an unstable arc and cause excessive spatter. A short extension will cause an excessive arc length at a particular voltage setting. With gas-shielded electrodes, excessive spatter may result, which can build up in the nozzle and restrict the shielding gas flow. Poor shielding gas coverage can result in porosity and surface oxidation of the weld bead.
The amount of electrode extension also has an effect on the deposition rate. Increasing the electrode extension will increase the preheating effect on the electrode and therefore increase the deposition rate. Figure 99 shows this for a gas-shielded flux-cored electrode.
Figure 99 Effect of electrode extension on deposition rate.
The angle at which the welding electrode is held with respect to the weld joint is called the electrode angles. These angles have an effect on the shape of the weld bead and the amount of penetration. The electrode angles are called the travel and work angles and are shown in Figure 100.
Figure 100 Travel angle and work angle.
The travel angle is the angle between the joint and electrode in the longitudinal plane. A push angle exists when the electrode points in the direction of travel. A drag angle exists when the electrode points in the direction opposite of travel. The work angle is the angle between the electrode and the plane perpendicular to travel.
The angle at which the electrode is held during welding determines the direction in which the arc force acts on the weld pool. The electrode angles are used to shape the weld bead and to prevent the slag from running ahead of the weld pool and becoming trapped in the weld. When making flat position fillet and groove welds, gravity tends to make the molten slag run ahead of the weld pool. To compensate for this, a drag angle is used, which forces the slag back. The proper travel angle depends on the method of FCAW being used, the thickness of the base metal, and the position of welding. Using gas-shielded electrodes, maximum weld penetration is obtained with a 10° drag angle. Drag angles ranging from about 2° to 15° are normally recommended, but a drag angle greater than 25° should not be used. Drag angles greater than this do not provide good control of penetration. As the drag angle is decreased, the bead height decreases and the width increases.
This effect continues into the push angle up to a point where the bead will start to narrow down again. Push angles are generally not recommended because of the greater chances of slag entrapment occurring. For self-shielded electrodes, the drag angles used are similar to those used in SMAW. Flat and horizontal position welding is done using drag angles ranging from 20° to 45°. Larger angles may be used for thin sections. As the thickness of the metal increases, smaller angles are used to increase the penetration. For vertical position, uphill welding, a push angle of 5° to 10° is recommended. When making fillet welds in the horizontal position, the weld metal tends to flow in both the horizontal and vertical directions. To compensate for the vertical flow, a work angle of 40° to 50° from the upper plate is used. The electrode should be centered about one diameter of the electrode below the center of the weld, as shown in Figure 101. This will prevent an unequal legged fillet weld from being formed.
Figure 101 Positioning the electrode for fillet welds.
- To Table of Contents -
The welding procedure schedules in this course give typical welding conditions that can be used to obtain high quality welds under normal welding conditions. FCAW uses a wide variety of operating conditions for welding mainly steels, some stainless steels, and some nickels. The procedure schedules presented in this course are in no way a complete guide to the procedures that can be used for FCAW and are not the only conditions that may be used to obtain a specific weld. Other conditions could be used because of factors such as weld appearance, welder skill, method of application, and the specific application that may require variations from the schedules. For example, automatic FCAW normally requires higher amperage settings and faster travel speeds than semiautomatic welding. The type of electrode wire has a significant effect on the conditions. This is because the type of electrode wire indicates whether a shielding is required, the recommended electrical polarity, the recommended amount of electrode extension, and other factors. As the particular requirements of the application become known, the settings may be adjusted to obtain the optimum welding conditions. Qualifying tests or finals should be made under the actual conditions before applying the information in the tables to actual production welding.
When changing or adjusting the variables for welding, the effect of the variables on each other must be considered. One variable cannot usually be drastically changed without adjusting or changing the other variables in order to obtain a stable arc and good overall welding conditions.
The following schedules are based on welding plain carbon steels using various types of electrode wires in appropriate positions. Generally, electrode wires over 1/16in. (1.6 mm) diameter are limited to the flat and horizontal positions. The welding schedules include the semiautomatic and automatic methods of application, using self-shielded and CO2-shielded electrode wires. The tables use the base metal thickness or fillet size, number of weld passes, electrode diameter, welding current, welding voltage, wire feed speed, gas flow rate (if used), and travel speed as variables. Each table contains the type of shielding gas (if used), type of joint, and the position of welding being used. All of the schedules are based on using DCEP. Both the welding current and wire feed speed values are given because, even through the welding current is set by the wire feed speed, it is sometimes more convenient to directly establish the welding current without exactly knowing the wire feed speed. Figures 102 and 103 show wire feed speeds and their corresponding welding currents for several sizes of tubular electrode wire.
Figure 102 Wire feed speed vs. welding current for externally-shielded tubular wires.
Figure 103 Wire feed speed vs. welding current for self-shielded tubular wires.
Many of the charts include welding conditions for both groove and fillet welds given on the same chart. Generally, fillet welds will use the higher current levels for the ranges given and groove welds will use the lower end of the current range.
Table 16 Flux cored arc welding of plain and low-alloy steels using external shielding.
- To Table of Contents -
Several operations may be required before making a weld. These operations include preparing the weld joint, setting up or fixturing the weldment, possible maintenance of welding gun and cable assembly, setting the variables, and in some cases preheating. The amount of preweld preparation depends upon the size of the weld, the material to be welded, the ease of fit-up, the quality requirements, the governing code or specification, and the welder.
There are different ways of preparing the edges of the joint for welding. The methods most often used for edge preparation are oxygen fuel gas cutting, plasma arc cutting, air carbon arc gouging, shearing, machining, grinding, and chipping. When they can be used, the thermal cutting methods, oxyfuel gas, plasma arc cutting, and air carbon arc cutting are generally faster than the mechanical cutting methods, with the exception of shearing. Oxygen fuel gas cutting is used on carbon and low-alloy steels. Plasma arc cutting is used on carbon, low-alloy, and stainless steels and is best for applications where high production rates are required. Air carbon arc cutting is used for preparing joints in most steels, including stainless steels. This process should not be used on stainless steels for critical corrosion applications because of the carbon deposited, unless the cut surfaces are cleaned by grinding and brushing. The surfaces cut by these thermal methods sometimes have to be ground lightly to remove scale or contamination. Common types of prepared weld joints are the square-, V-, U-, J-, bevel-, and combination grooves. The more complex types of bevels require a longer joint preparation time, which makes the joint preparation more expensive.
Since FCAW is used on all metal thicknesses, all of the different joint preparations are widely used. Joints for fillet or square-groove welds are prepared simply by squaring the edges of the members to be welded if the as-received edge is not suitable.
Next to the square edge preparation, the V-groove and single-bevel grooves are the types most easily prepared by oxygen fuel cutting, plasma arc cutting, chipping, or machining. These methods leave a smooth surface if properly done. The edges of U-and J-grooves can be done by using special tips and techniques with oxy-fuel cutting or by machining. Machining produces the uniform groove. Carbon arc cutting is used extensively for preparing U-grooves in steels and for removing part of root passes so that the joint can be welded from both sides. Chipping is sometimes done on the back side of the weld, when full penetration is required and a thermal cutting method is not being used.
Weld backings are commonly used in FCAW to provide support for the weld metal and to control the heat input. Copper, steel, stainless steel, and backing tape are the most common types of weld backing. Copper is a widely used method of weld backing because it does not fuse to thin metals. It also provides a fast cooling rate because of the high heat conductivity of copper, which makes this the best method of controlling the heat input. Steel backing is used when welding steels. These are fusible and remain part of the weldment unless they are cut off. Often, these are removed by oxy-fuel, aircarbon arc cutting, or grinding. Stainless steels are good backing materials for welding stainless steels. Backing tape is popular because it can be molded to any joint configuration, such as the inside of a pipe.
The welds made by FCAW are susceptible to contamination during the welding process. The surface of the base metal should be free of grease, oil, paint, plating, dirt, oxides, or any other foreign material. This is especially critical when welding stainless steel. FCAW is less sensitive to contaminants than GMAW because of the scavengers and deoxidizers present in the flux core. Some flux-cored electrodes are made specifically for welding over rust and scale. This is done to make preweld cleaning less expensive. Very dirty workpieces are usually cleaned by using solvent cleaners followed by vapor degreasing. Simple degreasing is often used for cleaning carbon and low-alloy steels that have oxide free surfaces. Acid pickling is generally used for cleaning scale and rust, and can be removed mechanically by grinding and abrasive blasting.
The type of cleaning operation will vary, depending on the type of metal. Carbon and low-alloy steels may be cleaned chemically in a hydrochloric acid solution. Nickel alloys and stainless steels may be cleaned by pickling, which removes iron, sand blast residue, and other contaminants. Welding should never be done near chlorinated solvents because the arc can create phosgene gas, which is toxic. Chemical cleaning can be done by pickling.
Just before welding, several other tasks should be performed. One is to grind or file the edges of the joint smooth so that there are no burrs present. Burrs can cause physical pain as well as create a place to trap contaminants in a weld joint. Grinding is often used on plain carbon and low-alloy steels to remove burrs and rust or mill scale from the area in and around the joint. The surfaces of the joint and surrounding area should be wire brushed. Mild steel brushes are used for cleaning plain carbon and low-alloy steel. Stainless steel wire brushes are used for cleaning stainless steel. The joint surfaces and surface of the previous weld bead should also be cleaned off between passes of a multiple-pass weld. Stainless steel brushes should be used on these metals to avoid contamination due to rust or carbon from the mild steel wire brushes. Welding should be done soon after cleaning, especially on metals that form surface oxides, such as stainless steel. Wire brushing does not completely remove the oxide but it reduces the thickness and makes them easier to weld. Gloves should be worn while cleaning stainless steels to prevent oil or dirt from the fingers or from getting on the joint surfaces, which can also cause contamination.
Fixturing can affect the shape, size, and uniformity of a weld bead. Fixtures are devices that are used to hold the parts to be welded in proper relation to each other. The alignment is called fit-up. When fixturing is not used, it usually indicates that the resulting weld distortion can be tolerated or corrected by straightening operations. The three major functions of fixtures are:
When a welding fixture is used, the components of a weldment can be assembled and securely held in place while the weldment is positioned and welded. The use of those devices is dependent on the specific application. These devices are more often used when large numbers of the same part are produced. When a fixture is used, the production time for the weldments can be greatly reduced. They are also good for applications where close tolerances must be held. Positioners are used to move the workpiece into a position so welding can be done more conveniently, which improves the appearance and the quality of the weld bead.
Positioning is sometimes needed simply to make the weld joint more accessible. The main objective of positioning is to put the joint in the flat or other more favorable position. Positioners are particularly important in FCAW because they allow the use of larger diameter flux-cored electrode wires when the weld joint can be rotated into the flat or horizontal fillet. The larger diameter electrodes produce higher deposition rates, are less expensive, and generally reduce the overall welding costs. Flat position welding usually increases the quality of the weld because it makes the welding easier.
The use of preheat is sometimes needed, depending on the type of metal being welded, the base metal thickness, and the amount of joint restraint. For a refresher, refer again to topic 7.0.0 and Table 12. The specific amount of preheat needed for a given application is often obtained from the welding procedure.
The preheat temperature of the base metal is often carefully controlled. Several good methods of doing this are furnace heating, electric induction coils, and electric resistance heating blankets. On thin metals, hot air blasts or radiant lamps may be used. With these methods, temperature indicators are connected to parts being preheated. Another method of preheating is using torches, which give more localized heating than the previously mentioned methods. However, when using torches for preheating, it is important to avoid localized overheating and deposits of incomplete combustion products from collecting on the surface of the parts to be welded. Colored chalks and pellets are often used to measure the preheat temperature. Chalks and pellets melt at a specific, predetermined temperature. Another method of measuring the temperature is by using a hand-held temperature indicator. These indicators can give meter readings, digital readings, or recorder readings, depending on the type of temperature indicator.
|Test Your Knowledge
9. Which of the following is NOT a major type of welding variable?
10. Fixtures and jigs are devices that are used to hold the parts to be welded in proper relation to each other. What is this alignment called?
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Flux cored arc welding, like other welding processes, has welding procedure problems that may develop, which can cause defects in the weld. Some defects are caused by problems with the materials. Other welding problems may not be foreseeable and may require immediate corrective action.
A poor welding technique and improper choice of welding parameters can cause weld defects. Defects that can occur when using the FCAW process are slag inclusions, wagon tracks, porosity, wormhole porosity, undercutting, lack of fusion, overlapping, burn through, arc strikes, craters, and excessive weld spatter. Many of these welding technique problems weaken the weld and can cause cracking. A poor welding technique and improper choice of welding parameters are major causes of weld defects. Some defects are caused by the use of improper base metal, filler metal, or shielding gas. The base metal and filler metal should also be cleaned to avoid creation of a discontinuity. Other problems that can occur and reduce the quality of the weld are arc blow, loss of shielding, defective electrical contact between the contact tube and the electrode, and wire feed stoppages.
FCAW produces a slag covering over the weld. Slag inclusions (Figure 104) occur when slag particles are trapped inside the weld metal, which produces a weaker weld.
Figure 104 Slag inclusions.
Slag inclusions can be caused by:
This defect can be prevented by:
Wagon tracks (Figure 105) are linear slag inclusions that run the longitudinal axis of the weld. They result from allowing the slag to run ahead of the weld puddle and by slag left on the previous weld pass. This is especially common when slag forms in undercuts on the previous pass. This discontinuity occurs along the toe line of the previous weld bead and can be corrected by correcting the electrode travel angles, increasing the travel speed, or by doing a better slag cleaning.
Figure 105 Wagon tracks.
Porosity (Figure 106) is gas pockets in the weld metal that may be scattered in small clusters or along the entire length of the weld. Porosity weakens the weld in approximately the same way that slag inclusions do. Porosity may be internal, on the surface of the weld bead, or both.
Figure 106 Porosity.
Porosity may be caused by:
Porosity can be prevented by:
Wormhole porosity (Figure 107) is the name given to elongated gas pockets and is usually caused by sulfur or moisture trapped in the weld joint The best methods of preventing this are to clean the surfaces of the joint and preheat to remove moisture. If sulfur in the steel is the problem, a more weldable grade of steel should be selected.
Figure 107 Wormhole porosity.
Undercutting (Figure 108) is a groove melted in the base metal next to the toe or root of a weld that is not filled by the weld metal. Undercutting causes a weaker joint and it can cause cracking.
Figure 108 Undercutting.
This defect is caused by:
This defect can be prevented by:
Lack of fusion (Figure 109) occurs when the weld metal is not fused to the base metal. This can occur between the weld metal and the base metal or between passes in a multiple-pass weld. This is less of a problem with FCAW than with SMAW and short-circuiting transfer GMAW because of the deeper penetration obtained. More care should be taken when using a weaving technique because there is a greater chance of creating this discontinuity. Incomplete fusion between passes in a multiple-pass weld often is the result of welding over a previous weld bead that has excessive convexity. If an excessively convex weld bead is created, the surface should be ground off enough so that complete fusion can be made in the next pass.
Figure 109 Lack of fusion.
Causes of this defect can be:
Lack of fusion can usually be prevented by:
Overlapping (Figure 110) is the protrusion of the weld metal over the edge or toe of the weld bead. This defect can cause an area of lack of fusion and create a notch that can lead to crack initiation. If overlapping is allowed to occur, grinding off the excess weld metal after welding can be done.
Figure 110 Overlapping.
Overlapping is often produced by:
Overlapping can be prevented by or corrected by:
Melt-through (Figure 111) occurs when the arc burns through the bottom of the weld. It is usually caused by the heat input being too high.
Figure 111 Melt-through.
This can be caused by:
This can be prevented by:
FCAW may produce a small amount of spatter but excessive weld spatter creates a poor weld appearance, wastes electrodes, causes difficult slag removal, and can lead to incomplete fusion in multipass welds. Excessive spatter can also block the flow of shielding gas from the nozzle that causes porosity. The amount of spatter produced by FCAW will vary, depending on the type of metal transfer, type of electrode, and the type of shielding gas used. (Electrode wires that produce a large droplet size globular metal transfer will produce more spatter than those that produce a fine globular transfer. Self-shielded electrodes tend to produce higher spatter levels than gas-shielding types.)
The shielding gas provides slightly better arc stability. A gas-shielded electrode that is used with carbon dioxide shielding will produce higher spatter levels than the same electrode used with argon-carbon dioxide or argon-oxygen mixtures. This is due to the coarser droplet size promoted by the carbon dioxide shielding. Excessive weld spatter may also result from operating the electrode wire outside the operating ranges of amperage, voltage, and electrode extension for which the manufacturer designed the electrode. Methods of reducing the amount of spatter would be to reduce the welding current, welding voltage, or electrode extension. When gas-shielded wires are being used, changing the shielding gas from carbon dioxide to an argon-carbon dioxide mixture will further reduce spatter levels. If spatter is caused, it can be removed by grinding or chipping.
Many codes prohibit striking the arc on the surface of the workpiece. Striking the arc on the base metal outside the weld joint can produce a hard spot on the base metal surface. Failures can then occur due to the notch effect. The arc strikes might create a small notch on the surface of the metal that can act as an initiating point for cracks.
Weld craters (Figure 112) are depressions on the weld surface at the point where the arc was broken. These craters are caused by the solidification of the metal after the arc has been broken. The weld crater often cracks and can serve as an origin for linear cracking back into the weld metal or into the base metal.
Figure 112 Weld crater.
These craters can usually be removed by chipping or grinding and the depression can be filled in with a small deposit of filler metal. The best way of preventing weld craters is to reverse the travel of the electrode a little way back into the weld bead from the end of the weld bead before breaking the arc. Another method is to stop the travel long enough to fill the crater before breaking the arc.
An improper welding procedure, welder technique, or materials may cause cracking. All types of cracking can be classified as either hot cracking or cold cracking, and these cracks can be oriented transversely or longitudinally to the weld. Transverse cracks are perpendicular to the axis of the weld, where longitudinal cracks are parallel to the axis of the weld. Transverse cracks are often the result of longitudinal shrinkage strains acting on excessively hard and brittle weld metal. Longitudinal cracks are often caused by high joint restraint and high cooling rates. Hot cracking is a defect that occurs at higher temperatures and generally happens just after the weld metal starts to solidify. This type of cracking is often caused by excessive sulfur, phosphorous, and lead contents in the base metal. It can also occur because of an improper method of breaking the arc or in a root pass when the cross-sectional area of the weld bead is small compared to the mass of the base metal. Hot cracking often occurs in deep penetrating welds and it can continue through successive layers if it is not repaired.
Hot cracking may be prevented or minimized by:
Crater cracks are shallow hot cracks that are caused by improperly breaking the arc. Several types are shown in Figure 113. Crater cracks may be prevented the same way that craters are prevented: by reversing the travel of the electrode a little way back into the weld from the end of the weld or stopping the travel before breaking the arc.
Figure 113 Crater cracks.
Cold cracking occurs after the weld metal solidification is complete. Cold cracking may occur several days after welding and is generally caused by hydrogen embrittlement, excessive joint restraint, and rapid cooling. Preheating, the use of a dry, high purity shielding gas, and a proper cleaning procedure can help reduce this problem. Cold cracking is often less of a problem with FCAW than GMAW because of the higher heat input of FCAW, which provides more of a preheating effect. The preheating helps to reduce slightly the problems with cold cracking due to excessive cooling rates.
Centerline cracks are cold cracks that often occur in single-pass, concave fillet welds. A centerline crack is a longitudinal crack that runs down the center of the weld, as shown in Figure 114.
Figure 114 Centerline crack.
This problem may be caused by:
The chief methods of preventing centerline cracks are:
Base metal and underbead cracks are cold cracks that form in the heat-affected zone of the base metal. Underbead cracks occur underneath the weld bead, as shown in Figure 115.
Figure 115 Underbead cracks.
Base metal cracks are those cracks that originate in the heat-affected zone of the weld. These types of cracking are caused by excessive joint restraint, hydrogen, and a brittle microstructure. Rapid cooling causes a brittle microstructure or excessive heat input. Underbead and base metal cracking can be reduced or eliminated by using preheat.
A number of other welding problems may occur, such as those caused by magnetic fields, improper moisture, or indirect electrode arc.
The electric current that flows through the electrode, workpiece, and work cable sets up magnetic fields in a circular path perpendicular to the direction of the current. When the magnetic fields around the arc are unbalanced, it tends to bend away from the greatest concentration of the magnetic field. This deflection of the arc is called arc blow.
Deflection is usually in the direction of travel or opposite to it, but it sometimes occurs to the side. Arc blow can result in an irregular weld bead and incomplete fusion.
Direct current is susceptible to arc blow, especially when welding is being done in corners and near the end of joints. Arc blow occurs with direct current because the induced magnetic field is in one direction. Arc blow is shown in Figure 116.
Figure 116 Arc blow.
Arc blow is often encountered when welding magnetized metal or near a magnetized fixture. This problem also occurs when welding complex structures and on massive structures with high currents and poor fit-up. Forward arc blow is encountered when welding away from the ground connection or at the beginning of a weld joint. Backward arc blow occurs toward the grounding connection, into a corner, or toward the end of a weld joint. Several methods can be used to correct the arc blow problem:
Many discontinuities that occur in FCAW are caused by inadequate shielding of the arc. Inadequate shielding can cause oxidation of the weld puddle and porosity in the weld bead. This will usually appear as surface porosity. This problem can easily be detected because the arc will change color, the weld bead will be discolored, and the arc will become unstable and difficult to control. The most common causes of this problem when using gas-shielded flux-cored arc wires are:
The most common causes of inadequate shielding for self-shielded electrodes are:
In general, inadequate shielding is more of a problem with gas-shielding electrodes. There are several ways that this problem can be corrected or prevented. The torch and hoses should be checked before welding to make sure that the shielding gas can flow freely and is not leaking. The nozzle and contact tube should be cleaned of spatter regularly. A very high travel speed may leave the weld puddle or part of it exposed to the atmosphere. This may be corrected, in some cases by inclining the gun in the direction of travel, using a nozzle that directs shielding gas back over the heated area, or by increasing the gas flow rate. The best method is to slow the travel speed. Increasing the gas flow rate will increase the expense of the welding. An improper flow rate may occasionally be a problem. For example, when using carbon dioxide shielding in the overhead position, highest gas flow rates may have to be used to provide adequate shielding.
Carbon dioxide is heavier than air and will tend to fall away from the weld area. An excessive gas flow rate can cause excessive turbulence in the weld puddle. When winds or air drafts are present, several corrective steps may be taken. One method is to switch from a gas-shielded electrode to a self-shielded electrode. Setting up screens around the operation is another method of solving this problem. Increasing the gas flow rate is helpful when using gas-shielded electrodes, or increasing the electrode extension when using self-shielded electrodes. An excessive distance between the end of the nozzle and the molten weld puddle will also create a problem in providing adequate shielding, which can be corrected by shortening this distance.
The power delivered to the arc in FCAW depends on a transfer of current from the tip of the contact tube to the electrode by means of a sliding contact tube. A clogged, dirty, or worn contact tube can cause changes for power transferred to the electrode, which can have an effect on the arc characteristics. It can also cause an irregular weld bead and possible incomplete fusion because of the power fluctuations. A clogged contact tube can stop the feed of the electrode wire, which stops the welding arc. A contact tube can become dirty or clogged by spatter from the arc, by rust, scale, drawing compounds left from the manufacture of the wire on the surface of the electrode, or by metal chips created by tight wire feed rolls. These problems can best be prevented by making sure that the electrode wire is clean and the wire feed rolls are tight enough to feed the wire without creating chips. A wire wipe made of cloth is often attached to the wire feeder to clean the electrode wire as it is fed
Wire feed stoppages are generally less of a problem with FCAW than with GMAW because of the larger diameter electrode wires used in FCAW. However, this can still be a problem. Wire feed stoppages cause the arc to be extinguished and can create an irregular weld bead because of the stops and starts. Wire stoppages can also cause a loss of welding time because many of the problems take a long time to correct when wire becomes wrapped around the wire feed rolls, wadded up in bird nests in the wire feeder, or broken. Wire feed stoppages can be caused by:
Wire feed stoppages, in many cases, must be corrected by taking the disassembling the gun and cutting and removing the wire, or by cutting and removing the wire from the wire feeder. Both result in time lost to locate the problem and feed the new length of wire through the assembly to the gun. Wire stoppages can be prevented by:
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Several operations may be required after welding, such as cleaning, inspection of the welds, and postheating. These items may or may not be part of the procedure, the operations performed will depend on the governing code or specification, type of metal, and the quality of the weld deposit.
FCAW produces a moderate slag covering that must be removed after welding. Slag removal is also required between passes of a multipass weld to prevent slag inclusions and incomplete fusion. Slag removal is generally done using a chipping hammer. A certain amount of spatter is created in FCAW, which can make slag removal slightly more difficult. If an excessive amount of spatter is created, slag removal may become very difficult. After the slag has been removed, wire brushing or buffing can be done to remove the loose slag particles and to remove discoloration around the bead. Mild steel brushes can be used on most steels but stainless steel brushes should be used on stainless steel to prevent contamination. Spatter can be removed by grinding or wire brushing. FCAW usually produces a smooth weld surface. If a different weld profile is needed, grinding can be used, although grinding of weld profiles should be avoided due to the expense. 1
Inspection and testing of the weld is done after cleaning to determine the quality of the weld joint. There are many different methods of inspection and testing which will not be covered in detail in this course. The uses of these methods wiII often depend on the code or specification that covered the welding. Testing of a weldment may be done nondestructively or destructively. Nondestructive testing is used to locate defects in the weld and base metal. There are many different nondestructive testing methods. Some of the most widely used methods are visual, magnetic particle, liquid penetrant, ultrasonic, and radiographic. Visual, magnetic particle and liquid penetrant inspection are used to locate surface defects, while ultrasonic and radiographic inspections are used to locate internal defects. Destructive testing is used to determine the mechanical properties of the weld, such as the strength, ductility, and toughness. Destructive testing is also done by several methods, depending on the mechanical properties being tested. Some of the most common types of destructive testing are tensile bar tests, impact tests, and bend tests.
Repairing the weld is usually needed when defects are found during inspection. When a defect is found, it can be gouged, ground, chipped, or machined out, depending on the type of material being welded. For steels, grinding and air carbon arc gouging are commonly used. When maximum corrosion resistance is required, air carbon arc gouging is used on stainless steels only when grinding or wire brushing of the groove face to remove carbon deposits is done. For stainless steels, chipping is a common method for removing defects. Air carbon arc gouging is preferred for many applications because it is usually the quickest method. Grinding is popular for removing surface defects and shallow-lying defects. Once the defects have been removed, the low areas created by the grinding and gouging can be rewelded using FCAW or some other welding process. The welds are then reinspected to make sure that the defects have been properly repaired.
Postheating is the heat treatment applied to the weld or weldment after welding. Postheating is often required after the weld has been completed, depending on the type of metal being welded, the specific application, and the governing code or specifications. Many of the low-carbon and low-alloy steels are rarely postheated. Various types of postheating are used to obtain specific properties. Some of the most commonly used postheats are annealing, stress relieving, normalizing, and quenching and tempering. Stress relieving is the most widely used heat treatment after welding. Postheating is accomplished by most of the same methods that are used for preheating, such as furnaces, induction coils, and electric resistance heating blankets. One method used for stress relieving that does not involve the reheating of the weldments is called vibratory stress relief. This method vibrates the weldment during or after welding to relieve the residual stresses during or after solidification.
Annealing is a process involving heating and cooling that is usually applied to induce softening. This process is widely used on steels that become very hard and brittle because of welding. There are several different kinds and when used on ferrous metals, it is called full annealing. Full annealing is the heating up of a material to cause recrystallization of the grain structure, which causes softening. This softening process is done by heating a ferrous metal to a temperature above the transformation range and slowly cooling to a temperature below this range. This process is usually done in a furnace to provide a controlled cooling rate.
Normalizing is a heat treatment that is applied only to ferrous metals. Normalizing occurs when the metal is heated to a temperature above the transformation range and is cooled in still air to a temperature below this range. The main difference between normalizing and annealing is that a normalized weldment is cooled in still air that produces a quicker cooling rate and an annealed weldment is slowly cooled in a furnace. A normalizing heat treatment will refine the metal grain size and give a tougher weld, while an annealing heat treatment will result in a softer weld.
Stress relieving is the uniform heating of a weldment to a high enough temperature, below the critical range, to relieve most of the residual stresses due to welding. This operation is performed on many steels after welding to relieve the residual stresses due to welding. This also reduces warpage during machining that may occur with a high residual stress buildup. On parts and metals that are likely to crack due to the internal stress created by welding, the parts should be put into stress relief immediately after welding, without being allowed to cool to room temperature. The terms normalizing and annealing are misnomers for this heat treatment.
Quenching and tempering is another postweld heat treatment commonly used. The metal is heated up and then quenched to form a hard and brittle metallurgical structure. The weldment is then tempered by reheating to a particular temperature, dependent on the degree of ductility, strength, toughness, and hardness desired. Tempering reduces the hardness of the part as it increases the strength, toughness, and ductility of the weld.
|Test Your Knowledge
10. What causes slag inclusions?
11. Which of the following is a nondestructive test?
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To become a fully certified welder, you must know the requirements for training and qualifications. While these requirements may differ somewhat from organization to organization, and you may need to demonstrate your skills to qualify for a particular **130 project and specific welding task, the basic guidelines are the same for achieving the training and qualifications.
FCAW requires a certain degree of skill to produce good quality welds. In semiautomatic welding, the welder has to manipulate the welding gun and control the speed of travel. Less skill is required to operate this process when compared to the manual welding processes because the machine controls the arc length and feeds the electrode wire. Welders skilled in manual welding processes and GMAW generally have less difficulty learning FCAW. This process uses similar equipment and welding techniques to those used in GMAW. At higher current levels, when using larger diameter wires, FCAW has a smoother arc and is easier to handle than larger diameter solid wires with a carbon dioxide shielding. Because of the deep penetrating characteristics of the process, lack of fusion and incomplete penetration are easier to avoid and compensate for than GMAW using short-circuiting transfer.
The exact content of a training program will vary, depending on the specific application of the process. A training program should have enough flexibility so that it can be adapted to changing needs and applications. Because of this, the emphasis may be placed on certain areas of training based on the complexity of the parts to be welded, type of metal, and governing code or specification. A welding course that covers all position welding requires more training time than one that simply covers flat position welding only. A welding course for pipe requires more training time than one for welding plate. The major purpose of the training program is to give the welder the skill and knowledge to be able to do the best job possible. A training program may be broken up into several areas, depending on the training requirements of the student.
The basic FCAW training program is used to teach the students the basic skills necessary to weld plate. This course provides training on how to make quality fillet and groove welds. The course also gives the students the knowledge of how to set up the equipment, clean the base metal, basic operating principles, and the difficulties that are commonly encountered. The training also covers the different welding techniques used for gas-shielded and self-shielded electrodes. Also covered are the techniques for welding out-of-position using small diameter electrodes. The training obtained by the student should give the skill to perform a job welding plate. This course should also provide the background skill and knowledge required to take an advanced course for a specific application, such as for welding pipe. The following is an outline for a course approximately 35 hours long:
Before a welder can begin work on any job covered by a welding code or specification, he or she must become certified under the code that applies. Many different codes are in use today, and it is exceedingly important that the specific code is referred to when taking qualification tests. In general, the following type of work is covered by codes: pressure vessels and pressure piping, highway and railway bridges, public buildings, tanks and containers that will hold flammable or explosive materials, cross country pipeline, aircraft, ordnance material, ships and boats, and nuclear power plants.
Several of the specifications include consideration of the FCAW process. These are:
These specifications do not provide qualifications of the FCAW process for all applications and service requirements. For applications where AWS or other specifications are not available or do not apply and general criteria for qualification is desired, AWS B2.1, Standard for Welding Procedure and Performance Qualification, is often used. Qualification is obtained differently under the various codes. Qualification under one code will not necessarily qualify a welder to weld under a different code. In most cases, qualification for one employer will not allow the welder to work for another employer. If the welder uses a different process or the welding procedure is altered drastically, requalification is required. In most cases, if the welder is continually; employed, welding requalification is not required, providing the work performed meets the quality requirements.
Responsible manufacturers or contractors may give qualifications tests. On pressure vessel work, the welding procedure must also be qualified and this will be done before the welders are qualified. Under other codes, this is not necessary. To become qualified, the welder must make specified welds using the required process, base metal, base metal thickness, electrode type, position, and joint design. For example, in the AWS Structural Welding Code (D1.1), certain joint designs are considered prequalified for FCAW. Test specimens must be made according to standardized sizes and under the observation of a qualified person. For most government specifications, a government inspector must witness the making of weld specimens. Specimens must be properly identified and prepared for testing. The most common test is a guided bend test. In some cases, radiographic examinations, fracture tests, or other tests are used. Satisfactory completion of test specimens, provided that they meet acceptability standards, will qualify the welder for specific types of welding. Again, the welding that will be allowed depends on the particular code. In general, the code indicates the range of thicknesses that may be welded, the positions that may be used, and the alloys that may be welded.
Qualification of welders is a highly technical subject and cannot be covered fully here. You should obtain and study the actual code prior to taking any tests.
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Safety is an important consideration when welding. Every welding shop should have a safety program and take adequate safety precautions to help protect welders. The welders should also be made aware of safety precautions and procedures. Employees who fail to follow adequate safety precautions can cause physical injury to themselves and others, and damage property. Any of these conditions can result in physical discomfort and loss of property, time, and money. Welding is a safe occupation when safety rules and common sense are followed. A set of safety rules is presented in the American National Standard Z49.1, "Safety in Welding and Cutting," published by the American Welding Society, which should be followed.
There are several types of hazards associated with FCAW. These hazards do not necessarily result in serious injuries. They can also be of a minor nature, which can cause discomforts that irritate and reduce the efficiency of the welders. These hazards are:
There are several precautions that should be taken to prevent an electrical shock hazard. The first item that should be done before welding is to make sure the arc welding equipment is installed properly, grounded, and in good working condition. The electrical equipment should be maintained and installed in accordance with the National Electrical Code and any state and local codes that apply. Power supplies should be connected to an adequate electrical ground, such as an approved building ground, cold water pipe, or ground rod. Power supplies are connected to ground through the cable that connects the power supply to the electrical system ground. Cables with frayed or cracked insulation and faulty or badly worn connections can cause electrical short circuits and shocks. If it is necessary to splice lengths of welding cable together, the electrical connections should be tight and insulated. The proper size welding cables should also be used because constantly overloading a welding cable that is too small can destroy the insulation and create bare spots in the insulation. This occurs because excessive heat builds up in the cable and destroys the insulation. An improperly insulated welding cable is both an electrical shock hazard and a fire hazard.
The welding area should be dry and free of any standing water, which could cause electrical shock. When it is necessary to weld in a damp or wet area, the welder should wear rubber boots and stand on a dry, insulated platform.
The welding arc of FCAW emits large amounts of invisible ultraviolet and infrared rays. Skin exposed to the arc, even for a short time, can suffer serious ultraviolet and infrared burns, which are essentially the same as sunburn, but the burn caused by welding can take place in a much shorter time and can be very painful. Because of this, the welder should always wear protective clothing suitable for the welding to be done. These clothes should be fairly heavy and not easily burned. Leather is often used to make jackets, capes and bibs, or other similar arrangements to shield the arms, shoulders, chest, and stomach from the arc radiation and arc spatter. Leather is also used to make gloves and gauntlets for the welder.
The eyes must also be protected from the radiation emitted by the welding arc. Arc-burn can result if the eyes are not protected. Arc-burn of the eye is similar to sunburn of the skin and it is extremely painful for about 24 to 48 hours. Usually, arc-burn does not permanently injure the eyes, but it can cause intense pain as though several grains of sand were in your eyes. There are several commercial solutions available to soothe the skin and eyes during the period of suffering.
Infrared arc rays can cause fatigue of the retina of the eye. The effects of infrared rays are not nearly as noticeable or immediate as the effects of ultraviolet rays. Infrared rays are probably more dangerous in that their effects can be longer lasting and result in impaired vision.
The flux-cored welding arc is a relatively high energy arc that is much brighter than lower current welding arcs. Even though more smoke is given off from the arc area, it does not shield arc rays effectively.
The best protection for the eyes and face is provided by a headshield that has a window set in it with a filter lens in the window. Headshields are generally made of fiberglass or a pressed fiber material that is lightweight. The filter lens is made of a dark glass capable of absorbing infrared rays, ultraviolet rays, and most visible light coming from the arc. The type of lens used varies for different welders but it should be dark enough so the arc can be viewed without discomfort, yet not so dark the welder cannot see what he or she is doing. Table 23 shows the different lenses commonly recommended for use in shielded metal arc welding. The higher the lens numbers, the darker the lens. A clear, replaceable glass should be put on the outside of the welding lens to protect it from spatter and breakage.
Table 23 Recommended Filter Lens Shades Used in
Shielded Metal Arc Welding
|Range-Amperes||Lens Shade Number|
|75-200||10 to 11|
|200-400||12 to 13|
One of the main problems with FCAW is that it gives off more smoke and fumes than processes such as GTAW, GMAW, and SAW. It even tends to produce higher smoke and fume levels than SMAW. A hazard warning for fume is placed on the electrode wire box.
The welding area should be adequately ventilated because fumes and gases, such as ozone, carbon monoxide, and carbon dioxide, are hazardous for the welder to breathe. When welding is done in confined areas, an external air supply is required. This is furnished by the use of a respirator on a special helmet. A second person should stand just outside the confined area to lend assistance to the welder, if necessary. Another method is to use an exhaust system to remove welding fumes. Special fume extractor nozzles attached to the welding gun are popular for use with FCAW to reduce the smoke levels produced. These nozzles are connected to a filter and an exhaust pump, which greatly reduce the smoke level as shown in Figure 117.
Figure 117 FCAW with fume extractor nozzle.
The shielding gas may displace the air that the welder needs for breathing. Because of this, welding should not be done in an enclosed area or hole, which can cause suffocation without the use of a respirator. Welding should never be done near degreasing and cleaning operations. The fumes from chlorinated solvents used for cleaning form a very toxic gas, called phosgene, when exposed to an arc. A mechanical exhaust system should be used when welding metals with lead, cadmium, and zinc coatings. AWS/ANSI Z49.1 should be consulted for ventilation requirements.
The shielding gas used for FCAW is compressed and stored in cylinders. One advantage of self-shielded flux-cored wire is that compressed gas cylinders are not required, so this is primarily a safety consideration when gas-shielded electrodes are used. Improper handling of compressed gas cylinders can create a safety hazard. When in use, gas cylinders should be secured to a wall or other structural support. The valve of the cylinder should be opened slowly and the welder should stand away from the face of the regulator when doing this. The welding arc should never be struck on a compressed gas cylinder. When not in use, gas cylinders should be stored with their caps on. Caps should also be on when they are moved. If the valve would get knocked off, the cylinder acts like a missile because of the escaping gas and can cause injury and damage. When compressed gas cylinders are empty, the valve should be closed and they should be marked as empty. This is done by marking the letters, "MT" or "EMPTY" on the cylinder.
Fires and explosions are hazards that can exist in a welding area if the proper precautions are not taken. The FCAW process produces sparks and spatters which can start a fire or explosion in the welding area if not kept free of flammable, volatile, or explosive materials. Welding should never be done near degreasing and other, similar operations. Welders should wear leather clothing for protection from burns because leather is fireproof. Fires can also be started by an electrical short or by overheated, worn cables. In case of a fire started by a flammable liquid or an electrical fire, a CO2 or dry chemical type of fire extinguisher is used. Fire extinguishers should be kept at handy spots around the shop and welders should make a mental note of where they are located.
Other precautions that have to do with explosions are also important. A welder should not weld on containers that have held combustibles unless it is absolutely certain there are no fumes or residue left. Welding should not be done on sealed containers without providing vents and taking special precautions. The welding arc should never be struck on a compressed gas cylinder. When the electrode holder is set down or not in use, it should never be allowed to touch a compressed gas cylinder.
Hazards can also be encountered during the weld cleaning process. Precautions must be taken to protect the skin and eyes from hot slag particles. FCAW produces a moderate slag covering which much be removed. The welding helmet, gloves, and heavy clothing protect the skin from slag chipping and grinding of the weld metal. Safety glasses should also be worn underneath the welding helmet to protect the eyes from particles that could get inside the welding helmet. Screens should be set up if there are other people in the area to protect them from arc burn. **136
This course has introduced you to the FCAW process, from the types of power sources, controls, and electrodes to the types of training and qualifications needed. It also described the industries that use the FCAW process and its applications. Welding metallurgy, weld and joint design, and welding procedure variables were also discussed. The course finished up with a description of weld defects and how to identify them, then covered welder training and the common safety precautions applicable to all welding processes. As always, use the manufacturers operator manuals for the specific setup and safety procedures of the welder you will be using.
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1. What type of current is used in flux cored arc welding?
2. What is the main advantage of self shielding flux cored electrodes?
3. An electrode that has a minimum tensile strength of 80,000 psi for use in all positions for low alloy has what designation?
4. A welding electrode that has an AWS classification of E700T should be used for a metal-arc welding job in what position(s)?
5. What is the largest diameter electrode that can be used for vertical and overhead welding?
6. Which of the following properties is the basic rule for selecting an electrode for a job?
7. When the electrode is positive and the workpiece is negative, the electrons flow from the workpiece to the electrode. What polarity is being used?
8. Which one of the following steps do you take to correct arc blow?
9. Of the following practices, which one is correct for breaking an arc with an electrode?
10. What drag angle is used for flat and horizontal position welding using self shielded electrodes?
11. When using gas shielded electrodes, what angle is used for maximum penetration?
12. For which of the following reasons do you use relatively small electrodes for overhead butt welding?
13. Which of the following mistakes can cause undercutting in welds?
15. Which of the following mistakes can cause cracked welds?
16. Which of the following mistakes can cause poor penetration?
17. Which of the following mistakes can cause brittle welds?
18. When pipe has _____ wall thickness, only the single U-type of butt joint should be used.
19. You do NOT need to do which of the following procedures when preparing a joint for welding?
20. What maximum nominal diameter of electrode should you NOT exceed when making the root pass of a multilayer weld on pipe?
21. The root of a fillet weld is where the _____.
22. The face of a fillet weld is the _____.
23. The toe of a fillet weld is the _____.
24. The leg of the weld is the _____.
25. The throat of a fillet is the shortest distance from the _____.
26. Welding machine installations should be _____.
27. Welding machine frames should be _____.
28. The welding arc gives off ultra-violet rays, which can cause eye injury. Injury can be prevented by _____.
29. Ultra-violet rays from the arc _____.
30. Vaporized metals, such as zinc, cadmium, lead, and beryllium _____.
31. After striking an arc, when should the travel angle start?
32. When welding over a previously deposited bead, _____.
33. At the completion of the weld, the crater should _____.
34. Horizontal position fillet welding is done from the_____.
35. In the flat position welding, the face of the weld is approximately _____.
36. At what angle should you hold the electrode when making lap joints with metal of differing thickness?
7. What determines the direction the arc force applies to the weld pool?
38. When reading current ranges in a welding schedule, fillet welds use the _____.
39. Tack welds should be _____.
40. You are responsible for performing all checks and procedure steps before, during, and after welding.
41. How do you clean the slag form a weld bead?
42. You must be certified under the code that applies to the type of welding you will be doing.
43. A sound weld can be made over dirt, paint, and grease if the correct electrode is used.
44. A destructive test is _____.
45. It is necessary to know the position in which welding is to be done _____.
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