Properties and Uses of Metal (Advanced)
As a steelworker, you will be regarded as the subject-matter expert on everything metal. You will be expected to build, repair, and refurbish almost everything metal. Knowing how to identify the metals you will be working with is one of the foundations of your rate. To carry out these responsibilities skillfully, you must possess a sound working knowledge of various metals and their properties.
Once you learn how to identify different metals confidently, beyond the ferrous/nonferrous determination you learned in steelworker basics, you can make the proper decisions pertaining to materials and tools you will need to complete the job. You will work mainly with the ferrous metals iron and steel; however, you must also become familiar with and be able to differentiate between the nonferrous metals. This coursel discusses the properties of different metals in greater detail and show how to use simple tests to help identify common metals.
When you have completed this lesson, you will be able to:
Metals in general have high electrical conductivity, thermal conductivity, luster and density, and the ability to be deformed under stress without cleaving. Chemical elements lacking these properties are classed as nonmetals. A few elements, known as metalloids, sometimes behave like a metal and at other times like a nonmetal. Some examples of metalloids are as follows: boron, arsenic, and silicon.
As you have already studied, metals are divided into two classes, ferrous and nonferrous. Ferrous metals are those in the iron class and are magnetic in nature. These metals consist of iron, steel, and alloys related to them. Nonferrous metals are those that contain either no, or very small amounts of, ferrous metals. These are generally divided into the aluminum, copper, magnesium, lead, and similar groups.
Although you will hardly ever work with pure metals, you need to be knowledgeable of their properties because the alloys you will work with are combinations of pure metals. Some of the pure metals discussed in this lesson are the base metals in these alloys, especially iron, aluminum, and magnesium. Other metals discussed are the alloying elements present in small quantities but important in their effect, including chromium, molybdenum, titanium, and manganese.
An alloy is a mixture of two or more elements in solid solution in which the main element is a metal. Most pure metals are either too soft, brittle, or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the properties of the resultant metals to produce desirable characteristics. The reason for making alloys is generally to create a less brittle, harder, corrosion resistant material, or one with a more desirable color and luster.
Of the metallic alloys in use today, the alloys of iron (steel, stainless steel, cast iron, tool steel, alloy steel) make up the largest proportion by both quantity and commercial value. Iron alloyed with various proportions of carbon gives low-, mid- and high-carbon steels, and as the carbon levels increase, ductility and toughness decrease. The addition of silicon will produce cast irons, while the addition of chromium, nickel, and molybdenum to carbon steels (more than 10%) results in stainless steels.
Aluminum, titanium, copper, and magnesium alloys are also significant in commercial value. Copper alloys have been around since prehistory—bronze gave the Bronze Age its name—and have many applications today, most importantly in electrical wiring. The alloys of aluminum, titanium, and magnesium are valued for their high strength-toweight ratios. These materials are ideal for situations where high strength-to-weight ratio is more important than material cost, such as in aerospace and some automotive applications.
Alloys specially designed for highly demanding applications, such as jet engines, may contain more than ten elements.
Table 1 is a list of various elements and their symbols that compose metallic materials.
Table 1 Symbols of Base Metals and Alloying Elements.
Since you will work mostly with alloys, you need to understand their characteristics. The characteristics of elements and alloys are explained in terms of physical, chemical, electrical, and mechanical properties.
When selecting stock for a job, your main concern is the mechanical properties of the metal. The various properties of metals and alloys were determined in the manufacturers’ laboratories and by various societies interested in metallurgical development. Charts presenting the properties of a particular metal or alloy are available in many commercially published reference books. The charts provide information on the melting point, tensile strength, electrical conductivity, magnetic properties, and other properties of a particular metal or alloy. Simple tests can be conducted to determine some of the properties of a metal; however, we normally use a metal test only as an aid for identifying a piece of stock. Some of these methods of testing are discussed later in this lesson.
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Strength, hardness, toughness, elasticity, plasticity, brittleness, and ductility and malleability are mechanical properties used as measurements of how metals behave under a load. These properties are described in terms of the types of force or stress that the metal must withstand and how these are resisted. Common types of stress are compression, tension, shear, torsion, impact, or a combination of these stresses, such as fatigue (Figure 1).
Figure 1 — Stress applied to a material.
Some materials are equally strong in compression, tension, and shear. However, many materials show marked differences; for example, cured concrete has a maximum strength of 2,000 psi in compression, but only 400 psi in tension. Carbon steel has a maximum strength of 56,000 psi in tension and compression but a maximum shear strength of only 42,000 psi; therefore, when dealing with maximum strength, you should always state the type of loading.
Strength is the property that enables a metal to resist deformation under load.
Hardness is defined as resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness (temper) or to resistance to scratching, abrasion, or cutting. It is the property of a metal which gives it the ability to resist being permanently deformed (bent, broken, or have its shape changed) when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation. There are several methods of measuring the hardness of a material, so hardness is always specified in terms of the particular test used.
The metals industry uses three types of hardness tests with accuracy: the Brinell, Rockwell, and Vickers hardness tests. Since the definitions of metallurgic ultimate strength and hardness are rather similar, it can generally be assumed that a strong metal is also a hard metal.
These hardness tests measure a metal's hardness by determining the metal's resistance to the penetration of a non-deformable ball or cone. The tests determine the depth to which such a ball or cone will sink into the metal under a given load within a specific period of time.
Of these three tests, Rockwell is the most frequently used, the basic principle being that a hard material can penetrate a softer one, so you measure the amount of penetration and compare it to a scale.
In regular Rockwell testing the minor load is always 10 kgf (kilograms of force). The major load can be any of the following loads: 60 kgf, 100 kgf, or 150 kgf. No Rockwell hardness value is specified by a number alone. It must always be prefixed by a letter signifying the value of the major load and type of penetrator (e.g., HRC 35). A letter has been assigned for every possible combination of load and penetrator, as given in Table 2. Each test yields a Rockwell hardness value on your tester. Testers with dial gauges have two sets of figures: red and black. When the diamond penetrator is used, the readings are taken from the black divisions. When testing with any of the ball penetrators, the readings are taken from the red divisions. Testers with digital displays have a scale selection switch, allowing an automatic display of the Rockwell hardness number on its screen.
Table 2 — Rockwell Hardness Scale.
in Kilograms-Force (Kgf)
The regular Rockwell scales are established such that an infinitely hard material will read 100 on the diamond penetrator scales and 130 on the ball penetrator scales. One regular Rockwell number represents a penetration of 0.002 mm (0.000080 inch). Therefore, a reading of C60 indicates penetration from a minor to major load of (100 to 60 Rockwell points) x 0.002 mm = 0.080 mm or 0.0032 inch. A reading of B80 indicates a penetration of (130 to 80 Rockwell points) x 0.002 = 0.100 mm or 0.004 inch (Figure 2).
Figure 2 — Rockwell testing.
A full explanation of the various methods used to determine the hardness of a material is available in commercial books. ASTM publishes standards for every type of hardness test. Use these standards for the type of testing you will be performing as they are the most up-to-date standards available.
Toughness is the property that enables a material to withstand shock and be deformed without rupturing. Toughness may be considered as a combination of strength and plasticity. Table 3 shows the order of some of the more common materials for toughness as well as other properties.
Table 3 — Mechanical Properties of Metals/Alloys.
|Copper||White cast iron||Gold||Gold||Gold|
|Nickel||Gray cast iron||Silver||Silver||Platinum|
Metals/alloys are ranked in descending order of having the property named in the column heading.
When a material has a load applied to it, the load causes the material to deform. Elasticity is the ability of a material to return to its original shape after the load is removed. Theoretically, the elastic limit of a material is the limit to which a material can be loaded and still recover its original shape after the load is removed.
All materials are elastic to some extent. It may surprise you to learn that a piece of steel is more elastic than a rubber band. The rubber band stretches more than the steel since it is more easily strained, but the steel returns more nearly to its original shape and size and is, therefore, more truly elastic.
Plasticity describes the ability of materials to undergo irreversible deformation without fracture or damage. This property is the opposite of strength. By careful alloying of metals, the combination of plasticity and strength is used to manufacture large structural members. For example, should a member of a bridge structure become overloaded, plasticity allows the overloaded member to flow, allowing the distribution of the load to other parts of the bridge structure. Sheet aluminum has a high plasticity, whereas tool steel has a very low plasticity.
Brittleness is the opposite of plasticity. A brittle metal will break or shatter before it deforms if bent or struck a sharp blow. Generally, brittle metals are high in compressive strength but low in tensile strength. For example, cast iron is very brittle, so you would not use cast iron for fabricating support beams in a bridge.
The properties known as ductility and malleability are special cases of plasticity.
Most metals that exhibit one of these properties also exhibit the other. However, this is not always true. Lead, for example, is very malleable (it can be permanently deformed in compression without breaking), but it is not ductile (it cannot be permanently deformed in tension to any great extent).
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Corrosion resistance is the property that enables a material to resist entering into chemical combination with other substances from attacks by atmospheric, chemical, or electrochemical conditions. A high degree of corrosion resistance is very desirable in all metals exposed to weather elements. Most metals are easily corroded, however, as shown by the fact that pure metals occur only rarely in nature. One of the most common examples of corrosion, sometimes called oxidation, is illustrated by the rusting of iron.
The presence of impurities or the presence of alloying elements may greatly alter the corrosion resistance of a metal. For example, the zinc that is known as “commercially pure” contains a small amount of impurities; this grade of zinc corrodes about 10,000 times as fast as zinc that is chemically pure. On the other hand, many alloys have been developed for the particular purpose of increasing the corrosion resistance of the material. For example, pure iron would be entirely unsuitable for use in boilers because it has very poor resistance to corrosion, particularly at high temperatures; yet alloys composed primarily of iron are used successfully for this service.
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Ferrous metals are metals that contain iron. Ferrous metals appear in the form of cast iron, carbon steel, and tool steel. The various alloys of iron, after undergoing certain processes, are pig iron, gray cast iron, white iron, white cast iron, malleable cast iron, wrought iron, alloy steel, and carbon steel. All these types of iron are mixtures of iron and carbon, manganese, sulfur, silicon, and phosphorous. Other elements are also present, but in amounts that do not appreciably affect the characteristics of the metal. Normally, ferrous metals are magnetic and nonferrous metals are nonmagnetic.
Pure iron rarely exists outside of the laboratory. Iron is produced by reducing iron ore to pig iron by using a blast furnace. From pig iron, many other types of iron and steel are produced by the addition or deletion of carbon and alloys. The following paragraphs discuss the different types of iron and steel that can be made from iron ore.
Pig iron is about 93% iron, from 3% to 5% carbon, with various amounts of other elements. Pig iron is comparatively weak and brittle; therefore, it has a limited use as is (cast iron pipe and some fittings and valves), and approximately ninety percent of it is refined to produce steel.
Wrought iron is almost pure iron. It is made from pig iron in a puddling furnace and has a carbon content of less than 0.08 percent. Carbon and other elements present in pig iron are taken out, leaving almost pure iron. In the process of manufacture, some slag is mixed with iron to form a fibrous structure in which long stringers of slag, running lengthwise, are mixed with long threads of iron. Because of the presence of slag, wrought iron resists corrosion and oxidation which cause rusting.
The chemical analyses of wrought iron and mild steel are just about the same. The difference comes from the properties controlled during the manufacturing process. Wrought iron can be gas and arc welded, machined, plated, and easily formed; however, it has a low hardness and a low fatigue strength.
Cast iron is a manmade alloy of iron, carbon, and silicon. A portion of the carbon exists as free carbon or graphite. Cast iron is any iron containing greater than 2% carbon alloy, with most cast irons ranging between 2.1% to 4% by weight. Cast iron has a high-compressive strength and good wear resistance; however, it lacks ductility, malleability, and impact strength. Alloying it with nickel, chromium, molybdenum, silicon, or vanadium improves toughness, tensile strength, and hardness. A malleable cast iron is produced through a prolonged annealing process.
Ingot iron is a commercially pure (99.85% iron), easily formed iron, with good ductility and corrosion resistance. The chemical analysis and properties of ingot iron are practically the same as the lowest carbon steel. The lowest carbon steel, known as dead-soft, has about 0.06% more carbon than ingot iron. Carbon content in iron is considered an impurity; carbon content in steel is considered an alloying element. The primary use for ingot iron is for galvanized and enameled sheet.
Steel is an alloy consisting mostly of iron, with carbon content between 0.2% and 2.1% by weight, depending on the grade. Steel contains less carbon than cast iron (2.1% to 4%), but considerably more than wrought iron (less than 0.08%). Basic carbon steels are alloyed with other elements, such as chromium and nickel, to increase certain physical properties of the metal. Steel can be machined, welded, and forged, all to varying degrees, depending on the type of steel.
Steels and other metals are classified based on method of manufacture, method of shaping, method of heat treatment, properties, intended use, and chemical composition. In addition, certain steels and other metals are often referred to by trade names.
Probably the most reasonable way to classify steels is by their chemical composition. Steels that derive their properties primarily from the presence of carbon are referred to merely as “steels” or sometimes as “plain carbon steels.” Steels that derive their properties primarily from the presence of some alloying element other than carbon are referred to as “alloys” or “alloy steels.”
Low-carbon steel (0.05% to 0.30% carbon) is tough and ductile, and can be rolled, punched, sheared, and worked when either hot or cold. It is easily machined and can readily be welded by all methods. It does not respond to heat-treating; however, it can easily be case hardened.
Medium-carbon steel (0.30% to 0.45% carbon) is strong and hard but cannot be welded or worked as easily as the low-carbon steels. It may be heat-treated after fabrication. It is used for general machining and forging of parts that require surface hardness and strength, such as crane hooks, axles, shafts, setscrews, and so on. Medium-carbon steel is made in bar form in the cold-rolled or the normalized and annealed condition. During welding, the weld zone will become hardened if cooled rapidly and must be stress-relieved after welding.
High-carbon steel (0.45% to 0.75% carbon) and very high-carbon steel (0.75% to 1.70% carbon) respond well to heat treatment and can be welded with difficulty, but the welding must be done using specific processes due to the hardening effect of heat at the welded joint. This steel is used for the manufacture of drills, taps, dies, springs, and other machine tools and hand tools that are heat-treated after fabrication to develop the hard structure necessary to withstand high shear stress and wear. It is manufactured in bar, sheet, and wire forms, and in the annealed or normalized condition in order to be suitable for machining before heat treatment.
Tool steel (0.70% to 1.40% carbon) refers to a special variety of carbon and alloy steels particularly well suited to be made into tools. Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, abrasion resistance is paramount, or the tool must withstand impact loading encountered with such tools as axes, pickaxes, and quarrying implements. Tool steel is used to manufacture chisels, shear blades, cutters, large taps, woodturning tools, blacksmith’s tools, razors, and similar parts where high hardness is required to maintain a sharp cutting edge. It is very difficult to weld due to the high carbon content. A spark test shows a moderately large volume of white sparks having many fine, repeating bursts.
This is a special low-carbon steel, containing specific small amounts of alloying elements, that is quenched and tempered to get a yield strength greater than 50,000 psi and tensile strengths of 70,000 to 120,000 psi. Structural members made from these high-strength steels may have smaller cross-sectional areas than common structural steels and still have equal or greater strength. Additionally, these steels are normally more corrosion and abrasion resistant. High-strength steels are covered by ASTM specifications.
This type of steel is much tougher than low-carbon steels. Shearing machines for this type of steel must have twice the capacity than that required for low-carbon steels.
This type of steel is classified by the American Iron and Steel Institute (AISI) into two general series named the 200-300 series and the 400 series. Each series includes several types of steel with different characteristics.
The 200-300 series of stainless steel is known as austenitic. Austenitic wrought stainless steel is classified in three groups:
Carbon content is usually low (0.15% or less), and the alloys contain a minimum of 16% chromium with sufficient nickel and manganese to provide an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.
Nitrogen-strengthened austenitic stainless steels are alloys of chromium-manganese=nitrogen; some grades also contain nickel. Yield strengths of these alloys (annealed) are typically 50% higher than those of the non-nitrogen-bearing grades. They are nonmagnetic, and most remain so, even after severe cold working.
Like carbon, nitrogen increases the strength of a steel, but unlike carbon, nitrogen does not combine significantly with chromium in a stainless steel. This combination, which forms chromium carbide, reduces the strength and corrosion resistance of an alloy.
Until recently, metallurgists had difficulty adding controlled amounts of nitrogen to an alloy. The development of the argon-oxygen decarburization (AOD) method has made possible strength levels formerly unattainable in conventional annealed stainless alloys.
Austenitic stainless steels are generally used where corrosion resistance and toughness are primary requirements. Typical applications include shafts, pumps, fasteners, and piping in seawater, and equipment for processing chemicals, food, and dairy products.
The most well known types of steel in this series are the 302 and 304. They are commonly called 18-8 because they are composed of 18% chromium and 8% nickel. The chromium nickel steels are the most widely used and are normally nonmagnetic.
The 400 series of steel is subdivided according to their crystalline structure into two general groups. One group is known as ferritic chromium and the other group as martensitic chromium.
Ferritic chromium contains 10.5% to 27% chromium and 0.08% to 0.20% carbon. Low in carbon content but generally higher in chromium than the martensitic grades, these steels cannot be hardened by heat treating and are only moderately hardened by cold working. Ferritic stainless steels are the straight chromium grades of stainless steel since they contain no nickel; they are magnetic and retain their basic microstructure up to the melting point if sufficient Cr and Mo are present. In the annealed condition, strength of these grades is approximately 50% higher than that of carbon steels.
Ferritic stainless steels are typically used where moderate corrosion resistance is required and where toughness is not a major need. They are also used where chloride stress-corrosion cracking may be a problem because they have high resistance to this type of corrosion failure. In heavy sections, achieving sufficient toughness is difficult with the higher-alloyed ferritic grades. Typical applications include automotive trim and exhaust systems and heat-transfer equipment for the chemical and petrochemical industries.
Martensitic chromium contains from 11.5 to 18% chromium, 0.15% to 1.2% carbon, and up to 2.5% nickel. They are magnetic, can be hardened by heat treatment, and have high strength and moderate toughness in the hardened-and-tempered condition. Forming should be done in the annealed condition. Martensitic stainless steels are less resistant to corrosion than the austenitic or ferritic grades. Two types of martensitic steels, 416 and 420F, have been developed specifically for good machinability.
Martensitic stainless steels are used where strength and/or hardness are of primary concern and where the environment is relatively mild from a corrosive standpoint. These alloys are typically used for bearings, molds, cutlery, medical instruments, aircraft structural parts, and turbine components. Type 420 is used increasingly for molds for plastics and for industrial components requiring hardness and corrosion resistance.
Steels that derive their properties primarily from the presence of some alloying element other than carbon are called alloys or alloy steels. Alloy steels always contain traces of other elements. Among the more common alloying elements are nickel, chromium, vanadium, silicon, and tungsten. One or more of these elements may be added to the steel during the manufacturing process to produce the desired characteristics.
Alloy steels may be produced in structural sections, sheets, plates, and bars for use in the “as-rolled” condition. Better physical properties are obtained with these steels than are possible with hot-rolled carbon steels. These alloys are used in structures where the strength of material is especially important, such as bridge members, railroad cars, dump bodies, dozer blades, and crane booms. The following paragraphs briefly describe some common alloy steels.
These steels contain from 3.5% nickel to 5% nickel. The nickel increases the strength and toughness of these steels. Nickel steel containing more than 5% nickel has an increased resistance to corrosion and scale. Nickel steel is used in the manufacture of aircraft parts, such as propellers and airframe support members.
These steels have chromium added to improve hardening ability, wear resistance, and strength. These steels contain between 0.20% to 0.75% chromium and 0.45% carbon or more. Some of these steels are so highly resistant to wear that they are used for the races and balls in anti-friction bearings. Chromium steels are highly resistant to corrosion and scale.
This steel has the maximum amount of strength with the least amount of weight. Steels of this type contain from 0.15% to 0.25% vanadium, 0.6% to 1.5% chromium, and 0.1% to 0.6% carbon. Common uses are for crankshafts, gears, axles, and other items that require high strength. This steel is used also in the manufacture of high quality hand tools, such as wrenches and sockets.
This is a special alloy that has the property of red hardness, that is, the ability to continue to cut after it becomes red-hot. A good grade of this steel contains from 13% to 19% tungsten, 1% to 2% vanadium, 3% to 5% chromium, and 0.6% to 0.8% carbon. Because this alloy is expensive to produce, its use is largely restricted to the manufacture of drills, lathe tools, milling cutters, and similar cutting tools.
This is often used as an alloying agent for steel in combination with chromium and nickel. The molybdenum adds toughness to the steel. It can be used in place of tungsten to make the cheaper grades of high-speed steel and in carbon molybdenum high-pressure tubing.
The amount of manganese used depends upon the properties desired in the finished product. Small amounts of manganese produce strong, free-machining steels. Larger amounts (between 2% and 10%) produce a somewhat brittle steel, while still larger amounts (11% to 14%) produce a steel that is tough and very resistant to wear after proper heat treatment.
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Nonferrous metals contain either no iron or only insignificant amounts used as an alloy. Some of the more common nonferrous metals Steelworkers work with include copper, brass, bronze, copper-nickel alloys, lead, zinc, tin, aluminum, and Duralumin. All nonferrous metals are nonmagnetic.
Copper and its alloys have many desirable properties. Copper is ductile, malleable, hard, tough, strong, wear resistant, machinable, weldable, and corrosion resistant. It also has high-tensile strength, fatigue strength, and thermal and electrical conductivity. Copper is one of the easier metals to work with, but you must be careful because it easily becomes work-hardened. However, this condition can be remedied by annealing, that is, heating it to a cherry red, and then letting it cool; this process restores it to a softened condition. Annealing and softening are the only heat-treating procedures that apply to copper. Seams in copper are joined by riveting, silver brazing, bronze brazing, soft soldering, gas welding, or electrical arc welding. Copper is frequently used to give a protective coating to sheets and rods and to make ball floats, containers, and soldering coppers.
Brass is an alloy of copper and zinc, with additional elements such as aluminum, lead, tin, iron, manganese, or phosphorus added to give the alloy specific properties. Rolled brass (Tobin bronze) commonly contains about 60% copper, 39% zinc, and 0.75% tin. This brass is highly corrosion resistant and is practically impurity free.
Brass sheets and strips are available in several grades: soft, 1/4 hard, 1/2 hard, full hard, and spring grades. The process of cold rolling creates hardness. All grades of brass can be softened by annealing at a temperature of 550°F to 600°F, then allowing it to cool by itself without quenching, but be careful not to overheat; overheating can destroy the zinc in the alloy.
Bronze is a combination of 84% copper and 16% tin, and was the best metal available before steel-making techniques were developed. Many complex bronze alloys are now available, containing such elements as zinc, lead, iron, aluminum, silicon, and phosphorus, so today, the name bronze is applied to any copper-based alloy that looks like bronze. In many cases, there is no real distinction between the composition of bronze and that of brass.
Nickel is used in these alloys to make them strong, tough, and resistant to wear and corrosion. Because of their high resistance to corrosion, copper-nickel alloys, containing 70% copper and 30% nickel or 90% copper and 10% nickel, are used for saltwater piping systems. Small storage tanks and hot water reservoirs are constructed of a copper-nickel alloy available in sheet form. Copper-nickel alloys should be joined by metal-arc welding or by brazing.
Lead is a heavy metal that weighs about 710 pounds per cubic foot. In spite of its weight, lead is soft, malleable, and available in pig and sheet form (in rolls). Lead’s surface is grayish, but after scratching or scraping it, you can see that the actual color of the metal is white. Because it is soft, lead is used as backing material when punching holes with a hollow punch or when forming shapes by hammering copper sheets. Sheet lead is also used to line sinks or protect bench tops where a large amount of acid is used. Lead-lined pipes are used in systems that carry corrosive chemicals. Frequently, lead is used in alloyed form to increase its low-tensile strength. Alloyed with tin, lead produces a soft solder; when added to metal alloys, lead improves their machinability.
When working with lead, you must take proper precautions because the dust, fumes, or vapors from it are highly poisonous.
You often see zinc used on iron or steel in the form of a protective coating called galvanizing. Zinc is also used in soldering fluxes and die-castings, and as an alloy in making brass and bronze.
Tin has many important uses as an alloy. It can be alloyed with lead to produce softer solders and with copper to produce bronze. Tin-based alloys have a high resistance to corrosion, low-fatigue strength, and a compressive strength that accommodates light or medium loads. Tin, like lead, has a good resistance to corrosion and has the added advantage of not being poisonous; however, it has a tendency to decompose when subjected to extremely low temperatures.
Aluminum is easy to work with and has a good appearance. It is light in weight with a high strength per unit weight. A disadvantage is that its tensile strength is only one third of iron’s and one fifth of annealed mild steel’s. Aluminum alloys usually contain at least 90% aluminum, while the addition of silicon, magnesium, copper, nickel, or manganese can raise the strength of the alloy to that of mild steel. In its pure state, aluminum is soft, with a strong affinity for gases. Alloying elements are used to overcome these disadvantages, but the alloys, unlike the pure aluminum, corrode unless given a protective coating. Threaded parts made of aluminum alloy should be coated with an anti-seize compound to prevent sticking caused by corrosion.
Developed in 1903, Duralumin is one of the first of the strong structural aluminum alloys; it was used in zeppelins, including the Hindenburg. Over the past hundred years, with the development of a variety of different wrought-aluminum alloys, a numbering system was adopted, with digits indicating the major alloying element and the coldworked or heat-treated condition of the metal. Today, the name Duralumin is rarely used, and it is now classified in the metal working industries as 2017-T4; the T4 indicates heat treated.
This is a protective covering consisting of a thin sheet of pure aluminum rolled onto the surface of an aluminum alloy during manufacture. Zinc chromate is a protective covering that can be applied to an aluminum surface as needed, or used as a primer on steel surfaces for a protective coating
Monel is an alloy in which nickel is the major element. It contains from 64% to 68% nickel, about 30% copper, and small percentages of iron, manganese, and cobalt. Monel is harder and stronger than either nickel or copper, and has high ductility. It resembles stainless steel in appearance and has many of its qualities. The strength combined with a high resistance to corrosion makes Monel an acceptable substitute for steel in systems where corrosion resistance is the primary concern. Nuts, bolts, screws, and various fittings are made of Monel. This alloy can be forged, welded, and worked cold. If worked in the temperature range between 1200°F and 1600°F, it becomes “hot short” or brittle.
K-monel is a special type of alloy developed for greater strength and hardness than Monel. In strength, it is comparable to heat-treated steel, and is used for instrument parts that must resist corrosion.
A high-nickel alloy often used in the exhaust systems of aircraft engines, Inconel is composed of 78.5% nickel, 14% chromium, 6.5% iron, and 1% of other elements. It offers good resistance to corrosion and retains its strength at high operating temperatures.
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Many methods are used to identify a piece of metal. Identification is necessary when selecting a metal for use in fabrication or in determining its weldability. Some common methods used for field identification are surface appearance, spark test, chip test, and use of a magnet.
It is possible to identify several metals by their surface appearance. Although examination of the surface does not usually give you enough information to classify the metal exactly, it will often give you enough information to allow you to identify the group to which the metal belongs. Even this much identification is helpful since it will limit the number of tests required for further identification. In trying to identify a piece of metal by its surface appearance, consider both the color and the texture of the surface. Table 4 indicates the surface colors of some of the more common metals.
Table 4 — Surface Colors of Some Common Metals.
|Metals||Color of unfinished, unbroken surface||Color and structure of newly fractured surface||Color of freshly filed surface|
|White cast iron||Dull gray||Silver white; crystalline||Silvery white|
|Gray cast iron||Dull gray||Dark gray; crystalline||Light silvery gray|
|Malleable iron||Dull gray||Dark gray; finely crystalline||Light silvery gray|
|Wrought iron||Light gray||Bright gray||Light silvery gray|
|Low-carbon and cast steel||Dark gray||Light gray||Bright silvery gray|
|Stainless steel||Dark gray||Medium gray||Bright silvery gray|
|Copper||Reddish brown to green||Bright red||Bright copper color|
|Brass and bronze||Reddish yellow, yellow-green, or brown||Red to yellow||Reddish yellow to yellowish white|
|Aluminum||Light gray||White; finely crystalline||White|
|Monel metal||Dark gray||Light gray||Light gray|
|Nickel||Dark gray||Off-white||Bright silvery white|
|Lead||White to gray||Light gray; crystalline||White|
Referring to the table, you can see that the outside appearance of a metal helps to identify and classify metal, while newly fractured or freshly filed surfaces offer additional clues.
When the surface appearance of a metal does not give enough information to positively identify it, other identification tests become necessary. Some of these tests are complicated and require equipment Seabees do not usually have; however, some tests are fairly simple and reliable when done by a skilled person. Three of these tests are the spark test, chip test, and magnetic test.
The spark test is a method of classifying steels and iron according to their composition by observing the sparks formed when the metal is held against a high-speed grinding wheel. This test does not replace chemical analysis, but it is a very convenient and fast method of sorting mixed steels whose spark characteristics are known.
When held lightly against a grinding wheel, the different kinds of iron and steel produce sparks that vary in length, shape, and color. The grinding wheel should be run to give a surface speed of at least 5000 ft (1525 m) per minute to get a good spark stream. Grinding wheels should be hard enough to wear for a reasonable length of time, yet soft enough to keep a free-cutting edge. Spark testing should be done in subdued light since the color of the spark is important. In all cases, it is best to use standard known metal samples to compare their sparks with that of the unknown test sample.
The spark resulting from the test should be directed downward and studied. The color, shape, length, and activity of the sparks relate to characteristics of the material being tested (Figure 1-3).
Figure 1-3 — Examples of spark-testing terms.
Spark testing is not of much use on nonferrous metals such as coppers, aluminums, and nickel-base alloys since they do not exhibit spark streams of any significance. However, this is one way to separate ferrous and nonferrous metals.
Low Carbon Steel
High Carbon Steel
Gray Cast Iron
Monel and Nickel
• Spark stream next to the wheel is straw colored.
• Spark stream at the end is white.
• Spark stream is about 50 inches long.
• Volume is moderate with few sparklers.
• Sparklers are forked.
Figure 1-4 — Spark patterns formed by common metals.
The chip test or chisel test may also be used to identify metals. The only tools required are a hammer and a cold chisel. Use the cold chisel to hammer on the edge or corner of the material being examined.
The ease of producing a chip is the indication of the hardness of the metal. If the chip is continuous, it is indicative of a ductile metal, whereas if chips break apart, it indicates a brittle material. On such materials as aluminum, mild steel, and malleable iron, the chips are continuous. They are easily chipped and the chips do not tend to break apart. The chips for gray cast iron are so brittle that they become small, broken fragments. On high-carbon steel, the chips are hard to obtain because of the hardness of the material, but can be continuous. Information given in Table 1-5 can help you identify various metals by the chip test.
Table 1-5 — Metal Identification by Chip Test.
|White cast iron||Chips are small brittle fragments. Chipped surfaces are not smooth.|
|Gray cast iron||Chips are about 1/8 inch in length. Metal not easily chipped; chips break off and prevent smooth cut.|
|Malleable iron||Chips vary from 1/4 to 3/8 inch in length. Metal is tough and hard to chip.|
|Wrought iron||Chips have smooth edges. Metal is easily cut or chipped, and a chip can be made as a continuous strip.|
|Low-carbon and cast steel||Chips have smooth edges. Metal is easily cut or chipped, and a chip can be taken off as a continuous strip.|
|High-carbon steel||Chips show a fine grain structure. Edges of chips are lighter in color than chips of low-carbon steel. Metal is hard, but can be chipped in a continuous strip.|
|Copper||Chips are smooth, with sawtooth edges where cut. Metal is easily cut as a continuous strip. l|
|Brass and bronze||Chips are smooth, with sawtooth edges. These metals are easily cut, but chips are more brittle than chips of copper. Continuous strip is not easily cut.|
|Aluminum and aluminum alloys||Chips are smooth, with sawtooth edges. A chip can be cut as a continuous strip.|
|Monel||Chips have smooth edges. Continuous strip can be cut. Metal chips easily.|
|Nickel||Chips have smooth edges. Continuous strip can be cut. Metal chips easily|
|Lead||Chips of any shape may be obtained.|
The magnetic test can be quickly performed using a small pocket magnet. With experience, it is possible to judge a strongly magnetic material from a slightly magnetic material. The nonmagnetic materials are easily recognized. Strongly magnetic materials include the carbon and low-alloy steels, iron alloys, pure nickel, and martensitic stainless steels. A slightly magnetic reaction is obtained from Monel and high nickel alloys and the stainless steel of the 18 chrome-8 nickel type when cold worked, such as in a seamless tube. Nonmagnetic materials include copper-base alloys, aluminum-base alloys, zinc-base alloys, annealed 18 chrome-8 nickel stainless, magnesium, and the precious metals.
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This manual discussed how to identify the various metals and their properties. You also learned how to describe corrosion resistance and identify different types of ferrous and nonferrous metals and alloys, and how to use simple tests to help identify common metals. As always, use the manufacturers’ operator manuals for the specific setup and safety procedures of the equipment you will be using, and wear the proper personal protective equipment.
1. Steelworkers work primarily with iron and steel.
2. Which symbol is NOT a chemical symbol for a metal?
3. An alloy is defined as a substance having metallic properties that is composed of two or more elements.
4. The characteristics of elements and alloys are terms of physical, chemical, electrical, and mechanical properties.
5. Which property is an electrical property of an alloy?
6. Tension stresses are also known as “tensile stresses.”
7. Having the capacity to conduct heat and electricity, to be lustrous, and to be deformed or permanently shaped at room temperature are properties of which substance?
8. Which elements sometimes behave like metals and at other times like nonmetals?
9. Which property is NOT a mechanical property of a metal alloy?
10. Within a column that is supporting a roof beam, internal stresses develop. This condition is referred to by what term?
11. Tensile stresses are developed when a material is subjected to what type of force?
12. Carbon steel has an ultimate tension and compression strength of what maximum psi?
13. What term is used to describe the tendency of a metal to fail after repeated stressing at the same point?
14. What term is used to describe the mechanical property of a metal that allows it to be drawn out into a thin wire?
15. What characteristic is responsible for the limited use of pig iron?
16. When cast iron is alloyed with nickel, chromium, molybdenum, silicon, or vanadium, which characteristic is enhanced?
17. What process is used to produce malleability in cast iron?
18. What group of steel is best suited for the manufacture of crane hooks and axles?
19. Steel containing 10.5% to 27% chromium, .08% to .20% carbon, and no nickel is in what group and series of stainless steel?
20. For what purpose is nickel added to low-alloy nickel steel?
21. Which metal is nonferrous?
22. What element or base metal is alloyed with copper to produce bronze and is alloyed with lead to produce soft solders?
23. When used in conjunction with a numbering system that classifies different aluminum alloys, the letter “T” signifies that what action has occurred?
24. What alloy contains 64% to 68% nickel, about 30% copper, and small percentages of iron, manganese, and cobalt?
25. When applying the spark test to a metal, you notice the spark stream has shafts and forks only. What does this condition indicate about the metal under test?
26. What metal produces a spark stream about 25 inches long with small and repeating sparklers of small volume that are initially red in color?
27. Which metal produces the shortest length spark stream?
28. On which metal does a chip test produce chips that have smooth surfaces and sawtooth edges?
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|Much of the content of this manual is adapted from handbooks prepared by the U.S. Department of Defense. Occasional references of military specifications and protocols do not affect the principles of similar civilian work.||
Copyright © David L.