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CORROSION RESISTANCE Corrosion resistance, although not a mechanical property, is important in the discussion of metals. Corrosion resistance is the property of a metal that gives it the ability to withstand attacks from atmospheric, chemical, or electrochemical conditions. Corrosion, sometimes called oxidation, is illustrated by the rusting of iron. Table 1-2 lists four mechanical properties and the corrosion resistance of various metals or alloys. The first metal or alloy in each column exhibits the best characteristics of that property. The last metal or alloy in each column exhibits the least. In the column labeled "Toughness," note that iron is not as tough as copper or nickel; however, it is tougher than magnesium, zinc, and aluminum. In the column labeled "Ductility," iron exhibits a reasonable amount of ductility; however, in the columns labeled "Malleability" and "Brittleness," it is last. METAL TYPES The metals that Steelworkers work with are divided into two general classifications: ferrous and nonferrous. Ferrous metals are those composed primarily of iron and iron alloys. Nonferrous metals are those composed primarily of some element or elements other than iron. Nonferrous metals or alloys sometimes contain a small amount of iron as an alloying element or as an impurity. FERROUS METALS Ferrous metals include all forms of iron and steel alloys. A few examples include wrought iron, cast iron, carbon steels, alloy steels, and tool steels. Ferrous metals are iron-base alloys with small percentages of carbon and other elements added to achieve desirable properties. Normally, ferrous metals are magnetic and nonferrous metals are nonmagnetic. Iron Pure iron rarely exists outside of the laboratory. Iron is produced by reducing iron ore to pig iron through the use of 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.- Pig iron is composed of about 93% iron, from 3% to 5% carbon, and various amounts of other elements. Pig iron is comparatively weak and brittle; therefore, it has a limited use and approximately ninety percent produced is refined to produce steel. Cast-iron pipe and some fittings and valves are manufactured from pig iron. WROUGHT IRON.- Wrought iron is made from pig iron with some slag mixed in during manufacture. Almost pure iron, the presence of slag enables wrought iron to resist corrosion and oxidation. 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.- Cast iron is any iron containing greater than 2% carbon alloy. 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.- Ingot iron is a commercially pure iron (99.85% iron) that is easily formed and possesses good ductility and corrosion resistance. The chemical analysis and properties of this iron and the lowest carbon steel are practically the same. The lowest carbon steel, known as dead-soft, has about 0.06% more carbon than ingot iron. In iron the carbon content is considered an impurity and in steel it is considered an alloying element. The primary use for ingot iron is for galvanized and enameled sheet. Steel Of all the different metals and materials that we use in our trade, steel is by far the most important. When steel was developed, it revolutionized the American iron industry. With it came skyscrapers, stronger and longer bridges, and railroad tracks that did not collapse. Steel is manufactured from pig iron by decreasing the amount of carbon and other impurities and adding specific amounts of alloying elements. Do not confuse steel with the two general classes of iron: cast iron (greater than 2% carbon) and pure iron (less than 0.15% carbon). In steel manufacturing, controlled amounts of alloying elements are added during the molten stage to produce the desired composition. The composition of a steel is determined by its application and the specifications that were developed by the following: American Society for Testing and Materials (ASTM), the American Society of Mechanical Engineers (ASME), the Society of Automotive Engineers (SAE), and the American Iron and Steel Institute (AISI). Carbon steel is a term applied to a broad range of steel that falls between the commercially pure ingot iron and the cast irons. This range of carbon steel may be classified into four groups: Low-Carbon Steel ........ 0.05% to 0.30% carbon LOW-CARBON STEEL.- Steel in this classification is tough and ductile, easily machined, formed, and welded. It does not respond to any form of heat treating, except case hardening. MEDIUM-CARBON STEEL.- These steels are strong and hard but cannot be welded or worked as easily as the low-carbon steels. They are used for crane hooks, axles, shafts, setscrews, and so on. HIGH-CARBON STEEL/VERY HIGH-CARBON STEEL.- Steel in these classes respond well to heat treatment and can be welded. When welding, special electrodes must be used along with preheating and stress-relieving procedures to prevent cracks in the weld areas. These steels are used for dies, cutting tools, mill tools, railroad car wheels, chisels, knives, and so on. LOW-ALLOY, HIGH-STRENGTH, TEMPERED STRUCTURAL STEEL.- A special lowcarbon steel, containing specific small amounts of alloying elements, that is quenched and tempered to get a yield strength of 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 abrasionresistant. High-strength steels are covered by ASTM specifications. NOTE: 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. STAINLESS STEEL.- This type of steel is classified by the American Iron and Steel Institute (AISI) into two general series named the 200-300 series and 400 series. Each series includes several types of steel with different characteristics. The 200-300 series of stainless steel is known as AUSTENITIC. This type of steel is very tough and ductile in the as-welded condition; therefore, it is ideal for welding and requires no annealing under normal atmospheric conditions. 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.-This type of steel contains 12% to 27% chromium and 0.08% to 0.20% carbon. These alloys are the straight chromium grades of stainless steel since they contain no nickel. They are nonhardenable by heat treatment and are normally used in the annealed or soft condition. Ferritic steels are magnetic and frequently used for decorative trim and equipment subjected to high pressures and temperatures. Martensitic Chromium.- These steels are magnetic and are readily hardened by heat treatment. They contain 12% to 18% chromium, 0.15% to 1.2% carbon, and up to 2.5% nickel. This group is used where high strength, corrosion resistance, and ductility are required. ALLOY STEELS.- Steels that derive their properties primarily from the presence of some alloying element other than carbon are called ALLOYS or ALLOY STEELS. Note, however, that 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. Bridge members, railroad cars, dump bodies, dozer blades, and crane booms are made from alloy steel. Some of the common alloy steels are briefly described in the paragraphs below. Nickel 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. Chromium Steels.-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 antifriction bearings. Chromium steels are highly resistant to corrosion and to scale. Chrome Vanadium Steel.- 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 also used in the manufacture of high-quality hand tools, such as wrenches and sockets. Tungsten Steel.- This is a special alloy that has the property of red hardness. This 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. Molybdenum.- 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. Manganese Steels.- 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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