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The Structure of Metals Mechanical Behavior, Testing and Manufacturing Properties of Materials Physical Properties of Materials Metal Alloys: Structure and Strengthening by Heat Treatment Ferrous Metals and Alloys: Production, General Properties and Applications.
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The Structure of Metals • Mechanical Behavior, Testing and Manufacturing Properties of Materials • Physical Properties of Materials • Metal Alloys: Structure and Strengthening by Heat Treatment • Ferrous Metals and Alloys: Production, General Properties and Applications
6. Nonferrous Metals and Alloys: Production, General Properties and Applications 7. Polymers: Structure, General Properties and Applications 8. Ceramics, Graphite and Diamond: Structure, General Properties and Applications 9. Composite Materials: Structure, General Properties, and Applications
Chapter Objectives • Introduction • Production of Iron and Steel • Casting of Ingots • Continuous Casting • Carbon and Alloy Steels • Stainless Steels • Tool and Die Steels
Chapter Outline • Primary production methods for iron and steel. • Types of ferrous alloys, their properties, and applications. • Specific applications for various types of steels and cast irons. • Characteristics of tool and die steels and their selection for specific applications.
5.1 Introduction • By virtue of their wide range of mechanical, physical, and chemical properties, ferrous metals and alloys are among the most useful of all metals. • Ferrous alloys are produced as • Sheet steel for automobiles, appliances, and containers • Plates for boilers, ships, and bridges • Structural members such as I-beams, bar products, axles, crankshafts, and railroad rails • Gears, tools, dies, and molds • Music wire • Fasteners such as bolts, rivets, and nuts
5.2.1 Raw Materials • The three basic materials used in iron- and steelmaking are iron ore, limestone, and coke. • The principal iron ores are taconite (a black flint-like rock), hematite (an iron-oxide mineral), and limonite (an iron oxide containing water). • The concentrated iron ore is referred to as beneficiated (as are other concentrated ores). • The function of limestone (calcium carbonate) is to remove impurities from the molten iron. • The limestone reacts chemically with impurities, acting like a flux (meaning to flow as a fluid) that causes the impurities to melt at a low temperature. • The limestone combines with the impurities and forms a slag.
5.2.2 Iron Making • The three raw materials described previously are carried to the top of a blast furnace. • Fig 5.1 shows the schematic illustration of a blast furnace.
5.2.3 Steelmaking • The steelmakin process is essentially one of refining the pig iron by the reduction of the percentage of manganese, silicon, carbon, and other elements and by controlling of the composition of the output with the addition of various elements. • The molten metal from the blast furnace is transported into one of three types of furnaces: open-hearth, electric, or basic-oxygen.
5.2.3 Steelmaking Electric Furnace • The source of heat in this furnace is a continuous electric arc that is formed between the electrodes and the charged metal. • Fig 5.2 shows the schematic illustration of types of electric furnaces: (a) direct arc, (b) indirect arc, and (c) induction. • For smaller quantities, electric furnaces can be of the induction type.
5.2.3 Steelmaking Basic-oxygen Furnace • The basic-oxygen furnace (BOF) is the fastest steelmaking process. • Fig 5.3 shows the schematic illustrations showing charging, melting, and pouring of molten iron in a basic-oxygen process.
5.2.3 Steelmaking Vacuum Furnace • Steel also may be melted in induction furnaces from which the air has been removed.
5.3 Casting Ingots • Traditionally, the next step in the steelmaking process is the shaping of the molten steel into a solid form (ingot) for such further processing as rolling it into shapes, casting it into semifinished forms, or forging it. • This process now is replaced largely by continuous casting, which improves efficiency by eliminating the need for ingots. • The cooled ingots are removed (stripped) from the molds and lowered into soaking pits, where they are reheated to a uniform temperature of about 1200°C for subsequent processing by rolling.
5.3 Casting Ingots • Depending on the amount of gas evolved during solidification, three types of steel ingots can be produced: killed, semi-killed, and rimmed. 1. Killed steel. Killed steel is a fully deoxidized steel; that is, oxygen is removed and porosity is thus eliminated. 2. Semi-killed steel. Semi-killed steel is a partially deoxidized steel. • 3. Rimmed steel. In a rimmed steel, which generally has a low carbon content, the evolved gases are killed partially by the addition of other elements, such as aluminum. The gases produce blowholes along the outer rim of the ingot—hence the term rimmed.
5.3 Casting Ingots • Depending on the amount of gas evolved during solidification, three types of steel ingots can be produced: killed, semi-killed, and rimmed. 1. Killed steel. Killed steel is a fully deoxidized steel; that is, oxygen is removed and porosity is thus eliminated. 2. Semi-killed steel. Semi-killed steel is a partially deoxidized steel. 3. Rimmed steel. In a rimmed steel, which generally has a low carbon content, the evolved gases are killed partially by the addition of other elements, such as aluminum. The gases produce blowholes along the outer rim of the ingot—hence the term rimmed.
5.3 Casting Ingots Refining • The properties and manufacturing characteristics of ferrous alloys are affected adversely by the amount of impurities, inclusions, and other elements present. • The removal of impurities is known as refining.
5.4 Continuous Casting • The inefficiencies and the problems involved in making steels traditionally in ingots are alleviated by the continuous-casting processes, which produce higher-quality steels at reduced costs. • Fig 4.5 (a) shows the continuous-casting process for steel. Typically, the solidified metal descends at a speed of 25 mm/s. • Note that the platform is about 20 m above ground level. • Fig 4.5 (b) Continuous strip casting of nonferrous metal strip.
5.5 Carbon and Alloy Steels • Carbon and alloy steels are among the most commonly used metals and have a wide variety of applications shown in Table 5.1.
5.5.1 Effects of Various Elements in Steels • Various elements are added to steels in order to impart properties such as hardenability, strength, hardness, toughness, wear resistance, workability, weldability, and machinability.
5.5.2 Residual Elements in Steels • During steel production, refining, and processing, some residual elements (trace elements) may still remain. • The following generally are considered unwanted residual elements: Antimony and arsenic cause temper embrittlement. Hydrogen severely embrittles steels; however, heating during processing drives out most of the hydrogen. Nitrogen improves strength, hardness, and machinability; in aluminum-deoxidized steels, it controls the size of inclusions, improves strength and toughness, and decreases ductility and toughness. Oxygen slightly increases the strength of rimmed steels; it severely reduces toughness. Tin causes hot shortness and temper embrittlement.
5.5.3 Designations for Steels • The present numbering system is known as the Unified Numbering System (UNS) and has been adopted widely by ferrous and nonferrous industries. • Typical letter designations are: G—for AISI and SAE carbon and alloy steels J—for cast steels K—for miscellaneous steels and ferrous alloys S—for stainless steels and superalloys T—for tool steels
5.5.4 Carbon Steels • Carbon steels generally are classified by their proportion (by weight) of carbon content. • The general mechanical properties of carbon and alloy steels are shown in Table 5.2.
5.5.4 Carbon Steels • Low-carbon steel, also called mild steel, has less than 0.30% C. It often is used for common industrial products (such as bolts, nuts, sheet, plate, and tubes) and for machine components that do not require high strength. • Medium-carbon steel has 0.30 to 0.60% C. It generally is used in applications requiring higher strength than is available in low-carbon steels, such as in machinery, automotive and agricultural equipment parts (gears, axles, connecting rods, crankshafts), railroad equipment, and parts for metalworking machinery.
5.5.4 Carbon Steels • High-carbon steel has more than 0.60% C. Generally, high-carbon steel is used for parts requiring strength, hardness, and wear resistance, such as cutting tools, cable, music wire, springs, and cutlery. After being manufactured into shapes, the parts usually are heat treated and tempered. The higher the carbon content of the steel, the higher is its hardness, strength, and wear resistance after heat treatment. • Carbon steels containing sulfur and phosphorus are known as resulfurized carbon steels (11xx series) and rephosphorized and resulfurized carbon steels (12xx series).
5.5.5 Alloy Steels • Steels containing significant amounts of alloying elements are called alloy steels; they usually are made with more care than are carbon steels. • Structural-grade alloy steels are used mainly in the construction and transportation industries because of their high strength.
5.5.6 High-Strength Low-Alloy Steels • In order to improve the strength-to-weight ratio of steels, a number of high-strength, low-alloy steels (HSLA) have been developed. Designations • Three categories compose the system of AISI designations for high-strength sheet steel (Table 5.3).
5.5.6 High-Strength Low-Alloy Steels Microalloyed Steels • These recently developed HSLA steels provide superior properties and can eliminate the need for heat treatment. • They have a ferrite-pearlite microstructure with fine dispersed particles of carbonitride.
5.5.6 High-Strength Low-Alloy Steels Nanoalloyed Steels • Now under development, these steels have extremely small grain sizes (10–100 nm), and are produced using metallic glasses.
5.5.7 Dual-Phase Steels • Dual-phase steels, designated with the letter “D” in Table 5.3, are processed specially to have a mixed ferrite and martensite structure.
5.6 Stainless Steel • Stainless steels are characterized primarily by their corrosion resistance, high strength and ductility, and high chromium content. • They are called stainless because, in the presence of oxygen (air), they develop a thin, hard, adherent film of chromium oxide that protects the metal from corrosion. • Stainless steels generally are divided into five types (see Table 5.4).
5.6 Stainless Steel Austenitic (200 and 300 series) • These steels generally are composed of chromium, nickel, and manganese in iron. • They are nonmagnetic and have excellent corrosion resistance, but they are susceptible to stress-corrosion cracking.
5.6 Stainless Steel Ferritic (400 series) • These steels have a high chromium content—up to 27%. • They are magnetic and have good corrosion resistance, but they have lower ductility than austenitic stainless steels. • They generally are used for nonstructural applications, such as kitchen equipment and automotive trim.
5.6 Stainless Steel Martensitic (400 and 500 series) • Most martensitic stainless steels do not contain nickel and are hardenable by heat treatment. • Their chromium content may be as much as 18%. These steels are magnetic, and they have high strength, hardness, and fatigue resistance, good ductility, and moderate corrosion resistance. • Martensitic stainless steels typically are used for cutlery, surgical tools, instruments, valves, and springs.
5.6 Stainless Steel Precipitation-hardening (PH) • These steels contain chromium and nickel along with copper, aluminum, titanium, or molybdenum. • They have good corrosion resistance and ductility, and they have high strength at elevated temperatures. • Their main application is in aircraft and aerospace structural components.
5.6 Stainless Steel Duplex structure • These steels have a mixture of austenite and ferrite. • They have good strength and have higher resistance to both corrosion (in most environments) and stress–corrosion cracking than do the 300 series of austenitic steels. • Typical applications are in water-treatment plants and in heat-exchanger components.
Example 5.1 Use of Stainless Steels in Automobiles The types of stainless steel usually selected by materials engineers for use in automobile parts are 301, 409, 430, and 434. Because of its good corrosion resistance and mechanical properties, type 301 is used for wheel covers. Cold working during the forming process increases its yield strength (by means of strain hardening) and gives the wheel cover a spring-like action.
Example 5.1 Use of Stainless Steels in Automobiles Type 409 is used extensively for catalytic converters. Type 430 had been used for automotive trim, but it is not as resistant as type 434 is to the de-icing salts used in colder climates in winter. As a result, its use is now limited. In addition to being more corrosion resistant, type 434 closely resembles the color of chromium plating, so it has become an attractive alternative to 430. Stainless steels are well-suited for use in other automobile components as well: exhaust manifolds (replacing cast-iron manifolds to reduce weight, increase durability, provide higher thermal conductivity and reduce emissions), mufflers and tailpipes (to offer better corrosion protection in harsh environments), and brake tubing.
5.7 Tool and Die Steels • Tool and die steels are specially alloyed steels (Tables 5.5 and 5.6) designed for high strength, impact toughness, and wear resistance at room and elevated temperatures. • They commonly are used in the forming and machining of metals.
5.7.1 High-Speed Steels • High-speed steels (HSS) are the most highly alloyed tool and die steels. They maintain their hardness and strength at elevated operating temperatures. • There are two basic types of high-speed steels: the molybdenum type (M-series) and the tungsten type (T-series). • The M-series steels contain up to about 10% molybdenum with chromium, vanadium, tungsten, and cobalt as other alloying elements. • The T-series steels contain 12 to 18% tungsten with chromium, vanadium, and cobalt as other alloying elements.
5.7.2 Die Steels • Hot-work steels (H-series) are designed for use at elevated temperatures. • They have high toughness as well as high resistance to wear and cracking. • Cold-work steels (A-, D-, and O-series) are used for cold-working operations. • Shock-resisting steels (S-series) are designed for impact toughness and are used in applications such as header dies, punches, and chisels. • Various tool and die materials for a variety of manufacturing applications are presented in Table 5.7.
Concept Summary • The major categories of ferrous metals and alloys are carbon steels, alloy steels, stainless steels, and tool and die steels. Their wide range of properties and general low cost have made them among the most useful of all metallic materials. • Steelmaking processes have been improved upon continuously, notably by the continuous-casting and secondary-refining techniques. These advances have resulted in higher-quality steels and in higher efficiency and productivity.
Concept Summary • Alloying elements greatly influence mechanical, physical, chemical, and manufacturing properties (hardenability, castability, formability, machinability, and weldability) and performance in service. • Carbon steels generally are classified as low-carbon (mild steel), medium-carbon, and high-carbon steels. Alloy steels contain a variety of alloying elements, particularly chromium, nickel, and molybdenum. Stainless steels generally are classified as austenitic, ferritic, martensitic, and precipitation-hardening.
Concept Summary • Tool and die steels are among the most important materials and are used widely in casting, forming, and machining operations for both metallic and nonmetallic materials. They generally consist of high-speed steels (molybdenum and tungsten types), hot- and cold-work steels, and shock-resisting steels.
