1.2 Ferrous Metals and Alloys

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 metals and alloys contain iron as their base metal; the general categories are cast irons, carbon and alloy steels, stainless steels, tool and die steels.

The term cast iron refers to a family of ferrous alloys composed of iron, carbon (ranging from 2.11% to about 4.5%), and silicon (up to about 3.5%). Cast irons are usually classified as follows:

1. Gray cast iron, or gray iron;

2. Ductile cast iron, nodular cast iron, or spherical graphite cast iron;

3. White cast iron;

4. Malleable iron;

5. Compacted graphite iron.

The equilibrium phase diagram relevant to cast irons is shown in Fig.1.1, in which the right boundary is 100% carbon, that is, pure graphite. The eutectic temperature is 1154 °C (2109 °F), and so cast irons are completely liquid at temperatures lower than those required for liquid steels. Consequently, iron with high carbon content can be cast at lower temperatures than can steels.

Carbon steels are generally classified by their proportion (by weight) of carbon content.

1. Low-carbon steel, also called mild steel, has less than 0.30% carbon. It is generally used for common industrial products, such as bolts, nuts, sheet, plate, and tubes, and for machine components that do not require high strength.

2. Medium-carbon steel has 0.30% to 0.60% carbon. It is generally used in applications requiring higher strength than is available in low-carbon steels, such as in machinery, in automotive and agricultural equipment parts (gears, axles, connecting rods, crankshafts), in railroad equipment, and in parts for metalworking machinery.

3. High-carbon steel has more than 0.60% carbon. It is generally used for parts requiring strength, hardness, and wear resistance, such as cutting tools, cable, music string, springs, and cutlery. After being manufactured into shapes, the parts are usually heat treated and tempered. The higher the carbon content of the steel, the higher is its hardness, strength, and wear resistance after heat treatment.

Alloy steels contain significant amounts of alloying elements. Structural-grade alloy steels, as identified by ASTM specifications, are used mainly in the construction and transportation industries, because of their high strength. Other alloy steels are used in applications where strength, hardness, creep and fatigue resistance, and toughness are required. These steels may also have been heat treated, in order to obtain the desired properties.

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 (passivation). This protective film builds up again in the event that the surface is scratched.[6] For passivation to occur, the minimum chromium content should be 10% to 12% by weight. In addition to chromium, other alloying elements in stainless steels typically are nickel, molybdenum, copper, titanium, silicon, manganese, columbium, aluminum, nitrogen, and sulfur.

Tool and die steels are specially alloyed steels. They are designed for high strength, impact toughness, and wear resistance at room and elevated temperatures. They are commonly used in forming and machining of metals.

High-speed steels (HSS) are the most highly alloyed tool and die steels. First developed in the early 1900s, 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).

Hot-work steels are designed for use at elevated temperatures. They have high toughness as well as high resistance to wear and cracking. The alloying elements are generally tungsten, molybdenum, chromium, and vanadium.

Cold-work steels are used for cold-working operations. They generally have high resistance to wear and cracking. These steels are available as oil-hardening or air-hardening types.

Shock-resisting steels are designed for impact toughness and are used in applications such as header dies, punches, and chisels. Other properties of these steels depend on the particular composition.