Pure iron is too soft to be used for structure, but adding small quantities of other elements (carbon, manganese, or silicon, for instance) greatly increases its mechanical strength. Alloys are usually stronger than pure metals, although they generally offer reduced electrical and thermal conductivity. Strength is the most important criterion by which many structural materials are judged. Therefore, alloys are used for engineering construction. The synergistic effect of alloying elements and heat treatment produces various microstructures and properties.
- Carbon. Carbon is a non-metallic element, an important alloying element in all ferrous metal-based materials. Carbon is always present in metallic alloys, i.e., in all grades of stainless steel and heat-resistant alloys. Carbon is a very strong austenitizing and increases the strength of steel. It is the principal hardening element essential to forming cementite, Fe3C, pearlite, spheroidite, and iron-carbon martensite. Adding a small amount of non-metallic carbon to iron trades its great ductility for greater strength. Suppose it combines chromium as a separate constituent (chromium carbide). In that case, it may have a detrimental effect on corrosion resistance by removing some of the chromium from the solid solution in the alloy and, consequently, reducing the amount of chromium available to ensure corrosion resistance.
- Chromium. Chromium increases hardness, strength, and corrosion resistance. The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The resistance of these metallic alloys to the chemical effects of corrosive agents is based on passivation. For passivation to occur and remain stable, the Fe-Cr alloy must have a minimum chromium content of about 11% by weight, above which passivity can occur and below is impossible. Chromium can be used as a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes to increased strength. The high-speed tool steels contain between 3 and 5% chromium, and it is ordinarily used for applications of this nature in conjunction with molybdenum.
- Nickel. Nickel is one of the most common alloying elements. About 65% of nickel production is used in stainless steel. Because nickel does not form any carbide compounds in steel, it remains in solution in the ferrite, thus strengthening and toughening the ferrite phase. Nickel steels are easily heat treated because nickel lowers the critical cooling rate. Nickel-based alloys (e.g., Fe-Cr-Ni(Mo) alloys) exhibit excellent ductility and toughness, even at high strength levels, and these properties are retained up to low temperatures. Nickel also reduces thermal expansion for better dimensional stability. Nickel is the base element for superalloys, a group of nickel, iron-nickel, and cobalt alloys used in jet engines. These metals have excellent resistance to thermal creep deformation and retain their stiffness, strength, toughness, and dimensional stability at temperatures much higher than the other aerospace structural materials.
- Molybdenum. In small quantities in stainless steel, molybdenum increases hardenability and strength, particularly at high temperatures. The high melting point of molybdenum makes it important for giving strength to steel and other metallic alloys at high temperatures. Molybdenum is unique in the extent to which it increases steel’s high-temperature tensile and creeps strengths. It retards the transformation of austenite to pearlite far more than the transformation of austenite to bainite; thus, bainite may be produced by continuous cooling of molybdenum-containing steels.
- Vanadium. Vanadium is generally added to steel to inhibit grain growth during heat treatment. Controlling grain growth improves the strength and toughness of hardened and tempered steels.
- Tungsten. Produces stable carbides and refines grain size to increase hardness, particularly at high temperatures. Tungsten is used extensively in high-speed tool steels and has been proposed as a substitute for molybdenum in reduced-activation ferritic steels for nuclear applications.