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Alloying of Steel

Alloying of Steel

Alloying of steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low-alloy steels and high-alloy steels. The difference between the two is somewhat arbitrary: Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. Most commonly, the phrase "alloy steel" refers to low-alloy steels.

properties is a mature science. The following summarizes the known effects of adding certain elements to steel:

  • Carbon: In general, increasing the carbon content of steel alloys produces higher ultimate strength and hardness but may lower ductility and toughness. Carbon also increases air-hardening tendencies and weld hardness. In low-alloy steel for high-temperature applications, the carbon content is usually restricted to a maximum of about 0.15 percent in order to assure optimum ductility for welding, expanding, and bending operations. An increasing carbon content lessens the thermal and electrical conductivities of steel.

  • Phosphorus: High phosphorus content has an undesirable effect on the properties of carbon steel, notably on shock resistance and ductility (see the sectionon temper embrittlement). Phosphorus is effective, however, in improving machineability. In steels, it is normally controlled to less than 0.04 weight percent.

  • Silicon: Used as a deoxidizing agent, silicon increases the tensile strength of steel without increasing brittleness when limited to less than about 2 percent.Silicon increases resistance to oxidation, increases electrical resistivity, and decreases hysteresis losses. Thus it is used for electrical applications. Adding siliconmay reduce creep rupture strength.

  • Manganese: Manganese is normally present in all commercial steels. The manganese combines with sulfur, thus improving hot-working characteristics. In alloysteels, manganese decreases the critical cooling rate to cause a hardened ormartensitic structure and thus contributes to deep-hardening.

  • Nickel: As an alloying element in alloy steels, nickel is a ferrite strengthener and toughener and is soluble in all proportions. Nickel steels are easily hardened because nickel lowers the critical cooling rate necessary to produce hardening on quenching. In heat-treated steel, nickel increases the strength and toughness. In combination with chromium, nickel produces alloy steels possessing higher impact and fatigue resistance than can be obtained with straight carbon steels.

  • Chromium: As an alloying element in steel, chromium is miscible in iron as a solid solution, and forms a complex series of carbide compounds. Chromium is essentially a hardening element and is frequently used with a toughening element such as nickel to produce superior mechanical properties. At higher temperatures, chromium contributes increased strength and is ordinarily used in conjunction with molydenum. Additions of chromium significantly improve the elevated temperature oxidation resistance of steels.

  • Molybdenum: In steel, molybdenum can form a solid solution with the iron and, depending on the molybdenum and carbon content, can also form a carbide. A deeper hardening steel results. The molybdenum carbide is very stable and is responsible for matrix strengthening in long-term creep service.

  • Vanadium: This element is one of the strong carbide formers. It dissolves to some degree in ferrite, imparting strength and toughness. Vanadium steels show a much finer grain structure than steels of a similar composition without vanadium.

  • Boron: Boron is usually added to steel to improve hardenability; that is, to increase the depth of hardening during quenching.

  • Aluminum: Aluminum is widely used as a deoxidizer in molten steel and for controlling grain size. When added to steel in controlled amounts, it produces a fine grain size.

  • Sulfur: Present to some degree in all steel (less than 0.04 weight percent), sulfur forms a nonmetallic inpurity that, in large amounts, results in cracking during forming at high temperatures (hot shortness). Combining it with managanese forms a MnS compound that is relatively harmless.

  • Copper: Copper dissolves in steel and strengthens the iron as a substitutional element. The use of copper in certain alloys increases resistance to atmospheric corrosion and increases yield strength. However, excessive amounts of copper (usually above 0.3 percent are harmful to elevated temperature performance since the lower melting point element segregates to grain boundaries and locally melts (liquates), causing intergranular separation under applied stress.

In general, when used in combination, alloying elements may complement each other and give greater overall benefits than when used singly in much larger quantities. #Little_PEng


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