Alloying element have the greatest influence on the steel. For example, chromium (Cr) makes steel hard whereas nickel (Ni) and manganese (Mn) make it tough. Alloying elements are added to the steel so that the steel satisfies various requirements, such as:

• Environmental: corrosion resistance, and nonrusting and nonscaling qualities
• Mechanical: hardness, strength, and toughness
• Thermal: strength and durability of a metal at either subzero or extremely high temperatures

Steel can be classified into two major groups: carbon steels and alloy steels. An alloy steel is steel which derives its principal properties from chemical elements other than carbon.

Since metallurgy became a science, more than one thousand alloys have been created and used in alloying steels. Chromium and nickel are the elements used most often, although copper, manganese, molybdenum, silicon, titanium, tungsten, and vanadium are also used, but in lesser amounts.

In the late 1960s, an entirely new group of front base, superalloys was developed to cope with some of the space age problems.

A property of great importance is the ability of alloying elements to promote the formation of a certain phase or to stabilize it. There are three main functions of alloying elements [29]:

(1) To substitute for iron atoms in solid solutions, or in cementite, to increase strength, hardness and toughness. Alloying elements may also restrict the growth of the crystals or grains during transformation or heat-treatment. Certain elements are added to combine with impurities in the iron, such as sulphur or nitrogen.

(2) To ensure the formation of martensite at lower cooling rates than those that operate when quenching in water. Heat can only travel from the center of a piece of cooling metal to the surface at a limited rate, and if a section is above a certain thickness, the rate of cooling at the center will be too slow for martensite to form. Small amounts (less than 5%) of chromium, nickel, molybdenum and vanadium, especially when used in combination, promote the formation of martensite so that thick sections can be cooled even in air and still form a martensitic structure in their centers. Alloying elements that exercise this function are said to increase the hardenability of the steel. Note that hardenability refers to a structural effect, not to the hardness level, which is dictated almost entirely by the carbon content.

(3) To form alloy carbides that are harder and more resistant to wear than cementite (Fe₃C) and, in addition, to check the tempering of martensite. The steep drop in hardness of unalloyed carbon steels occurs between 300°C and 400°C, but in steels containing tungsten, chromium, cobalt and vanadium this drop does not occur until the temperature reaches about 650°C. The difference means that such steels can be used in high-speed machining processes in which the tool may locally become red hot. Appropriate compositions of such steels are known as high-speed tool steels. Even with less highly alloyed materials, the functions of small amounts of additional elements in improving the toughness of tempered martensite is very important in engineering applications.

The alloying elements can be grouped as austenite-forming, ferrite-forming, carbide-forming and nitride-forming elements.

The most important Austenite-forming elements are C, Ni and Mn. Sufficiently large amounts of Ni or Mn render a steel austenitic even at room temperature. In a steel that contains 13% Mn, 1.2% Cr and 1% C so-called Hadfield steel), for example, both the Mn and C take part in stabilizing the austenite. As to Ni, the range of stability of austenite increases with increasing Ni-content. An alloy containing 10% Ni becomes wholly austenitic if heated to 700 °C. On cooling, transformation from y to a takes place in the temperature range 700-300 °C.

The most important elements in this group are Cr, Si, Mo, W and Al. Fe-Cr alloys in the solid state containing more than 13% Cr are ferritic at all temperatures up to incipient melting. Another instance of a ferritic steel is one that is used as transformer sheet material. This is a low-carbon steel containing about 3% Si.

Several ferrite formers also function as carbide formers, while the most carbide formers are also ferrite formers with respect to Fe. The affinity of the elements in the line below for carbon increases from left to right:

Cr, W, Mo, V, Ti, Nb, Ta, Zr

Alloy steels often contain more than one type of carbides. Several special carbides contains no iron, such as Cr7C3, W2C, VC, Mo2C. Double or complex carbides contain both Fe and a carbide-forming element, for example Fe4W2C.

How stable the carbides are depends on how the element is partitioned between the cementite and the matrix. The percentage of some alloying elements presented in the steel have a great contribution. Cr is the alloying element most commonly used as a carbide stabilizer. Element Mn, for example, though by itself is a very weak carbide former, is a relatively potent carbide stabilizer.

All carbide formers are also nitride former. Nitrogen may be introduced into the surface of the steel by nitriding. The contribution of several elements to surface hardness during precipitation hardening is illustrated in the Fig. 1. It can be seen Al and Ti re great contributor and provide very high hardnesses in amounts of about 1.5%.

Fig. 1: Effect of alloying element additions on hardness after nit riding. Basc composition 025% C, 0.30% Si, 0.70% Mn [38]

References

[29] W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J. Bradbury: Essential metallurgy for engineers, p63-71. 1985. Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5
[38] Karl-Erik Thelning: Steel and its Heat Treatment, 2nd ed. 1984. Butterworths. ISBN: 0-408-01424-5
[241] Donald V. Brown: Metallurgy Basics.Delmar Publishers Inc., 1983. ISBN 0-442-21434-0
[244] Stahleisen: Stahlschussel, 1992
[1026] W. Stevens and A.G. Haynes: The temperature of forming martensite and bainite in low-alloy steels. J. Iron & Steel Inst., 183, 349-359 (1956)
[1027] L.D. Jaffe and e. Gordon: Temperability of steels. Trans ASTM, 49 (1957).