Alloy Steels

Alloy steels have been economically developed for many uses. In some applications, alloy steels are the only materials that are able to meet engineering requirements. While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, silicon, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium.

The family of ferrous metals incorporates a vast number of alloys. Those alloys containing a very high proportion of carbon (over about 2%) are called cast irons. Virtually all of the remainder are termed steels and these can be found in either cast form (produced by pouring molten metal into a mould of the shape of the finished part) or wrought form (cast as ingots or continuous cast billets or slabs, but then hot rolled or forged to produce bars, plates or complex shapes such as rail sections and beams). They can also be formed to finished shape by sintering powdered metal at high temperature. Steels are categorised by their major alloying elements (carbon, manganese, chromium, nickel and molybdenum) and by the presence or absence of minor elements (silicon, sulphur, phosphorus, nitrogen and titanium), as shown in the table in Table 1.

Table 1

Effects of Alloying Elements in Steel

Alloying elements are added to plain-carbon steels for many purposes. Some of the most important of these are:

• To improve mechanical properties by increasing the depth to which steel can be hardened
• To allow higher tempering temperatures while maintaining high strength and good ductility
• To improve mechanical properties at high and low temperatures
• To improve corrosion resistance and elevated temperature oxidation
• To improve special properties such as abrasion resistance and fatigue behaviour

Effect of Alloying Elements on the Eutectoid Point of Steels

All common substitutional alloying elements in steel such as Ni, Cr, Si, Mg, W, Mo and Ti lower the eutectoid Carbon content. Ti, W and Mo are the most effective, whereas Ni and Cr are the least effective. For example a steel containing 5% Cr has the eutectoid carbon content reduced from 0.8 to 0.5 %.

Harden Ability

The hardenability of a steel is defined as that property which determines the depth and distribution of hardness induced by quenching. Hardenability is a characteristic of a steel and is principally determined by the following factors:

• Chemical composition of the steel
• Austenitic grain size
• Structure of the steel before quenching

Hardenability should not be confused with the hardness of steel, which is its resistance to plastic deformation. Hardness is usually measured by a hardness-testing machine that makes an indentation into the surface of the steel. Hardenability, on the other hand is a measure of the depth of hardening of a steel upon quenching from austenite.

Manganese Steels

Manganese is added to all commercial steels in the range of 0.25 to 1% to deoxidize it and to combine with sulphur to form globular MnS. Mn is increased the Tensile strength of steel. Thus, when higher strength than mild steel is required combined with weldability.1.6-1.9% Mn steels are widely used. The steel of 13xx have levels of carbon from 0.30 to 0.45% and 1.75% Mn.

Chromium-Molybdenum Steels

0.5 to 0.95 %Cr is added, along with a small amount of 0.13 to 0.20% Mo to make the 41xx series of alloy steels. The addition of Cr further increases the hardenability, strength and wear resistance of the plain –carbon steels of the same carbon content.

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