Heattreated alloy steels provide high strength, high yield point, combined with appreciable ductility even in large sections. The use of plain carbon steels frequently necessitates water quenching accompanied by the danger of distortion and cracking, and even so only thin sections can be hardened throughout. For resisting corrosion and oxidation at elevated temperatures, alloy steels are essential.

The Alloy Steels Research Committee adopted the following definition: “Carbon steels are regarded as steels containing not more than 0,5% manganese and 0,5% silicon, all other steels being regarded as alloy steels”.

The principal alloying elements added to steel in widely varying amounts either singly or in complex mixtures are nickel, chromium, manganese, molybdenum, vanadium, silicon and cobalt.

The effect of the alloying element in the steel may be one or more of the following:

(1) It may go into solid solution in the iron, enhancing the strength. The general effectiveness is shown in Fig. 1.
(2) Hard carbides associated with Fe,C may be formed.
(3) It may form intermediate compounds with iron, e.g. FeCr (sigma phase), Fe,W,.
(4) It may influence the critical range in one or more of the following ways:

(a) Alter the temperature. For example, 3% nickel lowers the Ac points some 30°C, while 12% chromium raises the Ac1, temperature to about 800°C and also forms a range of 150/200°C above this in which the pearlite changes to austenite. Fig. 2 shows the effect of alloys on the eutectoid temperature.

(b) Alter the carbon content of the eutectoid (Fig. 2). The carbon content of the pearlite in a 12% chromium steel is 0,33%, as compared with 0,87 in an ordinary steel. Nickel also reduces the amount of carbon in the pearlite and consequently increases the volume of this constituent at the expense of the weaker ferrite.
(c) Alter the “critical cooling velocity”, which is the minimum cooling speed which will produce bainite or martensite from austenite. Typical critical speeds obtained by quenching from 950°C are given in Table 1.

Table 1. Effect of alloying on the critical cooling speed of steel

Carbon, %	Alloying Element, %	Cooling Speed to form Martensite, °C per sec (650°C)
0.42	0.55 Mn	550
0.40	1.60 Mn	50
0.42	1.12 Ni	450
0.40	4.80 Ni	85
0.38	2.64 Cr	10

The efficiency of the additions of the various alloy elements in reducing the effect of mass during quenching may be judged by the relative reduction of the critical velocity of the steel. Chromium and manganese respectively are far more effective than nickel.

(5) Combinations of elements can be chosen so that the volume change is reduced and also the risk of quench cracking. It may produce effeets characteristic of the alloying element.

(a) It may render the alloy sluggish to thermal changes, increasing the stability of the hardened condition and so producing tool steels which are capable of being used up to 550°C without softening and in certain cases may exhibit an increase in hardness.

(b) It may have a chemical effect on the impurities. Under suitable slag conditions vanadium, in quite small quantities, "cleans" the steel and renders it free from slag inclusions. Manganese and zirconium form sulphides.

(c) Certain elements such as chromium, Aluminium, silicon and copper tend to produce adherent oxide films on the surface of the steel which increase its resistance to corrosion and oxidation at elevated temperatures.

(d) Creep strength may be increased by the presence of a dispersion of fine carbides, e.g. molybdenum.