Classification of alloying metals according to their effect in the steel is difficult, because the influence varies so widely with each addition depending on the quantity used and other elements present. A useful grouping, however, is based upon the effect of the element on (a) the stability of the carbides and (b) the stability of the austenite.

(1) Elements which tend to form carbides. Chromium,tungsten,titanium, columbium, vanadium, molybdenum and manganese. The mixture of complex carbides is often referred to as cementite.

(2) Elements which tend to graphitise the carbide. Silicon, cobalt, aluminium and nickel. Only a small proportion of these elements can be added to the steel before graphite forms during processing, with attendant ruin of the properties of the steel, unless elements from group 1 are added to counteract the effect.

(3) Elements which tend to stabilise austenite. Manganese, nickel, cobalt and copper.

These elements alter the critical points of iron in a similar way to carbon by raising the A4 point and lowering the A3 point, thus increasing the range in which austenite is stable, and they also tend to retard the separation of carbides. They have a crystal lattice (f.c.c.) similar to that of g-iron in which they are more soluble than in a-iron.

(4) Elements which tend to stabilise ferrite. Chromium, tungsten, molybdenum, vanadium and silicon .

These elements are more soluble in a-iron than in g-iron. They diminish the amount of carbon soluble in the austenite and thus tend to increase the volume of free carbide in the steel for a given carbon content. On the binary equilibrium diagram of these elements with pure iron the A4 point is lowered and A3 raised (although it may be lowered initially), until the two points merge to form a “closed gamma loop”.

Thus, with above, a certain amount of each of these elements the austenite phase disappears and ferrite exists from the melting-point down to room temperature. No critical points exist and such steels (e.g. 18% chromium irons) are not amenable to normal heat treatment, except recrystallisation after cold work. This effect, however, can be counteracted by adding elements from group 3. For example, 2% of nickel is added to the 18% chromium stainless steel to enable it to be refined by normal heat-treatment; carbon has the same effect. Aluminium has the reverse effect in 12 % chromium steel.