1. The purpose of adding alloying element is to make it a special purpose element.
2. If the element is added to steel with less than 5%, then it is added for providing strength and hardness while if more than 5% is added then it is added for special properties like corrosion resistance, heat resistant etc.
3. Following are the classification of elements:

I. Austinite Stabilizing Elements

• Ni, Mn, Co are austenite stabilizers and they tend to form solid solution with gamma iron.
• They have greater solubility in austenite.
• They have FCC crystal structure.
• They do not combine with carbon present in γ to form simple or complex carbide, therefore carbon remains in the solid solution in the γ.
• 13% Mn steels are austenitic at room temperature called as Hadfield Steel.
• C and N are also austenite stabilizers(interstitial solutes in FCC).

II. Ferrite Stabilizing Elements

• Al, Cr, Si, Mo, W, P, are ferrite stabilizers, they tend to form solid solution with alpha iron.
• They have greater solubility in ferrite – BCC.
• Generally have similar BCC structure.
• They decrease the amount of C present in γ-Fe.
• Favours formation of free carbides in steel.
• The ferrite form is Delta ferrite since it can exists from melting point to room temperature.

III. Carbide forming Elements.

• Some alloying elements, notably manganese, chromium, molybdenum, and vanadium, dissolve in theferrite phase and also form carbides.
• Such elements as nickel, copper, and silicon do not form carbides in steel. When present in the amounts usually found in alloy steels, they dissolve in and strengthen the ferrite.

IV. Carbide Graphitizing elements

• Si, Co, Al and Ni. 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. Their present makes the carbides unstable.