What is the effect carbon content on steel?

Carbon is primary hardening element in steel. Hardness and tensile strength increases as carbon content increases upto 0.85%. Ductility and weldability decreases as carbon content increases.

If the solution of carbon and liquid iron is solidified slowly the carbon tends to separate out in the form of graphite flakes (Grey cast iron). It is easily machinable. If the same iron is cast and colled quickly, it is hard and has a higher tensile strength, is difficult to machine (white cast iron).