Mechanical Engineering - Engineering Materials - Discussion

Free carbon is the part of the total carbon in steel or cast iron that is present in elemental form as graphite or temper carbon. In steels none of the carbon is present as free carbon; it is all dissolved in the iron.

As free carbon is a major impurity, the pig iron becomes brittle and hard. Pig iron may be made into steel (with up to about 2% carbon) or wrought iron (commercially pure iron) through oxidizing some or all of the carbon, together with other impurities.

The hardness of the steel depends on its carbon content: The higher the percentage of carbon, the greater the hardness and the lesser the malleability. Regardless of the heat treatment, higher carbon content reduces weld ability. In carbon steels, higher carbon content also lowers the melting point.

I have a practical case to discuss.

A part made from carbon steel become cracked after welding. We have checked the hardness of cracked part and an another part that was not cracked after welding. Hardness was almost same. (92~97 HRB). The only difference between two samples was one that was not cracked when flatten with hammer become flat easily and another one becomes further cracked and split into two pieces during flattening.

If the question were to ask regarding hardness of iron in general, then the presence of carbon induces propotional hardness, but in this case for steel carbon is in dissolved form (cementite) so, for steel amount of cementite affects the hardness.

Cementite is the main component which imparts hardness.

Steels containing >0.8% Carbon are hard because they contain cementite and pearlite.

Steels containing <0.8% Carbon are soft because they contain ferrite and pearlite.

Option B is right.

If carbon was not in the option then cementite will be the answer but out of cementite and carbon, carbon is the main reason for hardness of steel.

(b) is the correct answer because in question steel is give and for steel it is carbon. But for cast iron cementite is used.

Carbon up to 0.6% it will increase the hardness, then it depends upon cemented content in the metal, answer is cementite.

Amount of carbon can't decide hardness of steel because if it is in free form (graphite) it will impart softness.

If carbon present in combined form then its called cementite and cementite is present then steel is more harder.

I think carbon increase hardness of cast iron.

But cementite increase hardness of steel.