Surface hardening

Surface hardening, treatment of steel by heat or mechanical means to increase the hardness of the outer surface while the core remains relatively soft. The combination of a hard surface and a soft interior is greatly valued in modern engineering because it can withstand very high stress and fatigue, a property that is required in such items as gears and anti-friction bearings. Surface-hardened steel is also valued for its low cost and superior flexibility in manufacturing.

The various methods of surface hardening are:

1. Case hardening
• Pack hardening
• Liquid carburizing
• gas carburizing
2. Nitriding
3. Cyaniding
4. Flame hardening
5. Induction hardening.

          Mechanical means of hardening the surface of steel parts include peening, which is the hammering of the heated surface, as by iron pellets shot onto the surface or by air blasting, and cold-working, which consists of rolling, hammering, or drawing at temperatures that do not affect the composition of the steel.

Case hardening

Case hardening :

          As mentioned previously, only those carbon steels can be hardened whose carbon content is about 0.25% or more.

How do we harden dead mild steel? The answer is by case hardening. In this process, the work piece is packed in charcoal and heated as in annealing. It is kept at that high temperature for a few hours. The result is that carbon enters into the surface of the work piece to the depth of a mm or two depending upon the heating time. The work piece now has a case where carbon percentage is as per requirement for hardening. It is then heated and quenched in the usual manner. The result is a component whose surface acquires hardness, but core remains soft and tough.

The Objects of  case hardening are:

1. To obtain a hard and wear resistance surface on machine parts with enrichment of the surface layer with carbon to concentration of 0.75 to 1.2%.
2. To obtain a tough core.
3. To obtain close tolerance in machine parts.
4. To obtain a higher fatigue limit and high mechanical properties in the core.

          Case hardening consists in heating a steel in the presence of a solid, liquid or gas, rich in carbon in order to enable the surface to be hardened, while retaining a tough ductile core.

          There are three methods of adding carbon to the surface of the metal:

1. Pack hardening               2. Liquid Carburizing               3. Gas carburizing.