ANS:A - case hardening

Case hardening is a surface hardening process used to increase the hardness and wear resistance of metal components, particularly steel, while maintaining a tough and ductile core. This is achieved by introducing additional carbon or nitrogen into the surface layer of the metal, creating a hard outer layer while leaving the interior relatively soft and ductile. There are several methods of case hardening, including:

1. Carburizing: This involves introducing carbon into the surface of steel by heating it in a carbon-rich environment, such as with charcoal or gas containing carbon compounds. Carbon diffuses into the steel surface, forming carbides, which increase hardness.
2. Nitriding: Nitrogen is introduced into the surface of steel by heating it in a nitrogen-rich atmosphere or by immersing it in molten nitrogen-containing salts. Nitrogen diffuses into the steel surface, forming nitrides, which increase hardness and wear resistance.
3. Carbonitriding: This is a combination of carburizing and nitriding processes, where both carbon and nitrogen are introduced into the surface of steel simultaneously. It provides improved hardness and wear resistance compared to carburizing alone.
4. Cyaniding: Cyaniding involves introducing carbon and nitrogen into the steel surface by immersing it in a molten cyanide salt bath at high temperatures. It forms a hard surface layer rich in carbon and nitrogen compounds.

After the surface hardening process is completed, the component may undergo additional heat treatment steps such as quenching and tempering to achieve the desired hardness, toughness, and dimensional stability. Case hardening is commonly used in manufacturing industries for components subjected to high wear, such as gears, bearings, shafts, and machine parts, where a combination of surface hardness and core toughness is required.