Q1: The only suitable method for hardening the low carbon steel is case hardening. Which of the following is a case hardening process?

ANS:A - Cyaniding

Cyaniding is a case hardening process used to increase the surface hardness of low-carbon steels. It involves the introduction of carbon and nitrogen into the surface layer of the steel to form a hard, wear-resistant outer layer while maintaining a softer and tougher core. The cyaniding process typically involves the following steps:

1. Preparation of the Steel: The steel parts to be treated are cleaned thoroughly to remove any surface contaminants, such as oil, grease, or rust. Proper cleaning ensures good metallurgical bonding during the cyaniding process.
2. Cyanide Bath: The cleaned steel parts are then immersed in a molten bath of cyanide salts. The cyanide salt used is usually sodium cyanide (NaCN) or potassium cyanide (KCN). This bath is typically heated to temperatures ranging from 760°C to 950°C (1400°F to 1740°F).
3. Diffusion: At the elevated temperatures of the cyanide bath, carbon and nitrogen atoms from the cyanide salts diffuse into the surface layer of the steel. The cyanide salts dissociate, releasing carbon and nitrogen, which then diffuse into the steel surface.
4. Formation of Compound Layer: The carbon and nitrogen atoms diffuse into the steel lattice, forming hard carbonitride compounds such as Fe3C (iron carbide) and Fe4N (iron nitride) within the surface layer. These compounds contribute to the increased hardness and wear resistance of the treated steel.
5. Quenching: After the desired case depth is achieved, the steel parts are rapidly quenched in water or oil to cool them quickly. Quenching "freezes" the carbon and nitrogen atoms in the hardened surface layer, preventing them from diffusing back into the core of the steel.
6. Finishing Operations: Finally, the cyanided steel parts may undergo additional finishing operations such as grinding, polishing, or machining to achieve the desired surface finish and dimensional accuracy.

Cyaniding is commonly used for applications where improved wear resistance and surface hardness are required, such as gears, cams, shafts, and various machine components. It is particularly suitable for low-carbon steels that do not respond well to conventional heat treatment methods such as quenching and tempering.