Final answer:
Case hardening, nitriding, flame hardening, and induction hardening are all valid methods of hardening steel. These processes either alter the surface of the steel or change the composition to increase hardness and strength.
Step-by-step explanation:
Methods of Hardening Steel
The methods of hardening steel are processes designed to increase the hardness and strength of the steel. The correct options from the given choices for methods of hardening steel include:
- Case hardening: A heat treatment process that hardens the surface layer of steel while maintaining a softer, ductile core.
- Nitriding: A heat treatment process that diffuses nitrogen into the surface of steel to create a hard, wear-resistant case.
- Flame hardening: A process where a high-temperature flame is applied to the steel surface, followed by rapid cooling, to increase surface hardness.
- Induction hardening: A heat treatment technique that uses induction heating followed by rapid cooling to harden the steel surface.
Therefore, all options presented (A, B, C, and D) represent valid methods of hardening steel.