The heat treatment process used for softening hardened steel is

Answer: Option C

Definition: Annealing is a heat treatment process in which hardened steel is heated to a temperature above its critical point, held for a specific duration, and then cooled slowly, usually in a furnace.

Purpose: The primary aim of annealing is to soften hardened steel, improve its ductility, relieve internal stresses, and refine the grain structure for better machinability.

Reason: Slow cooling during annealing transforms the steel’s microstructure into a softer and more workable form, making it suitable for further machining or shaping operations.

Why not tempering: Tempering also reduces hardness but its main purpose is to reduce brittleness and improve toughness while retaining most of the hardness; it does not fully soften the steel like annealing.

Conclusion: For complete softening of hardened steel, annealing is the correct and standard heat treatment process.