Surface hardening methods — identify the case hardening family Which of the following heat treatment processes are classified as case hardening methods used to harden the surface while maintaining a tougher core?

Introduction / Context:
Case hardening creates a hard, wear-resistant surface layer (case) over a tougher, ductile core. It is crucial in gears, camshafts, and shafts where surface fatigue and wear dominate while impact resistance must be preserved in the core.

Given Data / Assumptions:

• Steel components requiring surface hardness and core toughness.
• Diffusional thermochemical processes considered.
• Objective: compare carburising, cyaniding, nitriding.

Concept / Approach:
Carburising enriches the surface with carbon at elevated temperatures, followed by quenching to form martensite in the case. Cyaniding introduces carbon and nitrogen from cyanide salts for a thin, hard case at lower times. Nitriding diffuses nitrogen at lower temperatures (typically 500–550°C), forming hard nitrides without quenching, with excellent dimensional control. All three are standard case hardening processes. (Note: induction hardening is a surface-hardening method but not a thermochemical “case” process; it relies on selective austenitisation and quenching.)

Step-by-Step Solution:

Verification / Alternative check:
Heat-treatment texts group carburising, cyaniding, and nitriding under thermochemical case hardening methods.

Why Other Options Are Wrong:

• Any single selection ignores the others which also satisfy the definition.
• “Induction hardening only” is not a thermochemical case but a thermal surface-hardening technique; it is not among the given three.

Common Pitfalls:
Conflating case hardening (diffusion) with surface hardening (thermal) processes; assuming quench is always required (nitriding is not).

Final Answer: