Surface Hardening Methods — Need for Quenching For which hardening process is quenching generally NOT required to obtain the hardened case?

Introduction / Context:
Surface hardening methods differ in mechanism and post-heat-treatment steps. Some depend on a martensitic transformation that requires rapid quenching, while others form hard compounds at relatively low temperatures and do not require quenching.

Given Data / Assumptions:

• Case hardening via carburising/flame/induction typically austenitises the surface.
• Martensite requires rapid cooling to form.
• Nitriding forms alloy nitrides at subcritical temperatures.

Concept / Approach:
Nitriding is performed at about 500–550°C, below the lower critical temperature. Nitrogen diffuses to form very hard nitride compounds (e.g., Fe2–3N, CrN, AlN) in alloy steels. Because no austenite is formed, quenching is unnecessary; the part can be cooled in still air with minimal distortion.

Step-by-Step Solution:
Identify transformation requirement: martensite needs quench; nitrides do not.Map processes: carburising, flame, induction → austenitise + quench; nitriding → low-temperature diffusion, no quench.Therefore, the process not requiring quenching is nitriding.

Verification / Alternative check:
Process charts show quench steps for flame/induction/carburised cases (often oil or polymer), whereas nitriding cycles end with a slow cool.

Why Other Options Are Wrong:

• Carburising/flame/induction: require quenching to obtain martensitic case hardness.
• “Any one of these”: incorrect because only nitriding omits quenching by design.

Common Pitfalls:
Assuming all surface hardening requires a quench; diffusion-based nitriding is the notable exception.

Final Answer:
nitriding