Heat treatment — classification of induction hardening Induction hardening of steels is basically which type of heat-treatment process?

Introduction / Context:
Induction hardening uses alternating electromagnetic fields to rapidly heat the surface layer of a steel component above the austenitising temperature, followed by immediate quenching to form martensite at the surface. The core remains relatively unaffected, preserving toughness. This places induction hardening squarely within the family of surface hardening processes.

Given Data / Assumptions:

• Steel composition contains sufficient carbon (typically >= 0.35% C) to form martensite upon quenching.
• Induction frequency and power are set to target a specific case depth.
• Rapid heating and quenching occur locally at the surface.

Concept / Approach:
Unlike carburising (which adds carbon to the surface at high temperature over hours), induction hardening does not change composition. It only transforms the existing surface microstructure by localized austenitising followed by quench. The result is a hard, wear-resistant martensitic case with a tough, ductile core—ideal for shafts, gears, and cams.

Step-by-Step Solution:
Apply alternating magnetic field → eddy currents heat the surface layer.Surface reaches austenitising temperature rapidly while the core stays cooler.Immediate quench converts surface austenite to martensite (hard case).Core retains original microstructure (toughness), confirming surface hardening classification.

Verification / Alternative check:
Case depth profiles and hardness gradients measured after induction confirm the hardened surface and softer core. No carbon enrichment is detected, differentiating from carburised cases.

Why Other Options Are Wrong:
Carburising adds carbon; induction does not. Core hardening implies through-hardening, which is not the objective here. Tempering is a secondary process used after quench to adjust hardness—not the primary mechanism of induction hardening.

Common Pitfalls:
Attempting to induction-harden very low carbon steel (insufficient martensite potential); mis-selecting frequency and power leading to too shallow or too deep a case.

Final Answer:
Surface hardening process (through rapid surface austenitising and quench)