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Surface hardening of metals

Short answer

Surface hardening creates a hard, wear-resistant skin on a tough, ductile core. It is achieved by carburising, nitriding, cyaniding, flame or induction heating followed by quenching, giving surface hardness without overall brittleness.

Explain surface hardening as a heat treatment process of metals.

Surface hardening (case hardening) is a heat treatment that makes the outer layer (case) of a steel part hard and wear resistant, while the inner core remains soft, tough and ductile. It is used for gears, cams, shafts, bearings and tools that need a hard surface to resist wear but a tough core to resist shock.

Methods

  1. Carburising: low carbon steel is heated at about 900-950 C in a carbon-rich medium (charcoal, gas or liquid) so that carbon diffuses into the surface (0.5-2 mm); then quenched and tempered, giving a high-carbon hard case.
  2. Nitriding: steel containing Al, Cr or Mo is heated at about 500-550 C in ammonia, forming very hard nitrides on the surface without quenching; little distortion.
  3. Cyaniding / carbonitriding: heated in a molten cyanide salt bath so carbon and nitrogen enter the surface; quenched.
  4. Flame hardening: the surface is heated with an oxy-acetylene flame and quenched immediately by water spray.
  5. Induction hardening: the surface is heated by high frequency induced current, then quenched.
   Hard case (high carbon)
   +---------------------+
   |  ...............    |
   |  .  Tough, soft  .  |
   |  .     core      .  |
   |  ...............    |
   +---------------------+

✓ Answer checked by Claude Opus 5.5 (high effort)

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