Case Hardening: Carburizing, Nitriding & Carbonitriding

Many parts need two conflicting properties: a hard wear-resistant surface and a tough core that won’t snap. Case hardening delivers both by hardening only the outer skin while the core stays tough.

It suits low-carbon steel that can’t be through-hardened, because we first add an element to the surface, then harden.

Carburizing holds the part in a carbon-rich atmosphere at ~850–940°C so carbon diffuses into the surface, then quenches it to form a hard martensitic case (a core process offered by V.S. Heat Treatment).

Case depth runs 0.2–2 mm at 58–62 HRC with a tough core — ideal for special-grade screws, pins, studs and nuts needing a hard case with a tough core.

Nitriding diffuses nitrogen at lower temperature (~500–550°C) with no quench, so distortion is minimal — great for high-precision parts.

The surface is very hard (equivalent to ~900–1100 HV) with good wear and fatigue resistance and a slight corrosion benefit, though the case is usually thinner than carburizing.

A hybrid: both carbon and nitrogen are added at ~800–870°C. Nitrogen improves hardenability and wear resistance, making it ideal for high-volume small parts such as self-tapping and set screws that need an economical hard surface.

• Carburizing — deep, very hard case; high temperature; moderate distortion; quench required.
• Nitriding — thin but very hard case; low temperature; least distortion; no quench.
• Carbonitriding — cost/quality balance; ideal for high-volume small parts.