case hardening

A gear tooth has two conflicting wishes: the surface should be glass-hard so it resists wear and denting, but the core should stay tough so the whole tooth does not snap under a shock load. Case hardening grants both wishes at once, it makes a hard skin (the case) over a soft, tough core. Think of a hard candy shell around a chewy center.

There are two broad ways to do it. One is to change the surface chemistry so only the skin becomes hardenable: carburizing packs extra carbon into the surface of a low-carbon steel by soaking it in a carbon-rich atmosphere hot enough for carbon to diffuse in (following Fick's laws, deeper cases need longer times), then quenching, so the high-carbon skin turns to hard martensite while the still-low-carbon core stays soft and tough. Nitriding instead diffuses nitrogen in at lower temperature to form hard nitrides, with little distortion. The other way changes nothing chemically but heats only the surface fast: induction or flame hardening runs a coil or flame over the part so just the skin reaches austenite and quenches to martensite before the core ever gets hot.

Case hardening is everywhere parts must resist both wear and impact: gears, camshafts, bearing races, crankshaft journals, and hand tools. The hard case also fights fatigue, because it puts the surface in residual compression that resists crack starting. The honest caveats: the case is thin (often a few tenths of a millimeter to a couple of millimeters), so it can be worn or ground through; and a chemistry-based case takes time to grow because diffusion is slow, so a deep case is expensive.