Hardening only the surface layer while leaving the core tough.
A carburised case gives a hard, wear-resistant skin over a core that stays ductile — the combination gears need, where the tooth surface must resist wear and the root must resist shock.
The case is produced by diffusing carbon (carburising, gas or pack) or nitrogen (nitriding, carbonitriding) into the surface at temperature, then hardening. Machining is done before treatment, and the part grows slightly during it — typically 0.001 to 0.002 in on a carburised part, which matters on a ground dimension.
Leaving a grinding allowance of zero on a carburised part. Case hardening grows the part, and a through-hardened core cannot be corrected afterwards.
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