A gear tooth lives two lives at once. Its surface fights sliding wear and contact fatigue; its core absorbs shock every time torque spikes. Hard skin, tough heart — and the two main heat-treatment paths trade these off differently.

Case hardening (carburizing) diffuses carbon into the outer 0.5–2 mm of a low-carbon steel like 8620, then quenches. The result is a file-hard case around 60 HRC over a ductile core in the 30s. It is the default for automotive and aerospace power gears, and the reason those gears shrug off millions of load cycles.

Through hardening treats the whole section of a medium-carbon steel like 4140 to a uniform 28–45 HRC. It is cheaper, dimensionally more predictable, and entirely adequate for moderate loads, larger gears, and anything that will be nitrided later for surface wear.

The practical rule we quote by: if contact stress drives the design, carburize and grind. If bending or budget drives it, through harden. And if you’re not sure, send the duty cycle — picking the recipe is part of the quote, not an extra.