About 8620|The gear steel. Low-carbon (0.20%) nickel-chromium-molybdenum alloy designed specifically for carburizing. You machine it soft in the annealed condition (easier than
4140, nice finish, reasonable tool life), then send it out for carburizing to get a hard, wear-resistant surface (HRC 58-62) over a tough, ductile core. This hard-shell/soft-core combination is exactly what gears, pins, camshafts, and bearing races need. It's the original and still most common case-hardening steel. Welds better than 4140 due to lower carbon. Cheap because automotive OEMs buy mountains of it. The catch: once carburized, it's extremely hard to machine, so finish all your features before heat treatment. Compare to 4140 (through-hardening, no case, stronger overall),
9310 (deeper case hardening for high-performance gearboxes), and
8620H (guaranteed hardenability version).
