AISI/SAE 8620 is a low-carbon, nickel-chromium-molybdenum alloy steel purpose-built for case hardening rather than through-hardening. With only about 0.18–0.23% carbon in the core, 8620 doesn't harden meaningfully on its own — its real value comes from carburizing, a process that diffuses additional carbon into the surface layer so it can be hardened to a wear-resistant case while the low-carbon core stays tough and ductile.

The nickel improves core toughness and impact resistance, while chromium and molybdenum contribute hardenability so the carburized case forms consistently and the core retains strength after heat treatment. This combination — a hard, wear-resistant surface over a tough, shock-resistant core — is exactly what's needed for parts that see both sliding or rolling contact wear and cyclic bending or impact loads. That makes 8620 a standard choice for gears, pinions, camshafts, wrist pins, bearing races, and similar power-transmission components across automotive, industrial gearbox, and heavy equipment applications. It's typically machined to near-final dimensions in the annealed or normalized condition before carburizing and hardening, with only light finish grinding afterward, since the low-carbon core machines considerably easier than higher-carbon alloy steels.

8620's low carbon content makes it noticeably more machinable than higher-carbon alloy steels like 4140 or 4340, and this is the condition shops almost always encounter it in — since the case-hardening step happens after rough and often finish machining. Cutting forces are moderate, and the material doesn't have the abrasive carbide volume of higher-carbon alloys, so tool wear rates are generally manageable with standard coated carbide grades.

The main machining challenge with 8620 isn't hardness or abrasion — it's a mild tendency toward built-up edge and tearing at low cutting speeds, since the softer, more ductile low-carbon matrix can smear rather than shear cleanly if the tool isn't kept sharp and speeds are too conservative. Running toward the higher end of the recommended speed range, with a sharp, positive-rake edge, typically produces better surface finish than backing off speed to protect the tool, which is the opposite of what many operators expect from an alloy steel.

Because most 8620 parts are machined complete before carburizing, dimensional stability during machining matters more than raw tool life — stress relief and consistent workholding help avoid distortion that shows up later after heat treatment. Chip formation is generally favorable, producing manageable chips with standard chipbreaker geometry, and coolant is mainly useful for heat and chip evacuation rather than to combat extreme tool wear.