SAE/AISI 9310 is a low-carbon nickel-chromium-molybdenum alloy steel best known as a premier aerospace carburizing and gear steel. Its relatively high nickel content, well above what's found in the 86xx or 87xx grades, gives it exceptional core toughness and fracture resistance, while chromium and molybdenum together provide the deep hardenability needed to carburize a hard, wear-resistant case over a large, tough section. The low base carbon content is deliberate — it keeps the core soft and ductile so the finished part can absorb shock and cyclic loading without cracking, while the carburized surface carries the wear and contact-fatigue load.

This combination makes 9310 the standard choice for highly loaded aircraft and helicopter transmission gears, pinions, and shafts, where failure simply isn't an option and both surface hardness and core toughness have to be excellent simultaneously. Outside aerospace, it also turns up in high-performance gearboxes and other precision power-transmission components where the extra alloy cost is justified by the combination of case hardness and core ductility it delivers.

9310's low carbon content actually makes it relatively cooperative to machine in the annealed or normalized condition most shops receive it in prior to carburizing — the nickel-chromium-molybdenum alloy content raises cutting forces above plain low-carbon steel, but nowhere near as much as a higher-carbon through-hardening grade would. Once a part has been carburized and case hardened, machining is limited to grinding or hard finishing on the hardened surfaces, while any remaining soft-core work follows the guidance below.

Chip control in the annealed state is generally good, producing manageable chips with standard chipbreaker geometries. The nickel content can introduce a bit more gumminess and built-up-edge tendency than a straight chromium-molybdenum steel, particularly at low cutting speeds or with a dull edge, so staying within the recommended speed range and keeping the edge sharp both help keep chip and surface quality consistent.

Because precision aerospace gear blanks often carry tight dimensional and surface-finish requirements before carburizing, a rigid setup and a sharp, well-supported edge matter as much for part accuracy as for tool life. Coated carbide with good toughness handles the alloy content well without sacrificing the finish these parts typically demand.