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8620 Case-Hardening Steel

8620 is a low-carbon nickel-chromium-molybdenum case-hardening steel designed to be carburized. Its low core carbon keeps it soft and machinable (635 MPa tensile, 22% elongation as supplied), then carburizing produces a hard 58-62 HRC wear-resistant case over a tough, ductile core — the ideal combination for gears and shafts.

How 8620 Case-Hardening Steel machines

At 3.5/5 in the soft, pre-carburized condition it machines well, allowing teeth, splines, and features to be cut cleanly before heat treatment. This soft-core machinability is the whole point: you machine easily first, then carburize to add surface hardness without having to cut a fully hardened part.

Manufacturing & processing

The defining process is carburizing (gas, pack, or vacuum) followed by quench and temper, which diffuses carbon into the surface to create the hard case while the core stays tough. It is supplied as forgings and bar. Corrosion resistance is low (1.5/5), so finished parts need oiling, plating, or paint.

Typical applications

The standard for carburized gears, pinions, camshafts, splined shafts, bearing races, and pins — anywhere a hard, wear-resistant surface must sit on an impact-tough core. Best where contact and wear loads demand surface hardness but the part must also survive shock without cracking through.

When to choose it

Choose 8620 when you need a hard wearing surface and a tough core, and you will carburize the finished part — classic for gears and shafts. If you want through-hardened high strength instead, use 4340 or 4140 Q&T. If no hardening is wanted at all, 1144 Stressproof machines and serves as-is.

Suitable surface finishes

Common finishes for 8620 Case-Hardening Steel: zinc plating, black oxide, powder coating, nickel plating. Use the finish selector →

FAQ

What does case hardening do for 8620?
Carburizing diffuses carbon into the surface, then quenching forms a hard 58-62 HRC case for wear resistance, while the low-carbon core stays soft and tough to absorb impact. This hard-skin-over-tough-core structure is exactly what gears, cams, and shafts need under contact loading.
Why machine 8620 before carburizing?
In the soft, pre-carburized state (3.5/5) gear teeth, splines, and features cut cleanly with normal tooling. Carburizing is done afterward to add only surface hardness. Machining a fully hardened case would be slow and would remove the very wear surface you created.
How is 8620 different from 4340?
8620 is a low-carbon steel hardened only at the surface by carburizing, giving a hard case and soft tough core. 4340 is a medium-carbon alloy that through-hardens to high strength everywhere. Use 8620 for surface wear with impact toughness; use 4340 for bulk high-strength loading.

Property values are typical/nominal for early guidance and vary by temper, grade, supplier and heat treatment. Confirm critical specs against a certified datasheet or with an mfgiq engineer.