Gear Steel Selection: Carburizing vs Through-Hardening

Two philosophies

A gear tooth needs a hard, wear-resistant surface for contact (pitting) strength and a tough core for bending strength. Two steel families deliver that balance differently.

Carburizing steels

Low-carbon alloy steels — 8620, 16MnCr5, 20MnCr5, 20CrMnTi — are machined soft, carburized, then quenched. Result: a case of roughly 58–62 HRC over a core of perhaps 30–45 HRC.

• Highest contact and bending fatigue strength of the common options.
• Case depth scales with tooth size: a widely used rule of thumb puts effective case depth at roughly 0.15–0.30 × module — a module 3 gear needs only a few tenths of a millimetre, a module 10 gear needs proportionally more.
• Cost: carburizing cycles are long, distortion must be managed (often finish-grinding), and very large gears become impractical to case uniformly.

Through-hardening steels

42CrMo4 / 4140 and similar medium-carbon alloy steels are quenched and tempered to a uniform ~28–36 HRC, then cut or finish-machined. Surface hardness — and therefore pitting resistance — is lower, but:

• No case-depth limit; suitable for large-module, large-diameter gears (wind, mill, marine auxiliaries).
• Teeth can be induction hardened afterwards (roughly 50–55 HRC on the flank) to recover surface durability while keeping a tempered core.
• Nitrided steels are a third route for moderate loads: very hard thin cases with almost no distortion, at limited case depth.

Selection logic

Situation	Typical choice
Small/medium module, high torque density, automotive and gearbox duty	Carburizing: 8620, 16MnCr5, 20CrMnTi
Large module, shock loads, one-off industrial gears	Through-hardened 42CrMo4, optionally induction hardened
Precision gears where distortion cannot be corrected	Nitriding steel
Low load, low cost, replaceable gears	1045 or cast steel, unhardened or lightly hardened

The practical boundary is size and stress: once modules grow and case depths would exceed a millimetre or so, carburizing cost and distortion rise steeply, and through-hardening with a surface treatment usually wins.

Heat treatment and finishing reality

Carburizing is a batch process measured in hours-to-days: gears are machined with a grinding allowance, cased, quenched, and finish-ground or hard-skived to restore tooth accuracy — that finishing step must be in the process plan and the tolerance budget from the start. Through-hardened gears are simpler: quench and temper first, then cut the teeth in the hardened blank where size allows, accepting the flank finish that hard cutting gives, or soft-cut and induction-harden with somewhat looser accuracy. For both routes, the steel's hardenability band (the H-band in the purchasing standard) is what guarantees the core hardness at the tooth root; specifying a grade without its hardenability band leaves the most important gear property to chance.