Two gears cut with the same hob can differ by more than 30 HRC at the surface and nearly an order of magnitude in contact fatigue life. The difference comes after hobbing, so gear hobbing heat treatment decides how long a gear survives.
Gear hobbing heat treatment covers the steps after the teeth are cut: carburizing and quenching, carbonitriding, induction hardening, nitriding and quench-and-temper, plus grinding, honing and shot peening. It walks you through process selection, case depth and core hardness, distortion and grinding allowance, and the parameters to lock before quotation. Yujiaxin Tech links hobbing, heat treatment, grinding and inspection in one chain; the six-process manufacturing matrix shows the capability.
Key takeaways
- Hobbing shapes teeth but does not harden them; surface hardness stays near 30-40 HRC until carburizing raises it to 58-62 HRC while the core holds 30-45 HRC.
- Effective case depth uses 550 HV (about 52 HRC) as the limit hardness, with a working rule of 15%-20% of the module.
- Carburizing with oil quenching shifts cumulative pitch by +0.05-0.15 mm, so reserve 0.10-0.20 mm of grinding allowance per flank.
- Root shot peening builds a 0.1-0.3 mm compressive layer and raises tooth root bending fatigue life by 3-5 times.
Hobbing is a cold forming step that sets profile, lead and pitch accuracy yet leaves surface hardness unchanged. Gear steels such as 20CrMnTi and 42CrMo sit near 30-40 HRC after hobbing, so using them as cut invites wear, pitting and tooth root fracture. Gear hobbing heat treatment raises surface carbon to 0.8%-1.0%, reaching 58-62 HRC at the surface while the core keeps 30-45 HRC.
Takeaway: Hobbing sets the tooth form, heat treatment sets the life.
Heavily loaded gears favor carburizing, tight-accuracy gears with small distortion favor nitriding, large modules favor induction hardening, and gears needing only through toughness favor quench and temper.
| Process | Temperature | Surface hardness | Case depth | Distortion | Cost |
|---|---|---|---|---|---|
| Carburizing + quench | 880-940 °C | 58-62 HRC | 0.8-2.5 mm | High | High |
| Carbonitriding | 820-880 °C | 58-63 HRC | 0.1-0.75 mm | Medium | Medium-high |
| Induction hardening | Local heating | 50-58 HRC | 1-5 mm | Low | Medium |
| Nitriding | 500-570 °C | above 650 HV | 0.1-0.5 mm | Very low | Medium |
| Quench and temper | Quench + high temper | 28-35 HRC | Through section | Medium | Low |
20CrMnTi and SCM420 are carburizing steels with a ceiling near 920 °C to limit grain growth. Process metallurgy details are covered by ASM materials and heat treating resources.
Takeaway: Load, accuracy and batch size drive the choice.
Carburized gears use 550 HV (about 52 HRC) as the limit hardness for effective case depth, while nitrided layers use 650 HV, and a working rule sets ECD near 15%-20% of the module. A surface at 58-62 HRC resists wear and pitting, a core at 30-45 HRC resists root bending, and retained austenite is held within 15%.
Takeaway: Process, surface hardness, core hardness and contact fatigue limit are confirmed as one set.
Distortion from gear hobbing heat treatment comes from phase transformation volume change, quenching thermal stress and fixturing or self-weight. Carburizing with oil quenching shifts cumulative pitch by +0.05-0.15 mm and lead by +0.02-0.06 mm. The allowance reserved during hobbing depends on hobbing cutter and cutting parameters.
Reserve 0.10-0.20 mm of grinding allowance per flank before finish grinding. Low-pressure carburizing with high-pressure gas quenching can cut lead variation by 48%-59%. Yujiaxin Tech folds heat treatment and grinding allowance into one process sheet in its precision gear and drive part capability.
Takeaway: Hobbing allowance, distortion, grinding correction and final accuracy run as one chain.
Grinding splits into generating and form grinding, used to correct distortion and restore the ISO 1328 accuracy grade, and grinding through the case is the main risk. Honing follows grinding and lowers mesh noise by improving surface roughness, and low-stress grinding preserves the compressive stress on the flank.
Takeaway: Choose grinding for large distortion and tight accuracy, and honing when noise and surface quality come first.
Shot peening after gear hobbing heat treatment is controlled by shot size, pressure, coverage and Almen intensity, and high-speed shot builds a 0.1-0.3 mm compressive layer. The root fillet is the weak point in bending fatigue, so peening the root returns more than peening the flank, and root peening can raise bending fatigue life by 3-5 times.
Takeaway: A 0.1-0.3 mm compressive layer multiplies root bending fatigue life by 3-5 times.
Surface hardness is measured in HRC. The case profile needs an HV1 or HV0.3 gradient, because an HRC indentation reaches about 1.5 mm and averages the case away. Conversion follows ISO 18265 and ASTM E140.
Metallography checks martensite level, retained austenite and grain size. Yujiaxin Tech covers hardness gradient, metallography and CMM in its end-to-end quality and heat-treat inspection. Load capacity is verified to ISO 6336 and ISO 9085, and traceability follows NIST manufacturing metrology.
Takeaway: Confirm the acceptance list item by item across item, method and criterion.
Pitting or spalling follows a case that is too shallow or too deep, or a core with weak support, so review the ECD and core hardness pairing. Root fracture follows insufficient core toughness, so hold core hardness at 30-45 HRC and shot peen the root fillet. Grinding burn, excessive quench rate and uncontrolled furnace atmosphere cause grinding cracks, quench cracks and decarburization.
Takeaway: Fix the cause before the measure.
For gear hobbing heat treatment, lock each item in writing:
Takeaway: Locking all ten at once prevents repeated quotations and production drift.
Why Yujiaxin Tech. Founded in 1998, Yujiaxin Tech coordinates six processes: MIM, PM, investment casting, gear hobbing, CNC and CIM. Hobbing, heat treatment, grinding and inspection close the loop inside one plant, so responsibility stays single-point and quality data stays traceable.
Conclusion. The selection logic for gear hobbing heat treatment comes down to four points. Pick the process by load and accuracy. Case depth follows the module. Distortion sets the grinding allowance. Fatigue demand decides whether shot peening is needed. To evaluate a specific gear, send your heat-treatment and sample data for process advice and a quotation. You can also read gear hobbing accuracy and acceptance.
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