Carburizing & Quenching for Gears: Key Parameters and Quality Control
Core Conclusion: Carburizing and quenching is the dominant heat treatment process for heavy-duty gears. Case depth, surface hardness, and effective case depth are the three critical indicators. Controlling carbon potential within ±0.05%C and quenching temperature at Ac3 + 30
50°C ensures core toughness ≥ 40J/cm² and surface hardness of 5862HRC — the engineering baseline for reliable gear performance.
1. Fundamentals of Carburizing & Quenching
Carburizing and quenching is a heat treatment process where low-carbon steel gears (typically 20CrMnTi, 20CrNiMo, 18CrNiMo7-6) are heated in a carbon-rich atmosphere at austenitizing temperature (900~950°C), allowing active carbon atoms to diffuse into the surface layer, followed by rapid cooling (quenching) to form a high-carbon martensitic structure. This creates a composite structure: a hard, wear-resistant surface with a tough, ductile core.
At Geyon Transmission, all high-precision heavy-duty gear projects strictly follow ISO 6336-5 requirements for case hardness quality assessment.
2. Three Core Process Parameters & Control
2.1 Case Depth Control
| Gear Module m (mm) | Effective Case Depth CHD (mm) | Recommended Carburizing Time (h) |
|---|---|---|
| 3 ~ 5 | 0.8 ~ 1.2 | 6 ~ 9 |
| 5 ~ 8 | 1.2 ~ 1.8 | 9 ~ 14 |
| 8 ~ 12 | 1.8 ~ 2.5 | 14 ~ 20 |
| 12 ~ 18 | 2.5 ~ 3.5 | 20 ~ 30 |
Empirical formula for lower limit: CHD ≈ 0.2 × (Module + 1).
2.2 Surface Carbon Content
Optimal surface carbon mass fraction: 0.75% ~ 0.95%C. Risks of over-carburizing (>1.0%C):
- Network carbide precipitation → reduced contact fatigue life
- Excessive retained austenite (>20%) → hardness drop
Under-carburizing (<0.7%C) cannot meet the 58HRC minimum surface hardness requirement.
2.3 Quenching Temperature & Cooling Rate
Recommended quenching temperature: Ac3 + 30~50°C (typically 810~840°C). Cooling medium selection:
- Oil quenching: suitable for module ≤ 8 gears, manageable distortion
- Salt bath / die quenching: recommended for thin-wall or large precision gears, distortion controlled within 0.05mm
3. Common Defects & Countermeasures
3.1 Excessive Distortion
- Cause: uneven heating, excessive cooling rate, asymmetric gear geometry
- Countermeasure: preheat at 500°C for 30 min + die press quenching
3.2 Insufficient Surface Hardness (<58HRC)
- Cause: low quenching temperature or inadequate cooling
- Countermeasure: check oil temperature (optimal: 40~80°C), replace aged cooling media
3.3 Case Delamination
- Cause: overly steep carbon gradient in transition zone
- Countermeasure: extend diffusion stage by 1~2h for gradual carbon decline
4. 💡 Practical Takeaways
Geyon Transmission’s shop-floor experience shows that 60% of carburizing success depends on furnace loading arrangement. Positioning gears with tooth faces upward and uniform spacing (≥10mm between gears) ensures uniform gas flow, preventing the common defect of “insufficient case at root, excessive carbon at tip.” We recommend destructive testing (microhardness traverse) on the first piece for process validation before batch production.
5. References
- ISO 6336-5: Calculation of load capacity of spur and helical gears — Part 5: Strength and quality of materials
- GB/T 3480.5-2008: Load capacity calculation of involute cylindrical gears
- AGMA 2004-B89: Gear Materials and Heat Treatment Manual
- Geyon Transmission Internal Specification Q/GY-HT-2024
Technical learning notes compiled by Geyon Transmission Co., Ltd. — www.geyontech.com