Gear Carburizing & Quenching Hardening: Core Methods for Deep Case Uniformity and Distortion Control
Executive Summary: Carburizing and quenching is the most dominant case-hardening process for gear transmission components to achieve a “hard case + tough core” microstructure. Low-carbon alloy steels such as 20CrMnTi, after carburizing at 920–950°C, quenching, and low-temperature tempering, yield tooth surface hardness of HRC 58–62 with core hardness of HRC 30–42, extending fatigue life by 3–5×. Controlling heat treatment distortion within 0.05–0.15 mm is the decisive factor determining final gear precision grade.
1. Process Fundamentals
1.1 Definition
Carburizing and quenching feeds carbon atoms into the surface layer of low-carbon steel (0.10%–0.25%C) in a carbon-rich atmosphere, then quenches and tempers to form a gradient structure of high-carbon martensite case + low-carbon tempered martensite core.
1.2 Comparison of Three Carburizing Media
| Method | Medium | Temp. Range | Infiltration Rate (mm/h) | Distortion Control | Application |
|---|---|---|---|---|---|
| Gas Carburizing | Propane + carrier gas | 920–950°C | 0.15–0.35 | ★★★★ | Batch production, gear carburizing services |
| Vacuum Carburizing | Acetylene | 950–1050°C | 0.25–0.50 | ★★★★★ | High-precision gears, internal rings |
| Salt Bath Carburizing | Cyanide salts | 900–930°C | 0.20–0.40 | ★★★ | Small-module gears |
Vacuum carburizing, with zero intergranular oxidation and minimal distortion, has become the preferred process for precision transmission gears.
2. Key Process Parameters & Acceptance Criteria
2.1 Effective Case Depth (ECD)
| Gear Module m (mm) | Recommended ECD (mm) | Inspection Method |
|---|---|---|
| m ≤ 3 | 0.4–0.8 | Vickers hardness HV1 |
| 3 < m ≤ 6 | 0.8–1.2 | Vickers hardness HV5 |
| m > 6 | 1.2–2.0 | Vickers hardness HV10 |
Acceptance standard: ECD is defined as the perpendicular distance from the surface to the position where hardness drops to 550 HV.
2.2 Factors Controlling Distortion
- Pre-heat treatment: Normalizing + high-temperature tempering reduces machining stress and lowers quench distortion by 30%–50%
- Quenching temperature: 30–50°C above Ac3 is optimal; higher temperature increases distortion risk
- Quenching medium: Hot oil (80–120°C) produces 40%–60% less distortion than cold oil
- Furnace loading: Gears arranged vertically or suspended on dedicated fixtures; avoid stacking
3. Common Defects & Countermeasures
| Defect | Symptom | Root Cause | Remedy |
|---|---|---|---|
| Insufficient hardness | Tooth surface < HRC 55 | Low carburizing concentration or inadequate quench | Raise carbon potential to 0.8%–1.0%C; check oil temperature |
| Excessive distortion | Lead deviation > 0.15 mm | Uneven quench stress, insufficient grinding allowance | Add stress-relief annealing; optimize gear blank design |
| Surface decarburization | Ferrite layer at surface | Low furnace carbon potential or poor seal | Control furnace pressure + regular oxygen probe calibration |
| Intergranular oxidation | Black network at grain boundaries | O₂/H₂O impurities in furnace atmosphere | Use high-purity carrier gas; switch to vacuum carburizing |
4. Practical Lessons
💡 Tip 1: In a recent m=5 spiral bevel gear project for a customer requiring DIN 6 precision, we adopted vacuum carburizing + martempering. After three rounds of process tuning, distortion was controlled within 0.08 mm, with a first-pass inspection yield of 92%.
💡 Tip 2: Adding a 650°C × 2h stress-relief annealing step before carburizing reduced grinding crack occurrence from 5% to below 0.3%, at an added cost of only ~$1.2 per piece.
💡 Tip 3: Recommended inspection frequency: at least 3 coupons per furnace load for metallography, hardness gradient profiling, and fracture analysis. For large-module gears, check tooth surface hardness on every tooth of every gear.
5. References
- ISO 2639:2002 — Steels — Determination and verification of the effective depth of carburized and hardened cases
- AGMA 2001-D04 — Hardening and Heat Treatment of Gears
- Geyon Transmission Process Manual — Heat Treatment Volume (Internal)
- Explore gear precision manufacturing capabilities and custom gear solutions
Daily Learning Note · Geyon Transmission · 2026-07-10