Gear Carburizing & Quenching: Case Uniformity Control
Carburizing and quenching of 20CrMnTi gears at 920-950°C yields HRC 58-62 cases and HRC 30-42 cores, with distortion held to 0.05-0.15 mm.
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
- Geyontech Process Manual — Heat Treatment Volume (Internal)
- Explore gear precision manufacturing capabilities and custom gear solutions
Daily Learning Note · Geyontech · 2026-07-10