Deep Case Carburizing for Gears: Process Parameters and Deformation Control
Key Takeaway: Deep case carburizing (CHD >2mm) is the essential heat treatment for heavy-duty transmission gears. Optimal parameters: carburizing temperature 920–950°C, carbon potential 1.0–1.2%C. Controlling heat treatment deformation (lead tolerance ≤0.02mm, profile tolerance ≤0.015mm) relies on synergistic optimization of pre-deformation compensation + quenchant selection + tempering cycle.
1. Process Fundamentals
1.1 What Is Deep Case Carburizing?
Deep case carburizing is a carburizing heat treatment process where the effective Case Hardening Depth (CHD) reaches ≥2mm. It is widely applied in heavy-duty transmissions such as wind turbine gearboxes, construction machinery, and mining equipment. The principle involves introducing carbon atoms into the gear surface at austenitizing temperature, forming a high-carbon surface layer, then quenching to obtain a high-hardness martensitic structure while the core retains good toughness.
1.2 Typical Process Parameters
| Parameter | Recommended Range | Effect |
|---|---|---|
| Carburizing Temperature | 920–950°C | Every +10°C increases diffusion rate ~20% |
| Carbon Potential | Boost: 1.0–1.2%C, Diffuse: 0.75–0.85%C | Excess causes network carbides |
| Duration | 12–50 hours (depth-dependent) | Depth ≈ proportional to √t |
| Quench Temperature | 820–860°C | Direct or reheat quench |
| Quench Oil Temp | 60–120°C | Affects martensite transformation rate |
| Tempering Temp | 160–200°C | Low-temp tempering relieves stress |
1.3 Material Selection for Carburizing
Recommended Material Grades:
- 18CrNiMo7-6 — wind turbine gearbox primary material
- 20CrMnTi — general transmission gears
- 17CrNiMo6 — ultra-high fatigue life applications
Effective CHD ≈ (0.15–0.25) × module. For module m=10 gear: recommended CHD = 1.5–2.5mm
2. Three Pillars of Deformation Control
Carburizing and quenching distortion is the most common heat treatment defect — manifested as lead deviation, profile distortion, and bore ovality.
2.1 Pre-Deformation Compensation
Reserving reverse deformation during the design phase is the most economical control method:
- Lead pre-crowning: 0.01–0.05mm reverse compensation based on L/D ratio
- Tooth space shrinkage compensation: 0.02–0.04mm negative tolerance for thin-wall gears
💡 Practical Insight: For one of our m=12 wind turbine gears, three trial runs revealed a “tapered” lead deformation pattern. By introducing 0.025mm reverse taper compensation at the rough machining stage, deformation was reduced from 0.06mm to 0.015mm.
2.2 Quenchant Selection and Cooling Curve Control
| Medium | Temp Range | Cooling Rate | Distortion Risk | Best For |
|---|---|---|---|---|
| Conventional Quench Oil | 60–80°C | Medium | Medium | Module ≤8, standard gears |
| Martempering Oil | 100–120°C | Slower | Low | Precision transmission gears |
| Vacuum High-Pressure Gas | Ambient | Controllable | Minimal | Small module, high-precision gears |
| Salt Bath Martempering | 180–220°C | Very slow | Extremely low | Ultra-precision gears |
💡 Practical Insight: For deep case carburized gears (CHD ≥3mm), martempering is strongly recommended. Although cycle time increases ~30%, profile distortion can be reduced by over 50%. For Geyon Transmission’s accumulated strain data from large gear carburizing, see our capabilities page.
2.3 Tempering Effects on Dimensional Stability
Low-temperature tempering (160–200°C) relieves ~60% of quench stress but does not fully decompose retained austenite. For gears requiring Grade D precision or higher:
- 1st temper: 180°C × 3h
- 2nd temper: 170°C × 4h (cool to room temperature between cycles)
- Deep cryogenic treatment (optional): −80°C × 1h to eliminate retained austenite
3. Quality Inspection and Standards
3.1 Key Inspection Metrics
| Inspection Item | Method | Standard | Typical Acceptance |
|---|---|---|---|
| Effective CHD | Microhardness (HV1) | ISO 2639 | CHD ≥2.0mm |
| Surface Hardness | HRC or HV | ISO 6508 | 58–63 HRC |
| Core Hardness | HRC | — | 30–42 HRC |
| Lead Tolerance | Gear Measuring Center | DIN 3962 / ISO 1328 | ≤0.02mm (Grade 6) |
| Profile Tolerance | Gear Measuring Center | DIN 3962 / ISO 1328 | ≤0.015mm (Grade 6) |
| Surface Carbon | Layer-by-layer analysis | GB/T 9450 | 0.75–0.95%C |
| Retained Austenite | XRD | — | ≤15% |
| Microstructure | Optical microscopy | ISO 13284 / GB/T 25744 | Grades 1–4 (martensite + fine carbides) |
3.2 Common Defects and Solutions
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Network Carbides | Excessive carbon potential during diffusion | Reduce diffusion carbon potential to 0.75–0.80%C |
| Surface Decarburization | Poor atmosphere control | Check oxygen probes; methanol + N₂ protection |
| Distortion Out of Tolerance | Uneven cooling / stress concentration | Optimize fixturing, add pre-deformation compensation |
| Insufficient Hardness | Quench temperature too low | Raise to 840–860°C; check quench oil aging |
💡 Practical Insight: Geyon Transmission has long supplied precision-machined gears for wind power drivetrains. For the correlation between gear precision grades and stock allowance for blanks, refer to our spur gear product guide.
4. References
- ISO 2639:2002 — Determination of Effective Case Hardening Depth
- ISO 1328-1:2013 — Cylindrical Gears — Accuracy System
- GB/T 25744-2010 — Metallographic Inspection of Carburized and Quenched Steels
- AGMA 2001-D04 — Gear Material and Heat Treatment Standards
- “Deformation Control for Gear Heat Treatment”, China Machine Press, 2021
Compiled from daily technical learning. For gear selection assistance or process consultation, visit Geyon Transmission.
Tags: Gear Heat Treatment · Deep Case Carburizing · Deformation Control · Precision Manufacturing