Gear Carburizing & Quenching: Parameters and Defect Prevention
Learn gear carburizing and quenching principles, parameter control, and defect prevention, covering 920-950°C carburizing and 58-62 HRC surface hardness.
Gear Carburizing & Quenching: Principles, Parameter Control, and Defect Prevention
Core Conclusion: Carburizing and quenching is the most critical and widely used heat treatment process for gears. It achieves a high surface hardness of 58–62 HRC with excellent wear resistance while maintaining a tough core at 30–42 HRC. The key to success lies in precise control of four parameters: carburizing temperature (920–950°C), carbon potential (0.75–1.05%C), quenching temperature (820–860°C), and tempering temperature (160–200°C). Field data shows over 80% of carburized gear failures are directly linked to process parameter deviations.
1. Fundamentals of Carburizing and Quenching#
Carburizing and quenching is a combined heat treatment process where gears are heated in a carbon-rich atmosphere above the austenitizing temperature, allowing carbon atoms to diffuse into the surface layer, followed by rapid cooling (quenching) to form a high-carbon martensitic structure.
Definition: Carburizing is a thermochemical surface hardening process that alters the chemical composition of the gear surface layer (raising carbon content from 0.15–0.25%C in the core to 0.70–1.05%C at the surface) and then hardens it through quenching.
Three Process Stages#
- Decomposition stage: Carburizing media (natural gas, propane, or kerosene) cracks at high temperature to produce active carbon atoms
- Adsorption stage: Active carbon atoms are absorbed by the austenite at the gear surface
- Diffusion stage: Carbon atoms diffuse from the surface inward, forming a concentration gradient
2. Key Process Parameters and Their Effects#
| Parameter | Typical Range | Effect on Gear Performance | Recommended Precision |
|---|---|---|---|
| Carburizing temperature | 920–950°C | +10°C increases diffusion rate ~20%, but raises grain growth risk | ±5°C |
| Carbon potential | 0.75–1.05%C | Higher CP accelerates carburizing but risks network carbide formation | ±0.05%C |
| Quenching temperature | 820–860°C | Affects retained austenite volume and martensite morphology | ±10°C |
| Tempering temperature | 160–200°C | Each +10°C reduces hardness by ~1–2 HRC | ±5°C |
| Case depth | 0.3–1.5 mm | Contact fatigue resistance improves with greater depth | ±0.1mm |
Parameter Correlation Analysis#
The relationship between carbon potential and case depth is nonlinear. At 920°C:
- Boost phase (CP 1.05%C): penetration rate ~0.15–0.20 mm/h
- Diffusion phase (CP 0.80%C): penetration rate ~0.10–0.12 mm/h
Engineering rule of thumb: Case depth ≈ K × √t, where K = temperature coefficient (K≈0.45 at 920°C) and t = boost time in hours.
3. Typical Process Types#
Commonly Used Carburizing Methods#
- Gas Carburizing (most common)
- Media: Propane + carrier gas (methanol cracked gas) or N₂-methanol atmosphere
- Features: Excellent carbon potential control, suitable for batch production
- Highest usage share in precision gear drive systems
- Vacuum Carburizing
- Media: Acetylene or ethylene
- Features: Zero internal oxidation, ideal for high-quality gears
- Best suited for custom gear manufacturing
- Salt Bath Carburizing
- Media: Cyanide salts
- Features: Fast carburizing speed but significant environmental concerns; being phased out
4. Common Defects and Prevention#
4.1 Incorrect Case Depth (too deep or too shallow)#
- Cause: Time/temperature control deviation or unstable carbon potential
- Solution: Closed-loop control with oxygen probe + carbon controller; periodic foil calibration
4.2 Network Carbides#
- Cause: Excessively high carbon potential (>1.10%C) or insufficient diffusion time
- Solution: Keep boost-phase CP ≤ 1.05%C; allocate adequate diffusion time
4.3 Internal Oxidation#
- Cause: High oxygen content in furnace atmosphere (improper dew point control)
- Solution: Maintain furnace dew point ≤ −30°C; use high-purity nitrogen gas
4.4 Excessive Distortion#
- Cause: Uneven quench cooling or asymmetric gear design
- Solution: Optimize fixturing design; use precision shaft components press-quenching process
4.5 Insufficient Hardness#
- Cause: Low quenching temperature or inadequate cooling rate
- Solution: Check quench oil temperature (60–80°C) and agitation system
5. Quality Inspection Standards#
| Inspection Item | Standard Method | Acceptance Criteria |
|---|---|---|
| Case depth | GB/T 9450 / ISO 2639 | Effective case depth (CHD) |
| Surface hardness | GB/T 230.1 / ISO 6508 | 58–62 HRC |
| Core hardness | GB/T 230.1 | 30–42 HRC |
| Microstructure | GB/T 25744 / ISO 6336-5 | Martensite + minor retained austenite |
| Distortion | CMM measurement | Per gear accuracy grade |
| Surface carbon content | EPMA / spectroscopy | 0.70–1.05%C |
Precision control in gear carburizing must be integrated with the overall gear machining process workflow.
Practical Insights#
- Never skip carbon potential calibration: Oxygen probes accumulate carbon deposits over time, causing reading drift. Calibrate with shim stock weekly; correct immediately if deviation exceeds ±0.05%C.
- Manage quench oil age: After 6 months or 100+ tons of throughput, quench oil cooling characteristics degrade. Test the cooling curve quarterly.
- Loading technique matters: Maintain ≥10mm spacing between gears for uniform atmosphere circulation. Place large gears in the furnace center, not near the fan side.
- Pre-oxidation step: A 30–60 minute pre-oxidation at 450–550°C before carburizing significantly improves case uniformity and reduces internal oxidation risk.
References#
- GB/T 9450-2018 — Determination and verification of carburized hardened case depth for steel parts
- ISO 6336-5:2016 — Calculation of load capacity of spur and helical gears — Part 5: Strength and quality of materials
- “Gear Heat Treatment Process Standards Manual” (China Machine Press)
- Geyontech internal process database (2025 edition)
Appendix: Consolidated Supplementary Process Data (merged from duplicate articles)#
The tables and figures below are reproduced verbatim from the duplicate same-topic articles (former ids 29, 35, 38, 41, 42, 43, 46). Numbers, units, grade designations and standard numbers are unchanged.
A1. Comparison of Three Carburizing Media (from former id 29)#
| 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 |
A2. Effective Case Depth (ECD) vs. Gear Module (from former id 29)#
| 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 |
A3. Key Factors Controlling Distortion (from former id 29)#
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
A4. Shop-floor Experience Data (from former id 29)#
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%.
A5. Three-Phase Process: Time Share & Heating Rate (from former id 35)#
| Phase | Temperature Range | Time Share | Key Parameters |
|---|---|---|---|
| Heat-up/Equalization | 850-930°C | 15-20% | Heating rate ≤100°C/h |
| Boost (Carburizing) | 920-950°C | 50-60% | CP 1.05-1.20%C, time from case depth formula |
| Diffusion | 880-920°C | 20-30% | CP reduced to 0.80-0.95%C, gradient homogenization |
A6. Case Depth Formula & Geyontech Coefficients (from former id 35)#
Effective case depth ( DS = k \times \sqrt{t} )
Geyontech production data:
- At 920°C boost: DS(mm) ≈ 0.65 × √t(h)
- At 940°C boost: DS(mm) ≈ 0.78 × √t(h)
Example: For DS=1.2mm, 920°C requires ~3.4h boost; 940°C needs only ~2.4h — but every 20°C increase raises grain coarsening risk significantly.
A7. Quench Media, Agitation and Core Hardness by Steel Grade (from former id 35)#
| Steel Grade | Quench Medium | Agitation | Core Hardness | Distortion |
|---|---|---|---|---|
| 20CrMnTi | Fast quench oil | High (1500rpm) | 35-42HRC | Medium |
| 20CrNi2Mo | Martempering oil | Medium (800rpm) | 38-45HRC | Minimal |
| 8620H | Hot oil (120°C) | Medium | 32-38HRC | Low |
A8. Microstructure Requirements (from former id 35)#
2.3 Microstructure Requirements (per ISO 6336 / GB/T 3480)#
- Martensite grade: ISO Class 3-5 (fine acicular martensite preferred)
- Retained austenite (RA): ≤15% (precision gears ≤10%)
- Carbides: No network carbides; isolated globular carbides (≤2μm) permissible
- Core ferrite: ≤1% (volume fraction)
A9. CP Verification, Cryogenic Treatment and Pre-oxidation Data (from former id 35)#
4. Practical Insights#
- Daily carbon potential verification is the #1 priority: Calibrate oxygen probes monthly and validate with steel foil shim carbon test (±0.05%C) every shift — otherwise all process parameters lose meaning.
- Cryogenic treatment is essential for high-precision gears: For DS≥1.5mm gears, -80°C×2h cryogenic treatment after quenching reduces RA from 15-20% down to 5-8%, dramatically improving dimensional stability.
- Pre-oxidation should not be overlooked: 380-420°C×30min pre-oxidation forms Fe₃O₄ nucleation sites on the gear surface, improving carburizing uniformity by 30%+.
- Distortion control starts at the blank: Post-forging normalization hardness uniformity (≤10HB scatter) has a greater impact on final distortion than the carburizing process itself.
A10. Case Depth Rule of Thumb (d = 0.1~0.2 × mn) (from former id 38)#
Case Depth Selection Rule of Thumb#
$$d = (0.1 \sim 0.2) \times m_n$$
Where (d) = effective case depth (mm) and (m_n) = gear normal module (mm). Example: for module 4 gears, recommended case depth is 0.4–0.8 mm.
A11. Carburizing Process Comparison (incl. plasma carburizing) (from former id 38)#
| Process | Temp. | Carbon Control | Distortion | Batch Size | Typical Use |
|---|---|---|---|---|---|
| Gas carburizing (pit furnace) | 920–950°C | ±0.05% C | Medium | Small–medium | Small–medium module gears |
| Low-pressure vacuum carburizing | 900–1050°C | ±0.02% C | Excellent | Multi-variety | Precision transmission gears |
| Salt bath carburizing | 900–950°C | ±0.08% C | Poor | Large batch simple parts | Low-precision applications |
| Plasma/ion carburizing | 850–950°C | ±0.03% C | Excellent | Single/small batch | Aerospace gears |
A12. Recommended Marquenching Parameters (from former id 38)#
| Step | Temperature Range | Holding Time | Purpose |
|---|---|---|---|
| 1. Austenitizing | 920–950°C | 30–60 min | Uniform austenite |
| 2. Hot oil quench | 150–200°C | 5–15 min | Temperature equalization |
| 3. Air cool to room temp | — | — | Martensite transformation |
| 4. Tempering | 160–200°C | 2–4 h | Stress relief + structure stabilization |
A13. Furnace Placement and Simulation Tools (from former id 38)#
- Teeth facing upward: Better gas flow into root area, but poorer oil flow during quench
- Teeth facing downward (suspended): More uniform quenching, but requires higher initial carbon potential
A14. Critical Process Parameter Triangle (from former id 38)#
| Parameter | Typical Range | Impact on Gear Performance |
|---|---|---|
| Effective case depth (CHD) | 0.3–2.5 mm | Too shallow → contact fatigue spalling; too deep → reduced impact toughness |
| Surface carbon concentration | 0.7%–1.0% C | Too low → insufficient hardness; too high → retained austenite, lower wear resistance |
| Quenching temperature | 800–860°C (direct quench) | Affects martensite morphology and residual stress distribution |
| Tempering temperature | 160–200°C | Relieves stress, stabilizes microstructure, maintains high hardness |
A15. Module–CHD–Carburizing Time Table and Empirical Formula (from former id 41)#
| 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).
A16. Quenching Temperature and Cooling Medium (Ac3 + 30~50°C) (from former id 41)#
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
A17. Defect Countermeasures and Furnace Loading Practice (from former id 41)#
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#
Geyontech’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.
A18. Typical Deep-Case Carburizing Parameters (12~50 h) (from former id 42)#
| 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 |
A19. Recommended Materials and CHD/Module Ratio (from former id 42)#
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
A20. Pre-deformation Compensation and Wind-Turbine Gear Data (from former id 42)#
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.
A21. Quenchant Selection and Distortion Risk (from former id 42)#
| 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 |
A22. Deep-Case Quenching, Tempering and Cryogenic Practice (from former id 42)#
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 Geyontech’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
A23. Deep-Case Inspection Metrics and Standards (from former id 42)#
| 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) |
A24. Deep-Case Common Defects and Solutions (from former id 42)#
| 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 |
A25. Boost/Diffusion/Quenching Stage Parameters and Cooling Rate (from former id 43)#
| Stage | Parameter Range | Key Control Indicator |
|---|---|---|
| Boost (Enrichment) | 930±10°C, CP 1.10-1.20%C | Case depth growth rate 0.15-0.25mm/h |
| Diffusion | 920±10°C, CP 0.80-0.90%C | Smooth carbon gradient, avoid cementite network |
| Quenching | 830-860°C (direct quench) | Cooling rate > critical martensitic rate (~50°C/s in oil) |
A26. CHD vs. Module Table with Hardness Gradient Requirements (from former id 43)#
| Gear Module (m) | Recommended CHD (mm) | Hardness Gradient |
|---|---|---|
| m ≤ 3 | 0.3-0.6 | Surface 58-62 HRC |
| 3 < m ≤ 6 | 0.6-1.2 | Transition zone ≤ 0.3mm |
| 6 < m ≤ 10 | 1.2-2.0 | Core 30-42 HRC |
| m > 10 | 1.8-2.5 | Carbon gradient ≤ 0.10%/0.1mm |
A27. Distortion Modes and Control Strategies (from former id 43)#
2.2 Distortion Control#
Carburizing distortion is a persistent challenge in precision gear transmission manufacturing. Common distortion modes:
- Ovality: Asymmetric gear sections under uneven quench cooling
- Taper: Axial shrink variation from cross-section differences along gear length
- Tooth profile distortion: Phase transformation stress from uneven carbon distribution
Control Strategies:
- Pre-heat treatment (normalizing + tempering) refines grain structure and homogenizes stress
- Intercritical quenching (slightly below Ac3) reduces thermal stress
- Press quenching or fixture quenching constrains geometric deformation
- Machining allowance with subsequent precision grinding
Practical Insight: In our precision gear machining practice, adding a stress relief annealing at 650°C/2h before carburizing reduces C22 (8620) gear distortion by over 40%. The key is that the annealing temperature must exceed the previous heat treatment temperature (approximately 100°C above the carburizing temperature); otherwise, stress relief is incomplete.
A28. Routine Inspection Items and Acceptance Criteria (from former id 43)#
| Inspection Type | Method | Acceptance Criteria |
|---|---|---|
| Effective case depth | Micro-Vickers hardness (HV1) | CHD ±0.1mm |
| Surface hardness | Rockwell HRC | 58-62 HRC |
| Microstructure | Optical microscopy ×500 | Martensite grade 1-5, RA ≤ 20% |
| Surface carbon content | Layer removal / OES | 0.75-0.95%C |
| Gear distortion | Gear measuring center | Runout ≤ 0.03mm, lead error ≤ 0.015mm |
A29. Decarburization, Coarse Martensite and Non-Martensitic Layer Remedies (from former id 43)#
3.2 Common Defects and Solutions#
- Surface decarburization: Low CP or poor atmosphere circulation → Raise CP setpoint, check O₂ probe accuracy
- Coarse martensite: Excessive quench temperature → Verify TC calibration, lower by 10-15°C
- Non-martensitic surface layer: Internal oxidation → Control O₂ content < 50ppm
- Insufficient core hardness: Low hardenability or slow cooling → Change quenchant or increase agitation
Practical Insight: Surface carbon concentration is the most underestimated control parameter in carburizing. Excess (>1.0%C) causes carbide network precipitation, significantly reducing contact fatigue life; too low (<0.7%C) yields insufficient surface hardness. In our custom gear projects, we insist on layer-removal carbon analysis per batch, combined with CMM full inspection for distortion — ensuring consistent delivery quality.
A30. Vacuum Carburizing vs. Gas Carburizing (from former id 43)#
| Parameter | Gas Carburizing (GP) | Vacuum (Low-Pressure) Carburizing (LP) |
|---|---|---|
| CP accuracy | ±0.05%C | ±0.03%C |
| Surface oxidation | Possible mild IGO | Virtually none |
| Distortion | Baseline | 30-50% reduction |
| Cycle time (per batch) | 8-14h | 6-10h |
| Equipment cost | Low-Medium | High |
A31. Application Outlook for LPVC (from former id 43)#
4.2 Application Outlook for LPVC#
With new energy and precision gear drives demanding ever-higher fatigue life, low-pressure vacuum carburizing (LPVC) is expanding from aerospace into premium industrial gears. By 2028, LPVC penetration in passenger vehicle transmission gears is projected to exceed 35%.
A32. Distortion Types, Typical Ranges and Correction Difficulty (from former id 46)#
| Distortion Type | Typical Range (mm) | Affected Precision Aspect | Correction Difficulty |
|---|---|---|---|
| Bore shrinkage | 0.02~0.15 | Assembly fit precision | Moderate |
| Tooth profile distortion | 0.01~0.05 | Meshing/transmission quality | High |
| Lead/helix deviation | 0.02~0.08 | Contact load distribution | High |
| Face runout/warping | 0.05~0.20 | Axial locating precision | Low |
A33. Carburizing Steel Grades: Distortion Tendency and Ms Data (from former id 46)#
| Steel Grade | Hardenability | Distortion Tendency | Typical Applications |
|---|---|---|---|
| 20CrMnTi | Moderate | Moderate | Automotive transmission gears, best cost-performance |
| 20CrMo | Low-Moderate | Low | Small-to-medium module gears, distortion-sensitive parts |
| 17CrNiMo6 | High | Low | Heavy-duty gears, wind power & construction machinery |
| 8620H | Moderate | Moderate | Export standard parts, narrow hardenability band control |
A34. Material Selection Insight for Large Thin-Walled Ring Gears (from former id 46)#
Practical Insight: For thin-walled ring gears with diameters >400mm, prioritize 17CrNiMo6 or 20CrMo. Their alloy composition yields a lower Ms point (martensite start temperature ~300~330°C), enabling more complete transformation stress relaxation and roughly 30% better distortion controllability compared to 20CrMnTi.
A35. Critical Carburizing Parameters (Temperature, CP, Case Depth, Cooling) (from former id 46)#
- Carburizing temperature: 920~950°C, recommended 930±5°C
- Carbon potential control: Boost stage 1.05~1.15%C, Diffusion stage 0.80~0.85%C
- Case depth selection: Based on module m, recommended 0.15×m~0.20×m
- Cooling method: Furnace cool to 840~860°C then direct quench — avoids re-austenitization
A36. Quenchant Selection and Agitation Speed Control (from former id 46)#
2.3 Quenching Process Control#
Quenching medium selection directly affects cooling rate and distortion:
- Martempering oil (recommended): 80~120°C, suitable for gear steels and alloy carburizing steels
- Fast quenching oil: For simple-geometry gears, higher distortion risk
- Austempering: 220~320°C salt bath, minimal distortion but longer cycle time
Practical Insight: Optimal agitation speed is 0.5~1.0m/s. Speeds below 0.3m/s cause insufficient cooling and low hardness; speeds above 1.5m/s increase cooling non-uniformity, raising bore shrinkage by 40%~60%. VFD-controlled agitation systems are recommended for adjusting based on gear cross-section.
A37. Distortion Allowance Design and Pre-deformation Compensation (from former id 46)#
3.1 Distortion Allowance Design#
Using FEA simulation to predict distortion trends, reverse distortions are pre-machined at the gear cutting stage. Geyontech employs an ANSYS-based carburizing-quenching distortion simulation module for module 3~6 gears:
- Bore: Pre-machine 0.05~0.15mm shrinkage allowance
- Lead: Pre-machine 0.02~0.05mm reverse helix correction
- Tooth thickness: Compensate per distortion simulation contour map
3.2 Pre-deformation Compensation#
- Die quenching: Apply axial pressure during quenching using a dedicated quench press — reduces face warping by 60%~80%
- Hot straightening: Straighten between quench and temper (150~200°C) to correct lead/helix deviation
- Sub-zero treatment: Deep cooling at -80~-120°C to transform retained austenite, improving dimensional stability
A38. Distortion Inspection Methods and Sampling Rates (from former id 46)#
| Inspection Item | Tool | Accuracy | Sampling Rate |
|---|---|---|---|
| Bore dimension | Air gauge / CMM | ±0.002mm | 100% |
| Profile / lead | Gear measuring center (Klingelnberg/Gleason) | ±0.003mm | 10%~20% |
| Face runout | Dial indicator | ±0.005mm | 100% |
| Case depth | Metallographic / Micro-hardness | ±0.05mm | 1~3 pcs per batch |
A39. Three Most Effective Distortion Control Levers (J9≤4HRC, 10~20 bar gas quench) (from former id 46)#
Practical Summary
Based on years of shop-floor tracking and process iteration, Geyontech identifies three most effective distortion control levers:
- Source control: Specify hardenability band J9≤4HRC at material procurement to eliminate heat-to-heat variation
- Temperature uniformity: Adopt vacuum carburizing + high-pressure gas quenching (10~20bar) to replace conventional atmosphere carburizing + oil quenching — distortion reduced by 50%+
- Pre-deformation design: Build a company-level distortion database; use historical data to guide new gear designs, closing the design-to-manufacturing loop