Gear Carburizing & Quenching: Parameters and Defect Prevention

Summary: Among the heat treatments applied to gearing, carburizing followed by quenching remains the workhorse. A wear-resistant outer layer measuring 58–62 HRC is produced while the interior stays ductile at 30–42 HRC. Four variables govern the outcome — furnace temperature (920–950°C), atmosphere carbon level (0.75–1.05%C), quench start temperature (820–860°C) and tempering heat (160–200°C). Shop records indicate that process drift accounts for more than 80% of failures seen in carburized gears.

1. How the Process Works

Carburizing plus quenching is a two-part thermal cycle. The gear sits in an atmosphere supplying carbon at a temperature above the austenitizing threshold, carbon migrates inward from the surface, and a fast cool-down then converts the enriched layer into high-carbon martensite.

Working definition: A thermochemical treatment that changes surface chemistry — carbon rises from 0.15–0.25%C in the interior to 0.70–1.05%C at the skin — after which quenching delivers the hardening effect.

Stage sequence

• Cracking: Feedstock gas (natural gas, propane or kerosene) breaks down at elevated temperature, freeing reactive carbon.
• Pick-up: Those reactive atoms are taken into the austenite lattice at the gear skin.
• Inward migration: A concentration slope develops as carbon travels progressively deeper.

2. Parameter Window and Consequences

• Furnace temperature: 920–950°C
  • Consequence for the gear: a 10°C rise lifts diffusion speed roughly 20%, yet coarsening of grains becomes more likely
  • Target accuracy: ±5°C
• Atmosphere carbon level: 0.75–1.05%C
  • Consequence for the gear: carbon enrichment proceeds faster at higher values, though carbide networks may appear
  • Target accuracy: ±0.05%C
• Quench start temperature: 820–860°C
  • Consequence for the gear: governs how much austenite stays untransformed and the shape of the martensite
  • Target accuracy: ±10°C
• Tempering heat: 160–200°C
  • Consequence for the gear: hardness falls roughly 1–2 HRC for every 10°C added
  • Target accuracy: ±5°C
• Depth of hardened case: 0.3–1.5 mm
  • Consequence for the gear: deeper cases resist contact fatigue better
  • Target accuracy: ±0.1mm

How the variables interact

Carbon level and case depth do not scale linearly. Taking 920°C as the reference point:

• During boost with 1.05%C, the layer advances about 0.15–0.20 mm each hour.
• During diffusion with 0.80%C, growth slows to roughly 0.10–0.12 mm per hour.

A useful approximation: depth grows with the square root of time — depth ≈ K × √t, with K being the temperature coefficient (about 0.45 when the furnace runs at 920°C) and t the boost duration measured in hours.

3. Process Routes in Common Use

Available carburizing routes

1. Atmosphere (gas) carburizing — the mainstream choice

   • Atmosphere: propane with a carrier gas from cracked methanol, or a nitrogen–methanol mix
   • Strengths: carbon level can be held tightly; works well for repeating batches
   • Dominant route across precision gear drive systems

2. Low-pressure (vacuum) carburizing

   • Atmosphere: acetylene or ethylene
   • Strengths: no internal oxidation at all; suited to demanding gear quality
   • Preferred for custom gear manufacturing

3. Salt bath carburizing

   • Atmosphere: cyanide-based salts
   • Strengths: rapid carbon pick-up, but environmental handling is difficult; the route is being retired

4. Defects, Root Causes and Remedies

4.1 Case depth outside specification

• Root cause: drift in time or temperature, or carbon level wandering
• Remedy: run an oxygen probe feeding a carbon controller in closed loop; verify with foil shims at intervals

4.2 Carbide networks

• Root cause: carbon level pushed above 1.10%C, or too little time allowed for diffusion
• Remedy: cap the boost at 1.05%C and schedule a sufficient diffusion period

4.3 Oxidation beneath the surface

• Root cause: too much oxygen in the furnace gas (dew point not held)
• Remedy: keep dew point at −30°C or below; supply nitrogen of high purity

4.4 Shape change beyond limits

• Root cause: non-uniform cooling after quench, or a gear form that is not symmetric
• Remedy: improve fixture layout; apply press-quenching for precision shaft components

4.5 Hardness falling short

• Root cause: quench started too cold, or the cooling rate was too slow
• Remedy: verify oil temperature (60–80°C) and confirm the agitator is working

5. Inspection and Acceptance

Tight control during carburizing has to be planned together with the wider gear machining process sequence.

Shop-floor Guidance

1. Carbon level checks are non-negotiable: deposits build up on oxygen probes and readings drift. Verify with shim stock every week; if the reading is off by more than ±0.05%C, correct it before running again.
2. Track quench oil condition: cooling performance falls away once oil has seen six months of service or 100+ tons of parts. Run a cooling-curve test each quarter.
3. Fixturing affects the result: leave at least 10mm between adjacent gears so gas can circulate evenly. Large gears belong near the middle of the load, away from the fan.
4. Add a pre-oxidation step: holding parts at 450–550°C for 30–60 minutes before carburizing improves case consistency noticeably and lowers the chance of internal oxidation.

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)
• PinyiGear internal process database (2025 edition)

Appendix: Additional Process Data

The content below consolidates tables and figures carried over from earlier same-topic articles (former ids 29, 35, 38, 41, 42, 43, 46). All numbers, units, grade designations and standard numbers are unchanged.

A1. Carburizing media compared

• Gas carburizing: propane with carrier gas
  • Temperature band: 920–950°C
  • Layer growth (mm/h): 0.15–0.35
  • Where it fits: batch work, gear carburizing services
• Vacuum carburizing: acetylene
  • Temperature band: 950–1050°C
  • Layer growth (mm/h): 0.25–0.50
  • Where it fits: high-precision gears, internal rings
• Salt bath carburizing: cyanide-based salts
  • Temperature band: 900–930°C
  • Layer growth (mm/h): 0.20–0.40
  • Where it fits: small-module gears

A2. Effective case depth against gear module

A3. What governs distortion

• Before carburizing: normalizing together with a high-temperature temper cuts residual machining stress and can lower quench-related shape change by 30%–50%
• Quench start temperature: sitting 30–50°C above Ac3 works best; going hotter raises the risk of distortion
• Cooling medium: hot oil held at 80–120°C yields 40%–60% less distortion than oil at room temperature
• Load arrangement: stand gears upright or hang them from purpose-made fixtures; do not stack them

A4. A case from the shop floor

Note: On a recent m=5 spiral bevel gear job with a DIN 6 accuracy requirement, the route chosen was vacuum carburizing combined with martempering. Three tuning iterations brought shape deviation inside 0.08 mm and the first inspection passed 92% of parts.

A5. Three-stage schedule: time split and heating rate

• Stage one — heat-up and equalization: 850–930°C
  • Share of cycle: 15–20%
  • Control point: heating no faster than 100°C/h
• Stage two — boost: 920–950°C
  • Share of cycle: 50–60%
  • Control point: carbon level 1.05-1.20%C; duration taken from the case depth formula
• Stage three — diffusion: 880–920°C
  • Share of cycle: 20–30%
  • Control point: carbon level eased to 0.80-0.95%C so the gradient flattens

A6. Depth formula and PinyiGear coefficients

Effective case depth ( DS = k \times \sqrt{t} )

Figures from PinyiGear production:

• Boosting at 920°C: DS(mm) ≈ 0.65 × √t(h)
• Boosting at 940°C: DS(mm) ≈ 0.78 × √t(h)

Illustration: reaching DS=1.2mm takes about 3.4h of boost at 920°C, whereas 940°C brings it down to roughly 2.4h — yet a 20°C step-up carries a meaningfully higher chance of grain coarsening.

A7. Quench medium, agitation and core hardness by steel grade

A8. Microstructure acceptance

Microstructure Requirements (per ISO 6336 / GB/T 3480)

A9. Carbon verification, cold treatment and pre-oxidation

1. Check atmosphere carbon every shift: oxygen probes need monthly calibration, and a steel foil shim test (±0.05%C) should confirm the reading at the start of each shift — without this, no other parameter can be trusted.
2. Cold treatment pays off on precision work: when DS reaches 1.5mm or more, a −80°C × 2h soak after quenching brings retained austenite down from 15–20% to 5–8%, which markedly improves dimensional behaviour.
3. Do not skip pre-oxidation: a 380–420°C × 30min hold creates Fe₃O₄ nucleation sites across the surface, lifting carburizing uniformity by 30% or more.
4. Distortion is decided early: hardness scatter after forging and normalizing (keep within ≤10HB) influences the final shape change more strongly than anything done in the carburizing furnace.

A10. Rule-of-thumb case depth selection

Case Depth Selection Rule of Thumb

$$d = (0.1 \sim 0.2) \times m_n$$

Here (d) stands for effective case depth (mm) and (m_n) for the gear normal module (mm). Illustration: a module 4 gear calls for a case of 0.4–0.8 mm.

A11. Carburizing routes compared, plasma included

A12. Marquenching schedule

A13. Orientation in the furnace

• Teeth pointing up: gas reaches the root more easily, though oil drainage during quenching is poorer
• Teeth pointing down (suspended): quenching is more even, but the starting carbon level must be set higher

A14. The four parameters that matter most

• Effective case depth (CHD): 0.3–2.5 mm
  • Effect on the gear: too thin invites contact fatigue spalling; too thick costs impact toughness
• Carbon concentration at the surface: 0.7%–1.0% C
  • Effect on the gear: a low value leaves hardness short; a high value leaves retained austenite and wears faster
• Quench start temperature: 800–860°C (direct quench)
  • Effect on the gear: shapes the martensite and how residual stress is distributed
• Tempering heat: 160–200°C
  • Effect on the gear: eases stress, settles the structure and preserves hardness

A15. Module, CHD and carburizing time

For the lower bound, use CHD ≈ 0.2 × (Module + 1).

A16. Quenching temperature and cooling medium

Quench temperature is set at Ac3 + 30~50°C, which in practice lands around 810~840°C. Medium options:

• Oil quenching: works for gears of module ≤ 8, with distortion that can be managed
• Salt bath or die quenching: the choice for thin-wall or large precision gears, holding shape deviation within 0.05mm

A17. Defect countermeasures and loading practice

Excessive Distortion

• Cause: heating that is not even, cooling that is too fast, or gear geometry without symmetry
• Countermeasure: preheat at 500°C for 30 min, then quench under die pressure

Surface Hardness Below 58HRC

• Cause: quench started too cold or cooling capacity lacking
• Countermeasure: verify oil temperature (optimal: 40~80°C) and swap out degraded media

Case Delamination

• Cause: the carbon slope through the transition zone is too abrupt
• Countermeasure: lengthen the diffusion stage by 1~2h so carbon declines gradually

Practical Takeaways

PinyiGear shop records indicate that how the furnace is loaded determines about 60% of carburizing success. Placing gears with tooth faces upward at even spacing (at least 10mm apart) lets gas travel freely and avoids the familiar problem of a thin case at the root combined with excess carbon at the tip. Before committing to a batch, validate the process on a first piece using microhardness traverse testing.

A18. Parameters for deep-case carburizing (12~50 h)

• Furnace temperature: 920–950°C
  • Effect: a 10°C rise speeds diffusion by about 20%
• Carbon level: boost at 1.0–1.2%C, diffuse at 0.75–0.85%C
  • Effect: going too high produces carbide networks
• Duration: 12–50 hours (depth-dependent)
  • Effect: depth tracks √t
• Quench temperature: 820–860°C
  • Effect: either direct or reheat quenching
• Quench oil temperature: 60–120°C
  • Effect: sets the pace of martensite transformation
• Tempering temperature: 160–200°C
  • Effect: low-temperature tempering eases stress

A19. Material grades and the CHD/module ratio

Grades typically specified:

• 18CrNiMo7-6 — the mainstay for wind turbine gearboxes
• 20CrMnTi — used widely for general transmission gears
• 17CrNiMo6 — chosen where fatigue life must be very high

In practice, CHD ≈ (0.15–0.25) × module. A module m=10 gear therefore calls for CHD = 1.5–2.5mm

A20. Pre-deformation compensation and wind-turbine data

Pre-Deformation Compensation

Building a reverse shape into the design is the cheapest way to control the outcome:

• Lead pre-crowning: 0.01–0.05mm of reverse compensation derived from the L/D ratio
• Tooth space shrinkage compensation: 0.02–0.04mm of negative tolerance applied to thin-wall gears

Note: On an m=12 wind turbine gear, three trial runs exposed a taper-shaped lead deviation. Introducing 0.025mm of reverse taper compensation at the rough machining stage cut deformation from 0.06mm to 0.015mm.

A21. Quenchant options and distortion risk

A22. Quenching, tempering and cold treatment for deep cases

Note: Gears with CHD ≥3mm are best run through martempering. Cycle time rises by around 30%, yet profile distortion drops by more than half. Accumulated strain data from PinyiGear’s large-gear carburizing work is available on our capabilities page.

Tempering and dimensional stability

A low-temperature temper (160–200°C) clears roughly 60% of quench stress, though it leaves retained austenite largely intact. Where Grade D precision or better is required:

• First temper: 180°C × 3h
• Second temper: 170°C × 4h (return to room temperature between the two)
• Optional deep cold soak: −80°C × 1h to remove retained austenite

A23. Inspection metrics for deep cases

A24. Deep-case defects and how to answer them

• Carbide networks: carbon level too high while diffusing
  • Fix: drop the diffusion carbon level to 0.75–0.80%C
• Decarburization at the surface: atmosphere poorly regulated
  • Fix: inspect oxygen probes; protect with methanol + N₂
• Distortion beyond tolerance: cooling that is uneven, or concentrated stress
  • Fix: improve fixturing and work pre-deformation compensation into the design
• Hardness below target: quench temperature set too low
  • Fix: bring it up to 840–860°C and check whether quench oil has aged

A25. Boost, diffusion and quench stages

• Boost stage: 930±10°C with carbon level 1.10-1.20%C
  • Watch: case growth of 0.15-0.25mm per hour
• Diffusion stage: 920±10°C with carbon level 0.80-0.90%C
  • Watch: keep the carbon slope smooth and prevent cementite networks
• Quench stage: 830–860°C (direct quench)
  • Watch: cooling must beat the critical martensitic rate (roughly 50°C/s in oil)

A26. CHD against module with hardness gradient requirements

A27. Forms of distortion and how to counter them

Distortion Control

Shape change after carburizing remains a stubborn problem in precision gear transmission work. The forms seen most often:

Countermeasures:

1. Normalize plus temper before machining — this refines grains and evens out internal stress
2. Quench from just under Ac3 (intercritical) so thermal stress stays lower
3. Hold the geometry with die pressure or fixtures at the quench station
4. Leave stock for a finish grinding operation after heat treatment

Note: Across PinyiGear’s precision gear machining work, inserting a stress relief anneal at 650°C for 2h ahead of carburizing has cut distortion on C22 (8620) gears by over 40%. The annealing temperature has to sit above whatever the previous thermal step used — roughly 100°C higher than the carburizing temperature — or the stress relief will not be complete.

A28. Routine checks and acceptance limits

A29. Decarburization, coarse martensite and non-martensitic layers

Defects and Their Solutions

Note: Among carburizing variables, surface carbon is the one most often underrated. A value above 1.0%C precipitates a carbide network that shortens contact fatigue life, while a value under 0.7%C leaves hardness too low. For custom gear jobs we run layer-removal carbon analysis on every batch and pair it with full CMM distortion checks, which keeps delivered quality consistent.

A30. Low-pressure vacuum versus gas carburizing

A31. Where LPVC is heading

Application Outlook for LPVC

As electrified drivetrains and precision gear drives call for longer fatigue life, low-pressure vacuum carburizing (LPVC) keeps moving out of aerospace and into premium industrial gearing. Projections put LPVC adoption in passenger vehicle transmission gears above 35% by 2028.

A32. Distortion types, ranges and how hard they are to fix

A33. Carburizing steels: distortion tendency and Ms data

• 20CrMnTi
  • Distortion Tendency: Moderate
  • Typical Applications: Automotive transmission gears, good cost-performance
• 20CrMo
  • Distortion Tendency: Low
  • Typical Applications: Small-to-medium module gears, distortion-sensitive parts
• 17CrNiMo6
  • Distortion Tendency: Low
  • Typical Applications: Heavy-duty gears, wind power & construction machinery
• 8620H
  • Distortion Tendency: Moderate
  • Typical Applications: Export standard parts, narrow hardenability band control

A34. Picking material for large thin-walled ring gears

Note: Where ring gears have thin walls and diameters beyond 400mm, 17CrNiMo6 or 20CrMo should come first. Their alloying gives a lower Ms point (martensite start around 300~330°C), so transformation stress relaxes more fully — roughly 30% better shape control than 20CrMnTi.

A35. Core carburizing variables

A36. Choosing a quenchant and setting agitation speed

Quenching Process Control

Cooling rate, and therefore distortion, follows directly from the medium chosen:

• Martempering oil (preferred): 80~120°C, a good match for gear steels and alloy carburizing steels
• Fast quenching oil: used on simple shapes only, with greater risk of distortion
• Austempering: 220~320°C salt bath, distortion is minimal but the cycle runs longer

Note: Agitation is best held between 0.5~1.0m/s. Below 0.3m/s cooling is inadequate and hardness suffers; above 1.5m/s cooling becomes uneven and bore shrinkage climbs by 40%~60%. VFD-controlled agitation lets the setting be matched to gear cross-section.

A37. Designing distortion allowance and compensating in advance

Distortion Allowance Design

FEA modelling predicts how the part will move, and the reverse of that movement is machined in at the gear cutting stage. For modules 3~6, PinyiGear runs an ANSYS-based carburizing-quenching distortion simulation module:

Pre-deformation Compensation

A38. How distortion is inspected and how often

• Bore dimension: air gauge or CMM
  • Accuracy: ±0.002mm
  • Sampling Rate: 100%
• Profile / lead: gear measuring center (Klingelnberg/Gleason)
  • Accuracy: ±0.003mm
  • Sampling Rate: 10%~20%
• Face runout: dial indicator
  • Accuracy: ±0.005mm
  • Sampling Rate: 100%
• Case depth: metallographic or micro-hardness
  • Accuracy: ±0.05mm
  • Sampling Rate: 1~3 pcs per batch

A39. Three levers that work best for distortion

Practical Summary

Years of tracking on the shop floor and iterating the process lead PinyiGear to single out three levers for controlling distortion:

1. Control at the source: write a hardenability band of J9≤4HRC into the material purchase spec so heat-to-heat scatter disappears.
2. Even out temperature: move from atmosphere carburizing plus oil quenching to vacuum carburizing with high-pressure gas quenching (10~20bar) — shape change falls by more than 50%.
3. Design for the movement: maintain a company-level distortion database and feed its history into new gear designs, so the loop from design through manufacturing closes.