Deep Case Carburizing of Gears: Parameters, Distortion Control and Inspection

Summary: Once the effective case hardening depth passes 2mm, carburizing becomes the default thermal route for gears carrying heavy loads. Two process windows govern the result: 920–950°C for the furnace atmosphere temperature and 1.0–1.2%C for the boost carbon potential. Holding lead deviation within 0.02mm and profile deviation within 0.015mm requires three things working together — reverse-distortion allowance built into the blank, a quench medium matched to the section, and a controlled tempering schedule.

1. Keeping Distortion Under Control

Dimensional movement after carburizing and quenching is one of the most common rejection causes in gear heat treatment. It manifests as lead error, profile warping and out-of-round bores. Three measures are available to counteract it.

1.1 Building Reverse Distortion Into the Blank

Compensating at the drawing and rough-machining stage is the lowest-cost option.

• Lead pre-crowning: apply 0.01–0.05mm of opposite-direction compensation, with the amount chosen according to the length-to-diameter relationship
• Tooth space shrinkage offset: allow 0.02–0.04mm of negative tolerance where wall sections are thin

Field Note: A wind turbine gear with module 12 was run three times before a tapered lead pattern became visible. Adding 0.025mm of reverse taper during rough machining cut the measured movement from 0.06mm to 0.015mm.

1.2 Choosing the Quench Medium and Shaping the Cooling Curve

Field Note: Once CHD reaches 3mm or more, martempering is the recommended route. It extends the cycle by about 30%, yet profile movement can be reduced by over 50%. PinyiGear accumulates strain records from large-gear carburizing; details are available on our capabilities page.

1.3 How Tempering Affects Dimensional Stability

Holding at 160–200°C releases roughly 60% of the stress left by quenching, although retained austenite does not break down completely at that point. Where Grade D accuracy or tighter is required, two tempering passes are used:

• First pass: 180°C for 3h
• Second pass: 170°C for 4h, with a return to room temperature in between
• Optional cryogenic step: −80°C for 1h to remove retained austenite

2. How the Process Works

2.1 Defining Deep Case Carburizing

A carburizing treatment counts as deep case carburizing when the effective Case Hardening Depth (CHD) reaches 2mm or more. It is applied to gearboxes in wind turbines, construction machinery, mining equipment and similar heavy-duty drives. At austenitizing temperature, carbon diffuses into the tooth surface, creating a carbon-rich outer layer; the subsequent quench turns that layer into hard martensite while the core retains its ductility.

2.2 Process Variables and Their Effects

• Carburizing Temperature: 920–950°C
  • Raising the setpoint by 10°C lifts the diffusion rate by approximately 20%
• Carbon Potential: boost stage 1.0–1.2%C; diffuse stage 0.75–0.85%C
  • Too high a value leads to carbide networks
• Duration: 12–50 hours, depending on target depth
  • Depth grows roughly with the square root of time
• Quench Temperature: 820–860°C
  • Either direct quenching or a reheating step can be used
• Quench Oil Temp: 60–120°C
  • This setting influences how fast martensite forms
• Tempering Temp: 160–200°C
  • A low-temperature temper relieves residual stress

2.3 Selecting a Carburizing Steel

Three grades handle the majority of carburized gear applications:

• 18CrNiMo7-6 — the standard choice for wind turbine gearbox gearing
• 20CrMnTi — used across general transmission gears
• 17CrNiMo6 — specified where very high fatigue life is needed

A useful rule: effective CHD sits between 0.15 and 0.25 times the module. For m=10, the recommended CHD falls in the 1.5–2.5mm band.

3. Inspection and Acceptance

3.1 What Gets Measured

3.2 Reject Causes and Corrective Actions

• Carbide network present: the diffuse-stage carbon potential ran too high
  • Remedy: lower the diffuse setpoint into the 0.75–0.80%C range
• Decarburized surface layer: furnace atmosphere was not held correctly
  • Remedy: verify oxygen probe calibration and maintain methanol plus N₂ shielding
• Distortion beyond the drawing limit: cooling was uneven or stresses concentrated
  • Remedy: revise the fixturing arrangement and add reverse-distortion allowance
• Hardness below specification: the quench temperature was set too low
  • Remedy: bring it up to 840–860°C and check whether the quench oil has aged

Field Note: PinyiGear has supplied machined gears to wind power drivetrains for many years. The relationship between final gear accuracy and the stock left on the blank is described in our spur gear product guide.

4. Source Documents

1. ISO 2639:2002 — Determination of Effective Case Hardening Depth
2. ISO 1328-1:2013 — Cylindrical Gears — Accuracy System
3. GB/T 25744-2010 — Metallographic Inspection of Carburized and Quenched Steels
4. AGMA 2001-D04 — Gear Material and Heat Treatment Standards
5. Heat Treatment Distortion Management for Gearing, China Machine Press, 2021

Assembled from routine technical study notes. To discuss gear selection or a heat treatment problem, visit PinyiGear.

Tags: Thermal Processing of Gears · Deep Carburizing · Dimensional Stability · Precision Manufacturing