Effective Case Depth Control in Gear Carburizing and Quenching

Key takeaway: When 20CrMnTi gear steel is carburized, an effective case depth (DS) between 0.8-1.5mm keeps the microstructure stable. Holding carbon potential at 1.0%C-1.2%C produces the desired acicular or fine acicular martensite, with surface hardness reaching 58-62HRC. Once case depth passes 2.0mm, retained austenite commonly exceeds the 15% critical limit and fatigue life drops.

1. Process Fundamentals

Carburizing followed by quenching is a combined heat treatment. A low-carbon steel gear is held in a carbon-rich atmosphere so carbon diffuses into the surface layer, then cooled rapidly to create a hard martensitic case. The purpose is to combine a wear-resistant hard surface with a tough core that absorbs impact — the two most conflicting demands placed on transmission gears.

1.1 The Three Stages

• Stage Heat-up / equalization: 850–930°C
  • Share of cycle: 15–20%
  • Main settings: Heating rate ≤100°C/h
• Stage Boost (carburizing): 920–950°C
  • Share of cycle: 50–60%
  • Main settings: CP 1.05-1.20%C, duration from case depth formula
• Stage Diffusion: 880–920°C
  • Share of cycle: 20–30%
  • Main settings: CP lowered to 0.80-0.95%C, gradient homogenization

1.2 Estimating Case Depth

A practical rule used on the shop floor is:

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

From PinyiGear production records:

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

For DS=1.2mm, the 920°C route needs roughly 3.4h of boost, while 940°C cuts that to about 2.4h. Each 20°C step up, however, adds a marked risk of grain coarsening.

2. Parameters That Drive the Result

2.1 Carbon Potential

Of all gas-carburizing variables, carbon potential matters most. It describes the equilibrium carbon content at the steel surface in contact with the furnace atmosphere. The control approach is:

• Boost: 1.05-1.20%C, so a steep gradient pushes carbon inward
• Diffusion: 0.80-0.95%C, which suppresses grain-boundary carbide networks
• Too high a CP leaves retained austenite above 20% and network carbides along grain boundaries
• Too low a CP slows carburizing, leaves the case shallow and the surface under-hardened

2.2 Quench Medium and Cooling

2.3 Microstructure Targets (ISO 6336 / GB/T 3480)

3. Defects, Causes and Fixes

• Defect Shallow case: DS below specification
  • Root cause: Short time or low CP
  • Countermeasure: Extend boost time, recalibrate O₂ probe
• Defect Low surface hardness: <58HRC
  • Root cause: Too much retained austenite
  • Countermeasure: Cryogenic treatment (-80°C) or longer diffusion
• Defect Distortion beyond tolerance: Profile or tooth trace deviation
  • Root cause: Uneven quench stress
  • Countermeasure: Adjust oil-entry orientation, add pre-oxidation
• Defect Network carbides: Continuous carbides at grain boundaries
  • Root cause: Diffusion time too short
  • Countermeasure: Lengthen diffusion, lower diffusion CP

4. Shop-Floor Practice

1. Verify carbon potential every day — this comes first. Oxygen probes need monthly calibration, and a steel foil shim carbon test (±0.05%C) should confirm the reading each shift. Without that, no process parameter can be trusted.
2. High-precision gears need cryogenic treatment. For gears with DS≥1.5mm, a -80°C×2h cryogenic step after quenching brings retained austenite from 15–20% down to 5–8%, which greatly improves dimensional stability.
3. Do not skip pre-oxidation. A 380–420°C×30min pre-oxidation step creates Fe₃O₄ nucleation sites on the surface and improves carburizing uniformity by more than 30%.
4. Distortion control begins with the blank. Hardness uniformity after forging normalization (≤10HB scatter) influences final distortion more than the carburizing process itself.

5. Standards and Sources

• GB/T 3480-2019 Calculation of Load Capacity of Gears
• ISO 6336-5:2016 Strength Calculation of Gears (Materials & Heat Treatment)
• JB/T 6141.2-2018 Gear Carburizing & Quenching Process Specification
• PinyiGear Factory Process Manual

Related reading: Machining requirements before carburizing are covered in Gear Blank Preparation & Normalizing. Heat treatment process cards for specific gear materials are available under Gear Machining Capabilities.

Doc No: GL-TN-20260713 | Category: Heat Treatment | Preview: Detection & Control of Gear Grinding Burns