Gear Steel Selection & Heat Treatment Deformation Control

1. Gear Steel Selection — Practical Guide#

Core principle: Material selection is not about strength alone — it’s about matching.

Key data:

• 20CrMnTi (carburized): Rm≥1080MPa, core 30~42HRC, impact energy ≥55J
• 42CrMo (Q&T): Rm≥1080MPa, hardness 269~321HB
• 20CrNi2Mo (carburized): Rm≥1180MPa, core 35~45HRC, impact energy ≥63J

Incoming inspection: Spectrometer (composition) + universal tester (mechanical) + acid etch (microstructure). Sampling: 1 per heat. Requirements: general porosity ≤Grade 2, inclusions A/B ≤Grade 2.5, C/D ≤Grade 2.0, grain size 5~8.

2. Heat Treatment Deformation Control#

Deformation is the “invisible killer” of gear manufacturing — harder to manage than dimensional overshoot because the patterns are inconsistent.

Deformation Types & Root Causes#

Practical Control Measures#

1. Pre-oxidation at 400~500°C — uniformizes case layer — do not skip this step
2. Quenching press — effective for controlling ovality
3. Oil temperature: 60~80°C, PAG concentration 5~15%
4. Deep freezing at -60~-80°C — significantly improves dimensional stability
5. At least 2 tempering cycles, each ≥2h — the real key to deformation stability

Inspection Standards#

• Bore ovality: air gauge, sample 5~10 pcs, standard ≤0.02mm
• Tooth profile/helix deviation: gear measuring center per GB/T 10095

3. Today’s Reflection: The Stress-Deformation-Precision Triangle#

Material selection → Heat treatment difficulty → Deformation magnitude → Final precision

These three links are interdependent: material choice determines heat treatment difficulty, heat treatment determines deformation magnitude, and deformation determines whether precision targets can be met.

Example: 20CrMo for precision gears — minimal distortion, 0.15mm grinding allowance sufficient. Same gear in 20CrNi2Mo — higher toughness but harder deformation control, at least 0.25mm allowance needed.

The “material cost vs. machining allowance vs. yield rate” balance is where real profitability lies in gear manufacturing.

Geyontech — Precision Gear Manufacturing & Transmission Components

Annex: Gear Material Classification, Case Depth & Carbon Potential Control (merged from Daily Learning Notes 2026-07-07)#

Annex 1: Common Gear Material Classification#

Annex 2: Carburizing Process Route & Case Depth Selection#

Carburized gears account for over 80% of heavy-duty gears. Process route:

Blanking → Forging → Normalizing → Rough turning → Semi-finish turning →
Hobbing → Carburizing → Quenching + Low-temperature tempering → Shot peening → Finish grinding

• Estimation by module: δ ≈ (0.15~0.25) × m (m is module)
• By tooth root stress: Case depth ≥ 1.2 × bending fatigue critical section depth
• Common reference:

Annex 3: Consequences of Incorrect Case Depth Selection#

Annex 4: Core Hardness Control#

The core hardness of carburized gears is equally critical:

• Recommended range: 30-45 HRC (pinion take upper limit, gear take lower limit)
• Too low (<25 HRC): High risk of tooth root fatigue fracture
• Too high (>48 HRC): Insufficient toughness, impact tooth breakage
• Control methods: Adjust quenching temperature, cooling rate, select appropriate hardenability material

Annex 5: 20CrMnTi & the case-depth misconception#

20CrMnTi accounts for over 60% of China’s gear industry, with clear advantages:

• Ti element refines grains, low grain growth tendency during carburizing → allows direct quenching after cooling (eliminates secondary heating)
• Moderate hardenability → core hardness easily controlled at 30-42 HRC
• Reasonable price, mature supply chain

But note: 20CrMnTi has a tendency for high-temperature carburizing (need to control carbon potential ≤1.1%), otherwise carbides may exceed limits.

Many technicians habitually run case depth to the upper limit or even overshoot. This is wrong:

• Excessive case depth → deteriorated surface compressive stress distribution → fatigue life may actually decrease
• Each additional 1mm of carburizing depth requires approximately 6-8h at 930°C, costs skyrocket
• Correct approach: Determine minimum safe case depth by tooth root stress field simulation, reserve only 0.1-0.2mm allowance

Annex 6: Shot peening parameters after carburizing#

Adding shot peening after carburizing and quenching:

• Creates 300-800 MPa residual compressive stress on tooth root surface
• Bending fatigue life can be improved by 2-5 times
• Key parameters: Almen intensity 0.3-0.5A, coverage ≥100%
• Large module gears need larger diameter shot media (S170-S230)

Annex 7: Three-stage carbon potential control#

In multi-purpose furnace carburizing, we commonly use three-stage carbon potential control:

1. Boost stage (carbon potential 1.0~1.2%C): Rapid carbon absorption, control layer depth
2. Diffusion stage (carbon potential 0.8~1.0%C): Carbon diffuses inward, surface concentration decreases
3. Cooling stage: Reduce to quenching temperature, maintain protective atmosphere

Annex 8: Material selection quick-reference table#

Annex 9: Material selection principle#

Material selection and case depth determination are the “foundation” of gear quality. If these are wrong, all subsequent precision grinding efforts are wasted. The industry data shows that >80% of gear early failures are related to heat treatment issues — insufficient case depth or excessive distortion.