Gear Materials & Heat Treatment: Selection & Carburizing
Covers gear material grades like 20CrMnTi and 42CrMo, hardness ranges, and the carburizing process route from forging to final machining.
Daily Learning Notes — Gear Materials & Heat Treatment: Scientific Material Selection & Carburizing Process
Date: 2026-07-07 | Topic: Gear Materials & Heat Treatment | Session: 2
I. Technical Highlights#
1. Common Gear Material Classification#
| Material Category | Representative Grades | Application Scenarios | Hardness Range |
|---|---|---|---|
| Quality Carbon Steel | 45#, 50# | Low-medium speed, light-load gears | Q&T 220-280 HB |
| Alloy Q&T Steel | 40Cr, 42CrMo, 35CrMo | Medium-load, medium-speed gears | Q&T 250-320 HB |
| Alloy Carburizing Steel | 20CrMnTi, 20CrNi2Mo, 20CrMo | High-speed heavy-duty gears (most common) | Carburized 58-62 HRC |
| Nitriding Steel | 38CrMoAl, 40Cr (nitrided) | Wear-resistant precision gears | Nitrided 850-1100 HV |
| Cast Steel | ZG310-570, ZG40Cr | Large gears | Normalizing/Q&T |
| Non-metallic | MC Nylon, POM | Low-noise, light-load gears | — |
2. Carburizing & Quenching Process (The Core Heat Treatment Method)#
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
Case Depth Selection Formula (Rule of thumb):
- 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:
| Module m (mm) | Recommended Effective Case Depth (mm) | Notes |
|---|---|---|
| 2-3 | 0.4-0.8 | Small module, too shallow risks crushing |
| 4-6 | 0.8-1.3 | Most common range for medium gears |
| 6-10 | 1.2-1.8 | Heavy-duty needs strict gradient control |
| 10-16 | 1.5-2.5 | Large module, carburizing time 20h+ |
| >16 | 2.0-3.5 | Extra-large gears, deep case carburizing |
3. Consequences of Incorrect Case Depth Selection#
| Too Shallow — Insufficient Case | Too Deep — Excessive Case |
|---|---|
| Tooth surface contact fatigue spalling | Long carburizing time, doubled cost |
| Insufficient tooth root bending strength | Increased retained austenite |
| Core hardness insufficient for support | Coarse surface carbides, increased brittleness |
| Short-term failure | Coarse grains, decreased impact toughness |
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
II. Practical Insights#
Insight 1: Why Is 20CrMnTi the “Jack of All Trades”?#
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.
Insight 2: “Better too deep than too shallow” Is a Misconception#
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
Insight 3: Shot Peening Cannot Be Ignored#
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)
Insight 4: Precise Carbon Potential Control Is a True Skill#
In multi-purpose furnace carburizing, we commonly use three-stage carbon potential control:
- Boost stage (carbon potential 1.0~1.2%C): Rapid carbon absorption, control layer depth
- Diffusion stage (carbon potential 0.8~1.0%C): Carbon diffuses inward, surface concentration decreases
- Cooling stage: Reduce to quenching temperature, maintain protective atmosphere
III. Today’s Reflection#
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.
For Geyontech, building a standardized case depth selection table (like the module-case depth table above) and integrating it into the quotation and production system can save a lot of trial-and-error time.
Geyontech · Precision Gears & Transmission Systems · Technical Knowledge Accumulation