π Daily Learning Notes β 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 | β |
2. Carburizing & Quenching Process (The Core Heat Treatment Method)
Carburized gears account for over 80% of heavy-duty gears. Process route:
Blanking β Forging β Normalizing β Rough turning β Semi-finish turning β
Hobbing β Carburizing β Quenching + Low-temperature tempering β Shot peening β Finish grinding
Case Depth Selection Formula (Rule of thumb):
- Estimation by module: Ξ΄ β (0.15~0.25) Γ m (m is module)
- By tooth root stress: Case depth β₯ 1.2 Γ 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 β Insufficient Case | Too Deep β 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 β allows direct quenching after cooling (eliminates secondary heating)
- Moderate hardenability β 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 β€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 β deteriorated surface compressive stress distribution β fatigue life may actually decrease
- Each additional 1mm of carburizing depth requires approximately 6-8h at 930Β°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 β₯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 β 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 Β· Precision Gears & Transmission Systems Β· Technical Knowledge Accumulation