Daily Learning Note — Gear Heat Treatment Processes
Date: 2026-07-09 | Topic: Heat Treatment (Core Gear Process)
Technical Highlights
1. Carburizing & Quenching
- Applicable Materials: Low-carbon steel / low-carbon alloy steel (e.g., 20CrMnTi, 20CrMo)
- Process: Carbon atoms penetrate at 900–950°C in a carburizing atmosphere → quench cooling → low-temperature tempering
- Case Depth: 0.8–1.5mm (based on module and load)
- Hardness: Surface HRC 58–64; Core HRC 30–42
- Features: Hard, wear-resistant surface with tough, impact-resistant core — ideal for automotive transmission gears
2. Nitriding
- Applicable Materials: Medium-carbon alloy steel / quenched-and-tempered steel (e.g., 40Cr, 38CrMoAl)
- Process: Nitrogen atoms penetrate at 500–550°C in an ammonia decomposition atmosphere — no quenching needed
- Case Depth: 0.2–0.5mm (relatively shallow)
- Surface Hardness: HV 800–1200 (very high)
- Features: Minimal distortion, usable as a final process; suitable for precision gears and thin-walled parts
3. Induction Hardening
- Applicable Materials: Medium-carbon steel / medium-carbon alloy steel (e.g., 45 steel, 40Cr)
- Process: High-frequency induction heats the tooth flank to quenching temperature → spray cooling
- Hardened Layer: Only the tooth surface is heated; the core retains its quenched-and-tempered state
- Features: High efficiency, localized heating, controllable distortion; suitable for large-module gears
4. Key Process Parameter Control
| Parameter | Carburizing & Quenching | Nitriding | Induction Hardening |
|---|---|---|---|
| Temperature | 900–950°C | 500–550°C | 800–950°C |
| Time | 4–10h | 24–80h | Seconds |
| Case Depth | 0.8–1.5mm | 0.2–0.5mm | 1–4mm |
| Distortion | Moderate | Very small | Controllable |
Practical Insights
- Choosing case depth: Empirical formula based on gear module (m): case depth ≈ (0.15–0.25)×m. Too deep increases brittleness; too shallow reduces wear resistance.
- Nitriding limitations: Thin case depth makes it unsuitable for heavy impact loads; however, wear resistance and anti-scuffing properties are excellent — ideal for high-speed, light-load gears.
- Quenching medium: Oil quenching has slower cooling and less distortion; water quenching is faster but risks cracking.
- Timely tempering: Tempering must be completed within 2 hours after quenching, otherwise retained austenite transformation increases cracking risk.
- Inspection items: After heat treatment, always check — surface hardness, core hardness, effective case depth (at HV 550), and metallographic structure (martensite grade, retained austenite content).
One-sentence summary: A gear’s service life is half material and half heat treatment. Choose the right process and control the parameters tightly to unlock the material’s full potential.