Carburizing and Quenching of Gears: Quality Control and Process Variables

Carburizing and Quenching of Gears: Quality Control and Process Variables

Key takeaway: Carburizing plus quenching continues to be the go-to heat treatment for gearing under heavy duty. Acceptance rests on three measurements: case depth, surface hardness, and effective case depth. Keep carbon potential inside ±0.05%C and quench at Ac3 + 30~50°C, and the result is core toughness of at least 40J/cm² alongside 58~62HRC at the surface — the minimum standard a dependable gear has to satisfy.

1. What the Process Involves

Gears made from low-carbon grades — typically 20CrMnTi, 20CrNiMo and 18CrNiMo7-6 — sit in a carbon-rich atmosphere at austenitizing temperature (900~950°C). Carbon migrates into the outer layer, after which fast cooling turns that layer into high-carbon martensite. What emerges is a composite: a wear-resistant hard skin wrapping a core that remains ductile and tough.

Whenever a high-precision heavy-duty gear is involved, the case hardness quality assessment set out in ISO 6336-5 is the method PinyiGear uses.

2. Defect Modes and How They Are Addressed

2.1 Distortion Beyond Limits

Typical triggers are heating that is not uniform, cooling that is too aggressive, or gear geometry that is asymmetric. Counter it with a 500°C preheat lasting 30 min, then die press quenching.

2.2 Surface Hardness Below 58HRC

Usually the cause is a quenching temperature set too low, or cooling capacity that falls short. Verify the oil temperature (optimal: 40~80°C) and swap out cooling media that has aged.

2.3 Case Delamination

Responsibility lies with a carbon gradient that drops too sharply across the transition zone. Add 1~2h to the diffusion stage and the carbon decline becomes more gradual.

3. Controlling the Three Core Parameters

3.1 Quenching Temperature and Cooling Rate

Ac3 + 30~50°C is the recommended quenching temperature, which in practice means 810~840°C. Which cooling medium to use follows from the part:

• Oil quenching — works for module ≤ 8 gears, distortion stays manageable
• Salt bath / die quenching — the choice for thin-wall parts or large precision gears, keeping distortion within 0.05mm

3.2 Surface Carbon Content

A surface carbon mass fraction of 0.75% ~ 0.95%C is optimal. Push past 1.0%C and two problems appear: network carbide precipitation, which cuts contact fatigue life, and retained austenite above 20%, which pulls hardness down. Drop below 0.7%C and the 58HRC surface hardness floor becomes unreachable.

3.3 Case Depth

Empirical formula for the lower limit: CHD ≈ 0.2 × (Module + 1).

4. Shop-Floor Notes

Drawing on PinyiGear’s production experience, 60% of carburizing success depends on furnace loading arrangement. Position gears with tooth faces upward and keep spacing uniform (≥10mm between gears) so gas flow remains even; this prevents the familiar defect of a thin case at the root paired with too much carbon at the tip. Validate the process before batch production by running destructive testing (microhardness traverse) on the first piece.

5. References

• PinyiGear Factory Specification

Notes assembled for technical study by PinyiGear Co., Ltd. — www.dgpinyi.com