title: “How Deep Should the Hardened Case Be? Matching CHD, Module, and Core Hardness for Carburized Gears” date: 2026-08-14 tags: [gear heat treatment, carburizing, case hardening depth, CHD, core hardness, technical learning] category: Heat Treatment
How Deep Should the Hardened Case Be? Matching CHD, Module, and Core Hardness for Carburized Gears
Bottom line: Carburizing and quenching is the standard heat treatment route for heavy-duty gears. Effective case hardening depth (CHD — the distance from the flank surface to the point where hardness drops to 550 HV) is usually sized at 0.15–0.25 × normal module: 0.6–0.9 mm for modules up to 2.5, and 1.2–1.8 mm for modules of 5–10. Too shallow, and the flank collapses under Hertzian contact stress; too deep, and the tooth root turns brittle. A typical 20CrMnTi combination is 58–62 HRC at the surface and 30–42 HRC in the core.
1. What carburizing and quenching actually does
Carburizing and quenching: heating a low-carbon alloy steel gear into the austenite range (920–950°C) in a carbon-rich atmosphere so carbon diffuses into the surface, then quenching to form a hard, high-carbon martensite case over a tough, low-carbon core. The hard surface resists wear, pitting and scuffing; the tough core carries bending loads at the tooth root.
The usual sequence is carburize → quench → temper at 160–200°C for 2–3 h. Tempering removes quench stress; surface hardness drops little while root toughness recovers.
Typical steels: 20CrMnTi, 20CrMo, 17CrNiMo6, SAE 8620. In China, 20CrMnTi is a widely used choice for small and medium modules — medium hardenability, and oil quenching is enough.
2. Sizing the effective case depth (CHD)
CHD (Case Hardening Depth): the perpendicular distance from the surface to the depth where Vickers hardness falls to 550 HV. It is more useful than “total case depth” because only the part hard enough to 550 HV actually carries contact load.
Rule of thumb: CHD ≈ 0.15–0.25 × normal module.
| Normal module mn (mm) | Recommended CHD (mm) | Typical application |
|---|---|---|
| ≤ 2.5 | 0.6 – 0.9 | Automotive gearboxes, small reducers |
| 2.5 – 5 | 0.9 – 1.3 | General industrial gearboxes |
| 5 – 10 | 1.2 – 1.8 | Heavy-duty gearboxes, rolling mill drives |
| > 10 | 1.6 – 2.5 | Mining and cement mill gears |
Too shallow, and Hertzian contact stress exceeds case capacity — the flank fails by collapse-type spalling. Too deep, and the residual stress pattern shifts, making tooth-root brittle fracture more likely. Deeper is not automatically better.
3. Surface hardness and core hardness go together
| Item | Typical range | Role |
|---|---|---|
| Surface hardness | 58–62 HRC (≈650–700 HV) | Wear, scuffing and pitting resistance |
| Core hardness | 30–42 HRC | Root bending fatigue strength, toughness |
| Surface carbon content | 0.75–1.05% C | Martensite hardness, retained austenite control |
Core hardness is the item people forget. Above about 45 HRC, the root has little plastic reserve and a crack, once started, runs fast. Below about 28 HRC, root strength drops and bending fatigue life falls. For 20CrMnTi in normalized or quenched-and-tempered condition, core hardness usually lands in the 30–38 HRC band.
4. Process parameters and common defects
Carburizing runs at 930 ± 10°C with a carbon potential of 0.75–1.05% C. Time depends on target depth and furnace load — roughly 1–1.5 h per 0.1 mm in a batch furnace. Two quench routes exist: direct quench (cool from carburizing temperature to 830–850°C, then oil quench) or re-heat quench (cool slowly after carburizing, re-austenitize and quench — better distortion control, longer cycle).
| Defect | Symptom | Main cause | Fix |
|---|---|---|---|
| Surface decarburization / internal oxidation | Soft spots, low flank hardness | High dew point, low carbon potential | Control furnace atmosphere, raise CP |
| Coarse grain | Coarse microstructure | Too hot or too long | Control temperature, normalize if needed |
| Distortion out of tolerance | Tooth alignment error 0.02–0.08 mm | Uneven loading, quench stress | Symmetric loading, grinding allowance |
5. How case depth is measured
- Metallographic method: etch and measure the martensite or carbide boundary — fast, but depends on etch quality.
- Hardness gradient method (the arbiter): run a hardness traverse from the surface and read the depth at 550 HV. Measure at mid-flank and at the root; the two requirements can differ.
💡 Tip 1: When the drawing says “case depth 1.0 mm”, confirm whether it means CHD or total case depth. The two differ by 0.2–0.4 mm; skipping this check sends the supplier to the wrong standard.
💡 Tip 2: Count grinding allowance into the case. With 0.2–0.3 mm stock per flank, carburize to CHD + grinding allowance, or the hardened layer is gone after grinding. Many ground-gear scrap parts were not ground wrong — the case was simply left too thin.
💡 Tip 3: Threaded holes, oil holes and splines need anti-carburizing protection (copper plating or stop-off paint, 0.03–0.05 mm). Skip this and the threads come out hard and brittle; they chip during assembly.
6. Final notes
Case depth follows module, core hardness follows material hardenability, and grinding allowance follows the downstream precision gear machining step. Settle all three at drawing approval and you save a lot of rework later. For material and process support, see the gear product range and our capabilities pages.
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