Matching Case Hardening Depth to Gear Module and Core Hardness
CHD is sized at 0.15-0.25 x normal module: 0.6-0.9 mm for modules up to 2.5 and 1.2-1.8 mm for modules of 5-10, with 20CrMnTi at 58-62 HRC surface.
Why the hardened layer must be sized, not guessed
Heavy-duty gearing still relies on carburizing followed by quenching as its default thermal route. The controlling dimension is the effective case hardening depth, or CHD: the distance measured perpendicular from the flank surface until hardness drops to 550 HV. Practice ties this figure to the normal module, using a band of 0.15–0.25 × module. That works out to 0.6–0.9 mm where modules reach 2.5, and 1.2–1.8 mm where modules span 5–10. Go too thin and Hertzian contact stress crushes the flank; go too thick and the tooth root turns brittle. A common 20CrMnTi pairing delivers 58–62 HRC at the surface and 30–42 HRC beneath it.
What the carburizing and quenching cycle does
A low-carbon alloy steel gear is held in a carbon-rich atmosphere inside the austenite range, 920–950°C, so carbon migrates into the surface layer. Quenching then converts that enriched skin into high-carbon martensite while the interior stays low-carbon and tough. Wear, pitting and scuffing are resisted by the hard skin; bending loads at the root are carried by the ductile core.
Carburize, then quench, then temper at 160–200°C for 2–3 h — that is the standard order. Quench stresses relax during tempering, root toughness comes back, and surface hardness gives up only a little.
Among the steels normally chosen are 20CrMnTi, 20CrMo, 17CrNiMo6 and SAE 8620. For small and medium modules in China, 20CrMnTi sees wide use: its hardenability is moderate, and oil quenching alone is enough.
Choosing the effective case depth
CHD is not the same as total case depth. Only the zone hard enough to hold 550 HV actually bears contact load, which is why the 550 HV boundary is the meaningful one.
Working rule: CHD ≈ 0.15–0.25 × normal module.
| Normal module mn (mm) | 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 |
When the layer is undersized, contact stress outruns what the case can support and collapse-type spalling takes the flank. When it is oversized, residual stresses redistribute and the root becomes more prone to brittle fracture. Thicker is not safer by default.
Surface and core hardness must be specified together
| Parameter | Value | Function |
|---|---|---|
| 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 value most often overlooked. Once it climbs past roughly 45 HRC, the root has almost no plastic reserve and any crack propagates quickly. Drop below about 28 HRC and root strength falls away, shortening bending fatigue life. With 20CrMnTi in normalized or quenched-and-tempered condition, the core normally settles between 30 and 38 HRC.
Measurement methods and practical cautions
Two routes exist for establishing case depth. Etching to reveal the martensite or carbide boundary is the metallographic approach — quick, but its accuracy rides on etch quality. The hardness gradient traverse is the deciding method: hardness is plotted inward from the surface and the depth at 550 HV is read off. Readings should be taken at mid-flank and at the root, since the two locations can call for different values.
Caution 1: A drawing note reading “case depth 1.0 mm” is ambiguous — establish whether CHD or total case depth is intended. The gap between them runs 0.2–0.4 mm, and guessing wrong points the supplier at the wrong specification.
Caution 2: Grinding stock must be added to the case target. Where 0.2–0.3 mm per flank will be removed, carburize to CHD plus that allowance; otherwise the hardened zone disappears during grinding. Plenty of scrapped ground gears were ground correctly — the case was simply too thin from the start.
Caution 3: Threaded holes, oil holes and splines require anti-carburizing protection, such as copper plating or stop-off paint at 0.03–0.05 mm. Without it, threads harden and embrittle, then chip during assembly.
Process window and typical defects
A carbon potential of 0.75–1.05% C and a temperature of 930 ± 10°C define the carburizing atmosphere. Duration is set by target depth and furnace load; in a batch furnace, budget roughly 1–1.5 h for each 0.1 mm. Quenching can follow one of two paths. Direct quench cools from carburizing temperature down to 830–850°C before the oil quench. Re-heat quench cools slowly first, then re-austenitizes and quenches — distortion is better controlled, but the cycle runs longer.
| 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 |
Closing points
Module governs case depth; material hardenability governs core hardness; the downstream precision gear machining step governs grinding allowance. Lock all three down when the drawing is approved and later rework is largely avoided. Material and process questions can be directed through the gear product range and our capabilities pages.
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