Gear Heat Treatment Selection Guide: Induction Hardening, Nitriding and Carburizing Compared

The short version

Whether a gear survives its duty cycle or fails by pitting comes down largely to heat treatment. Three routes cover most industrial work:

• Carburizing — 20CrMnTi, surface HRC 58–62 over a case of 0.8–2.0 mm. The standard answer for high-load gears at module ≥ 3.
• Gas nitriding — 40Cr or 38CrMoAl, HV 550–900, with almost no dimensional movement. Chosen for slender, tight-tolerance gears.
• Induction hardening — 42CrMo, HRC 50–55, cheapest of the three. Fits medium-duty gears made from medium-carbon steel.

PinyiGear operates all three routes and delivers finished gears at DIN 6–8.

What the treatment changes inside the tooth

The workpiece is taken above its critical temperature — or soaked within a set band — held, then cooled, with or without an alloying element diffusing inward, so that the microstructure transforms. What you gain is a flank that fights wear while the material below stays tough.

Requirements come from three failure modes: pitting under contact fatigue, bending-fatigue cracking at the root, and wear or scuffing. Both the contact-fatigue strength of the flank and the bending strength of the root are set by the treatment, which explains why drives running harder than HRC 50 cannot be produced without one.

Soft-faced and hard-faced compared

Effective case depth: the governing figure on the drawing

Effective case depth is measured at right angles to the flank, from the surface down to where hardness drops to a specified level — usually HV 550. On a heat-treatment drawing, it is the acceptance value that carries the most weight.

For carburized parts the rule of thumb is case depth ≈ module m × 0.15–0.25.

• M2: 0.5–0.8 mm
• M4: 0.8–1.2 mm
• M6: 1.2–1.6 mm
• M8–M10: 1.6–2.5 mm

Three cautions apply. Too shallow a case collapses at the flank and pits prematurely. Too deep a case, or too hard a core, leaves the tooth brittle and more likely to fracture. And the hardness gradient needs to fall off smoothly — a steep transition invites case spalling.

Three processes, side by side

On value alone, carburizing stays the default for industrial gearing. Nitriding is picked when distortion has to stay close to zero. Induction hardening takes the job when price and turnaround time are the deciding factors. For spur gears and helical gears alike, PinyiGear matches the route to your tooth geometry and order quantity.

Process detail and parameter control

Carburizing and quenching

The base materials are low-carbon and low-carbon alloy steels such as 20CrMnTi and 20CrMo. At 900–950 °C in a carbon-rich atmosphere, carbon diffuses into the surface layer; quench cooling and a low-temperature temper follow. Depending on module and load, case depth is set between 0.8 and 1.5 mm. Surface hardness reaches HRC 58–64 while the core sits at HRC 30–42. A wear-resistant skin over an impact-tolerant core is the reason this route is standard for automotive transmission gears.

Nitriding

Here the starting stock is medium-carbon alloy or quenched-and-tempered steel — 40Cr and 38CrMoAl, for instance. Nitrogen diffuses at 500–550 °C in a decomposed-ammonia atmosphere, and there is no quench afterwards. Case depth stays shallow, 0.2–0.5 mm, but surface hardness is very high at HV 800–1200. Because distortion is minimal, the operation can be placed last in the routing, which suits precision gears and thin-walled components.

Induction hardening

Medium-carbon and medium-carbon alloy steels such as 45 steel and 40Cr work well here. A high-frequency field brings the flank up to quenching temperature, after which spray cooling follows. Heating is confined to the tooth surface while the core retains its quenched-and-tempered condition. The method is fast, localized and gives distortion you can control — a good fit for large-module gears.

Parameter control

Position in the manufacturing sequence

1. Cutting → forging → normalizing / quench & temper (HB 160–190)
2. Rough turning → hobbing or gear shaping (grinding allowance left at 0.2–0.4 mm)
3. Carburizing or gas nitriding, per the selection made earlier
4. Finishing: gear grinding (DIN 5–6) or shaving plus honing (DIN 6–7)
5. Final inspection: hardness, case depth, metallography, tooth profile and lead accuracy

Nitriding moves the part so little that it can be scheduled after finish machining, with no grinding step needed. Carburizing moves it more, so leaving allowance and grinding afterwards are both mandatory. PinyiGear batches shaft parts alongside gears in one treatment cycle, which reduces re-clamping error.

Practical notes and inspection

• Case depth choice: apply the module-based formula, case depth ≈ (0.15–0.25)×m. Extra depth buys brittleness; insufficient depth costs wear life.
• Nitriding limits: the shallow layer rules it out where impact is severe, yet its wear and anti-scuffing performance is excellent — well suited to high-speed, lightly loaded gears.
• Quenching medium: oil cools slowly and distorts less; water cools faster but raises the risk of cracking.
• Tempering timing: finish tempering within 2 hours of the quench, otherwise retained austenite transformation increases the chance of cracking.
• Inspection scope: after treatment, verify surface hardness, core hardness, effective case depth (at HV 550), and metallographic structure (martensite grade, retained austenite content).

Selection checklist

1. Start from module: m ≥ 3 → carburize; thin high-precision gears at m ≤ 2 → nitride.
2. Accuracy target: DIN 5–6 needs grinding to remove carburizing distortion; nitriding can be used without grinding.
3. Order quantity: carburizing cost is absorbed well by large batches; single-piece work points to induction hardening.
4. Loading pattern: for severe impact, carburizing develops favourable compressive stress at the surface; for steady medium loads, induction hardening is enough.
5. Material availability: 20CrMnTi is easy to source locally; 42CrMo forgings are inexpensive and readily obtainable.
6. Documentation: where hardness-gradient and metallographic reports are specified, use controlled-atmosphere carburizing furnaces.

PinyiGear capability

Accuracy class, module, tooth count and material follow the customer drawing.

• Module range M0.5–M12, max OD Φ600 mm
• Tooth accuracy DIN 5–8

Worm gears, racks and the full range of capabilities are also available, with the material–process–accuracy package matched to your duty cycle.

Looking for heat-treatment guidance and pricing? Reach the PinyiGear technical team for a no-charge evaluation covering material choice, case-depth design and machining cost. dgpinyi.com English site

In one line: a gear’s service life is half material and half heat treatment. Choose the right process and control the parameters closely, and the material will deliver everything it has.