Choosing Gear Steel and Optimizing Heat Treatment — An Engineering Handbook

Selecting Gear Steel and Tuning Heat Treatment — An Engineering Handbook

Key point: Roughly seven tenths of a gear’s in-service behaviour traces back to how the steel grade and the thermal cycle are paired. General-purpose work is covered by two grades in most shops: 20CrMnTi, a case-carburizing steel picked when shock loads dominate, and 40Cr, a quench-and-temper steel picked when medium or low speeds and smooth running matter more. Match the two properly and the part can stay in service 3-5× longer.

1. Comparing the Heat Treatment Routes

1.1 Carburizing plus quenching (the workhorse route)

Normalizing → Rough machining → Carburizing (920–950°C) → Quenching (820–860°C) → Low-temp tempering (180–200°C) → Finish machining

1.2 Quench-and-temper (Q&T)

Rough machining → Q&T (quench + high-temp temper) → Finish machining → (optional tooth surface induction hardening)

• Process is straightforward, movement during treatment stays small, and the cost is modest
• Hardness aimed for: HB 240-320

1.3 Nitriding

Q&T → Finish machining → Nitriding (500–530°C) → Lapping/polishing

• Movement is minimal (0.01-0.03mm), so no grinding step follows
• The trade-off: a 20–60 hour cycle and a shallow hardened layer

1.4 Induction hardening

Where large gears need either individual teeth or the whole part hardened, this route applies. Depth of the hardened zone can be set anywhere from 1-5mm. Throughput is high, though the equipment bill is substantial.

2. The Four Families of Gear Steel

2.1 Carburizing grades

• 20CrMnTi: ≥1080
  • Case Depth (mm): 0.8-1.2
  • Surface Hardness (HRC): 58-62
  • Typical Applications: Auto transmission gears, drive axle gears
• 20CrMo: ≥980
  • Case Depth (mm): 0.7-1.1
  • Surface Hardness (HRC): 56-60
  • Typical Applications: Medium-load transmission gears
• 20CrNiMo: ≥1180
  • Case Depth (mm): 0.9-1.5
  • Surface Hardness (HRC): 58-63
  • Typical Applications: Heavy-duty gears, mining machinery

2.2 Quench-and-temper grades

• 40Cr: 28-32
  • Best For: Low/medium speed gears
  • Key Feature: Excellent comprehensive mechanical properties
• 42CrMo: 32-38
  • Best For: Large gears
  • Key Feature: Deep hardenability
• 45#: 25-30
  • Best For: Low-load gears
  • Key Feature: Low cost, easy to machine

2.3 Nitriding and through-hardening grades

Once 38CrMoAl has been nitrided, its surface reads 900-1100 HV (roughly 65-70 HRC) — wear resistance is outstanding, yet the hardened zone measures only 0.3-0.6mm, which suits precision gearing. For small-module precision parts, GCr15 bearing steel at 60-65 HRC is the usual answer.

2.4 Plastics and other non-metals

Light loads and quiet running are the domain of engineering plastics such as POM and nylon (PA66+GF30). Continuous service temperature for these materials generally tops out at 120°C.

3. Matching Material, Treatment and Accuracy

4. Defects That Show Up and How to Answer Them

• Excess carbides in the case: either the carbon potential ran too rich or the diffusion stage was cut short
  • Fix: bring carbon potential down to 0.8-1.0%C and give diffusion more time
• Quench cracking: the cooling rate was too aggressive, or the steel carries too much carbon
  • Fix: cool the part partially before quenching, or switch to a martempering oil
• Hardness falling short: quench temperature was low, or cooling proved inadequate
  • Fix: check that the furnace heats evenly, and raise agitation
• Distortion beyond limits: heating was uneven, or parts were loaded poorly
  • Fix: load vertically and insert a preheat stage
• Grinding burn: too much stock was removed per pass, or cooling was weak
  • Fix: hold depth to ≤0.03mm per pass

5. Shop-Floor Advice

1. Carbon potential is the variable that decides everything: with a methanol+kerosene atmosphere, holding fluctuation inside ±0.05%C is what keeps case depth uniform. Layer-by-layer analysis every 4 hours tells you the real carbon level.
2. Low-temperature tempering is not optional: skip it and root fracture becomes far more likely. For 20CrMnTi, 180°C × 2h is the floor.
3. Prep treatment sets the ceiling: a normalized structure that is uniform yields a carburized layer that is uniform. Banding left in place produces uneven case depth, so remove it during normalizing.
4. Treat grinding and heat treatment as one chain: how much a part moves during carburizing decides where grinding stock ends up. Running 3-5 trial pieces before the batch starts cuts reject rates noticeably.

6. Standards and Sources

• JB/T 7516-2018 — Gear Materials and Heat Treatment (Chinese industry standard)
• GB/T 9450-2020 — Determination of Carburized Case Depth
• PinyiGear Gear Manufacturing Capabilities & Material Guide | Custom Gear Solutions
• ISO 6336-5:2016 — Calculation of Load Capacity of Spur and Helical Gears — Part 5: Strength and Quality of Materials

Compiled by the PinyiGear technical team as part of our ongoing notes. New entries appear in the Gear Design & Precision Manufacturing series. Need help picking a gear? The Spur Gear Parameter Tool is the place to start.