Gear Material Choice and Distortion Management in Heat Treatment

A. How Distortion Arises and How to Hold It Back

Among the problems encountered in gear production, distortion ranks as the hardest to tame. Dimensional overshoot can be corrected by adjustment; distortion cannot, because it never repeats the same way twice.

• Distortion Mode Ovality: Bore or OD turns elliptical
  • Why It Happens: Heating and quenching stresses that are not uniform
• Distortion Mode Taper: Ends measure at different diameters
  • Why It Happens: Differences in the cooling sequence
• Distortion Mode Warping: The axis bends
  • Why It Happens: Residual stress release combined with quenching stress
• Distortion Mode Tooth profile change: Precision grade drop
  • Why It Happens: Case depth variation plus structural stress

Measures that work in practice:

1. Pre-oxidation at 400~500°C — this evens out the case layer, and skipping it is not advisable
2. A quenching press — the practical answer to ovality
3. Oil held at 60~80°C, with PAG concentration 5~15%
4. Deep freezing at -60~-80°C — dimensional stability improves markedly
5. Two tempering cycles at minimum, each ≥2h — this is what actually locks in deformation stability

How we verify the result:

• Bore ovality checked with an air gauge on a sample of 5~10 pcs, standard ≤0.02mm
• Tooth profile and helix deviation measured on a gear measuring center per GB/T 10095

B. The Triangle: Stress, Deformation, Precision

Material selection → Heat treatment difficulty → Deformation magnitude → Final precision

Each link feeds the next. The material chosen sets how difficult heat treatment will be; heat treatment sets how much deformation appears; deformation decides whether the precision target is reachable at all.

Take precision gears in 20CrMo: distortion stays small, and 0.15mm grinding allowance is enough. Run the same gear in 20CrNi2Mo and toughness rises, but deformation becomes harder to hold, so at least 0.25mm allowance is required.

Profitability in gear manufacturing comes down to balancing material cost against machining allowance against yield rate.

PinyiGear — Precision Gear Manufacturing & Transmission Components

C. Matching Steel Grade to the Job

The guiding idea: strength by itself decides nothing — the match between steel and application does.

• Application Auto transmission gears: 20CrMnTi
  • Rationale: Good cost-performance: core toughness + 60HRC+ surface after carburizing
• Application Heavy-duty gear shafts: 42CrMo
  • Rationale: Strength 1080MPa+, good hardenability for large sections
• Application Small-module precision gears: 20CrMo
  • Rationale: Minimal carburizing distortion, controllable grinding allowance (Grade 6+)
• Application Heavy truck axle gears: 20CrNi2Mo
  • Rationale: Ni addition delivers impact energy 63J+, handles heavy shock loads

Numbers behind the choices:

• 20CrMnTi (carburized): Rm≥1080MPa, core 30~42HRC, impact energy ≥55J
• 42CrMo (Q&T): Rm≥1080MPa, hardness 269~321HB
• 20CrNi2Mo (carburized): Rm≥1180MPa, core 35~45HRC, impact energy ≥63J

Before material enters production we run incoming inspection: spectrometer for composition, universal tester for mechanical properties, acid etch for microstructure. One sample is drawn per heat. Acceptance limits are general porosity ≤Grade 2, inclusions A/B ≤Grade 2.5, C/D ≤Grade 2.0, grain size 5~8.

D. Annex: Material Families, Case Depth and Carbon Potential

D1. Gear material families at a glance

• Material Category Quality Carbon Steel: 45#, 50#
  • Application Scenarios: Low-medium speed, light-load gears
  • Hardness Range: Q&T 220-280 HB
• Material Category Alloy Q&T Steel: 40Cr, 42CrMo, 35CrMo
  • Application Scenarios: Medium-load, medium-speed gears
  • Hardness Range: Q&T 250-320 HB
• Material Category Alloy Carburizing Steel: 20CrMnTi, 20CrNi2Mo, 20CrMo
  • Application Scenarios: High-speed heavy-duty gears (most common)
  • Hardness Range: Carburized 58-62 HRC
• Material Category Nitriding Steel: 38CrMoAl, 40Cr (nitrided)
  • Application Scenarios: Wear-resistant precision gears
  • Hardness Range: Nitrided 850-1100 HV
• Material Category Cast Steel: ZG310-570, ZG40Cr
  • Application Scenarios: Large gears
  • Hardness Range: Normalizing/Q&T
• Material Category Non-metallic: MC Nylon, POM
  • Application Scenarios: Low-noise, light-load gears
  • Hardness Range: —

D2. Carburizing route and how deep the case should go

More than 80% of heavy-duty gears are carburized. The route runs:

Blanking → Forging → Normalizing → Rough turning → Semi-finish turning →
Hobbing → Carburizing → Quenching + Low-temperature tempering → Shot peening → Finish grinding

Three ways to arrive at a case depth:

• From the module: δ ≈ (0.15~0.25) × m
• From tooth root stress: case depth ≥ 1.2 × bending fatigue critical section depth
• From the reference table below

D3. When the case depth is wrong

• Tooth surface contact fatigue spalling: Long carburizing time, doubled cost
• Insufficient tooth root bending strength: Increased retained austenite
• Core hardness insufficient for support: Coarse surface carbides, increased brittleness
• Short-term failure: Coarse grains, decreased impact toughness

D4. Core hardness

Core hardness in a carburized gear matters just as much as the case. Aim for 30-45 HRC, with pinions at the upper end and gears at the lower end. Below 25 HRC the risk of tooth root fatigue fracture climbs. Above 48 HRC toughness falls away and impact breakage becomes likely. Quenching temperature, cooling rate and a suitably hardenable material are the levers used to hold this range.

D5. 20CrMnTi and the case-depth misconception

Over 60% of China’s gear industry works in 20CrMnTi, and the reasons are clear. Ti refines the grain, so grain growth during carburizing stays low — direct quenching after cooling is possible, with no secondary heating. Hardenability is moderate, which keeps core hardness at 30-42 HRC without difficulty. Price is reasonable and the supply chain is mature.

One caution applies: 20CrMnTi tends to carburize at high temperature, so carbon potential must be held ≤1.1%, or carbides may exceed limits.

A habit worth breaking is running case depth to the upper limit or beyond. Excess case depth degrades the surface compressive stress distribution, and fatigue life can actually fall. Every additional 1mm of carburizing depth costs roughly 6-8h at 930°C, so expense rises steeply. The sound approach is to derive the minimum safe case depth from tooth root stress field simulation and leave only 0.1-0.2mm allowance.

D6. Shot peening after carburizing

Applying shot peening once carburizing and quenching are done brings 300-800 MPa residual compressive stress to the tooth root surface, and bending fatigue life can rise by 2-5 times. The parameters that matter are Almen intensity 0.3-0.5A and coverage ≥100%. Large module gears call for larger diameter shot media (S170-S230).

D7. Carbon potential in three stages

Multi-purpose furnace carburizing at PinyiGear follows a three-stage carbon potential profile:

1. Boost stage (carbon potential 1.0~1.2%C): rapid carbon absorption, layer depth controlled here
2. Diffusion stage (carbon potential 0.8~1.0%C): carbon moves inward, surface concentration drops
3. Cooling stage: temperature reduced to quenching level, protective atmosphere maintained

D8. Material selection quick-reference table

D9. Why this foundation matters

Choosing the material and fixing the case depth form the foundation of gear quality. Get either wrong and every later precision grinding step is wasted. Industry data shows that more than 80% of early gear failures trace back to heat treatment — a case that is too shallow, or distortion that is too large.