date: 2026-07-22 topic: Gear Materials — Case Hardening Steel vs Through-Hardening Steel tags: [geyontech, gear transmission, materials, heat treatment, learning note]
Case Hardening vs Through-Hardening Steel: The Core Decision in Gear Material Selection
Bottom line: Gear material selection determines the transmission system’s service life, load capacity, and manufacturing cost. In industrial applications, 20CrMnTi (case hardening steel) and 40Cr (through-hardening steel) represent the two most typical gear material categories — the former excels in high-impact, high-wear scenarios, while the latter suits medium-to-low loads and mass production. Wrong material choice can lead to premature pitting or tooth fracture, directly compromising equipment reliability.
1. Definition and Typical Grades
1.1 Case Hardening Steel
Case hardening steel refers to low-carbon alloy steels (typically 0.15%~0.25% C) that achieve a “hard shell, tough core” property distribution through carburizing + quenching heat treatment. Common grades:
| Grade | Standard | Surface Hardness After Carburizing | Core Hardness |
|---|---|---|---|
| 20CrMnTi | GB/T 3077 | 58~62 HRC | 30~42 HRC |
| 20CrMo | GB/T 3077 | 58~62 HRC | 30~40 HRC |
| 8620H | SAE J1268 | 58~62 HRC | 30~42 HRC |
| 18CrNiMo7-6 | EN 10084 | 58~62 HRC | 35~43 HRC |
1.2 Through-Hardening Steel
Through-hardening steel refers to medium-carbon steels (typically 0.35%~0.55% C) that achieve uniform tempered martensite/sorbite structure across the entire cross-section after quenching + high-temperature tempering.
| Grade | Standard | Hardness Range After Q&T |
|---|---|---|
| 40Cr | GB/T 3077 | 28~35 HRC |
| 42CrMo | GB/T 3077 | 32~39 HRC |
| 4140 | ASTM A29 | 28~38 HRC |
| S45C | JIS G4051 | 20~28 HRC |
2. Case Hardening vs Through-Hardening — Key Comparison
| Dimension | Case Hardening (20CrMnTi) | Through-Hardening (40Cr) |
|---|---|---|
| Surface Hardness | 58~62 HRC (very high) | 28~35 HRC (moderate) |
| Core Toughness | High (low-C martensite/bainite) | Moderate (tempered sorbite) |
| Case Depth | 0.8~1.5 mm (controllable) | Uniform throughout |
| Contact Fatigue Strength | Very high (≥1500 MPa) | Moderate (800~1000 MPa) |
| Bending Fatigue Strength | High (supported by tough core) | Medium-high (uniform) |
| Wear Resistance | Excellent | Fair |
| Impact Resistance | Excellent | Moderate |
| Machining Difficulty | Requires grinding (hard after HT) | Can use hobbing directly |
| Relative Cost Ratio | 1.0 (baseline) | 0.55~0.70 |
| Typical Applications | Auto transmissions, construction machinery | General gearboxes, agricultural machinery |
💡 Practical Insight
During design, always perform contact stress verification first: if σ_H ≥ 1200 MPa, choose case hardening steel; if σ_H ≤ 900 MPa, through-hardening steel is sufficient. The middle zone requires a combined assessment with the impact factor K_A.
3. Selection Decision Flow
graph TD
A[Input: Operating Conditions] --> B{Contact Stress σ_H?}
B -->|σ_H ≥ 1200 MPa| C[Case Hardening Steel]
B -->|σ_H ≤ 900 MPa| D[Through-Hardening Steel]
B -->|900 < σ_H < 1200| E{High Impact / K_A?}
E -->|Yes| C
E -->|No| D
C --> F[Requires Gear Grinding]
D --> G[Hobbing + Finish OK]
3.1 Case Hardening Process Route
Blank → Forging → Normalizing → Rough Turning → Hobbing → Carburizing+Quench → Gear Grinding → NDT Inspection
Geyon Transmission offers high-precision gear grinding and machining services for post-carburizing finishing.
3.2 Through-Hardening Process Route
Blank → Forging → Q&T Treatment → Rough Turning → Finish Turning → Hobbing → Surface Treatment (blackening/phosphating) → Inspection
4. International Material Cross-Reference
When designing for export or switching materials, use this cross-reference:
| China GB | USA SAE/AISI | EU EN | Type |
|---|---|---|---|
| 20CrMnTi | No direct match | 18CrMo4 (approx.) | Carburizing |
| 20CrMo | 4118 / 4120 | 18CrMo4 | Carburizing |
| 18CrNiMo7-6 | 4817 (approx.) | 18CrNiMo7-6 | Carburizing (high toughness) |
| 40Cr | 5140 | 41Cr4 | Q&T |
| 42CrMo | 4140 | 42CrMo4 | Q&T (high strength) |
💡 Note: 20CrMnTi is a China-specific grade. For export projects, substitute with 18CrMo4 or 8620H with a confirmed technical agreement.
For a complete range of custom gear solutions including spur gears, bevel gears, and helical gears, consult Geyon Transmission’s engineering team.
5. References
- GB/T 3077-2015 “Alloy Structural Steels”
- AGMA 2001-D04 “Rating the Pitting Resistance and Bending Strength”
- ISO 6336 “Calculation of Load Capacity of Spur and Helical Gears”
- “Gear Materials and Heat Treatment” — Gear Handbook, Chapter 3
Daily Learning Note — July 22, 2026