Case Hardening vs Through-Hardening Steel for Gears
Compare case hardening vs through-hardening gear steels, including 20CrMnTi and 40Cr, load capacity, wear resistance, and material selection impact.
topic: Gear Materials — Case Hardening Steel vs Through-Hardening Steel
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
Geyontech 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 Geyontech’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