Gear Material Selection: Case Hardening Steel Compared with Through-Hardening Steel

topic: Gear Materials — Case Hardening Steel vs Through-Hardening Steel

Gear Material Selection: Case Hardening Steel Compared with Through-Hardening Steel

Material choice sits at the centre of gear design. It governs how long a transmission lasts, how much load it can carry, and what it costs to manufacture. Two families dominate industrial practice: 20CrMnTi, a case hardening steel, and 40Cr, a through-hardening steel. Where impact and wear are severe, the carburizing route is normally the right answer; where loads are medium-to-low and volumes are high, through-hardening is usually sufficient. Choose wrongly and the result is premature pitting or tooth fracture, which undermines equipment reliability.

1. How the Two Families Compare

The table below sets the two routes side by side across the properties that matter most during design.

• Dimension Surface Hardness: 58~62 HRC (very high)
  • Through-Hardening (40Cr): 28~35 HRC (moderate)
• Dimension Core Toughness: High (low-C martensite/bainite)
  • Through-Hardening (40Cr): Moderate (tempered sorbite)
• Dimension Case Depth: 0.8~1.5 mm (controllable)
  • Through-Hardening (40Cr): Uniform throughout
• Dimension Contact Fatigue Strength: Very high (≥1500 MPa)
  • Through-Hardening (40Cr): Moderate (800~1000 MPa)
• Dimension Bending Fatigue Strength: High (supported by tough core)
  • Through-Hardening (40Cr): Medium-high (uniform)
• Dimension Wear Resistance: Excellent
  • Through-Hardening (40Cr): Fair
• Dimension Impact Resistance: Excellent
  • Through-Hardening (40Cr): Moderate
• Dimension Machining Difficulty: Requires grinding (hard after HT)
  • Through-Hardening (40Cr): Can use hobbing directly
• Dimension Relative Cost Ratio: 1.0 (baseline)
  • Through-Hardening (40Cr): 0.55~0.70
• Dimension Typical Applications: Auto transmissions, construction machinery
  • Through-Hardening (40Cr): General gearboxes, agricultural machinery

What This Means in Practice

Contact stress verification comes first. A design where σ_H ≥ 1200 MPa calls for case hardening steel. Where σ_H ≤ 900 MPa, through-hardening steel is adequate. Designs falling between those two figures need a combined assessment that brings in the impact factor K_A.

2. Material Definitions and Grade Tables

2.1 Case Hardening Grades

Case hardening steels are low-carbon alloy steels, generally 0.15%~0.25% C, that are carburized and quenched to produce a hard shell over a tough core.

2.2 Through-Hardening Grades

Through-hardening steels are medium-carbon steels, typically 0.35%~0.55% C. Quenching followed by high-temperature tempering gives a uniform tempered martensite or sorbite structure across the whole cross-section.

3. Cross-Referencing Grades Internationally

Export projects and material substitutions both depend on a reliable cross-reference.

One point deserves attention: 20CrMnTi is a China-specific grade. Export projects should substitute 18CrMo4 or 8620H, with the change confirmed through a technical agreement.

PinyiGear supplies a full range of custom gear solutions, including spur gears, bevel gears, and helical gears. The engineering team is available to discuss material selection for specific applications.

4. Choosing Between the Two Routes

The decision follows operating conditions through to a material family.

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]

4.1 Process Route for Case Hardening

Blank → Forging → Normalizing → Rough Turning → Hobbing → Carburizing+Quench → Gear Grinding → NDT Inspection

For post-carburizing finishing, PinyiGear provides high-precision gear grinding and machining services.

4.2 Process Route for Through-Hardening

Blank → Forging → Q&T Treatment → Rough Turning → Finish Turning → Hobbing → Surface Treatment (blackening/phosphating) → Inspection

5. Source Documents

• 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

PinyiGear — Gear Materials & Capabilities Overview

Daily Learning Note — July 22, 2026