8620 Case-Hardening Steel vs 4140 Alloy Steel
8620 and 4140 are alloy steels with different hardening strategies. 8620 is a low-carbon nickel-chromium-molybdenum case-hardening steel — carburized to a hard ~58-62 HRC case over a tough, soft core. 4140 is a medium-carbon Cr-Mo steel that through-hardens by quench-and-temper to a uniform ~197 HB (and much higher when fully hardened). Case-hardened wear surface versus uniform through-hardened strength.
The verdict
Choose 8620 for carburized gears and shafts that need a hard, wear-resistant case over a tough, impact-absorbing core. Choose 4140 when you need uniform through-hardened strength and toughness across the whole section — for highly stressed shafts, gears, and tooling hardened throughout.
Side-by-side data
| Property | 8620 Case-Hardening Steel | 4140 Alloy Steel |
|---|---|---|
| Category | Steel | Steel |
| Density (g/cm³) | 7.85 | 7.85 |
| Tensile strength (MPa) | 635 | 655 |
| Yield strength (MPa) | 360 | 415 |
| Elongation (%) | 22 | 20 |
| Hardness | 58-62 HRC case / soft core | 197 HB |
| Max service temp (°C) | 425 | 425 |
| Machinability | ●●●●● | ●●●●● |
| Corrosion resistance | ●●●●● | ●●●●● |
| Relative cost | ●●●●● | ●●●●● |
| Thermal cond. (W/m·K) | 46 | 42 |
| Typically used for | Carburized gears & shafts (hard case, tough core) | High-strength heat-treatable shafts/gears |
Which should you choose?
Choose 8620 Case-Hardening Steel when…
- Making carburized gears, pins, and shafts needing a hard wear surface
- A tough, ductile core is essential to absorb shock (22% elongation)
- Surface hardness of ~58-62 HRC over a soft core is the goal
- Parts see surface wear plus impact loading simultaneously
- Good machinability before carburizing matters (3.5 index)
- The classic case/core combination of carburized gear steel is wanted
Choose 4140 Alloy Steel when…
- Uniform strength through the whole section is required
- Through-hardening by quench-and-temper fits the part
- Highly stressed shafts, gears, and machine components
- Higher core strength than 8620's soft core is needed
- Tooling, axles, and fixtures benefiting from heat-treatable Cr-Mo
- Bulk strength matters more than a distinct hard surface case
Key differences that matter
- 8620 is a low-carbon (~0.20% C) Ni-Cr-Mo carburizing steel; 4140 is a medium-carbon (~0.40% C) Cr-Mo through-hardening steel
- 8620 develops a hard case (~58-62 HRC) over a tough soft core; 4140 hardens uniformly through the section
- Core ductility: 8620 lists 22% elongation (tough core) vs 4140's 20% — both tough, but 8620's core stays soft for impact
- 4140 reaches higher uniform core strength (and much higher when fully Q&T) than 8620's low-carbon core
- Machinability: 8620 (3.5) machines easier than 4140 (2.5) in the pre-hardened state
- Both are poor in corrosion (1.5/5) and need protective finishes
- Cost is similar (8620 ~1.9, 4140 ~1.7 index); choice hinges on case-hardening vs through-hardening need
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Open the Material SelectorGet a Quote →Frequently asked questions
What is the difference between 8620 and 4140 steel?
8620 is a low-carbon case-hardening steel that's carburized to give a hard surface (~58-62 HRC) over a tough soft core. 4140 is a medium-carbon alloy steel that through-hardens by quench-and-temper to uniform strength. Choose 8620 for a wear-resistant surface with impact-absorbing core; choose 4140 for consistent strength throughout the part.
Is 8620 harder than 4140?
At the surface, yes — carburized 8620 reaches about 58-62 HRC on the case, harder than typical through-hardened 4140. But that hardness is only skin-deep over a soft core, while 4140 hardens uniformly through the section. So 8620 wins on surface wear; 4140 wins on uniform core strength.
Why is 8620 used for gears?
8620 is ideal for gears because carburizing creates a hard, wear-resistant tooth surface that resists pitting and abrasion, while the low-carbon core stays tough and ductile to absorb shock and bending loads. This case-hardened combination handles the surface wear and impact fatigue gears experience, which a uniformly hard steel would tolerate less well.
Can 4140 be case-hardened like 8620?
4140 is usually through-hardened rather than carburized, because its medium carbon content (~0.40%) already lets it harden throughout by quench-and-temper. It can be surface-hardened by nitriding or induction hardening, but it isn't a carburizing grade like low-carbon 8620. For a deep carburized case over a soft core, 8620 is purpose-built.
Property values are typical/nominal figures for early-stage guidance only and vary by temper, grade, supplier and heat treatment. Confirm critical specifications against a certified datasheet or with an mfgiq engineer before production.