Alloy Steel 4140 vs 4340: Medium vs Heavy-Duty Applications
4140 is the versatile, cost-effective alloy steel for most machine components. 4340 adds nickel for exceptional toughness and deeper hardenability, making it essential for aerospace landing gear and heavy-duty crankshafts.
Side-by-Side Property Comparison
| Property | Alloy Steel 4140 | Alloy Steel 4340 |
|---|---|---|
| Tensile Strength | 655 MPa (95 ksi) | 745 MPa (108 ksi) |
| Yield Strength | 415 MPa (60 ksi) | 470 MPa (68 ksi) |
| Hardness | 197 HB | 217 HB |
| Density | 7.85 g/cm³ | 7.85 g/cm³ |
| Machinability Rating | 66/100 | 57/100 |
| Thermal Conductivity | 42.6 W/m·K | 44.5 W/m·K |
| Melting Point | 1,416–1,460 °C | 1,427–1,477 °C |
| Tolerance | ±0.02 mm | ±0.02 mm |
Key Differences
| Property | Alloy Steel 4140 | Alloy Steel 4340 | Winner |
|---|---|---|---|
| Tensile Strength | 655 MPa | 745 MPa | Alloy |
| Impact Toughness | Good | Exceptional | Alloy |
| Hardenability Depth | Good | Excellent (deep) | Alloy |
| Machinability | 66/100 | 57/100 | Alloy |
| Cost | ~$1.00/lb | ~$1.50/lb | Alloy |
| Fatigue Strength | Good | Excellent | Alloy |
Which Material Should You Choose?
Choose Alloy Steel 4140 when:
- General-purpose shafts and gears
- Toolholders and collets
- Moderate-stress components
- Budget-conscious alloy steel needs
- Oil and gas downhole tools
Choose Alloy Steel 4340 when:
- Aircraft landing gear
- Heavy-duty crankshafts
- High-impact structural parts
- Large cross-section parts needing through-hardening
- Extreme fatigue applications
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