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.
| 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 | ±0.01 mm | ±0.01 mm |
| 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 |
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