Stainless Steel 304 vs 4140 Alloy Steel for CNC Parts
304 stainless steel provides corrosion resistance without coatings, ideal for food, medical, and chemical environments. 4140 alloy steel offers higher strength and hardenability at lower cost for industrial machinery where coatings are acceptable.
Side-by-Side Property Comparison
| Property | Stainless Steel 304 | Alloy Steel 4140 |
|---|---|---|
| Tensile Strength | 515 MPa (75 ksi) | 655 MPa (95 ksi) |
| Yield Strength | 205 MPa (30 ksi) | 415 MPa (60 ksi) |
| Hardness | 201 HB | 197 HB |
| Density | 8.00 g/cm³ | 7.85 g/cm³ |
| Machinability Rating | 45/100 | 66/100 |
| Thermal Conductivity | 16.2 W/m·K | 42.6 W/m·K |
| Melting Point | 1,400–1,450 °C | 1,416–1,460 °C |
| Tolerance | ±0.02 mm | ±0.02 mm |
Key Differences
| Property | Stainless Steel 304 | Alloy Steel 4140 | Winner |
|---|---|---|---|
| Corrosion Resistance | Excellent (no coating) | Poor (needs coating) | Stainless |
| Tensile Strength | 515 MPa | 655 MPa | Alloy |
| Machinability | 45/100 | 66/100 | Alloy |
| Through-Hardenable | No | Yes | Alloy |
| Material Cost | ~$2.50/lb | ~$1.00/lb | Alloy |
| Maintenance Cost | Low (no rust) | Higher (coating needed) | Stainless |
Which Material Should You Choose?
Choose Stainless Steel 304 when:
- Corrosive environments
- Food and medical equipment
- Parts where coatings are impractical
- Marine applications
- Chemical processing
Choose Alloy Steel 4140 when:
- High-strength structural parts
- Shafts and gears (with coatings)
- Cost-sensitive projects
- Hardened and ground components
- Industrial machinery
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