4140 is a chromium-molybdenum alloy steel valued for strength and toughness. Machine it in the annealed or pre-hardened (HT) condition; fully hardened 4140 needs much slower speeds or grinding.
Key properties
| Property | Typical value |
|---|---|
| Density | 0.284 lb/in³ (7.85 g/cm³) |
| Hardness | ~197 HB annealed |
| Tensile strength | ~95,000–120,000 psi |
| Machinability | Fair |
| Composition | Fe with Cr and Mo |
Cutting speeds
| Operation | HSS (SFM) | Carbide (SFM) |
|---|---|---|
| Milling | 50 – 80 | 200 – 350 |
| Turning | 60 – 90 | 250 – 450 |
| Drilling | 40 – 70 | 150 – 280 |
Convert to RPM: RPM = (SFM × 3.82) / cutter diameter. Use the Feeds & Speeds calculators.
Tooling and tips
- Use TiAlN carbide, flood coolant, and a very rigid setup.
- Pre-hardened (28–32 HRC) stock machines fine; over ~40 HRC, slow right down.
- Maintain feed pressure — rubbing glazes the surface and burns the edge.
See Cutting Speeds for Steel and the full Feeds and Speeds Chart.