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.
FAQ
Should 4140 be machined in the annealed or hardened state? Machining is far easier and tool life much longer in the annealed or normalized condition. If higher hardness is required in the final part, most shops rough machine soft, heat treat, then finish machine or grind to final dimensions.
See Cutting Speeds for Steel and the full Feeds and Speeds Chart.