AISI O1 Oil-Hardening Tool Steel: Classic General-Purpose Grade
ASTM A681 · Updated: 2026-05-25
AISI O1 is a manganese-tungsten-chromium oil-hardening tool steel (0.85-1.0% C, 1.0-1.4% Mn, 0.4-0.6% Cr, 0.4-0.6% W) that is the classic general-purpose tool steel for low-to-medium production tooling. Oil quenching from 790-815°C produces 60-64 HRC with minimal distortion compared to water-hardening W1. Excellent machinability in the annealed condition (spheroidized carbide structure), making it the preferred grade for prototype tooling, gauges, fixtures, and short-run stamping dies. The tungsten addition improves wear resistance and helps maintain a fine grain size during hardening. Tempering at 175-260°C for cutting tools, 260-370°C for cold work dies requiring higher toughness.
Quick Facts
| Category | Tool Steel |
| Standard | ASTM A681 |
| Density | 7.80 g/cm³ |
| Yield Strength | 1,450 MPa (210 ksi) at 58-60 HRC |
| Tensile Strength | 1,760 MPa (255 ksi) at 58-60 HRC |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN 95MnWCr5 | Compare |
| DIN 1.2510 | Compare |
| SKS3 | Compare |
| 9CrWMn | Compare |
| BS BO1 | Compare |
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Frequently Asked Questions
When should O1 be chosen over A2 or D2?
O1 is preferred for: (1) prototype and short-run tooling where machinability and lower material cost matter more than production longevity, (2) tools with thin sections that would distort excessively in A2/D2's higher hardening temperatures, (3) applications where the oil quench is simpler than controlled-atmosphere air hardening. For production tooling above 10,000 cycles, A2 or D2 generally provide better value through longer tool life despite higher material cost.