AISI O1: Oil-Hardening General-Purpose Tool Steel
ASTM A681 · Published: 2026-06-03 · Updated: July 2026
AISI O1 is an oil-hardening, non-shrinking tool steel with good machinability and minimal distortion during hardening — making it the preferred grade for gauge blocks, jigs, fixtures, and prototype tooling. Its tungsten content (0.5%) provides...
AISI O1 is an oil-hardening, non-shrinking tool steel with good machinability and minimal distortion during hardening — making it the preferred grade for gauge blocks, jigs, fixtures, and prototype tooling. Its tungsten content (0.5%) provides fine grain size and wear resistance without the grinding difficulty of high-chromium D2.
Quick Facts
| Category | Tool Steel |
| Standard | ASTM A681 |
| Density | 7.80 g/cm³ |
| Yield Strength | N/A (hardened) |
| Tensile Strength | 1,380-1,860 MPa |
Global Equivalents & Cross-Reference
Related Materials
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Frequently Asked Questions
Why oil-quench O1 rather than water-quench?
Oil quenching reduces the thermal shock and resulting distortion compared to water — critical for precision tools like gauges and dies where post-hardening grinding should be minimized. The 'O' designation specifically indicates oil-hardening: the alloy composition is formulated for full hardening in oil, unlike water-hardening W-series steels.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu