O1 Oil-Hardening Tool Steel: Precision Tooling & Wear Resistance Guide
ASTM A681 / UNS T31501 · Published: 2026-06-02 · Updated: July 2026
O1 is the most widely used oil-hardening tool steel — a general-purpose cold-work steel offering excellent wear resistance, good machinability in the annealed condition, and low distortion during hardening due to the oil quench (vs water quench...
O1 is the most widely used oil-hardening tool steel — a general-purpose cold-work steel offering excellent wear resistance, good machinability in the annealed condition, and low distortion during hardening due to the oil quench (vs water quench for W-series steels). O1 achieves full hardness with a relatively low austenitizing temperature (790-815°C), which minimizes grain growth and decarburization during heat treatment. The 0.90% carbon, 1.0-1.4% manganese, 0.50% chromium, and 0.50% tungsten composition produces a fine martensitic microstructure with dispersed tungsten carbides for wear resistance.
Heat treatment: preheat at 650°C, austenitize at 790-815°C (hold 30 min + 15 min per additional 25mm thickness), quench in warm oil (50-80°C), and temper immediately. Temper at 150-200°C for 62-64 HRC (for gages and measuring tools), 200-300°C for 58-62 HRC (for cutting tools, punches, dies), or 300-450°C for 54-58 HRC (for components requiring toughness). O1 is the standard material for: blanking dies, forming dies, trim dies, gages, master tools, precision measuring instruments, and cutting tools where the tool will not be subjected to high cutting temperatures (the temper resistance is poor above 200°C — hardness drops sharply above this temperature).
Quick Facts
| Category | Tool Steel |
| Standard | ASTM A681 / UNS T31501 |
| Density | 7810 kg/m³ |
| Yield Strength | 1,900-2,100 MPa (hardened & tempered) |
| Tensile Strength | 2,100-2,400 MPa |
Physical Properties
| Melting Point | 1427°C |
| Thermal Conductivity | 32 W/m·K |
| Density | 7.76 g/cm³ |
Global Equivalents & Cross-Reference
Heat Treatment & Processing
| Austenitizing | 790-815°C, soak 10-30min |
| Quenching | Oil quench |
| Tempering | 150-260°C (cutting); 260-370°C (cold work) |
| Annealing | 760-780°C, furnace cool |
Welding & Fabrication
| Preheat | 300-400°C mandatory |
| Filler Metal | O1 filler or E312 |
| Pwht | Anneal + re-harden |
| Weldability Rating | Poor — repair only |
Related Materials
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Frequently Asked Questions
When should I use O1 vs A2 tool steel?
O1 requires oil quenching — distortion is higher than air-hardening A2, especially in complex geometries with thin sections. A2 air-hardens with significantly less distortion. However, O1 has slightly better wear resistance (due to tungsten carbides) and costs ~20-30% less. Use O1 for simple geometries where distortion is manageable; use A2 for complex geometries or where the extra cost is justified by reduced post-heat-treatment grinding.
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