O1
1.2510 / BO1 · EN ISO 4957 · 1.2510
O1 (1.2510 / BO1) is a tool steel (Oil-Hardening Cold Work family) to EN ISO 4957 (1.2510). Supplied hardened in service; for analysis treat it as elastic (E = 210 GPa) with a hardness- or fracture-based limit. It is formulated for hardness, wear resistance and dimensional stability in tooling.
Bilinear material model — nonlinear FEA
This grade is supplied hardened in service and is not represented as a bilinear material — it has a negligible uniform-plastic range. Model it as elastic (E = 210 GPa) with an appropriate fracture / hardness-based limit.
Ratings — engineering judgement
Weldabilitypoor
Not for welding in service — high carbon cracks readily and welding destroys the temper.
Formabilitypoor
Shaped by machining/grinding in the annealed state, then hardened.
Machinabilitygood
A key O1 advantage — machines cleanly and predictably in the soft (annealed) condition, which is why it is the toolroom favourite for one-offs.
Corrosion resistancepoor
None — oil, wax or plate for storage.
Hardenabilitygood
Oil-hardening — hardens on an oil quench with modest distortion. Shallower hardening than the air-hardening grades, so best in smaller sections.
Toughnessmoderate
Tougher than the high-chromium cold-work steels (D2) at similar hardness, trading away some wear resistance.
Expert notes
O1 is the general-purpose toolroom steel — if you buy “gauge plate” or “ground flat stock”, you are almost certainly buying O1. It machines cleanly and predictably in the annealed state, then oil-hardens to around 60 HRC, which makes it the easy, forgiving choice for gauges, jigs, form tools and short-to-medium-run press tools.
It sits opposite D2 on the cold-work trade-off. O1 is tougher and far easier to machine and grind, but hardens shallower (oil quench, so smaller sections) and wears faster. Choose O1 for one-offs and general tooling; choose D2 (or A2) when abrasive wear over long runs is the priority and you will accept a fussier, more brittle steel.
Its oil quench distorts more than an air-hardening grade — leave grinding allowance and keep sections reasonably uniform to avoid cracking on quench.
Like other tool steels it is used fully hardened with negligible uniform plasticity, so there is no meaningful bilinear model — treat it as elastic (E ≈ 210 GPa) up to a hardness/fracture-based limit.
Physical properties
| Density | 7850 | kg/m³ |
| Elastic modulus | 210 | GPa |
| Shear modulus | 81 | GPa |
| Poisson's ratio | 0.30 | |
| Thermal expansion | 12.0 | ×10⁻⁶/°C |
| Thermal conductivity | 30 | W/(m·K) |
| Specific heat | 460 | J/(kg·K) |
| Magnetic | Yes |
Chemical composition (wt %)
Available forms
Equivalent grades
| Standard | Grade | Note |
|---|---|---|
| AISI | O1 | |
| EN ISO 4957 | 1.2510 / 100MnCrW4 | |
| JIS | SKS3 | |
| BS 4659 | BO1 |
Historical designations
| Standard | Grade | Note |
|---|---|---|
| BS 4659 | BO1 | British oil-hardening tool-steel designation. |
| Trade | Gauge plate | The everyday name for ground O1 flat stock. |
Common applications
- Gauges, jigs and fixtures
- Blanking and forming tools (short–medium runs)
- Form tools, taps and reamers
- Knives, blades and cutting edges
- General toolroom one-offs
History
Oil-hardening manganese tool steels became the general-purpose toolroom standard — easy to machine, forgiving to heat treat, and available as precision-ground “gauge plate”.
- 1920Oil-hardening (Mn) cold-work tool steels established
- 1960Ground flat stock (“gauge plate”) in O1 becomes a toolroom staple