O1 tool steel equivalents

O1 tool steel is nearest to EN 1.2510 (100MnCrW4), JIS SKS3 and China GB 9CrWMn. These are nearest equivalents, not identical grades: the governing standard or mill certificate decides.

Designations follow EN 10027 and ISO 4957. Updated 6 October 2026.

EN steel number for O1

1.2510

100MnCrW4 · SKS3 · 9CrWMn

O1 = EN 1.2510 100MnCrW4 = JIS SKS3 = GB 9CrWMn

AISI / ASTM / SAE US: O1
EN steel number EN 10027-2: 1.2510
EN steel name EN 10027-1: 100MnCrW4
JIS Japan: SKS3
GB China: 9CrWMn
Family: Cold-work tool steel, oil-hardening

Nearest-equivalent designationsCheck the mill certificate

About O1

Cold-work tool steel, oil-hardening. General characteristics; check the standard and the supplier for anything critical.

What it is

An oil-hardening cold-work tool steel. It hardens at a relatively low temperature, which makes it a favourite for small shops and toolrooms.

Typical uses

Gauges, punches, short-run dies, knives, bushings and general toolroom parts.

Machining notes

Machines well in the annealed condition. Ground flat stock is widely available.

Related grades

Grades people often compare with this one.

A2 tool steelAir-hardening, less distortion1.2363 · SKD12 · Cr5Mo1V D2 tool steelHigher wear resistance1.2379 · SKD11 · Cr12Mo1V1

Drilling and tapping

Tap drills

Drill and tap hardenable tool steel in the annealed condition, before heat treatment. A slightly larger tap drill, around 60–70 % thread, eases the load on the tap.

Tap drill chart

Drill sizes

Number, letter and fractional drill sizes with metric equivalents, for dowel, clearance and pilot holes in tooling.

Drill size chart

All tool steel grades

Nearest equivalents across the five designation systems.

Tool steel grade equivalents
AISI / ASTM	EN name	EN number	JIS	China GB
D2	X153CrMoV12	1.2379	SKD11	Cr12Mo1V1
H13	X40CrMoV5-1	1.2344	SKD61	4Cr5MoSiV1
O1	100MnCrW4	1.2510	SKS3	9CrWMn
A2	X100CrMoV5	1.2363	SKD12	Cr5Mo1V
M2	HS6-5-2	1.3343	SKH51	W6Mo5Cr4V2
P20	40CrMnMo7	1.2311	—	3Cr2Mo

Equivalents are nearest matches, not identical grades. Composition limits, mechanical properties and test requirements differ in detail between standards and product forms. For anything that matters, the governing standard, the drawing and the mill certificate decide.

Common questions

What is the European equivalent of O1 tool steel?

The nearest EN grade is 1.2510, steel name 100MnCrW4.

What is the JIS equivalent of O1 tool steel?

The nearest Japanese grade is SKS3.

What is the Chinese GB equivalent of O1 tool steel?

The nearest Chinese GB grade is 9CrWMn.

Is 1.2510 the same as O1?

1.2510 (100MnCrW4) is the nearest European equivalent of O1, not an identical grade. Composition limits and required properties differ in detail, so check the governing standard or the mill certificate when it matters.

What is O1 used for?

Gauges, punches, short-run dies, knives, bushings and general toolroom parts.