Tool steelsUNS T31501

O1 Tool Steel

Also: AISI O1, UNS T31501, W.Nr. 1.2510, DIN 1.2510, 100MnCrW4, SKS3 (JIS, near), Böhler K460, Uddeholm Arne, O-1, oil hardening tool steel, O1 drill rod, O1 ground flat stock, gauge plate

O1 is the oil-hardening general-purpose tool steel: cheap, easy to machine, hardens to 57-62 HRC, stocked everywhere as drill rod and ground flat stock.

ASTM A681 (alloy tool steels)

Sourcing O1 parts or stock?

Attach a drawing (PDF or STEP) or describe the part; a U.S. shop that fits quotes it.

At a glance

O1 Tool Steel typical properties
Density	7.81 g/cm³ (0.282 lb/in³)
Tensile strength	About 630-730 MPa (90-105 ksi) annealed, estimated from hardness. Not a published property at 57-62 HRC, where O1 is rated by hardness.
Yield strength	n/a: tensile yield is not commonly published for hardened O1, which fractures with little yielding in tension.
Hardness	183-212 HB annealed; 57-62 HRC working hardness (about 63-65 HRC as quenched)
Elongation	Not meaningful at working hardness; annealed stock bends, straightens and takes light cold forming
Modulus of elasticity	207-214 GPa (30-31 Msi)
Thermal	Solidus roughly 1,400 °C (2,550 °F); softens above its tempering temperature, about 175-260 °C (350-500 °F), so it is not for hot work
Composition	Fe, 0.85-1.00% C, 1.00-1.40% Mn, 0.40-0.70% Cr, 0.40-0.60% W, up to 0.30% V, 0.10-0.50% Si

Typical values, Annealed as stocked (183-212 HB); working hardness after an oil quench from 790-815 °C (1,450-1,500 °F) and a temper at 175-260 °C (350-500 °F) to 57-62 HRC. Confirm against the mill certificate and the purchase spec.

MachinabilityGood: About 90% of the W1 (1% carbon tool steel) benchmark annealed, the easiest of the common tool steels to machine, drill and tap. Hardened O1 is ground or wire-cut.
WeldabilityPoor: An oil-hardening steel: the heat-affected zone hardens and cracks. Repair only, with a preheat of about 200-315 °C (400-600 °F) on annealed stock (below the tempering temperature on a hardened tool), matching filler and an immediate temper. Small O1 tools are usually remade instead.
FormabilityPoor: Thin annealed stock bends gently before hardening; bar and plate are machined. Nothing is formed after hardening.
Corrosion resistancePoor: Rusts quickly, especially on ground surfaces and from fingerprints. Oil, black oxide or a rust-preventive coating for storage.

O1 is a low-alloy tool steel with about 0.9% carbon, 1.2% manganese and small additions of chromium and tungsten. It is hardened by quenching in oil from a low hardening temperature of about 800 °C (1,475 °F), which any small shop furnace can reach. It is the oldest general-purpose tool steel in U.S. tool rooms and the cheapest to buy and machine, stocked everywhere as drill rod and precision ground flat stock (gauge plate).

Annealed at 183-212 HB it machines, drills and taps easily. Hardened and tempered at 175-260 °C (350-500 °F) it reaches 57-62 HRC, takes a fine edge, and is the usual choice for short-run blanking and forming dies, punches, gauges, jig and fixture details, knives and woodworking tools.

The oil quench is its weakness. O1 moves and bows more than air-hardening A2 in heat treatment, and sharp corners and uneven sections crack more easily. It has the lowest wear resistance of the common cold work grades, it rusts, and it softens above about 200-260 °C, so it is not for hot work. When distortion is the problem, go to A2; when wear is, D2.

Heat treatment

Preheat large parts at about 650 °C (1,200 °F). Austenitize at 790-815 °C (1,450-1,500 °F), soak until through temperature, and quench in warm oil to about 50-65 °C (125-150 °F). Temper immediately: about 175 °C (350 °F) for 61-62 HRC or 260 °C (500 °F) for 57-58 HRC, and temper twice for gauges. Protect the surface with an atmosphere, a foil wrap or anti-scale compound, or leave grind stock for the decarburized skin. Anneal at 760-790 °C (1,400-1,450 °F) with a slow furnace cool; stress relieve at about 650 °C (1,200 °F) after rough machining.

Tempers and conditions

Annealed (183-212 HB), as stocked
Hardened and tempered, 57-62 HRC

Stock forms

Drill rod (precision ground round, annealed)
Precision ground flat and square stock (gauge plate)
Decarb-free (DF) oversize flat bar
Round and flat bar
Plate and blocks

Relative cost

Tier 2 of 5. The cheapest mainstream tool steel: roughly 2-3x 1018 per pound and below A2 and D2. Drill rod and ground flat stock are off-the-shelf in fractional and metric sizes.

Typical applications

Short-run blanking, forming and trim dies
Punches, pilots and die buttons
Gauges, templates and measuring tools
Jig and fixture details and locators
Knives, chisels and woodworking tools
Taps, reamers and form tools for soft materials
Short-run plastic mold inserts

Process compatibility

How O1 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.

O1 Tool Steel process compatibility
Process	Rating	Notes
CNC Machining	GoodGood	Annealed, the easiest tool steel to machine. Leave grind stock for the movement in the oil quench.
CNC Grinding	ExcellentExcellent	Grinds freely and takes a fine finish; the standard finish after hardening.
Centerless Grinding	GoodGood	Drill rod is centerless ground; pins, pilots and punches finish the same way after hardening.
Wire EDM	GoodGood	Cuts hardened O1, but oil-quenched parts carry more residual stress than air-hardened ones; stress relieve or temper before deep cuts and skim the recast layer.
Heat Treating	ExcellentExcellent	A low hardening temperature and a plain oil quench: any tool room or commercial heat treater can do it.
Vacuum Heat Treating	FairFair	Needs a vacuum furnace with an oil quench; a standard nitrogen gas quench is too slow to harden O1 fully.
Annealing	GoodGood	Before re-hardening or reworking a hardened part: 760-790 °C (1,400-1,450 °F) and a slow furnace cool.
Stress Relieving	GoodGood	At about 650 °C (1,200 °F) after roughing, to reduce movement in the oil quench.
Black Oxide	GoodGood	The usual finish on O1 tooling details and fixtures; oil it for rust resistance.
Hard Chrome Plating	FairFair	Possible on wear surfaces; bake for hydrogen immediately after plating and keep plating off sharp edges.
Waterjet Cutting	GoodGood	Cuts blanks in any condition with no heat-affected zone.
Laser Cutting	FairFair	Thin annealed plate cuts cleanly, but the edge hardens; anneal, grind or machine it off before forming or hardening.
Welding	PoorPoor	Repair only: preheat about 200-315 °C (400-600 °F), matching filler, and temper right after.
Nitriding	Not recommendedNot recommended	Nitriding runs near 500-550 °C, far above the temper that gives O1 its hardness, so the core would soften to well below 50 HRC. Use D2, A2 or H13 tempered high for a nitrided tool.

Design and sourcing tips

Use O1 for short runs, gauges and one-off tools where cost and machinability matter more than wear life; step to A2 when distortion is the problem and to D2 when wear is.
Leave about 0.25-0.4 mm (0.010-0.015") per side for grinding after hardening, more on long and thin parts that bow in the oil quench.
Keep sections balanced and avoid sharp internal corners, holes close to an edge and stamped markings: they start quench cracks in oil.
Specify a hardness range and a temper ("O1, 58-60 HRC"), not just "hardened".
Start from drill rod or precision ground flat stock to skip squaring and truing operations.

Frequently asked questions

What is O1 tool steel used for?: General-purpose tooling where cost and ease of machining matter most: short-run blanking and forming dies, punches, gauges, jig and fixture parts, knives, chisels and woodworking tools, and short-run mold inserts. It is the steel most tool rooms keep on the shelf as drill rod and ground flat stock.
Can you heat treat O1 at home?: Small parts, yes, which is part of its appeal. Heat evenly to about 800-815 °C (1,475-1,500 °F), a little past the point where it stops attracting a magnet, soak briefly, quench in warm oil in a lidded metal container away from anything flammable, and temper right away in an oven at 175-205 °C (350-400 °F) for an hour or two. Wrap it in stainless foil or leave grind stock for scale and decarburization. Large or precise parts belong at a heat treater.
Is O1 stainless?: No. It has only about 0.5% chromium and rusts quickly, even from handling. Oil it, black oxide it, or store it with a rust preventive. For a hardenable steel that resists corrosion, look at 440C or 420 stainless.
How hard is O1 tool steel?: Annealed, about 183-212 HB. As quenched in oil, about 63-65 HRC in normal sections. Tempered for use at 57-62 HRC: about 61-62 HRC after a temper at 175 °C (350 °F) and about 57-58 HRC at 260 °C (500 °F).
O1 vs 1095 for knives?: Both are simple oil-hardening high-carbon steels that take a keen edge. O1 adds manganese, chromium and tungsten, so it hardens deeper and more forgivingly in oil and usually holds an edge a little longer; 1095 is cheaper and easier to find as thin strip. Both rust. Makers who want a predictable oil quench in thicker blades tend to prefer O1.

Processes that commonly use it

Wire EDM Sinker EDM CNC Grinding Black Oxide Heat Treating

Related materials

A2 Tool Steel D2 Tool Steel 1095 Steel 52100 Steel S7 Tool Steel

Where to verify these values

ASTM A681 (chemistry and annealed hardness limits)
Producer data sheets: Böhler K460, Uddeholm Arne (O1 / 1.2510)
ASM Handbook Vol. 4, Heat Treating (tool steel hardening and tempering)
Roberts, Krauss and Kennedy, Tool Steels, 5th ed. (ASM International)

O1 Tool Steel

Also: AISI O1, UNS T31501, W.Nr. 1.2510, DIN 1.2510, 100MnCrW4, SKS3 (JIS, near), Böhler K460, Uddeholm Arne, O-1, oil hardening tool steel, O1 drill rod, O1 ground flat stock, gauge plate

O1 is the oil-hardening general-purpose tool steel: cheap, easy to machine, hardens to 57-62 HRC, stocked everywhere as drill rod and ground flat stock.

ASTM A681 (alloy tool steels)

At a glance

O1 Tool Steel typical properties
Density	7.81 g/cm³ (0.282 lb/in³)
Tensile strength	About 630-730 MPa (90-105 ksi) annealed, estimated from hardness. Not a published property at 57-62 HRC, where O1 is rated by hardness.
Yield strength	n/a: tensile yield is not commonly published for hardened O1, which fractures with little yielding in tension.
Hardness	183-212 HB annealed; 57-62 HRC working hardness (about 63-65 HRC as quenched)
Elongation	Not meaningful at working hardness; annealed stock bends, straightens and takes light cold forming
Modulus of elasticity	207-214 GPa (30-31 Msi)
Thermal	Solidus roughly 1,400 °C (2,550 °F); softens above its tempering temperature, about 175-260 °C (350-500 °F), so it is not for hot work
Composition	Fe, 0.85-1.00% C, 1.00-1.40% Mn, 0.40-0.70% Cr, 0.40-0.60% W, up to 0.30% V, 0.10-0.50% Si

MachinabilityGood

About 90% of the W1 (1% carbon tool steel) benchmark annealed, the easiest of the common tool steels to machine, drill and tap. Hardened O1 is ground or wire-cut.

WeldabilityPoor

An oil-hardening steel: the heat-affected zone hardens and cracks. Repair only, with a preheat of about 200-315 °C (400-600 °F) on annealed stock (below the tempering temperature on a hardened tool), matching filler and an immediate temper. Small O1 tools are usually remade instead.

FormabilityPoor

Thin annealed stock bends gently before hardening; bar and plate are machined. Nothing is formed after hardening.

Corrosion resistancePoor

Rusts quickly, especially on ground surfaces and from fingerprints. Oil, black oxide or a rust-preventive coating for storage.

Heat treatment

Process compatibility

How O1 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.

O1 Tool Steel process compatibility
Process	Rating	Notes
CNC Machining	GoodGood	Annealed, the easiest tool steel to machine. Leave grind stock for the movement in the oil quench.
CNC Grinding	ExcellentExcellent	Grinds freely and takes a fine finish; the standard finish after hardening.
Centerless Grinding	GoodGood	Drill rod is centerless ground; pins, pilots and punches finish the same way after hardening.
Wire EDM	GoodGood	Cuts hardened O1, but oil-quenched parts carry more residual stress than air-hardened ones; stress relieve or temper before deep cuts and skim the recast layer.
Heat Treating	ExcellentExcellent	A low hardening temperature and a plain oil quench: any tool room or commercial heat treater can do it.
Vacuum Heat Treating	FairFair	Needs a vacuum furnace with an oil quench; a standard nitrogen gas quench is too slow to harden O1 fully.
Annealing	GoodGood	Before re-hardening or reworking a hardened part: 760-790 °C (1,400-1,450 °F) and a slow furnace cool.
Stress Relieving	GoodGood	At about 650 °C (1,200 °F) after roughing, to reduce movement in the oil quench.
Black Oxide	GoodGood	The usual finish on O1 tooling details and fixtures; oil it for rust resistance.
Hard Chrome Plating	FairFair	Possible on wear surfaces; bake for hydrogen immediately after plating and keep plating off sharp edges.
Waterjet Cutting	GoodGood	Cuts blanks in any condition with no heat-affected zone.
Laser Cutting	FairFair	Thin annealed plate cuts cleanly, but the edge hardens; anneal, grind or machine it off before forming or hardening.
Welding	PoorPoor	Repair only: preheat about 200-315 °C (400-600 °F), matching filler, and temper right after.
Nitriding	Not recommendedNot recommended	Nitriding runs near 500-550 °C, far above the temper that gives O1 its hardness, so the core would soften to well below 50 HRC. Use D2, A2 or H13 tempered high for a nitrided tool.

Design and sourcing tips

Use O1 for short runs, gauges and one-off tools where cost and machinability matter more than wear life; step to A2 when distortion is the problem and to D2 when wear is.

Leave about 0.25-0.4 mm (0.010-0.015") per side for grinding after hardening, more on long and thin parts that bow in the oil quench.

Keep sections balanced and avoid sharp internal corners, holes close to an edge and stamped markings: they start quench cracks in oil.

Specify a hardness range and a temper ("O1, 58-60 HRC"), not just "hardened".

Start from drill rod or precision ground flat stock to skip squaring and truing operations.

Frequently asked questions

What is O1 tool steel used for?

General-purpose tooling where cost and ease of machining matter most: short-run blanking and forming dies, punches, gauges, jig and fixture parts, knives, chisels and woodworking tools, and short-run mold inserts. It is the steel most tool rooms keep on the shelf as drill rod and ground flat stock.

Can you heat treat O1 at home?

Small parts, yes, which is part of its appeal. Heat evenly to about 800-815 °C (1,475-1,500 °F), a little past the point where it stops attracting a magnet, soak briefly, quench in warm oil in a lidded metal container away from anything flammable, and temper right away in an oven at 175-205 °C (350-400 °F) for an hour or two. Wrap it in stainless foil or leave grind stock for scale and decarburization. Large or precise parts belong at a heat treater.

Is O1 stainless?

No. It has only about 0.5% chromium and rusts quickly, even from handling. Oil it, black oxide it, or store it with a rust preventive. For a hardenable steel that resists corrosion, look at 440C or 420 stainless.

How hard is O1 tool steel?

Annealed, about 183-212 HB. As quenched in oil, about 63-65 HRC in normal sections. Tempered for use at 57-62 HRC: about 61-62 HRC after a temper at 175 °C (350 °F) and about 57-58 HRC at 260 °C (500 °F).

O1 vs 1095 for knives?

Both are simple oil-hardening high-carbon steels that take a keen edge. O1 adds manganese, chromium and tungsten, so it hardens deeper and more forgivingly in oil and usually holds an edge a little longer; 1095 is cheaper and easier to find as thin strip. Both rust. Makers who want a predictable oil quench in thicker blades tend to prefer O1.