Tool steelsUNS T30102

A2 Tool Steel

Also: AISI A2, UNS T30102, W.Nr. 1.2363, DIN 1.2363, X100CrMoV5, SKD12 (JIS, near), Böhler K305, Uddeholm Rigor, A-2, air hardening tool steel, A2 ground flat stock

A2 is the air-hardening, 5% chromium cold work tool steel: tougher than D2, stable in heat treatment, hardened to 57-62 HRC for dies, punches and gauges.

ASTM A681 (alloy tool steels)ISO 4957 / EN ISO 4957 (X100CrMoV5)

Sourcing A2 parts or stock?

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

At a glance

A2 Tool Steel typical properties
Density	7.86 g/cm³ (0.284 lb/in³)
Tensile strength	About 700-800 MPa (100-115 ksi) annealed, estimated from hardness. Not a published property at 57-62 HRC, where A2 is rated by hardness, toughness and compressive strength.
Yield strength	n/a: tensile yield is not commonly published for hardened A2. Compressive yield exceeds 2,000 MPa (290 ksi) at 58-62 HRC.
Hardness	About 200-240 HB annealed; 57-62 HRC working hardness (about 63-64 HRC as quenched)
Elongation	Not meaningful at working hardness; hardened A2 fails in tension with little plastic strain, though it absorbs more impact than D2
Modulus of elasticity	207 GPa (30 Msi)
Thermal	Solidus roughly 1,400 °C (2,550 °F); softens above its tempering temperature, usually 175-260 °C (350-500 °F), so it is not used for hot work
Composition	Fe, 0.95-1.05% C, 4.75-5.50% Cr, 0.90-1.40% Mo, 0.15-0.50% V, 0.40-1.00% Mn, 0.10-0.50% Si

Typical values, Annealed as stocked (about 200-240 HB); working hardness after an air or gas quench from about 955 °C (1,750 °F) and a double temper at 175-260 °C (350-500 °F) to 57-62 HRC. Confirm against the mill certificate and the purchase spec.

MachinabilityGood: About 65% of the W1 (1% carbon tool steel) benchmark annealed: slower than O1, easier than D2. Use carbide and rigid setups; hardened A2 is ground, wire-cut or hard milled.
WeldabilityFair: Repair welding is routine in tool rooms: TIG with an A2-type filler at a preheat of about 315-425 °C (600-800 °F) for annealed stock, or just below the last tempering temperature for a hardened tool, then a slow cool and a temper. Welded cold, the heat-affected zone air-hardens and cracks.
FormabilityPoor: Machined from bar, flat and plate, not formed. Annealed stock can be straightened; hardened A2 cannot be bent.
Corrosion resistancePoor: The 5% chromium gives slightly better rust resistance than O1, but A2 still rusts readily in humid air. Oil, black oxide or a coating.

A2 is a cold work tool steel with about 1% carbon, 5% chromium and 1% molybdenum. The A stands for air hardening: it hardens by cooling in still air or a gas quench from its hardening temperature, so it distorts less and cracks less than an oil-hardening steel such as O1. That dimensional stability makes it the default for die sections, punches, forming tools, gauges and precision tooling details that must hold size through heat treatment.

It sits between O1 and D2. It wears longer than O1 and is noticeably tougher than D2, because it carries far less carbide. Bought annealed at about 200-240 HB, it machines reasonably well, and it is hardened and tempered to 57-62 HRC, most often 58-60 HRC. Precision ground flat and square A2 is stocked in a wide range of sizes, which is why so many tool rooms reach for it first.

Its limits: on long runs of abrasive stock D2 outwears it, and under heavy impact S7 outlasts it. Very large sections cool slowly in air and may need a faster gas quench to reach full hardness at the center. It is not corrosion resistant, and it softens if it runs above its tempering temperature, so it is not a hot work steel.

Heat treatment

Preheat at about 790 °C (1,450 °F), austenitize at 925-980 °C (1,700-1,800 °F), usually 955 °C (1,750 °F), and soak until the section is through temperature. Cool in still air or a gas quench to about 50-65 °C (125-150 °F) and temper immediately, twice: about 175 °C (350 °F) for 60-62 HRC, about 260 °C (500 °F) for about 58 HRC. A high temper near 510-540 °C (950-1,000 °F) gives about 56-57 HRC with better stability for wire EDM and nitriding. A sub-zero treatment helps gauges hold size. Anneal at 845-870 °C (1,550-1,600 °F) with a slow furnace cool; stress relieve at 650-675 °C (1,200-1,250 °F) after rough machining.

Tempers and conditions

Annealed (about 200-240 HB), as stocked
Hardened and tempered, 57-62 HRC (58-60 typical)
Hardened, high temper, about 56-57 HRC (for EDM and nitriding)

Stock forms

Precision ground flat and square stock
Decarb-free (DF) oversize flat bar
Round and flat bar
Drill rod
Plate and saw-cut blocks

Relative cost

Tier 3 of 5. About 1.2-1.4x O1 per pound and similar to D2. Widely stocked, so ground flat stock is usually available off the shelf.

Typical applications

Blanking, forming and trim dies for medium runs
Punches and die buttons
Gauges, master gauges and precision tooling details
Coining and embossing dies
Mandrels, arbors and fixture wear parts
Plane irons, chisels and knife blades

Process compatibility

How A2 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.

A2 Tool Steel process compatibility
Process	Rating	Notes
CNC Machining	GoodGood	Annealed, it is the tool-room workhorse. Leave grind stock for finishing after hardening.
CNC Grinding	ExcellentExcellent	Grinds more freely than D2; the usual finish for punches, die sections and gauges after hardening.
Wire EDM	ExcellentExcellent	Cuts hardened die openings and punches; temper high or stress relieve large blocks first and skim-cut the recast layer.
Sinker EDM	GoodGood	Burns forms and cavities in hardened stock; remove the white layer and temper after heavy roughing.
Vacuum Heat Treating	ExcellentExcellent	Air hardening makes it a natural for vacuum with a gas quench: bright, decarb-free and minimal movement.
Heat Treating	GoodGood	Any tool-room furnace with atmosphere control or a foil wrap; open-air hardening decarburizes the surface.
Stress Relieving	GoodGood	At 650-675 °C (1,200-1,250 °F) after heavy roughing, for parts with tight flatness after hardening.
Annealing	GoodGood	Needed before a hardened part is reworked or re-hardened: 845-870 °C (1,550-1,600 °F) and a slow furnace cool.
Nitriding	FairFair	Only on parts tempered high, around 510-540 °C, which costs a few points of hardness; D2 and H13 are the usual nitrided grades.
Black Oxide	GoodGood	A common, cheap finish on fixtures and tooling details; oil it for rust resistance.
Hard Chrome Plating	FairFair	Used against galling on forming tools; bake for hydrogen right after plating.
Welding	FairFair	Tool-room repair welding with a 315-425 °C (600-800 °F) preheat on annealed stock, then a temper.
Laser Cutting	PoorPoor	The cut edge air-hardens and micro-cracks; saw, waterjet or wire-cut blanks instead, or machine the edge off.
Carburizing	Not recommendedNot recommended	A2 already carries 1% carbon; carburizing adds nothing but a brittle, over-carbided surface.

Compare A2

D2 vs A2 Tool Steel

Design and sourcing tips

Call out "A2, 58-60 HRC" and the heat treat route; add "temper 510 °C (950 °F) min" if the part will be wire EDM cut after hardening or nitrided.
Leave about 0.15-0.3 mm (0.006-0.012") per side for finish grinding; air hardening keeps size change small and predictable.
Choose A2 over O1 when the part must hold size through hardening, and over D2 when chipping or cracking is the failure mode.
Balance sections, add radii at internal corners and keep holes away from edges; even a gentle air quench cracks parts at sharp stress raisers.
Buy precision ground flat stock for plates, punches and gibs: it arrives square and decarb-free, which saves milling and surface grinding setups.

Frequently asked questions

Why is A2 called air hardening?: In the AISI system the letter describes the quench or the use: W for water, O for oil, A for air. A2 carries enough chromium and molybdenum that it hardens fully by cooling in still air or a gas quench from about 955 °C (1,750 °F), instead of a plunge into oil or water. The slower quench means less distortion and less cracking risk, which is why A2 is chosen for tools that have to hold size. Very large sections may need a faster gas quench to harden through the middle.
A2 vs O1: which should I use?: O1 is cheaper, machines more easily and hardens at a lower temperature in any small furnace, which makes it good for short runs, one-off tools and gauges. A2 costs a little more but moves less in hardening, wears longer and is tougher, so it wins for production dies, punches and anything that must hold size through heat treatment. When distortion or wear is the problem with an O1 tool, A2 is the usual step up.
Is A2 tougher than D2?: Yes. With about 1% carbon and 5% chromium against about 1.5% and 12% in D2, A2 has much less carbide, so at the same hardness it absorbs noticeably more impact before it chips or cracks. D2 wins on abrasive wear. Punches and dies that chip in D2 are commonly remade in A2, and those that still chip in A2 go to S7.
How hard is A2 tool steel?: Annealed, about 200-240 HB. Hardened and tempered it is used at 57-62 HRC, most often 58-60 HRC. As quenched it reaches about 63-64 HRC; a temper around 175 °C (350 °F) gives the top of the working range and around 260 °C (500 °F) the lower end.
Can A2 be welded?: Yes, for repair. Preheat to about 315-425 °C (600-800 °F) for annealed stock, or just below the last tempering temperature for a hardened tool so it does not soften, weld by TIG with an A2-type filler, cool slowly and temper. Tool rooms routinely build up chipped edges and modify dies this way. Welding cold or skipping the temper afterward leads to cracks.

Processes that commonly use it

CNC Milling Wire EDM Sinker EDM CNC Grinding Black Oxide Heat Treating Vacuum Heat Treating Stress Relieving Annealing

Related materials

D2 Tool Steel O1 Tool Steel S7 Tool Steel H13 Tool Steel M2 High-Speed Tool Steel

Where to verify these values

ASTM A681 (chemistry and annealed hardness limits)
Producer data sheets: Crucible A2, Böhler K305, Uddeholm Rigor
ASM Handbook Vol. 4, Heat Treating (tool steel hardening and tempering)
Roberts, Krauss and Kennedy, Tool Steels, 5th ed. (ASM International)

A2 Tool Steel

Also: AISI A2, UNS T30102, W.Nr. 1.2363, DIN 1.2363, X100CrMoV5, SKD12 (JIS, near), Böhler K305, Uddeholm Rigor, A-2, air hardening tool steel, A2 ground flat stock

A2 is the air-hardening, 5% chromium cold work tool steel: tougher than D2, stable in heat treatment, hardened to 57-62 HRC for dies, punches and gauges.

ASTM A681 (alloy tool steels)ISO 4957 / EN ISO 4957 (X100CrMoV5)

At a glance

A2 Tool Steel typical properties
Density	7.86 g/cm³ (0.284 lb/in³)
Tensile strength	About 700-800 MPa (100-115 ksi) annealed, estimated from hardness. Not a published property at 57-62 HRC, where A2 is rated by hardness, toughness and compressive strength.
Yield strength	n/a: tensile yield is not commonly published for hardened A2. Compressive yield exceeds 2,000 MPa (290 ksi) at 58-62 HRC.
Hardness	About 200-240 HB annealed; 57-62 HRC working hardness (about 63-64 HRC as quenched)
Elongation	Not meaningful at working hardness; hardened A2 fails in tension with little plastic strain, though it absorbs more impact than D2
Modulus of elasticity	207 GPa (30 Msi)
Thermal	Solidus roughly 1,400 °C (2,550 °F); softens above its tempering temperature, usually 175-260 °C (350-500 °F), so it is not used for hot work
Composition	Fe, 0.95-1.05% C, 4.75-5.50% Cr, 0.90-1.40% Mo, 0.15-0.50% V, 0.40-1.00% Mn, 0.10-0.50% Si

MachinabilityGood

About 65% of the W1 (1% carbon tool steel) benchmark annealed: slower than O1, easier than D2. Use carbide and rigid setups; hardened A2 is ground, wire-cut or hard milled.

WeldabilityFair

Repair welding is routine in tool rooms: TIG with an A2-type filler at a preheat of about 315-425 °C (600-800 °F) for annealed stock, or just below the last tempering temperature for a hardened tool, then a slow cool and a temper. Welded cold, the heat-affected zone air-hardens and cracks.

FormabilityPoor

Machined from bar, flat and plate, not formed. Annealed stock can be straightened; hardened A2 cannot be bent.

Corrosion resistancePoor

The 5% chromium gives slightly better rust resistance than O1, but A2 still rusts readily in humid air. Oil, black oxide or a coating.

Heat treatment

Process compatibility

How A2 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.

A2 Tool Steel process compatibility
Process	Rating	Notes
CNC Machining	GoodGood	Annealed, it is the tool-room workhorse. Leave grind stock for finishing after hardening.
CNC Grinding	ExcellentExcellent	Grinds more freely than D2; the usual finish for punches, die sections and gauges after hardening.
Wire EDM	ExcellentExcellent	Cuts hardened die openings and punches; temper high or stress relieve large blocks first and skim-cut the recast layer.
Sinker EDM	GoodGood	Burns forms and cavities in hardened stock; remove the white layer and temper after heavy roughing.
Vacuum Heat Treating	ExcellentExcellent	Air hardening makes it a natural for vacuum with a gas quench: bright, decarb-free and minimal movement.
Heat Treating	GoodGood	Any tool-room furnace with atmosphere control or a foil wrap; open-air hardening decarburizes the surface.
Stress Relieving	GoodGood	At 650-675 °C (1,200-1,250 °F) after heavy roughing, for parts with tight flatness after hardening.
Annealing	GoodGood	Needed before a hardened part is reworked or re-hardened: 845-870 °C (1,550-1,600 °F) and a slow furnace cool.
Nitriding	FairFair	Only on parts tempered high, around 510-540 °C, which costs a few points of hardness; D2 and H13 are the usual nitrided grades.
Black Oxide	GoodGood	A common, cheap finish on fixtures and tooling details; oil it for rust resistance.
Hard Chrome Plating	FairFair	Used against galling on forming tools; bake for hydrogen right after plating.
Welding	FairFair	Tool-room repair welding with a 315-425 °C (600-800 °F) preheat on annealed stock, then a temper.
Laser Cutting	PoorPoor	The cut edge air-hardens and micro-cracks; saw, waterjet or wire-cut blanks instead, or machine the edge off.
Carburizing	Not recommendedNot recommended	A2 already carries 1% carbon; carburizing adds nothing but a brittle, over-carbided surface.

Design and sourcing tips

Call out "A2, 58-60 HRC" and the heat treat route; add "temper 510 °C (950 °F) min" if the part will be wire EDM cut after hardening or nitrided.

Leave about 0.15-0.3 mm (0.006-0.012") per side for finish grinding; air hardening keeps size change small and predictable.

Choose A2 over O1 when the part must hold size through hardening, and over D2 when chipping or cracking is the failure mode.

Balance sections, add radii at internal corners and keep holes away from edges; even a gentle air quench cracks parts at sharp stress raisers.

Buy precision ground flat stock for plates, punches and gibs: it arrives square and decarb-free, which saves milling and surface grinding setups.

Frequently asked questions

Why is A2 called air hardening?

In the AISI system the letter describes the quench or the use: W for water, O for oil, A for air. A2 carries enough chromium and molybdenum that it hardens fully by cooling in still air or a gas quench from about 955 °C (1,750 °F), instead of a plunge into oil or water. The slower quench means less distortion and less cracking risk, which is why A2 is chosen for tools that have to hold size. Very large sections may need a faster gas quench to harden through the middle.

A2 vs O1: which should I use?

O1 is cheaper, machines more easily and hardens at a lower temperature in any small furnace, which makes it good for short runs, one-off tools and gauges. A2 costs a little more but moves less in hardening, wears longer and is tougher, so it wins for production dies, punches and anything that must hold size through heat treatment. When distortion or wear is the problem with an O1 tool, A2 is the usual step up.

Is A2 tougher than D2?

Yes. With about 1% carbon and 5% chromium against about 1.5% and 12% in D2, A2 has much less carbide, so at the same hardness it absorbs noticeably more impact before it chips or cracks. D2 wins on abrasive wear. Punches and dies that chip in D2 are commonly remade in A2, and those that still chip in A2 go to S7.

How hard is A2 tool steel?

Annealed, about 200-240 HB. Hardened and tempered it is used at 57-62 HRC, most often 58-60 HRC. As quenched it reaches about 63-64 HRC; a temper around 175 °C (350 °F) gives the top of the working range and around 260 °C (500 °F) the lower end.

Can A2 be welded?

Yes, for repair. Preheat to about 315-425 °C (600-800 °F) for annealed stock, or just below the last tempering temperature for a hardened tool so it does not soften, weld by TIG with an A2-type filler, cool slowly and temper. Tool rooms routinely build up chipped edges and modify dies this way. Welding cold or skipping the temper afterward leads to cracks.