A2 Air Hardening Tool Steel | DIN 1.2363 | X100CrMoV5

A2 tool steel is a high carbon, high chromium, air-hardened cold working tool steel, which belongs to the American standard ASTM A681.

As a star in the field of cold work applications, A2 tool steel is not the best in wear resistance or toughness, but it has achieved a very practical and reliable balance between the two. Coupled with the advantages of low deformation and low cracking risk brought by air-cooled hardening, as well as good machinability and dimensional stability, A2 steel is an ideal choice for manufacturing various cold work dies, tools and gauges with medium wear, certain impact resistance, complex or precise shapes.

A2 STEEL CHEMICAL COMPOSITION

Grade	C	Si	Mn	P	S	Cr	Mo	V
A2	0.95-1.05	0.1-0.5	0.4-1.0	≤0.03	≤0.03	4.75-5.5	0.9-1.4	0.15-0.5
1.2363/X100CrMoV5	0.95-1.05	0.1-0.4	0.4-0.8	≤0.03	≤0.03	4.8-5.5	0.9-1.2	0.15-0.35

Core Element Characteristics

Carbon (C): Provides high hardness, wear resistance and strength.
Chromium (Cr): Provides hardenability, wear resistance, resistance to temper softening and some corrosion resistance.
Molybdenum (Mo): Enhances hardenability (especially thick sections), wear resistance, toughness and resistance to temper softening.
Vanadium (V): Refines grains, improves wear resistance, toughness and resistance to temper softening. Low content, mainly plays an auxiliary role.
Manganese (Mn): Improves hardenability.
Silicon (Si): Deoxidizer, improves strength.

A2 STEEL RELATED STANDARD & EQUIVALENT GRADE

China: Cr5Mo1V (GB/T 1299)

Europe: X100CrMoV5(DIN EN ISO 4957)

Japan: SKD12(JIS G4404)

A2 STEEL FORGING

A2 steel contains high carbon and alloying elements (such as Cr, Mo), so special attention should be paid to controlling the temperature, deformation and cooling rate during the forging process to avoid cracking, structural defects or performance degradation.

Forging process	Temperature range	Operation requirements
Preheating temperature	750°C - 850°C	Slowly and evenly heat (≤100°C/h recommended) to avoid thermal stress cracking (especially for large cross-section billets).
Initial Forging Temperature	1050-1100°C	Maximum temperature should not exceed 1150°C to prevent coarse grains or overburning (grain boundary melting).
Final Forging Temperature	≥900°C	Plasticity drops sharply below 900°C, and cracks are prone to occur (forging below 850°C is strictly prohibited).
Deformation	Single deformation 30%-50%	Avoid uneven grains due to small deformation (<20%); large cross-sections require multiple forgings.
Cooling After Forging	Furnace cooling/sand cooling	After forging, it must be cooled slowly (no air cooling or water cooling), otherwise it is easy to crack (the air-cooling hardening characteristics of A2 steel are a risk after forging!).

A2 STEEL HEAT TREATMENT

Pretreatment: Spheroidizing Annealing For A2 Steel

Annealing purpose:
1. Eliminate forging stress;
2. Reduce hardness (to 20 HRC) for machining;
3. Prepare uniform spheroidal carbide structure for final quenching.
Annealing process:
– Heat to 870°C, hold for a sufficient time and then slowly cool to 730°C isothermal for 4-6 hours, furnace cool to 500°C and then air cool.

A2 steel quenching (austenitizing + cooling)

Parameters	Control range	Functions and precautions
Austenitizing temperature	980°C - 1010°C	● Optimum temperature: 995°C (balance carbide dissolution and grain growth) ● Grain coarsening above 1010°C, overburning risk above 1050°C
Holding time	≈ 30 minutes per 25mm thickness	● Start timing from when the workpiece reaches the austenitizing temperature as a whole ● Too short: carbides are not dissolved → insufficient hardness; too long: grain growth → reduced toughness
Cooling method	Still air cooling (air cooling)	● Core advantages of A2 steel! ● Air cooling speed is sufficient for hardening (cross section ≤ 100mm), with small deformation and low risk of cracking ● Large cross section or dense parts can be accelerated by air cooling

A2 steel tempering (stress relief + performance adjustment)

Parameter	Control range	Hardness and performance changes
Tempering timeliness	≤30 minutes after quenching	● Temper immediately! Prevent cracking caused by residual austenite transformation or quenching stress
Tempering temperature	175°C - 540°C	● Low temperature tempering (175-250°C): hardness peak 60-63 HRC, high wear resistance, medium toughness ● Medium temperature tempering (400-450°C): hardness 58-60 HRC, improved toughness ● High temperature tempering (500-540°C): hardness 55-58 HRC (secondary hardening peak), best toughness, impact resistance
Number of tempering	At least 2 times	● After the first tempering, the residual austenite → martensite, and the second tempering is required to stabilize the structure ● Heat preservation 1.5-2 hours/25mm each time
Cooling method	Air cooling to room temperature	● After each tempering, it is necessary to cool thoroughly before the next one

A2 steel tempering temperature-hardness-toughness

Tempering temperature	175°C	250°C	400°C	500°C	540°C
Hardness (HRC)	62-63	60-62	58-60	56-58	55-57
Toughness	Low	Medium	Medium-high	High	Highest

Secondary hardening peak: When tempering at 500-540°C, fine carbides (Mo₂C/V₄C₃) are precipitated, causing a slight increase in hardness, while significantly improving toughness.

A2 Steel Comparison with other tool steels

Steel type	Quenching temperature	Cooling method	Tempering characteristics	Maximum hardness (HRC)
A2	980-1010°C	Air cooling	Secondary hardening (500°C)	63
D2	1020-1040°C	Oil/air cooling	Secondary hardening (520°C)	64
O1	790-820°C	Oil cooling	No secondary hardening	65
H13	1020-1050°C	Air/oil cooling	Secondary hardening (550°C)	54