A2 TOO STEEL | 1.2363 | SKD12

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A2 tool steel is a deep-hardened, air-hardened tool steel. The deformation caused by air hardening is approximately one-quarter that of tungsten-based oil-hardened tool steel. Its wear resistance falls between that of chromium-type and high-carbon, high-chromium-type tool steels, but its toughness is superior. This makes it especially suitable for applications requiring good wear resistance, toughness, and dimensional stability. A2 steel is widely used in blanking dies, forming dies, rolling dies, punch dies, calendering dies, thread rolling dies, and specific cutting blades.

1. Applications

A2 tool steel is widely used for tooling operations performed at temperatures below 200°C (typically room temperature). It is an economical choice for general shop purposes. Its applications span situations demanding extreme hardness, friction resistance, durability, and strength, and it’s suitable where improved toughness and reasonable abrasion resistance are needed.

  • Bending dies
  • Blanking dies and punches (for long runs or thin/medium-thickness stock)
  • Coining dies
  • Cold forming tools
  • Drawing dies
  • Extrusion dies and punches for aluminum
  • Forming dies
  • Gages and precision measuring tools (due to dimensional stability)
  • Knurls
  • Mandrels
  • Master hubs
  • Molds
  • Planer tools
  • Shear knives and blades (cold shears for light and medium duty, trimming dies)
  • Slitter knives
  • Spindles
  • Punches (even over wear-resisting D2)

[References: Totten, G. E., Xie, L., & Funatani, K. (Eds.). (2004). Handbook of Mechanical Alloy Design (p. 169). CRC Press.]

2. A2 Steel Composition

Element Carbon (C) Chromium (Cr) Molybdenum (Mo) Vanadium (V) Manganese (Mn) Silicon (Si) Phosphorus (P) Sulfur (S)
Percentage (%) 0.95 – 1.05 4.75 – 5.50 0.90 – 1.40 0.15 – 0.50 0.40 – 1.00 0.30 – 0.90 ≤ 0.03 ≤ 0.03

3. A2 steel equivalent

  • DIN/ISO: 1.2363 (X100CrMoV5),
  • JIS (Japan): SKD12
  • China(GB/T 1299 standard): Cr5Mo1V

4. A2 Steel Properties

4.1 Hardness

The hardness after annealing is 235 HB to 269 BHN.

In the as-quenched condition, A2 can achieve a surface hardness of 60 HRC. As-quenched hardness is influenced by austenitizing temperature.

The hardness after tempering is 58–64 HRC. Tempering at 200°C (390°F) can achieve a hardness of 60 HRC.

4.2 Wear Resistance

The high carbon and chromium content give A2 excellent wear resistance. Compared to O1 steel, A2 steel has slightly better wear resistance. Compared to A6 steel, its wear resistance is improved by 20-25%. However, its wear resistance is lower than that of D2 steel or high-speed steel.

4.3 Toughness

A2 steel has moderate toughness, higher than oil-hardening die steels and D-series tool steel. Toughness and wear resistance are inversely proportional. The large amount of carbides in D-series tool steel increases wear resistance, but reduces toughness.

4.4 Dimensional Stability

A2 steel undergoes minimal deformation during hardening. The expansion is approximately 0.001 inches/inch (0.001 mm/mm).

4.5 Machinability

A2 steel is an easy-to-machine tool steel after appropriate annealing treatment. If the machinability of tool steel with a carbon content of 1% is set to 100, the machinability rating of A2 is 65.

A2 tool steel

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5. Heat Treatment

Below, we outline the critical steps and considerations for a successful A2 tool steel heat treatment, based on industry best practices.

5.1 Initial Preparation

  1. Assessing Initial Condition: Before performing any heat treatment on A2 material, it is essential to ensure that the material surface is thoroughly degreased.
  2. Pre-Hardening Stress Relief (If Applicable): For A2 materials that have undergone extensive machining, we strongly recommend stress relief treatment, which minimizes deformation during heat treatment. Procedure for unhardened material: Heat the steel slowly and uniformly to 1200–1250°F (649–677°C). Hold at this temperature for approximately 2 hours per inch of the thickest section. Follow this with a slow cool, preferably within the furnace.

5.2 Protecting A2 Tool Steel During Heat Treatment

Surface Integrity (Preventing Decarburization):

A2 tool steel is prone to decarburization when heated to high temperatures. To prevent decarburization and ensure the integrity of the heat treatment of A2 tool steel, we recommend securely wrapping the steel A2 with stainless steel foil and then placing it in a controlled neutral atmosphere furnace or a vacuum furnace. Sealing the material in stainless steel foil is a widely adopted and effective method.

5.3 Key Stages of A2 Tool Steel Heat Treatment

5.3.1 Preheating

The purpose of preheating A2 tool steel is to:

  1. Adjust the molecular structure of the material.
  2. Ensure that the temperature of the entire component is uniform, reducing thermal shock.
  3. Eliminate internal stress before reaching a higher austenitizing temperature.

The preheating temperature for A2 steel is 1200°F (650°C). Hold for 10-15 minutes to ensure that the temperature of the material is consistent with the temperature inside the furnace before proceeding to the next step. This can be determined by observing whether the color of the material and the furnace wall remains consistent.

5.3.2 Austenitizing (Hardening):

During this process, the alloy carbides within the material dissolve into the matrix, which determines the final quenching properties of A2 steel.
The standard quenching temperature is approximately 1775°F (968°C), with some processes ranging from 1775°F to 1825°F (970-995°C). The soaking time is 1 hour per inch (25mm) of the component’s thickest cross-section.
The soaking time should NOT be too long, as excessive soaking time can have a negative impact on the material’s microstructure.

5.3.3 Quenching:

Air quenching significantly reduces thermal shock and internal stress and is the mildest common quenching method. Use foil to seal the parts until all visible red-hot phenomena have completely dissipated.

However, air quenching of A2 has limitations. If the cross-sectional dimensions exceed approximately 5 inches (127 mm), air quenching may NOT achieve complete hardening.

5.4 Tempering: Refining A2 Tool Steel Properties

When A2 tool steel material is air cooled to 125-150°F (52-65°C), tempering should begin immediately. Delayed tempering may increase the risk of cracking or shorten the life of the tool. If straightening is required, it can be performed when the material temperature is above 400°F (205°C), at which point the martensitic transformation has not yet been completed.

The purpose of tempering is:

  1. To eliminate internal stresses in the material.
  2. To increase toughness and reduce brittleness.
  3. To minimize residual austenite and transform residual austenite into a more stable microstructure.

5.4.1 Single Temper

Heat the material to 400°F (205°C), then soak at that temperature for 2 hours per inch (25 mm) of cross-section, and finally allow to cool naturally.

The first tempering helps to stabilize the newly formed martensite and transform the residual austenite.

5.4.2 Multi-Tempering (Double/Triple Tempering)

We strongly recommend a double-tempering process, or even a triple-tempering process.

The first tempering temperature for the double-tempering process is 400°F (205°C). The second tempering temperature is 375°F (190°C). Ensure that the material is cooled to room temperature between each tempering cycle.

Multiple tempering cycles can significantly refine the grain structure, enhance wear resistance, and provide excellent stress relief. This is particularly beneficial for components with complex geometries or sharp angles.

The tempering temperature directly affects the final hardness of A2 tool steel.

5.5 Post-Hardening Stress Relief

After completing the above heat treatment steps, additional stress relief treatment may be required for A2 material, especially when the material has undergone grinding, welding, or electrical discharge machining (EDM).

This method involves tempering the material at a temperature 25-50°F (14-28°C) lower than the final tempering temperature used in the main heat treatment.

6. Compare A2 Steel With Other Steels

6.1 A2 steel vs. 1095 steel

6.1.1 Hardenability and Heat Treatment

A2 tool steel’s primary advantage is its air-hardening capability and minimal distortion during heat treatment, making it highly reliable for intricate tooling. The hardenability of 1095 carbon steel is more dependent on section size and cooling severity, making it challenging to achieve consistent through-hardness in larger sections. Both are sensitive to heat treatment, but A2 is engineered for greater predictability in hardening.

6.1.2 Mechanical Properties

Compared to other tool steels and carbon steels, including 1095 carbon steel, A2 has better toughness.

In terms of wear resistance, A2 steel is also superior to 1095 steel, especially under long-term working conditions. 1095 steel generally has average wear resistance when untreated or in its simple hardened state.

6.1.3 Cost

A2 steel contains expensive alloys, making it more costly than 1095 steel, which does not contain these expensive alloys.

6.1.4 Applications

A2 is preferred for demanding cold-work tooling applications requiring high wear resistance, good toughness, and minimal distortion, such as long-run dies and molds. 1095 is used when high hardness is the primary requirement, such as for springs, gears, and simpler cutting tools, and is often chosen for its cost-effectiveness in certain wear applications. Based on our export experience, A2 steel is commonly used in markets outside China, especially in the United States.

In general, A2 steel is a more advanced and specialized tool steel containing expensive alloys. It exhibits excellent dimensional stability, comprehensive wear resistance, and toughness, making it suitable for high-precision and high-volume mold manufacturing. While 1095 steel is a high-hardness carbon steel with a simple composition, its performance is more dependent on the base carbon content and quenching conditions.

[References: Bringas, J. E. (Ed.). (2004). Handbook of Comparative World Steel Standards (3rd ed). ASTM International.]

6.2 A2 Steel vs. D2 Steel

Here’s a summary comparing the two steels:

FeatureA2 SteelD2 Steel
AISI ClassificationAir-hardening, medium-alloy, cold-work tool steelHigh-carbon, high-chromium, cold-work tool steel
Hardening MethodAir-hardening (principal air-hardening tool steel)Air-hardening (most D-type steels)
Wear ResistanceVery good; more abrasion resistance than S-series, but less than D-seriesExcellent; superior to A2 (30-40% better)
ToughnessHigher than oil-hardening steels and D2lower than A2; low impact strength
MachinabilityRelatively easy machining Difficult to work and grind 
Dimensional StabilityMinimal movement/low distortionMinimal movement/low distortion
Typical Hardness58-60 HRC60-62 HRC
Carbide ContentVery small formation of carbidesLarge amount of carbides (~16% by volume), primarily Cr-rich M7C3
Retained AusteniteCan have high amounts; high-temperature tempering helpsProne to retaining significant amounts (up to 20%); often requires cold treatment/double tempering

Overall, if you need higher toughness and easier machining for moderately demanding applications, A2 steel is the choice. However, if your priority is maximum wear resistance and dimensional stability for very long production runs, and you can manage the increased difficulty in machining and grinding, then D2 steel is the option.

6.3 O1 Vs. A2 steel

A direct comparison of O1 and A2 tool steels:

FeatureO1 Tool SteelA2 Tool Steel
Classification & HardeningAn oil-hardening tool steel, specifically a manganese oil-hardening type.An air-hardening die steel.
Heat TreatmentHardened by heating to an austenization temperature of 802-816°C (1475-1500°F) and then quenching in oil. Hardened by air cooling from temperatures around 955°C (1750°F). It can be heated in a preheated furnace, which is faster than O1 tool steel. 
Hardness (HRC)Can easily harden to 62-63 HRC. The typical working range is 57-62 HRC.The normal working hardness is 58–60 HRC, and it can be tempered to 59-61 HRC.
ToughnessHas slightly higher toughness than other oil-hardening steels and can offer a great combination of high hardness and toughness.Possesses higher toughness than oil-hardening die steels and offers good toughness in general.
Wear ResistanceGood wear resistanceVery good wear resistance compared to O1.
MachinabilityExcellent machinabilityParticularly poor.
Dimensional StabilityIs subject to cracking and distortion during oil quenching.Minimum movement in hardening and low distortion. Offers safety and dimensional stability.
Applications Blanking, coining, and forming dies, as well as punches, shear blades, and woodworking knives.Bending dies, blanking dies, and punches. It is an excellent choice for tools where O1 might have hardenability issues and for designs prone to cracking.

In summary, O1 steel offers good machinability and an excellent balance of hardness and toughness at a low cost, but it is more prone to distortion and cracking during oil quenching. A2 steel, while less machinable, provides superior dimensional stability and wear resistance due to its air-hardening characteristics, making it suitable for applications requiring higher precision and longer tool life, even at a slightly higher cost.

FAQs

1. What is A2 steel?
A2 steel is an air-cooled, medium-alloyed cold-worked tool steel known for its high wear resistance, good toughness, and small heat treatment deformation. It is commonly used in the manufacture of a variety of cold-worked molds.

2. Is A2 a good knife steel?
A2 steel is a good knife steel because it combines high wear resistance, good toughness, and easy heat treatment. It is so commonly used in the production of cutting tools.

3. Is A2 steel better than A4?
A4 steel generally exhibits slightly better wear resistance than A2 steel and can be hardened at lower austenitizing temperatures. However, both exhibit good toughness and low heat-treat distortion.

4. Is A2 better than D2 steel?
D2 steel has better wear resistance than A2 steel, but is less tough than A2 steel.

5. Does A2 steel rust easily??
A2 steel will rust, but its rust resistance is somewhat better than that of ordinary carbon steel because it contains a moderate amount of chromium.

6. Is A2 steel the same as 304?

In the ISO 3506 standard, “A2” refers to 304 stainless steel. This “A2” is not the A2 tool steel under the AISI standard on this page.

7. Can A2 steel be hardened?
Yes, it can indeed be hardened, and it is known explicitly as an air-hardening tool steel.
8. Can A2 be oil quenched?
Yes, A2 steel can be oil quenched. Specifically, for sections of A2 steel greater than 125 mm (5 inches) square, it is recommended to oil quench them from the austenitizing temperature down to 540 °C (1000 °F). After this oil quench, the material should then be air quenched to 65 °C (150 °F) before tempering.

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