Skip to main content

Heat Treating A2 Air-Hardening Tool Steel: Austenitize, Quench, and Temper

AISI A2 is the middle ground in the cold-work tool steel family — tougher than D2, harder and more wear-resistant than O1, and air-hardening in sections up to roughly 4 inches so that the quench distortion that plagues oil-hardening grades is largely absent. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. At ~1.0% carbon, 5% chromium, and 1% molybdenum, A2 is specified for blanking and forming dies, shear blades, punches, thread rolls, and trim dies where a working hardness of 58-62 HRC is needed but the shock and impact loading would chip a D2 edge. This article walks through the production heat-treatment cycle as it is actually run — anneal condition, preheat, austenitize at 1,750-1,800 °F, still-air quench, and the double temper at 350-450 °F that delivers the final hardness.

What is A2 tool steel and where is it used?​

AISI A2 is a medium-alloy air-hardening cold-work tool steel per ASTM A681, with nominal composition of approximately 1.00% carbon, 5.00% chromium, 1.00% molybdenum, 0.25% vanadium, 1.00% manganese, and 0.30% silicon, balance iron. The "A" in the AISI designation places it in the air-hardening cold-work family; the "2" identifies this specific chemistry within that family. A2 is specified for tooling that needs a hard, wear-resistant working edge at moderate-to-low impact levels: blanking and piercing dies for sheet metal, trim dies, forming punches, shear blades for cold-formed stock, thread rolls, coining tools, and plastic mold cores that run against abrasive filled resins. Its combination of 58-62 HRC working hardness, reasonable toughness (better than D2, worse than S7), and low distortion on air-hardening makes it the default choice when D2 would be too brittle and O1 would distort too much in the oil quench. Typical as-annealed hardness of A2 from the mill is 201-229 HB — the starting condition for die making (ASTM A681; ASM Handbook, Vol. 4A, ASM International, 2013).

What is the full heat treatment cycle for A2?​

The production cycle for A2 runs in four phases. Anneal condition is the starting point: A2 arrives from the mill spheroidize-annealed at 201-229 HB, suitable for rough and semi-finish machining. Preheat brings the part up to temperature in stages — typically a single equalizing soak at 1,200-1,250 °F — to avoid cracking on the austenitize ramp. Austenitize holds at 1,750-1,800 °F (955-980 °C) for 30-45 minutes per inch of section thickness after the part equalizes at temperature, dissolving alloy carbides into the austenite without grain coarsening. Air quench cools from austenitize temperature in still air, below the Ms temperature (approximately 400 °F for A2) quickly enough to form martensite through sections up to 4 inches. Temper is performed as a double cycle at 350-450 °F for 2 hours per cycle with a cool-to-room-temperature step between tempers. Typical delivered hardness after double temper at 375-400 °F is 60-62 HRC; tempers at 500-600 °F produce 57-59 HRC when improved toughness is required (Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).

Why is A2 called "air-hardening" and what does it mean for distortion?​

A2's 5% chromium and 1% molybdenum give it enough hardenability that still-air cooling from 1,750-1,800 °F produces a fully martensitic structure in sections up to roughly 4 inches in diameter or thickness — there is no oil or water quench in the cycle. This matters in practice because the quench step is the largest source of dimensional distortion in tool-steel heat treatment: non-uniform cooling between surface and core, between thick and thin sections, and between the quench-facing and quench-sheltered faces of a die block all produce differential transformation and differential thermal contraction, which in turn cause warping, cupping, and out-of-flatness on finished dies. Oil-hardening grades like O1 typically distort 0.002-0.005 inches per inch of dimension on the quench, enough that production dies must carry 0.015-0.030 inches of grind stock on critical surfaces to clean up after hardening. A2's air quench typically produces 0.0005-0.0015 inches per inch of dimensional change — less than half the distortion of comparable O1 tooling — allowing tighter pre-hardening machining tolerances and less grind stock. The result is lower total manufacturing cost for A2 dies versus O1 dies of comparable size, even though A2 bar stock costs more per pound. The distortion advantage of air-hardening is why A2 displaced O1 as the default general-purpose cold-work tool steel in most production die shops (ASM Handbook, Vol. 4A, ASM International, 2013).

What preheat does A2 need before austenitizing?​

Preheat is standard practice on A2, though less critical than on the heavier-alloy D2 or H13 grades. A single-stage equalizing soak at 1,200-1,250 °F for 30-60 minutes brings the part through the low-temperature stress range and equalizes the cross-section before the austenitize ramp. On thin sections under 1 inch and on symmetrical parts without sharp geometric features, preheat can sometimes be omitted, but omitting it is risky on any production part with abrupt thickness transitions, deep pockets, or blind holes — cracks initiate at these stress concentrations on rapid heating. On heavier sections above 4 inches, a two-stage preheat with an additional hold at 1,500 °F ensures uniform temperature through the part before reaching 1,750-1,800 °F. Heating rate between stages is typically 300-500 °F per hour. The purpose of preheat is thermal equilibration, not transformation: the part enters the austenitize range at uniform temperature so the transformation happens uniformly and the still-air cool afterward produces uniform martensite. UTEC Industrial's car-bottom furnace runs A2 preheat and austenitize cycles under programmable ramp-and-soak control, with the furnace thermocouple chart forming part of the job documentation package (ASM Handbook, Vol. 4A, ASM International, 2013).

What austenitizing parameters produce the target hardness in A2?​

Austenitize A2 at 1,750-1,800 °F (955-980 °C), with the specific temperature within this range chosen to balance maximum carbide dissolution against grain coarsening. At the low end (1,750 °F), less carbide dissolves, leaving more carbide as discrete particles in the final microstructure — which contributes to wear resistance but caps the as-quenched martensite hardness. At the high end (1,800 °F), more carbide dissolves into the austenite, increasing as-quenched martensite hardness but also increasing grain size and retained austenite content. Production practice on general cold-work tooling uses 1,775 °F as the default, producing 63-65 HRC as-quenched and 60-62 HRC after double temper at 375-400 °F. Soak time at austenitizing temperature is 30-45 minutes per inch of section thickness, minimum 30 minutes. Longer soaks do not improve properties and risk grain coarsening; shorter soaks leave carbides partially undissolved and produce softer-than-target hardness. After the austenitize soak, the part is removed from the furnace and cooled in still air — on large dies in a cool shop, this produces an adequate cooling rate to form martensite through sections up to 4 inches. On heavier sections or in a warm shop, a fan-assisted forced-air cool is sometimes used to accelerate cooling through the 1,500-1,000 °F transformation range (Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).

Why is A2 double-tempered, and at what temperature?​

A2 is double-tempered for the same reason as D2: retained austenite. As-quenched A2 contains 8-15% retained austenite (somewhat lower than D2 at 15-30%, because A2 has less total alloy in solution), which is soft (25-30 HRC) and metastable — it will transform to untempered martensite during service under load or thermal cycling, producing localized dimensional change and hardness variation that degrades tool performance. The first temper at 350-450 °F heats the as-quenched part, relieving residual stress and tempering the initial martensite; during cooldown from the first temper, most of the retained austenite transforms to fresh (untempered) martensite. The second temper tempers this fresh martensite, producing a microstructure with less than 3% retained austenite and uniform tempered-martensite hardness. Temperature selection within the 350-450 °F range controls final hardness: 350 °F produces approximately 62 HRC, 400 °F produces approximately 60 HRC, 450 °F produces approximately 58 HRC. Each temper runs 2 hours minimum at temperature, with the part cooled to below 150 °F between cycles to ensure complete martensite transformation. For applications requiring improved toughness at some hardness cost, tempering at 500-600 °F produces 56-58 HRC with roughly twice the Charpy V-notch impact energy of the 400 °F condition. The 350-450 °F range is standard for wear-dominated applications (blanking, trim, thread rolls); 500-600 °F is used when the service loads include moderate impact (Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).

How does A2 compare to D2 and S7 for die and punch selection?​

A2 occupies a middle position in the cold-work tool-steel family. Compared to D2 (1.5% C, 12% Cr), A2 has significantly lower carbide volume fraction — approximately 5-8% carbides by volume versus 15-18% in D2 — which means A2 has less abrasive wear resistance but substantially better toughness. A2 dies typically last 30-50% as long as D2 dies in pure abrasive wear applications (stamping thick gauge or abrasive materials), but A2 edges do not chip under the shock that breaks D2 edges. Compared to S7 (0.50% C, 3.25% Cr, 1.4% Mo), A2 has more carbon and more wear resistance but less impact toughness — A2 at 60 HRC has a Charpy V-notch impact energy of about 15-25 ft·lb, while S7 at 57 HRC runs 60-100+ ft·lb. S7 is the choice for heavy impact tooling (chisels, breaker tools, hot or cold header dies) where A2 would crack. The selection rule in general die-making practice: wear-dominated, no significant impact → D2; wear with some impact (blanking dies, forming dies, shear blades) → A2; impact-dominated with secondary wear concern (chisels, punches in severe service) → S7. Where the workload sits in the middle — a production blanking die running moderate-thickness steel — A2 is the default and usually correct choice (ASM Handbook, Vol. 4A, ASM International, 2013).

What dimensional change should be expected during A2 heat treatment?​

A2 is dimensionally more stable through heat treatment than most oil-hardening grades, though not as stable as H13. Overall dimensional change from annealed to fully hardened (austenitize + air quench + double temper at 400 °F) is typically +0.0005 to +0.0015 inches per inch of dimension — the part grows slightly as the ferrite-carbide aggregate transforms to tempered martensite, but the change is predictable and directionally consistent. Distortion (non-uniform dimensional change) depends on geometry and cooling uniformity: symmetrical blocks cooled in uniform still air typically hold 0.001-0.002 inches per inch of flatness; asymmetrical tooling, thin sections adjacent to heavy sections, and parts with internal stress from rough machining can distort enough to require post-hardening straightening or additional grind stock. Standard practice on production A2 dies: leave 0.010-0.030 inches of finish-grind stock on critical surfaces, specify symmetrical orientation on the furnace car, and perform a pre-hardening stress relief at 1,200 °F on rough-machined blocks to release residual machining stress before the austenitize soak releases it uncontrollably. The pre-HT stress relief adds an overnight cycle but measurably reduces post-HT distortion on complex geometries — it is cheap insurance against a ruined expensive die block (Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).

Related Articles

References​

  • ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes, ASM International, 2013.
  • Heat Treater's Guide: Practices and Procedures for Irons and Steels, 2nd edition, ASM International, 1995.
  • Totten, G.E., ed., Steel Heat Treatment Handbook, 2nd edition, CRC Press / Taylor & Francis, 2006.
  • ASTM A681, Standard Specification for Tool Steels Alloy, ASTM International.
  • AMS 2759, Heat Treatment of Steel Parts, General Requirements, SAE Aerospace.

Need In-House Heat Treating for Heavy Industrial Parts?​

UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.

Request a Quote →

Questions? Call (509) 922-1832 or email sales@utec.co