A2 Air Hardening Tool Steel
Medium-alloy, air-hardening cold-work tool steel with excellent dimensional stability during heat treatment, high wear resistance, and good machinability in the annealed condition. Widely used for blanking dies, forming rolls, shear blades, and plastic mould inserts.
Material Overview & Common Uses
A2 is a 5 % chromium, air-hardening tool steel that offers an excellent balance of wear resistance, toughness, and machinability. Unlike oil- or water-hardening grades, A2 hardens with minimal distortion, making it the首选 grade for precision tooling and dies. Typical applications include:
- Blanking, piercing, and trimming dies
- Forming rolls and thread rolling dies
- Shear blades, slitter knives, and granulator knives
- Plastic injection mould inserts and core pins
- Gauges, mandrels, and master tools
Supplied in the annealed condition (≤ 248 HBW) for maximum machinability; after hardening and tempering, surface hardness reaches 57–62 HRC depending on tempering temperature.
Core Technical Properties & Sizing
| Property | Value / Specification |
|---|---|
| Density | 7.86 g/cm³ (0.284 lb/in³) |
| Modulus of Elasticity (Tension) | 200 GPa (29 × 10⁶ psi) |
| Annealed Hardness (max) | 248 HBW (24 HRC max) |
| Hardened Hardness Range | 57–62 HRC (depending on temper) |
| Typical Chemical Composition (wt%) | C: 0.95–1.05, Mn: 0.60–1.00, Si: 0.20–0.50, Cr: 4.75–5.50, Mo: 0.90–1.40, V: 0.15–0.50 |
| Thermal Conductivity (at 20 °C) | 24.5 W/(m·K) |
| Mean Coefficient of Thermal Expansion (20–200 °C) | 11.5 × 10⁻⁶ /K |
| Available Forms | Round bar (Ø 6–300 mm), flat bar / plate (6–100 mm × 25–610 mm), square block |
| Tolerance (ground stock) | h6 / h7 per ASTM A681 |
Machinability, Tooling & Feeds
In the annealed condition A2 offers 70–80 % machinability relative to AISI 1112 free-cutting steel. Carbide tooling is recommended for production runs; HSS (M2, M42) performs well for prototypes and low-volume work. Use positive rake angles and rigid setups to minimize chatter.
Recommended Cutting Parameters — Annealed Condition (≤ 248 HBW)
| Parameter | HSS (M42 / T15) | Carbide (C6 / C2) |
|---|---|---|
| Cutting Speed (turning) | 20–30 m/min | 55–85 m/min |
| Feed rate (turning) | 0.15–0.35 mm/rev | 0.20–0.45 mm/rev |
| Cutting Speed (milling) | 15–25 m/min | 45–70 m/min |
| Feed per tooth (milling) | 0.04–0.10 mm/tooth | 0.06–0.14 mm/tooth |
| Depth of cut (roughing) | 1.5–3.0 mm | 2.0–4.0 mm |
| Coolant | Soluble oil 5–8 % | Soluble oil or MQL |
Grinding: Use aluminum oxide (Al₂O₃) wheels for rough grinding and CBN (cubic boron nitride) wheels for finish grinding after hardening. Avoid overheating — maintain wheel speed ≤ 30 m/s and use generous coolant flow.
Common Defects & Quality Control
| Defect | Description | QC Method / Rejection Criterion |
|---|---|---|
| Decarburization (surface) | Carbon depletion at the surface during hot forming or heat treatment; reduces case hardness. | Microstructural examination per ASTM E1077. Max depth 0.25 mm per side for finished tools. Reject if > 0.40 mm. |
| Carbide Banding / Segregation | Non-uniform distribution of carbides; increases risk of edge chipping and anisotropic shrinkage. | Rated per ASTM E768 (Carbide Banding Index). Acceptable: ≤ 3.0 for general tooling; ≤ 1.5 for high-load dies. |
| Excessive Retained Austenite | Incomplete transformation after quench; leads to dimensional instability and reduced hardness. | X-ray diffraction (XRD) or magnetic saturation. Maximum 5 vol% retained austenite for hardened tools. |
| Quench Cracking | Thermal stress fracture during air cooling, typically at sharp corners or thin sections. | Visual + magnetic particle inspection (MPI). Zero cracks acceptable. Section geometry must have radii ≥ 1 mm. |
| Soft Spots (low hardness) | Localized areas below 57 HRC due to uneven austenitizing or slow cooling. | Hardness traverse per ASTM E18. Reject if any single reading < 55 HRC within the working zone. |
Storage, Handling & Racking Guidelines
Proper storage and handling prevent surface corrosion, mechanical damage, and dimensional drift. Follow these protocols to maintain material integrity from receiving to fabrication.
- Receiving inspection: Verify mill cert — check heat number, composition, hardness (≤ 248 HBW), and surface condition. Flag any rust, pits, or heavy scratches.
- Storage environment: Store indoors in dry, well-ventilated area. Relative humidity ≤ 60 %. Temperature range 10–30 °C. Avoid direct contact with concrete floors — use wooden dunnage or steel racks with at least 100 mm clearance.
- Racking / segregation: Separate by grade and size. Use designated racks for A2 round bar and flat stock. Do not store with copper or brass to avoid galvanic corrosion in humid conditions.
- Handling: Use nylon slings or padded clamps for lifting. Never drop or drag bars across each other — surface nicks can act as stress risers during hardening.
- Shelf life: Indefinite if stored properly. Apply light rust-preventive oil (e.g., LPS 3) on ground surfaces if RH exceeds 60 % for more than 48 hours.
Recommended Storage Conditions
| Parameter | Specification |
|---|---|
| Storage Temp | 10–30 °C |
| Relative Humidity | ≤ 60 % |
| Rack Material | Mild steel (painted) or stainless steel; no copper alloys |
| Bar Support Spacing | Max 1.2 m for bars ≤ 50 mm dia; max 0.8 m for bars > 50 mm dia |
| SupplyGrid Reorder Point | When on-hand quantity falls below 2 weeks of average consumption (adjustable per tool-room policy) |