HAP40 steel is a powder metallurgy (PM) high-speed steel (HSS) developed by Hitachi Metals Ltd., renowned for its exceptional edge retention and wear resistance. This guide examines its metallurgical properties, performance benchmarks, and optimal applications, supported by published research and industry data.
1. Metallurgical Composition and Manufacturing
1.1 Alloy Design
HAP40 belongs to the high-speed steel (HSS) family, specifically classified as a cobalt-bearing PM steel. Its composition is optimized for hardness and thermal stability:
| Element | % Composition | Role |
|---|---|---|
| Carbon (C) | 1.3–1.5 | Forms hard carbides (MC, M₆C) for wear resistance. |
| Tungsten (W) | 5.0–6.0 | Enhances hot hardness via W₂C carbides. |
| Molybdenum (Mo) | 4.0–5.0 | Refines grain structure, substitutes for tungsten. |
| Vanadium (V) | 2.5–3.0 | Forms ultra-hard VC carbides (HV 2800+). |
| Cobalt (Co) | 7.0–8.0 | Elevates red-hardness by delaying tempering effects. |
| Chromium (Cr) | 3.5–4.5 | Improves corrosion resistance and hardenability. |
1.2 Powder Metallurgy Process
HAP40 is produced via gas atomization:
- Alloy melting at ~1500°C under inert gas.
- Rapid solidification into fine powder (particle size <100 µm).
- Hot isostatic pressing (HIP) to eliminate porosity.
- Annealing at 850–900°C for machinability.
Result: A homogeneous microstructure with minimal segregation, critical for edge stability.
2. Mechanical Properties
2.1 Hardness and Edge Retention
HAP40 reaches 64–67 HRC after heat treatment, with 30% higher wear resistance than VG-10 in CATRA tests. Its high-volume MC-type carbides provide excellent abrasive wear resistance.
2.2 Toughness and Chipping Resistance
Moderate toughness (~12 MPa√m), comparable to CPM-3V, but lower than AEB-L. High hardness reduces toughness, making 15–20° edge angles optimal for kitchen knives.
2.3 Corrosion Behavior
Moderate corrosion resistance (PRE ~12), prone to staining with acidic foods (pH <4.5).
3. Heat Treatment Optimization
Recommended protocol:
- Austenitizing: 1190–1210°C (hold 5–10 min).
- Quenching: Oil or air (critical cooling rate ~5°C/s).
- Tempering: 2–3 cycles at 540–560°C (target 65 HRC).
Caution: Over-austenitizing (>1220°C) can cause grain growth and embrittlement.
4. Performance vs. Competing Steels
Comparison with other popular steels:
| Property | HAP40 | ZDP-189 | SG2 (R2) | CPM-M4 |
|---|---|---|---|---|
| Hardness (HRC) | 64–67 | 66–67 | 62–64 | 62–65 |
| Edge Retention | ★★★★★ | ★★★★★ | ★★★★☆ | ★★★★☆ |
| Toughness | ★★★☆ | ★★☆ | ★★★☆ | ★★★★ |
| Corrosion Resistance | ★★☆ | ★★☆ | ★★★★ | ★★☆ |
5. Practical Applications
- High-end kitchen cutlery (gyutos, santokus)
- Industrial cutting tools (drills, end mills)
- Custom/hunting knives (where edge longevity > corrosion resistance)
