HPM31 Material Detailed Information
HPM31 is a high-performance pre-hardened plastic mold steel developed by Hitachi Metals. It is a modified version of P21 (JIS) and 1.2738 (DIN) steel, designed for excellent machinability, uniform hardness, and high toughness.
As a pre-hardened steel, HPM31 is supplied at a hardness of ~30-34 HRC, eliminating the need for further heat treatment, making it ideal for large plastic mold bases, automotive parts molds, and general-purpose plastic molds.
Key Features
- Pre-Hardened Material: Delivered at 30-34 HRC, eliminating the need for additional heat treatment, reducing lead time and processing costs.
- Excellent Machinability: Optimized composition for faster cutting speeds, reduced tool wear, and easy grinding.
- Good Toughness & Hardness Uniformity: Nickel and chromium enhance structural uniformity, reducing risk of deformation or cracking in large molds.
- Good Wear Resistance: Suitable for high-volume plastic molding applications.
- Decarburization-Resistant: Maintains surface integrity during processing.
Chemical Composition
| Element | Composition (%) |
| Carbon (C) | 0.35 – 0.45 |
| Silicon (Si) | 0.20 – 0.50 |
| Manganese (Mn) | 1.30 – 1.60 |
| Chromium (Cr) | 1.50 – 2.00 |
| Molybdenum (Mo) | 0.15 – 0.35 |
| Nickel (Ni) | 0.80 – 1.20 |
| Sulfur (S) | ≤ 0.030 |
Key Features in Composition:
- Nickel (Ni) Addition: Enhances toughness and hardness uniformity, making it more resistant to deformation.
- Chromium (Cr) & Molybdenum (Mo): Improves wear resistance and strength, allowing for longer tool life.
- Balanced Carbon (C) Content: Provides good hardness while maintaining machinability.
Mechanical Properties
| Property | Value |
| Hardness (Pre-Hardened) | 30 – 34 HRC |
| Tensile Strength | ~1,000 – 1,200 MPa |
| Yield Strength | ~900 MPa |
| Elongation | ~10-15% |
| Impact Toughness | High |
| Machinability | Excellent |
- Pre-hardened to 30-34 HRC → No need for additional heat treatment
- High toughness and uniform hardness distribution
- Excellent machinability for easy milling and cutting
- Good polishability for plastic molding applications
HPM31 vs. Other Mold Steels
| Property | HPM31 | 1.2738 (DIN) | P20 (AISI) | HPM1 |
| Hardness (HRC) | 30-34 | 28-32 | 28-32 | 30-34 |
| Toughness | High | High | Moderate | High |
| Machinability | Excellent | Good | Moderate | Excellent |
| Polishability | Good | Good | Moderate | Excellent |
| Applications | Large plastic molds, mold bases, automotive molds | General molds | General molds | High-precision plastic molds |
Why Choose HPM31?
- Better machinability and toughness than standard P20 (AISI 1.2311).
- Pre-hardened, eliminating the need for additional heat treatment.
- More uniform hardness and better wear resistance than 1.2738.
Applications
HPM31 is widely used in plastic molding and general-purpose mold applications, particularly where good machinability and moderate hardness are required.
Plastic Injection Molds
- Large automotive parts molds
- Household appliance molds (TV frames, washing machine parts)
- Electronic plastic components
Blow Molding & Compression Molds
- Plastic container molding
- Thin-walled plastic parts
Mold Bases & Structural Components
- Mold bases for large-scale injection molds
- Support structures and mold frames
General Engineering Applications
- Precision mechanical components
- High-strength machine parts
Heat Treatment Process
However, since HPM31 is already pre-hardened, additional heat treatment is usually not required.
Annealing
- Heat to 820–860°C, then slowly cool in a furnace.
Hardening
- Preheat at 600–700°C, then heat to 850–900°C, followed by oil or air quenching.
Tempering
- Temper at 500–600°C to adjust hardness while maintaining toughness.
Alternative Materials
- HPM1 (Hitachi Metals): Similar to HPM31 but optimized for better polishability.
- 1.2738 (DIN): A similar pre-hardened mold steel, but lower machinability compared to HPM31.
- P20 (AISI 1.2311): Lower toughness and hardness than HPM31, commonly used for general plastic molds.
- NAK80 (Daido Steel): Pre-hardened stainless mold steel with better corrosion resistance but lower toughness.
Conclusion
HPM31 is a pre-hardened plastic mold steel offering excellent machinability, good toughness, and wear resistance, making it ideal for large plastic injection molds, mold bases, and general-purpose molds.
Best Choice for: Automotive molds, large plastic molds, and mold bases
Why Choose HPM31? Pre-hardened (30-34 HRC), easy machining, good wear resistance, and uniform hardness distribution
