HPM38 Steel CNC Machining: Pre-Hardened Stainless Steel for Plastic Injection Molds

Key Takeaways

• HPM38 is a Hitachi pre-hardened stainless steel designed for mold applications.

• Requires no post-machining heat treatment, reducing costs and shortening lead times.

• Offers corrosion resistance, machinability, and mirror-finish performance.

• Ideal for plastic injection molds and precision tooling.

What is HPM38 Steel?

HPM38 is a pre-hardened stainless mold steel developed by Hitachi Metals. Classified as a 13Cr-series stainless steel with molybdenum, it was designed as an upgraded alternative to SUS420J2. Because HPM38 is delivered pre-hardened, it does not require post-machining heat treatment, helping manufacturers save time, lower costs, and reduce delivery lead times.

Compared to SUS420J2, HPM38 offers superior corrosion resistance and is highly suitable for long-term mold storage and management since it resists rusting. It also provides excellent machinability and a strong mirror-finish capability, making it ideal for plastic injection mold applications.

Why Choose HPM38 for Mold Applications?

• Pre-Hardened: Shipped at 29–33 HRC, no heat treatment needed after machining.

• Excellent Corrosion Resistance: Protects against rust during storage and operation.

• Mirror-Finish Capability: Suitable for molds requiring high gloss and optical clarity.

• Cost-Effective: Reduces heat treatment costs, machining time, and distortion.

• Versatile Applications: Works with transparent plastics, flame-retardant resins, and precision molding.

Unlike general SC-series steels, HPM38 can be machined directly in its supplied condition. Its stability after machining minimizes dimensional changes, making it ideal for precision molds.

HPM38 Technical Data (Reference Values)

Chemical Composition

C: 0.4% | Si: 0.4% | Mn: 0.4% | Cr: 13.5% | Mo: 0.6%

Hardness

• Pre-hardened: 29-33 HRC

• Hardened & tempered: 50 to 55 HRC

Coefficient of Thermal Expansion

• RT-100°C/212°F (×10⁻⁶/°C): 10.4

• RT-200°C/392°F (×10⁻⁶/°C): 11.1

• RT-300°C/572°F (×10⁻⁶/°C): 11.5

• RT-400°C/752°F (×10⁻⁶/°C): 11.8

Tensile Strength

• 20°C/68°F (MPa): 1910

• Elongation (%): 13

• Reduction of area (%): 35

• 0.2% Yield Strength (MPa): 1620

Thermal Conductivity

• 20°C/68°F (W/(m·K)): 22.1

• 100°C/212°F (W/(m·K)): 25.5

• 200°C/392°F (W/(m·K)): 26.7

• 300°C/572°F (W/(m·K)): 28.5

• 400°C/752°F (W/(m·K)): 29.6

At Yumoto Electric, we provide end-to-end support for HPM38 with ease and other mold steels.

HPM38 Stainless Steel CNC Machining at Yumoto Electric

Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals using milling, turning, and specialized metal processing. We have experience with over 200 materials including aluminum alloys, stainless steel, molybdenum, and titanium. 

If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.