P20 Tool Steel
P20 is a widely used pre-hardened plastic mold steel known for its excellent machinability, good polishability, and consistent mechanical properties. It is specifically designed for plastic injection molds, die casting dies, and other tooling applications where dimensional stability and surface finish are critical. As a low-alloy steel, P20 offers a balance of hardness, toughness, and cost-effectiveness, making it a staple in the mold-making industry.
Basic Information
Definition: P20 is a chromium-molybdenum alloy tool steel supplied in the pre-hardened condition (typically 28–32 HRC), eliminating the need for post-machining heat treatment. This pre-hardened state ensures dimensional stability during machining and use, critical for precision molds.
Corresponding Standards: Defined by ASTM A681 (Standard Specification for Tool Steels) in the United States. International equivalents include:
European: 1.2311 (DIN)
Japanese: SK50 (JIS)
Chinese: 3Cr2Mo (GB/T)
Key Characteristics: Pre-hardened for immediate use, good machinability, excellent polishability (up to mirror finishes), and moderate corrosion resistance, suitable for most plastic molding applications.
Chemical Composition
P20’s composition is balanced to achieve pre-hardened strength while maintaining machinability and polishability:
Element Content Range Role in the Steel
Carbon (C) 0.28–0.40% Provides a foundation for hardness and strength; balanced to avoid brittleness.
Chromium (Cr) 1.40–2.00% Enhances hardenability, wear resistance, and polishability; forms fine carbides.
Molybdenum (Mo) 0.30–0.55% Improves hardenability, toughness, and resistance to tempering; reduces grain growth.
Manganese (Mn) 0.60–1.00% Enhances hardenability and strength; aids in deoxidation.
Silicon (Si) 0.20–0.80% Acts as a deoxidizer; improves strength and oxidation resistance.
Phosphorus (P) ≤0.030% Strictly controlled to prevent brittleness, especially in pre-hardened condition.
Sulfur (S) ≤0.030% Minimized to avoid hot cracking and reduce impact on polishability.
Physical Properties
Density: ~7.85 g/cm³ (consistent with most alloy tool steels).
Melting Point: ~1420–1460°C.
Thermal Conductivity: ~35 W/(m·K) at room temperature, ensuring efficient heat transfer in mold cooling systems.
Coefficient of Thermal Expansion: ~11.5×10⁻⁶/°C (20–100°C), minimizing dimensional changes during temperature fluctuations in molding cycles.
Elastic Modulus: ~210 GPa, providing rigidity for maintaining mold precision under clamping pressures.
Mechanical Properties
P20’s mechanical properties are optimized for plastic mold applications, with consistent performance in the pre-hardened state:
Property Pre-Hardened Condition (28–32 HRC)
Tensile Strength (Rm) 850–1000 MPa
Yield Strength (Rp0.2) 700–850 MPa
Elongation (A) 12–18%
Impact Toughness (Charpy V-Notch) ~25–35 J
Hardness 28–32 HRC (typical; can be supplied up to 35 HRC)
Dimensional Stability: Low residual stress in the pre-hardened state minimizes distortion during machining or mold use.
Wear Resistance: Moderate, suitable for plastic molding where abrasive wear from fillers (e.g., glass fibers) is minimal.
Heat Treatment
P20 is unique among tool steels in that it is typically used in the pre-hardened state, eliminating the need for heat treatment by the end-user:
Pre-Hardening Process: Performed by the manufacturer—heated to 850–880°C, quenched in oil, then tempered at 550–650°C to achieve 28–32 HRC. This process ensures uniform hardness across thick sections.
Stress Relief Annealing: Optional for large or complex molds after machining, performed at 500–550°C for 2–4 hours to reduce residual stresses from machining.
Avoid Reheating: Exposing P20 to temperatures above 300°C during use may cause gradual softening, so it is not recommended for high-temperature plastic molding (e.g., engineering resins requiring mold temperatures >200°C).
Processing Performance
Machinability: Excellent in the pre-hardened state (28–32 HRC), allowing for precise machining of complex mold geometries. It can be milled, turned, drilled, and EDM’d (electrical discharge machining) with good surface finish.
Polishability: Superior—can be polished to mirror finishes (up to Ra 0.02–0.05 μm), critical for plastic parts requiring high surface quality (e.g., automotive interiors, consumer electronics).
Weldability: Moderate—can be welded for mold repair using matching filler metals, followed by stress relief annealing to avoid cracking.
Formability: Limited cold formability due to pre-hardened state; typically machined to final shape rather than formed.
Application Fields
P20 is the material of choice for plastic injection molds and related tooling, where precision, surface finish, and cost-effectiveness are key:
Plastic Injection Molds:
Molds for thermoplastic materials (e.g., PP, PE, ABS, PS) used in automotive, consumer goods, and packaging industries.
Cavity and core inserts, mold bases, and ejector plates for medium to large-sized molds.
Die Casting Dies:
Molds for low-pressure zinc or aluminum die casting (where high-temperature strength is not critical).
Other Tooling Applications:
Blow molding dies, compression molds, and thermoforming tools.
Jigs, fixtures, and gauges requiring moderate wear resistance and dimensional stability.
Advantages and Limitations
Advantages
Ready-to-Use: Pre-hardened state eliminates post-machining heat treatment, reducing production time and costs.
Excellent Machinability: Enables precise machining of complex mold details with tight tolerances.
Superior Polishability: Achieves high-gloss finishes required for cosmetic plastic parts.
Cost-Effective: More affordable than high-alloy mold steels (e.g., H13, S136) while meeting most plastic molding needs.
Limitations
Limited High-Temperature Performance: Softens at temperatures above 300°C, making it unsuitable for high-temperature resins (e.g., PEEK, LCP) or hot runner systems.
Moderate Corrosion Resistance: Susceptible to rust in humid environments; requires proper storage and maintenance (e.g., rust inhibitors, painting).
Wear Resistance: Not ideal for molds processing abrasive materials (e.g., glass-filled plastics) without surface treatments (e.g., nitriding).
Surface Treatments for Enhanced Performance
To extend P20’s capabilities, surface treatments are often applied:
Nitriding: Increases surface hardness (up to 65 HRC) and wear resistance, suitable for molds processing filled plastics.
Chrome Plating: Enhances corrosion resistance and provides a smooth, wear-resistant surface.
Polishing: Achieves mirror finishes for high-quality plastic parts, reducing friction and improving part release.
In summary, P20 tool steel is a versatile, cost-effective solution for plastic mold making, offering an unbeatable combination of pre-hardened strength, machinability, and polishability. Its widespread use across industries underscores its reliability as a go-to material for precision tooling applications.