NAK80 Mold Steel
NAK80 is a premium pre-hardened plastic mold steel renowned for its excellent polishability, high hardness in the as-delivered state, and good machinability. It is specifically designed for plastic injection molds requiring high surface finish and dimensional accuracy, eliminating the need for post-machining heat treatment in many applications.
Basic Information
Definition: NAK80 is a nickel-based precipitation-hardening mold steel that achieves its properties through a specialized aging process. It is supplied in a pre-hardened state (typically 37–43 HRC), making it ready for direct machining without additional heat treatment—significantly reducing production time for mold manufacturing.
Corresponding Standards: Developed in Japan (JIS standard), NAK80 is equivalent to grades like DIN 1.2738 (Germany) and GB 3Cr2NiMo (China) in some applications, though its unique composition offers superior polishability.
Key Characteristics: High pre-hardened hardness, exceptional mirror polishability (up to Ra 0.02–0.04 μm), good toughness, minimal distortion during machining, and resistance to galling.
Chemical Composition
NAK80’s composition is optimized for pre-hardening and polishability, with key elements as follows:
Element Content Range Role in the Steel
Carbon (C) 0.06–0.12% Low carbon content ensures good toughness and machinability while allowing precipitation hardening.
Nickel (Ni) 3.00–3.50% A primary alloying element; enables precipitation hardening (aging) to achieve high hardness without quenching. Enhances toughness and polishability.
Chromium (Cr) 1.50–2.00% Improves corrosion resistance, hardenability, and surface finish quality.
Molybdenum (Mo) 0.20–0.50% Enhances strength, toughness, and precipitation hardening response.
Copper (Cu) 0.80–1.20% Aids in precipitation hardening; forms fine intermetallic compounds during aging to increase hardness.
Manganese (Mn) ≤0.50% Improves strength and machinability.
Silicon (Si) ≤0.35% Acts as a deoxidizer; maintains clean steel for better polishability.
Phosphorus (P) ≤0.030% Strictly controlled to avoid brittleness and surface defects.
Sulfur (S) ≤0.030% Controlled to prevent reduced toughness and surface imperfections.
Physical Properties
Density: Approximately 7.85 g/cm³ (consistent with most alloy steels).
Melting Point: Around 1450–1500°C.
Thermal Conductivity: ~40 W/(m·K) at room temperature.
Coefficient of Thermal Expansion: ~11.5×10⁻⁶/°C (20–100°C), contributing to good dimensional stability.
Elastic Modulus: ~210 GPa.
Mechanical Properties
NAK80’s properties are defined by its pre-hardened state, with minimal need for further heat treatment:
As-Delivered (Pre-Hardened)
Hardness: 37–43 HRC (ideal balance of machinability and wear resistance for mold applications).
Tensile Strength: ~1100–1300 MPa.
Yield Strength: ~1000–1200 MPa.
Elongation: ~12–15% (good ductility for complex machining).
Impact Toughness: ~30–40 J (Charpy V-notch, ensuring resistance to impact during mold operation).
Polishability: Excellent—can achieve mirror finishes (Ra ≤0.02 μm) with proper polishing techniques, critical for plastic parts requiring high surface quality.
Heat Treatment Process
Unlike many tool steels, NAK80 is supplied pre-hardened through a proprietary aging process, eliminating the need for post-machining heat treatment in most cases:
Aging (Precipitation Hardening)
Process: After hot rolling, the steel is heated to 500–550°C and held for several hours to promote the formation of fine copper-nickel-molybdenum precipitates, which strengthen the matrix.
Result: Achieves the desired pre-hardened hardness (37–43 HRC) with uniform properties throughout the material.
Stress Relief Annealing (Optional)
Purpose: Reduce internal stresses after heavy machining to prevent distortion in critical mold components.
Process: Heat to 200–250°C, hold for 2–4 hours, then air cool. This does not significantly reduce hardness.
Processing Performance
Machinability: Excellent in the pre-hardened state (37–43 HRC). It can be machined with high-speed steel or carbide tools, producing smooth surfaces that require minimal post-processing.
Weldability: Moderate; requires careful control of heat input to avoid cracking. Preheating to 200–300°C and post-weld stress relief are recommended.
Polishability: Superior to most pre-hardened steels. Its low carbon content and minimal non-metallic inclusions allow it to be polished to mirror finishes, essential for transparent or high-gloss plastic parts.
EDM (Electrical Discharge Machining) Performance: Good—produces a smooth recast layer with minimal cracking, suitable for intricate mold details.
Application Fields
NAK80 is the material of choice for high-precision plastic injection molds where surface finish and dimensional stability are critical:
Plastic Injection Molds:
Molds for transparent plastics (e.g., PMMA, PC, PET) requiring mirror-like surfaces (e.g., lenses, optical components, and clear packaging).
Molds for high-gloss or textured plastic parts (e.g., automotive interiors, consumer electronics casings).
Complex molds with intricate details, where pre-hardened status reduces production time.
Die Casting:
Molds for non-ferrous die casting (e.g., aluminum, zinc) requiring good polishability and moderate wear resistance.
Precision Components:
Jigs, fixtures, and gauges requiring high dimensional accuracy and surface quality.
In summary, NAK80 mold steel stands out for its unique combination of pre-hardened hardness, exceptional polishability, and ease of machining. Its ability to eliminate post-machining heat treatment makes it a cost-effective and time-saving choice for high-quality plastic molds, particularly those demanding superior surface finishes.