NAK80 Mold Steel Explained Properties, Machining & Pitfall Checklist

What Is NAK80 Mold Steel?

NAK80 Mold Steel is a pre-hardened mirror-finish plastic mold steel produced by Daido Steel (Japan). It is typically supplied at
about 38–42 HRC, designed for no overall heat treatment, lower risk of machining distortion,
and strong suitability for mirror polishing and precision machining.

Key Features of NAK80

1. High hardness & wear resistance: Pre-hardened at 37–43 HRC, offering more stable wear resistance and polish retention for production plastic molds.
2. Good machinability: Uniform hardness; complex geometries remain machinable even around 40 HRC.
3. Convenient weld repair: Uniform aging response after welding; no full heat treatment required, reducing repair cost.
4. Excellent texturing/etching performance: Uniform texture quality, suitable for high-end requirements.
5. Outstanding mirror-polishing potential: Can reach a high mirror-finish level (often referenced as #4000–#8000 in polishing-grit terminology) with a clean surface.
6. Pre-hardened delivery: Ready for machining without overall heat treatment, reducing deformation risk and shortening lead time.

Note: “#4000–#8000” refers to polishing system grit/industry wording and is not a universal acceptance standard. Actual acceptance should follow
SPI grades, Ra/Rz, or customer-approved samples.

NAK80 Property Tables (Daido Steel Official)

1) Chemical Composition

2) Physical Properties

3) Mechanical Properties

Typical Applications of NAK80 Mold Steel

• High mirror-finish / high-gloss appearance requirements
• Transparent parts or optical-grade appearance requirements
• High clarity requirements for micro-textures / micro-structures
• EDM-intensive cavities/inserts requiring high surface quality
• Large-area surfaces requiring consistent polishing appearance

Applications Not Recommended for NAK80

• High impact / high shear/edge or cutting areas with high stress
• Ultra-long service life targets (> 1,000,000 cycles)
• High-abrasion materials (glass fiber / mineral-filled resins)
• Strongly corrosive plastics (e.g., PVC)
• Long-term high-temperature operation (> 150°C)
• Cost-sensitive projects without high surface requirements

NAK80 Selection Decision Comparison Table

Recommended Cutting Tools for CNC Machining NAK80

Logic for Selecting CNC Cutting Parameters (NAK80)

NAK80 Machining Notes (Pitfall Checklist)

• Pre-hardened delivery: Supplied at ~38–42 HRC; typically machinable without overall heat treatment. Plan finishing allowance by process needs to avoid unnecessary extra work.
• Toolpath sensitivity in deep cavities: Deep-cavity machining can be more sensitive to toolpath direction. Evaluate the rolling direction and deflection risk; adjust the toolpath strategy and tool support as needed.
• Surface quality improvement: With adequate machine rigidity, tooling, and coolant, higher RPM / reasonable cutting speed can reduce built-up edge and drag marks, helping reduce polishing workload.
• Anti built-up edge: Due to Ni/Cu alloying, a lower cutting speed is more likely to cause built-up edge. Keep cutting speed in a reasonable range and tune by tool and cooling method.
• Rust prevention: Rust prevention management is important. Water-based emulsions should be evaluated carefully; clean, dry, and apply rust preventive protection after machining to avoid long exposure.
• Weld repair note: Weld repair has crack risk. Typically, preheating to around 200°C and controlled, slow cooling are recommended to reduce the risk of cracking (depending on the filler, repair area, and heat input).

Typical Problems & Solutions

Q1: What if haze, pinholes, and scratches appear during NAK80 polishing, making it hard to reach “Mirror Level 3”?

Try the following approaches:

1. Refine the polishing process into 8 steps, progressing sandpaper grit as:
   180# → 320# → 400# → 600# → 800# → 1000# → 1500# → 2000#.
2. Check hardness and consistency. If hardness is uneven, perform two stabilization treatments before entering the mirror-polishing stage.
3. Use imported mirror polishing fluid or diamond polishing paste (W5 → W1.5). Finish with a wool wheel and chromium oxide compound for final luster.
4. Control pressure and direction: reduce pneumatic polishing pressure to about 0.25 MPa and maintain a consistent polishing direction to prevent repeated cross-scratches.

Q2: What if local “white haze” appears after polishing (especially at corners), affecting appearance?

Try the following approaches:

1. Add cold-air cooling for the polishing area to avoid local overheating (keep temperature below about 60°C).
2. Use smaller polishing tools at corners and reduce pressure.
3. After polishing, clean with alcohol to remove polishing compound residue.

Q3: What if pinholes cause dimples on molded plastic parts after polishing?

Try the following approaches:

1. Select higher-purity NAK80 with S ≤ 0.005%.
2. Perform fluorescent penetrant inspection before polishing.
3. Use a compatible mold repair compound to fill existing pinholes, then re-polish.

Q4: How to improve efficiency and reduce cost for mirror polishing of NAK80 molds?

Try the following approaches:

1. Introduce robotic polishing for rough and mid polishing, keeping only final mirror finishing as manual work.
2. Use dedicated NAK80 polishing wheels to improve cutting efficiency.
3. Build a polishing parameter database to standardize operations.

Q5: What if cracks appear after weld repair on NAK80?

Try the following approaches:

1. Preheat to 200–250°C using ceramic heaters or induction heating for uniform temperature rise.
2. Use low-hydrogen / crack-resistant consumables such as WE600 special alloy steel electrodes or ERNiCrMo-3 nickel-based filler wire.
3. Use low current with multi-layer multi-pass welding or segmented back-step technique; control interpass temperature at 200–220°C.
4. Immediately cover with asbestos cloth for slow cooling, then stress-relief at 280–300°C × 1.5–2 hours.

Q6: What if hardness/strength is not acceptable after weld repair?

Try the following approaches:

1. If hardness is insufficient: use “base layer + WC-reinforced top layer” overlay welding, then temper at 250°C for 1 hour.
2. If hardness is too high and machining is difficult: use low-carbon filler, temper at 350°C × 2 hours, and machine with carbide tools at high speed and low feed.
3. If strength is insufficient: increase groove angle to 60°, use three-layer welding to ensure penetration, choose ER110S-G high-strength wire, preheat to 280°C, then temper at 320°C × 2 hours.

Q7: What if porosity, color difference, or mirror finish is not achieved after weld repair?

Try the following approaches:

1. Remove porosity: clean the area with acetone; bake electrodes at 350°C for 1 hour; use 2–3 mm short-arc uphill welding and increase travel speed to reduce molten pool residence time.
2. Reduce color difference: use filler with similar composition; control heat input to reduce oxidation; apply local heat treatment and tone-matching polishing.
3. Meet mirror finish: use pulsed TIG and dedicated filler wire; leave 0.1 mm stock after precision grinding; polish progressively with diamond paste 800#–8000#, then finish with wool wheel for mirror appearance.

Q8: What if hardness is insufficient after heat treatment?

Try the following approaches:

1. Low-temperature aging strengthening: 500–530°C staged holding for 2–3 hours; strictly control heating rate ≤ 100°C/hour; furnace cooling.
2. Double aging: 500°C × 2 hours air cool, then 530°C × 3 hours furnace cool; use polymer quench fluid to increase cooling rate when applicable.
3. Local/overall re-hardening: for machined parts, local induction heating (850°C × 30 s + spray cooling + 200°C temper). For unmachined parts, re-austenitize at 840–870°C for 1.5–2.5 hours, then isothermal quench and age at 520–540°C.
4. Surface nitriding: gas nitriding at 500°C for 3 hours; 180°C low-temperature temper; control case depth at 0.1–0.2 mm.

Q9: What if hardness is uneven after heat treatment?

Try the following approaches:

1. Vacuum quenching + aging: 840°C vacuum heating for 1.5 hours; oil cool to 150°C then temper at 200°C; age at 520°C for 3 hours; maintain temperature uniformity ±3°C.
2. Stress relief + overall aging: 550°C × 2 hours anneal, then 530°C overall aging for 3 hours.
3. Local heating + overall aging: locally induction heat low-hardness areas to 840°C, then age overall at 530°C; repeat local treatment if still not acceptable.
4. Adjust process parameters: increase quench temperature (860–870°C), extend holding time, and use two-stage tempering to optimize precipitate distribution.

Q10: What if hardness is abnormal (too high/brittle)?

Try the following approaches:

1. High-temperature temper to reduce hardness: 600°C × 2 hours to relieve stress, then age at 520°C for 3 hours; heating rate ≤ 50°C/hour to reduce cracking risk.
2. Vacuum slow-cooling aging: hold at 520°C for 3 hours, cool slowly at 20°C/hour to reduce thermal stress.
3. Targeted adjustments: confirm alloy composition first, then fine-tune austenitizing temperature, cooling rate (control at 50–60°C/s), and tempering process to balance hardness and toughness.
4. Low-temperature pre-treatment: add 150°C × 1 hour pre-treatment before aging to reduce deformation and stabilize hardness.

NAK80 Datasheet Download