Steel NAK55 Prehard
Heat-treated state
Prehard = pre-hardened
Standard commercial supply condition: machine and EDM directly, no further heat treatment or distortion risk.
Values for the Prehard condition
Other NAK55 tempers and conditions
NAK55 is listed in 4 conditions; see the grade overview at NAK55.
Compare all 4 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
AnnealedDefault soft condition for heavy machining or re-heat-treatment | Rarely stocked; used only when very heavy stock removal or re-heat-treatment of a used block is planned, aging afterwards restores 40 HRC. | 560 | 800 | 20 | 24 HRC | — |
PrehardThis page Pre-hardened | Standard commercial supply condition: machine and EDM directly, no further heat treatment or distortion risk. | 1,100 | 1,270 | 10 | 40 HRC | — |
prehard core plus gas/plasma nitrided case (~600–750 HV surface) | Use for abrasive glass- or mineral-filled resins and sliding faces; nitriding at 480–530 °C does not soften the aged core. | 1,100 | 1,270 | 8 | 40 HRC | 6.6 % IACS |
Quenched and tempered | Choose when a repaired, welded or annealed block must be restored to full working hardness; aging is a low-temperature, low-distortion treatment. | 1,120 | 1,290 | 9 | 41 HRC | 6.6 % IACS |
What is NAK55 Prehard?
NAK55 Prehard is NAK55 heat treated to Prehard — standard commercial supply condition: machine and EDM directly, no further heat treatment or distortion risk. NAK55 Prehard has a yield strength of 1,100 MPa (160 ksi) and a tensile strength of 1,270 MPa (184 ksi) — stronger than 93% of steel grades. Elongation at break is 10% and the elastic modulus is 206 GPa (29.9 Msi). NAK55 Prehard has a density of 7.8 g/cm³ (0.282 lb/in³), lighter than 87% of steel grades. It melts at 1,420°C (2,588 °F). Annealed is the default condition FabDigit quotes; Prehard is available on request or by drawing note.
Advantages
- High strength — 1,100 MPa (160 ksi), better than 93% of steel grades
Limitations
- Limited ductility — 10%, worse than 91% of steel grades
- High embodied carbon — 2.5 kg CO₂/kg, worse than 82% of steel grades
- Low fracture toughness — 60 MPa·√m (54.6 ksi·√in), worse than 78% of steel grades
NAK55 Prehard properties
Typical room-temperature values for NAK55 Prehard; 13 are specific to this condition. Other listed values are NAK55 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
NAK55 Prehard has a density of 7.8 g/cm³ (0.282 lb/in³), lighter than 87% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
NAK55 Prehard has a yield strength of 1,100 MPa (160 ksi) and a tensile strength of 1,270 MPa (184 ksi) — stronger than 93% of steel grades. Elongation at break is 10% and the elastic modulus is 206 GPa (29.9 Msi).
Thermal5
NAK55 Prehard is rated for continuous service to 430°C (806 °F). It conducts heat at 39 W/m·K (22.5 BTU/hr·ft·°F), worse than 67% of steel grades. Thermal expansion is 12.4 µm/m·K (6.89 µin/in·°F).
Electrical2
NAK55 Prehard has an electrical resistivity of 2.6×10⁻⁷ Ω·m.
Chemical & Environmental2
Corrosion resistance is poor (20/100), better than 87% of steel grades.
Sustainability4
Producing a kilogram of NAK55 Prehard takes about 34 MJ of energy and emits 2.5 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 30%.
Manufacturability6
NAK55 Prehard's machinability is good (55/100, better than 62% of steel grades); weldability fair (40/100); formability poor (30/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with NAK55 Prehard
Is NAK55 easy to machine?
Machines readily at 37–43 HRC thanks to the sulfur addition — use coated carbide, moderate speeds and rigid setups; drilling/tapping is far easier than P20 at equal hardness, and EDM finishing needs a light polish of the recast layer.
Can NAK55 be welded?
TIG/laser repair with matched Ni-Cu-Al or maraging filler after preheat ~250 °C; re-age the area at 500–550 °C to recover hardness, expect somewhat lower toughness at sulfide-rich zones.
Can NAK55 be formed, bent or molded?
Supplied fully hardened — no cold forming or bending; shape only by machining, EDM or grinding, and stress-relieve at ~500 °C after heavy roughing on thin cores.
What surface finishes work on NAK55?
Takes a good general polish (A2/B-grade texture and #4000 finish) but sulfide stringers can show as pinholes in mirror work — choose NAK80 for optical finishes; chrome/EN plating, nitriding and PVD all work, keep plating bake below 250 °C.
NAK55 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Cproducer typical | 0.1 – 0.2 | 0.15 |
| Si | 0.15 – 0.5 | 0.3 |
| Mn | 1.2 – 1.8 | 1.5 |
| Niwith Cu+Al forms age-hardening precipitates | 2.5 – 3.5 | 3 |
| Cu | 0.8 – 1.3 | 1 |
| Al | 0.8 – 1.3 | 1 |
| Sintentional free-machining addition (absent in NAK80) | 0.04 – 0.12 | 0.08 |
| Presidual | ≤ 0.03 | |
| Fe | balance |
NAK55 Prehard — frequently asked
What is NAK55 Prehard used for?
NAK55 Prehard is typically used for injection mould cavities and cores, mould inserts, mould cavity and bolster plates, ejector blocks and slides, textured-finish cosmetic mould components and precision gauge and fixture blocks. In short: premium mirror-polish mold.
What is the yield strength of NAK55 Prehard?
NAK55 Prehard has a typical yield strength of 1,100 MPa (160 ksi) and a tensile strength of 1,270 MPa (184 ksi) — stronger than 93% of steel grades. Strength varies by condition: see the 4 listed tempers.
Is NAK55 Prehard easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). Machines readily at 37–43 HRC thanks to the sulfur addition — use coated carbide, moderate speeds and rigid setups; drilling/tapping is far easier than P20 at equal hardness, and EDM finishing needs a light polish of the recast layer.
Can NAK55 Prehard be welded?
With care — weldability is rated fair (40/100). TIG/laser repair with matched Ni-Cu-Al or maraging filler after preheat ~250 °C; re-age the area at 500–550 °C to recover hardness, expect somewhat lower toughness at sulfide-rich zones.
What surface finishes work on NAK55 Prehard?
Takes a good general polish (A2/B-grade texture and #4000 finish) but sulfide stringers can show as pinholes in mirror work — choose NAK80 for optical finishes; chrome/EN plating, nitriding and PVD all work, keep plating bake below 250 °C.
Can NAK55 Prehard be formed, bent or molded?
Supplied fully hardened — no cold forming or bending; shape only by machining, EDM or grinding, and stress-relieve at ~500 °C after heavy roughing on thin cores.
What is the maximum service temperature of NAK55 Prehard?
NAK55 Prehard is rated for continuous use to about 430°C (806 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between NAK55 Annealed and NAK55 Prehard?
Annealed is the default condition — rarely stocked; used only when very heavy stock removal or re-heat-treatment of a used block is planned, aging afterwards restores 40 HRC. Prehard: standard commercial supply condition: machine and EDM directly, no further heat treatment or distortion risk. Yield strength is 560 MPa in Annealed versus 1,100 MPa in Prehard.
Can FabDigit make parts in NAK55 Prehard?
Yes — NAK55 Prehard is available for CNC machining; every part is priced by an engineer rather than instantly. Upload a STEP file to get a price and a DFM check.
Sources
- NAK55 / NAK80 Pre-hardened Plastic Mold Steel data sheet — Daido Steel Co., Ltd. (2021)
- NAK55 free-machining prehardened mold steel technical bulletin — International Mold Steel (Daido distributor) (2020)
- ASM Handbook Vol. 1: Properties and Selection — Irons, Steels and High-Performance Alloys — ASM International (1990)
- Tool Steels, 5th ed. (mold steel chapters) — ASM International (1998)
- GB/T 1299 (mold and tool steels, 10Ni3MnCuAl class) — SAC (2014)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.