Steel P21 Nitrided
Finish state
AISI/SAE P21 · UNS T51621
Choose for abrasive glass- or mineral-filled resins where cavity wear, not core strength, governs life.
Values for the Nitrided condition
Other P21 tempers and conditions
P21 is listed in 5 conditions; see the grade overview at P21.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
soft annealed for machining/forming stock | Softest supply state for heavy stock removal or re-forging before solution + aging. | 480 | 720 | 22 | 220 HB | — |
Solution annealed | Pick when the cavity is to be fully machined soft and then age-hardened with near-zero distortion. | 740 | 950 | 18 | 30 HRC | — |
PH 38-42 HRCDefault Pre-hardened (solution treated + aged), as-supplied mold plate | Standard stock condition: machine directly to finished cavity, no further heat treatment or distortion. | 1,150 | 1,300 | 14 | 40 HRC | — |
NitridedThis page aged core with gas/plasma nitrided case | Choose for abrasive glass- or mineral-filled resins where cavity wear, not core strength, governs life. | 1,150 | 1,300 | 11 | 40 HRC | 4.9 % IACS |
High-hardness aged mirror-polish grade | Slightly harder aged variant for optical/mirror cavities and long-run high-gloss parts. | 1,230 | 1,380 | 12 | 42 HRC | — |
What is P21 Nitrided?
P21 Nitrided is P21 supplied in the Nitrided condition — choose for abrasive glass- or mineral-filled resins where cavity wear, not core strength, governs life. P21 Nitrided has a yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,300 MPa (189 ksi) — stronger than 94% of steel grades. Elongation at break is 11% and the elastic modulus is 205 GPa (29.7 Msi). P21 Nitrided has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F). PH 38-42 HRC is the default condition FabDigit quotes; Nitrided is available on request or by drawing note.
Advantages
- High strength — 1,150 MPa (167 ksi), better than 94% of steel grades
- Good corrosion resistance — 25/100, better than 92% of steel grades
- Polishes to a fine finish — 70/100, better than 87% of steel grades
- High service temperature — 450°C (842 °F), better than 86% of steel grades
Limitations
- Difficult to machine — 10/100, worse than 99% of steel grades
- Limited formability — 5/100, worse than 99% of steel grades
- Difficult to weld — 12/100, worse than 96% of steel grades
- Limited ductility — 11%, worse than 89% of steel grades
- Low electrical conductivity — 4.9 % IACS, worse than 82% of steel grades
P21 Nitrided properties
Typical room-temperature values for P21 Nitrided; 23 are specific to this condition. Other listed values are P21 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
P21 Nitrided has a density of 7.83 g/cm³ (0.283 lb/in³), lighter than 80% of steel grades. It melts at 1,420°C (2,588 °F).
Mechanical16
P21 Nitrided has a yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,300 MPa (189 ksi) — stronger than 94% of steel grades. Elongation at break is 11% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
P21 Nitrided is rated for continuous service to 450°C (842 °F). It conducts heat at 33 W/m·K (19.1 BTU/hr·ft·°F), worse than 80% of steel grades. Thermal expansion is 12.4 µm/m·K (6.89 µin/in·°F).
Electrical3
P21 Nitrided conducts electricity at 4.9 % IACS, worse than 82% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (25/100), better than 92% of steel grades.
Sustainability4
Producing a kilogram of P21 Nitrided takes about 34 MJ of energy and emits 2.4 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 30%.
Manufacturability8
P21 Nitrided's machinability is not recommended (10/100, worse than 99% of steel grades); weldability not recommended (12/100); formability not recommended (5/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with P21 Nitrided
Is P21 easy to machine?
Machines cleanly at 38–42 HRC with coated carbide (rigid setup, moderate feed); machining in the solution-treated state and aging afterwards gives the best surface plus near-zero size change.
Can P21 be welded?
TIG repair with matching Ni-Al or maraging filler, preheat ~200–250 °C, then re-age at ~510–550 °C to restore weld-zone hardness and avoid polish streaks.
Can P21 be formed, bent or molded?
Hot work 1050–850 °C then slow cool/anneal; no cold forming — the alloy is used as machined mold plate.
What surface finishes work on P21?
Excellent diamond polish to mirror (Ra <0.02 µm) and good photo-etch texturing; add hard chrome, electroless Ni or PVD for corrosive or abrasive resins.
P21 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 |
|---|---|---|
| C | 0.15 – 0.25 | 0.2 |
| Si | 0.15 – 0.4 | 0.3 |
| Mn | 0.2 – 0.6 | 0.35 |
| P | ≤ 0.03 | — |
| Smax; free-machining versions add controlled S | ≤ 0.03 | — |
| Niforms NiAl precipitate with Al | 3.85 – 4.25 | 4 |
| Alage-hardening element | 1 – 1.3 | 1.2 |
| Cr | 0.1 – 0.35 | 0.25 |
| Mooptional, producer dependent | 0.1 – 0.3 | 0.2 |
| Fe | balance | — |
P21 Nitrided — frequently asked
What is P21 Nitrided used for?
P21 Nitrided is typically used for precision plastic mold cavities, optical/lens molds and components requiring low distortion after hardening. In short: age-hardening mold steel; low distortion after hardening; mirror-polish cavity grade.
What is the yield strength of P21 Nitrided?
P21 Nitrided has a typical yield strength of 1,150 MPa (167 ksi) and a tensile strength of 1,300 MPa (189 ksi) — stronger than 94% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is P21 Nitrided easy to machine?
Not especially — machinability is rated not recommended (10/100, worse than 99% of steel grades). Machines cleanly at 38–42 HRC with coated carbide (rigid setup, moderate feed); machining in the solution-treated state and aging afterwards gives the best surface plus near-zero size change.
Can P21 Nitrided be welded?
Not readily — weldability is rated not recommended (12/100). TIG repair with matching Ni-Al or maraging filler, preheat ~200–250 °C, then re-age at ~510–550 °C to restore weld-zone hardness and avoid polish streaks.
What surface finishes work on P21 Nitrided?
Excellent diamond polish to mirror (Ra <0.02 µm) and good photo-etch texturing; add hard chrome, electroless Ni or PVD for corrosive or abrasive resins.
Can P21 Nitrided be formed, bent or molded?
Hot work 1050–850 °C then slow cool/anneal; no cold forming — the alloy is used as machined mold plate.
What is the maximum service temperature of P21 Nitrided?
P21 Nitrided is rated for continuous use to about 450°C (842 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between P21 PH 38-42 HRC and P21-ST?
PH 38-42 HRC is the default condition — standard stock condition: machine directly to finished cavity, no further heat treatment or distortion. ST: pick when the cavity is to be fully machined soft and then age-hardened with near-zero distortion. Yield strength is 1,150 MPa in PH 38-42 HRC versus 740 MPa in ST.
Sources
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels, and High-Performance Alloys (mold steels) — ASM International (1990)
- ASTM A681 — Alloy Tool Steels (P-type mold steels) — ASTM (2015)
- GB/T 1299 / GB/T 1299-2014 — SAC (2014)
- Ni-Al age-hardening plastic mold steel datasheets (NAK80/P21-class) — Daido Steel / Baosteel Special Steel (2022)
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.