Steel P21
AISI/SAE P21 · UNS T51621
P21 (T51621) is a Ni-Al age-hardenable plastic mold steel: it is machined in the solution-treated state, then precipitation hardened to 38–43 HRC with minimal distortion and near-zero size change.
Reference datasheet — P21 is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the PH 38-42 HRC condition
P21 tempers and conditions
P21 is supplied in 5 conditions, soft to hard. PH 38-42 HRC is the default FabDigit quotes; the values on this page are for it.
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 | — |
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?
P21 (T51621) is a Ni-Al age-hardenable plastic mold steel: it is machined in the solution-treated state, then precipitation hardened to 38–43 HRC with minimal distortion and near-zero size change. It is supplied as annealed, solution-treated, pre-hardened or nitrided plate for precision cavities; at 40 HRC it runs about 1150 MPa yield, 1300 MPa tensile, 14% elongation, 375 HB and service to 450 °C. Machinability is moderate (score 45) and the steel is not stainless, so corrosive resins call for plating or a corrosion-resistant grade such as S136. P21 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). P21 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 14% and the elastic modulus is 205 GPa (29.7 Msi).
What is P21 used for?
- precision plastic mold cavities
- optical/lens molds
- components requiring low distortion after hardening
Advantages
- Polishes to a fine finish — 90/100, better than 100% of steel grades
- High strength — 1,150 MPa (167 ksi), better than 94% of steel grades
- High service temperature — 450°C (842 °F), better than 86% of steel grades
Limitations
- Limited formability — 15/100, worse than 87% of steel grades
- Low electrical conductivity — 4.9 % IACS, worse than 82% of steel grades
- Poor heat conductor — 33 W/m·K (19.1 BTU/hr·ft·°F), worse than 80% of steel grades
- Limited ductility — 14%, worse than 75% of steel grades
P21 properties
Typical room-temperature values for P21-PH 38-42 HRC; bars are the position among steel grades.
Physical3
P21 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 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 14% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
P21 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 conducts electricity at 4.9 % IACS, worse than 82% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (16/100), better than 79% of steel grades.
Sustainability4
Producing a kilogram of P21 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's machinability is fair (45/100, worse than 65% of steel grades); weldability good (55/100); formability poor (15/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. P21 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with P21
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.
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 |
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.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Organic Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel P21 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- T51621
How much does P21 cost?
P21 is a premium-cost steel ($$$), cheaper than about 51% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
P21 — frequently asked
What is P21 used for?
P21 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?
P21 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 easy to machine?
Reasonably — machinability is rated fair (45/100, worse than 65% 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 be welded?
Yes — weldability is rated good (55/100). 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?
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 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?
P21 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.
How much does P21 cost?
P21 is a premium-cost steel ($$$), cheaper than about 51% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
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; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.