Steel P21-ST (Solution annealed)
Heat-treated state
AISI/SAE P21 · UNS T51621
Pick when the cavity is to be fully machined soft and then age-hardened with near-zero distortion.
Values for the ST 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 | — |
STThis page 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-ST?
P21-ST is P21 heat treated to ST — pick when the cavity is to be fully machined soft and then age-hardened with near-zero distortion. P21-ST has a yield strength of 740 MPa (107 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 83% of steel grades. Elongation at break is 18% and the elastic modulus is 205 GPa (29.7 Msi). P21-ST 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; ST is available on request or by drawing note.
Advantages
- 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
- High strength — 740 MPa (107 ksi), better than 83% 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
P21-ST properties
Typical room-temperature values for P21-ST; 12 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-ST 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-ST has a yield strength of 740 MPa (107 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 83% of steel grades. Elongation at break is 18% and the elastic modulus is 205 GPa (29.7 Msi).
Thermal6
P21-ST 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-ST 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-ST 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-ST's machinability is good (58/100, better than 75% of steel grades); weldability good (55/100); formability poor (15/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with P21-ST
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-ST — frequently asked
What is P21-ST used for?
P21-ST 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-ST?
P21-ST has a typical yield strength of 740 MPa (107 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 83% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is P21-ST easy to machine?
Reasonably — machinability is rated good (58/100, better than 75% 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-ST 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-ST?
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-ST 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-ST?
P21-ST 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.