Steel P91
AISI/SAE P91
P91 is a 9Cr-1Mo-V-Nb-N creep-strength-enhanced ferritic (CSEF) steel supplied as seamless pipe, tube, forgings and plate for main steam lines, headers, superheaters and reheaters in supercritical power plants.
Values for the HR condition
P91 tempers and conditions
P91 is supplied in 5 conditions, soft to hard. HR 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 |
|---|---|---|---|---|---|---|
subcritical/full anneal ~760–800 °C, softened for machining or forming | Softening treatment for heavy machining, cold forming or before re-normalizing; creep strength is not certified in this state. | 350 | 560 | 28 | 180 HB | — |
Normalized and tempered | The only ASME/ASTM-acceptable service condition for P91 pipe, tube, forgings and plate — tempered martensite with balanced creep strength, toughness and 190–250 HB hardness. | 500 | 650 | 24 | 19 HRC | — |
Quenched and tempered | Used for heavy-wall forgings, valve bodies and thick headers where liquid quenching is needed to get full martensite through-thickness; properties and hardness limits match N&T. | 520 | 670 | 22 | 20 HRC | — |
HRDefault Hot rolled | Mill intermediate stock only: air-cooled hot-worked material with mixed hard martensite; P91 must be normalized and tempered before it can be used or certified. | 750 | 1,000 | 14 | 35 HRC | 2.7 % IACS |
austenitized ~1050 °C and air cooled, untempered martensite | Intermediate mill/repair condition only — very hard untempered martensite; must be tempered before service or further fabrication. | 950 | 1,300 | 10 | 45 HRC | — |
What is P91?
P91 is a 9Cr-1Mo-V-Nb-N creep-strength-enhanced ferritic (CSEF) steel supplied as seamless pipe, tube, forgings and plate for main steam lines, headers, superheaters and reheaters in supercritical power plants. Its tempered-martensite structure with fine MX and M23C6 precipitates gives roughly twice the allowable stress of 1.25Cr-0.5Mo up to about 600 °C, plus good steam-oxidation resistance and low thermal expansion. Limits are strict thermal-processing control: normalizing/tempering windows, mandatory preheat and 750–780 °C PWHT after welding, hardness control, and a soft type-IV HAZ that governs long-term weld creep life. P91 has a density of 7.77 g/cm³ (0.281 lb/in³), lighter than 92% of steel grades. It melts at 1,450°C (2,642 °F). P91 has a yield strength of 750 MPa (109 ksi) and a tensile strength of 1,000 MPa (145 ksi) — stronger than 84% of steel grades. Elongation at break is 14% and the elastic modulus is 218 GPa (31.6 Msi).
What is P91 used for?
- Main steam piping
- Boiler headers
- Superheater and reheater tubing
- Forged pipe fittings and elbows
- Thick-wall pressure vessel plate
- High-temperature valve and pump bodies
Advantages
- Good corrosion resistance — 28/100, better than 94% of steel grades
- High strength — 750 MPa (109 ksi), better than 84% of steel grades
Limitations
- Low electrical conductivity — 2.7 % IACS, worse than 96% of steel grades
- Difficult to machine — 22/100, worse than 95% of steel grades
- Limited service temperature — 200°C (392 °F), worse than 93% of steel grades
- Low fracture toughness — 40 MPa·√m (36.4 ksi·√in), worse than 92% of steel grades
- Difficult to weld — 15/100, worse than 91% of steel grades
P91 properties
Typical room-temperature values for P91-HR; bars are the position among steel grades.
Physical3
P91 has a density of 7.77 g/cm³ (0.281 lb/in³), lighter than 92% of steel grades. It melts at 1,450°C (2,642 °F).
Mechanical17
P91 has a yield strength of 750 MPa (109 ksi) and a tensile strength of 1,000 MPa (145 ksi) — stronger than 84% of steel grades. Elongation at break is 14% and the elastic modulus is 218 GPa (31.6 Msi).
Thermal6
P91 is rated for continuous service to 200°C (392 °F). It conducts heat at 25 W/m·K (14.4 BTU/hr·ft·°F), worse than 88% of steel grades. Thermal expansion is 10.9 µm/m·K (6.06 µin/in·°F).
Electrical3
P91 conducts electricity at 2.7 % IACS, worse than 96% of steel grades.
Chemical & Environmental2
Corrosion resistance is poor (28/100), better than 94% of steel grades.
Sustainability4
Producing a kilogram of P91 takes about 34 MJ of energy and emits 2.5 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 30%.
Manufacturability7
P91's machinability is poor (22/100, worse than 95% of steel grades); weldability poor (15/100); formability poor (20/100).
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. P91 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with P91
Is P91 easy to machine?
Machines like ~215 HB low-alloy steel: rigid setups, carbide tooling, moderate speeds and heavy coolant; work-hardens locally and any cold-worked surface should be considered in later heat treatment.
Can P91 be welded?
Requires 200–250 °C preheat, matched 9CrMoV filler, tight interpass control (≤300 °C), cool to below Ms (~80–100 °C) before PWHT, then mandatory 750–780 °C PWHT; type-IV HAZ softening limits long-term creep life.
Can P91 be formed, bent or molded?
Hot form at 950–1150 °C; any hot bending or significant cold work must be followed by full re-normalizing and tempering to restore creep properties.
Manufacturability
The process limits FabDigit applies to P91. Every P91 part is priced by an engineer; design inside these and the quote comes back without a redesign loop.
CNC machining
| Machinability | Poor22 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 180 / 210 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 5× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Sheet metal
| Formability | Fair20 / 100 |
|---|---|
| Weldability | Better15 / 100 |
P91 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 |
|---|---|---|
| Ccarbide/carbonitride former, creep strength | 0.08 – 0.12 | 0.1 |
| Si | 0.2 – 0.5 | 0.35 |
| Mn | 0.3 – 0.6 | 0.45 |
| P | ≤ 0.02 | |
| S | ≤ 0.01 | |
| Crsteam oxidation resistance | 8 – 9.5 | 8.75 |
| Mosolid-solution creep strength | 0.85 – 1.05 | 0.95 |
| VMX precipitation | 0.18 – 0.25 | 0.21 |
| NbMX precipitation, grain refinement | 0.06 – 0.1 | 0.08 |
| Nnitride strengthening | 0.03 – 0.07 | 0.05 |
| Nimax; limited to keep Ac1 high | ≤ 0.4 | |
| Almax; excess Al ties up N | ≤ 0.02 | |
| Ti | ≤ 0.01 | |
| Zr | ≤ 0.01 | |
| Fe | balance |
What surface finishes work on P91?
Not corrosion resistant at ambient — shot blast plus high-temperature aluminium paint, aluminizing or chromizing; no anodizing.
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).
Plating
CVD Coating
Black / Phosphate
Thermal Spray
How much does P91 cost?
P91 is a standard-cost steel ($$), cheaper than about 71% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
P91 — frequently asked
What is P91 used for?
P91 is typically used for main steam piping, boiler headers, superheater and reheater tubing, forged pipe fittings and elbows, thick-wall pressure vessel plate and high-temperature valve and pump bodies. In short: high-temp power-plant pipe.
What is the yield strength of P91?
P91 has a typical yield strength of 750 MPa (109 ksi) and a tensile strength of 1,000 MPa (145 ksi) — stronger than 84% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is P91 easy to machine?
Not especially — machinability is rated poor (22/100, worse than 95% of steel grades). Machines like ~215 HB low-alloy steel: rigid setups, carbide tooling, moderate speeds and heavy coolant; work-hardens locally and any cold-worked surface should be considered in later heat treatment.
Can P91 be welded?
Not readily — weldability is rated poor (15/100). Requires 200–250 °C preheat, matched 9CrMoV filler, tight interpass control (≤300 °C), cool to below Ms (~80–100 °C) before PWHT, then mandatory 750–780 °C PWHT; type-IV HAZ softening limits long-term creep life.
What surface finishes work on P91?
Not corrosion resistant at ambient — shot blast plus high-temperature aluminium paint, aluminizing or chromizing; no anodizing.
Can P91 be formed, bent or molded?
Hot form at 950–1150 °C; any hot bending or significant cold work must be followed by full re-normalizing and tempering to restore creep properties.
What is the maximum service temperature of P91?
P91 is rated for continuous use to about 200°C (392 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
How much does P91 cost?
P91 is a standard-cost steel ($$), cheaper than about 71% 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.
Can FabDigit make parts in P91?
Yes — P91 is available for CNC machining and sheet metal; every part is priced by an engineer rather than instantly. Upload a STEP file to get a price and a DFM check.
Sources
- ASTM A335/A335M — Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service (Grade P91) — ASTM (2023)
- ASTM A213/A213M (T91) and A182/A182M (F91) — ASTM (2022)
- EN 10216-2 — Seamless steel tubes for pressure purposes, X10CrMoVNb9-1 (1.4903) — CEN (2020)
- GB/T 5310 — (10Cr9Mo1VNbN) — SAC (2017)
- ASME BPVC Section II Part D — allowable stresses and physical properties — ASME (2023)
- The Grades 91 Steel Handbook / CSEF steel guidelines — EPRI (2015)
- ASM Handbook Vol. 1 — Properties and Selection: Irons, Steels — ASM International (1990)
- Seamless P91/T91 tube datasheets — Vallourec / Tenaris / Sandvik (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.