ASTM A335 P91 Alloy Seamless Pipes – ASME SA335 P91 (9Cr-1Mo-V, UNS K91560)
ASC LLP, Mumbai, supplies ASTM A335 P91 pipe (ASME SA335 P91, UNS K91560) – seamless 9Cr-1Mo-V alloy steel pipe for high-temperature steam and process lines. Pipes are available from ½” to 24″ NB in Type 1 and Type 2, normalised and tempered, with EN 10204 3.1 certificates. See our full alloy steel seamless pipe range.
What is ASTM A335 P91 Pipe?
ASTM A335 covers seamless ferritic alloy steel pipe for high-temperature service. Grade P91 is a 9% chromium, 1% molybdenum steel with small, controlled additions of vanadium, niobium and nitrogen. These form fine carbonitrides that give P91 roughly twice the allowable stress of P22 at 550 – 600 °C. This lets designers use much thinner walls for main steam and hot reheat lines. P91 is a creep-strength-enhanced ferritic (CSEF) steel: its strength depends on correct normalising and tempering, both at the mill and after every weld.
P91 Type 1 vs Type 2
The current ASTM A335 lists P91 as Type 1 and Type 2. Both types have the same mechanical properties and heat treatment. Type 2 has tighter chemistry: lower limits on residual elements such as aluminium (0.020% max.), arsenic (0.010% max.) and boron (0.001% max.), plus a minimum nitrogen-to-aluminium ratio. This improves long-term creep performance. Many power-plant specifications now call for Type 2 – state the type on your enquiry.
Properties of P91 Pipe
- High creep strength: much stronger than P11 and P22 at 550 – 620 °C, allowing thinner, lighter piping.
- Oxidation resistance: 9% chromium gives good steam-side oxidation resistance up to about 600 °C.
- Lower thermal stress: thinner walls and lower thermal expansion than austenitic stainless steel mean less thermal fatigue during start-ups and shutdowns.
- Hardness window: 190 – 250 HBW. Hardness below the minimum usually indicates wrong tempering and lost creep strength.
Read more: role of alloying elements in P91.
Welding & PWHT of P91
P91 needs a qualified welding procedure. Typical practice is:
- Preheat of about 200 – 250 °C
- Matching 9Cr-1Mo-V-Nb filler with controlled Ni + Mn
- Cooling to complete martensite transformation
- Mandatory post-weld heat treatment, usually around 745 – 775 °C, with hardness checks afterwards
Over-tempering or skipping PWHT both damage creep life. Follow the governing code (ASME B31.1, B31.3 or Section I) and the client’s specification.
Related: welding considerations for T91 / P91 and inspection and testing of P91 pipes.
ASTM A335 P91 Pipe Specifications
| Item | Details |
| Specification | ASTM A335 / ASME SA335, Grade P91 Type 1 or Type 2 |
| UNS | K91560 |
| Type | Seamless – hot finished or cold drawn |
| Size | ½” – 24″ NB (larger sizes and non-standard walls on request) |
| Wall | SCH 40, 80, 120, 160, XXS or specified wall, per ASME B36.10M |
| Length | Single random, double random, cut lengths |
| Ends | Plain end or bevelled end (threading not normal for P91) |
| Heat treatment | Normalised 1040 – 1080 °C and tempered 730 – 800 °C |
| Testing | Tensile, hardness, flattening/bend, hydrostatic or NDE, PMI; UT and impact tests on request |
| Certificate | EN 10204 3.1; 3.2 / third-party inspection on request |
ASTM A335 P91 Pipe Size, Schedule & Weight Chart
| NPS | OD (mm) | SCH 40 (mm) | SCH 80 (mm) | SCH 160 (mm) | XXS (mm) | SCH 80 weight (kg/m) | SCH 160 weight (kg/m) |
| ½” | 21.34 | 2.77 | 3.73 | 4.78 | 7.47 | 1.60 | 1.93 |
| ¾” | 26.67 | 2.87 | 3.91 | 5.56 | 7.82 | 2.17 | 2.86 |
| 1″ | 33.40 | 3.38 | 4.55 | 6.35 | 9.09 | 3.20 | 4.19 |
| 1¼” | 42.16 | 3.56 | 4.85 | 6.35 | 9.70 | 4.42 | 5.55 |
| 1½” | 48.26 | 3.68 | 5.08 | 7.14 | 10.15 | 5.35 | 7.17 |
| 2″ | 60.33 | 3.91 | 5.54 | 8.74 | 11.07 | 7.41 | 11.01 |
| 2½” | 73.03 | 5.16 | 7.01 | 9.53 | 14.02 | 11.30 | 14.77 |
| 3″ | 88.90 | 5.49 | 7.62 | 11.13 | 15.24 | 15.12 | 21.13 |
| 4″ | 114.30 | 6.02 | 8.56 | 13.49 | 17.12 | 22.09 | 33.19 |
| 5″ | 141.30 | 6.55 | 9.53 | 15.88 | 19.05 | 30.65 | 48.61 |
| 6″ | 168.28 | 7.11 | 10.97 | 18.26 | 21.95 | 42.12 | 66.86 |
| 8″ | 219.08 | 8.18 | 12.70 | 23.01 | 22.23 | 63.98 | 110.12 |
| 10″ | 273.05 | 9.27 | 15.09 | 28.58 | 25.40 | 95.01 | 170.54 |
| 12″ | 323.85 | 10.31 | 17.48 | 33.32 | 25.40 | 130.72 | 236.29 |
| 14″ | 355.60 | 11.13 | 19.05 | 35.71 | – | 156.49 | 278.83 |
| 16″ | 406.40 | 12.70 | 21.44 | 40.49 | – | 201.46 | 361.63 |
| 18″ | 457.20 | 14.27 | 23.83 | 45.24 | – | 252.07 | 454.91 |
| 20″ | 508.00 | 15.09 | 26.19 | 50.01 | – | 308.00 | 559.06 |
| 24″ | 609.60 | 17.48 | 30.96 | 59.54 | – | 437.27 | 799.40 |
Wall thicknesses per ASME B36.10M. Weights are plain-end values calculated for P91 density (7.77 g/cm³). XXS is not standard above 12″. Other schedules (SCH 100, 120, 140) and specified walls on request.
Mechanical Properties of A335 P91 Pipe
| Property | ASTM A335 P91 (Type 1 & Type 2) |
| Tensile strength, min. | 585 MPa (85 ksi) |
| Yield strength, min. | 415 MPa (60 ksi) |
| Elongation in 50 mm, min. | 20% longitudinal / 13% transverse |
| Hardness | 190 – 250 HBW (196 – 265 HV) |
P91 vs P11, P22 & P92
| Grade | Nominal composition | Tensile / yield min. (MPa) | Typical use |
| P11 | 1¼Cr-½Mo | 415 / 205 | Steam and refinery lines to about 540 °C |
| P22 | 2¼Cr-1Mo | 415 / 205 | Steam, hydrogen and refinery service |
| P91 | 9Cr-1Mo-V-Nb-N | 585 / 415 | Main steam, hot reheat, headers to about 600 °C |
| P92 | 9Cr-0.5Mo-1.8W-V-Nb | 620 / 440 | Ultra-supercritical steam lines |
Also see P5 / P5b / P5c pipes and our blog P5 vs P11 vs P91 – which to choose.
Heat Treatment of ASME SA335 P91
| Process | Temperature |
| Normalising | 1040 – 1080 °C (1900 – 1975 °F) |
| Tempering | 730 – 800 °C (1350 – 1470 °F) |
| Alternative | Quench from 1040 – 1080 °C and temper at 730 – 800 °C |
Hold times depend on wall thickness and are set by the mill procedure. Any repair-welded pipe must be re-tempered at 730 – 800 °C.
P91 Chemical Composition (UNS K91560)
| Element | P91 Type 1 (%) |
| Carbon (C) | 0.08 – 0.12 |
| Manganese (Mn) | 0.30 – 0.60 |
| Phosphorus (P) | 0.020 max. |
| Sulphur (S) | 0.010 max. |
| Silicon (Si) | 0.20 – 0.50 |
| Chromium (Cr) | 8.00 – 9.50 |
| Molybdenum (Mo) | 0.85 – 1.05 |
| Vanadium (V) | 0.18 – 0.25 |
| Niobium (Nb) | 0.06 – 0.10 |
| Nitrogen (N) | 0.030 – 0.070 |
| Nickel (Ni) | 0.40 max. |
| Aluminium (Al) | 0.02 max. |
| Titanium (Ti) | 0.01 max. |
| Zirconium (Zr) | 0.01 max. |
Type 2 has tighter limits on residual elements (including Al 0.020% max., As 0.010% max., B 0.001% max.) and a minimum N/Al ratio – refer to the current ASTM A335 for the full Type 2 table.
Physical Properties of P91
| Property | Value (approx., at 20 °C) |
| Density | 7.77 g/cm³ |
| Modulus of elasticity | ≈ 218 GPa |
| Thermal conductivity | ≈ 26 W/m·K |
| Thermal expansion (20 – 100 °C) | ≈ 10.5 – 11 µm/m·K |
| Magnetic | Yes (ferritic / martensitic) |
P91 Equivalent Grades
| Standard | Equivalent |
| ASTM / ASME | A335 / SA335 P91 (K91560) |
| EN 10216-2 | X10CrMoVNb9-1 (1.4903) |
| JIS G3458 | STPA28 |
| GB 5310 | 10Cr9Mo1VNbN |
Equivalent grades are similar, not identical – check heat treatment, testing and code acceptance before substituting.
Applications of ASTM A335 P91 Pipes
| Industry | Typical P91 pipe applications |
| Power plants | Main steam, hot reheat, headers and turbine bypass lines in supercritical units |
| HRSG / combined cycle | HP superheater headers, steam links and interconnecting piping |
| Refineries & petrochemicals | High-temperature process and furnace piping |
| Industrial steam | High-pressure, high-temperature steam distribution |
For boiler and superheater tube, see T91 tubes (ASTM A213). P91 is not intended for oil and gas production pipelines or low-temperature service.
Why Buy P91 Pipe from ASC LLP?
- Correct paperwork: A335 / SA335 P91 Type 1 or Type 2 with EN 10204 3.1 certificates showing heat treatment, hardness and full chemistry including Nb and N.
- Checks before dispatch: PMI and hardness verification; third-party inspection on request.
- Complete P91 system: WP91 fittings, F91 flanges, T91 tubes and SA387 Gr 91 plates.
- Export packing with bevel protectors and end caps.
Send size, schedule, type (1 or 2), length and quantity to sales@amardeepsteel.com, call 022-67439130 or contact us.
Frequently Asked Questions – ASTM A335 P91 Pipe
What is A335 P91 material?
A335 P91 (UNS K91560) is a seamless 9% chromium, 1% molybdenum alloy steel pipe with vanadium, niobium and nitrogen additions. It is a creep-strength-enhanced ferritic steel used for high-temperature steam piping in power plants.
What is the chemical composition of P91?
P91 contains C 0.08–0.12%, Mn 0.30–0.60%, Si 0.20–0.50%, Cr 8.00–9.50%, Mo 0.85–1.05%, V 0.18–0.25%, Nb 0.06–0.10% and N 0.030–0.070%, with P 0.020% max., S 0.010% max. and Ni 0.40% max.
What are the mechanical properties of P91 pipe?
Minimum tensile strength is 585 MPa (85 ksi), minimum yield strength 415 MPa (60 ksi), elongation 20% longitudinal or 13% transverse, and hardness 190–250 HBW.
What is the difference between P91 Type 1 and Type 2?
Both have the same strength and heat treatment. Type 2 has tighter limits on residual elements such as aluminium, arsenic and boron, and a minimum N/Al ratio, for better long-term creep performance.
What is the heat treatment for P91 pipe?
A335 P91 is normalised at 1040–1080 °C and tempered at 730–800 °C (or quenched and tempered in the same ranges). After welding, post-weld heat treatment is mandatory.
What is the equivalent of A335 P91?
The European equivalent is EN 10216-2 X10CrMoVNb9-1 (1.4903). The Japanese equivalent is JIS STPA28. The matching tube grade is ASTM A213 T91, fittings A234 WP91 and flanges A182 F91.
Global Reach
ASC LLP supplies alloy steel pipes, fittings and flanges to power, refinery and EPC customers in the USA, Europe, the Middle East and Asia, with secure packing and full documentation. Buyers worldwide rely on our ASTM A335 P91 alloy seamless pipes for certified material and traceability.
Countries We Export To
UNITED STATES, EUROPE, UK, UAE, SAUDI ARABIA, OMAN, KUWAIT, JORDAN, QATAR, INDONESIA, MALAYSIA, BRAZIL, SPAIN, ITALY, FRANCE, GERMANY, BAHRAIN, SRI LANKA, AZERBAIJAN, AUSTRALIA, AUSTRIA, PHILIPPINES, TURKEY, RUSSIA, IRAN, IRAQ, CANADA, KOREA, JAPAN, YEMEN, FINLAND, ZIMBABWE, HUNGARY, PORTUGAL, VIETNAM, MOROCCO, THAILAND, SOUTH AFRICA, MEXICO, SINGAPORE, GREECE, KUWAIT, POLAND, NEW ZEALAND, SWEDEN, SWITZERLAND, BELGIUM, BHUTAN, BANGLADESH, TAIWAN, ROMANIA, CAMBODIA, KAZAKHSTAN, AFGHANISTAN.
