ASTM A335 P91 Pipes
Giriraj Impex stocks, supplies and exports ASTM A335 P91 pipe – seamless ferritic alloy steel pipe to ASTM A335 / ASME SA335 Grade P91 (9Cr-1Mo-V) for high-temperature service. We supply A335 P91 pipes from approved mills, normalised and tempered, in schedules and wall thicknesses to ASME B36.10M or to specified minimum wall, with IBR certification when the order requires it, and with mill test certificates.
P91 pipe is used for main steam, hot reheat and header piping in supercritical and combined-cycle power plants, and in refinery and petrochemical high-temperature service, where its creep strength allows much thinner walls than P22.
Specifications
| Item | Details |
|---|---|
| Standard | ASTM A335 / ASME SA335, Grade P91 (UNS K91560) |
| Type | Type 1 and Type 2 as defined in the current edition of ASTM A335, as ordered |
| Production | Seamless, hot finished or cold drawn |
| Heat treatment | Normalised and tempered |
| Dimensions | ASME B36.10M schedules, or specified minimum wall thickness and inside diameter |
| Ends | Plain, bevelled to ASME B16.25 or to the customer's weld preparation |
| Options | IBR certification, ultrasonic test, hardness survey, PMI – on request |
| Documents | Mill test certificate (MTC); third-party inspection on request |
Chemical composition and mechanical properties
The tables include other A335 grades for comparison; P91 is the last row.
| Grade | UNS | C | Mn | P | S | Si | Cr | Mo |
|---|---|---|---|---|---|---|---|---|
| P5 | K41545 | 0.15 | 0.30–0.60 | 0.025 | 0.025 | 0.50 | 4.00–6.00 | 0.45–0.65 |
| P9 | S50400 | 0.15 | 0.30–0.60 | 0.025 | 0.025 | 0.25–1.00 | 8.00–10.00 | 0.90–1.10 |
| P11 | K11597 | 0.05–0.15 | 0.30–0.60 | 0.025 | 0.025 | 0.50–1.00 | 1.00–1.50 | 0.44–0.65 |
| P22 | K21590 | 0.05–0.15 | 0.30–0.60 | 0.025 | 0.025 | 0.50 | 1.90–2.60 | 0.87–1.13 |
| P91 | K90901 | 0.08–0.12 | 0.30–0.60 | 0.020 | 0.010 | 0.20–0.50 | 8.00–9.50 | 0.85–1.05 |
P91 also has limits on V, Nb, N, Ni, Al and other elements, as given in the standard.
| Grade | Tensile strength, MPa (ksi) | Yield strength, MPa (ksi) |
|---|---|---|
| P5, P9, P11, P22 | 415 (60) | 205 (30) |
| P91 | 585 (85) | 415 (60) |
Elongation and P91 hardness requirements as given in the standard.
Background on chrome-moly grades is on the alloy steel page.
Why P91
P91 is a creep-strength-enhanced ferritic steel. Its 9% chromium and 1% molybdenum, with small additions of vanadium, niobium and nitrogen, form fine, stable precipitates during normalising and tempering that give much higher creep strength than conventional chrome-moly steels. At typical main steam temperatures, P91 allows wall thicknesses roughly half those of P22 for the same pressure, which reduces weight, support loads and thermal stresses during start-up and shutdown. It also has good resistance to oxidation in steam.
P91 replaced P22 in many new power plants because thinner walls reduce the weight of large main steam and reheat lines, and lower thermal stresses allow faster start-ups and more cycles. It is also used to replace ageing P22 piping in existing plants. Grade P92 offers still higher creep strength for the most advanced ultra-supercritical plants and is available on request.
Fabrication and heat treatment
The properties of P91 depend on its microstructure, so fabrication must be controlled closely. Welding requires preheat and interpass temperature control, matching filler metals, and post-weld heat treatment within a narrow temperature band; too low a temperature leaves hard welds, and too high a temperature, or bending without re-heat treatment, can reduce creep strength. Hardness testing of welds and base metal is commonly used to check heat treatment. Fabricators should follow qualified procedures and the design code's rules for P91.
IBR certification
For steam piping in India subject to the Indian Boiler Regulations, P91 pipe is supplied with IBR certification when the order requires it, with the mill documents and inspection arranged accordingly. Please state the IBR requirement in the enquiry so it can be included in the quotation.
Ordering A335 P91 pipe
Please give the size, schedule or minimum wall, length, end finish, quantity, the A335 type if specified, and any IBR, ultrasonic, hardness or third-party inspection requirement. Pipes are marked with specification, grade, size, wall, heat number and the mill's identification, and are supplied with end caps.
Applications and related products
A335 P91 pipe is used for main steam, hot reheat and header piping in power plants, heat-recovery steam generators, high-temperature refinery and petrochemical lines and boiler components. Lower-alloy grades such as P11 and P22 are on the ASTM A335 P11 and P22 pipes page, other alloy steel pipes on the alloy steel pipes and tubes page, and all materials on the seamless pipes and tubes page.
Frequently Asked Questions
ASTM A335 P91 is a seamless 9Cr-1Mo-V alloy steel pipe for high-temperature service, with high creep strength from vanadium, niobium and nitrogen additions.
P91 has much higher creep strength than P22 (2¼Cr-1Mo), allowing thinner walls at high temperatures, but needs stricter control of welding and heat treatment.
Yes. P91 pipe can be supplied with IBR certification when the order requires it.
P91 pipe is normalised and tempered at the mill. Welds require post-weld heat treatment within a narrow temperature range.
P91 pipe is used for main steam, hot reheat and header piping in power plants, HRSGs and high-temperature refinery service.
ASTM A335 P91 is UNS K91560.
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