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ASTM A335 Grade P91 seamless pipes and tubes are premium-quality ferritic-martensitic alloy steel products designed for high-temperature, high-pressure applications in power generation and industrial facilities. Composed of 9% chromium, 1% molybdenum, and small additions of vanadium, niobium, and nitrogen (9Cr-1Mo-V-Nb-N), this alloy combines excellent creep resistance, thermal fatigue strength, and oxidation resistance. Conforming to ASTM A335 and ASME SA-335, it is a critical material for transporting hot gases and fluids in demanding environments.
The seamless manufacturing process ensures a homogeneous microstructure and precise dimensional control, essential for maintaining integrity in pipelines subjected to extreme thermal and mechanical stresses. Grade P91 was developed as an upgrade to older 9Cr-1Mo steels, offering improved toughness and reduced carbon migration at elevated temperatures.
Outstanding High-Temperature Strength: Maintains creep rupture strength up to 650°C, significantly higher than traditional low-alloy steels, enabling thinner wall designs and weight savings.
Oxidation & Corrosion Resistance: The 9% chromium content forms a protective Cr2O3 oxide layer, resisting scaling and corrosion in steam and flue gas environments.
Low Thermal Expansion & High Thermal Conductivity: Balances thermal properties to minimize stress in cyclic temperature applications, such as boiler tubes and headers.
Weldability with Precaution: Requires preheating (200-300°C) and post-weld heat treatment (730-780°C) to prevent hydrogen-induced cracking and restore mechanical properties, though modern welding techniques have improved its fabricability.
ASTM A335 /A335M ASME SA335 pipes shall be made by the seamless process and shall be either hot-finished or cold drawn with the finishing treatment.
ASTM A335 /A335M covers nominal wall and minimum wall seamless ferritic alloy-steel pipe intended for high-temperature service.
ASTM A 999/A999M specification for general requirements for alloy and stainless-steel pipe
E92 Test method for Vickers hardness of metallic materials
E213 practice for ultrasonic testing of metal pipe and tubing
E309 practice for Eddy-Current examination of steel tubular products using magnetic saturation
E381 method of macro etch testing steel bars, billets, blooms, and forgings
| Grade | C | Mn | P | S | Si | Cr | Mo | V | Nb | N | Al | Others |
| P11 | 0.05-0.15 | 0.30-0.60 | ≤0.025 | ≤0.025 | 0.50-1.00 | 1.0-1.50 | 0.44-0.65 | / | / | / | / | / |
| P12 | 0.05-0.15 | 0.30-0.61 | ≤0.025 | ≤0.025 | ≤0.50 | 0.80-1.25 | 0.45-0.65 | / | / | / | / | / |
| P22 | 0.05-0.15 | 0.30-0.60 | ≤0.025 | ≤0.025 | ≤0.50 | 1.90-2.60 | 0.87-1.13 | / | / | / | / | / |
| P91 | 0.08-0.12 | 0.30-0.60 | ≤0.020 | ≤0.010 | 0.20-0.50 | 8.0-9.50 | 0.85-1.05 | 0.18-0.25 | 0.030-0.070 | ≤0.40 | ≤0.02 | Cb 0.06-0.10 Ti 0.01max Zr 0.01max |
Grade | P11 | P12 | P22 | P91 |
UNS Number | K11597 | K11562 | K21590 | K91560 |
Heat Treat Type | full or isothermal anneal normalize and temper | full or isothermal anneal normalize and temper subcritical anneal | full or isothermal anneal normalize and temper | normalize and temper quench and temper |
Austenitizing/ Solutioning Temperature, min or range °F [°C] | … | … | … | 1900-1975[1040-1080] 1900-1975[1040-1080] |
Subcritical Annealing or Tempering Temperature, Min or range °F [°C] | 1200 [650] | 1200 [650] 1200-1300 [650 to 705] | 1250 [675] | 1350-1470[730-800] 1350-1470[730-800] |
Grade | Tensile | Yield | Elongation | Hardness |
Strength (Mpa) | Strength (Mpa) | (%) | ||
P11 P12 | ≥415 | ≥220 | ≥30 | ≤89HRB |
P22 | ≥415 | ≥200 | ≥30 | ≤89HRB |
P91 | ≥585 | ≥415 | ≥20 | 91HRB-25HRC |
Flattening Test—One flattening test shall be made on specimens from each end of one finished tube, not the one used for the flaring test, from each lot
Flaring Test—One flaring test shall be made on specimens from each end of one finished tube, not the one used for the flattening test, from each lot
Hydrostatic or Nondestructive Electric Test—Each tube shall be subjected to the nondestructive electric test or the hydrostatic test. The type of test to be used shall be at the option of the manufacturer, unless otherwise specified in the purchase order.
Power Plants: Used in superheaters, reheaters, and main steam pipelines of coal-fired and combined-cycle power plants, operating at temperatures up to 625°C and pressures exceeding 300 bar.
Petrochemical Industry: Employed in hydrogen reformers, ethylene heaters, and high-pressure gas pipelines, resisting hydrogen-induced corrosion and thermal fatigue.
Industrial Boilers: Ideal for waste heat boilers, chemical recovery boilers, and process heaters in pulp & paper, refinery, and cement industries.
Turbocharging Systems: Used in high-temperature gas paths of gas turbines and turbochargers, where resistance to creep and oxidation is non-negotiable.
Q: Why is Grade P91 preferred over older 9Cr-1Mo steels?
A:P91 incorporates vanadium and niobium for precipitation strengthening, offering higher creep resistance, better toughness, and lower carbon loss during long-term service compared to its predecessors.
Q: What is the maximum allowable service temperature for P91?
A:It is typically rated for continuous service up to 650°C, though design codes may specify lower limits based on stress conditions.
Q: Does P91 require special non-destructive testing (NDT)?
A:Due to its martensitic microstructure, ultrasonic testing (UT) and magnetic particle inspection (MPI) are recommended to detect any welding defects or material inconsistencies.
Q: Can P91 be used in sub-zero temperatures?
A:While it has better low-temperature toughness than some martensitic steels, it is not optimized for cryogenic use; Austenitic stainless steels or nickel alloys are more suitable for such applications.
