Products Features
Ultra-high temperature strength and creep resistance (core advantages)
The core advantage of ASTM A335 P91 lies in its ultra-high high-temperature strength and creep resistance. Thanks to micro-alloying with vanadium (V) and niobium (Nb), along with a tempered martensitic microstructure, P91 exhibits significantly superior high-temperature creep rupture strength compared to P22 and conventional chromium-molybdenum steels within the 550-650°C temperature range. At 600°C operating conditions, the allowable stress of P91 is approximately 1.5-2 times that of P22, and its creep resistance is about 3-5 times that of P22. This enables it to effectively resist slow plastic deformation (creep) of materials under high temperature and pressure, ensuring the safety and reliability of pipelines during long-term high-temperature service. With a minimum tensile strength of 585 MPa and a minimum yield strength of 415 MPa-far exceeding those of P22 (≥415 MPa / ≥205 MPa)-P91 allows for a reduction in wall thickness of approximately 40-50%, significantly lowering pipeline self-weight, thermal stress, and overall cost.
Excellent high-temperature oxidation resistance
The 9% chromium content in ASTM A335 P91 provides excellent high-temperature oxidation resistance. In high-temperature steam environments, P91 can form a dense and stable Cr₂O₃ oxide layer, effectively resisting high-temperature oxidation and steam oxidation. Its upper oxidation temperature limit reaches 593-620°C, which is significantly better than P22 (≤565°C). This property makes it an ideal material for main steam pipes in ultra-supercritical power plants (steam temperature ≥593°C).
Good machinability and formability
ASTM A335 P91 exhibits good cold and hot working properties and can undergo various forming processes such as hot bending, cold bending, and forging. It should be noted that since P91 has a martensitic structure, its tendency to work harden is higher than that of ferrite/pearlite steels (e.g., P22). Therefore, it is recommended to include appropriate intermediate annealing steps during forming processes. Its machinability is comparable to that of P22, allowing processing using conventional machining methods. It is suggested to use carbide cutting tools.
Strict welding performance and post-weld heat treatment requirements
ASTM A335 P91 exhibits good weldability. However, due to its martensitic microstructure and high chromium content (9%), it has a strong tendency to harden in the heat-affected zone (HAZ) of welding, imposing extremely strict requirements on welding processes and post-weld heat treatment (PWHT). It is recommended that preheating temperature before welding be 200-300°C. After welding, PWHT must be performed by heating to 730-780°C, holding at temperature, and then cooling slowly to eliminate residual welding stresses, soften the heat-affected zone, and ensure the toughness and crack resistance of the welded joint. The welding process requirements for P91 are more stringent than those for P22, and it is recommended that experienced welders perform the operation.
Excellent organizational stability and long-term service reliability
ASTM A335 P91 exhibits good organizational stability when in long-term service at temperatures ≤593°C. Its 9% Cr-1% Mo-V-Nb composition design effectively enhances the material's high-temperature creep strength and oxidation resistance. The tempered martensite microstructure ensures the maintenance of organizational stability and mechanical properties during prolonged high-temperature service. After normalizing and tempering (N+T) heat treatment, P91 obtains a uniform and fine tempered martensite structure, guaranteeing organizational stability and reliability under high-temperature service conditions.
Technical Specifications
Chemical Composition(ASTM A335 / ASME SA335)
|
Element |
Standard requirements |
Note |
|
C |
0.08 - 0.12% |
Balancing strength and resilience |
|
Mn |
0.30 - 0.60% |
Solid solution strengthening, improve strength |
|
P |
≤ 0.020% |
Harmful elements, strictly controlled |
|
S |
≤ 0.010% |
Harmful elements, strictly controlled |
|
Si |
0.20 - 0.50% |
Deoxidizer, improve high-temperature oxidation resistance |
|
Cr |
8.00 - 9.50% |
Core elements that provide excellent high-temperature oxidation resistance and corrosion resistance |
|
Mo |
0.85 - 1.05% |
Core elements to improve high-temperature creep fracture strength |
|
V |
0.18 - 0.25% |
Microalloying elements, precipitation strengthening, significantly improve high-temperature creep strength |
|
Nb |
0.06 - 0.10% |
Microalloying elements, grain refinement, improve high-temperature strength and toughness |
|
N |
0.03 - 0.07% |
Solid solution strengthening to improve high-temperature strength |
|
Al |
≤ 0.02% |
Strict control to ensure purity |
|
Fe |
Remaining quantity (≈ 89%) |
Matrix element |
Physical Properties
|
Performance |
Numerical Value/Range |
Test conditions |
|
Density |
≈ 7.75 g/cm³ |
Room Temperature (20°C) |
|
Melting Point |
≈ 1360-1400°C |
- |
|
Electrical resistance |
≈ 0.45 μΩ·m |
Room Temperature |
|
Thermal Conductivity |
≈ 32 W/(m·K) |
Room Temperature |
|
Coefficient of Thermal Expansion (20-100°C) |
≈ 11.0 × 10⁻⁶ /K |
- |
|
Elastic modulus (Young's modulus) |
≈ 195 GPa |
Room Temperature |
|
Poisson's ratio |
≈ 0.28 |
Room Temperature |
|
Magnetism |
Ferromagnetic |
Martensitic structure, which is magnetic |
Mechanical Properties (Normalized + Tempered, Typical Values, ASTM A335)
|
Performance |
Typical value |
Implementation standards |
|
Tensile Strength |
≥ 585 MPa |
ASTM A335 |
|
Yield Strength (0.2% Offset) |
≥ 415 MPa |
ASTM A335 |
|
Elongation (gauge length 2in/50mm) |
≥ 20% |
ASTM A335 |
|
Hardness |
≤ 250 HB (typical value) |
- |
Note: The strength level of P91 is significantly higher than that of P22 (≥415 MPa / ≥205 MPa). Its core advantages lie in the ultra-high high-temperature strength and creep resistance brought about by V-Nb micro-alloying and tempered martensite microstructure, which can achieve a wall thickness reduction of approximately 40-50%. For higher temperature grades (>620°C), it is recommended to select P92.
High-temperature performance
ASTM A335 P91 maintains a high level of strength at elevated temperatures and is suitable for long-term high-temperature service conditions up to ≤593°C:
|
Temperature |
Tensile strength (MPa, approximately) |
Performance retention rate |
|
Room temperature (20°C) |
≥ 585 |
100% |
|
400°C |
≈ 520 |
≥ 89% |
|
500°C |
≈ 480 |
≥ 82% |
|
593°C |
≈ 400 |
≥ 68% |
|
620°C |
≈ 350 |
≥ 60% |
Long-term Thermal Stability and Microstructural Stability
ASTM A335 P91 exhibits good microstructural stability during long-term service at temperatures ≤593°C. The V-Nb microalloying and tempered martensite microstructure effectively enhance high-temperature creep strength and oxidation resistance. However, microstructural stability decreases and creep strength significantly drops when used long-term above 620°C; thus, it is not recommended for long-term service above 620°C. For higher temperature grades, P92 is recommended.
Heat treatment system
The final performance of ASTM A335 P91 is highly dependent on the heat treatment regime, with common specifications as follows:
1. Normalizing Treatment (Normalizing)
Heat to 1040-1065°C, hold for sufficient time (approximately 1 hour per inch of wall thickness), then air cool or fan cool. This results in a uniform, fine martensitic structure, preparing it for subsequent tempering.
2. Tempering treatment(Tempering)
Heat to 730-780°C, hold for sufficient time, then air cool. This eliminates stresses induced by normalizing, obtains tempered martensite microstructure, optimizes toughness and plasticity, and achieves the final service properties.
3. Full Annealing (Optional)
Heat to 850-900°C, hold at temperature, then furnace cool to approximately 650°C, followed by air cooling to room temperature. This is used to eliminate cold working stresses or improve cold working formability.
Attention：
The heat treatment temperature of P91 (normalizing 1040-1065°C) is much higher than that of P22 (900-960°C) and must be strictly controlled.
Post-weld heat treatment (PWHT) must be performed after welding. It involves heating to 730-780°C, holding at temperature, and then slow cooling to eliminate residual welding stresses and soften the heat-affected zone.
The welding process requirements for P91 are stricter than those for P22. It is recommended to strictly control the welding heat input and preheating temperature (200-300°C).
The heat treatment temperature and time must be strictly controlled to avoid grain coarsening or performance fluctuations.
Applicable Media
High-temperature steam
Main steam pipelines of ultra-supercritical/supercritical power plants (≤593°C), boiler headers, superheater tubes, reheater tubes
High-temperature and high-pressure process gases
High-temperature and high-pressure pipelines for chemical equipment such as ammonia synthesis, methanol production, and chemical fertilizers
Nuclear Power Plant
Nuclear Island Heat Exchanger Pipes, Steam Generator Pipes
Hot water
Heating network, heating system
Heat transfer oil and thermal carriers
Pipelines for chemical heat transfer systems
Note: ASTM A335 P91 is not suitable for strongly corrosive media such as strong oxidizing acids and high-temperature, high-concentration chlorides. Its upper limit for oxidation resistance is approximately 593-620°C; P92 is recommended when this temperature is exceeded. P91 has high requirements for welding processes and is advised to be operated by experienced welders.
ASTM A335 P91 is suitable for ultra-high temperature and high-pressure conditions ranging from 550 to 593°C. It is one of the preferred materials for ultra-supercritical power plants and nuclear power projects, recommended for use in the following areas:
Application Fields
-
Fossil Fuel Power Generation (Core Application)
● Main Steam Piping for Ultra-Supercritical/Supercritical Power Plants (≤593°C)
● Boiler drum, superheater drum
● High-temperature reheater pipes, high-temperature superheater tubes
● High-temperature steam pipeline system -
Nuclear Power Generation (Core Application)
● Nuclear power plant heat exchanger pipes
● Nuclear island steam generator piping
● High-temperature and high-pressure process pipelines in nuclear power plants -
Petrochemical
● High-temperature and high-pressure pipelines for hydrotreating/hydrocracking units in refineries
● Catalytic Reforming Unit Process Piping
● High-temperature pipelines in ethylene cracking units -
Chemical and Fertilizer Industry
● High-Temperature Process Piping for Ammonia Synthesis Plants
● Methanol Plant Reformate Gas Pipelines
● High-Temperature and High-Pressure Piping in Fertilizer Factories -
High-temperature and high-pressure pipe fittings
● High-temperature elbows, tees, and reducers
● Forged pipe fittings, flanges
Delivery and Customization
We offer flexible and reliable supply chain solutions to ensure you receive ASTM A335 P91 alloy steel pipes that perfectly match your project requirements.
Customizable material forms
We support producing various basic and special shapes according to drawings or specifications.
|
Form |
 Seamless Steel Pipe
|
 Large-diameter Thick-walled Pipe
|
 Alloy Steel Pipe
|
|---|---|---|---|
|
Conventional Range |
Outer Diameter: 10.3 - 1067 mm; Wall Thickness: 1 - 150 mm |
Outer Diameter: 219 - 1067 mm; Wall Thickness: 20 - 150 mm |
Outer Diameter: 10.3 - 610 mm; Wall Thickness: 1 - 50 mm |
|
Customization Capability |
Hot-rolled / cold-drawn seamless tubes, fixed-length cutting, bevel machining, pipe end threads, special heat treatment conditions (normalizing + tempering / annealing) |
Hot expansion, fixed-length cutting, bevel machining, special heat treatment conditions |
Cold-drawn precision tubes, fixed-length cutting, special surface treatment |
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Full traceability across key QC checkpoints throughout production. 100% compliant with ASTM/ASME, EN, and GB standards. Material test reports available with every shipment.
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FAQ
Q: What are the main advantages of ASTM A335 P91?
Q: What is the difference between ASTM A335 P91 and P22? How to choose between them?
Q: What are the differences between ASTM A335 P91 and P92? How to choose between them?
Q: What is the maximum service temperature for ASTM A335 P91?
Q: What is the welding performance of ASTM A335 P91?
Q: What is the corrosion resistance of ASTM A335 P91?
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