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ASTM A335 P11, P22, and P91 are among the most widely used alloy steel pipe grades for high-temperature service. They are all ferritic Cr-Mo alloy steels, but their performance levels are very different. Choosing between them requires more than comparing chromium and molybdenum content. Engineers must consider creep strength, design temperature, pressure, oxidation resistance, weldability, post-weld heat treatment, and long-term operating risk.
This article provides a technical comparison of ASTM A335 P11 vs P22 vs P91 for high-temperature piping systems, especially for power plants, refineries, petrochemical units, and boiler applications.
|
Grade |
Alloy Type |
Typical Description |
General Position |
|
ASTM A335 P11 |
1.25Cr-0.5Mo |
Low-alloy Cr-Mo pipe |
Economical grade for moderate high-temperature service |
|
ASTM A335 P22 |
2.25Cr-1Mo |
Medium Cr-Mo pipe |
Higher strength and oxidation resistance than P11 |
|
ASTM A335 P91 |
9Cr-1Mo-V-Nb |
Modified 9Cr creep-resistant pipe |
Advanced grade for critical high-temperature and high-pressure service |
P11 and P22 are conventional Cr-Mo steels. P91 is a modified 9Cr steel strengthened by vanadium, niobium, and nitrogen. This difference gives P91 much higher creep strength, but also makes it more sensitive to welding and heat treatment quality.
|
Element |
P11 |
P22 |
P91 |
Technical Meaning |
|
Chromium |
Approx. 1.00-1.50% |
Approx. 1.90-2.60% |
Approx. 8.00-9.50% |
Improves oxidation and corrosion resistance |
|
Molybdenum |
Approx. 0.44-0.65% |
Approx. 0.87-1.13% |
Approx. 0.85-1.05% |
Improves high-temperature strength and creep resistance |
|
Vanadium |
Not primary |
Not primary |
Present |
Strengthens microstructure for creep resistance |
|
Niobium |
Not primary |
Not primary |
Present |
Supports precipitation strengthening |
|
Carbon |
Controlled |
Controlled |
Strictly controlled |
Affects strength, hardness, and weldability |
The jump from P22 to P91 is not only an increase in chromium. P91 is a different metallurgical system. Its creep strength depends heavily on a tempered martensitic microstructure. If welding or PWHT is poorly controlled, the expected long-term performance may not be achieved.
|
Performance Factor |
P11 |
P22 |
P91 |
|
High-temperature strength |
Moderate |
Good |
Excellent |
|
Creep resistance |
Moderate |
Better than P11 |
Much higher than P11 and P22 |
|
Oxidation resistance |
Moderate |
Better than P11 |
Strong due to high chromium |
|
Wall thickness reduction potential |
Limited |
Moderate |
High in suitable designs |
|
Long-term critical service |
Limited |
Common |
Excellent when properly fabricated |
For main steam piping material selection in power plants, P91 is often preferred where high creep strength allows thinner wall designs and improved thermal efficiency. However, the design code, allowable stress values, and service temperature must always be checked before substitution.
|
Application |
P11 |
P22 |
P91 |
|
Low/intermediate-pressure steam lines |
Very suitable |
Suitable |
Usually unnecessary |
|
Refinery process piping |
Suitable |
Very suitable |
Used in selected severe conditions |
|
Boiler piping |
Suitable |
Very suitable |
Suitable for critical high-temperature sections |
|
Main steam lines |
Limited |
Common in older/conventional units |
Common in high-efficiency power plants |
|
Hot reheat lines |
Limited |
Common |
Common in advanced units |
|
Petrochemical high-temperature service |
Suitable |
Very suitable |
Case-dependent |
|
Ultra-supercritical power units |
Not preferred |
Limited |
More suitable |
|
Fabrication Factor |
P11 |
P22 |
P91 |
|
Welding difficulty |
Lower |
Medium |
High |
|
Preheat requirement |
Usually required |
Usually required |
Strictly required |
|
PWHT sensitivity |
Important |
Important |
Critical |
|
Hardness control |
Required |
Required |
Very important |
|
Risk if procedure is poor |
Moderate |
Moderate to high |
High |
|
Welder/procedure qualification |
Standard control |
Strong control |
Strict control essential |
P91 requires more discipline during welding because its creep strength depends on achieving the correct tempered martensitic structure. Poor PWHT, incorrect interpass temperature, or improper filler selection can lead to premature failure. Buyers comparing P11 P22 P91 pipe weldability and PWHT requirements should evaluate not only material cost but also fabrication capability.
|
Cost Factor |
P11 |
P22 |
P91 |
|
Material cost |
Low |
Medium |
High |
|
Fabrication cost |
Low to medium |
Medium |
High |
|
Inspection cost |
Standard |
Standard to higher |
Higher |
|
Maintenance sensitivity |
Moderate |
Moderate |
High if fabrication quality is poor |
|
Lifecycle value |
Good for moderate service |
Strong balance |
Excellent for critical high-temperature service |
P91 is more expensive, but in some high-temperature designs it may reduce required wall thickness. This can lower pipe weight, reduce thermal stress, and improve system efficiency. However, these benefits only apply when the engineering design, welding procedure, heat treatment, and inspection system are properly controlled.
|
Selection Question |
Recommended Direction |
|
Is the service temperature moderate and cost-sensitive? |
Consider P11 |
|
Is higher oxidation resistance and creep strength required? |
Consider P22 |
|
Is the line part of critical high-temperature, high-pressure steam service? |
Consider P91 |
|
Is welding control limited or field PWHT difficult? |
Be cautious with P91 |
|
Is long-term creep life the main design concern? |
P91 is usually more suitable |
|
Is the application a refinery or petrochemical unit with moderate severity? |
P11 or P22 may be more practical |
|
Is the project governed by strict power plant material specifications? |
Follow the design code and approved material list |
A simplified rule is: P11 equals economical moderate-temperature alloy pipe; P22 equals stronger 2.25Cr-1Mo pipe for higher-temperature service; P91 equals advanced 9Cr creep-resistant pipe for critical high-temperature power and process piping.
· Choosing P91 only because it is stronger. P91 is better only when the application needs its creep strength and the project can control welding, heat treatment, inspection, and documentation properly.
· Replacing P22 with P11 to reduce cost without checking allowable stress, corrosion conditions, and design temperature.
· Treating PWHT as a formality. For Cr-Mo alloy steel pipes, especially P91, PWHT is essential for controlling hardness, residual stress, and microstructure.
ASTM A335 P11, P22, and P91 are not interchangeable grades. P11 is economical and practical for moderate high-temperature service. P22 provides a stronger balance of creep strength, oxidation resistance, and cost. P91 offers superior high-temperature creep performance but requires strict welding and heat treatment control.
For engineering teams evaluating ASTM A335 P11 P22 P91 pipe for boiler and steam piping applications, the right choice depends on operating temperature, pressure, design life, fabrication capability, and inspection requirements. If your project requires certified seamless alloy steel pipe for high-temperature service, our ASTM A335 pipe range can be matched to project specifications, including P11, P22, P91, and related Cr-Mo grades.
