ASTM A335 covers seamless ferritic alloy-steel pipe for high-temperature service. P11, P22, and P91 are distinct grades with different alloy systems, heat-treatment requirements, fabrication controls, and design properties; they are not a simple price or strength ladder.
Engineering and purchasing summary
These grades are not substitutes for one another
P11, P22, and P91 must be selected from the governing design code and approved material specification. Their alloy systems, heat treatment, design properties, welding controls, and post-weld requirements differ, so availability or nominal strength alone is not a valid substitution basis.
Practical purchasing comparison for ASTM A335 grades
| Grade | Material family | Selection context | Procurement focus |
|---|---|---|---|
| P11 | Chromium-molybdenum ferritic alloy steel | High-temperature service where approved by the design specification | Full grade, heat treatment, dimensions, tests, and fabrication route |
| P22 | Higher chromium-molybdenum alloy basis than P11 | High-temperature service requiring the P22 design basis | Do not substitute for P11 without engineering approval |
| P91 | Modified high-chromium creep-strength-enhanced ferritic steel | Demanding high-temperature systems with a qualified P91 fabrication program | Chemistry, heat treatment, PMI, welding, hardness, and PWHT control |
| All grades | Seamless ferritic alloy-steel pipe under ASTM A335 | Service and allowable stresses follow the governing code and edition | Maintain heat and grade traceability through processing and packing |
Start with the design specification
The piping design code, calculated temperature and pressure, approved material list, corrosion allowance, creep requirements, and service environment determine the permitted grade. Procurement should not substitute P11, P22, or P91 solely because one is available sooner.
P11 and P22 are different chromium-molybdenum grades
P11 and P22 belong to the chromium-molybdenum alloy family, but their compositions and required properties differ. P22 has a higher chromium-molybdenum alloy basis than P11. The applicable standard edition controls chemistry, heat treatment, tensile properties, testing, and marking.
P91 requires especially disciplined control
P91 is a modified high-chromium alloy grade whose performance depends strongly on chemistry, heat treatment, microstructure, welding procedure, consumables, preheat, interpass temperature, and post-weld heat treatment. Positive material identification and hardness or other project checks may be specified.
Fabrication requirements belong in the RFQ
State whether the pipe will be bent, welded, machined, or heat treated after delivery. Provide the applicable code, welding and PWHT requirements, supplementary tests, hardness limits where specified, and any restrictions on repair or re-heat treatment.
Verify traceability and documentation
Confirm full grade, dimensions, schedule or wall, heat-treatment condition, heat number, MTC, NDT, hydrotest, marking, and third-party inspection. Keep grade identity traceable during cutting, beveling, and mixed-grade packing.
Procurement checklist
- Confirm the approved grade from the piping material specification
- State ASTM A335 edition, dimensions, schedule or wall, and length
- Define heat-treatment condition and supplementary requirements
- Coordinate welding procedure, consumables, preheat, and PWHT
- List PMI, hardness, NDT, hydrotest, and third-party inspection
- Keep heat and grade identity through cutting and mixed-grade packing
Related steel pipe products
Technical references
Use the edition required by the purchase order or project specification. The links below identify the relevant standards organizations and public standard overviews.
Technical note: This guide supports purchasing preparation and does not replace the governing design code, current standard edition, client specification, or approval by the responsible engineer.
Send a clearer requirement
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