ASTM A335 P11 vs P22 vs P91 Technical Comparison

How do ASTM A335 P11, P22 and P91 differ for high-temperature piping? P11 is nominal 1.25Cr-0.5Mo, P22 is 2.25Cr-1Mo and P91 is modified 9Cr-1Mo with additional alloying controls. P91 can provide higher creep-strength capability but demands tighter welding, heat treatment and hardness control; the grades are not interchangeable by size.

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.

For an approved piping specification, the corresponding ASTM A335 alloy seamless steel pipe supply range provides the product and quotation starting point. Use the comparison below to identify what still needs engineering or supplier confirmation.

1. Basic Grade Overview

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.

2. P11, P22 and P91 Material Composition

Table basis: selected reference ranges from the publicly available ASTM A335/A335M-11 chemistry table; the P91 values are also cross-checked against Nippon Steel’s boiler-pipe catalogue. These are not a complete acceptance table for every current grade/type. Specify the contract edition and any applicable P91 type, then check all required elements, residual limits and analysis rules against that standard and the MTC. A dash means not listed in this comparison, not zero content. See the linked sources below.

What is the difference between P11 and P22 material?

P22 has more chromium and molybdenum than P11: nominally 2.25Cr-1Mo versus 1.25Cr-0.5Mo. This changes the high-temperature material options available to the designer. It does not establish a pressure rating or prove that an existing P11 line can be replaced with P22 without review. Check allowable stress at design temperature, required wall thickness, service environment and the qualified welding procedure.

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.

3. Temperature and Pressure Performance

There is no single maximum operating temperature or pressure for a grade name. The following ratings are qualitative comparisons, not design limits. Use the governing code, design metal temperature, allowable stress, wall allowance, weld factors and required life for the actual piping system.

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.

4. Application Comparison

These are common areas of use, not automatic approvals. The material specification and design review govern suitability for each duty.

5. Weldability and Fabrication

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.

For P91, separate pipe-mill heat treatment from fabrication PWHT. Ask for the specified delivery condition and traceable mill records, then verify the fabricator’s qualified welding and PWHT procedure separately. Weld heat-affected zones can suffer Type IV creep damage; a satisfactory parent-pipe certificate alone does not demonstrate the life of a fabricated joint. Do not adopt a generic preheat or PWHT temperature from a comparison article.

6. Cost and Lifecycle Considerations

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.

7. How to Choose Between P11, P22, and P91

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.

8. Common Selection Mistakes

· 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.

9. Turn the Grade Comparison Into an RFQ

Copyable inquiry line: Seamless alloy pipe to [ASTM A335 / ASME SA335 and edition], Grade [P11 / P22 / P91 and applicable type], [NPS/OD] × [schedule or WT and wall basis], [length], [quantity], [end preparation]; delivery condition, test scope, certificate type and witness points per attached specification. State the destination and required delivery date. Any unconfirmed field should remain open for review.

Questions About P11, P22 and P91 Material

Can P22 replace P11, or can P91 replace P22?

Only after the responsible engineer approves the change. Compare the governing material specification, allowable stress, dimensions, service environment, welding consumables, procedure qualification and heat treatment. A higher chromium content is not a substitution approval.

What is a P22 material equivalent?

EN 10CrMo9-10 is often cross-referenced to the same 2.25Cr-1Mo family. A cross-reference is not proof of interchangeable pipe: product standard, composition limits, heat treatment, mechanical properties, dimensions and certification must all be compared. Keep the specified ASTM/ASME grade unless the alternative is formally accepted.

Are P91 and T91 the same product?

They belong to the same modified 9Cr alloy family, but A335 P91 is a pipe designation and A213 T91 is a boiler, superheater or heat-exchanger tube designation. Order the required product standard and dimensional basis; do not change P to T on the purchase order without review.

Does a hardness test prove P91 is acceptable?

No. Hardness is one check within an agreed inspection plan. It does not establish the complete chemistry, heat-treatment history or long-term creep performance. Review it with the MTC, traceability and applicable heat-treatment and inspection records.

10. Conclusion

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.

Related Reading

1．Alloy Steel Pipe Standards and Grade Selection Guide

2．Steel Pipe Heat Treatment: How Grade, Manufacturing Route and Service Determine the Correct Process

3．Boiler Tube Replacement Guide: Material Matching, Damage Review and Code-Controlled Installation

4．Heat Exchanger Retubing Specification: How to Replace Tubes Without Repeating the Original Failure

5．Chemical Plant Process Piping: MTC, NDT and Coating Checks

References & Sources

• ASTM A335/A335M — seamless ferritic alloy-steel pipe scope and current edition listing
• ANSI Webstore — historical ASTM A335/A335M-11 chemical requirements (reference table basis; not the current purchase specification)
• Nippon Steel — Seamless Steel Tubes and Pipes for Boilers (alloy families and Grade 91 reference data)
• ASTM A213/A213M — boiler, superheater and heat-exchanger tube scope
• TWI — Type IV cracking of 9–12Cr creep-strength-enhanced ferritic steel weldments