ASTM A335 Alloy Steel Pipe: P11, P22 & P91 Grades Explained
When plant piping must survive temperatures above 425°C, ordinary carbon steel is no longer a safe choice. This is where ASTM A335 chrome-molybdenum alloy steel pipe comes in.
What Is ASTM A335?
ASTM A335 (also published as ASME SA335) covers seamless ferritic alloy-steel pipe intended for high-temperature service. The most popular grades belong to the P-series of chrome-molybdenum (Cr-Mo) alloys.
The chromium gives oxidation resistance; the molybdenum provides strength at high temperature. Together they give the pipe excellent creep resistance — the property that matters most in hot piping.
The Three Grades Buyers Ask About Most
| Grade | Cr (%) | Mo (%) | Min. Yield (MPa) | Typical Use |
|---|---|---|---|---|
| P11 | 1.0–1.5 | 0.44–0.65 | 205 | 1.25Cr-0.5Mo piping, superheater tubes |
| P22 | 1.9–2.6 | 0.87–1.13 | 205 | 2.25Cr-1Mo, power & refinery piping |
| P91 | 8.0–9.5 | 0.85–1.05 | 415 | High-strength, high-temperature headers |
- P11 is a workhorse grade for temperatures up to about 540°C, widely used in power plants and refineries.
- P22 offers better high-temperature strength than P11 and resists hydrogen attack.
- P91 is a 9Cr-1Mo-V grade with roughly double the creep strength of P22 — popular for modern ultra-supercritical power stations.
Chinese Equivalent Grades
Chinese national standards use their own designations. Common equivalents we export alongside ASTM A335 include:
- 15CrMoG (GB/T 5310) — similar to P11/P12 service
- 12Cr1MoVG (GB/T 5310) — similar to P22 service
Supplying under both systems lets the same mill batch serve API, ASTM and GB specifications with the right test plan.
Key Specifications for Ordering
When you send an enquiry for A335 alloy pipe, include:
- Grade (P11, P22, P91 or a Chinese equivalent)
- OD and wall thickness (or NPS + schedule)
- Length (fixed or random)
- Heat treatment required (normalized + tempered, etc.)
- NDT requirements (ultrasonic, hydrostatic, PMI)
- Test documents (MTC per EN 10204 3.1, heat traceability)
Why Heat Treatment and Testing Matter
A335 grades are supplied in the normalized and tempered condition. A mill that skips proper heat treatment cannot meet the guaranteed mechanical properties. Always require:
- Mill test certificate with chemical analysis
- Tensile and impact results
- Hydrostatic test per pipe
- Ultrasonic examination when required by the project
- PMI (positive material identification) — this is cheap insurance and widely required on alloy lines
Xbang Supply Supply
Xbang Supply (山东信邦金属材料有限公司) supplies seamless ASTM A335 P11, P22, P91 and P92 pipe, plus GB/T 5310 grades 15CrMoG and 12Cr1MoVG, with full certification and third-party inspection available. Send us your temperature, pressure and size range, and our engineers will confirm the correct grade within 24 hours.
