Sep 12, 2026Procurement Guides
ASTM A335 Alloy Steel Pipe: Grades P11, P22 and P91 Explained
ASTM A335 covers seamless chrome-moly pipe for high-temperature service. Compare grades P11, P22 and P91 and what to verify when ordering.
Quick answer: ASTM A335 is the standard specification for seamless ferritic alloy steel pipe for high-temperature service, and P11, P22 and P91 are its featured chrome-moly grades. They are chosen for power and petrochemical duty, where chromium and molybdenum provide strength and oxidation resistance at elevated temperature.
What ASTM A335 Pipe Is
ASTM A335 pipe is also called ASME S/A335 chrome-moly pipe. It is a seamless pipe used in high-temperature environments of roughly 540 to 750 C. Because the chemistry carries a high level of chromium and molybdenum, the grade family is commonly called chrome moly pipe. The specification covers nominal wall and minimum wall pipe made from alloy material in the seamless type, and the pipe is suitable for bending, flanging and similar forming operations as well as fusion welding. That is why A335 material is also used to make fittings such as elbows, tees and reducers.
Why Chromium and Molybdenum Matter
Molybdenum raises overall strength, resistance, elasticity, hardenability and general quality. It also makes the material more resistant to softening, restrains grain growth and reduces the chance of embrittlement, which is why it is the single addition most responsible for high-temperature resistance. It also improves corrosion resistance. Chromium is the other key element. Above roughly 12 percent chromium a steel can be classified as stainless, and chromium's oxidation resistance is difficult to replace even at elevated temperature. Hardness, tensile strength and yield strength are also better than plain carbon steel at standard temperature. This combination makes the grade family ideal for power plants, refineries, petrochemical plants and oilfield service where fluids are transported at high temperature and pressure.
Standard Scope and Referenced Standards
ASTM defines A335 as the standard specification for seamless ferritic alloy steel pipe for high-temperature service. Supporting standards referenced in the specification include ASTM A999/A999M for common requirements, A92 for hardness test methods, E213 for ultrasonic testing of pipe and tube, E309 for eddy-current examination, E381 for inspection of bars, beams, profiles, billets and forgings, E527 for numbering metals and alloys, and E570 for flux leakage examination of ferromagnetic pipe. Dimensions follow ASME B36.10M for welded and seamless steel pipe.
Raw Material and Manufacture
Pipe material is either hot finished or cold drawn, with the heat treatment required by the grade. For certain low-alloy grades the steel is made by coarse-grain melting practice. The finished pipe must meet the chemical composition, tensile property and hardness requirements of the specification, and lengths are subject to hydrostatic testing with non-destructive examination where the specification calls for it.
Featured Grades P11, P22 and P91
The featured grades in this standard are P5, P9, P11, P22 and P91, which is why the family is sometimes simply called P-grade pipe. P11 and P22 are widely used in the power industry and in petrochemical plants, while P5 and P9 generally find use in refineries. ASTM A335 P91 is a strong choice for high-pressure boiler service, and it performs well in bending, flanging and similar operations such as welding. P91 is also supplied as high-pressure boiler pipe. The grade and size range depend on the intended use.
P11 and P22 serve general high-temperature power and petrochemical duty, while P91 is the grade to consider when high-pressure boiler service and high creep strength are the priority.
Tests Normally Involved
Common tests include transverse and longitudinal tension tests, flattening tests, a hardness test and a bend test. When heat testing is performed on material from a blast furnace, it is typically done on 5 percent of the pipes made from each lot. For diameters above NPS 25 the diameter-to-wall-thickness ratio should typically be 7.0 or less. For NPS 10 and thicker pipe, the flattening test is normally performed only when the purchaser approves it.
What to Verify When Ordering
Before placing an order, confirm the length per piece and the quantity in feet, metres or pieces; the material name, meaning seamless alloy steel pipe; the grade, such as P5, P9, P11, P22 or P91; the manufacturing type, either hot finished seamless or cold drawn seamless; and the size. Size can be given as NPS and schedule number under ASME B36.10, for example NPS 7 inch Schedule 40, or as OD or ID with nominal or minimum wall thickness in inches or millimetres.
How SPAI STEEL Supplies Alloy Pipe
SPAI STEEL, part of the SP Group and founded in 1996, produces and exports alloy steel and stainless steel pipe from its Longwan, Wenzhou base and carbon steel pipe from Jinghai, Tianjin. Products can be supplied to GB, API, ASTM, ASME/ANSI, BS, DIN, JIS and GOST standards and verified by third-party inspection such as SGS, BV, TUV or TUBOSCOPE. Additional services include cutting, slotting, coating, threading, grooving, custom fitting fabrication, packing, marking, warehousing and logistics.
FAQ
What is ASTM A335 pipe used for?
It is used for high-temperature, high-pressure service in power plants, refineries, petrochemical plants, heat exchangers and high-pressure vessels.
Which A335 grade is best for a high-pressure boiler?
P91 is a strong candidate for high-pressure boiler pipe and where higher creep strength at elevated temperature is needed. P11 and P22 cover many general high-temperature duties.
Is ASTM A335 pipe seamless?
Yes. The specification covers seamless ferritic alloy steel pipe for high-temperature service.
Can A335 pipe be bent and welded?
Yes. The specification states the pipe is suitable for bending, flanging, fusion welding and similar forming operations, and the material is also used for fittings.
For alloy steel pipe to ASTM A335, tell SPAI STEEL the grade, size, schedule and end use. We can confirm availability, third-party inspection and documentation before you order.
