What is ASTM A335 Grade P91 Alloy Steel Seamless Pipe?

ASTM A335 P91 / ASME SA335 P91 pipe is a seamless ferritic chrome-moly alloy steel pipe specifically designed for high-temperature service and corrosion resistance. Containing 8.00–9.50% chromium and 0.85–1.05% molybdenum, this alloy provides excellent creep strength, oxidation resistance, and long-term stability under elevated temperatures. With a minimum yield strength of 205 MPa and a tensile strength of 415 MPa, ASTM A335 P91 offers mechanical properties superior to conventional carbon steels, and it is often compared to ASTM A335 P9 pipe due to its similar alloying base but enhanced performance.

P91 material is part of a larger family of chrome moly steels used across multiple product forms:

• ASTM A234 WP91 – seamless pipe fittings (elbows, tees, reducers)

• ASTM A182 F91 – forged flanges

• ASTM A387 Grade 91 – chrome moly alloy steel plates

• ASTM A691 91 – welded chrome moly pipe fabricated from plates

P91 Pipe Applications

A335/SA335 P91 is suitable for bending, flanging (vanstoning), and similar forming operations and for fusion welding. P91 is most commonly used in the power industry and petrochemical plants where fluids and gases are transported at extremely high temperatures and pressures.

P91’s strength and high temperature endurance makes it ideally suitable for plants that operate on a cyclic basis – like combined cycle plants. Also, the reduction in thickness suits HRSG designers, in an HRSG the temperature head is limited and locating the coils in the heat transfer path is very critical.

P91 is the correct grade for:

• Power generation
• Petrochemicals
• Gas processing
• Petrochemical Plants
• Heaters
• Oilfield services

What is the difference between P91 Type 1 and Type 2?

ASTM A335 P91 Type 1 & Type 2 have subtle differences mainly in elements Mn, S, Si, N, Sn and Sb content, and Type 2 has stricter requirements on element content than Type 1. P91 type 1 and 2 shall have a hardness within the range of 190 to 250 [HBW] or 196 to 265 [HV] or 91 HRBW to 25 HRC.