ASTM A335

Covers — seamless alloy steel pipe:

• Common low-alloy grades: P11 (1.25Cr-0.5Mo), P22 (2.25Cr-1Mo).
• Intermediate-alloy grades: P5 (5Cr-0.5Mo), P9 (9Cr-1Mo).
• Advanced high-strength grades: P91 (9Cr-1Mo-V).
• High-temperature steam, hydrogen, and hot oil services.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Welded alloy pipe → ASTM A691.
• Standard high-temperature carbon steel → ASTM A106.
• Austenitic stainless steel → ASTM A312.

A335 alloys are tailored for structural stability at red-hot temperatures:

• Chemical Composition — Chromium provides oxidation resistance and high-temperature strength. Molybdenum significantly increases high-temperature creep strength.
• Manufacture — strictly seamless.
• Heat Treatment — requires full annealing, isothermal annealing, or normalizing and tempering depending on the grade, to achieve the specific microstructure required for creep resistance.

Above 800°F (427°C), carbon steel rapidly loses strength and is susceptible to graphitization. Chrome-moly pipes (A335) safely extend the operating envelope to 1000°F+ (537°C+).

Grade P91

P91 is a modified 9Cr-1Mo steel with Vanadium and Niobium. It offers extremely high creep strength, allowing for significantly thinner pipe walls in high-pressure steam lines. Thinner walls reduce thermal fatigue during plant startup/shutdown. However, P91 requires flawless temperature control during welding and Post-Weld Heat Treatment (PWHT); any deviation ruins its microstructure.

Where to find it in the code: Chemical requirements → Table 1 · Heat treatment → Table 2.