ASTM A335 / ASME SA335 UNS K90901 P91 Boiler Tube EN 1.4903 Alloy Steel Seamless Pipe

ASTM A335 / ASME SA335 P91 Alloy Steel Seamless Pipe for High-Temperature Service​

ASTM A335 P91 (UNS K90901 / EN 1.4903) is a high-alloy martensitic steel with ~9% Chromium, Molybdenum, Vanadium, Niobium, and Nitrogen. Designed for boiler tubes and main steam piping, it offers exceptional creep strength up to 620°C and superior oxidation resistance—allowing thinner, lighter walls than traditional low-alloy grades. It is mandatory to use this material in the normalized + tempered condition. Critical warning: It is air-hardening; therefore, all welded joints require strict preheating (200–250°C) and mandatory Post-Weld Heat Treatment (730–760°C) to prevent cracking.

Key Features & Advantages (Why High-Alloyed Grade P91?)

• Superior Oxidation Resistance: The ~9% Chromium content provides significantly better resistance to steam-side oxidation and high-temperature flue gas corrosion compared to low-alloy grades (e.g., T22/P22).

• High Thermal Stability: Retains strength and structure integrity at extreme temperatures.

• Low Thermal Expansion: Lower than austenitic stainless steels (e.g., 304/316), allowing for thinner wall thicknesses and better fatigue resistance under thermal cycling.

• Reduced Wall Thickness: Excellent creep strength allows designers to use thinner pipe walls than P22/T22, reducing weight and thermal inertia.

Equivalent Material of ASTM A335 / ASME SA335 UNS K90901 P91 Boiler Tube EN 1.4903 Alloy Steel Seamless Pipe

A335/SA335 P91 is suitable for bending, flanging (vanstoning), and similar forming operations and for fusion welding. Its commonly used in below usages.

1. Superheaters & Reheaters (Boiler tubes).

2. Main Steam Piping and hot reheat piping.

3. Headers and manifolds in supercritical and ultra-supercritical coal-fired power plants.

4. Nuclear power heat exchangers.

5. Petrochemical high-temperature furnace tubes.