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?)
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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).
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High Thermal Stability: Retains strength and structure integrity at extreme temperatures.
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Low Thermal Expansion: Lower than austenitic stainless steels (e.g., 304/316), allowing for thinner wall thicknesses and better fatigue resistance under thermal cycling.
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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
| ASTM | ASME | Equivalent material |
|---|---|---|
| A335 P 91 | ASME SA 335 P91 | 1.4903, K90901, X10CrMoVNb9-1 |
Chemical Composition of ASME SA335 UNS K90901 P91 Boiler Tube EN 1.4903 Alloy Steel Seamless Pipe
| Element | Grade | P5 | P9 | P11 | P22 | P91 | P91 shall also include the following: |
|---|---|---|---|---|---|---|---|
| UNS | K41545 | S50400 | K11597 | K21590 | K91560 | ||
| Carbon | 0.15 max | 0.15 max | 0.05 – 0.15 | 0.05 – 0.15 | 0.08 – 0.12 | V at 0.18 – 0.25 | |
| Manganese | 0.30 – 0.60 | 0.30 – 0.60 | 0.30 – 0.60 | 0.30 – 0.60 | 0.30 – 0.60 | N at 0.030 – 0.070 | |
| Phosphorous, max | 0.025 | 0.025 | 0.025 | 0.025 | 0.020 | Ni at 0.40 max | |
| Sulfur, max | 0.025 | 0.025 | 0.025 | 0.025 | 0.010 | Al at 0.02 max | |
| Silicon | 0.50 max | 0.25 – 1.00 | 0.50 – 1.00 | 0.50 max | 0.20 -0.50 | Cb at 0.06 – 0.10 | |
| Chromium | 4.00 – 6.00 | 8.0 – 10.0 | 1.00 – 1.50 | 1.90 – 2.60 | 8.00 – 9.50 | Ti at 0.01 max | |
| Molybdenum | 0.45 -0.65 | 0.90 – 1.10 | 0.44 – 0.65 | 0.87 – 1.13 | 0.85 – 1.05 | Zr at 0.01 max | |
Mechanical Properties of ASME SA335 UNS K90901 P91 Boiler Tube EN 1.4903 Alloy Steel Seamless Pipe
| Grade | Tensile strength(MPa) | Yield strength(MPa) | Elongation(%) | Impact energy(J) | Hardness |
| P11 | ≥ 415 | ≥ 205 | ≥ 22 | ≥ 35 | ≤ 163HB |
| P12 | ≥ 415 | ≥ 220 | ≥ 22 | ≥ 35 | ≤ 163HB |
| P22 | ≥ 415 | ≥ 205 | ≥ 22 | ≥ 35 | ≤ 163HB |
| P5 | ≥ 415 | ≥ 205 | ≥ 22 | ≥ 35 | ≤ 187HB |
| P91 | 585~760 | ≥ 415 | ≥ 20 | ≥ 35 | ≤ 250HB |
| P92 | ≥ 620 | ≥ 440 | ≥ 20 | ≥ 35 | 250HB |
Application
A335/SA335 P91 is suitable for bending, flanging (vanstoning), and similar forming operations and for fusion welding. Its commonly used in below usages.
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Superheaters & Reheaters (Boiler tubes).
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Main Steam Piping and hot reheat piping.
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Headers and manifolds in supercritical and ultra-supercritical coal-fired power plants.
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Nuclear power heat exchangers.
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Petrochemical high-temperature furnace tubes.
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