ASTM A335 P91 (9Cr-1Mo-V) Chrome-Moly Steel: High-Temperature Boiler Tube Material
ASTM A335/A335M / ASME SA-335 / EN 10216-2 X10CrMoVNb9-1 · Published: 2026-05-31 · Updated: 2026-05-31
ASTM A335 P91 is a creep-strength-enhanced ferritic (CSEF) steel developed by Oak Ridge National Laboratory for advanced fossil fuel power plants operating at steam temperatures up to 593°C (1100°F). Compared to its predecessor P22 (2.25Cr-1Mo),...
ASTM A335 P91 is a creep-strength-enhanced ferritic (CSEF) steel developed by Oak Ridge National Laboratory for advanced fossil fuel power plants operating at steam temperatures up to 593°C (1100°F). Compared to its predecessor P22 (2.25Cr-1Mo), P91's addition of vanadium and niobium with controlled nitrogen creates fine MX carbonitride precipitates that pin grain boundaries at high temperature, providing approximately 50% higher allowable stress at 550-600°C. This enables thinner-walled piping, reduced weight, lower thermal stresses during startup/shutdown cycling, and improved thermal fatigue life in combined-cycle and supercritical power plants.
Quick Facts
| Category | Alloy Steel |
| Standard | ASTM A335/A335M / ASME SA-335 / EN 10216-2 X10CrMoVNb9-1 |
| Density | 7.77 g/cm³ |
| Yield Strength | 415 MPa (60 ksi) minimum at room temperature |
| Tensile Strength | 585 MPa (85 ksi) minimum at room temperature |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN X10CrMoVNb9-1 | Compare |
| DIN 1.4903 | Compare |
| JIS STPA28 | Compare |
| GB 10Cr9Mo1VNb | Compare |
| ASME SA-213 T91 | Compare |
Related Materials
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Frequently Asked Questions
Why is P91 so sensitive to welding and heat treatment?
P91's creep strength comes from a precisely controlled tempered martensite microstructure with MX (V/Nb carbonitride) and M23C6 precipitates. Improper welding or PWHT destroys this microstructure: (1) PWHT below 730°C fails to re-form sufficient MX precipitates — creep life reduced by 50-90%; (2) PWHT above 800°C partially transforms martensite to ferrite — immediate loss of room-temperature tensile strength below code minimum; (3) cooling too fast from PWHT can form fresh untempered martensite — brittle, crack-susceptible, and must be re-tempered. P91 welding requires strict procedural qualification, controlled preheat (200-300°C), and PWHT at 740-770°C with precise ramp rates and hold times verified by thermocouples.
References & International Standards
- ASTM International. Standard Specifications for Steel & Metal Alloys. astm.org
- International Organization for Standardization (ISO). Metallic Materials — Cross-Reference Database. iso.org
- American Iron and Steel Institute (AISI). Steel Grade Designations & Equivalents. steel.org
- European Committee for Standardization (CEN). EN Steel Standards & Numbering System. cencenelec.eu