P91 (9Cr-1Mo-V): Creep-Strength Enhanced Ferritic Steel for USC Boilers
ASME SA335 P91 / ASTM A335 P91 · Published: 2026-06-02 · Updated: 2026-06-02
P91 (Grade 91) enabled ultra-supercritical power plants at 620°C steam — 50-60°C higher than 2.25Cr-1Mo. MX carbonitride precipitates (V/Nb(C,N)) provide creep strength. Welding demands precise PWHT at 750°C — failure causes Type IV cracking...
P91 (Grade 91) enabled ultra-supercritical power plants at 620°C steam — 50-60°C higher than 2.25Cr-1Mo. MX carbonitride precipitates (V/Nb(C,N)) provide creep strength. Welding demands precise PWHT at 750°C — failure causes Type IV cracking within 10,000 hours.
Quick Facts
| Category | Alloy Steel |
| Standard | ASME SA335 P91 / ASTM A335 P91 |
| Density | 7770 kg/m³ |
| Yield Strength | 415 MPa (min) |
| Tensile Strength | 585 MPa (min) |
Global Equivalents & Cross-Reference
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Frequently Asked Questions
Why is P91 welding so sensitive?
MX precipitates dissolve during welding. PWHT too low: incomplete re-precipitation. PWHT too high: martensite→ferrite transformation, permanently reducing strength.
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