ASTM A335 P91 (9Cr-1Mo-V): Creep-Strength-Enhanced Ferritic Steel for Power Plants
ASTM A335/A335M · Published: 2026-06-03 · Updated: July 2026
ASTM A335 P91 (T91 for tube) is a creep-strength-enhanced ferritic (CSEF) steel that revolutionized power plant design — enabling main steam temperatures of 580-620°C at 250-300 bar pressure, dramatically improving thermal efficiency over the P22...
ASTM A335 P91 (T91 for tube) is a creep-strength-enhanced ferritic (CSEF) steel that revolutionized power plant design — enabling main steam temperatures of 580-620°C at 250-300 bar pressure, dramatically improving thermal efficiency over the P22 (2.25Cr-1Mo) steel it replaced. Its 9% chromium, 1% molybdenum, plus microalloying with V, Nb, and N create a stable MX precipitate structure that resists creep for 200,000+ hours at design temperature.
Quick Facts
| Category | Alloy Steel |
| Standard | ASTM A335/A335M |
| Density | 7.80 g/cm³ |
| Yield Strength | 415 MPa |
| Tensile Strength | 585 MPa |
Global Equivalents & Cross-Reference
| Alternative Standard / Grade | Action |
|---|---|
| EN X10CrMoVNb9-1 | Compare |
| JIS STPA28 | Compare |
| GB 10Cr9Mo1VNb | Compare |
Related Materials
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Frequently Asked Questions
What is the post-weld heat treatment requirement for P91?
P91 requires PWHT at 730-760°C for 2 hours minimum (1 hour per 25mm thickness). This is critical: without proper PWHT, the heat-affected zone will have untempered martensite with hardness >400 HV and near-zero toughness — a condition that has caused catastrophic steam pipe failures. PWHT for P91 is NOT optional under any code (ASME B31.1, ASME Section I, EN 13480).
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