WP91 (Grade 91 — Modified 9Cr-1Mo-V) is the dominant grade for ultra-supercritical (USC) power generation piping. The addition of vanadium and niobium to the 9Cr-1Mo base gives exceptional creep stren...
WP91 (Grade 91 — Modified 9Cr-1Mo-V) is the dominant grade for ultra-supercritical (USC) power generation piping. The addition of vanadium and niobium to the 9Cr-1Mo base gives exceptional creep strength at 600–649°C — approximately 58% higher allowable stress than WP22 at the same temperature. This means thinner walls, less weight, and better thermal response in modern power plants.
| Property | Value |
|---|---|
| Grade | WP91 (P91) |
| ASTM / ASME | ASTM A234 WP91 |
| UNS Number | — |
| Chromium (Cr) | 8.0–9.5% |
| Nickel (Ni) | ≤0.40% |
| Molybdenum (Mo) | 0.85–1.05% |
| Max/Min Service Temp | 649°C |
| Fitting Standard | ASME B16.9 / MSS SP-43 |
| Size Range | ½" NB to 48" NB |
| Certificate | EN 10204 3.1 MTC |
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