The most common question in high-temperature power piping: WP91 or WP22? Here is the definitive technical comparison to help you specify correctly.
In a modern ultra-supercritical power plant, the main steam temperature can reach 620°C at pressures of 300+ bar. At these conditions, the choice between WP91 and WP22 determines whether your piping system is viable at all — WP22 simply doesn't have adequate creep strength above 600°C. But for maintenance and replacement on existing P22 systems, using P91 without understanding its specific requirements can cause catastrophic failures.
This guide covers everything you need to make the right specification decision.
WP22 is a simple 2¼Cr-1Mo ferritic alloy — developed in the 1950s and proven over decades of service. WP91 is Modified 9Cr-1Mo — developed in the 1970s at Oak Ridge National Laboratory specifically to improve on WP22's limitations at higher temperatures. The addition of vanadium (V) and niobium (Nb) to the 9Cr-1Mo base creates fine carbide and nitride precipitates that dramatically increase creep strength.
| Parameter | WP91 (P91) | WP22 (P22) |
|---|---|---|
| Alloy System | 9Cr-1Mo-V-Nb (Modified) | 2¼Cr-1Mo |
| Max Service Temperature | 649°C | 600°C |
| Allowable Stress at 550°C | ~105 MPa | ~85 MPa |
| Allowable Stress at 600°C | ~87 MPa | ~55 MPa |
| Creep Rupture Strength | Significantly higher | Moderate |
| Wall Thickness (same P&T) | ~35% thinner than P22 | Baseline |
| PWHT Temperature | 760°C ±15°C minimum 2hr | 675–760°C |
| Weld Preheat | 200–250°C minimum | 150–200°C |
| Hardness (BHN) | 195–265 (must verify) | No strict limit |
| Failure Risk if PWHT Wrong | High — Type IV cracking | Lower |
| Installed Base | New plants (post-1990s) | Massive — global legacy |
| Material Cost | Higher | Lower |
Any new design above 565°C main steam temperature should use WP91. Above 600°C, WP22 is not viable from a code allowable stress perspective.
WP91's ~35% higher allowable stress means thinner walls, less weight, and better thermal flexibility — significant for large-diameter hot piping.
For new plants designed for 30+ years service, WP91's superior creep properties offer a better long-term investment despite higher initial cost.
If the plant was built with P22, maintenance and replacement should use P22. Mixing P91 and P22 in the same circuit creates weld compatibility issues.
For applications between 450–565°C where both grades are adequate, WP22's lower cost and simpler weld procedures make it the preferred economic choice.
P91 welding requires specially qualified procedures and welders. If qualified P91 welding is unavailable, P22 is significantly more tolerant of less strict procedures.
âš P91 PWHT is NOT Optional
P91 must be post-weld heat treated at exactly 760°C ±15°C for minimum 2 hours. Under-tempered P91 (PWHT below 750°C) is brittle and has failed catastrophically in service. Over-tempered P91 (above 790°C) loses strength. The hardness after PWHT must be between 195–265 BHN — verify by hardness testing. Never weld P91 without a qualified WPS and PWHT procedure.
We stock both grades with full heat treatment documentation. Tell us your grade, type, size, and schedule.