According to a 2026 review in Welding in the World, post-weld heat treatment (PWHT) of Grade 91 (P91) steel is critical for achieving the required creep strength in high-temperature piping systems.
P91 steel is widely used in power plants and petrochemical facilities for its excellent creep resistance at elevated temperatures. However, improper PWHT can lead to premature failures. The standard requires a controlled cooling rate from the PWHT temperature to prevent re-hardening and to ensure the formation of tempered martensite.
Key requirements from ASME B31.1 and EN 13445:
• Minimum PWHT temperature: 730°C, with a holding time of at least 1 hour per 25 mm of thickness.
• Maximum cooling rate: 150°C per hour, measured at the surface.
• Thermocouple placement: at least two per weld, positioned 50 mm from the weld toe.
Auditors typically check the heat treatment records for compliance with these parameters. Deviations can result in a repair weld or even component replacement.
The takeaway: PWHT for P91 is not a generic process. It demands strict adherence to temperature and cooling rate specifications.
How does your team verify PWHT compliance for creep-resistant steels?
Source: Welding in the World, 2026