According to ASTM A335/A335M-22 and ASME Section I, Grade P91 (9Cr-1Mo-V) steel is widely used in high-temperature creep service in power plants and refineries. A critical requirement is post-weld heat treatment (PWHT) within a narrow temperature range of 730-760°C (1350-1400°F).
Deviation outside this window risks two failure modes. Underheating leaves untempered martensite, which reduces creep ductility and can lead to Type IV cracking in the heat-affected zone. Overheating above the lower critical temperature (Ac1, approximately 800°C) can re-austenitize the microstructure, forming fresh untempered martensite on cooling and negating the PWHT benefit.
Most codes require a minimum holding time of 2 hours per inch of thickness, with a controlled cooling rate to avoid residual stress. Thermocouple placement and furnace uniformity are often audited during WPQR qualification.
A common shop-floor issue is insufficient soak time for thick-wall components, especially when multiple welds are PWHTed in the same furnace cycle. Verification of heating and cooling rates per the WPS is essential.
How do you ensure PWHT compliance for P91 when furnace capacity limits simultaneous treatment of multiple welds?
Source: ASTM A335/A335M-22, 2022