Integrity Assessment of Grade 92 Welded Joint under Creep Condition

Integrity of tempered martensitic grade 92 (9Cr-1.8W-0.5Mo-VNb) steel welded joint has been assessed by carrying out creep test at 923 K over a wide stress range. The failure location in the joint was found to shift from base metal to heat affected zone (HAZ) of the weld joint with decrease in applied stress. The HAZ of the weld joint consisted of coarse prior-austenite grain (CGHAZ), fine prior-austenite grain (FGHAZ) and intercritical (ICHAZ) regions in a sequence from the fusion boundary to unaffected base metal. The failure was associated with localized creep deformation coupled with creep cavitation. To have a better insight to the creep failure behaviour of the joint, the different microstructural constituents of HAZ of the joint has been simulated. Creep and tensile tests were carried out on the simulated HAZs specimens. The steel suffered degradation in creep rupture strength on subjecting heating in the intercritical (between Ac1 and Ac3) (ICHAZ) as well as at temperature just above the Ac3 (FCHAZ). The extensive creep cavitation has been observed in the simulated FGHAZ of the joint. Similarly, localized extensive creep cavitation and precipitation of tungsten rich Laves phase has been observed in the FGHAZ of the weld joint. This caused the type IV failure in the FGHAZ of the weld joint. The integrity of grade 92 steel joint under creep condition is governed by the above microstructural modifications of the steel in HAZ by weld thermal cycle.