Abstract: Many dissimilar steel joints are used in the high-temperature section of boiler heating surface tubes of ultra-supercritical (USC) thermal power units and their welding quality and performance directly determines the operation security of the units. Macroscopic and microscopic examinations on the failure specimen of T91/HR3C dissimilar steel welded joints are conducted for the high-temperature reheater in a 3 033-t/h USC boiler. The results show that the failure part is located in the heat affected zone of T91 side with the cracks spreading along the circumferential direction of the tubes, starting from the CGHAZ and ending at the FGHAZ of the outer wall. The insufficient post-welding heat treatment, unstable microstructure in the HAZ and high hardness are the major causes leading to the cracking failure. Post-welding heat treatment(PWHT) is suggested to improve the ductility and reduce the residual stress for the welded joints. Furthermore, the delta ferrite of lower hardness white and bright is observed near the fusion line of T91 side. It is not directly related to the failure, but further study is required to evaluate its influence on the long-term performance of the welded joints.

Key words: T91 steel, HR3C steel, dissimilar steel welding, delta ferrite, utility boiler