Intergranular Cracking Analysis in Heat-Affected Zone on TP347H Steel Side of T91/TP347H Dissimilar Steel Joint in Service

Intergranular cracks were found in T91/TP347H dissimilar steel joint of a boiler panel superheater in a power plant after 80 000 h operation. The cracks were located in the heat-affected zone on TP347H austenitic steel side. The features and forming causes of the intergranular cracking and its effect on the mechanical properties were analyzed by microstructure observation, hardness test and tensile test. The results show that the intergranular cracks existed at the fusion line on TP347H steel side, and were intergranular corrosion cracks. The cracks penetrated through 3-4 grains and were filled with oxides inside. The hardness and tensile strength of the panel superheater tube joint with intergranular cracks met the requirements, but the plasticity was poor. Coarse grains and fast cooling rates after welding made the heat affected zone on the TP347H steel side of the panel superheater tube joint prone to forming depletion of chromium at grain boundaries during high-temperature service, causing intergranular corrosion. The high hardness gradient near the fusion line on the TP347H steel side led to stress concentration here, accelerating the generation of intergranular corrosion cracks.