The 12Cr1MoVG/TP347H dissimilar metal welded joint of reheater tube in a utility boiler had fractured in weld fusion line of 12Cr1MoVG steel side after 17 000 hours' running. Inspection and analysis including visual examination

chemical composition analysis

mechanical property test

stress calculation

metallographic structure and fracture observation were carried out. The results show that

the composition

strength

plasticity and hardness of the parent material were in accordance with the relevant standards. No abnormal situations were found in the metallographic structure of the parent material and the heat affected zone(HAZ) of the pipe section. The main components of the weld metal were in normal. The weld fracture was characterized by brittle fracture without obvious step and plastic deformation. So it can identify that the fracture failure is a typical early failure of A/P dissimilar metal welded joints. The early failure of the joint was mainly caused by the brittle and soft layers formed at two sides of the weld metal and the three axis tensile stress resulted from mechanical properties difference. Meanwhile

these factors such as poor weld shape and strong stress concentration due to the unreasonable groove and dramatic structural changes(over-standard overlap and root reinforcement) played a promoting role to the joint early failure.