The weld joint of TG700A and HT700T which is used in superheater of ultra supercritical power plant boiler was studied after 10 000 h service on 700 ℃ ultra supercritical coal-fired power unit key component verification test platform. The evolution of microstructure and microhardness of the in-service weld joint was investigated. The results show that

after being put into service

the Mo/Ti-rich primary carbides distributed in interdendrite

and there was no obvious inter diffusion on both sides of the fusion line. Coarsening of grain boundary carbides occurred in the heat affected zone(HAZ)

and microhardness of the HAZ was consistent with the base metal. The steam oxide layer on inner wall was mainly chromium oxide

and the morphology of the HAZ was the same as that of the oxide layer of the base material. The average thickness of the oxide layer was about 7 μm

which was excellent oxidation resistance under the service conditions. The microstructure stability of the welded joint was good after 10 000 h service.