After being put into service for 6 200 hours

the microstructure of S30432 austenitic stainless steel used in boiler tubes for an ultra-supercritical 620 ℃ double-reheat unit changed. Through the SEM

EDS and TEM analysis

the surface morphology and element distribution characteristics of the oxide scale were investigated

after the high pressure reheater

the final stage superheater and the low pressure reheater were put into service. Moreover

the precipitation behaviors both in grains and along grain boundaries were compared. The results demonstrate that

after operation

the high temperature heating surface has steady microstructure

and a dense Cr2O3 protective layer formed on the inner surface of the tubes

with an average thickness of about 200~400 nm. Quite a lot of tiny ε-Cu particles and Nb C(N) compounds precipitated

with size of about 10~20 nm and 50 nm respectively

which were scattered in the matrix uniformly. Some M23C6 typed carbides with size of 100~400 nm precipitated along the grain boundaries discontinuously. Due to the high working temperature

the high pressure reheater tubes had larger precipitation particles than other tubes.