Through metallographic analysis

carbon replica extraction

transmission electron microscopy(TEM)

and energy dispersive spectrometer(EDS)

etc.

the microstructure analysis of T91 steel after 130 000 h in service has been carried out

and the factors affecting deterioration of T91 steel in service under high temperature being further studied.Results of study show that the structure of T91 steel after 130 000 hin service has been largely changed:(1)the Laves-phase precipitates along grain bounderies and lath bounderies;(2)coarsening of precipitation-phases M23C6 and MX;(3)the Z-phase precipitares on grain bounderies and sub-grain bounderies

and grows rapidly there

paying cost of consuming MX;(4)the tempered martensite recovers

the dislocation density reduces

and widely coarsening the sub-grains.The precipitation of Laves-phase has little effect upon the performance of T91 steel after long-term operatin under high temperature

but the coarsening of precipitation-phase leading to decrease the strengthening effect

resisting the decrease of growing ability for sub-grains.In addition

the separation out of Z-phase consumes large amount of precipitation-strengthening phase MX

leading to recovery of tempered marttensite

and to coarsening of sub-structure

affecting seriously the creep rupture strength of T91 steel under high temperature.