Aiming at finding out the relationship between microstructural evolution and degradation of creep rupture strength

the microstructural evolution of the weld joint metal 12%Cr martensite steel used for superheater header after 45 000hours service at 600℃ has been investigated

by means of carbon replica extraction method

transmission electron microscopy(TEM)and energy dispersive spectroscopy(EDS).The results show that

the microstructure of weld joint metal 12%Cr martensite steel after 45 000hours service at high temperature changed significantly:(1)the Laves phase precipitated along the martensite lath boundaries and the prior austenite grain boundaries;(2)the M23 C6 carbides coarsened;(3)the Z phase formatted around the prior austenite grain boundaries and the packet boundaries by consuming the MX carbonitrides.Coarsen of the precipitated phase resulted in decrease in strengthening effect and inhibiting ability for subgrain's growth.Furthermore

the precipitation of Z phase consumed large amount of MX carbonitrides

causing the tempering martensite recovered and the subgrain structure coarsened

which affected the high temperature creep rupture strength of the 12%Cr martensite steel seriously.