CREEP BEHAVIOR AND MICROSTRUCTURE OF A PROSPECTIVE RE-CONTAINING 10%CR-3%CO-3%W MARTENSITIC STEEL

9-10%Cr-3%Co martensitic steels are the prospective materials for elements of boilers, tubes and pipes for fossil power plants which are able to work at ultra-supercritical parameters of steam (1=620-650 degrees C, P=25-30 MPa). The effect of creep on the microstructure of the 10 wt.%Cr-3Co-3W-0.2Re martensitic steel was investigated in the condition of 650 C and an applied stress of 140 MPa, time to rupture was more than 8500 h. Previously, this steel was subjected to the normalizing at 1050 degrees C and tempering at 770 degrees C. This heat treatment provided the hierarchical tempered martensite lath structure with the mean size of prior austenite grains of 59 p.m and with high dislocation density (2x10(14) m(-2),) within martensitic laths. Boundary M23C6 and M6C carbides and randomly distributed within matrix Nb-rich MX carbonitrides were detected after final heat treatment. The addition of Re in the steel studied positively affected creep at 650 degrees C/140 MPa and stabilized the tempered martensite lath structure formed during 770 degrees C-tempering. The formation of the subgrains in the gage section was accompanied by the coarsening of M23C6 carbides and precipitations of Laves phase with fine sizes during creep. No depletion of Re and Co from the solid solution during creep was revealed whereas W content decreased from 3 to 1 wt.% for first 500 h of creep. Reasons of improved creep as well as mechanisms of grain boundary pinning by precipitates are discussed.