Microstructure changes responsible for the degradation of the 10CrMo9-10 and 13CrMo4-5 steels during long-term operation

Abstract

The paper presents results of microstructure and mechanical testing examinations performed using optical and transmission electron microscopy, tensile tests and Charpy tests on 10CrMo9-10 and 13CrMo4-5 steels, before and after they were long-term operated at elevated temperatures in a steam heater. In the 10CrMo9-10 steel, the optical microscopy detected a degradation of original bainite that was accompanied by the formation of ferrite, precipitates and micropores. The transmission electron microscopy revealed that the precipitates are M23C6 and M7C3 type carbides, which are located mainly at the boundaries of former austenite grains, and M3C type carbides, which appear inside the grains. The 13CrMo4-5 steel contained a relatively high amount of ferrite in the ferritic-bainitic/perlitic microstructure already in the originally state. The degradation of the microstructure was less serious than for the 10CrMo9-10 steel. The thermal treatment of the 13CrMo4-5 steel led mainly to the precipitation of carbides. The M23C6 and M7C3 type carbides form in perlitic-bainitic areas, while M3C and M6C type carbides precipitate in ferrite. The higher density of the grain boundary precipitates in the long-term operated 10CrMo9-10 steel facilitated the formation of creep-induced micropores and contributed to the hardness reduction.