Microstructural Evolution of Grade 91 Steel upon Heating at 760~1000 oC

Abstract

The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at 760~1000 o C for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite(760~850 o C), and to fresh martensite(900~1000 o C), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at 925 o C), indicating grain recovery upon intercritical heating. The main precipitate, M 23 C 6 , was found to be coarsened slightly at 760~850 o C; it then dissolved at 850~1000 o C. Besides this, M 3 C cementite was formed at 900~1000 o C. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.