Crystallography of the martensitic transformation in friction-stir processed 10 % chromium martensitic steel

In this study, the martensitic transformation that occurred in the friction-stir processed (FSPed) 10 % Cr martensitic steel was studied. Due to the specific character of the FSP technique, the processed material undergoes very large plastic strains at an elevated temperature and a high strain rate. It is important to emphasize that material flow during FSP of martensitic steels usually occurs in the austenitic phase field. During subsequent cooling cycle, the processed material typically experiences martensitic transformation. It is expected that the heavily-deformed and fine-grained microstructure produced in the austenitic phase during FSP may exert an influence on the following martensitic transformation. Specifically, deviations from the “conventional” orientation relationship and selection of crystallographic variants are anticipated. To the best of authors’ knowledge, this issue is still not studied well. Therefore, the present work aimed to shed some light on this phenomenon. To this end, the electron backscatter diffraction (EBSD) technique was applied to characterize the microstructure produced during FSP of 10 % Cr creep-resistant steel. It was found that the evolved microstructure was indeed dominated by martensitic phase, though it also contained a minor fraction (≈0.1 %) of retained austenite. It was also shown the orientation relationship between the martensite and austenite deviated from the “classical” Kurdjumov-Sachs (K-S) one and could be described in terms of a mixture of K-S and Nishiyama-Wasserman (N-W) relationships. This observation was attributed to the essential orientation spread within the prior austenite grains. Moreover, a pronounced variant selection was revealed in the martensitic phase. The reconstruction of the microstructure of high-temperature austenite had demonstrated that the latter observation was likely due to the pronounced grain refinement and development of a distinct B / -B {110}<112> simple shear texture occurring in the austenitic phase during FSP.