Revealing the reverse austenite and martensite transformation pathways in P91 steel through in situ MEMS-heating EBSD investigations

In situ electron back scatter diffraction (EBSD) analysis of martensitic P91 (modified 9Cr-1Mo) steel during heating and after cooling down was carried out. Crystallographic information about α’-to-γ phase transformation with heating reveals that orientation relationships (ORs) between initial martensite and reversed austenite likely follows Pitsch OR. The γ-to-α' phase transformation that occurred after cooling exhibits multiple possible orientation relationships, with the Kurdjumov–Sachs (K-S) OR showing a slightly better fit. In situ EBSD analysis also provided insights into the nucleation and growth of reversed austenite during heating. γ nucleation initially occurs at high-angle grain boundaries and is characterized by rapid (massive) growth. In addition, it was found that the setup used with the microelectromechanical system (MEMS) heater does not allow for accurate control of cooling rates, and the structural inhomogeneity of the steel samples after rapid cooling affects the interpretation of OR analysis.