In situ Observations of Transformation Behavior upon Heating for a 1.5Mn-1.5Si-0.2C Steel -Comparison between Neutron Diffraction, XRD, EBSD and Dilatometry

The austenite reverse transformation behavior in a 1.5Mn-1.5Si-0.2C steel was in situ monitored using dilatometry, electron back scatter diffraction (EBSD), X-ray diffraction and neutron diffraction. The austenite reversion kinetics showed excellent agreements between dilatometry and neutron diffraction, whereas the austenite formation was observed to start at much higher temperature in cases of EBSD and X-ray diffraction measurements. Such discrepancy in transformation temperature is attributed to the change in chemical compositions near the surface of a specimen heated to elevated temperatures either in vacuum (EBSD) or in a helium gas atmosphere (X-ray); Mn and C concentrations were found to decrease with heating. In situ neutron diffraction enables us to investigate the changes in lattice constants of ferrite and austenite, showing not only thermal expansion but also suggesting carbon enrichment and phase stresses during the decomposition of the retained austenite and austenite reversion upon heating.