Abstract: Reverse transformation from ferrite to austenite has been utilized to obtain fine austenite grains. Initial structures as well as heating rates and chemical compositions of steels strongly influence formation behaviors of reversely transformed austenite. For example, when the lath martensite is reheated at a heating rate ≦10℃/sec, a formation of granular and acicular austenite has been reported. However, the formation mechanism of the acicular austenite has not been fully clarified by using quenched specimens. In this study, we investigated formation behaviors of granular and acicular austenite through detailed microstructural observations by means of the in-situ SEM/EBSD techniques in which a specimen is reheated in a SEM chamber and observed its surface while heating. Through in-situ SEM/EBSD investigations on a 0.1C-9Ni (mass%) tempered martensite, it was confirmed that austenite nucleated on high angle grain boundaries in the initial structure. With increasing the temperature, austenite grew into grain gradually. On the other hand, nucleation within grain could hardly be confirmed.
Authors: Natsumi Ooura, Kazuki Fujiwara, Kaori Kawano, and Nobuhiro Tsuji
Keywords: Reverse transformation, Acicular austenite, Granular austenite