Abstract: Mechanical property of heat affected zone (HAZ) in large heat input welding drastically changes by formation of Martensite-Austenite constituent (M-A). The influence of boron on microstructure formation was evaluated by simulated HAZ. Boron distribution at interface of M-A in simulated HAZ, and also prior austenite grain boundary before transformation were investigated by 3 Dimensional Atom Probe (3DAP) to clarify the mechanism of M-A formation by boron in microstructure formed under continuous cooling. Formation of M-A did not increase in coarse grain sample, which consist of mainly upper bainite. On the other hand, M-A increased in fine grain sample, which is mainly ferrite. It is found by 3DAP analysis that boron segregates at prior austenite grain boundary and interface of M-A in ferrite structure but not at interface of M-A in bainite structure. These results indicate that, boron segregates at grain boundary of untransformed austenite only during ferrite transformation but not in bainite transformation.
Authors: Ryo Arao, Katsuyuki Ichimiya, Yoshiaki Murakami, Naoki Takayama, Koichi Nakashima, Kazukuni Hase, Keiji Ueda, and Satoshi Igi
Keywords: large heat input welding, boron, segregation, Martensite, Austenite constituent, HAZ