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Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations. Usually, decreasing the grain size is the only approach by which both the strength and the toughness of a steel are increased. However, low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study. By direct tracking of the morphological evolution during phase transformation, it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite, whereas small austenite grain size leads to grain boundary nucleation of bainite. This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.