2007 年 25 巻 4 号 p. 532-536
A study was made on the relation between CTOD value and M-A constituent for the single-heat-cycled weld HAZ of YS 320 to 360 N/mm2 class TMCP steels with increased heat input. It was found that the M-A constituent disappeard and lost its deterioration effect on the HAZ CTOD toughness when the heat input exceeded about 15kJ/mm, although this boundary heat input depended on the steel chemistry. On the other hand, the austenite grain size increased monotonically with increasing the heat input. But the austenite grain size could not be the controlling factor of the HAZ toughness, and its effect deferred between base metals. However, the HAZ toughness was related to the fracture facet size for the large heat input conditions. This fracture facet size, which represents the fracture toughness, is considered to be a measure of the uniformity of the transformed microstructure from austenite in the HAZ.