Determination of microstructural criterion for cryogenic toughness variation in actual HAZs using microstructure-distribution maps

It is well known that heat-affected zone (HAZ)-notched fracture mechanics tests, such as crack tip opening displacement (CTOD) test, are very sensitive to the microstructural gradient in the HAZ. This study aimed to investigate the distribution of microstructures, including local brittle zones (LBZs), in the actual HAZ of advanced 9% Ni cryogenic steel, and to determine a microstructure criterion for the change in cryogenic toughness of the steel HAZs using microstructure-distribution maps. To estimate the microstructural influence more systematically, K-grooved HAZ specimens were prepared. Analysis using these microstructure-distribution maps constructed from the specimens tested at 111 K revealed that the weakest-link-type fracture occurs mainly in regions having a large fraction of LBZs (over 17%), whereas the fracture toughness of the other regions was found to be governed by a mixture-rule of the microstructures existing at the precrack tip front of CTOD specimen.