Evaluation of cryogenic fracture toughness in SMA-welded 9% Ni steels through modified CTOD test

As the first step of the study for the safety performance of LNG storage tank based on the concept of fitness-for-purpose, the change of cryogenic toughness within the X-grooved weld HAZ (heat-affected zone) of SMA (shielded metal arc)-welded QLT (quenching, lamellarizing, and tempering)-processed 9% Ni steels, was investigated qualitatively and quantitatively. In general, CTOD (crack tip opening displacement) test is widely used to determine the fracture toughness of steel weldments. But there is no standard or draft for evaluating the toughness of thick weldment with X-groove such as in this case. Therefore, in this study, modified CTOD testing method for fatigue precracking, calculation of CTOD, examination of fractured specimen was proposed and used. And the results of modified test were compared with those of conventional CTOD test and Charpy V-notch impact test. In addition, the relationship between the fracture toughness and microstructure was analyzed by OM, SEM and XRD. The cryogenic toughness in HAZ decreased as the evaluated region approached the fusion line from base metal. The decrease in toughness was apparently caused by the reduction of the retained austenite content and the absence of grain refinement effect in the coarse-grained zone in HAZ. The austenite reduction resulted from the decrease in nucleation sites for alpha --> gamma reverse transformation due to the increase in fraction of coarse-grained zone within HAZ. More complex thermal cycles in the mixed zone of weld metal and base metal caused the poor stability of retained austenite in the zone by the redistribution of alloying element in retained austenite. Due to this reason, the toughness drop with decreasing test temperature in F.L. (fusion line)-F.L.+3 mm was larger than that in F.L.+5 mm and F.L.+7 mm.