A study of fracture toughness and microstructures in the weld heat-affected zone of QLT-processed 9% Ni steel

The objective of this study, based on the concept of fitness-for-purpose, is to evaluate the fracture toughness in X-grooved weld HAZ(heat-affected zone) of QLT(quenching, lamellarizing and tempering)-processed 9% Ni steel, qualitatively and quantitatively, and analyze the relation between the fracture toughness and microstructure. In general, CTOD test is widely used to determine the fracture toughness of steel weldments. But several problems of accuracy have been brought up. To avoid those, in this paper, modified CTOD testing method is proposed and used for weld HAZ for 9% Ni steel. In addition, microstructure of HAZ is observed and analyzed by OM, SEM and XRD. From the results, HAZ toughness of QLT-9% Ni steel decreased as the evaluated region approaches the fusion line from base metal. The decrease in toughness was apparently partly caused by the reduction of the retained austenite content resulting from the decrease in nucleation sites for alpha-->gamma reverse transformation due to the increasing fraction of coarse-grained region. On the other hand, due to the poor stability of retained austenite in the mixed zone of weld metal/base metal, toughness drop in F. L.similar to F. L.+3mm was larger than that in F. L.+5mm and F.L.+7mm with decreasing test temperature.