STUDY OF THE MARTENSITE STRUCTURE AT THE WELD INTERFACE AND THE FRACTURE-TOUGHNESS OF DISSIMILAR METAL JOINTS

In austenitic-ferritic dissimilar metal welded joints, the content of alloying elements in the transition zone varies continuously from the heat-affected zone (HAZ) to the weld metal. Due to the low level of Ni content, a martensite layer is formed in this zone during the welding process. The Charpy impact test performed previously by other researchers indicated that the martensite layer was the weakest zone in toughness in the joints. In this study, color metallography and transmission electron microscope (TEM) analysis were used to show the martensite structures at the weld interface. The results show that the structures of the martensite layer at the weld interface are lath martensite. The martensite starting points vary with the distance from the fusion line and are controlled by composition gradient. The boundary of the transition zone could be divided into two types: the ''blurred'' type and the ''sharp'' type, which are controlled by diffusion of elements. A simulation test, in which specimens were prepared by casting steels in accordance with the compositions of the martensite layer, was conducted to evaluate the fracture toughness of the martensite layer. The experiments were also made to investigate the distribution of toughness in the different regions of the joints. The results show that the weakest region in toughness in the dissimilar metal joints is not the martensite layer but the overheated zone in HAZ. The fracture in the overheated zone is caused by the coarse-grained bainite, and it appears as a quasicleavage fracture; however, in the martensite layer, it appears as a tear fracture.