Investigation on the Fatigue Crack Propagation Behavior of L360MS Pipeline Steel Welded Joints with Inconel 625 Weld Metal

Abstract

This study investigated the fatigue crack propagation (FCP) behavior of different regions (base metal (BM), heat-affected zone (HAZ) and weld metal (WM)) of dissimilar metal welded joints of L360MS pipeline steel with Inconel 625 welding wire as the filler material. The effect of the microstructure on FCP was discussed. The results indicated that the fatigue crack growth rates (da/dN) for the WM were lower than those for the BM and the HAZ under the same stress ratio. The microstructures of these regions had different characteristics, such as equiaxed austenite, ferrite and pearlite, which could explain the observed differences in properties. For instance, the FCP path revealed that the existence of ductile equiaxed grains and coarse columnar grains caused the WM to have higher fatigue crack growth resistance and a more tortuous crack propagation path than the BM and the HAZ. The microstructure, grain orientation, strain distribution and grain boundaries around the fatigue crack in the HAZ specimens that propagated through the WM and the fusion line were analyzed by electron backscatter diffraction (EBSD). The results showed that the fatigue cracks followed a zigzag path into the HAZ along the high-angle Type II boundary and Type I boundary in the WM.