Microstructure and the fatigue crack propagation in the dissimilar low alloy/stainless steel GMAW welded joints

Joining dissimilar metals is a frequent task for engineers in the power industry. The resulting dissimilar joints are then critical parts of the constructions and have to be able to withstand various loads during the operation of power devices. Due to the metals dissimilarity, the interface between ferritic side and the austenitic side of the joint is considered to be a critical area regarding the joint resistance to failure under static or cyclic loading. Proper understanding of the microstructures formed at the interface is key for production of solid and reliable dissimilar joints. In the present study, two dissimilar joints were prepared by gas metal arc welding (GMAW) using different low alloy steels. The microstructure of both joints is extensively studied using electron microscopy methods, with the emphasis taken on the interface between low alloy steel and austenitic weld metal. It was shown, that the interface between low alloy steel and weld metal is not a preferential crack path in case of non-environmental fatigue crack propagation, and the cracks are rather propagating through to the heat-affected zones of the low alloy steel side of the dissimilar joint. There were recorded significant differences in the threshold stress intensity factor between two tested welded joints (5.8 MPa × m1/2 vs. 8.7 MPa × m1/2. Therefore, the choice of the low alloy steel has a significant effect on the resistance to the fatigue crack propagation, and even small differences in the microstructure of low alloy steel may affect the resistance to the fatigue crack propagation at low stress ratios of the whole dissimilar joint.