Fatigue crack propagation and metallographic mechanism for steel ADB610 welded joints

Abstract

High-strength steel ADB610 is a low carbon bainite steel and was newly developed in China. Fatigue crack propagation rate experiments and metallographic experiments for butt welded joints of steel ADB610 were investigated. The welding method is shielded metal arc welding. The fatigue crack propagation experiments were done under a constant load using compact tension samples and MTS810 TestStar materials testing facility at the stress ratio of 0·1. Seven data polynomial method was used to calculate fatigue crack propagation rate (da/dN) of each compact tension samples. All da/dN and △K of base metal samples, heat-affected zone samples and weld metal samples were fitted, respectively, and the formula based on Paris for base metal, heat-affected zone and weld metal were obtained. Fatigue crack propagation experimental results indicate that fatigue crack propagation rate in base metal is higher than that in heat-affected zone and weld metal, and the one in weld metal is the slowest during the early and middle stage of crack propagation, but during the end stage of crack propagation the fatigue crack propagation rates for the three regions of steel ADB610 welded joints were nearly at the same level with slight fluctuations with each other. The metallographic experiments show that the crystal grains in the weld metal were finer than those in the heat affected zone and base metal, and the heat-affected zone microstructure was meshy. So the resistance against fatigue crack propagation in the weld metal is higher than that in the heat-affected zone and base metal, and the resistance against fatigue crack propagation in the heat-affected zone is higher than that in the base metal. The fatigue crack propagation difference of steel ADB610 welded joins from metallographic experiments is consistent with the result from fatigue crack propagation experiments.