ANALYSIS OF THE CRACK IN THE BUTT JOINT OF Q345D STEEL STRUCTURAL BEAM

There was a crack, about 100mm long, in the heat affected zone (HAZ) of Q345D steel butt joint. The metallurgical structure of HAZ and matrix metal, along with pattern of the crack were observed using metallographic analysis. Scanning electronic microscope was used to analyze the microscopic shape of the crack and micro-area chemical composition. The results show that HAZ doesn′t form the quench-hardening structure. Decarburized layer appears in the two sides of the crack. There is some secondary oxide existing in the decarburized layer. The flaw isn′t the hot crack or cold crack resulted from welding. Microcrack has initiated early in the surface of billet steel, then the crack appears after welding.