Fracture Analysis Of Laser-Welded MW9124 Steel In Cold-Rolling Process And Study On Its Welding Technology

The Laser Filler Wire Welding is characterised with high speed, high efficiency and good welding quality, it has been successfully applied to joining the general carbon steel sheet on the tandem rolling production lines at Bao Steel Group Co. Ltd. But on the cold-rolling production line of silicon steel, the fracture occurs in the laser welding seam from time to time, which greatly lowers the production effieciency. In order to fint out the fracture cause and the effective solution to this problem exactly, a series of researches that focused on the fracture problem in the cold-rolling process of the laser welded silicon steel MW9124 were carried out in this thesis.Firstly a series of failure analysis of the broken steel sheet parts were performed. By means of various kinds of chemical and physical analysis of these parts, as well as the study of some possible failure causes, it was found that the low-cycle fatigue is the main possible reason, and the local stress and strain concentration in a small area induced by surface defects of welding seams have also a great effect on the initiation of the fatigue crack. For this sake a finite element simulation software ANSYS is used to simulate and analyse the stress concentration caused by welding deformation, inner and surface welding defects and intensity mismatch. The surface welding defects such as unfitness of butt joint have much larger influence on stress and strain concerntration than the inner welding defects. Therefore more attentions should be paid on such surface defects. Besides, ANSYS was used to simulate the propagation process of fatigue crack. In the last part of this thesis, the technology parameters, which influence the forming, structure and performance of the welding seam, was studied and optimized through the laser welding experiment of MW9124 silicon steel, and the relationship between structure, hardness grandient and line input energy was obtained. The laser welding characteristics of MW9124 silicon steel was also concluded as a reference to raise the welding quality and fracture toughness property, and to lower the cracking probability in the cold rolling process.