Study Of Laser Welding Of 400 MPa Super Steel And Its Weldability

Ultra steel is characterized by its higher cleanness and ultra-fine grain, thus it has higher strength, toughness and longer life even without addition of more alloying elements. The research and development of ultra steel is presently one of the most important scientific and technological fields in the world, and also meets the requirement of the state and the sustainable development strategy of china. The solution of the welding problems is one of the keys to the development and application of the ultra steel. Therefore, research on the weldability of ultra steel is of great significance in theoretical study and industrial application. Laser welding is one key method to carry out the welding of ultra steel. In this paper, on the basis of studying the thermal cycle of laser welding, the grain growth and the microstructure transformation of the welded joint in laser welding of 400MPa grade ultra steel were studied. The influence of the cleanness, grain size and microstructure on the mechanical properties of the laser-welded joint was investigated. The major conclusions are summarized as the followings:1. On the basis of compound heat source that is a simplified point-line thermal model for deep penetration laser welding, the term of diagnostic grain was proposed and a model of prior austenite grain growth for HAZ in laser welding was established. The calculated result matches well with the experimental result. The simulated and experimental result show that due to the effect of the welding thermal cycle, the grain growth of the welded joint of the 400MPa ultra steel with ultra-fine grain and higher cleanness was more severe than low carbon steel. The grain growth could be controlled by conducting low heat input welding.2. The microstructure of laser welded ultra-steel joint was observed with SEM and TEM. The microstructure of the weld metal and HAZ consists of martensite, lower bainite, upper bainite, similar pearlite, pearlite and ferrite. The content percent of the different microstructures with varied welding heat input was measured.3. Large quantities of similar pearlite (SP) that has good toughness was found in the laser welded ultra-steel joint. The appearance, formation condition and mechanism of the SP were theoretically analyzed. The theoretical and experimental results show that the occurrence of SP transformation is closely related with the cleanness of the base metal and the welding cooling conditions. 4. The influence of the grain size and the microstructure percent on the strength and toughness of the welded joint was acquired. What should be emphasized is that although the grain of welded joint has largely grown up under usual laser welding conditions, the strength and toughness of the welded joint were much higher than that of base metal. It was first found that the cleanness of 400MPa ultra steel has greater influence on the toughness of the welded joint than grain size. 5. By experimental study and theoretical analysis, it was stated that controlling the S, Si and Mn content of ultra-steel base metal is very important to preventing pro-entectoid ferrite and grain boundary impurity occurring, avoiding intercrystalline fracture during low temperature impact test, promoting the occurrence of SP transformation and improving the toughness of the joint. As a summary, with the study on the grain growth, microstructure and mechanical properties of the laser-welded joint of 400MPa ultra steel, the weldability of ultra steel was evaluated. It provides experimental and theoretical basis for the development and industrial application of the ultra steel with good weldability.