Microstructure and toughness of laser welded joint of 400MPa ultra-fine grained steel

Abstract

The variation with welding conditions in the microstructure and toughness of laser welds of 400MPa ultra-fine grained steel was synthetically studied in this paper. Because ultra-fine grained steel is characterized by ultra-fine grain and high purity, the laser-welded joint of ultra-fine grained steel shows unique features. The microstructures of the weld metal varie with different heat input. Besides common low carbon martensite, lower bainite and side-plate ferrite exist in weld metal, a kind of similar pearlite that is polygonal ferrite containing quantities of dispersed cementite particles occurs in the laser weld. The tensile strength of laser welds is higher than that of base metal, and monotonously increases as the heat input decreases. The impact energy of weld metal is not lower than that of base metal, too; furthermore, as the heat input increases, the toughness of the weld metal rises to a maximum value, at which point the percentage of lower bainite in microstructure is the highest, and then drops.The variation with welding conditions in the microstructure and toughness of laser welds of 400MPa ultra-fine grained steel was synthetically studied in this paper. Because ultra-fine grained steel is characterized by ultra-fine grain and high purity, the laser-welded joint of ultra-fine grained steel shows unique features. The microstructures of the weld metal varie with different heat input. Besides common low carbon martensite, lower bainite and side-plate ferrite exist in weld metal, a kind of similar pearlite that is polygonal ferrite containing quantities of dispersed cementite particles occurs in the laser weld. The tensile strength of laser welds is higher than that of base metal, and monotonously increases as the heat input decreases. The impact energy of weld metal is not lower than that of base metal, too; furthermore, as the heat input increases, the toughness of the weld metal rises to a maximu...