Microstructure and Toughness of Laser Welded Joint of 400MPa Ultra-Fine Grained Steel

摘要

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