Microstructures and mechanical properties of powder-filled laser welding joints composed of high-nitrogen steel

In this work, a permanent connection of a high-nitrogen steel plate is achieved by using layer-by-layer powder-filled laser welding of high-nitrogen steel powder, and the stability levels of the welding process, microstructures and mechanical properties are analyzed. Compared with other processes, powder-filled laser welding is smooth without bursts and spatters, and the nitrogen content of the welding seam is obviously improved. The welding seam consists mainly of austenite, with few ferrite and manganese oxides. There is a nitrogen concentration gradient between the inside and the upper part of the melt pool, which leads to nitrogen escape. Laser remelting exacerbates this process, resulting in more severe nitrogen loss in the bottom layer than in the upper layer of the welding seam. The differences in the nitrogen solubilities in austenite and ferrite and the high-nitrogen content in the melt pool are the main reasons for the formation of micropores. Tensile experiments and impact experiments are conducted on the welding joint. The average tensile strength and elongation of the welding joint are 1032.36 MPa and 19.70%, respectively, contributing to 94% and 53.4% of the total values for the base metal. The grain refinement and relatively high nitrogen content are the reasons for the better tensile strength relative to other processes. The average impact energy of the welding joint is 24.68 J, which is approximately 63.74% of the base metal. Pores are the main reason for the reduced impact energy of the welding joint