Research On Process And Mechanism Of Wet Welding Of 304L Stainless Steel And 16Mn Steel

When 304 L stainless steel and 16 Mn steel were welded by underwater wet welding, due to the lower thermal diffusivity of 16 Mn steel and the higher cooling rate of water, the joint of 16 Mn steel side was melted insufficiently. So it was difficult to choose the optimal welding process parameters; Because of the different chemical component of 304 L and 16 Mn base materials, the microstructure of 304L/16 Mn dissimilar steel joints was complex. According to the problems, the special self-shielded flux-cored wire and the reasonable welding process were used to obtain a sound 304L/16 Mn joint in the present work. At the same time, the influences of welding environment and different flux-cored wire on the microstructure and mechanical properties of 304L/16 Mn dissimilar steel joints were mainly investigated in this work.In order to clarify the effects of different welding environment on the 304L/16 Mn dissimilar steel joints, the 304L/16 Mn dissimilar steel joints welded underwater and in air with ER308 flux-cored wire were compared. Based on the observation of microstructure, it can be concluded that for the 304L/16 Mn joints welded in air, in the HAZ of the 16 Mn base metal, microstructure of the overheated zone was widmannstatten structure, microstructure of the recrystallization phase transition zone was fine grain ferrite and pearlite and that of the incompleted recrystallization zone was pearlite and ferrites with different size. And a sound weld can be obtained in air without defect. For the fusion zone of 304 L stainless steel, the direction of interactive crystallization of the joints was not obvious and a large amount of equiaxed grains was formed. While for the 304L/16 Mn joints welded underwater, in HAZ of the 16 Mn base metal, microstructure of the overheated zone was coarse martensite, microstructure of the recrystallization phase transition zone was fine martensite and a small amount of granular bainite and that of the incompleted recrystallization zone was finer martensite,a large amount of granular bainite and ferrite with different sizes. The zonal segregation and the intergranular crack were observed in the weld metal. For the fusion zone of 304 L stainless steel, the direction of interactive crystallization of the joints was obvious and only a small amount of equiaxed grains were formed. The tensile strength of the joints welded underwater was 165 MPa, which was lower than that of the jointswelded in air. While the average hardness of the joints was 237 HV, which was higher than that of the joints welded in air.The self-shielded flux-cored wire was selected to weld 304 L stainless steel and16 Mn steel underwater. Then, the comparation was made between the kinds of joints welded underwater. Firstly, under the special boundary conditions, the starting position was calculated to be 0.11 mm next to 16 Mn base metal through the one-dimension unsteady heat conduction differential equation. And the accuracy of the optimal starting position was verified by experiments, then the best process parameter was determined. It can be issued that for the joints welded with self-shielded flux-cored wire, in overheated zone, the microstructure was coarse martensite and granular bainite;in recrystallization phase transition zone, the microstructure was a large amount of granular bainite and in incompleted recrystallization zone, the microstructure was granular bainite and ferrite with different sizes. The defect-free weld metal was obtained. Compared with the joints welded with ER308 flux-cored wire underwater,the mechanical properties have been improved. The tensile strength of the joints increased to 400 MPa and the average hardness of the joints decreased to 192 HV.