Microstructures And Low Temperature Tensile Properties Of Friction Stir Welding Joints Of Nitrogen Strengthening High Manganese Steel

In this study,we chose nitrogen strengthening high manganese steel as research object and obtained a defect-free welded joint using friction stir welding(FSW).The microstructures and the mechanical performances at cryogenic temperature were systemically investigated for friction stir welding joints of nitrogen strengthening high manganese austenitic steel.By post-weld heat treatment,the low temperature mechanical properties of the joints were improved.Furthermore,the mechanism of grain abnormal growth in the heat affected zone(HAZ)was thoroughly analyzed.The study on the structure and properties of welded joints showed that the average nitrogen-content of the FSW joint was very close to that of base material(BM),almost no loss.A small amount of the sigma phase was found in the banded structure area of the nugget zone(NZ)of the joint,the rest microstructures in the joint were austenitic phases.The NZ exhibited fine grains due to the dynamic recrystallization occurred during FSW.Consequently,the inevitable differences in the microstructural distribution resulted along the transversal direction of the FSW joints.The joint hardness was obviously higher than that of the BM.At low temperatures,the joint fractured in the base metal zone and was characterized by the cryogenic intergranular fracture,and there was no obvious yield phenomenon,which showed that the tensile strength was 862 MPa.In order to improve the low temperature mechanical properties of the joint,the heat treatment was adopted after welding.The results showed that after water quenching treatment,the differences in the structure and hardness of the FSW joints were basically eliminated.However,the problems of cryogenic intergranular fracture in the joints with water quenching treatment also were found,and the fracture was located in the heat-affected zone of the joint,the yield phenomenon was not obvious,and the tensile strength decreased,which was 794 MPa.Compared with the as-welded joints,no obvious change occurred in the annealed joint structure,and it was still distributed in a gradient.The hardness of heat affected area decreased,and the hardness of other areas was not significantly changed.The low temperature mechanical properties of the annealing joints was obviously improved,The fracture was located in the banded structure area,The annealing joint shown obvious yield phenomenon and was characterized by low temperature ductile fracture,and the yield strength was 1080.9MPa,the tensile strength of the annealing joint was 1360 MPa.In addition,the elongation of the annealing joint was 15%.After natural aging treatment,the joint hardness was homogenized,and the hardness of the base material and the heat affected zone was increased to the level of the nugget zone.Low temperature tensile properties of natural aging joints were similar to annealed joint.And fracture position was also located in the area of banded structure,apparent yield phenomenon,yield strength was 1142 MPa,tensile strength was 1350 MPa,elongation was 13%,low temperature tensile performance for ductile fracture characteristics.As a result,the cryogenic intergranular fracture was effectively restricted by low temperature annealing and natural aging,and the overall mechanical properties of the FSW joints were significantly improved.The investigations on the abnormal growth of grain in the heat affected zone revealed that this problem may relate to the non-equilibrium segregation of the solute atoms at grain boundaries.Because of heat cycle and restraint stress in the welding process,the function of solute atoms at the grain boundary of partial poly state changed in heat affected zone,which affected the grain boundary mobility,appeared abnormal grain growth phenomenon.