Research On Microstructure And Performance Of High Nitrogen Steel Flash Butt Welding Joint

Steel with nitrogen content of more than 0.4%in austenitic Matrix are called high nitrogen steel（high nitrogen austenitic stainless steel）.Nitrogen element can increase the effect of solution strengthening so that the toughness,strength,strain hardening ability and corrosion resistance ability of the Material are significantly improved.The nitrogen element is pollution-free,low price,so high nitrogen steel has advantages of economic and environmental protection.However,whether steel can be widely used or not also depends on whether it has good weldability.The results show that the phenomenon of nitrogen loss is easy to happen in loss welding seam and heat-affected zone,which reduces the mechanical properties and corrosion resistance of the joint.Comprehensive domestic and international research status of high nitrogen steel welding,at present,we did not see the high nitrogen steel flash butt joint research.So in this article we used high nitrogen steel as test Materials to design and develop flash butt welding tools and carry out welding process and microstructure research to achieve the control of the microstructure and mechanical properties of high-nitrogen steel flash butt welding joints,as well as explore its corrosion mechanism,and finally to promote the application of this technology.First,the thermodynamic calculation software JMat Pro was used to calculate the equilibrium phase diagram of high-nitrogen steel,the incubation period of the precipitated phase,and the most sensitive precipitated temperature to reduce or avoid the precipitation of impurity phases in the joint.The results showed that the content of the impurity phase is least in the equilibrium microstructure at the temperature range of 960℃～1010℃,which contained an austenite phase and a little MNS phase.The sensitive temperature range of the second phase precipitates is 670℃～880℃.The finite element simulation of the flash welding process of high nitrogen steel showed that the weld temperature distribution state,maximum weld temperature is 1383℃,the plastic strain is mainly concentrated in the center of the interface zone,and the maximum plastic deformation value reaches 0.7289.The flash butt welding of high-nitrogen steel with different welding process parameters was completed using a self-created copper electrode fixture.The welded joints were divided into interface zone,overheated zone and heat-affected zone.We used a numerical simulation temperature field to analyze the phase composition of each region of the joint.The changes in microstructure and mechanical properties of the joints with different technological parameters were analyzed.The experimental results showed that the Thyristor angle（flash current）is 45°,the CAM rotation angle（flash allowance）is 100°,and the maximum forging pressure is 0.4 MPa.The dynamic recrystallization grains in the interface area were uniform and fine.The microhardness and tensile properties of the joint of high nitrogen steel were tested.The results showed that the joint hardness presents a"V"shape distribution along the weld,and the maximum tensile strength of the joint sample is 902 MPa,which reaches 86.4%of the base metal.After optimizing the welding process parameters,we obtained the optimal parameters:under the conditions of the Thyristor angle is 48.39°,the CAM rotation angle is 96.21°,the top forging pressure is 0.48 MPa we obtained the best tensile strength of the joint 923 MPa.The fracture morphology of the optimal parameter tensile specimen shows that the joint tensile specimen has a ductile fracture with noticeable dimples on the surface,and the joint has good plasticity and toughness.The optimal parameters of the joint were etched in 10%H₂SO₄ solutione for 1 h at room temperature.The corrosion rate reached 0.0208 g/cm²,which was much higher than that of the base metal.Based on in situ monitoring of electrochemical noise and corrosion kinetics analysis,the corrosion rate of high nitrogen steel joints is greatly accelerated by the internal lattice defects.