Study On Nitrogen Behavior And Properties Of Hybrid Welded Joints With High Nitrogen Steel

The wide application of high nitrogen steel depends on whether it has good weldability.During the atmospheric pressure welding process,changes in the nitrogen equilibrium solubility of the high-nitrogen steel welds cause problems such as porosity escape,reduction of the solid-solution nitrogen content,and precipitation of nitrides,resulting in degradation of joint performance.In view of the above problems,this paper carried out a laser-arc hybrid welding test on a 12 mm thick high nitrogen steel.The nitrogen behavior(nitrogen pores,nitrogen content and nitrides)and joints in high nitrogen steel joints were tested for different filler materials and welding heat input.The effects of microstructure and mechanical properties were studied.The test results show that there are process-type pores and precipitation-type pores in the weld of high nitrogen steel.Technological pores are irregular in shape and large in size,and are related to the instability of the keyhole,which reduces the cross-sectional area of the joint and leads to poor mechanical properties of the joint.The shape of the precipitation pores is elliptical and small in size,which is related to the change of the solubility of nitrogen during the solidification process,and has little effect on the mechanical properties of the joint.If the air bubbles fail to escape before the molten pool solidifies,pore defects will form.Both types of stomatal forces are related to the difference between the gas density G and the liquid metal density L and the bubble size.As the welding heat input increases,the viscosity coefficient CD of the molten pool decreases,and bubbles easily escape from the molten pool,which reduces the porosity of the weld.The nitrogen content of the weld is related to the transition of the nitrogen in the base metal,the precipitation,dissolution of the nitrogen in the welding wire,and the fusion ratio of the weld.The proper filling material can increase the equilibrium solubility of nitrogen.The formula for calculating the nitrogen content in the molten pool is based on the thermodynamics of nitrogen reaction.The measured values of nitrogen content in different positions of high nitrogen steel welds are basically the same as the calculated value of the empirical formula for the nitrogen content in the molten pool.The heat sensitive temperature of nitrides in high nitrogen steel welds is between 850 ～ 900 ℃.With the increase of the aging temperature of high-nitrogen steel welds,the grain boundary precipitates grow from granular to discontinuous cells,and the precipitate phase in the crystal cells continues to grow into long strips,eventually forming pearlite-like precipitates.Under the same welding heat input conditions,the nitrogen-containing welding wire has a higher nitrogen content and lower porosity than the stainless steel welding wire.The tensile stress of pores in stainless steel wire welds becomes the source of cracks,resulting in a lower tensile strength of stainless steel wire welds compared to nitrogen-containing wires.When the laser power is P = 3.25 k W,the tensile strength of the stainless steel welding wire is 867 MPa at the maximum,and when the laser power is P = 3.5k W,the tensile strength of the nitrogen-containing welding wire is 953 MPa at the maximum.The microstructure of high nitrogen steel welds has a dendritic structure and is inferior in toughness compared to the austenite structure such as the base metal.The impact fractures of the two types of welding wires are similar to the fracture fractures but the depth is significantly smaller.There are many sources of cracks in the tensile fracture of the nitrogencontaining welding wire,and the dimples are deep and uniform.When the laser power P = 3.5k W,the maximum impact absorption energy of the stainless steel wire welding joint is 181 J.When the laser power is P = 3k W,the impact absorption energy of the nitrogencontaining welding wire is 182 J.