Investigation Of Laser-MAG Hybrid Welding Of High Nitrogen Steel And Nitrogen Enrichment In The Weld

High nitrogen steels have high mechanical strength,good impact toughness and corrosion resistance.Replacing nickel with nitrogen in steels also allows economical production and good biological compatibility.They have wide application in armors,transportation pipelines,and medical devices.However,due to the high nitrogen feature,they are inferior during the welding process,causing nitrogen loss,air bubble formation,and nitride precipitation.This work investigated the laser-MAG hybrid welding technique based on powder filling and its application on high nitrogen steel welding.We analyzed the transition stability of molten droplet,dissolving and precipitating process of nitrogen,structural transformation of the weld and the improvement of mechanical performance.We conducted laser-MAG hybrid welding of high nitrogen steels and achieved relatively good surface morphology and welding stability with little splash.However,the nitrogen content in the welds was only 73% of the original steels.Welding speed is sensitive to the bubble formation in nitrogen steel.The defocus amount in laser also has impact on welding stability,and high-quality welding was achieved at zero defocus.Additionally,it is important to control the welding current and the distance between welding wire and laser due to the combined effect of arc and laser.Manganese nitride was used in our experiments to increase nitrogen concentration.Powder feeding angle mainly influenced welding process stability.Only feeding below welding torch could sustain stable welding process.The more powder feeded,the lower welding quality we achieved.But nitrogen content increased at beginning and reached plateau afterwards.It shows that highest nitrogen content could reach 0.55%(~84.6% in terms of original material).Nitrogen primarily existed in the form of solid solution,and a few in the form of nitrogen bubble.There were no obvious precipitation in the weld.During the process,manganese nitride dissolves completely and therefore enter molten pool through droplet surface,resulting higher possibility of nitrogen bubble precipitation,but overall nitrogen solid solution tends to increase in the weld.As increasing welding speed,air bubbles tend to decrease,and nitrogen bubbles tend to increase.In general,the amount of bubbles in the welding seam increased,but overall size decreased.The weld mainly contains austenite and ?-ferrite.As nitrogen content increased,?-ferrite decreased.Austenite body contains few Ti N particle phase.Higher nitrogen content was observed at the interface between austenite and ?-ferrite.There were also large amount of microscopic bubbles.the mechanical strength of the weld is significantly improved using this technique,but observed non-uniformity along thickness orientation.Tensile strength of the weld connection is 1072 MPa,(14% increase),and reached 73% of the original material.Failure analysis shows ductile fracture behavior.Impact toughness only shows mild increase.Fractures of the weld exhibit mixed features of quasi-cleavage fracture and dimple fracture.