Research On Characteristics And Quality Control Of Laser-arc Hybrid Welding Of High Nitrogen Steel

High nitrogen steel(N?0.5%wt)uses"nitrogen"instead of"nickel"and"carbon",adopts Cr-Mn-N composition system,has high strength,good plastic toughness,strong corrosion resistance,and high-speed impact resistance.High advantage is a new type of high strength and toughness stainless steel.In this paper,for the 8-20mm medium-thick high-nitrogen steel butt joints,Ar-O₂-N₂ ternary activated PMIG welding(N-MIG welding for short),keyhole laser welding and laser+(N-MIG)hybrid welding process research were carried out,which initially revealed high-nitrogen steel welding wire.The arc behavior and droplet transition characteristics,mastered the influence of shielding gas,process specifications,etc.on the weld formation,microstructure and mechanical properties of high nitrogen steel,forming a relatively reasonable high nitrogen steel welding process window.Using VEO410 high-speed camera and Smart laser backlight system,the arc behavior and droplet transition characteristics of N-MIG welding wire of high-nitrogen steel welding wire were studied.It was revealed that the droplet transition in the N-MIG welding process was an atypical blasting transition mode.The rapid escape of elements is the main source of the droplet blasting transition.The N-MIG welding test shows that during the welding process,the droplets show a blasting transition,the viscosity of the molten pool is high,and the stability is not high.Spatter cannot be completely avoided.The nitrogen content of the weld can be controlled at more than 0.5%.The appearance of the weld is formed in the weld.Nitrogen pores are distributed in fine and micro-distribution,the strength of the weld joint is 950?1050MPa;the keyhole laser welding test of high nitrogen steel shows that the welding process is stable and less spatter,the loss of nitrogen content in the weld can be controlled within 0.15%,in the weld The pores are also distributed in fine and micro distribution,and the strength of the weld joint is 900?1000MPa.In this paper,a large number of laser+(N-MIG)hybrid welding process tests were carried out for 8?20mm thick high nitrogen steel,and the process parameters such as laser power,defocusing amount,welding speed,and optical fiber spacing,etc.,on weld formation,welding quality,The influence of microstructure and mechanical properties was investigated on the pore behavior and control model of laser+(N-MIG)hybrid welding of medium-thick plate high-nitrogen steel.For the typical thickness of high-nitrogen steel laser+(N-MIG)hybrid welded joints,a more in-depth study of microstructure and mechanical properties was carried out The results show that,under the guarantee of the penetration process specification,the lateral input of Ar-O₂-N₂ shielding gas,the use of smaller laser power and lower welding speed are effective technical means to reduce the pores in the weld of high nitrogen steel.On this basis,the best process parameters were obtained.Taking 12mm thick high-nitrogen steel butt joint as an example:laser power is 8000W,current is 220A,welding speed is 14mm/s,defocus amount is-2mm,filament spacing is 3mm,laser lateral protection is Gas 88%Ar+2%O₂+10%N₂,flow rate is 20L/min,N-MIG protective gas is 93%Ar+2%O₂+5%N₂.The high-nitrogen steel laser-N-MIG hybrid welding head is composed of a weld zone,a heat affected zone and a base metal zone.The weld is divided into two parts,the arc zone and the laser zone.The structure of the weld zone is dendrite,and the nitrogen content has a significant effect on the dendrite spacing and grain size.The XRD test results of the weld zone of joints with different nitrogen contents show that the joint phases are all austenite and carbide.The hardness value of the base material area is the highest,followed by the heat affected area,and the weld area is the lowest.The tensile strength of welded joints with different nitrogen content is different,the highest can reach 986.8MPa,which is 94%of the base material,and the lowest is 929.9MPa.Tensile fractures and impact fractures are distributed with equiaxed dimples,which are ductile fractures.