Research Of The High Nitrogen Stainless Steels By Fiber Laser-MIG Hybrid Welding

The high nitrogen stainless steels use nitrogen instead of nickel and have the characteristics of high strength, good toughness and excellent resistance of corrosion. However, the weld porosity, nitrogen loss and other problems often occur in the fusion welding of the high nitrogen stainless steels. For the technical difficulty, the paper used 307 Mo wire and adopted the process of zero clearance butt welding for the 7mm and 12mm high nitrogen stainless steels fiber laser-MIG hybrid welding, at the same time explored the influence of laser-MIG hybrid welding process parameters on the quality of the weld appearance molding, and the micro-organization phase components and its mechanical properties were accurately tested and analyzed by standard tensile test and Charpy impact test.Results are as following:Since the high nitrogen stainless steel is a new kind of materials, the available data for reference is little. Therefore, the project obtained the system of key parameters by a series of basic technology experiments, which provides more basic data to solve the problem of weld porosity and nitrogen loss. The regular pattern of laser-MIG hybrid welding various process parameters on the quality of the weld surface formed was summarized as follows: laser power is the most important factor, and with the increasing of the laser power, the weld penetration increased, substantially linear relationship. The second factor is welding speed. For single MIG welding, when the welding speed reaches 0.03m/s, undercut and spatter are serious, and humping bead appears.However, under complex conditions, when the speed reaches 0.045m/s, the weld still has a certain penetration and the characteristic of fish-scale pattern. The third factor is defocus amount, which determines the width of HAZ. The forth factor is welding current. When welding current is in the range of 60-135A,with its increasing, the weld penetration increased, but the law is the opposite when welding current more than 135A. The last factor is DLA. Pores were found near surface of weld when DLA is 2mm. But when DLA is greater than 3mm, the weld appearance performed no change.This article explores the formation mechanism and influencing factors of air holes in the 7mm thickness high nitrogen stainless steels fiber laser-MIG composite welding joint. In the case of welding penetration, selecting smaller laser power and welding speed and using Ar as the protecting gas, which can effectively reduce the internal porosity of joints. The optimal parameters obtained on the basis of following conditions:lase power is 2000w, welding current is 120A, welding speed is 0.01m/s, defouse amount is Omm, DLA is 3mm, the protective gas is 25L/min of Ar.The high nitrogen stainless steels fiber laser-MIG composite welding joint consists of base metal, HAZ and weld bead. The weld bead consists of two parts, which are sector zone and straight seam zone.The microstructure of the weld zone is dendnte.When the process parameters varies, dendrite spacing and grain size are obviously different. Then XRD was detected in the weld zone under different process parameters, and the phase of the joint were found to be austenite and carbide. The hardness of the base metal is the highest, and the second is the HAZ, and the hardness of the weld zone is the lowest.The maximum tensile strength of welded joint is up to 1049MPa, which is 95.4% of base metal, and the lowest is 918.8MPa. The tensile and impact fracture surfaces are distributed with equal axis and ductile fracture, which shows that the joint is ductile fracture.