Research Of The High Nitrogen Stainless Steels By Fiber Laser Welding

Laser welding provides an unprecedented combination of excellent beam quality, high welding speed, narrow HAZ, little welding heat distortion and no vacuum. Possessing the advantage of flexible fiber-optic transmission, fiber laser welding is becoming a hotspot of research and applications. Usually, the high nitrogen stainless steels are fabricated by using the element N instead of the alloy element Ni in order to require the single-phase austenite of the base material in the room temperature or even low temperature so that it possesses high strength、good toughness and excellent resistance of corrosion. This paper was carried out basing on the 12mm high nitrogen stainless steels fiber laser welding in order to overcome the loss of nitrogen which occurs during arc welding as a big problem. Results are as following:By exploring the main process parameters that affecting the quality of HNS laser welding, laser power is proved to be the most important factor which determines the state of penetration welds. Welding speed is the second important factor. Welding tumor will occur on the bead surface as well as severe porosities in the bead. Defocus amount is the third key factor which influences the width of HAZ greatly. And the last one is the kind of shielding gas. When it comes to N2 as the shielding gas, the welding surface morphology of fish-scale pattern is obviously observed while the severe stoma is no longer saw. By controlling variables method, the optimal parameters are summarized as following:laser power 8kW, defocus amount +2mm, welding speed 0.84m/min and the protective gas is 20L/min of N2. In this case, the welding bead contains 0.556% nitrogen, reaching above 90% of the base metal. What’s more, the value can be improved up to 0.564% by leaning the laser. That is to say, the nitrogen loss is quite low in laser welding. However the increment is limited.The pores during the thick high nitrogen steel plate in fiber laser welding process are investigated according to the laser deep penetration welding mode as well as analyzing its generation mechanism, model characteristics and influencing factors. Under the premise of deep penetration hole welding, changing the inclination of the laser beam within the range of 10°～30° can effectively decrease the porosity forming probabilities. Welding was preferred to be carried out under the procedure of low laser power and slow welding speed.As a result, the micro-organization、phase components and its mechanical properties are accurately tested and analyzed by standard tensile test and Charpy impact test. The results showed that high nitrogen steel fiber laser welding seam is of single dendrite phase austenite. The grain grows perpendicularly to the weld center along the direction of maximum temperature with significant direction. The size of the bead grain is quite fine. There are substantially no precipitations of nitride in both the HAZ zone and the welding bead. The width of HAZ is as narrow as 50μm. The bead hardness is about 320HV decreasing compared with the base material while the hardness of HAZ is harder than the bead but slightly lower than of the base metal. Ductile fracture occurs in the welding joints and the tensile strength is about 1064MPa, slightly lower than that of the base material. The joint has a certain degree of toughness.