| Austenitic stainless steel is an important corrosion-resistant material,which is widely used in a variety of corrosive environment.Normally,this type of stainless steel has a little ferrite which can improve the thermal cracking resistance and corrosion resistance of steel.The best range of ferrite content for austenitic stainless steel should be less than 3%.The ferrite content of austenitic stainless steel weld metal obtained by traditional welding method can reach 5-7%.The heat input of laser welding process is higher,the cooling rate is faster,therefore,the ferrite content in laser welding joint is higher,which is detrimental to the corrosion resistance of the joint.In this paper,the method of increasing the nitrogen content in the weld metal by side assist gas was realized to control the ferrite content in the joint.A model of side assist gas and metallic vapor was established for austenitic stainless steel high power fiber laser welding.The influence of side assist gas type and flow rate was analyzed both on the metallic vapor temperature field and velocity vector field,as well as the microstructure and properties of the laser welding of the austenitic stainless steel by different experiments.First,the temperature field and velocity vector field in the welding process were analyzed by numerical simulation.The results showed that the size and height of the metallic vapor decreased as the side assist gas flow rate increased with the two gases,nitrogen and argon.However,the metallic vapor size outside of the key hole was slightly larger when using argon as shielding gas.This indicated that nitrogen had a strong ability to suppress metallic vapor.Also the weld forming and microstructure were tested by metallographic observation and element analysis.The results showed that with these two gases,nitrogen and argon,the welding joints were all “nail-shaped”,the change of penetration and melting width was not obvious.The type of side assist gas did not affect the interaction process between fiber laser and material.At the same time,nitrogen gas could significantly increase the content of nitrogen in the weld,the increase of nitrogen content could effectively promote the transformation of ferrite to austenite and reduce the ferrite content in the welding joint.In addition,nitrogen could also reduce the number of pores in the weld greatly.The mechanical properties and corrosion resistance of austenitic stainless steel fiber laser welding joints were tested.The results showed that the tensile strength of the welding joints was higher than that of the base metal when nitrogen and argon were used as shielding gases,while the micro-hardness of the weld zone was larger when using nitrogen.At the same time,the self-corrosion potential of these two kinds of welds was similar to that of the base metal.The pitting potential of argon-protected welds was closed to that of the base metal,while the pitting potential was slightly increased in the case of nitrogen protection.That is,nitrogen can improve the pitting resistance.In a word,the nitrogen content in the weld could be effectively increased by using nitrogen as side assist gas,so as to reduce the ferrite content.At the same time,nitrogen could also enhance the hardness and pitting corrosion resistance of the welds. |
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