Study On Key Technologies Of GMAW With Argon-nitrogen Mixture Shielding For Stainless Steel

In the deposited metal of austenitic stainless steel welded joints,maintaining an appropriate amount of ferrite plays an important role in its crack resistance and mechanical properties.However,the joint is very sensitive to the content of ferrite.If the content is high,the low-temperature toughness of the joint will be reduced,and too low content will increase the risk of welding hot crack.How to conveniently and accurately regulate the ferrite content of weld is a technical problem.Since nitrogen is a strong austenitizing element,this study is to achieve the purpose of flexibly adjusting the ferrite content of weld in gas metal arc welding（GMAW）of austenitic stainless steel by changing the nitrogen content in argon-nitrogen mixture welding shielding gas.The main research contents and results are as follows:Firstly,in order to effectively add nitrogen to the deposited metal,a coaxial gas mixer of welding shielding gas was designed,the coaxial gas mixing structure was simulated and the key parameters were optimized.The gas mixing process was simulated by using the hydrodynamics calculation software ANSYS fluent.The results showed that the optimized structure can achieve rapid,uniform and stable mixing of binary gas.The test results proved that the mixed gas of this mixer meets the test requirements and provides a basis for the follow-up tests.Secondly,the transition of nitrogen to deposited metal in GMAW process was analyzed by using orthogonal test method and Back Propagation neural network,and the relationship between nitrogen content in shielding gas,nitrogen content in deposited metal and Ferrite Number（FN）was established.Using the established artificial neural network,the required nitrogen addition can be given accurately under the requirements of given welding parameters and FN.Further research showed that the change of nitrogen content in weld metal will lead to a change of the solidification mode.For304L stainless steel,when the nitrogen content of weld metal is more than 0.15 wt.%,the solidification mode changes from FA mode to AF mode.The classical FN prediction phase diagram WRC-1992 can not accurately predict the FN of multilayer welding,and its prediction result is higher than the actual measurements.Finally,the mechanical properties and corrosion resistance of 304L stainless steel weld metal by multi-pass GMAW were tested and analyzed.The results showed that by adding 10.0 vol.%N₂ to the shielding gas,the FN of deposited metal can be effectively controlled between 3～4,and it is still with FA solidification mode.The yield strength of the joint increased by 14%and the elongation decreased slightly.The electrochemical test showed that the pitting corrosion resistance of the deposited metal with adding nitrogen of 10.0 vol.%is significantly improved,and the passivation potential range is nearly twice as high.