Study On The Gtaw Welding Process Of 304L Stainless Steel With Ar-N₂ Mixed Gas

304L austenitic stainless steels are widely used in the liquefied natural gas corridor,petrochemical and shipbuilding industries due to its good corrosion resistance,low-temperature property and weldability etc.Especially in recent years,large amounts of 304L stainless steel are applied in the construction of related devices for transporting and storing liquefied natural gas with the dramatically increasing demands for liquefied natural gas.Nitrogen element as austenite stabilizer can substitute for nickel element to expand austenite phase,and reduce ferrite content of weld metal in the reasonable limit to control the corrosion resistance and low-temperature property of weld joint.Therefore,mixed gas precision proportioning system was developed in this paper,304L austenitic stainless steels were welded under different N₂ concentrations,weld joints were analyzed by feritscope,optical emission spectrometer,microscopy,scanning electron microscope and micro-hardness tester,and the effects of nitrogen addition in shielding gases on the nitrogen content,solidification mode,ferrite number,microstructure morphologies and micro-hardness distribution of 304L weld metals that were produced by GTAW welding were studied to find a suitable nitrogen-adding process,this is advantageous to provide theoretical support for low cost welding process.The main research conclusions of this paper can be drawn as follows:?1?Self-developed high precision proportioning system is easy to use,and has the high accuracy and function of adjusting dynamically.The gas flow of precision proportioning system is stable and fluctuation range is small in the welding process,which indicates that the system is suitable for the actual construction requirements.?2?The differences between practical FN and FN predicted by Delong and WRC-1992 diagram are larger in the surface overlay cladding.Besides,the solidification mode,ferrite content and morphologies of surfacing layers are not only associated with the nitrogen concentration in the mixed gas,but also affected by arc length.?3?The solidification mode and ferrite number of 304L stainless steel weld metals are not consistent with the predicted solidification mode and ferrite number by WRC-1992 diagram in the multi-pass welding,and the deviation is gradually enlarged with the increasing nitrogen concentrations in the Ar-N₂ mixed gas.In addition,the solidification mode and ferrite number of different parts of weld joint are also different,which induces that ferrite morphologies and distributions of micro-hardness show a big difference in the different parts of weld joint.?4?It's possible to control the microstructure and property of weld metal during multi-pass welding process,when the nitrogen concentration in the shielding gas is adjusted dynamically by dynamic precision gas proportioning technology,whereas traditional method increasing the nitrogen content by welding consumables doesn't have the function of real-time adjustment of N₂ content.