| Since it was developed in the early 20 th century,stainless steel,as a new functional material,has played an indispensable role in modern industrial and technological advancement with its unique corrosion resistance.Among them,austenitic stainless steels are extraordinarily excellent in corrosion resistance,heat resistance and biocompatibility,combined with good comprehensive mechanical properties,processing properties and welding properties.And thus they are widely used in such many industries as atmospheric vessels like wine cans,fermentation equipment,petrochemicals,ships,medical equipment,bioengineering as well as food machinery.However,SUS304 stainless steels(equivalent to Chinese 06Cr18Ni9 or American ASTM304)as one of the austenitic stainless steels take on good welding performance.But both the small thermal conductivity and large coefficient of linear expansion cause residual stress during welding.And subsequently there are such appearances as corrosion,heat cracking and embrittlement in the joints.At present,pressure vessels are increasingly lightweight,which makes the applications of stainless steel sheets common.Therefore,it is of great economic significance to study the weldability and defect mechanism of SUS304 stainless steel sheet and optimize the existing process for the development of austenitic stainless steel weldments.SUS304 stainless steel is the most widely used austenitic stainless steel.In many cases,its application requires the use of an important processing method-welding.Compared with the traditional welding method,TIG welding has the advantages of low heat input,high energy density,small heating range,uniform heat input,small welding deformation,etc.,which can better control the line energy;on the other hand,its protective airflow has The cooling effect can reduce the surface temperature of the molten pool and increase the surface tension of the molten pool,so that high-quality SUS304 stainless steel welded joints can be obtained.Since the joint quality is closely related to the microstructure performance,it is necessary to study the morphology and microstructure of the SUS304 stainless steel TIG welded joint.In this paper,TIG welding of SUS304 stainless steel sheets was carried out by applying wire ER308 L and without adding fillers,in order to explore suitable welding process parameters to obtain satisfactory welded joints.On this basis,the microstructure of SUS304 stainless steel welded joints was observed by OM and SEM.The morphology changes of austenite structure were analyzed in detail.The mechanical properties(microhardness,weld hardness)of welded joints obtained under two welding methods were tested.Tensile,curved)and corrosion resistance(pitting),and mechanism analysis and evaluation of the occurrence and possible occurrence of welding defects at various stages.The research results are as follows:(1)The optimal parameters of TIG welding are obtained through multiple optimization tests: welding voltage 18~25V,welding current(DC positive connection)130~140A,base current 75~80A,welding speed 1.83~2.00mm/s,The argon flow rate was 15 L/min.Based on this,TIG welding can realize the welding of 3mm thick SUS304 stainless steel sheet,which can be formed without beveling and double-sided welding.At the macro level,the welding surface is smooth and flat,free from burrs,splashes and other defects;the weld is well fused,without defects such as porosity,collapse,undercut,etc.,so the quality of the welded joint obtained is good.(2)Through the observation of metallographic microstructure,the analysis results show that when the ER308 L austenitic stainless steel welding wire is used as the filling material,since the chemical composition is basically the same as that of the parent material SUS304 stainless steel,the microstructure of the weld is dense,and the austenite is dense.The body is composed of a small amount of worm-like ferrite,and there is a significant difference in the morphology and content of the microstructure in different areas of the weld;when the butt welding is directly carried out without any filler,the obtained weld is also made of austenite + iron.The composition of the body structure,but in the weld fusion zone equiaxed crystal(weld center)and columnar crystal(interface)distribution is obvious.(3)Through the microhardness test,tensile and bending performance test,the mechanical properties of the welded joints were comprehensively analyzed and found that the microhardness distribution of different joints is consistent with the microstructure distribution,when ER308 L austenitic stainless steel is used.When the welding wire is used as the filling material,the microhardness of the obtained weld is greatly changed due to the occurrence of worm-like δ-ferrite,so the microhardness of the obtained weld is large,and the highest microhardness value appears at the center of the weld,which is 392 HV.,obviously higher than the base metal;for the weldment formed by the weld self-melting,although the microhardness of the joint is also improved,the maximum hardness 368 HV is not in the center of the weld,but appears in the interface area at the edge of the weld,which it is also consistent with the fine equiaxed crystal results of the interface.The microhardness of the heat affected zone on both sides is lower than that of the base metal,but it is not greatly reduced.Compared with welded joints without filler material,the average yield strength,tensile strength and elongation of ER308 L welded joints increased by 71.30%,16.67% and 2.33%,respectively.The fracture occurred in the far away from the weld.The affected base metal area,and the fracture of the self-welding weldment occurs in the fusion zone,but the tensile fracture scan of the two weldments shows a typical plastic fracture of the dimple.The macroscopic test results of the positive and negative bends(approximate U-shape)of the transverse joint of the welded joint did not show cracks or the defect size was larger than 1.5 mm.According to the relevant evaluation criteria,the quality of the two joints obtained was qualified.(4)Evaluating the pitting corrosion resistance of SUS304 stainless steel and the obtained TIG welding joint transition layer weld in 3.5% Na Cl,10% oxalic acid and 10% Na2CO3 solution by electrochemical test method.After comprehensive analysis,the results show that at 3.5 Among the %Na Cl and 10% oxalic acid,the base metal has the strongest electrochemical corrosion resistance,and the self-welding welded joint is the worst;in the 10% Na2CO3 solution,the corrosion resistance of the base metal and the welded joint filled with ER308 L has no obvious discrimination.,but both are superior to self-fusion welded joints.In summary,the high-quality welding of 3mm SUS304 stainless steel sheet can be achieved under the conditions of TIG wwelding process,rational processing parameters and φ1.6mm ER308 Lwire. |
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