Study On Welding Process And Post Weld Heat Treatment Of 400 Series Stainless Steel Plate

400 series stainless steel is widely used in daily life and industrial production,which is different from austenitic stainless steel easy to be welded.A large number of ferrite in 400 series stainless steel will inevitably change in structure and performance under the high temperature heat cycle of welding,which leads to the decline of mechanical properties at the welding joint,which makes it limited in application.MAG welding and Laser+MAG combined heat source welding have the advantages of good quality,stable welding process and high production efficiency.Therefore,the above methods are used to weld 400 series stainless steel.However,due to the different characteristics of the weldment and the fluctuation of welding parameters,the quality defects of the welding joints will occur.Generally,reasonable post welding heat treatment is required on the basis of mastering the best welding parameters technology to obtain high performance welding seams and achieve good material welding.In this paper,MAG welding and laser+MAG composite heat source welding are used to weld 5.0mm thick SUS430 stainless steel.The influence of welding parameters on the weld forming is studied,and the optimal parameters are obtained.The parameters are applied to the welding of 410L and 50Cr15MoV stainless steel,and the difference of weld forming of three steels under the same welding parameters is compared.Then,according to the thermal expansion curve results,the corresponding post-welding heat treatment temperature was designed.By means of Metallographic observation,EBSD,hardness and tensile test,the weldability of the three stainless steels under two different welding methods and the mechanical properties of the welded joints were studied.The main research results are as follows:It can be found from the measurement results of weld width and residual height:the increase of welding voltage in MAG welding will lead to the increase of arc power,increase of arc length,and arc distribution in umbrella shape,the heat source radius increases and the weld width increases;The increase of wire feeding speed will lead to the increase of welding current,and the width and residual height of the weld will not change much,but due to melting There are too many welding wires,resulting in reflow phenomenon,which leads to a large change in the overall width of the weld,and a large radius of curvature in the fusion line;With the increase of welding speed,the weld width and residual height both show a decreasing trend.Finally,the optimal parameters of MAG welding 5.0mm thick stainless steel strip are welding voltage 26.5V,wire feeding speed 12.5m/min,welding speed 0.65m/min.Because the weld formation is affected by the difference of physical properties between base metal and welding wire,the weld formed by three kinds of steel under the same welding parameters is also different.In laser+MAG welding,the laser power is mainly used to melt the base metal and fill the welding wire.Small laser power can not get enough weld width,which leads to less filling of welding wire.With the increase of welding speed,the weld width,laser penetration and arc penetration will decrease.Theoretically,when the value of positive and negative defocusing is the same,the power of workpiece surface is the same,but negative defocusing is more suitable for the base metal At the same time,the smaller the defocusing value is,the greater the arc penetration is,and the wire filling is sufficient.Finally,the optimal parameters of Laser+MAG hybrid heat source welding for 5.0mm thick strip steel are 6.7kW laser power,2.0m/min welding speed and -5mm defocus.The laser penetration part of the weld formed by this welding method is obtained by the direct melting of the base metal,which is less affected by the difference of physical properties between the base metal and the welding wire.The weld formed by the three kinds of steel under the same welding parameters has little difference.The phase transformation temperatures of SUS430,410L and 50Cr15MoV were measured by thermal expansion method.According to the volume change diagram,it was found that the phase transformation temperature of SUS430 could not be measured by thermal expansion method.The austenite phase transformation temperature of SUS430 was proved to be 926?by metallographic method.The Ac₁ and Ac₃points of 410L and 50Cr15MoV are 845?and 910?,M_sand M_fpoints are 430?and 350?,respectively;the Ac₁and Ac₃points of 50Cr15MoV are 815?and 890?,M_sand M_fpoints are 335? and 120?,respectively.Five groups of post weld heat treatment temperatures were designed for the welded joints of three kinds of strip steel.Metallographic results show that with the increase of heat treatment temperature,the ferrite grains in heat affected zone grow up gradually and the content of martensite decreases;when the heat treatment temperature exceeds the transformation point,the content of martensite increases and the ferrite grains no longer grow up.Through EBSD results analysis,it is found that when the ferrite grains undergo austenite transformation under the influence of welding heat,the austenite structure will not be retained,and the transformed retained austenite will shear to form martensite with the decrease of temperature.The results of mechanical properties test show that with the increase of heat treatment temperature,the elongation of the welded joint and the hardness near the fusion line of the three steels all show a trend of first increase and then decrease.Among them,the yield strength and tensile strength of SUS430 and 410L welded joints also decrease first and then increase with the increase of heat treatment temperature,but the yield strength and tensile strength of 50Cr15MoV welded joints are just opposite to the former two.This is because the mechanical properties of the welded joints are far superior to that of the base metal.In the mechanical experiments,the fracture location is often in the base metal,while the martensite in the base metal After heat treatment,the bulk structure has a certain aging strengthening,which leads to the change of its strength.The results show that the SUS430 welded joint has the lowest strength and the best plasticity after holding at 900?for 10min.The results show that the410L welded joint after air cooling at 820?for 10min has the lowest strength and the best plasticity.The results show that the strength of 50Cr15MoV welded joint is the lowest and the plasticity is the best after air cooling at 790?for 10min.