Study On Microstructure And Mechanical Properties Of S30408 Steel Strain Strengthening Joint By PAW+TIG Combined Welding

Strain strengthening of austenitic stainless steel is one of the methods for the lightweight of pressure vessels,which can reduce the wall thickness of pressure vessels and achieve the balance of economy and safety under the condition of guaranteeing the mechanical properties of pressure vessels.At present,there are many researches on strain strengthening of austenitic stainless steel,but as the weak area of pressure vessels,there is a lack of research on the properties of strain strengthening joints.Therefore,it is of great significance to study the microstructure and properties of strain strengthened joints for evaluating the safety of S30408 austenitic stainless steel cryogenic vessels.In this paper,the welding test of thickness of 10mm S30408 steel is carried out plasma arc and tungsten argon arc welding(PAW+TIG),and the strain-strength test is applied to the joint.Research techniques including optical microscope(OM)?scanning electron microscope(SEM)and X-ray spectrometer(EDS)were used to study the joint microstructure before and after the strain strengthening,and the properties of the joints before and after the strengthening were tested by micro-hardness tester and universal tensile tester.By comparing and analyzing the change of microstructure of joint before and after strain strengthening,the microstructure evolution rule of strain strengthening joint of austenitic stainless steel was explored.The test results of PAW+TIG combined welding show that the increase of welding heat input(from 22.5kJ/cm to 25.7kJ/cm)makes the weld microstructure of the plasma arc welding bead become coarse,and the elements Fe,Cr,Ni present an upward trend from the weld to the heat affected zone,but the distribution of Fe,Cr,Ni and other elements is more steep under the large welding heat input.With the increase of welding heat input,the distribution of 8 ferrite in the TIG welding seam and fusion zone is more uniform,and the elements such as Fe,Cr and Ni fluctuate less from the weld to the heat affected zone.The tensile strength of the PAW+TIG combined welding joint reaches 761 MPa,and the distribution of microhardness on the tungsten argon arc welding joint increases from the center of the weld to the base metal,and the microhardness distribution on the side of the plasma arc welding is characterized by a drop from the center of the weld to the base metal.After the strain strengthening of PAW+TIG combination welding joint,the strain induced martensite transformation occurred in various areas of the joint(especially the welding seam and heat affected area),but there were great differences due to the different composition of each area.The microstructure of the weld zone before strain strengthening is mainly composed of austenite and 8 ferrite(worm-like,skeleton,etc.).After strain strengthening,the grains in the weld zone tend to be elongated along the stretching direction.It was observed that martensite with parallel cross-arrangement was formed in the austenite grains,but this transformation was observed only in the local region.The EDS point composition test results showed that the content of main elements of deformation induced martensite phase and austenite phase is consistent.The martensitic transformation is non-diffusion phase transition.The fusion zone is composed of unmixed zone and semi-melting zone,and also has a small amount of deformed martensite formed after strain strengthening,and is mainly distributed in the near of the edge of the semi-melting zone.The microstructure of the heat-affected zone is composed of austenite and a small amount of ? ferrite.After strain-strengthening,some grains in the heat-affected zone are refined,and parallel or parallel-cross arranged martensite can be observed on the austenite matrix,a portion of the austenite grains contain a plurality of lath bundles,and the martensite lath bundle has obvious of floating on the surface.By testing the micro-hardness of the weld,the fusion zone and the heat-affected zone under strain strengthening,and comparing it with the micro-hardness of the corresponding zone of the joint without strain strengthening,it is found that the micro-hardness of the joint increases with strain strengthening,but the degree of increase in micro-hardness is different in each area of joint.The improvement of micro-hardness of strain strengthened joints is affected by two factors,on the one hand,is a joint in the process of our pre-stretching strain induced martensite,on the other hand is the increase of dislocation density in the process of stretching and dislocation plug product hindered the dislocation glide,will also increase the strength of stainless steel.The increase in the microhardness of the weld zone is small,which is related to the lower strain of the weld zone and the small amount of martensite produced during the pre-stretching process.The increase of the fusion zone is close to that of the weld zone,and the microhardness of the heat-affected zone increases the most.The heat-affected zone has less 8 ferrite than the weld and fusion zone,and is composed of a large amount of austenite,which is more likely to induce martensite phase.Moreover,the stain generated by the heat-affected zone is large,the dislocation density increases after stretching,dislocation plugging and delivery occur,and plastic deformation is hindered,and the combined effect of the two increases the microhardness of the heat-affected zone.