Study On Microstructure Performance And Strain Strengthening Characteristic Of PAW+GTAW Combined Welding Joints For 304 SS

Austenitic stainless steel is widely used in pipeline of petrochemical,nuclear construction,shipbuilding and mechanical industries due to its excellent corrosion resistance,low temperature toughness,and so on.But thermal conductivity of austenitic stainless steel is smaller and coefficient of linear expansion is bigger,which caused residual stress by general arc welding.Further,it may generate hot crack and grain coarsening,hurting mechanical properties of joint.Especially for thick plate,those phenomenons are more serious.Therefore,the study with high efficiency welding of medium-thick plate is essential.Strain strengthening technology can improve yield strength of austenitic stainless steel with the advantage of saving,which is used in pressure vessel of low temperature.However,the most of strain strengthening is aimed at base material.There are fewer reports on joints.Especially,the analysis of strengthening feature among weld metal,heat affected zone and base material is less.In addition,it must be taken into account the influence of low temperature toughness after improving strength of joint.In this paper,how to improve the welding efficiency of medium-thick plate and the safe application of strengthening joint in low temperature environment were studied.Firstly,combined welding of PAW+GTAW with different heat input was used to 304 SS plate of 12mm thick with Y-groove.Secondly,part of combined joints were subjected to strain strengthening test by 9%at room temperature.The relationships between microstructure and mechanical properties of joint such as tensile strength,impact toughness,corrosion resistance were established by heat input.The influence of strain on microstructure,strength and low temperature toughness(-196?)of joint were investigated.Moreover,the microstructure and microhardness of joint after impact test with low temperature were researched.The results of combined welding test(PAW+GTA/W)with 304 SS showed that the weld metal was T-shaped,contained austenite and ferrite.There were two typical forms,lathy and skeletal 8-ferrite.Heat affected zone(HAZ)was mainly consisted of austenite,and there was short ferrite which extended along grain boundary from fusion zone to base material.With the increase of heat input(20.0-24.2kJ/cm),the direction of dendrite in weld metal was more obvious,but size and spacing changed little.Moreover,the feathery shape of ferrite gradually disappeared,formed strip of austenite at the area of weld junction.Microhardness of joint decreased from the center of weld metal to base material.The maximum value was existed by 22.5kJ/cm.Tensile strength increased little,which maintained between 700MPa and 709MPa at room temperature.Impact absorption energy of weld metal was reduced from 281J to 269J at room temperature.But heat affected zone showed a different change,the minimum was close to base material(205J).In addition,corrosion resistance of weld metal in PAW was increased with the increase of heat input.Besides,GTAW metal was excellent,the worst was HAZ.The joint with 21.7kJ/cm under 9%by strain strengthening test showed that the deformation rate of joint was disparity.The rate of weld metal was for 4%-5%and HAZ was up to 12%.The strain test had no effect on microstructure of weld metal,grains were refined.A small amount of deformed martensite was formed in HAZ.The microhardness of weld metal with strain strengthening joint was lower than that of ordinary joint,but,HAZ and base material was higher than ordinary joint.In a word,the distribution of microhardness in strain joint was more uniform.Moreover,tensile strength of strain joint was 804MPa,improved by 100MPa than that of ordinary joint.And it was 1677MPa at low temperature,which was twice than strain joint at room temperature.Furthermore,impact absorbing energy of weld metal with strain joint was reduced by 21J than ordinary joint,and HAZ was decreased by 31J.Therefore,the strain strengthening test of joint can not only improve strength,but also maintain a good low temperature toughness.The study of fracture characteristic at-196? low temperature with strain strengthening joint showed that weld metal fracture after impact test was mainly composed of large dimples zone,and found secondary crack.There were river pattern in HAZ.In a word,strain strengthening increased the size of dimples,reduced low temperature toughness.Fracture mechanism was changed from ductile fracture of dimple to cleavage fracture.Moreover,about tensile fracture of strain joint,there was necking phenomenon at room temperature.However,it was flat at low temperature.The size and depth of dimples were reduced as the decrease of tensile temperature.The research of base material,non-strain joint and strain joint after impact test under-196? low temperature showed the boundaryof austenite was not obvious and grain was fragmented in HAZ and se materia.But there was no change in weld metal.The microhardness of base material was increased by 40HV0.5 after impact test.The microhardness of weld metal in non-strain joint after impact test was lower,HAZ was improved.However,the distribution of microhardness in strain joint after impact test was close to the un-impacted joint.