Microstructure And Properties In Strain Strengthened Welded Joint Of Austenitic Stainless Steel At Low-temperature

SUS 304 austenitic stainless steel exhibits favorable mechanical properties and corrosion resistance,widespread used in pressure vessels,petrochemical equipment and marine accoutrement.Stainless steel pressure vessels based on strain strengthening technology can utilize good plasticity and toughness to improve the strength of stainless steel.On one hand,it can improve the load-bearing capacity of pressure vessels,and on the other hand,it can effectively reduce wall thickness with lightweight.After strain strengthening,the deformation rates in various parts of weld joint are different owing to the inhomogeneity of the microstructure in each region.Especially in the low-temperature environment of-196℃,more distinct heterogeneity of the joint properties after strain strengthening.In the thesis,PAW+GTAW technics are applied for welding SUS 304 austenitic stainless steel.SEM,EBSD,XRD,TEM are employed to analyze the influences of strain strengthening and low-temperature treatment on microstructures and properties in PAW+GTAW joint.The microstructural evolution of various regions of joint after strain strengthening are further studied,which has guiding significances to improve the service reliability of SUS 304 austenitic stainless steel as cryogenic vessels.The combination welding process of PAW and GTAW can achieve good stainless steel joint formation.PAW can achieve single sided welding and double sided forming,while GTAW has a good weld morphology on the cover surface.According to the structural characteristics of the weld joint,the joint is divided into PAW weld,GTAW weld,PAW heat-affected zone,GTAW heat-affected zone and weld bead junction zone.As the total pre tensile strain of stainless steel weld joint is up to 10%,the deformation rate of the weld is less than that of the heat-affected zone,and the tensile strength of the joint is significantly improved.The tensile strength at room temperature is increased by 14%and the tensile strength at low-temperature(-196℃)is increased by 135%.Ferrite and austenite exist in the weld seam of stainless steel weld joint.PAW weld grains grow vertically into the molten pool,GTAW weld grains crystallize vertically on the weld surface.Strain strengthening can change the grain orientation and refine the grain size in the weld seam.GTAW weld seam exhibits strong texture and high dislocation density.While PAW weld seam exhibits texture in multiple directions with weak texture strength,dislocations mainly exist in the center of the weld seam.Strain strengthening increases the dislocation density in the weld seam area,significantly increasing the ferrite slip system in the PAW weld seam.In the meantime,the strain strengthened GTAW heat-affected zone has more sub-grains with patchy growth.In the process of strain strengthening,the increase of sub-grains boundaries will also increase the dislocation value.Many deformed grains are observed in the Heat-affected zone of PAW,most of which are presented as large angle grain boundaries,and the dislocation density near the base metal is high.After low-temperature treatment,the microhardness of the stainless steel base material and joint is significantly improved.The microhardness of joint after strain strengthening as well increases with the prolongation of cryogenic treatment time.Subjected to cryogenic treatment for 36 h,the microhardness of the joint reaches the highest value.Austenite transforms to martensite and carbide precipitates at the austenite grain boundary.With the extension of lowtemperature treatment time,martensite transforms from discontinuous to continuous.TEM results show that the nucleation particles of martensite in the heat-affected zone of the strain strengthening joint increase and a large quantity of continuous martensite is formed.Dislocations and stacking faults accumulated in austenite grains also induce the transformation of martensite.The results of corrosion resistance potentiodynamic polarization curves indicate that strain strengthening is beneficial for increasing the pitting potential in various areas of stainless steel joint and delaying the corrosion rate,but has little effect on the corrosion potential.In EIS testing,strain strengthening and low-temperature treatment significantly increase the thickness of the passivation film on the surface of the base metal and weld seam,which can effectively prevent the occurrence of pitting corrosion.