| As more attentions have been paid to the requirement of lightweight andintegration of structural components in modern manufacturing industry,tailor-welded tubes process as a new technology for manufacturing and formingtubes with good performance developed rapidly. For its excellent corrosionresistance and plastic deformation ability, SUS304austenitic stainless steel isutilized to make tailor-welded tubes which are usually processed into many kindsof components with complex shapes and hollow cross-section by hydroforming,and widely used in such fields as automotive, aviation and aerospace.The microstructures and mechanical properties of welding joint are differentfrom the one of the base metal for304austenitic stainless steel tailor-weldedtube, all these will make the deformation coordination ofbese metal, HAZ andweld metal get worse in the process of plastic deformation by high-pressureforming and make the welding joint become a weak position, thereforetailor-welded pipe is easily fractured in welding seam under lower pressure.Moveover deformation induced martensite transformation (DIMT) will occurwhen304austenitic stainless steel undergoes large plastic deformation at theroom temperature, and the martensite will cause stress corrosion cracking underspecial environment and delay cracking for the products made through largedeformation.In this paper, in order to enhance hydroforming formability of tailor-weldedtube and reduce the harmful effects of weld, the microstructure and mechanicalnon-uniformity of the304stainless steel welded joint was studied firstly, thenthe relationship between strain, microstructural homogeneity and martensitictransformation were researched, after that the reason f formation failure fortailor-welded tube in hydroforming was analyzed, finally the correspondingoptimization technology to improve the hydroforming formability oftailor-welded tube were proposed.To fully understand the microstructure evolution and mechanicalnon-uniformity of the304stainless steel welded joint, thermal simulationexperiment, optical microscope analysis, EDS, tensile experiment were appliedto analyze themicrostructure and mechanical properties of bese metal, HAZ andweld beam respectively. Experimental results showed that grain degrees present atrend from big to small and then to big again from the base met al to HAZ for the whole welding joint, the yield strength of metals in different micro area of HAZshowed the same trend. OM and EDS results indicated that the solidificationmode of weld beam metal is FA and composition segregation in dendrite is notobvious.Martensite was characterized by opticmicroscope analysis and XRD, thevolume fraction of martensite transformed from austenite in the process ofplastic deformation was measured by using ferrite measuring instrument, theresults showed that the amount increases with the increasing of tensile strain, andthe transformation of martensite is closely related to uniformity of microstructurei, e., the higher the uniformity the less the amount of martensite transformedfrom austenite.The welding quality has a great effect on the quality of tailor-welded tubeby high-pressure forming. Welding defects were identified as the main failurereasons of the tube fracture in the process of hydroforming. Experimental resultsshowed that welding quality used TIG with no wire, single pass, small current,fast welding speed, low heat input is better compared with that used TIG withfilling wire and multiple pass. Weld metal obtained by TIG without wire do notappear such welding defects as blowholes, shrinkage cavity and incompletefusion; The microstructure and mechanical properties of welding joint increasedobviously after heat treatment at1050℃for30minutes. Tensile tests at275℃indicated that martensite transformation induced by deformation can berestrained in the process of plastic deformation at this temperature, the amount ofmartensite is almost0under such conditions. |
PDF Full Text Request