| Hyper duplex stainless steel(HDSS)with a PREN close to 50,belonging to the fourth generation of duplex stainless steel,has superior mechanical properties and corrosion resistance than existing super duplex stainless steel.The service temperature is about 90 ℃,and the yield strength could reach800 MPa.HDSS is increasingly applied in deepwater oil and gas fields.However,when it comes to structural material,the promotion in the field of engineering depends on the weld ability.As the further development of duplex stainless steel extension,when hyper stainless steels are welded,many problems exist as such in DSSs.Moreover,Undesirable intermetallic phase,typically σ phase,is more prone to precipitate during the welding of hyper stainless steel in comparison to the DSSs resulting from the higher alloy content.All of those would restrict the realization of its excellent properties.To avoid these problems,friction stir welding(FSW)as an innovative welding technology —— solid-state joining technology,has lots of advantages than conventional fusion welding technology,such as lower peak temperature and higher cooling rate and applicability.The application of FSW has extended to high melted materials,such as stainless steels and carton steel,and effectively solves the problem existing in stainless steels,especially the welding of duplex stainless steel.FSW has shown great potential in welding of DSS.Thus,this paper conducted the welding of HDSS by FSW,and investigated the effect of rotational speeds on the performance ofweldments,aimed to provide a promising welding technology to HDSS.In the process of FSW,the selection of process parameters is four different rotational speeds with a fixed walking speed of υ=100mm·min-1,and the four rotational speeds are ω=200rpm,300 rpm,400rpm and 500 rpm.By means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD)and electron back scattering diffraction test results with electron backscattered diffraction(EBSD),the precipitated phases and the microstructure variation of FSW butt joint in various process parameters has been investigated and analyzed.Mechanical performance of weldments has been evaluated through microhardness tests and tensile tests.Moreover,corrosion resistance of FSW joint performance has been valued using potentiodynamic polarization curve and impedance spectroscopy test.The object of this investigation is aimed to explore the applicability of the FSW welding to HDSS and select the most optimum process parameter by means of the above analysis and tests.The main conclusions are as follows:All of the weldments in various rotational speeds have a small of buckling deformation.No obvious groove defects are observed on the surface of FSW joint,and only some flashes are observed.Except the surface of 200 rpm,the rest of joint surfaces all present good smooth and community.Some parts of no populated areas between peaks appear on the surface of 200 rpm.Macro observation of FS welded joint cross section showed that the butt joint of FSW could be divided into three regions: stir zone(SZ),thermal-mechanical affected zone(TMAZ)and the base metal(BM),and no obvious heat affected zone(HAZ)was observed.Moreover,for four rotation speeds,the cross section presents good community,and no empty and internal defects appear.The stir zone is located in the heart of the butt joint,and the microstructure of SZ becomes refiner in comparison to the base metal.However,the grain size of duplex phase is inhomogeneous throughout the SZ,from the top to bottom andthe advancing side to retreating side.In accordance to the advancing side and retreating side of the working tool,the thermal-mechanical affected zone lying right next to stir zone can be classified into TMAZ-AS and TMAZ-RS.Electron back-scattered diffraction(EBSD)analysis reveals that fine-equiaxed grains are formed in stir zone due to dynamical recrystallization during friction stir weld process,more pronounced in the austenite phase.The microstructure observation of bottom of SZ and root surface of joint show some precipitated phases exist on the bottom of joint for 200 rpm and 300 rpm,and false connection is observed on the root surface of joint for 200 rpm.Compared to the base metal,the hardness of welded joint is higher,and maximum hardness exists in SZ.As the rotational speed decreases,the average hardness value of FSW joints gradually increases.When it comes to a single welding parameter,the values gradually decrease as the increase of distance away from SZ,and the most variation appears in TMAZ.For the rotational speeds ω=200rpm and 300 rpm,the failure position of weldments is located in the SZ,but for the rotational speeds ω=400rpm and 500 rpm,weldments are failed at the base metal close to the thermal-mechanical zone.Besides the rotational speed ω=200rpm,the rest of weldments have more high tensile strength in comparison to base metal,which implies the welded joint is overmatch to the base metal.However,when compared to the base metal,the elongation of all weldments declines.SEM morphology of weldment fracture surface demonstrates that except 200 rpm,fracture surface of the rest of weldments is charactive of uneven dimples.The corrosion resistance of welded joints is assessed by means of potentiodynamic polarization curves and impedance spectroscopy tests.It is found that joint surface corrosion resistance could meet or present better than that of the parent metal in addition to 200 rpm,and as the rotational speed declines,the corrosion resistance of joint surface becomes more outstanding.Microstructure observation of 200 rpm has found that no populated area similar to groove defect is observed on the joint surface due to the low heat input. |
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