| High nitrogen austenitic stainless steels(HNAS)exhibit a favorable combination of strength and toughness and excellent corrosion resistance.It has broad application prospects.The wider application of HNAS for engineering field is largely dependent on its welding characteristics.Conventional fusion welding processes of HNAS exhibit a number of welding defects,which seriously deteriorate mechanical and corrosion properties of the welds.Therefore,the welding technology is still restricting the development and engineering applications of HNAS.Friction Stir Welding(FSW)is an innovative solid-state welding technology,which does not undergo phase transition from solid to liquid in the welding process,and has potential advantages for the welding of the HNAS.Therefore,the research on the FSW of HNAS can provide a potential way for its welding,thus promoting its development and application.In this article,a high nitrogen austenitic stainless steel of 18Cr-18Mn-0.96N is friction stir welded with fixed welding speed(100 mm·min-1)and three different rotating speed(400 rpm,500rpm and 600rpm).Following FSW,the joint is first evaluated on the basis of visual and dye penetration inspection to reveal the weld defects.Then,the nitrogen content of the SZ is detected to determine whether nitrogen desorption occurred in the weld.The microstructure evolution of the weldments are well studied using optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron backscatter(EBSD),combined with Thermo-Calc software to calculate the thermodynamic phase diagram of the alloy.The mechanical properties of weldments are evaluated by hardness and tensile tests.The pitting and intergranular corrosion resistance of the welds are determined by anodic polarization curve and potentiodynamic reactivation(EPR),respectively.The effect of microstructure evolution on the mechanical properties and corrosion resistance is also studied to explore the influence of rotating speed on the mechanical and corrosion resistance of friction stir processed HNAS.The main conclusions are as follows:The friction stir welds of HNAS at different rotating speed have little amount of deformation and no obvious surface defects such as grooves,holes are observed.The results of dye penetrant inspection further confirm the absence of surface cracks in the weld.The nitrogen content tests show that the FSW processed HNAS did not exhibit nitrogen loss.The cross-section of joint at rotating speed of 400 rpm has compact structure and has no internal defects,however when the rotating speed increase to 500 rpm and 600 rpm,the cracks and holes are formed in the stir zone due to excessive heat input.The cross-section of each weld under three rotating speed can be divided into four areas:base metal(BM),stir zone(SZ),the heat-affected zone(HAZ)and the thermal-mechanical Affected Zone(TMAZ).The grains in the SZ has been significantly refined,and there is decrease in grain size from top to the bottom direction;HAZ has a microstructure similar to that of BM and the parent grains does not experience significant growth.;The two sides of TMAZ are asymmetry.The grains in the TMAZ-AS are severely-elongated and re-oriented perpendicular to the taper direction of the tool and the transition of TMAZ-RS is not obvious.The cracks and "boundary" type defects are formed in the TMAZ-RS at 500 rpm and 600 rpm,respectively.TEM observation showed that the dislocation density of BM is lower than the SZ at different rotating speed,and the dynamic recrystallization is incomplete during welding process.Combined with SEM observation,it has been confirmed that no secondary phases are precipitated during the FSW process.The hardness of SZ is significantly higher than that of the BM,the value increase from the as-received BM region toward the TMAZ,reaching a maxima at the SZ.Tensile tests show that the tensile samples of FSW joint at 400 rpm and 500 rpm fractured in the BM near the TMAZ,the yield strength and tensile strength are higher than that of the BM specimens,however,the elongation is lower.The FSW joints at 400 rpm have the maximum strength specimens,and the internal microcracks formed in the SZ at 500 rpm has litter effect on its tensile properties.The FSW joint at 600 rpm indicates characteristics of brittle fracture,and the tensile strength and elongation are very low.Anodic polarization curve and EPR methods show pitting potential and Ir/Ia of SZ are lower than that of the BM,indicating its resistance to pitting and intergranular corrosion resistance are lower.With the increasing of rotating speed,the pitting potential the SZ is gradually reduced and intergranular corrosion susceptibility Ir/Ia increases,indicating the corrosion resistance has been further reduced.The EBSD analysis indicates that the amount of?-ferrite in the SZ is higher compared with the BM due to the transformation from austenitic to ferrite during the welding process.The SEM-EDS results showed that the Cr-depletion zone is formed at the interface between the austenitic and ?-ferrite,resulting in the decreases of corrosion resistance of the SZ.With the increasing of rotating speed,the amount of 8-ferrite in the SZ further increases and deteriorates the corrosion resistance. |
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