Study On Thermal Treatment Parameters And Properties Of Bimodal Structured304Stainless Steel

Bulk nanocrystalline austenitic stainless steel (SS) by severe plastic deformation usuallyexhibits unprecedented strength and hardness but very poor ductility compared to itscoarse-grained counterpart. This drawback becomes an insurmountableobstacle in bringingbulk nanocrystalline SS into practical application. Previous studies indicated that the lowductility could be improved by introducing micron-grains of a certain volume fraction toachieve bimodal grain size distribution through suitable annealing. The nanocrystallinematrixmay ensure the high mechanical strength, while the micron grains may improve the ductilityofsuch materials, and a highstrengthaccompanied by an enhanced ductilityis expected. Inaddition, the introduction of bimodal grain size distribution,as an inhomogenousstructure, willcertainly affect the corrosion resistance of the material, which should not be ignored.The bulk nanocrystalline304SS prepared by ECAP was used as the investigation objectin this study. Firstly, the recrystallization temperature range of nanocrystalline SS wasdetermined by the variation in hardness and metallographic analysis, then several annealingtemperatures were picked out to achieve bimodal structure with different volume ofmicrocrystalline grains. The microstructure evolution after annealing was observed andanalyzed by optical microscopy (OM), electron channel contrast (ECC),electron backscatterdiffraction (EBSD) and the annealing parameters were settled for a typical bimodaldistribution. Tensile testing was conducted to measure the mechanical properties of bimodalstructured SS, and the plasticizing mechanism was proposed. Finally, the corrosion behaviorsof bimodal structured SS were investigated through electrochemical experiments.Hardness test and OM observationindicate that the recrystallization temperature ofnanocrystalline304SS（grain size of80-120nm）ranges from650℃to900℃. Therefore700℃,720℃,750℃,780℃and800℃for30min were finalized as the annealingparameters to achieve bimodal structure.The results of OM, ECC, and EBSD observation and analysis show that the averagegrain size is190nm after annealed at700℃. When annealed at720℃, coarse grains (＞1μm)emerge with a volume fraction of12%, presenting an unconspicuous bimodal distribution.When annealed at750℃, micron grains become larger and more obvious, constituting38%in volume, with an average grain size of～1.4μm, and the ultra-fine grains with an average sizeof～345nm occupy the rest of the volume, presenting a typical bimodal distribution. Theamount of coarse grains increases to62%and72%, and average grain size to1.5μm and1.7μm respectively, when annealed at780℃and800℃.Tensile test results show that the strength declines and the ductility recovers a lot afterannealing. A combination of1.0GPa of ultimate tensile strength with an uniform elongation of35%is achieved in the nanostructured304SS after annealed at750℃for30min. With theincrease of annealing temperature, there is no notable increase in ductility, butcontinuousdecline in strength.After comparing the work hardening behavior of different state SSs, the plasticizing ofbimodal structure is attributed to the improvement of work-hardening rate, which may comefrom two aspects:①the intrinsic work hardening rate introduced by micron-grains;②theextra work hardening appended by the deformation strain gradient of bimodal structure. Thefeature of tensile fractographies of different state SSs is consistent with the laws ofmechanical properties.The results of OCP, potentiodynamic polarization and EIS show that the corrosionresistance of bimodal structure is better than the coarse grained counterpart, but inferiortonanocrystalline SS. The results of potentiostatic polarization lgI-lgt curves show that thepassive film is porous on the corase grain sample (k=-0.38), and compact on thenanocrystalline SS (k1=-1.10, at the initial stage, and k2=-2.67at the later stage of filmformation). In the two bimodal structured samples, the slopes of lgI-lgt curves are-0.42and-0.46at the initial stage of film formation, which means the passive film is porous at earlystage of film formation; At the later stage of filmformation, the whole surface is graduallycovered with compact film (k2=-1.09~-1.12)since the the parallel growth of compact passivefilm on nanocrystalline region is faster than that of the porous film on coarse grained region.The corrosion morphology observation after anodic polarization further confirms theconclusion of electrochemical tests.