Fatigue And Corrosion Properties Of 316L Stainless Steel With A Gradient Nanostructured Layer

As an engineering material,austenitic 316L stainless steel are widely used in industries such as nuclear power,chemical engineering,medical equipment and bio-medical implants.However,its mechanical properties,especially fatigue resistance is not satisfying in some cases.Grain refinement is an effective way to enhance mechanical properties and fatigue resistance of 316L stainless steel under stress-controlled condition.However,strain-controlled fatigue resistance was negatively affected by nano-crystallization due to its poor ductility.Meanwhile,nanocrystallization induced by severe plastic deformation might has detrimental influence on its corrosion resistance.Recently,metallic materials with gradient nanostructures(GNS),in which grain sizes are in nanometer scale at sample surface and increase gradually to micrometer scale in interior,were widely studied in recent years because of its excellent synthesis between strength and ductility.In this works,surface mechanical rolling treatment(SMRT)equipped with temperature controlled device(W-SMRT)is developed to suppress the deformation induced martensite on the surface and manufacture a complete austenite GNS layer on the surface of 316L stainless steel.Fatigue properties and mechanism under both stress-and strain-controlled condition are analyzed on W-SMRT samples.Furthermore,strain controlled fatigue resistance is optimized by regulating the grain size and martensite volume fraction on the surface.Besides,microstructure and element distribution are adjusted and corrosion resistance is enhanced by SMRT and tempering as follows.The main results are followed:1)A GNS surface layer with full austenitic phase is synthesized on AISI 316L stainless steel by means of W-SMRT at 280? and deformation induced martensite(DIM)is suppressed during the process.After W-SMRT,the thickness of the deformed layer is around 800 ?m.Austenitic grains with a mean size of 45 nm(along short axis)are formed at the top surface.And the mean grain size increases gradually with increasing depth.2)Axial tension-compression fatigue tests show that fatigue properties are significantly enhanced in the GNS samples both in stress-and strain-controlled tests.The fatigue limit increases from 180 MPa in the CG samples to 320 MPa in the GNS samples.Simultaneously,fatigue strength and endurance are also enhanced in the GNS samples in strain-controlled tests.The fatigue lives of the GNS samples are more than 20 times of those the CG samples at the plastic strain amplitude of 0.1%.This is very different from the decreased fatigue endurances of conventional nanostructured 316L stainless steel in strain-controlled tests.For example,fatigue endurance of ECAPed(Equal-Channel Angular Pressing)316L is only 15%of that of CG counterparts at 0.1%plastic strain amplitude.3)Besides contributions from the enhanced mechanical properties and suppressed formation of surface defects,the present work demonstrates that the promoted formation of DIM during cyclic strain plays a key role in enhancing fatigue properties of the GNS samples in both stress-and strain-controlled fatigue tests4)Further enhancement can be achieved by regulating the grain size and phase constitution on the GNS surface layer.Fatigue life of CG and W-SMRT samples at 0.5%strain amplitude is around 7.0×103 cycles,which is 1.1 ×104 for W-SMRT-2 with larger grain size on the surface(80 nm)and 2.3×104 for RT-SMRT ones with large volume fraction of martensite phase on the surface.5)Deformation induced martensite gradually reverses into austenite as the temperature elevated.Cr enrichment on the surface is observed on the surface of RT-SMRT samples tempered at 700? and structural analysis shows that the Cr-rich phase is carbides with the average grain sizes of 100 nm.Surface segregation is driven by the thermodynamic process.Furthermore,temperature rise and nanocrystallization favor the Cr transference.Eventually,Cr is accumulated on the surface in the form of carbides which is the most stable phase at the given circumstance.6)After tempering 700?,RT-SMRT700 sample shows an enhanced corrosion resistance compared with CG samples.The passive current density of RT-SMRT700 is only 1/9 of that of CG samples.Meanwhile,mechanical properties of RT-SMRT700 also promoted compared with CG samples.Yield strength is enhanced from 212 MPa for CG samples to 320 MPa for RT-SMRT700.Corrosive and mechanical properties are enhanced because of Cr surface segregation and thermal stability of GNS.