The Preparation And Properties Of Inverse-gradient Nanostructure On 316L Stainless Steel

316L austenite stainless steel is of excellent corrosion resistance and ductility,but relative low strength.It is significant to require to the optimal strength-ductility-corrosion resistance synergy,which would expand its applied areas.The report showed that gradient nanostructure(GNS)prepared by surface nanocrystallization technology would solve the problems about strength-ductility synergy well,however,its surface structure is composed by poor corrosion resistant nanocrystal,which is harmful for its whole corrosion resistance.In addition,the improvement in strength-ductility synergy is restricted because of limited portion of gradient layers in traditional gradient nanostructure.In this research,inverse-gradient nanostructure on 316L stainless steel through high strain cold rolling and electromagnetic induction heating(EMIH)was obtained.Microstructure,strength and corrosion resistance in inverse-gradient nanostructure was paid attention to obtain optimal match.Meanwhile,compared to random mixtures of coarse grains with nano-grains(CG +NG)on 316L stainless steel fabricated by high strain cold rolling and isochronal annealing(IA),the influence of distributions of recrystalline grains on mechanical properties of 316L stainless steel was researched.Besides,the main results were listed as follow:1)Preparation of inverse-gradient nanostructure on 316L stainless steelInverse-gradient nanostructure on 316L stainless steel through high strain cold rolling and electromagnetic induction heating(EMIH)was prepared.Its gradient microstructure was that the grains decreased from micron-sized coarse grains on the surface to nanocrystal in the center.As the heat transfer decreased from outside to inside,different level of recrystallization occurred in the grain at different depth which gave rise to gradient relation between depth and microhardness.The key to control the inverse-gradient nanostructure are output power and heating time,and the influence of heating time is more sensitive.2)Research on mechanical properties and corrosion resistance of inverse-gradient nanostructure on 316L stainless steelThe layer thickness of inverse-gradient nanostructure on 316L stainless steel increased with heating time,and heat expanded to whole sample in the end,which the gradient of hardness disappeared.Uniaxial tensile tests show that,inverse-gradient structure is of outstanding strength-ductility synergy and linear relationship between the recovery of uniform elongation and yield strength is obtained.Electrochemical corrosion tests indicate that pitting corrosion resistance of inverse-gradient nanostructure on 316L stainless steel is better than coarse grain with the same chemical composition at 3.5%NaCl solution.When the size of surface grains is～3μm,pitting corrosion resistance of inverse-gradient nanostructure is better than that of coarse grain at complete annealing state.While the heating time is 1.0s,the optimal strength-pitting corrosion resistance synergy is obtained.3)The influence of distributions of recrystalline grains on mechanical properties of 316L stainless steelDifferent distributions of recrystalline grains were obtained on a cold-rolled 316L stainless steel through isochronal annealing(IA)and electromagnetic induction heating(EMIH),respectively.Recrystalline grains were uniformly distributed in the IA sample.While a gradient distribution of recrystalline grains was obtained in the EMIH sample.Uniaxial tensile tests show that similar strength-ductility synergy was achieved in the IA and EMIH samples when the volume fraction of recrystalline grains is close.The difference in the distribution of recrystalline grains imparts no significant effects on the mechanical properties of 316L stainless steel.Microstructural analysis indicates that tensile ductility of the annealed 316L is mainly depended on the network formed by recrystalline grains and nano-twins.And the deformation ability of the network is insensitive to the distribution of the recrystalline grains.Therefore,the effects of the distribution of recrystalline grains on the mechanical properties are suppressed in 316L stainless steel.