Effect Of Gradient Structure On Fatigue And Corrosion Properties Of 316L Stainless Steel

Fatigue and corrosion are the most common failure forms of metal materials.Refining the grains of metal materials into ultra-fine crystals or nanocrystals can greatly improve their fatigue performance and corrosion resistance.In addition,repairing metal materials that have fatigued to a certain number of cycles is also one of the common methods to improve their fatigue performance.Under cyclic loading,the nanocrystals on the surface of the material may be affected by working conditions(such as heat,chemistry,stress,etc.),which may cause the surface grains to coarsen and cause the surface to soften.The surface softening of materials will accelerate the initiation of fatigue cracks and cause the degradation of fatigue performance.However,the effect and mechanism of the effect of surface softening on the fatigue performance of materials are still lacking in in-depth research.The grain size distribution has a significant impact on the mechanical properties of metal materials such as strength and plasticity.When discussing the corrosion resistance of materials,the average grain size is generally used.At present,the influence of grain size distribution on corrosion performance is limited to simulation analysis,and there is no experimental verification yet.In this paper,a reverse gradient layer was prepared on the surface of bulk 316 L stainless steel,the surface softening was simulated and the effect of surface softening on the fatigue properties of 316 L stainless steel was studied.At the same time,electromagnetic induction heating is performed on the samples that have been fatigued to a certain number of times,and the effect of EMIH on the fatigue damage repair of 316 L stainless steel is analyzed.In this paper,a nanocrystalline-coarse-grained gradient transition structure was prepared on the surface of rod-shaped 316 L stainless steel by surface mechanical roller compaction(SMGT),and the effect of the grain size gradient distribution on the corrosion performance of 316 L was studied.The main research results are as follows:(1)The reverse gradient nanostructure(IGNS)316L stainless steel prepared by rolling and electromagnetic induction heating treatment simulates the occurrence of surface softening.The thickness of the softening layer is 250 ?m.The fatigue properties of the reverse gradient structure and the homogeneous sample(annealed after rolling)were compared and analyzed.Studies have shown that the reverse gradient structure sample and the rolled annealed sample exhibit similar fatigue strength in the low cycle fatigue zone;while in the high cycle fatigue zone,compared with the rolled and annealed sample,the fatigue strength of the reverse gradient structure sample is significantly reduced.The main reason for the decrease in fatigue performance of the reverse gradient structure sample in the high-cycle fatigue zone is due to the softened surface layer,which leads to the acceleration of crack initiation;while in the low-cycle fatigue zone,although it will also cause the acceleration of crack initiation,it is due to the greater stress,The crack initiation life accounted for a small proportion of the total fatigue life,and the relatively coarse grains on the surface layer showed enhanced crack growth resistance.Therefore,the reverse gradient structure and rolled annealed samples showed similar fatigue strength in the low cycle fatigue zone..(2)Perform electromagnetic induction heating treatment on the coarse-grained and reverse gradient structure samples whose fatigue cycle reaches 50% of the total life,and repair the fatigue damage of the samples.The study found that the remaining fatigue life of the coarse-grained samples has been greatly improved,while the remaining fatigue life of the reverse gradient structure samples has been reduced.Electromagnetic induction heating and water quenching were performed on the reverse gradient structure samples whose fatigue cycle reached 70% of the total life.It was found that the remaining fatigue life of the reverse gradient structure samples was slightly improved.Electromagnetic induction heating treatment can effectively reduce the number of fatigue damage on the surface of coarse-grained samples and reduce the plastic deformation during fatigue,and the repaired hardness value is higher than that of the initial coarse-grained samples,which leads to a great increase in the remaining fatigue life.The reverse gradient structure sample undergoes electromagnetic induction heating and water quenching treatment,and the increased surface hardness,the healing of microcracks,the recovery of strain and the reduction of surface damage make up for the negative impact of the decrease in internal hardness on the fatigue life,so it shows a slight Improved remaining fatigue life.(3)The nanocrystalline-coarse-grained gradient transition structure was prepared on the surface of rod-shaped 316 L stainless steel by mechanical surface rolling.The average grain size of nanocrystalline was about 40 nm,and the surface hardness increased from 3.68 GPa for coarse grain to 4.88 GPa for nanocrystalline.The immersion experiment and the electrochemical corrosion test show that the nanocrystalline-coarse-crystalline gradient zone sample exhibits a corrosion resistance that exceeds the uniformly distributed nanocrystalline and coarse-grained grain size.The improvement of the corrosion resistance of the samples in the gradient zone is mainly due to the grain size gradient distribution,which can effectively reduce the stress concentration in the passivation film and improve the integrity of the passivation film.