Study On The Intergranular Corrosion And The Optimization Of Grain Boundary Character Distribution Of 316L(N)Austenitic Stainless Steel

316L(N)austenitic stainless steels(ASS)have excellent mechanical properties and corrosion resistance properties.Thus they have been widely used in petrochemical,chemical,power and nuclear industries as structural material.However,the steels are still possible failure during their service conditions.The austenite grain and grain boundary have a tremendous effect on the mechanical properties and corrosion resistance of the 316L(N)stainless steels.Therefore,the optimizations of the microstructure and the grain boundary character distribution(GBCD)can improve 316L(N)stainless steels of the corresponding service performance.In this paper,the hot deformation process of 316LN stainless steel was optimized.The influence of the hot deformation parameters on the grain size of 316LN stainless steel was studied,and the optimized thermal deformation parameters were also verified.The sensitization temperature of 316L(N)austenitic stainless steels was choose based on the simulated phase diagram.The effect of the sensitization treatment conditions on the intergranular corrosion of 316LN stainless steel were studied by the electrolytic etching test,electrochemical test and tranmission electron microscope(TEM)test.The effects of the grain size on the intergranular corrosion of 316LN stainless steel were also studied through the experiments and theory.The precipitates of 316LN stainless steel were always various and complex due to the high nitrogen content of the steel.Therefore,316L stainless steel was selected to study the grain boundary character distribution and the properties in this thesis.The effect and mechanism of the thermo-mechanical processing on the grain boundary character distribution were studied.And the effect and mechanism of special boundaries and random boundaries on the tensile properties and corrosion fatigue properties of the materials were investigated by the tensile tests and corrosion fatigue tests.The main research contents and conclusions are as follows:(1)The effects of hot deformation parameters on the grain sizes of 316LN stainless steel were investigated by the Taguchi method.The results suggested that the optimum conditions of the hot deformation test for minimizing the mean grain size of the 316LN austenitic stainless steel are as follows,temperature of 1100?,strain rate of 0.01 s-1 and strain of 70%.The strain had the most significant effect on the mean grain size of 316LN austenitic stainless steel,and it reached up to 93.33%.The higher the deformation temperature and the strain rate,the higher the dynamic recrystallization volume fraction of the sample when the strain rate was lower.Therefore,the microstructure of 316LN stainless steel was more uniform,and the mean grain size was smaller.The mean grain size of the sample under the optimum conditions of the hot deformation test was 11.4 ?m rather than the original state 147 ?m.The refinement effect was very obvious.(2)The sensitization treatment led to 316LN stainless steel intergranular corrosion based on the simulated phase diagram and electrolytic etching test.Therefore,the effects and mechanism of the sensitization temperature,sensitization time and grain size on the intergranular corrosion of 316LN stainless steel were studied.The results showed that the intergranular corrosion of 316LN stainless steel was most serious at the sensitization time for 48 h,the sensitization temperature up to 700? and the sensitization temperature 650?,the sensitization time reached 400 h.Meanwhile,the the a phase,M23C6 and M7C3 phase were appeared in the grain boundaries and the grains were surrounded by ditches.In addition,with the decrease of grain size,the quantity of the grain boundaries increased,and the distance of the carbon and nitrogen atoms diffusing to grain boundaries decreased,which led to the increase of the degree of intergranular corrosion.(3)The effects and mechanism of the thermo-mechanical processing on the grain boundary character distribution of 316L stainless steel were investigated.The results showed that small deformation was easier to induce the special grain boundary nucleation.And the low-energy special boundaries were formed primarily through the partial recrystallization in annealing process when the strain was 5%.Subsequently,the grain grew up and the high-energy random grain boundaries reduced preferentially.The ?3n reactions between the grain boundaries,?3+?3n??3n+1 were dominant.Hence,the special grain boundary proportion of the specimen increased significantly.And the special grain boundary proportion of the specimen increased to 74%at the annealing time 90 min.Due to the large deformation storage energy,high-energy random grain boundaries were formed primarily through complete recrystallization in annealing process when the strain was greater than or equal to 10%.Therefore,the proportion of the special boundaries was low and increased slowly only with the grain growth.The final step of the multiple thermo-mechanical processing played an important role in the special grain boundary proportion.The 5%strain can promote the special grain boundary nucleation,and the 10%strain can lead to the generation of the random grain boundaries.(4)The effects and mechanism of the solution treatment,sensitization and thermo-mechanical processing on the tensile properties and corrosion fatigue properties of 316L stainless steel were studied.And the effects of the special boundaries on the mechanical properties,also combinated mechanical and corrosion properties were explored.The results showed that the solution treatment at 1100? was beneficial to the dissolution of the precipitated phases.The dimple number of the samples was increased and deepened.And the toughness was increased.The sensitization treatment at 700?,made the grain boundary precipitates increase and the grain boundaries became hard and brittle.Futher more,the tensile stress and tensile strength of the material increased obviously and the toughness decreased.With the increase of special grain boundary proportion,the strengthening effect of sensitization treatment on 316L stainless steel reduced.Therefore,the strength decreased and the toughness increased.However,the high proportion(74%)of special boundaries could not effectively prevent the propagation of fatigue bands in corrosion fatigue experiments.And the cracks produced earlier.The low proportion(55%)of special boundaries caused too much large angle random grain boundaries and good connection.So the crack tip was corroded easily.Therefore,the crack propagation rate was too fast.By contrast,the samples at 5%strain,1000? annealing temperature for 45 min annealing time had higher special grain boundary ratio(66%)and the grain size was moderate.Therefore,the maximum corrosion fatigue life for the sample was obtained.