Research On The Effect Of Nitrogen On Microstructure And Properties Of ITER 316LN Austenite Stainless Steel

With the development of the fourth generation nuclear power industry,nuclear power plants require more and more materials with higher properties.The magnet support structure of ITER project requires that the materials have complete austenite structure,excellent strength and toughness as well ascorrosion resistances.The conventional commercial 316LN austenitic stainless steel is difficult to meet these requirements of ITER.Therefore,it is of great significance to strengthen the research onthe new 316LN austenitic stainless steel for ITER(ITER 316LN austenitic stainless steel).In this study,the effect of nitrogen content on microstructures,mechanical properties and corrosion resistance of ITER 316LN austenitic stainless steel was investigated systematically.The influence of nitrogen content on precipitation behavior of the experimental steels was calculated by Thermo-Calc software.Microstructure characteristics of ITER 316LN with various nitrogen content were analyzed by optical microscope(OM),field emission scanning electron microscope(SEM),energy dispersive spectrometer(EDS)and X-ray diffraction(XRD).Mechanical properties of ITER 316LN were measured by Vickers microhardness tester,tensile testing machine and instrumented impact testing machine.Corrosion resistance was evaluated by electrochemical and chemical immersion methods.The main conclusions are obtained as follows:The size of inclusions in the three experimental steels(0.006N,0.16N and 0.29N)was small,and these inclusions mainly include Ce2O2S and Ce2O2S-Al2O3-MnS composite inclusions.In addition,AIN phase formed in the 0.29N steel.ThermoCcalc calculation results indicate that increasing nitrogen content inhibited the formation of ? ferrite and a phase and promoted the precipitation of Cr2N and AIN phase.When the solution temperature was 1050? the0.006N steel has complete austenite structure,and the 0.16N and 0.29N steels possess austenite and AlN structure.The Al-N equilibrium curve further confirms the formation of AlN in the 0.29N steel during solidification.Microstructure analysis results show that the grain size reduced and the dislocation density and microhardness increased with the nitrogen content increasing.Tensile test results illustrate that with the increase of nitrogen content,the yield and tensile strength were improved,and the elongation were reduced.The strengthening mechanism of nitrogen element on the steel is mainly solid solution strengthening and grain boundary strengthening.With the increment of nitrogen content,tensile-plastic deformation mechanism changes from dislocation slip to the combined action of dislocation slip and deformation twin.Instrumented impact test results display that,with the increment of nitrogen content,the impact toughness of the steel gradually increased at room temperature.This is because high nitrogen content promoted grain refinement and increased work hardening rate.However,at liquid nitrogen temperature,the impact toughness of the steel gradually reduced with the nitrogen content increasing.This may be attributed to that high nitrogen content restrained the formation of deformation-induced martensite.The analysis results of impact plastic deformation exhibit that,with the increase of nitrogen content,the deformation mechanism changed from dislocation slip and deformation twin to dislocation plane slip at room temperature.However,at liquid nitrogen temperature,the deformation mechanism changed from deformation-induced martensite and dislocation slip to dislocation slip and deformation twin.Instrumented impact test results display that with the increment of nitrogen content,the impact toughness of steel gradually increased at room temperature.The main reason was that high nitrogen content promoted grain refinement and increased work hardening rate.However,at liquid nitrogen temperature,,the impact toughness of steel gradually reduced with nitrogen content.This may be attributed tothat high nitrogen content restrained the formation of deformation-induced martensite.The analysis results of impact plastic deformation exhibit that with the increase of nitrogen content,the deformation mechanism changed from dislocation slip and deformation twin to dislocation plane slip at room temperature.However,at liquid nitrogen temperature,the deformation mechanism changed from deformation-induced martensite and dislocation slip to dislocation slip and deformation twin.Electrochemical and immersion test results indicate that with the increment of nitrogen content,corrosion resistance of steels was gradually improved.The XPS analysis of passive film formed on ITER 316LN indicate that nitrogen element promotes the formation of stable compounds such as Cr2O3 and MoO2 in passivation film.These compounds could prevent the diffusion of corrosive ions through passivation film and reduce the corrosion rate and anode current density.So the stability of passivation film was improved.Moreover,NH3 and NH4+formed in passivation film could consume H+generated during pitting formation and improve local corrosive environment.That is,the NH3 and NH4+ could inhibit acidification of pits and improve the corrosion resistance.