Research On Microstructure Evolution And Corrosion Property Of 316LN Stainless Steel For Nuclear Primary Pipe

Ultralow carbon 316LN austenitic stainless steel?SS?is a good candidate material for AP1000 nuclear primary pipe,due to its good mechanical property,excellent corrosion resistance and superior welding performance.While the grain size inhomogeneity and precipitation are often aroused during the production and processing?such as forging,welding and heat treatment?,because the complex shape and huge size of the nuclear primary pipe.All these microstructure evolutions could bring effect to mechanical and corrosion properties of primary pipe.This paper takes 316LN SS as the research object,and systematically studied the microstructure evolution of the SS during heating,cooling and exposure at high temperature.Moreover the effect of microstructure evolution on corrosion behavior of the SS in simulated primary circuit pipe water environment was also studied.The main research content and conclusions are as follows:?1?The relationship between the solution treatment,grain size,mechanical and corrosion performances of 316LN SS with three different primary grain sizes?30.3?m,66.5?m,121.4?m?was investigated in detail.The results show that:solution temperature is the major factor that affects grain growth and microstructure uniformity.The microstructures of the three different primary grain sizes are homogenous when the solution temperature at 1050?¹100?.With the increase of grain size,the tensile strength and yield strength of 316LN SS decreased,but the electronic stability,homogeneity and hardness of passive film increased,defect concentration of passive film also decreased.So the electrochemical corrosion and intergranular corrosion resistance improved.The optimum solution treatment technological parameters for AP1000 nuclear primary pipe was obtained based on a series of experiments,at 1050?¹100?for 120 min⁶0 min.Both the grain size and the mechanical properties of this process can meet the design requirements of Westinghouse Corporation.?2?The investigations of cooling rate and microstructure transformation of AP1000 nuclear primary pipe during cooling process after solution treatment were investigated systematically by the combination method of finite-element simulation and thermal simulation experiment.The results show that:a)The temperature difference of primary pipe in different parts during cooling process is very large.Cooling rate is fastest in primary pipe and outside surface,and the central part where the filler neck is connected to the pipe body has the slowest cooling rate.b)Both cooling rate and composition fluctuation can affect microstructure of 316LN SS after solution heat treatment.When all alloy elements take intermediate value,with the decrease of cooling rate,the dislocation density of316LN SS decreases.In addition,the Cr and Mo element segregate on the austenite grain boundaries seriously.When the cooling rate is less than 1?/min,second phase will be precipitated in grain boundaries,non-coherent twin boundaries and in the austenite grains.The grain boundary precipitated phase was mainly?-phase,non-coherent twin boundaries and within austenite grains were mainly Z-phase.These precipitated phases will reduce the properties of passive film formed on the surface of316LN SS and corrosion resistance of the SS.When the upper limit of Nb and C is taken,the cooling rate should be accelerated,should not be lower than 96?/min.Otherwise the Z phase will be precipitated.?3?The aging behaviour of 316LN SS with high Nb content at 650⁹50?was analyzed by means of transmission electron microscope?TEM?and scanning electron microscope?SEM?.Furthermore,the type and sequence of precipitated phase,the sensitive precipitation temperature and the precipitation mechanism were also discussed.The aging kinetic curve was established finally.The results show that:a)The first precipitated phase is Z phase of 316LN SS with high Nb content,which has wide temperature,fast speed,long duration and high dimensional stability.Moreover,there is a orientation between the Z phase and austenitic matrix:[010]z//[110]?,?1^—02?z//?1^—11?]?.b)The preferential precipitation of Z phase due to the addition of Nb element,leads to the reduces of saturation of N elements in the austenite,thus accelerating the precipitation of?phase,?phase and?phase,inhibiting the precipitation of M₂₃C₆.c)At the sensitive precipitation temperature?750??,with the aging time increase,the content and size of precipitated phase increases gradually.The precipitated phases eventually cover the whole grain boundary in chainlike.The precipitation reaction sequence in the SS was identified:particle or short bar-like Z phase precipitated on grain boundaries and dislocations inside austenitic?block and strip?phase precipitated on grain boundaries?particle?phase precipitated inside austenitic?block?phase precipitated on grain boundaries and dislocations inside austenitic?irregular strip M₂₃C₆ precipitated on grain boundaries.d)There is a cube-cube relationship between M₂₃C₆ and austenite:[^—211]M₂₃C₆//[2^—11]?,?111?M₂₃C₆//?111??.The orientation relationship between?phase and austenite meet Nishiyama-Wassermann:[001]?//[011]?,?110??//?111??.There are three styles of the growth and the formation for?phase:the first one is precipitated directly on grain boundaries especially the triangle grain boundary,and the high-density dislocation within the grain.The second one is precipitated in the nitrogen depleted region around Z phase.The third one is transformed from metastable?phase.?4?The properties?compactness,thickness,structure,composition?of passive film formed on the surface of SS in boric acid were analyzed by polarization curve,electrochemical impedance spectroscopy,Mott-Schottky,X-ray photoelectron spectroscopy,the resistances of intergranular corrosion and stress corrosion were evaluated by double loop electrochemical potentiodynamic reactivation and slow strain rate corrosion test respectively.The relationship between precipitates and the properties of passive film,intergranular corrosion and stress corrosion were also discussed in detail.The results show that:a)The precipitated phase has no effect on the composition of passive film.The passive films are mainly composed of Fe and Cr oxides or hydroxide,besides a small amount of Ni and Mo oxides.With the increases of the content of precipitated phase,the defect concentration in the passive film increases,the density and thickness decrease,leading to deterioration of corrosion property.The causes for performance change were also discussed.b)The precipitation of Z phase also increases the defect concentration,reduces the stability and uniformity of passive film,so increases the pitting sensitivity,but it has none effect on intergranular corrosion.c)With the extension of aging time,the stress corrosion sensitivity of 316LN SS in the service environment increases.The fracture mode changes from mechanical fracture in solution state to intergranular fracture,transgranular fracture or mixed fracture in aging state.This is closely related to the content and distribution of the precipitated phase.In addition,the chromium depleted region act as crack initiation site.