Study On Precipitates Of Nuclear Grade 316NG Austenitic Stainless Steel Under Long-term Thermal Aging

The main pipeline of the reactor coolant in the nuclear reactor environment during long-term service and nuclear power radiation will precipitate carbon and nitrogen compounds and intermetallic compounds and other phases,resulting in the gradual thermal aging of the pipeline material,greatly reducing its service life and causing damage to the main pipeline.In this paper,the precipitation conditions of316NG austenitic stainless steel in the nuclear environment for a long time were studied by thermal aging simulation experiment,electrolytic extraction experiment and CALPHAD method.First,microstructure morphology and precipitation phase composition of the precipitated phase of 316NG austenitic stainless steel precipitated at 450?and 650?for 0-1000h were studied by using electronic scanning microscope-energy spectrometer,electrochemical analyzer and X-ray diffractometer,and calculated the mass percentage of its precipitated phase.Then,the thermodynamic database of 316NG austenitic stainless steel containing Fe,Cr,Ni,Mn,Mo,Si,Ti,Nb,C and N as components was established by the CALPHAD method.Based on this database,the precipitation phase composition and mass percentage of 316NG austenitic stainless steel at 450?and 650?equilibrium were calculated.Finally,the phase diagram calculation of 316NG austenitic stainless steel was performed by using this database to simulate the composition and the temperature under nuclear radiation,and the composition and content of the precipitated phase were predicted.The main conclusions obtained are as follows:1)The results of thermal aging simulation experiment and electrolytic extraction experiment show that:316NG austenitic stainless steel had no phase precipitation during annealing at 450?for 100 hours;Cr₂₃C₆ was the main precipitation phase and the content of precipitated phase is 2.9wt%during annealing at 450?for 500 hours;The main precipitated phases were Z-Cr Nb N,Cr₂₃C₆,and Laves?C14 and the content of the precipitated phase is 5.3wt%during annealing at450?for 1000 hours.The main precipitated phases of 316NG austenitic stainless steel were Z-Cr Nb N and Cr₂₃C₆ and the content of the precipitated phase is 2.4wt%during annealing at 650?for 100h;the main precipitated phases were Z-Cr Nb N,Cr₂₃C₆and Sigma???and the content of the precipitated phase is 6.2wt%during annealing at 650?for 500h;the main precipitated phases were Z-Cr Nb N,Cr₂₃C₆,Laves?C14???and Sigma???and the content of the precipitated phase is 10.5wt%during annealing at 650?for 1000h.In addition,the volume fraction of the precipitated phase was calculated using Image Pro image processing software.2)CALPHAD calculation results show that:the main precipitated phases of316NG austenitic stainless steel were Z-Cr Nb N,Cr₂₃C₆ and Laves?C14???,and the content of the precipitation phase is 5.58wt%at the equilibrium state of 450?;the main precipitated phases were Z-Cr Nb N,Cr₂₃C₆,Laves?C14???and Sigma???,and the content of the precipitation phase is 10.59wt%at the equilibrium state of 650?.Obviously,the composition and content of the precipitated phase calculated by the CALPHAD method are consistent with the results of the thermal aging experiment after 1000h of isothermal annealing.3)Based on the database,it is predicted that the main precipitated phases of nuclear grade 316NG austenitic stainless steel in the nuclear environment are Z-Cr Nb N,M₂₃C₆,Laves?C14???,Ni₃Si??,?and G-phase.Ni₃Si??,?has the most precipitated content,up to 8.5at%;Laves?C14???and G-phase precipitate less,a and the precipitated content is only 1.9at%and 0.65at%respectively;The carbon and nitrogen compounds M₂₃C₆ and Z-Cr Nb N precipitated in trace amounts,and the precipitated contents account for only 0.16at%and 0.26at%,respectively.