Study On Sample Size Effect Of Grain Boundary Engineering And Molten Salt Corrosion Behavior Of 304 Austenitic Stainless Steel

Grain boundary engineering(GBE)has become an important method for improving the performance of polycrystalline materials and has been widely used.304 austenitic stainless steel has excellent comprehensive performance and is one of the candidate materials for molten salt reactor nuclear power plants.In this study,the grain boundary structure of 304 austenitic stainless steel was controlled by cold rolling and high temperature annealing.The microstructure,grain size and grain boundary character distribution(GBCD)of the annealed samples were compared in order to investigate the sample size effect of grain boundary engineering.The molten salt corrosion behavior of 304 austenitic stainless steel before and after grain boundary controlment were studied by molten salt corrosion experiment.The main research work and results are as follows:(1)Three 304 austenitic stainless steel samples with different sizes were subjected to 5%deformation by cold rolling,and then annealed at 947?for different time.The microstructure and grain boundary character distribution of the annealed samples were analyzed by scanning electron microscopy and EBSD system.It is found that the special grain boundary ratio of the grain boundary engineering material(GBEM)is more than 20%higher than that of the base materials(BM).When the annealing time is short,the grain boundary structure regulation can not be fully realized in the large-size samples.The GBCD between the edge region and the central region of the large-size sample is different.The corrosion resistance of the edge region is better than that of the central region.After 7h annealing,the tensile mechanical properties of the small-sized sample are better than that of the large-size sample.Indirectly proves that the size effect of the sample exists during the annealing process of GBE which should be considered in practical applications.(2)The molten salt corrosion behavior of 304 austenitic stainless steel optimization of GBCD in 35%NaCl-35%KCl-30%MgCl₂ molten salt at 650?was studied.The results show that the corrosion rate(the weight gain rate)of GBEM is obviously slower than that of BM.The surface of the corroded BM is rough,and there is obvious phenomenon of corrosion along grain boundaries.The surface of GBEM is relatively flat,indicating that the GBE can improve the corrosion resistance of molten salt of 304 austenitic stainless steel.Because the GBE produces a large number of low-energy grain boundaries in the structure.The low-energy grain boundaries interrupted the connectivity of the large-angle free grain boundaries,making the diffusion of atoms on the grain boundaries more difficult and hindering the inward expansion of the corrosive medium along the grain boundaries.Thereby the resistance to molten salt corrosion of the material are improved.