The Investigation On Solution Treatment Microstructure And Thermal Aging Properties Of316LN Austenitic Stainless Steel

316LN austenitic stainless steel is used in the third-generation nuclear power plant. Inthe forging process there are always uneven distribution of deformation and temperature,which results in the difference of microstructure and grain size in different areas. Thesolution treatment have a significant impact on Homogeneous and corrosion resistance.Thermal aging phenomenon will occur in the material due to the long-term working at288-327℃, which affects the safety of nuclear power. Therefore, the study ofmicrostructure after soluted and thermal aging is significant.In this paper, the influence of deformation amount on dynamic recrystallization. Theresults showed that there was a critical deformation amount above which recrystallizationwould occur. The critical value decreased from28.3%to4.9%with the increasing oftemperature from1000℃to1150℃.The rule of recrystallized microstructure evolution in the solution process was studiedby Gleeble thermal simulation experiment and the solution heat treatment. In the processof solution heat treatment, static recrystallization and sub dynamic recrystallization wouldoccur. The average grain size was affected by deformation parameters and solution heattreatment process:D1025℃=25.0t0.038; D1050℃=30.1t0.042; D1075℃=32.2t0.042; D1100℃=34.4t0.072;D1125℃=41.3t0.117. Under the same heat treatment process, the grain size increased with thedecreasing of deformation amount and the increasing of deformation temperature.The influence of the solution cooling rate on SCC sensitivity was studied by slowstrain rate tensile test. The results showed that the larger the cooling rate was, the smallerSCC sensitivity was. With the increasing of the cooling rate, lattice constant, the Vickershardness and the resistivity increased, and the thermal conductivity decreased. In thematerial of rapid cooling, the dislocation density and the degree of dislocation tangleswere both larger. While in the material of slow cooling, the dislocation densitysignificantly decreased, and there were lots of twins, stacking faults and extendeddislocations.By tensile test at room temperature and350℃and the temperature shock test, theinfluences of thermal aging on the mechanical properties of the steel were analyzed. With extension of thermal aging time, the strength significantly increased, the ductilitydecreased, and moreover the mechanical hardening index n on various stages increased.The impact energy of316LN stainless steel declined13.7%after the aging for5000h. Thedimples were the characteristic of the appearances of the micro-fracture. The fracturemechanism was fracture of the micro-void coalescence. With the extension of aging time,the dimples became smaller and lighter.