The Investigation On Microstructure And Properties Of Thermal Aging And Stress Corrosion Sensitivity Of The Forged 316LN Steel

Nuclear power main pipe belongs to one of the key equipments of nuclear power and provides an important channel for the transmission of nuclear energy.Its service conditions are high temperature and high pressure water environment.During the long-term service,solute atoms will diffuse and segregate,not only the performance degradation but also the stress corrosion problem will occur.As the main pipe material of the third generation nuclear power reactor,316 LN austenitic stainless steel has a great influence on the safe operation of nuclear power.Therefore,it is of great engineering significance to study the thermal aging microstructure and stress corrosion cracking of 316 LN main pipe material.In this paper,the thermal aging microstructure evolution and mechanical properties of main pipe material 316 LN were analyzed from the perspective of thermal aging temperature and thermal aging time,and the problems of stress corrosion cracking under different conditions were explored.Optical microscopy(OM)and scanning electron microscopy(SEM)were used to observe and compared the microstructure of 316 LN solid solution,350 ? and 400? in different thermal aging states,which showed that the thermal aging structure did not change obviously.The lattice of austenite matrix constants near the grain boundary and the characteristic grain boundary led to trace changes during thermal aging,thermal aging increased the lattice constant and raised the proportion of special grain boundaries.The effect of thermal aging time on the impact toughness under the room temperature conditions after thermal aging at 350? and 400? was obtained through impact test,and it was concluded that the impact power value decreased with the increase of thermal aging time;From the impact load and displacement curve,the crack was mainly promoted in the form of stable expansion.With the increase of thermal aging time,the crack growth was increased and the crack propagation work decreases.Through fitting the impact energy value,the activation energy Q=44.07kJ/mol of forging 316 LN was obtained,while the equation of between thermal aging parameter P and the impact energy were obtained.Microhardness analysis also showed that thermal aging made the material embrittlement.Slow-strain-rate tensile tests showed that grain coarsening increased the SCC susceptibility,the source of the crack was formed from the surface of the tensile specimen and expands into the interior.After a long time of thermal aging,the mechanical properties and stress corrosion susceptibility of the material had stabilized.Electron backscattered diffraction(EBSD)technique was used to obtain the crack formation rule: the initial stage of crack initiation was easy to extend along the grain boundary with the external force axis by 50° to 70°,and the orientation difference was 50°~60°.Cracks were more easily generated and expanded.The above research not only have enriched the related data of thermal aging and stress corrosion,and provided a theoretical basis for its safe service.It has important engineering guiding significance for the steady development of China's nuclear power safety.