Study Of Stress Corrosion Cracking Initiation And Propagation Behavior Of Two Typical Nuclear Key Metal Materials

In order to ensure the rapid development of the national economy and reduce the emission targets,China is accelerating the development of nuclear power.Currently,the number of units under construction ranks first and the total number of unit's ranks third around the world.China will rank first around the world by 2025 and the development speed is unparalleled among other countries of the world.With the rapid development of nuclear power plants(NPPs)in China,especially after the Fukushima nuclear accident,the safety of nuclear power has become an important issue.The operating experience of NPPs shows that stress corrosion cracking(SCC)has become one of the main reasons of environmental failure of key metal materials in nuclear power plants.The initiation life of SCC accounts for about 90%of the total time of SCC failure.In this paper,SCC initiation behaviour of pre-deformed 316LN stainless steel(SS)in high temperature and high pressure water is studied.The obtained relevant data is of great significance for the safety operation and life management of NPPs.Main pipelines are serviced at 290?350 ? for long-time and the thermal aging problems of the materials will lead to increasing its SCC sensitivity,This will further affects the safe and stable operation of NPPs.Fewer papers has studied SCC behaviour of thermally aged SSs in high temperature and high pressure water.The SCC behavior of thermally aged 316LN SS and SA351-CF8A SS in simulated oxygenated/hydrogenated high temperature and pressure water is studied.By optical microscopy(OM),scanning electron microscopy(SEM)and electron back scattering diffraction(EBSD),SCC initiation behaviour of pre-deformation 316LN SS in high temperature and high pressure is studied.Results show surface cracks are initiated at the slip bands and grain boundaries,and most of the cracks are approximately perpendicular to the tensile direction.The SCC behaviour of thermally aged 316L SS for 2000 h in simulated primary high temperature and high pressure water is tested.Results show the CGR of thermally aged 316L SS is decreased gradually with the DO content in the solution decreasing from 2000 ppb to 1000,500,100 and<5 ppb.When the DO content decreasing from 500 ppb to the lower,the CGR of SCC is decreased significantly.The CGR is further decreased,when the solution is changed from DO to DH.SEM and EBSD are used to observe the fractures and crack propagation paths of the tested specimens.SCC fracture mainly presents a typical characteristic of intergranular stress corrosion cracking(IGSCC).The change of DO/DH mainly affects the mass transfer process of cracks tip and the repassivation process of the bare surface at cracks tip,which further affects the CGR.The thermal aging of 316L SS for 2000 h has a small effect on its microstructure and CGR of SCC in high temperature and high pressure water.The SCC behaviour of thermally aged SA351-CF8A SS for 2000 h in simulated primary high temperature and high pressure water is also tested.Results show the CGR of thermally aged SA351-CF8A SS is increased with increasing DO content.SCC fracture mainly presents a typical characteristic of transgranular stress corrosion cracking(TGSCC)and high residual strain is observed in the region near the crack.