Investigation On The Fracture Of Stainless Steel For Nuclear Primary Pipes Considering The Effect Of Thermal Aging Embrittlement And Crack Tip Constraint

In the primary circuit system of nuclear power pressurized water reactor,the main pipe is the primary safety component that transfers the heat energy generated by nuclear fuel in the core to the secondary loop through circulating water.The main function of the main pipe is to transfer energy through the working medium,and convert most of the kinetic energy of the medium into pressure energy.Austenitic stainless steel used for main pipeline is easy to produce thermal aging embrittlement effect in harsh service environment such as high temperature,high pressure and radioactivity,which leads to fracture toughness decrease,thus greatly increasing the risk of reactor operation.Therefore,the investigation on the influence of thermal aging on the fracture behavior of stainless steel in the main pipe will provide technical support for the safety design and reliability evaluation of the main pipe,which has important theoretical significance and engineering application value.In this thesis,the thermal aging treatment at different time of 316LN stainless steel which was applied for the main pipeline material of AP1000 reactor had been made.The influence of thermal aging term and sample size on the fracture property of 316LN stainless steel were investigated.The main work of this thesis contains:(1)JSM-6610LV scanning electron microscope was used to observe the microstructure of 316LN stainless steel with different thermal aging time,and the mechanism of thermal aging of 316LN stainless steel was explored.The results show that,the precipitation of Cr-rich?'phase in ferrite phase is the main cause of thermal aging embrittlement of 316LN stainless steel.The content of the precipitated?'phase increase with the increase of aging time.(2)Uniaxial tensile test and hardness test of 316LN stainless steel with different thermal aging time were completed at room temperature.The experimental results show that the tensile stress-strain curve of 316LN stainless steel is significantly affected by thermal aging time.When the aging time is less than 2000h,the elastic modulus of 316LN stainless steel increases gradually,and when the aging time is more than 3000h,the elastic modulus decreases significantly.When the thermal aging time is 1000h,the yield strength and ultimate strength of the material are significantly higher than the material without thermal aging treatment.However,when the thermal aging time is 2000h,the yield strength and ultimate strength decrease significantly.The yield strength and tensile strength increase when the aging time exceeds 2000h.However,after the aging time exceeds 3000h,the yield strength and ultimate strength do not change significantly.Both the Rockwell hardness and Brinell hardness of 316LN stainless steel decrease obviously with the increase of thermal aging time.(3)Based on the principle of energy equivalence,the semi-analytical expressions of stress intensity factor K and J integral of SEB samples were established,and the quasi-static fracture test method based on Mini-SEB samples was established.The quasi-static fracture tests of316LN stainless steel with different thermal aging time were carried outby using SEB samples with different size.The effects of thermal aging time and sample size on the fracture properties of the materials were investigated.The results show that at the same aging time,the conditional crack initiation toughness J_0.2BL of 316LN stainless steel decreases obviously with the increase of sample thickness.Although the values of conditional crack initiation toughness J_0.2BL of316LN stainless steel are different due to different sample sizes,it decreases obviously with the increase of thermal aging time.(4)Based on the analysis of equivalent stress field ahead of crack tip,a model of the relationship between fracture toughness of 316LN stainless steel and crack tip constraint parameter was proposed considering the effect of thermal aging embrittlement,and the validity of the model was verified.