Stress Corrosion Cracking Behavior And Crack Growth Rate Prediction Of Cold Worked 316L Stainless Steel In Pressurized Water Reactor Primary Environments

Operating experiences show that cold working has been one of the major factors to enhance the stress corrosion cracking?SCC?failure of pressurised water reactor?PWR?primary loop components that are made of austenitic stainless steels and Ni-base alloys.However,the mechanism is still unclear and the models to quantify and predict the crack growth rates need to be improved.Therefore,the present work studied SCC of cold worked stainless steels in PWR primary loop environments.The results are of great significance in corrosion science,and provide basic data for the safety evaluation of primary components,decision making during operational maintenance and lifetime prediction of a PWR nuclear power plant.The austenitic stainless steel?SS?type 316L,one of the most commonly used material to make PWR primary pressure boundary components,was selected as the testing material in this work.The SCC crack growth rates?SCCGR?of 0,10,20 and40%cold worked 316L SS under various simulated PWR high temperature and pressure water environments were carefully measured to study the effects of plastic deformation,testing temperature,concentration of dissolved oxygen and impurities?Cl^-?,and external load on crack growth,systematically.Modified compact tensition specimen and high resolution online crack length measurement technology were employed to aquire high confidence data under testing conditions changing“on the fly”.The SCC model and crack growth rate prediction formula were established by integrating the fracture surfaces and crack tip morphology,high confidence crack growth rate data,crack tip water chemistry.The following conclusions are drawn.?1?The SCCGR increased with increasing level of cold work plastic deformation in both oxygenated and hydrogenated water and a more dramatic change in hydrogenated water.The relations between crack growth rates and microhardness,yield strength,reisidual strain/stress,stess relieved by heat treatment,specimen cracking plane orientation provide sound evidence that enhanced yield strength and high residual strain/stress induced by cold work at grain boundaries are the essential factors that cause the high crack growth rate of cold worked stainless steel.?2?It was also observed that SCCGR of 316L stainless steel increase with the increase of stress intensity factor?K?applied on the crack tip,but the dependence is affected by the level of cold work and parameters of water chemistry.To be specific,the effect of K on SCCGR becomes weaker with the increase of deformation level of material,or corrosiveness of water environment.?3?The effects of environment parameters,such as temperature,dissolved oxygen,chloride,B-Li,and Zinc,on the SCCGR were studied by using 20%cold worked 316L SS specimens.The results show that SCCGR increases with increasing temperature in the range 210³25oC,and increasing dissolved oxygen in the range of 0¹0 ppm.A rapid rise in SCCGR was observed when 10 ppb of chloride was added into the testing environment,however,SCCGR ramp slowly when further increase chloride concectration from 10 up to 100 ppb.The addition of B?as H₃BO₃?and Li?as LiOH?affect SCCGR by changing pH of the testing environments.The SCCGR under PWR normal water chemisty condition is lower than that in the corresponding pure water.The SCCGR was reduced when 60 ppb Zn was added to the testing environment and careful microscopic analysis revealed the formation of ZnCr₂O₄ at the crack tip.?4?Successive SCC and creep expriments using 40%cold worked specimens demonstrate that the SCC mechanism of cold worked austenitic stainless steel is the strain enhanced oxidation and dissolution at the grain boundary,creep or hydrogen has a negligible effect on cracking under the present testing conditions.Different types crack growth rate predicton formula were proposed based on the recursion of experimental data and discussion on previous SCC models and prediction formulas.