Experimental And Predictive Study Of Stress Corrosion Cracking Initiation On Machined Surface Of Austensite Stainless Steels

Austenitic stainless steels,the most used structural materials for nuclear power,petroleum,and offshore engineering equipments,are highly susceptible to stress corrosion cracking（SCC）.Although the austenitic stainless steels have excellent mechanical strength and corrosion resistance,the machining operations can cause changes in the surface properties during manufacturing process,affecting the surface mechanical,physical and chemical properties,and thereby affecting the SCC resistance of the structure component.Nevertheless,the impact of machining on the SCC behavior of austenitic stainless steels still lacks of indepth study.Focusing on the austenitic stainless steels,this dissertation is aimed at exploring the effects of machining-induced residual stress and microstructure changes on SCC initiation and constructing an analytical prediction model for SCC initiation on machined surface.The main work and innovations are carried out as follows,In order to explore the correlation between machining-induced residual stress and SCC initiation,SCC tests were designed and carried out for machined surface of austenitic stainless steels in boiling MgCl₂ solution and in simulated pressurized water reactor（PWR）environment.The characteristics of SCC micro-crack initiation and propagation in the early phase were revealed by quantitatively analyzing the relationship between the crack density,crack depth and the residual stress level in boiling MgCl₂ solution.The results revealed that only when residual stress was greater than a critical stress,the micro-cracks induced by residual stress could be initiated and developed significantly.The critical stress was about 200 MPa for 304 and 316stainless steels.In addition,the tensile specimens of 316 stainless steel with different levels of machining-induced residual stress were subjected to a constant strain SCC test in PWR environment for 3600 hours.The results revealed that when the specimens were subjected to both residual stress and external load,the resultant critical stress for SCC crack initiation was about 200 MPa.This result indicates that the critical stress values of SCC crack initiation in a strong corrosion environment and under a certain external load are the same.In addition,the strength of the passivation film of 316 stainless steel was determined by the relationship between the residual strain and the surface crack density.In order to establish the influence of machining-induced microstructure changes on the development of SCC micro-crack,SEM,TEM and EBSD techniques were used to investigate the microstructure alterations in the deformed layer.Based on EBSD,an analysis method was proposed to characterize the machining-induced microstructure changes.The misorientation parameters,grain boundary parameters were adopted to evaluate the microstructure changes,and a characterization model for the deformed zone was established.On this basis,the correlation between the microstructure characteristics and the micro-crack development was analyzed.The results showed that the SCC micro-cracks initiation and development in the near surface zone were affected by the dislocations and slip bands caused by machining.When the dislocation density was high enough and the local misorientation was greater than 2°,the micro-cracks could be developed.Additionnaly,the early micro-cracks mainly extended along the slip bands.The influence of machining on SCC initiation in austenitic stainless steels is summaried.The interactions of the microstructure changes,the residual stress and the critical strain of passivation film on SCC initiation are studied.Based on the slip-dissolution model for crack propagation,an analytical prediction model is proposed for SCC initiation on the machined surface,which is the first time to establish the relationship between the SCC micro-crack propagation and the surface micro-characteristic parameters.In order to analyze the SCC initiation on the machined surface,a predictive model for machining-induced residual stress and microstructure changes is constructed for austenitic stainless steels.By analyzing the thermo-mechanical coupling effects under orthogonal cutting condition,the method of calculating the multi-physics fields of the workpiece surface is proposed.Then,according to the strain induced martensitic transformation kinetics and dislocation density model,the model of evaluating machining-induced microstructure changes is proposed,and the predictions of the slip band volume fraction,dislocation density,martensite content and micro-hardness are realized.In addition,based on the mechanism of residual stress and the loading-unloading model,the prediction of residual stress is achieved.The proposed model was verified through orthogonal cutting experiments on 304 stainless steel.Synthetically applying the predictive model for SCC initiation and the predictive model for machining-induced residual stress and microstructure changes,the calculation flow of SCC crack sensitivity is constructed.The prediction and analysis of the SCC initiation on the machined surface are realized.Through simulating the microstructure changes and residual stress under different cutting conditions,the SCC initiation sensitivity is predicted for different surfaces,and the synthetical effects of residual stress and microstructure changes on SCC initiation are analyzed.At last,the influence of processing parameters on SCC initiation is evaluated.Through experimental testing,theoretical research and simulation analysis,this dissertation elaborates the effects of machining-induced surface properties on the SCC initiation.The prediction and analysis of SCC initiation on machined surface are realized.The research results can provide theoretical support and scientific basis for improving the processing technology,optimizing processing parameters and enhancing the surface SCC resistance.