Stress Corrosion Cracking And Fatigue Behavior Of Cold Drawn 316 Austenitic Stainless Steel

With the development of nuclear power industry,we have increasingly paid attention to the safety of key materials in nuclear power plants.316 austenitic stainless steel has been widely used for main circuit pipes and structural components in the pressurized water reactor,because of its excellent corrosion resistance and mechanical properties.The cold working can significantly improve the yield strength and tensile strength of austenitic stainless steel.Meanwhile,the cold working can produce some defects,such as dislocations,deformation bands,mechanical twins and deformation induced martensite,which affects stress corrosion cracking?SCC?and fatigue behavior of 316 austenitic stainless steel in different degrees.Cold-drawing 316 austenitic stainless steel is mainly used for high strength bolt material in the pressurized water reactor.In this paper,a large number of dislocations,deformation bands and mechanical twins were formed in cold-drawn 316 stainless steel.Recently,the SCC behavior of cold-drawn 316 austenitic steel in lithium boron solution containing Cl at high temperature and pressure needs to be further studied.The studies on the evolution of the SCC crack tip of cold-drawn 316 austenitic steel in lithium boron solution containing Cl are not perfect until now.Additionally,the evolution of dislocation structures in cold-drawn 316 austenitic stainless steel during cyclic deformation needs to be further studied.The understanding of interaction between mechanical twin and dislocation motion during cyclic deformation needs to be further improved.Research on these issues can further improve understanding of SCC and fatigue behavior of cold-drawn 316 austenitic stainless steel,and then can provide service performance evaluation of cold-drawn 316 austenitic stainless steel with theoretical basis and experimental evidence.The SCC in simulated pressurized water reactor environment and fatigue behavior at room temperature for cold-drawn 316 austenitic stainless steel used for high strength bolts in the pressurized water reactor was studied in this paper.The microstructure and evolution of SCC crack tip was investigated by scanning electron microscopy and transmission electron microscopy.The effect of cold drawing on the chloride-induced SCC behavior of 316 austenitic stainless steel in simulated pressurized water reactor environment was studied.The evolution law of the dislocation structures in cold-drawn 316 austenitic stainless steel during cyclic deformation was analyzed and discussed.The mechanism of interaction between mechanical twin and dislocation motion was discussed.The main conclusions are as follows:?1?A mass of dislocations are produced in austenite grains of the cold-drawn 316austenitic stainless steel.The networks of mechanical twins and deformation bands segment and refine the austenitic grains.No deformation induced martensite was found in the cold-drawn 316 stainless steel.?2?SCC slow tensile experiments for solution-treated and 30%cold-drawn 316austenitic stainless steel were conducted in simulated pressurized water reactor environment.Both cold drawing and Cl addition in corrosion environment observably increase SCC susceptibility of 316 austenitic stainless steel.In lithium boron solution,the SCC behavior of solution-treated 316 austenitic stainless steels didn't occur,while the quasi-cleavage morphology was observed at the rim of the fracture surface of the cold-drawn 316 austenitic stainless steel showing TGSCC characteristics,with I?=16.7%.In lithium boron solution containing Cl,both solution-treated and cold-drawn 316 austenitic stainless steel exhibited TGSCC characteristics.The SCC susceptibility?I?=27.0%?of the cold-drawn 316 austenitic stainless steels is much higher than that?I?=11.5%?of the solution-treated 316 austenitic stainless steel.The deformation twins and deformation bands activated by cold-drawing provide paths for the oxygen diffusion,and accelerate the crack initiation and propagation during SCC.The anodic dissolution of the cold-drawn 316 austenitic stainless steels is related to the oxides formed on the crack tip.The oxides containing higher Cr concentration with higher corrosion resistance initially nucleate on crack tip during the TGSCC crack initiation,and then react with dissolved oxygen and H₂O to form the Fe-rich oxides on crack flank in lithium boron solution containing Cl.?3?The fatigue tests for cold-drawn 316 austenitic stainless steel were performed at room temperature.The softening ratios of 30%cold-drawn steel are significantly lower than those of 20%cold-drawn steel at strain amplitudes below 0.5%.Above the strain amplitudes of 0.5%,the softening ratios of 30%cold-drawn steel are slightly higher than,but very close to those of 20%cold-drawn steel.The threshold strain is required for cyclic softening to occur during fatigue behavior for 316 austenitic stainless steel.The high cycle fatigue life of cold-drawn 316 austenitic stainless steel is significantly higher than that of solution-treated 316 austenitic stainless steel.The 30%cold-drawn type 316 stainless steel shows higher elastic strain amplitudes,resulting in higher high-cycle fatigue life at strain range below about 0.80%,while the 20%cold-drawn steel exhibits higher plastic strain,leading to higher low-cycle fatigue resistance at strain range above about 0.80%.?4?The mechanical twin spacing can significantly affect the evolution of dislocation structures in cold-drawn 316 stainless steel during cyclic deformation,and the constraint effect of mechanical twin on dislocations increases as the mechanical twin spacing decreases.The network of mechanical twins and deformation bands can effectively reduce the formation of persistent slip band?PSB?,and decrease the opportunity for PSB to traverse austenitic grain,and delay the fatigue crack initiation.?5?The observation by HRTEM reveals that the dislocations could glide along the twin boundary?TB?under cyclic loading.Additionally,the dislocations are blocked by TB,and dislocation reactions occur.The atomic layer steps are formed at the TB,and the atomic layer steps glide along the TB and gradually coalesce to form a large-scale step.?6?The deformation bands in cold-drawn 316 stainless steel move by cross slip mode under further cyclic loading.The annihilation of screw dislocations makes a contribution to the decrease of dislocation density and the formation of dislocation cells in deformation bands.