Research On Fatigue Fracture Properties Of Inconel 690 Heat Transfer Tube

The steam generator heat transfer tube is an important part of the primary circuit pressure boundary,and its structural integrity is essential for the safe operation of the nuclear power plant.At present,Inconel 690 alloy and Incoloy 800 alloy are widely used in steam generator heat transfer tubes.There have been a lot of studies on the fatigue fracture properties and corrosion resistance of these alloys in-depth,but it is relatively few about the research on fatigue fracture properties of Inconel 690 heat transfer tubes after forming.Therefore,carrying out the corresponding research has important theoretical and engineering significance for the fatigue design and structural integrity assessment of 690 heat transfer tubes.In this paper,two kinds of 690 alloy notch tube specimens were designed,and the fatigue life test and fatigue crack growth test of the notched component were carried out.The main research contents and conclusions are as follows:(1)If the thin-walled tube is used for fatigue test directly,the crack initiation will occur at the clamping position of the specimen due to the stress concentration,and the effective fatigue life can not be obtained.Therefore,the method of notching is adopted to ensure the crack initiation position,and the tube specimen with a through-wall circular hole is designed.The fatigue life test was carried out with this specimen.There is an obvious cyclic hardening phenomenon in the initial stage of cyclic loading.The fatigue fracture morphology analysis shows that the fatigue crack initiates near the outer wall of the pipe at the edge of the hole,penetrates the wall firstly and propagates along the circumferential direction.The Neuber’s method is used to calculate the local stress and strain of the circular notch,and the equivalent fatigue life of the pipe structure is predicted.The fatigue life test results are in good agreement with the predicted values.It is shown that notched tube specimens can obtain effective fatigue life data,and the method of estimating the equivalent fatigue life of tubular structures by local stress-strain method is feasible.The equivalent fatigue life of 690 heat transfer tube is higher than the fatigue curve of austenitic stainless steel and nickel-based alloy.Thus,Inconel 690 heat transfer tube made in China has better fatigue performance.(2)In fatigue crack growth test,the stress intensity factor of specimen needs to be calculated.By comparing the geometric coefficients of the different stress intensity factors solutions for cylinder with through-wall crack,the deviations between the solutions can be neglected only in the case of short cracks(θ/π≤0.3)and thick-walled(R_m/t≤10).Therefore,the stress intensity factor(SIF)of specimens is calculated by finite element method.The models of different crack lengths are established and the stress intensity factors of a tube with a circumferential through-wall crack under uniaxial tension are calculated.The simulated SIF agrees well with some solutions,which verifies the correctness of the finite element model and boundary conditions.The geometric shape coefficient polynomial of SIF and crack length polynomial for heat transfer tube specimens about crack opening displacement compliance are obtained by fitting.(3)The fatigue crack growth test was carried out on a tube specimen with circumferential through-wall crack.The experimental load-displacement relationship is in good agreement with the results of finite element simulation,which verifies the correctness of the six-order fitting polynomial obtained by simulation.The crack growth rate(CGR)under different stress ratios R and average stress conditions was calculated based on the polynomial fitted by finite element analysis.Under the same maximum nominal stress condition,the larger the stress ratio,the smaller the CGR of690 heat transfer tube specimen;under the same stress ratio condition,the larger the maximum nominal stress,the larger the CGR of 690 heat transfer tube specimen.The CGR of thick-walled specimens under the same conditions is predicted by using ANL superposition model of 690 alloy.Compared with the experimental values of tube specimens,the data points fall below the isoline.The results show that the crack growth rate of thin-walled tube specimen is lower,and Inconel 690 heat transfer tube has better fatigue crack growth resistance.Based on finite element simulation and experimental study,the notched fatigue life of Inconel 690 heat transfer tube structure is obtained,and the feasibility of estimating the equivalent fatigue life of tube structure by local stress-strain method is verified.The crack growth rate of 690 heat transfer tube specimens was obtained by using the geometric shape coefficient polynomial of crack stress intensity factor and crack length prediction formula fitted by finite element analysis.The effects of different stress ratios and average stresses on CGR of 690 heat transfer tube were analyzed.The research has high value for engineering application and structural integrity evaluation of 690 heat transfer tube.