Microscopic Mechanism And Deformation Model Research Of High Temperature Low Cycle Fatigue Of Sanicro 25 Steel And Welded Joint

Sanicro 25 novel austenitic heat-resistant steel has a wide broad application prospest in 700?/35 MPa ultra-supercritical units.Welding is the key technology to ensure its connection.Combined with microstructure analysis and cyclic deformation behavior and based on the interaction between dislocation and precipitate,this dissertation establishes the cyclic hardening model based on the microstructure mechanism of cyclic hardening for Sanicro 25 steel and its welded joints.Furthermore,the analytical models of fatigue life based on the microstructure are established according to the microstructure mechanism of cyclic damage.All these investigations provide theoretical basis for design of materials under fatigue conditions,analysis of fatigue failure mechanism,and fatigue life assessment.The main conclusions of this study are as follows:(1)Based on the dislocation morphology and Cr₂₃C₆distribution,the cyclic hardening mechanism is revealed and the cyclic hardening model is established.When the strain rate is less than 4×10^-3s^-1,the Cr₂₃C₆ precipitates around Nb Cr N precipitate can bend the dislocations and generate the modied FHP back stress,which leads to the cyclic hardening.When the strain rate is 4×10^-3s^-1,the Cr₂₃C₆ precipitates nucleated at the interface between Nb Cr N precipitate and matrix can pin the dislocations,which leads to the cyclic hardening.The cyclic hardening model based on the distribution of Cr₂₃C₆ precipitates can accurately calculate the maximum cyclic stress.(2)Based on the interaction between dislocation and precipitate,the microstructure mechanism of cyclic damage is proposed.When the total strain amplitude increases,the interaction between dislocation and precipitate increases.It leads to the concentration of fatigue cracks around the precipitates,namely,the increase in the Gibbs free energy change of crack nucleation.Therefore,the dispersed damage turns into the concentrated damage,which leads to the significant decrease in the fatigue life.When the strain rate decreases,the interaction between dislocation and precipitate increases.It leads to the increase in the number of creep cavity around the precipitates,namely,the increase in the creep damage.Therefore,the increase in the creep damage leads to the decrease in fatigue life.(3)The influence mechanism of the softening behavior(dislocation annihilation)during the II stage of cyclic hardening on the fatigue life is proposed.The dislocation annihilation mechanisms during compressive loading are revealed:annihilation caused by cross-slip of screw dislocations and annihilation of adjacent edge dislocation dipoles.The analytical model of fatigue life based on the dislocation annihilation mechanisms indicates that when the ratio of the dislocation annihilation rate to the average rate of change in dislocation density increases,the faituge life increases.(4)The influence mechanism of different postweld heat treatment conditions on fatigue properties is researched.The evolution of the dislocation morphology in the weld metal determines the fatigue life of welded joint.Under the optimal PWHT condition,the microstructure mechanism of cyclic deformation of welded joints is revealed.When the total strain amplitude incrases,the dislocation morphology in the weld metal changes from the distribution parallel to the loading axis(0.2%)to the dislocation tangle(0.3%and 0.4%),and finally to the entanglement of dislocation on the precipitates(0.5%).The cyclic hardening model based on the dislocation morphology in the weld metal can accurately calculate the maximum cyclic stress of welded joints.(5)Considering the positive cyclic yield strain and symmetric triangular wave,the fatigue damage eqution based on the maximum cyclic stress is established.Combining the cyclic hardening model and fatigue damage eqution,the analytical model of fatigue life of welded joints is established.This model can accurately represent the influence of the microstructure in weld metal on the fatigue life of welded joints.