Low Cycle Fatigue Life Prediction And Residual Stress Analysis Of High Strength Steel

In order to design and produce advanced high strength steel with higher application requirements,the scientific laws of microstructure and property control of advanced high strength steel are studied deeply.In order to evaluate the reliability of components,the fatigue life prediction model and cyclic elastoplastic constitutive model are established to predict the fatigue behavior and fatigue life of high-strength steels.The performance potential of steel is explored by adjusting composition or new heat treatment process.It is necessary to study the relationship between microstructures and properties of high-strength steel,and enrich the basic theory of high-strength steel performance evaluation,which lays a foundation for the production technology and performance improvement of high-strength steel.Taking martensitic high-strength steel as the research object,the effects of heat treatment parameters on the microstructure and properties of high-strength steel are systematically studied by using scanning electron microscopy,transmission electron microscopy and X-ray characterization techniques,tensile and fatigue testing methods and finite element simulation methods.The relationship between microstructure and properties of high strength steel as well as the prediction methods of fatigue life and residual stress are studied deeply.The relationship between tempering process and low cycle fatigue behavior of high strength steel is studied.The low cycle fatigue properties and microstructure of the new low carbon martensitic steel are tested at different tempering temperatures(320℃,350 ℃,380 ℃).The results show that with the tempering temperature increasing,the width of martensite lath increases,the retained austenite decompositions,and the dislocation density in the lath decreases.The fatigue life of martensitic steel decreases with the tempering temperature increasing.A plastic strain energy fatigue life prediction model considering tempering temperature is proposed and the fatigue properties of martensitic steel under different tempering temperatures are evaluated.The fatigue life model can predict the fatigue life of martensitic steel at different tempering temperatures.A cyclic elastoplastic constitutive model of high strength steel considering hysteresis damage and tempering temperature is established.The hysteresis loop,damage accumulation and cyclic elastoplastic constitutive model of low carbon martensitic steel are studied.When the cumulative damage exceeds 0.70 or so,the cyclic stress rapidly decreases.By introducing the damage accumulation model of hysteresis energy method,an improved kinematic and isotropic composite hardening model is proposed.This model is used to simulate the cyclic stress-strain behavior of martensitic steel before fracture and to predict the fatigue life of martensitic steel with applied strain amplitude and tempering temperature.The results show that the modified model can describe the acceleration of cycle softening and the reduction of cycle times at different tempering temperatures.The classical cyclic elastoplastic constitutive model is often difficult to simulate the rapid decline stage of cyclic stress.The established hysteresis damage constitutive model can simulate the rapid decline stage of cyclic stress well and improve the prediction accuracy of fatigue life.A calculation method of residual stress considering martensitic transformation is proposed so as to predict the residual stress of quenched high strength steel more accurately.At the same time,the evolution process of residual stress during heat treatment can be obtained by finite element method,which is impossible to do by test method.The effects of quenching medium,cooling rate,martensitic transition starting temperature and tempering temperature on the residual stress of martensitic steel are analyzed by using the established residual stress prediction model.By studying the influence of quenching medium on the residual stress of martensitic steel,it is found that the residual stress of martensitic steel is minimum under the condition of air cooling.Through the study on the effect of quenching temperature on the residual stress,it is found that the reduction of cooling rate is beneficial to reduce the residual stress.Reducing the Ms temperature of martensitic steel is also beneficial to reducing the residual stress.The effect of tempering on residual stress is studied,and it is found that the residual stress decreases after tempering.The residual stress decreases with increasing tempering temperature.The influence of mechanical property parameters on low cycle fatigue behavior and low cycle fatigue life of high strength steel is studied.The influence of tensile properties on low cycle fatigue life was systematically studied.The tensile mechanical properties and low cycle fatigue properties of high strength steels with different strengths are tested.The results show that the increase of tensile strength is helpful to improve the fatigue life of high strength steel under the condition of low strain amplitude.At higher strain amplitudes,higher elongation is helpful to improve the fatigue life of high strength steels due to the dominant cumulative plastic strain.Using the damage hysteresis energy method,the fatigue life can be modeled to show its dependence on tensile strength and elongation.The results show that the tensile strength is positively correlated with the inherent fatigue toughness,and the elongation is positively correlated with the damage transition index.The variation of low cycle fatigue life of high strength steel with different strength can be predicted effectively by mathematical analysis of fatigue life with tensile strength and elongation.The fatigue life prediction model based on tempering temperature,the cyclic elastoplastic constitutive model coupled with hysteretic energy damage and tempering temperature,the residual stress prediction model considering martensitic transformation and the fatigue life mathematical analysis model based on mechanical properties are established in this paper..The research results provide basic theoretical support for the development and application of high strength steel.