Study On The Cyclic Characteristics Of Surface Modified Layer Of 18CrNiMo7-6 Alloy Steel

After carburizing heat treatment,a certain thickness of modified layers will be formed on the surface of the material.The introduction of modified layers will change the microstructure and mechanical properties of the material.Under cyclic loading,materials often showed different mechanical response characteristics from monotonic loading,such as Bauschinger effect,cyclic softening and hardening,cumulative damage and so on.Therefore,the research on the mechanical properties,especially the cyclic characteristics,of the modified layer on the surface of materials is of great significance to improve the surface quality of workpieces and improve the fatigue life of key components.In thisdissertation,through static mechanical test,cyclic loading test and simulation,the effect of strain rate on the tensile properties of 18 CrNiMo7-6alloy steel with different surface modified layers thickness is analyzed,the variation laws of mechanical properties,hysteretic properties and energy dissipation capacity of18 CrNiMo7-6 alloy steel with different surface modified layers thickness are studied,the skeleton curve under cyclic loading is fitted and compared with the monotonic tensile curve.Based on Chaboche cyclic constitutive model,the cyclic constitutive parameters of 18 CrNiMo7-6 alloy steel with different surface modified layers thickness were calibrated and verified by simulation.The main research contents are as follows:(1)Monotonic tensile test and cyclic loading test were carried out on 18 CrNiMo7-6 alloy steel samples with different surface modified layers thickness.The differences of mechanical properties,hysteretic properties and energy dissipation capacity are analyzed.The results show that under monotonic tensile loading,the thicker the surface modified layers,the higher the yield strength and tensile strength and the lower the elongation after fracture;The strain rate has little effect on the tensile properties of heat-treated specimens.Under cyclic loading,the hysteretic curve shows Bauschinger effect,and gradually plumps with the increase of strain amplitude,and the yield surface gradually increases with the increase of strain amplitude;Under the same strain amplitude,the thicker the surface modified layers is,the greater the response stress is,the lower the energy dissipation coefficient is,and the weaker the energy dissipation capacity of the material is;The sample size has little effect on the hysteretic properties.(2)The skeleton curves of 18 CrNiMo7-6 alloy steel samples with different surface modified layers thickness were fitted and compared with the monotonic tensile stressstrain curve.The results shows that the cyclic strengthening coefficient increases and the cyclic strain strengthening index decreases with the increase of the thickness of the surface modified layers;The matrix sample shows the characteristics of cyclic hardening under cyclic loading.With the increase of the thickness of the surface modified layers,the sample gradually shows the characteristics of cyclic softening;The Chaboche cyclic constitutive model was used to fit the cyclic constitutive parameters of 18 CrNiMo7-6 alloy steel with different surface modified layers thickness.Importing the parameters into the simulation software can better simulate the actual mechanical response of the sample under cyclic loading.It shows that the Chaboche cyclic constitutive model has good applicability to 18 CrNiMo7-6 alloy steel with different surface modified layers thickness and can be applied to engineering practice.