Study On Tool Fatigue Damage And Life Prediction For Heavy Milling Water Chamber Head

As a key component of AP1000 evaporator,the head of water chamber is mainly cut by heavy milling.The water chamber head material is made of 508? steel,which has the characteristics of high strength,high hardness,high reduction of area and other difficult machining characteristics.As well as large cutting parameters,the tool is seriously damaged and failed in heavy milling process.Through the study of the failure mode of heavy milling tools,it is found that the fatigue cumulative damage is the main cause of tool failure.As the base material of cutting tool,cemented carbide endures large thermo mechanical alternating load continuously in heavy milling process,which makes the initial damage defect in cutting tool material evolve into macro micro crack gradually.With the accumulation and expansion of macro-micro crack and the cutting vibration produced by cutting in and cutting out,the fatigue damage failure of cutting tool is accelerated and the service life of cutting tool is reduced,the tool is broken ultimately,which seriously affects the machining efficiency.Therefore,based on the theory of damage mechanics,this paper studies the fatigue failure mechanism and damage evolution process of heavy milling tools,and analyzes and predicts the fatigue life of tools.First of all,milling experiments are carried out on the head material of the water chamber under the laboratory conditions by using the on-site machining parameters to analyze the influence of the cutting parameters on the cutting force and the cutting temperature;the milling process is simulated and analyzed to determine the cutter stress distribution state during the cut in and cut out process;Based on the irregularity of the contact area and the inhomogeneity of the milling thickness,the dynamic cutting force model of the water chamber head material milling is established.The above research provides the technical basis for the analysis of the influence of the thermal mechanical cycle load on the damage and failure of the tool material and the prediction of the tool life.Secondly,the failure test of heavy-duty milling tool of water chamber head was carried out,the failure morphology and vibration frequency of the tool were analyzed,and the failure form and the type of fatigue load were determined;Based on the irreversible thermodynamics and continuum damage mechanics,the low cycle fatigue damage model and high cycle fatigue damage model of cemented carbide materials are established to describe the evolution process of tool material fatigue damage,and the failure mechanism of tool material is analyzed according to the distribution of mechanical stress and thermal stress.Thirdly,the fatigue crack growth test and high temperature mechanical properties of cemented carbide tool materials are studied.According to the crack growth theory and Paris formula,the condition of tool fatigue damage and the rate of fatigue crack growth are analyzed;through the high temperature tensile test,the influence of temperature on the mechanical properties of cemented carbide is analyzed.At the same time,the morphology of pull-down fracture in high temperature state is analyzed,which provides the basis for the failure analysis and fatigue life prediction of cutting tools.Finally,based on the local stress-strain life analysis method and linear fatigue cumulative damage theory,using ANSYS finite element software and the high temperature mechanical property experimental data,the low cycle fatigue simulation analysis of the tool material is carried out,and the parameters of the fatigue life model are determined;at the same time,the low cycle fatigue life and high cycle fatigue life of the tool are predicted.Based on the effective stress of the tool in the maximum fatigue life,the damage limit value of the tool is calculated.