Investigations On Fatigue Properties And Damage Mechanisms Of Two Compacted Graphite Irons At Different Temperatures

Fatigue damage is one of the common manifestations of engineering materials failure during working.Compacted graphite irons(CGI),as an important material for the preparation of engine cylinder head,often suffers from fatigue damage due to the cyclic impact of high-temperature and high-pressure gas under working condition.The harsh working environment combined with the special microstructure makes the fatigue damage process of CGI extremely complicated.The keys to ensure the long-term safe service of diesel engine cylinder head of CGI are to clarify the fatigue damage mechanism of CGI at different temperatures,to propose a simple and effective fatigue performance prediction model,and to find a feasible way to optimize fatigue performance.Based on the actual service environment of the diesel engine cylinder head,this thesis has studied the high-and low-cycle fatigue(HCF and LCF)damage processes of CGI at different temperatures and different loading conditions;The corresponding fatigue damage behavior,fracture morphology and fatigue crack initiation and propagation have been investigated,and the key factors affecting the fatigue performance of CGI have been analyzed;Based on these,the fatigue strength/life prediction model and the corresponding optimization scheme have been discussed.This thesis aims to provide experimental basis and theoretical reference for producing high-performance diesel engine cylinder head and high-reliability CGI materials.The main results are as follows.(1)As the temperature increases,the HCF strength of CGI(RuT400)tends to increase first and then decrease.At room temperature,the HCF cracks initiated from the tip of vermicular graphite and propagated along the ferrite matrix.With the increase of temperature,the fatigue damage mechanism gradually transformeds into the mutual competitions among graphite clusters,oxidation,carbide precipitation,and grain boundary sliding.The hindering effect of precipitated carbides on dislocation motion is the main reason for the increase of fatigue strength at 400?.While at 500?,high-temperature oxidation damage and sub-grain boundary softening accelerate the process of fatigue crack initiation and propagation,resulting in a decrease of fatigue life.According to the fatigue damage behavior of CGI at different temperatures,a fatigue strength prediction model based on interphase corrosion depth is proposed.This model avoids the influence of graphite morphology and content on fatigue performance,and provides a new method for the optimization of fatigue performance of CGI.(2)When the strain rate remains unchanged,the LCF life of CGI(RuT500)increases slowly with the increase of temperature and then decreases significantly.The LCF cracks of CGI at different temperatures initiated from graphite clusters at the edge of the specimen.At400? and strain amplitudes of 0.25%and 0.3%,the matrix of CGI shows obvious cyclic hardening during cyclic loading due to the influence of precipitated carbides.Cyclic hardening increases the driving force for fatigue crack propagation and adversely affects the fatigue life.When the strain amplitude drops below 0.2%,the uniform strain of CGI significantly inhibits cyclic hardening.At the same time,the oxidation-induced crack closure mechanism also counteracts the negative effect of oxidation on the fatigue crack initiation life to a certain extent,and eventually leads to a significant increase of fatigue life.When the temperature is increased to 500?,the grain boundary softening and serious oxidative damage cause a significant decrease of the LCF life of CGI.(3)The decrease of strain rate has little effect on the LCF life of CGI(RuT500)at room temperature,but it leads to a significant decrease of that at 400? and 500?.At 400?,the initial cyclic hardening caused by dynamic strain aging effect combined with the serious damage localization of CGI is the main reason for the decrease of LCF life with the decrease of strain rate.While at 500?,the dynamic strain aging is inhibited by serious oxidation damage.And the main reason for the reduce of fatigue life is that oxidation-induced crack propagation and subgrain boundary cracking increase the fatigue crack propagation distance in a single cycle.(4)The hysteretic energy model can well describe and predict the LCF life of CGI.Among them,the intrinsic fatigue toughness W₀,can be used to measure the capacity of CGI to accommodate fatigue damage.While the damage transition exponent?can be used to judge the ability of CGI to resist fatigue crack propagation.The relationship between the linear function and the quadratic function established between W₀,?and temperature can effectively predict the trend of LCF life of CGI with temperature,but the prediction accuracy of the quadratic function relationship is higher.(5)The tensile strength of CGI decreases linearly with the increase of ferrite content and vermicularity,and linearly increases with the increase of pearlite content.The tensile strength and fatigue strength of CGI show a quadratic function relationship at different temperatures.The fatigue strength of CGI is mainly related to the overall uniformity of the sample,which depends largely on the difference between the vermicular graphite in the cluster and the spherical graphite outside the cluster on the matrix separation,and on the difference in mechanics properties between low-strength ferrite in the cluster and high-strength pearlite outside the cluster.The combined effect of the above differences and the cumulative damage caused in the performance transition area are the root cause for the HCF crack initiation of CGI.At room temperature,the fatigue strength of CGI has a quadratic function relation with the quotient of the area percentage of phase inside and outside the cluster.The fitting result shows that when (w_p+w_g)/(w_f+w_v)?3.2(w_p:pearlite content,w_f:the ferrite content,w_s:spherical graphite content,w_v:the vermicular graphite content),the fatigue strength approaches the lowest value at room temperature.At high temperatures,the fatigue strength of CGI shows a quadratic function relationship with the quotient of the area percentage of graphite inside and outside the cluster,while the effect of ferrite and pearlite content on fatigue strength decreases significantly compared to that at room temperature.The fitting results show that the fatigue strength of CGI approaches lowest at 400? and 500? when the values of w_v/w_s are 3.5 and 3.2,respectively.According to the above quadratic function fitting results at different temperatures,reasonable allocation of the percentage of each phase area in CGI can improve the fatigue strength of CGI to a certain extent.