| The service temperature of exhaust manifolds is the highest in automobile components,which could be up to 950 ?.The major failure mode is out-of-phase thermo-mechanical fatigue(OP-TMF)assisted by oxidation and creep.Currently,Nb-strengthened austenitic cast steels are the potential candidates for this component.For this kind of materials,three typical as-cast microstructural models("Chinese-script",flake-blocky and blocky type)have been proposed in our previous work based on the morphology of primary Nb(C,N),which is dependent on the N/C ratio,and their damage mechanisms in high temperature creep have been illuminated.However,the high temperature fatigue behavior of austenitic cast steels,which has been studied barely in the past,is much more important for exhaust manifolds in comparison to its creep behavior.In this thesis,the Nb-strengthened austenitic cast steels developed by our research group previously were investigated,chiefly with respect to their high temperature fatigue behaviors,in three steps:1)The influence of the morphology of primary Nb(C,N)on the high temperature properties of this kind of alloys,including thermal stability,oxidation resistance and fatigue resistance.In the first step,several alloy-screening rules were established and alloy 3C2N was selected for the following investigation due to its superior properties.2)The mechanical behaviors and the damage mechanisms of alloy 3C2N in low cycle fatigue(LCF),OP-TMF,LCF with hold time(during which the mechanical strain was held)and OP-TMF with hold time.3)The applicability of several popular methods used for predicting high temperature fatigue lifetime in austenitic cast steels.The results in the first step show that "Chinese-script" primary Nb(C,N)could increase the oxidation resistance in the interdendritic region via supplying a high density of fast diffusion channels,owing to its smaller eutectic lamellar spacing.Secondly,the alloys with "Chinese-script" primary Nb(C,N)demonstrated superior room temperature tensile properties under the as-cast state and after hot exposure at 950 ? for 1000 h,in comparison to the alloys with the flake-blocky one.After hot exposure,the "Chinese-script" primary Nb(C,N)changed slightly and the volume fraction of M23C6,which precipitated in the interdendritic region and the dendritic rim,was lower in the "Chinese-script" alloy as well as its coarsening level,indicating the better thermal stability at 950 ?."Chinese-script" primary Nb(C,N)also improved the LCF resistance of the model alloys at 950 ? with a high strain amplitude,although such effect was weakened with lowering the strain amplitude.Based on the results above,alloy 3C2N with"Chinese-script" primary Nb(C,N)was chosen for the following steps,owing to its well performance.The results regarding LCF behavior of alloy 3C2N at 600-950 ? in the second step show that when the plastic strain amplitude was higher than 0.05%,the logarithm of LCF lifetime of alloy 3C2N was decreased linearly with increasing the plastic strain amplitude,independent of temperature.As the LCF temperature was increased from 600 ? to 950 ?,the primary cyclic stress response might change from cyclic hardening,cyclic softening to cyclic stress saturation,corresponding to the deformation substructure of dislocation cells,slip bands and subgrains,respectively.Besides,the crack initiation on the surface and the crack propagation along grain boundaries were both promoted by increasing LCF temperature and the total strain amplitude.This part of study is the foundation for understanding OP-TMF behavior in the same temperature range.The results regarding OP-TMF behavior of alloy 3C2N at 600-950 ? in the second step show that the OP-TMF lifetime was significantly shorter than the LCF lifetime although their maximum temperature and mechanical strain amplitude were the same.Unlike that in LCF,the cyclic stress response in OP-TMF presented cyclic hardening and cyclic softening in the tensile and compression part,respectively.Multiple slip bands and stacking faults were formed in the dendritic rim and the interdendritic region in OP-TMF,as a result of increased strain heterogeneity,in addition to subgrains which were also formed in the LCF specimen at 950 ?.The OP-TMF crack propagated in the dendritic region at the primary stage but preferred to propagate along the interdendritic region when the crack length was long enough.The results regarding the influence of hold time on LCF behavior and OP-TMF behavior of alloy 3C2N in the second step show that either tensile hold or compression hold was significantly detrimental to the LCF property at 950 ?and the OP-TMF lifetime was decreased linearly with increasing the logarithm of compression hold time at 950 ?.Creep damage in the grain boundary and the interdendritic region was strongly promoted by inducing tensile hold,while the oxidation layer cracking on the surface was intensified by compression hold.Moreover,very high strain concentration was formed in some local interdendritic region in OP-TMF with a long compression hold time,resulting in an increased tendency of crack propagation along the interdendritic region.All of results and damage mechanisms acquired in the second step were the foundation for the study in the next step.The results in the third step show that the simple Manson-Coffin model was suitable for the lifetime prediction in LCF and OP-TMF without hold time at 600-950 ?,while the Ostergren strain energy density model was effective in LCF and OP-TMF with hold time.In comparison,the Sehitoglu model was capable of predicting lifetime under complicated fatigue loading conditions with a more precise and conservative result.In summary,this thesis not only supplements the high temperature fatigue database and alloy screening rules for austenitic cast steels,but also illuminates the fatigue damage mechanisms of austenitic cast steels at high temperature,providing well foundation for the alloy development and the establishment of microstructure-based fatigue lifetime prediction models for austenitic cast steels. |
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