| Gears are essential component of transmission system for mechanical equipment and often bear the risk of fatigue failure under complex cyclic loading conditions such as bending,torsion,and contact stress states.In fact,fatigue failure is most common in gearing.Generally,various case-hardening treatment technologies,such as carburizing,have been widely applied to these components which require high wear and fatigue resistance at the surface while maintaining high toughness in the center part.With the development of modern gear transmission system in the direction of high-speed,heavy-duty,low-noise and light-weight,there have been increasingly demand for the development of high performance gear steels with excellent fatigue failure resistance.In this dissertation,four groups of commercially produced low-carbon alloy steel20Cr2Ni4A for heavy-duty gears were adopted to systematically explore the influence of vacuum carburizing treatment and metallurgical quality on its high-cycle fatigue(HCF)failure behavior by using vacuum carburizing,electrochemical hydrogen charging,rotating bending fatigue testing,scanning electron microscopy(SEM)and transmission electron microscopy(TEM).A new fatigue strength prediction model for carburized steel incorporating the influence of non-metallic inclusion and case parameters has been developed.Moreover,a new non-metallic inclusion inspection strategy using normal tensile testing with properly hydrogen-charged tensile specimens combined with the application of the statistics of extreme value(SEV)method is proposed.The main results are listed as following:The HCF behavior of the tested 20Cr2Ni4A steel subjected to different heat treatments including vacuum carburized treatment and the conventional treatment of quenching and low-temperature tempering(QT)was studied.The results show that both the high-cycle fatigue strength and the threshold value of stress intensity factor range controlling macroscopic crack growth were significantly enhanced on condition that the crack initiation site was still within the carburized case layer after vaccum carburizing treatment.The fatigue strength of the carburized specimen was 915 MPa compared with664 MPa of the QT one,i.e,an increase by?38%.Moreover,it was found that the vacuum carburizing treatment significantly promoted the formation of a particular area of granular bright facet in the vicinity of non-metallic inclusion in the fatigue crack initiation region with fatigue life beyond?10~6 cycles and thus caused mainly an interior inclusion-induced failure mode compared with the QT treatment.These beneficial enhancements are mainly ascribed to the introduction of a case layer with much higher hardness and compressive residual stress which cause higher resistance against crack initiation and propagation and a transition from a predominant surface failure mode to an interior inclusion-induced failure mode for the carburized specimen.The HCF behavior of vacuum carburized 20Cr2Ni4A steel with three different case depths was studied.The results show that the fatigue strength of the vacuum carburized steel shows an increase-decrease pattern with an increase in effective case depth,and a maximum fatigue strength of 919 MPa was obtained with an effective case depth of?0.86 mm.Based on the obtained data and data in available literature,a dimensionless parameter of relative case depth,which is defined as the ratio of effective case depth to the section thickness of specimen,was proposed to reflect the effect of case depth on fatigue strength.Further analysis reveals that the change in case depth also resulted in variations of other parameters in the case layer,such as the retained ausntenite fraction,prior austenite grain size and residual stress.Therefore,it is concluded that both relative case depth and other parameters should be comprehensively considered in obtaining optimized fatigue performance of vacuum carburized steel.The results of the investigation of the HCF behavior of four groups of vacuumcarburized 20Cr2Ni4A steel with different metallurgical qualities demonstrated that the fatigue performance of the carburized gear steel can be greatly enhanced with the improvement of metallurgical quality,and the content of oxygen in steel may not always correctly reflect the fatigue performance of the tested steels.It was found that the type and size of non-metallic inclusion play controlling role on the fatigue failure mechanism.Therefore,based on the fatigue failure mechanism of carburized gear steel,a fatigue strength prediction model through optimizing the parameters in the Tanaka-Akiniwa model and considering the influence of inclusion and case parameters is proposed.The estimated values of fatigue strength are in well accordance with the experimental values and those available in literature with absolute deviations being lower than 10%.Due to the rather low-rate emergence of large size inclusions in high cleanness steels,which seriously affect the the fatigue properties,an appropriated inclusion inspection method is particularly important.Therefore,a new inclusion inspection method named hydrogen embrittlement(HE)-tensile was proposed.Standard tensile specimens of20Cr2Ni4A steel after QT treatment was properly electrochemical hydrogen-charged.It is found that there were many embrittled platforms generally with large inclusions inside presented on the fracture surfaces of the specimens after normal tensile testing due to the trapping of the charged hydrogen around inclusion and the occurrence of HE.The size,composition and distribution of these inclusions could be conveniently analyzed using a SEM,and thus the maximum inclusion size in the tested steel as well as its fatigue strength could be predicted using the SEV method.It was confirmed that the predicted maximum inclusion size and fatigue strength using the proposed method coincided well with that obtained by rotating bending fatigue test.Therefore,it is suggested that the proposed method is a promising efficient and reliable method to be used for the inspection of the maximum size inclusion in steel and the prediction of the fatigue strength in high-strength steels with high cleanness. |
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