Competitive Failure Mechanisms And Strength–Life Prediction Approaches Of Carburized/Nitrided Gear Steel Under Very High Cycle Fatigue

As a key driving part,the gear is used widely in the fields of automobile,aerospace,intelligent manufacturing equipment,etc.Its quality and performance?especially the fatigue performance?to a great extent control the safety and reliability of equipment.In view of the fact that the gear is actually subjected to the very high cycle fatigue?VHCF?loading condition beyond 10⁷ cycles,to carry out the study on the very high cycle fatigue property of gear material has a profound guiding significance for the high-performance,long-life reliable design of gear.Focusing on the failure feature of gear with a primary bending fatigue,and based on the actual surface hardening technology of gear,as well as the equivalent design method of gear tooth root and notch specimen,the VHCF properties of two gear steels were studied under different hardening technologies including carburizing and nitriding from the viewpoint of the bending fatigue design of the case hardening gear.The crack initiation and propagation behaviors were discussed.The influences of microstructure,surface hardening,notch,load,etc.on the VHCF failure mechanism of gear steels were analyzed.Finally,the VHCF strength-life evaluation methods of carburized/nitrided gear steels were established.The main contents and conclusions are summarized as follows:?1?The VHCF S-N properties and failure mechanisms of carburized CrMn steel were studied.The formation mechanisms of FGA?Fine Granular Area?and fisheye were analyzed,and the acceleration effect of residual tensile stress on the interior crack initiation was clarified.The failure modes can be divided into three types:?i?surface induced failure in low cycle fatigue regime,?ii?interior induced failure without FGA in the high cycle fatigue?HCF?regime and?iii?interior induced failure with FGA in the VHCF regime.The duplex S-N property for interior induced failure occurs between the HCF regime and the VHCF regime.The coexistence phenomenon of surface crack and interior crack can be observed on the fracture surface with surface failure,and the inclusions causing interior failure are limited in the non-carburizing area.The value of effective stress intensity factor at the front of the interior inclusion is much less than the threshold value controlling the long crack growth,the small crack effect plays a very important role in determining the VHCF life of carburized Cr Mn steel.?2?The notch specimen was designed by using the equivalent design between the gear tooth root and notch specimen,and the notch effect on the VHCF properties of nitrided Cr Ni gear steel under axial tension-compression loading was analyzed.For the smooth specimen and the notch specimen with the stress concentration factor K_t=1.20,the failure modes can be classified as surface induced failure and the interior induced failure where the defects are limited in the non-nitriding area.For the notch specimen with K_t=1.55,within the nitrided layer the inhomogeneous matrix area?IMA?induced interior failure as a new failure characteristics is observed.The duplex S-N property of smooth specimen is not obvious.However,the duplex S-N property with surface induced failure and interior induced failure for the notch specimen with K_t=1.20 is observed,while the duplex S-N property only with surface induced failure for the notch specimen with K_t=1.55 occurs between the short life regime and the VHCF regime.With the shape of specimen from surface to notch,the phenomenon of multiple interior crack sites is replaced by the single interior crack site.With the increase of K_t,the interior defect position gradually moves towards the surface of specimen,the value of effective stress intensity factor at the front of the FGA tends to decrease,and the shape of fisheye first approximately exhibits the elliptical shape with major axis at x direction,followed by the circular shape,and then the the elliptical shape with major axis at y direction.The K_t has a large effect on fatigue life with surface failure,but has a little effect on fatigue life with interior failure.?3?The influence of defect-fisheye failure behavior on fatigue strength was studied,and a fatigue strength prediction model based on defect size and a fatigue design method based on defect-fisheye failure behavior were established.The ratio of fisheye size to inclusion size has a great effect on fatigue life under low stress amplitude,whereas that has a little effect under high stress amplitude.The predicted results by using this model are in good agreement with the experimental ones.The distribution curves plotted by using four distribution functions including Gumbel distribution?Gumbel?,generalized Pareto distribution?GP?,generalized extreme values?GEV?and exponential generalized Pareto distribution?EGP?are well consistent with the test data.Under a given probability,the minimum inclusion size is estimated by using the GEV distribution,and then followed by using Gumbel distribution,GP distribution and EGP distribution in sequence.The results about inclusion sizes are in inverse proportion to the results about fatigue strength.In view of the fact that the agreement between the predicted and experimental results,the proposed fatigue design method can be acceptable.?4?In regard to the competitive failure mode of carburized/nitrided gear steel,the parameter solution method of local failure probability prediction model established by using the“weakest-link”concept was modified,and the fatigue strength and the local surface-interior failure probability were estimated.The prediction fatigue strength and the predicted local surface failure probability are in good agreement with the experimental results,but the predicted local interior failure probability deviates slightly from the experimental results.?5?In view of the multiple failure modes of carburized CrMn steel under VHCF,an approach based on the mixed Weibull distribution was proposed to establish the probabilistic S-N curve associated with multiple failure modes.The established median S-N curve is in agreement with the experimental results,and can well represent the VHCF property with the multiple failure modes.The predicted result based on the established S-N curve under a survival probability of 99%is relatively safe in the VHCF regime.?6?Taking the effect of stress ratio and mean stress into consideration,the interior crack initiation life prediction model was established.In consideration of the effect of stress ratio,surface residual stress and notch,the surface crack initiation life prediction model was established.At a given stress amplitude,the crack nucleation life tends to increase with the increase of crack size.For a given crack size,the needed crack nucleation life is relatively short at higher stress amplitudes.Conversely,even if the crack size is much small,the needed crack nucleation life also can occupy the vast majority of the total fatigue life at lower stress amplitudes.Furthermore,the predicted crack nucleation lives associated with the sizes of FGA are in good agreement with the experimental data.Compared with the percentage of the interior crack propagation life on the total fatigue life,the surface crack propagation life in the short life regime takes up a bigger proportion.In comparison with the experimental results,the total fatigue lives estimated by the present models involved the crack initiation and propagation behaviors are reasonable,and the agreement is fairly good within the factor-of-three boundaries.