Microstructure Fatigue Life Prediction Model Based On The Effect Of Inclusions In Bearing Steel

The oxide inclusions significantly affect the fatigue property of bearing steels.Therefore,to improve the fatigue property of bearing steel,it is important to study the differences between the foreign bearing steels with high quality and the domestic bearing steels,the critical size of each type of oxide inclusions under different loading conditions and different production processes,and further predict the effect of oxide inclusions with different features on the fatigue property,providing guidance for production of bearing steel.Aiming at these goals,experimental researches,hot tests,industrial tests were adopted in this study.The comparison between the metallurgical qualities and fatigue properties of foreign bearing steels with high quality and domestic bearing steel shows that the fatigue properties of some domestic bearing steels are close to the foreign bearing steels.Nevertheless,the control of calcium aluminate inclusions and the Ti content in domestic bearing steels still needs to be improved.The results of 50 kg total oxygen content controlled hot tests of bearing steels with different deoxidation methods and the corresponding fatigue tests results show that the inclusions observed in the fatigue crack initiation sites are mainly calcium aluminate inclusions and some spinel inclusions.It is noting that there are no silicate inclusions observed in the fatigue crack initiations.In addition,the minimum calculated stress intensity factors(SIF)of calcium aluminate inclusions is smaller than that of spinel inclusions.Based on this fact,it is deduced that the critical inclusion sizes of calcium aluminate inclusions and spinel inclusions are 8.5 ?m and 13.5 ?m under the fatigue stress of 1200 MPa.The analysis of the contribution of inclusions with different types and shapes on the total oxygen content also show that the total oxygen content cannot serve as the only criteria for the bearing steels since the effect of decreasing the total oxygen content on improving the fatigue property is getting less with the decrease of the total oxygen content.To increase the ratio of silicate inclusions,bearing steels were deoxided by different deoxidation agents with 100 t industrial tests.The comparisons of inclusion revolutions and fatigue characterizations show that the fatigue life of the two bearing steels are close even though there is non-negligible variation of the total oxygen contents.In addition,the ratio of fatigue cracks caused by inclusions is 62.5%for bearing steels deoxided by Al,while there is no fatigue cracks caused by inclusions for bearing steels deoxided by Si,which proves that the effect of oxide inclusions generated during the process of Si deoxidization on the fatigue property is lower than the effect of inclusions generated during the process of Al deoxidization.Based on the above-mentioned researches,a microstructure-based model for fatigue behavior prediction is proposed.In this model,a new method to introduce inclusions into the model is provided,which considers the residual stress distribution around inclusions generated during the cooling process in heat treatment induced by the different thermal expansion coefficients of inclusions and steel matrix.The fatigue crack initiation sites caused by oxide inclusions with different sizes and the fatigue life can be predicted.Based on the comparison of the experimental data and prediction results,this model corrects the over estimation of the effects of the inclusion sizes on the fatigue life and provides more accurate prediction results of fatigue crack initiation sites and fatigue life.