Contribution of different types of inclusions to fatigue failure of bearing steel and quantitative evaluations
Inclusion is an important factor affecting the fatigue properties of bearing steel.Taking an ultraclean GCr15(100Cr6)bearing steel produced in China as the research object,four types of inclusions in the steel were in-vestigated and statistically analyzed,including oxide,oxide-sulfide,sulfide,and TiN using the ASPEX scanning e-lectron microscope.The maximum characteristic size of each type of inclusions in the steel was predicted separately using the methods Statistical Extreme Value(SEV)and the Generalized Pareto Distribution(GPD).In addition,in-clusions causing fatigue failure were found by rotating bending fatigue test.Comparative analysis of the results ob-tained from these two approaches shows that the most frequent fatigue failure is caused by oxide-sulfide inclusions,and the maximum size of this type of inclusions measured on the fatigue fracture is closer to that predicted by the GPD method.Although there are the largest number of sulfide with the coarsest size in the ultraclean bearing steel,they do not induce the fatigue fracture at all,indicating that they are the least harmful to fatigue failure.There are very few large TiN inclusions in the steel,but TiN-induced fatigue crack is indeed observed,indicating that TiN in-clusions are detrimental to fatigue properties even though they are few.No oxide inclusions are found on the fatigue fracture and the measured oxide inclusions have the largest size of 10.7 μm and the predicted maximum size by GPD is 12.4 μm,suggesting that the critical size of the oxide inclusion not to induce fatigue cracking is probably between 10.7 μm and 12.4 μm.The results of the study provide theoretical guidance for the improvement of the fatigue properties of GCr15 bearing steel.
GCr15 bearing steelASPEXinclusionstatistics of extreme values methodgeneralized Pareto distri-bution methodrotating bending fatigue test
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