High Cycle Fatigue Of Ultrahigh-strength Nanostructured Bainite Steel

The aim of the present dissertation is to investigate the fatigue behavior ofultrahigh-strength nanostructured bainite steel by low-temperature austempering.The high-carbon Si-Al-rich steel was austempered by austenitizing at1000C for30min and rapidly quenched into salt bath for different hours at220C,240C and260C,which are a little higher than Ms(194C). Comparison tests are desiged. The tested steelwas austempered by austenitizing at850C for30min and oil quenching，then temperedat240C and260C for2hours. The microstructure of the samples were studied byoptical microscopy, X-ray diffractometry and transmission electron microscopy. At thesame time, the hardness, tensile strength and impact toughness have been determined. Thefatigue limit is estimated assuming no failure in107cycles, based on extrapolation of datein which the maximum number of cycles permitted was105. And the fatigue testdetermines the high-cycle push-pull fatigue S-N curves of the austempered samples andthe quenched and tempered ones. The fracture character and fracture mechanism werestudied by scanning electron microscopy.Results show that the low-temperature bainitic microstructure composed of bainiticferrite plates dispersed in retained austenite matrix can be obtained in high-carbonSi-Al-rich steel by austempering at different temperatures for different time afteraustenitized at1000C. The thickness of bainitic ferrite laths, The fraction of retainedaustenite and impact toughness increase as the isothermal austempered temperatureincrease, while the hardness, strength and fatigue performance decrease. Correspondingfatigue limit for samples by isothermal transformation at220C,240C and260C is889MPa，877MPa and710MPa respectively. The corresponding fatigue limit for thetested steel tempered at240C and260C is801MPa and782MPa respectively.Although the samples quenched and tempered have the same hardness as the specimensaustempered, the latter have the higher fatigue limit than the former.Fractography revealed that the failure of few samples austempered initiated frominclusions while most cracks apparently began from the polished surface. The fatiguefracture mode of the fatigue crack propagation regions is transgranular brittle quasi-cleavage fracture.What’s more, a lot of secondary cracks s and fatigue striation canbe seen of the crack propagation region in the fatigue fractures. Fatigue final ruptureregion showed that tearing ridge and dimple fracture areas were identified by the presenceof dimples. Observing the axial secondary cracks, it is easy to found that plasticdeformation happen in the microstructure near the cracks.