Microstructurre And Mechanical Properties Of Nanostructure Bainite In Surface Layer Of Alloy Steel

A method is proposed for the development of nanostructure bainitic microstructure inthe surface layer of low-carbon alloy steel, which is based on carburization and succedentlow-temperature austempering. The microstructure and property of the nanostructurebainite also been investigated. With the potential use of bearing and gear, the property ofwear and rolling contact fatigue was also tested.The samples were carburized in a gas carburizing furnace at930℃for8h. Thecarbonized samples were heated for austenitization and then austempered at200-300℃insalt bath. The holding time for austemper is from0.5h to150h. The microstructureswithin different depth of carburized steel were investigated by optical microscopy, fieldemission scanning electron microscopy and transmission electron microscopy. The phaseanalysis was carried out using X-ray Diffraction. The residual stress distribution on thesurface of the carburized samples was analyzed using sinψ method by XRD. The hardnessof different depth was tested by HV-sclerometer. Dry sliding friction was applied to thesurface of the plate-like samples with nanostructure bainite in the carburized layer to testwear resistance on a wear test machine which especially built for laboratory use. A rollingcontact fatigue machine was especially built for laboratory use.The results show that carbon content in the surface layer increases to a high levelafter carburization, making the martensite starting point depressed. Nanostructure bainiticmicrostructure was obtained in the surface layer of the low-carbon steel after isothermalholding. Low carbon lath martensite was obtained in the center and mixstructures of thenanostructure bainite and low carbon martensite in the transition layer.The microstructure of nanostructure bainite consists of an intimate mixture of ferrite(α) and film-like austenite (γ). The plate of nanostructure bainitic ferrite was as thin as50nm and film of retained austenite can reach a thickness of several nanometers. The surfacehardness with nanostructure bainite exceeds600HV. A compression residual stress of216MPa and196MPa were produced in the surface layer austempered at200℃and230℃.The results also show that adding of Al as alloy elements and reduction of Mn canenhance the driving force of the bainitic transformation and reduce the incubation periodof the isothermal transformation. However, this has no negative effect to the carburization ability of steels. With the transformation is accelerated, the bainite ferrite is slightlyrefined and the the hardness raises.The wear tests under the loads of50N,200N and300N show that the nanostructurebainitic samples exhibit a lower wear rates than the martensitic samples which istraditionally usd for gear. The bainitic and martensitic samples exhibit different wearbehaviours under the applied loads. Three factors are considered for the lower wear ratesof the nanostructure bainitic microstructure: i) the nanostructure bainitic microstructureexhibits high strength and toughness due to the refinement of the microstructuretransformed at low temperatures; ii) the refinement of the microstructure and the carbonenriched film-like austenite between the ferrite can effectively prevent crack propagationduring sliding friction; iii) the extremely fine α-phase microstructure induced in the drysliding friction surface layer of nanostructure bainitic steel leads to a higher hardness,which can improve wear resistance.The rolling contact fatigue lives of nanostructure bainite reach3.8×10~7cycles and2.3×10~7cycles under the test load of1550MPa and1900MPa, which is exceed twotimes of the martensitic samples. Compression residual stress produced in the surfacelayer of nanostructure bainitic sample is considered as a positive factor to RCF.