| EA4T steel has been widely used to produce high-speed train axle because it has excellent mechanical properties and can meet the requirements of the strength and toughness design of the axle steel.With the development of the high-speed and heavy-haul freight transportation,mechanical properties,especially fatigue properties,of the axle materials become more and more important to the safety of railway transportation.However,different microstructures of the axle materials got from different heat treating regimes affect their fatigue properties directly.So it is of vital importance to explore and choose appropriate heat treating regimes and get suitable microstructures to improve the fatigue properties of the EA4T steel.In this study,bainite/martensite duplex-phase microstructure and tempered-sorbite microstructure of the EA4T steel were investigated comparatively,which obtained by isothermal quenching process and the quenching and tempering process,respectively.Tensile test and tension-compression fatigue test were conducted to compare the basic mechanical properties and fatigue resistance of the EA4T steel with different microstructures.In addition,four kinds of bainite/martensite duplex-phase EA4T steel with different volume fractions of bainite got by controlling the heat treatment parameters were investigated.Then tension-compression fatigue tests and three-point bending tests of fatigue crack propagation were conducted to investigate the effects of microstructures on crack initiation and crack propagation behavior of the steels with different volume fractions of bainite.Stress controlled tension-compression fatigue tests indicates that the fatigue strength of the bainite/martensite duplex-phase EA4T steel is apparently higher than that of the tempered sorbite EA4T steel owing to the unique bainite/martensite duplex-phase microstructure.Fatigue strength of these samples with different volume fractions of bainite increase first with increasing content of bainite,and then drop afterwards.It is the evolutions of the microstructures of the bainite/martensite duplex-phase steel which affect variation tendency of the fatigue strength of the EA4T steels.It is found that bainite/martensite duplex-phase EA4T steel austempered in 440 ? for 2 minutes has the highest fatigue strength,and the bainite content of which is about 50 vol%.Three-point bending tests of fatigue crack propagation tests show that the fatigue crack growth resistance of bainite/martensite duplex-phase EA4T steel is mainly related to the content ratios of bainite and martensite.When the content of the bainite is very low,the bainite/martensite dupex-phase EA4T steel is brittle,and bainite/martensite duplex-phase EA4T steel cracks in intergranular fracture mode,so fatigue cracks grow fast.At the other extrem,when bainite accounts for the vast majority proportion,the bainite/martensite duplex-phase EA4T steel is so soft that it can not tolerate high stress,which resulting in fast crack propagation.Appropriate volume fractions of bainite can not only make bainite/martensite duplex-phase EA4T steel has a good combination of strength and toughness,but also make the fatigue crack propagation paths tortous,which lead to a lower fatigue crack growth rate.So,when the volume fraction of the bainite is about 50-60 vol%,the bainite/martensite duplex-phase steel has a superior fatigue crack growth resistance. |
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