| There has been an increasing tendency of replacing quenched and tempered(Q&T)steels by microalloyed medium-carbon forging steels for modern automotive forgings,and it is estimated that almost all the forgings will be forged by the later in the future.Most of the forgings,such as connecting rod,crankshaft,wheel hub and steering and support arms,are characterized by corners,steps,keyways,etc.,which are easy to induce stress concentrations and fatigue failures under cyclic loading.In this paper,the microstructural characteristics and mechanical properties of two types of microalloyed medium-carbon forging steels,30 Mn VS and 49 Mn VS,which are commonly used for automobile forgings,are analyzed by using optical microscopy,scanning electron microscopy(SEM),transmission electron microscopy(TEM)and phase analysis.Then,high-cycle fatigue behaviors using smooth sliding and notched fatigue specimens(stress concentration factors Kt=1,2.1 and 3.9)were systematically studied by rotating bending fatigue and fatigue crack growth rate tests.Structural steels 40 Cr as well as 30 Mn VS with quenched & tempered microstructure were used for comparison.The following conclusions could be obtained.Both the as-forged steels 30 Mn VS and 49 Mn VS exhibit a ferritic-pearlitic microstructure,in which the ferrite especially the intragranular ferrite fraction of the30 Mn VS steel is much higher than that of the 49 Mn VS steel.The mass fraction of M(C,N)precipitates of 30 Mn VS steel is ~44% higher than that of 49 Mn VS steel,which is primarily due to the comparatively higher C and N contents and lower C content of the former.Thus,the microhardness of the ferrite in 30 Mn VS steel is notably higher than that in 49 Mn VS steel.The tensile strength of 30 Mn VS steel is lower than that of49 Mn VS steel,but the yield strength,ductility,toughness and yield strength ratio of the former are higher than those of the latter,indicating a better strength,ductility and toughness combination of the former.The results of rotating bending fatigue tests show that both the fatigue strength and fatigue strength ratio at each Kt of the as-forged 30 Mn VS steel is notably higher than that of the as-forged 49 Mn VS steel,and this difference tends to decrease with an increase in Kt.Moreover,the as-forged 30 Mn VS steel has comparable fatigue properties at each Kt to the Q&Ted 30 Mn VS and 40 Cr steels,while the as-forged 49 Mn VS steel exhibits the lowest at each Kt.The fatigue notch sensitivity at each Kt of the as-forged30 Mn VS steel is slightly higher than that of the as-forged 49 Mn VS steel.The steels with ferritic-pearlitic microstructure have slightly higher fatigue notch sensitivity than steels with high-temperature tempered martensitic microstructure.For smooth specimens,all the fatigue cracks initiated from the specimen surface,while for notched specimens,all the cracks initiated from notch root surface.Based on the results of this study and those in literature,a simple and practical method which incorporates only microstructural parameters(constituent fraction and microhardness)and hardness was proposed to estimate ?w of ferritic-pearlitic medium-carbon forging steels.Notched fatigue strength can be expressed by a power function of Kt with its coefficient almost equals to ?w,which can be further expressed by two groups corresponding to MA ferritic-pearlitic forging steels with relatively lower and higher C contents,respectively.The results of fatigue crack growth rate tests show that the da/d N of the as-forged30 Mn VS steel is slightly higher than that of the as-forged 49 Mn VS steel,and this difference is more notable at higher ?K.This difference is ascribed mainly to the lower resistance to crack propagation of ferrite than that of pearlite.This result further confirms that the high-cycle fatigue property of the tested steels is mainly controlled by the fatigue crack initiation and early propagation. |
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