The Microalloying Non-quenched And Tempered Steel Fatigue Performance

There are an increasingly use of microalloyed forging steels for the production of automotive components for the purpose of higher productivity and thus cost saving compared with quenched and tempered (QT) steels. Most of these components such as crankshaft, connecting-rod and front axle experience cyclic loading in service. Therefore, There is an increasing demand of improving the fatigue properties of microalloyed steels. In this paper, the fatigue fracture behaviors of four kinds of commercial microalloyed forging steels with different carbon content (22MnVS, 38MnVS, 38MnS and 48MnV) at different conditions and one quenched and tempered (QT) medium carbon steel 40Cr for comparison were studied by using rotating-bending fatigue test. Besides to the investigation of fatigue crack initiation mechanism, emphasis was given to the effect of vanadium and microstructure on the fatigue fracture behaviors.Carefully observation of the fatigue specimen surfaces of as-rolled microalloyed forging steels 22MnVS, 38MnVS and 48MnV reveals that their fatigue cracks mainly initiate and propagate along the ferrite-pearlite boundaries, whereas, the fatigue cracks of QT steel 40Cr initiate either from specimen surface matrix or surface inclusion.The study of the fatigue properties of V-microalloyed ferrite-pearlite steel 38MnVS and 38MnS steel at different states shows that V can improve the fatigue properties of ferrite-pearlite steel mainly through precipitation strengthening and grain refining mechanism. TEM examination shows that in the as-hot forged condition, the fine V(C,N) particles with about 4 nm in diameter, have specific orientation relationship with ferrite matrix and significant effect on precipitation strengthening and microstructure refinement. The strengthening effect of soft ferrite raised the slip band initiation stress and thus delayed crack initiation, the fatigue property of 38MnVS steel in the as-hot forged condition is much higher than that of 38MnS steel. In the as-annealed condition, the initial fine V(C,N) particles coarsen and are incoherent with ferrite matrix, and thus have little effect on precipitation strengthening. Therefore, the fatigue property of 38MnVS steel in the as-annealed condition is lower than that in the as-hot forged condition and is comparable to thatof 38MnS steel.The investigation of the effect of microstructures on the fatigue properties of microalloyed medium-carbon steel 38MnVS shows that 38MnVS steel with coarse bainite microstructure obtained through fast cooling after high temperature austenitizing has the worst fatigue property. The formation of film-like ferrite along coarse prior austenite grain boundary and pearlite microstructure for either as-rolled condition or high temperature annealed condition also deteriorated the fatigue property of 38MnVS steel. However, 38MnVS steel with much fine ferrite and pearlite microstructure has the best fatigue property, which is even better than that of quenched and tempered 40Cr steel. Therefore, the fatigue property of microalloyed steel after appropriate controlling of its microstructure after rolling and/or forging is equal to or even better than that of quenched and tempered low alloyed steel at same strength level.