Effects Of Micro-nano Scale Twins On Mechanical Properties Of 51CrV4 Spring Steel

In this paper,the formation mechanism of twins at micro-nano scale level was comprehensively investigated in spring steel 51CrV4 with high stacking-fault-energy during the rapid cyclic quenching processing.Meanwhile,microstructure was systematically characterized by optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM),to reveal the effect of the evolution of grain,carbide and twin on the corresponding mechanical properties.The main results are as following:(1)With the increase of the number of quenching processing,grain refining and coarsening successively occur.The minimum size of ~1.76 μm is achieved in the 51CrV4 steels after six-time treatments with the heating rate of 1000 ℃ /s to austenitized temperature of 820℃,holding time of 2 seconds and cooling rate of 300℃/s.(2)The cyclic quenching treatment results in the gradual dissolution of carbides with low stability.This dissolution leads to the significant increase in local carbon concentration in matrix,which is attributed to the formation of twins during the quenching in 51CrV4 steels.The area fraction of the twins increases from ~3.59% after the first treatment to ~4.01% at the third treatment and to ~6.78% for the six time treatment.However,the corresponding length of twin lamellae is refined from the ~202.49 nm to ~196.75 nm and to ~148.61 nm;and their corresponding thicknesses are ~17.41 nm,~15.27 nm and ~11.71 nm,respectively.(3)Compared with single-quenching sample,the sample after six time treatments possesses strong twin strengthening and grain refinement strengthening,with ~68% and ~43% relative contribution to total strength incensement,respectively,while shows the slightly decreasing precipitation strengthening.A excellent comprehensive mechanical properties with a tensile strength of ~2036 MPa and area reduction of ~44.0% are achieved in the 51CrV4 steels.These results provide a guideline for designing strategies to achieve high performance in the high stacking-fault-energy materials using the micro-nano sized twins.