Effects of ultra-refine grain and micro-nano twins on mechanical properties of 51CrV4 spring steel

The cyclic quenching with both the rapid heating and cooling was applied on 51CrV4 spring steels by a DIL805A/D dilatometer. The microstructures were observed by a scanning electron microscope (SEM) and a transmission electron microscope (TEM), whereas the strengthening mechanism of the test steels was discussed by the general equation of Rp0.2i=σ0+Δσpi+Δσri+ΔσTi. The results demonstrated that the grain size was the lowest with only 1.76 µm, the area fraction of the twins was the highest up to 6.78% and the thickness of the twins was fined to 11.71 nm at six times. Also, an excellent comprehensive performance was obtained, such as the yield strength, the tensile strength and the area reduction of the test steels was increased substantially as the cyclic quenching was conducted from 1 to 6 times, where the maximum values were up to 1792.93 MPa, 2036.2 MPa and 44%, respectively. As a result, the strength increase was mainly attributed to the ultra-refine grain formation, the area fraction increase of the micro-nano scale twins and the size refinement decrease; whereas it was observed that the highest contribution to the strength and ductility was from the micro-nano scale twins.