Abstract
A series of austempering heat treatments were carried out on the designed new spring steel to study the influence of various parameters on the microstructure and mechanical properties of 55Si2MnMoV steel. Experimental results indicate that excellent comprehensive mechanical properties can be obtained at 275 °C for 30 min, with ultimate tensile strength (UTS) and product of strength and elongation (PSE) being 2144 MPa and 25.26 GPa%, respectively. Transmission electron microscope (TEM) shows that the multiphase microstructure is composed of lath bainite ferrite (BF), martensite, and retained austenite (RA), and the RA with film-like structure is mainly distributed between bainite laths, while the fine carbides are embedded in the matrix. As the austempering temperature increases, the grain size increases gradually, while the strength decreases slowly. Austempering at 325 °C will produce a long-term transformation induced plasticity (TRIP) effect during tensile strain, making total elongation (TE) reach the maximum. While having ultra-high strength and good plasticity, the average rotating bending fatigue strength of the sample at 275 °C is 952 MPa and the fatigue ratio is 0.444, which is mainly determined by the reasonable partition of the multiphase microstructure at low temperature.
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