Effect of pre-treatment and austempering on microstructure and mechanical properties of 55Si2MnMoV steel

To ensure the high strength of 55Si2MnMoV steel and greatly improve the ductility, three samples with various initial microstructures (annealing, quenching, and austempering) were subjected to intercritical annealing (820 °C) and austempering (325 °C) for 25 min, respectively. The effects of the initial microstructure on the final microstructure and mechanical properties were systematically studied. XRD revealed that the volume of retained austenite (RA) of the three decreased significantly with the location close to the tensile fracture surface. While the conversion proportion of RA in the quenched sample peaked at 16.5%, indicating that RA transforms into martensite to a greater extent during tensile, thus delaying the occurrence of necking. The ferrite grains in the quenched sample were significantly refined and uniformly distributed, and a few were polygonal. Additionally, the grains of the austempered sample were mostly lath because of the inheritance of the initial microstructure. The ultimate tensile strength (UTS) of the quenched sample reaches 1093.61 MPa, which is slightly lower than that of the annealed sample, but the plasticity is greatly improved. The corresponding product of strength and elongation (PSE) reaches the peak value of 67.05 GPa%, which is 26.22% higher than that of the annealed sample, this is mainly attributed to the transformation of RA in a broad range of strain and the uniform equiaxed microstructure. Also, the strength ratio of the three samples fluctuates between 0.67 and 0.71, and their failure modes are all ductile fractures.