Microstructure Transformation on Pre-Quenched and Ultrafast-Tempered High-Strength Multiphase Steels

High-strength, multiphase steels consisting of pearlite surrounded by tempered martensite were prepared by pre-quenching and ultrafast tempering heat treatment of high-carbon pearlitic steels (0.81% C). The microstructures were analyzed by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. With an increasing quenching temperature from 120 °C to 190 °C, the quenched martensite variants nucleated via autocatalytic nucleation along the interface. Furthermore, the tempered nodules exhibited a distinct symmetrical structure, and the tempered martensite and pearlitic colonies in the group also showed a symmetrical morphology. In addition, a reasonable model was formulated to explain the transformation process from quenching martensite to the multiphase microstructure. When the quenching temperature was set to 120 °C, followed by ultrafast heating at 200 °C/s to 600 °C and subsequent isothermal treatment for 60 s, the multiphase structure showed highest strength, and the pearlite volume fraction after tempering was the lowest. The microhardness softening mechanism for the tempered structures consisted of two stages. The first stage is related to martensitic sheets undergoing reverse transformation and the nucleation of cementite on dislocations. The second stage involves the transformation of austenite into pearlite and continued carbide coarsening in the martensitic matrix.