The Microstructure of Cast Steel Subjected to Austempering and B-Q&P Heat Treatment

The aim of this work was to characterize the microstructure and properties of cast steel after austempering heat treatment, which would make it possible to produce a carbide-free nanobainite. In order to design heat treatment parameters, dilatometric measurements of phase transformation kinetics were performed on cast steel samples. This study made it possible to construct a time-temperature-transformation diagram and to design the heat treatment parameters. Following this, the most suitable heat treatment parameters were chosen. The heat treatment consisted of austenitizing, followed by quenching with an isothermal stop at the temperature at which bainitic transformation occurred, and final quenching to room temperature. Four different durations of austempering were used at the same temperature of 250 degrees C to obtain a 55, 75, 85 and 100 pct degree of advancement of bainitic transformation. After 55 and 75 pct bainitic transformation, a martensitic transformation occurred in the remaining austenite during quenching to room temperature. In these cases, a partitioning process consisting of additional annealing at 250 degrees C, was then applied This kind of heat treatment has not been used in cast steel earlier All heat-treated samples were subjected to microscopic studies at various scales of observation with the use of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The observations revealed that the heat treatment produced a strongly refined multiphase microstructure and revealed its dependence on the austempering duration. The obtained microstructures provided high tensile strength, however toughness strongly depended on applied heat treatment parameters.

Automated summary

The aim of this study was to characterize the microstructure and properties of cast steel after austempering heat treatment in order to produce a carbide-free nanobainite. The heat treatment parameters were designed based on dilatometric measurements of phase transformation kinetics. Microscopic studies revealed that the heat treatment resulted in a refined multiphase microstructure, with the obtained microstructures providing high tensile strength. However, toughness was found to be strongly dependent on the applied heat treatment parameters.