Influence of the As-quenched State and Tempering Heating Rate on Microstructural Characteristics of a High-Strength Steel

This study aims to investigate the influence of as-quenched microstructures and heating rates to the tempering temperature on microstructure evolution and hardness of a high-strength steel. Two initial as-quenched microstructures, bainitic and martensitic, and two different heating rates, 0.2 and 5 °C/s, were used to reach the tempering temperature of 550 °C. A combination of high-resolution dilatometry, scanning electron microscopy, and chemical analysis was used in the investigation. The analysis revealed that, in both as-quenched microstructures, a higher heating rate to the tempering temperature, resulted in a shift of transformation temperatures to a higher range. In martensitic samples, increasing the heating rate altered carbide morphology from elongated structures at grain boundaries to a more uniform distribution within grains. In bainitic samples, decomposition of martensite/austenite constituent zones resulted in the formation of aggregated chromium carbides at grain boundaries. It was determined that heating rate to tempering temperature had no significant influence on hardness level. The results are discussed in terms of the fundamental mechanisms governing the evolution of bainitic and martensitic microstructures and a mechanism is proposed to interpret the obtained results.