Diagnostics of the bainite transformation mechanism and the effect of normalizing and tempering on the hardness and microstructure of the new Cr-Mo-V-Ti steel for operation at elevated temperature

Abstract

The Cr-Mo-V-Ti based low alloy steels are widely used in thermal power plants because of their ability to withstand elevated temperatures and high pressure under continuous service. In the present work conventional heat treatment like normalizing and tempering of the alloys has been performed. The material used in this study was the laboratory prepared experimental low alloy Cr-Mo-V-Ti steel. Samples were austenitized at 980oC for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750oC for 1 hour. Mechanism of bainite transformation has been studied in Fe-C-Cr-Mo-V-Ti steel using high speed dilatometry. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure and the growth of the carbides. The magnitude of these changes depended on the temperature of tempering.