Fig. 1 Dilatometry curves a to measure the AC1 and AC3 temperatures and b to measure the MS1 temperature, c thermal cycle and dilatation curves showing the kinetics of bainitic transformation at 315 °C

Fig. 2 Dilatometry curves a to determine the time at which almost 75% bainite formed at 315 °C and b to measure martensite start temperature for remaining austenite (MS2) being present at the end of partial bainite transformation

Fig. 3 One-, two- and three-step austempering heat treatment cycles as well as anticipated microstructural features during the heat treatment procedures. γb, M, αb1, αb2 and αb3 represent austenite blocks, martensite, bainitic ferrite formed at the first step, bainitic ferrite formed at the second step and bainitic ferrite formed at the third step, respectively

Fig. 4 SEM micrographs of a one-step, b two-step, c three-step austempering samples and d high-magnification image of the selected region in c. GB, γb, γf, M and αb represent prior austenite grain boundary, austenite blocks, austenite films, martensite and bainitic ferrite, respectively

Fig. 5 Nanoscale bainitic ferrites and austenite films formed at the end of the third step of austempering heat treatment

Fig. 6 Mean thicknesses and size distributions of bainitic ferrite plates at the end of different heat treatment conditions

Fig. 7 Mean thicknesses and size distributions of austenite films at the end of different heat treatment conditions

Fig. 8 XRD patterns of samples after one-, two- and three-step austempering heat treatments

Fig. 9 Changes in hardness values and the volume fraction of retained austenite after one-, two- and three-step austempering heat treatments

Fig. 10 Engineering stress-strain curves for heat-treated materials

Fig. 11 Toughness variations after applying different heat treatment procedures

Fig. 12 SEM images of fracture surfaces of impact test samples subjected to a one-step, b two-step, c three-step austempering heat treatment conditions, d high-magnification image from the rectangular region in c