Control of morphology and dimension of blocky retained austenite in medium-carbon steel

Four different heat treatments, one-step isothermal bainitic transformation (OIT), two-step isothermal bainitic transformation (TIT), adding quenching during two-step austempering (BQB) and pre-deformation austempering (AIT), were used to evaluate their effect on the dimension of blocky retained austenite (BRA) in medium-carbon enriched-chromium steel. The experimental results show that the four heat treatments could effectively reduce the dimension of BRA and narrow its distribution. Compared with the average BRA dimension under OIT processed at 330 °C, the reduced austempering temperature of 270 °C and the more complicated three other processes all narrow the BRA dimension (changed from 6.2 μ m to 1.1 μ m–1.9 μ m). Though the demision and its distribution of the AIT sample is larger than other three methods, it has the shortest bainitic transformation time of 65 min.