Effect of Cooling Rate on the Microstructure of a Pressure Vessel Steel

The present article thoroughly explores the effect of cooling rate on various microstructural features of a reactor pressure vessel steel (20MnMoNi55). For this, dilatometric experiments were performed at different cooling rates. Optical microscopy and electron backscatter diffraction were used to measure different microstructural parameters. Continuous cooling from 0.15 to 0.3 °C/s revealed ferrite and bainite; only bainite is seen from 1 to 2 °C/s, both bainite and martensite were found in the regime 5 to 15 °C/s, and complete martensitic transformation was observed at cooling rate 20 °C/s and above. With increase in cooling rate, the fraction of low angle boundary decreased and that of high angle boundary increased. The nature of grain size distribution was found to be cooling rate sensitive.