Effects of cooling media on the formation of Martensite-Austenite microconstituent in a HSLA steel

Abstract

The effect of different cooling conditions on the formation of Martensite-Austenite (MA) in a High - Strength Low - Alloy (HSLA) steel was assessed. The MA constituent is detrimental to impact toughness in pipeline applications, so the purpose of this research was to minimize its presence through the choice of effective cooling media and optimal parameters such as the cooling rate and final cooling temperature. The volume fraction, size and morphology of MA was evaluated by selective etching and corroborated trough SEM and EBSD. Vickers hardness testing agreed with the measured MA volume fractions. The sample cooled with helium gas and salt bath with the lowest final cooling temperature of 460 °C, exhibited a fine mixture of ferritic bainite, granular bainite and the lowest volume fraction of MA, along with MA smaller particle average size. A high cooling rate and a decrease in the final cooling temperature resulted in a decrease in the volume fraction and average particle size of MA.