Effect of M–A constituents formed in thermo-mechanical controlled process on toughness of 20CrNi2MoV steel

The effect of martensite–austenite (M–A) constituents formed in thermo-mechanical controlled process on impact toughness of 20CrNi2MoV steel was investigated. The variation in fraction, size and morphology of M–A constituent and its effect on toughness under different cooling rates were carried out. The result shows that there was no significant change in the fraction of M–A constituent under different cooling rates, but the distribution and size of M–A constituent were greatly influenced by cooling rate, which consequently influenced toughness. The amount of large blocky M–A constituents decreased from 4.7 to 1.7%, while that of elongated M–A constituents increased from 3.8 to 8.6% with the cooling rate increasing from 7 to 26 °C/s, and the corresponding impact energy decreased from 132 to 84 J. The deterioration of impact toughness could be related to the increase in the elongated M–A constituents. The elongated M–A constituents existing along the prior austenite grain boundaries in samples of 26 °C/s could easily lead to the formation of cleavage crack, which then results in the lower crack initiation energy than that of low cooling rate samples.