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 degrees 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 degrees 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.