Dilatometric study of the secondary martensitic transformation in cobalt high-speed cutting steel

Abstract

1. We confirmed the data published in [2] concerning the increase in the temperature of the secondary transformation of austenite under the effect of cobalt. 2. In the steel investigated the cobalt decreases the stability of residual austenite to such an extent that after quenching from 1240°C and tempering 45 min at 600°C the bainitic transformation occurs at 340–290°C during continuous slow cooling. 3. The increase of the quenching temperature to 1260°C changes the transformation rate during subsequent tempering: the temperature of the intermediate transformation of austenite increases and the martensitic transformation begins at lower temperatures (140–150°C). Thus, the decrease of the temperature of martensitic transformation is not equivalent to the increase of the stability of residual austenite in steels containing high concentrations of cobalt. 4. An increase of the tempering temperature to 600–650°C leads to a more complete transformation of residual austenite in the intermediate region. 5. We confirmed the results obtained in [9] concerning the intensification of the transformation of austenite as the result of rapid cooling after tempering: rapid cooling partially or completely eliminates the intermediate transformation of austenite or increases the effect of the martensitic transformation. 6. The martensitic transformation occurring during tempering of the cobalt steel investigated results from the preceding bainitic transformation. The rate of the secondary martensitic transformation can be changed by changing the bainitic transformation.