Influence of melt undercooling on the microstructure of levitated Cr–Mo–V tool steel

The paper deals with microstructure and phase analysis of bulk samples from Cr–Mo–V tool steel of ledeburite type deeply undercooled by the electromagnetic levitation method and subsequently solidified. In quasi-equilibrium conditions, solidification of investigated steel starts by austenite formation and it is finished by eutectic reactions resulting in the development of morphologically different M 4C 3 and M 7C 3 carbides which are located in the interdendritic positions. During levitation experiments the melt undercoolings from 50 °C to 290 °C below equilibrium liquidus temperature were achieved. In the deeply undercooled and spontaneously solidified samples, grain refined microstructures and carbide phases of the M 7C 3, M 4C 3 and M 23C 6 types were observed. Carbide phases were located at the grain boundaries of the metastable austenite as eutectic colonies and carbide skeletons. Based on the morphological features, it is proposed that grain refined microstructures were created by dendrite fragmentation after recalescence. Moreover, relatively small carbide phases were detected inside the austenite grains. It is supposed that these carbide phases in austenite grains are formed during postrecalescence stage of solidification from austenite supersaturated by carbon and carbide forming elements.