Microstructure and mechanical characterization of rapidly solidified Cr-C tool steel: Annealing effects

The effect of isochronal annealing on D2 tool steel powder, rapidly solidified via both Impulse Atomization and Water Atomization, has been evaluated using high-resolution scanning electron microscopy and Vickers microhardness. The amount of supersaturation of the alloying elements inside the retained austenite phase as a function of eutectic undercooling was calculated. The fraction of austenite transformed to ferrite at different annealing temperatures (from 350°C to 810°C) was also determined, using Rietveld analysis. The results show that although the particles with larger eutectic undercooling have larger supersaturation of alloying elements within the retained austenite phase, they have a smaller fraction of austenite to ferrite transformation at the temperature in which transformation starts. The maximum hardness was achieved at an annealing temperature of 550°C, due to the formation of fine and well-distributed carbides.