Microstructural investigation of D2 tool steel during rapid solidification

Impulse atomisation in helium and nitrogen and water atomisation have been utilised to produce powders of D2 tool steel. It was determined that higher cooling rates result in a lower percentage of eutectic. Scanning electron microscopy image analysis, along with coarsening model, was used to predict eutectic and primary phase undercooling of particles. Small particles exhibited a higher amount of undercooling. The particles exposed to a He atmosphere during atomisation had a larger amount of eutectic undercooling. The fraction of primary phase that solidified during the recalescence was then calculated based on the amount of primary phase undercooling under adiabatic conditions. In smaller particles, there was a larger amount of primary phase solidified during recalescence due to a higher amount of primary undercooling. Based on primary phase undercooling values, critical nuclei radius of austenite and assuming homogenous nucleation, the number of austenite unit cells in the stable nucleus was calculated.