The robustness of the two-colour assumption in pyrometry of solidifying AISI D2 alloy droplets

A drop tube-impulse atomization technique was used to produce D2 tool steel powders. Shadowgraph (Sizing Master Shadow from LaVision GmbH in Gottingen, Germany) was utilized to measure in-situ velocity and droplet size. Simultaneously, the DPV-2000 (Tecnar Automation Ltee, St. Hubert Quebec, Canada) was used to measure the radiant energy and droplet size of atomized droplets. These devices were mounted on a 3D translation stage which was designed, constructed and installed in the drop tube. The measurements were done at three different distances from the molten metal crucible, 4 cm, 18 cm and 28 cm. A thermal model of droplet cooling was coupled with the temperature of primary phase undercooling for D2 tool steel. The results from this model were used in order to find the emissivity behaviour of the droplets. It is shown that emissivity of the droplets is a function of size and temperature. It was concluded that the DPV-2000 should be considered as a single color pyrometer since the gray body assumption for D2 tool steel falling droplets using DPV-2000 during is invalid for the range of measurements taken.