Microstructural Quantification of Rapidly Solidified Undercooled D2 Tool Steel

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  • Rapid solidification of D2 tool steel is investigated experimentally using ElectroMagnetic Levitation (EML) under terrestrial and reduced gravity conditions and Impulse Atomization (IA), a drop tube type of apparatus. IA produces powders 300 to 1400 μm in size. This allows the investigation of a large range of cooling rate (~100-10,000 K/s) with a single experiment. On the other hand EML allows direct measurements of the thermal history, including primary and eutectic nucleation undercoolings, for samples ~ 6-7 mm in diameter. The final microstructures at room temperature consist of retained supersaturated austenite surrounded by eutectic of austenite and M7C3 carbides. Rapid solidification effectively suppresses the formation of ferrite in IA, while a small amount of ferrite is detected in EML samples. High primary phase undercoolings and high cooling rates tend to refine the microstructure, which results in a better dispersion of the eutectic carbides. Evaluation of the cell spacing in EML and IA samples show that the scale of the final microstructure is mainly governed by coarsening. EBSD analysis of both IA and EML samples reveal that IA powders are single crystal in the primary phase with texture. EBSD on EML samples reveal strong differences between the microstructure of droplets solidified on ground and in microgravity conditions. While the former ones are polycrystalline with many different grains, the EML sample solidified in µg shows a strong texture with very few much larger grains having twinning relationships. This indicates that fluid flow has a strong influence on grain refinement in this system.

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    Attribution-NonCommercial-NoDerivatives 4.0 International