Microstructural Quantification of Rapidly Solidified Undercooled D2 Tool Steel
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steels are widely used in industry for dies, punches, forming rolls, blades, etc. because of their good wear and abrasion properties.[1] This is due to the high volume fraction of carbides that precipitates during the eutectic reaction. However, as can be seen in Figure 1, conventional casting methods result in the formation of coarse carbides. The mechanical properties can be further improved by reducing the size and evenly distributing carbides.[2] This can be achieved by using a
J. VALLOTON is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 1H9, Canada, and also with the Institut fu¨r Materialphysik im Weltraum, Deutsches Zentrum fu¨r Luft- und Raumfahrt, Cologne, Germany, 51170. Contact e-mail: [email protected]; [email protected] D.M. HERLACH is with the Institut fu¨r Materialphysik im Weltraum, Deutsches Zentrum fu¨r Luft- und Raumfahrt. H. HENEIN is with the Department of Chemical and Materials Engineering, University of Alberta. D. SEDIAKO is with the School of Engineering, University of British Columbia, Kelowna, V1V 1V7, Canada. Manuscript submitted July 12, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
rapid solidification technique on D2 tool steel to refine the microstructure. Understanding the microstructural evolution during solidification is key to manufacturing products with desired properties. The microstructure evolution during rapid solidification processes depends on the velocity of the solid-liquid interface, which in turn depends on the undercooling DT prior to solidification of individual phases in the alloy and the mode of heat extraction. Containerless solidification refers to a class of solidification processes in which large undercoolings are achieved by the complete avoidance of heterogeneous nucleation on container walls. An undercooled melt corresponds to a nonequilibrium state of metastable liquid. During undercooling, driving forces occur such that in contrast to near-equilibrium solidification, there is more than one solidification pathway. The number of possible solidification modes increases with undercooling, making accessible a broad range of metastable microstructures and structurally different phases. In this study, two containerless techniques, electromagnetic levitation (EML) and impulse atomization (IA), are applied to D2 tool steel. EML enables direct measurement of the primary and eutectic undercoolings for samples ~6 to 7 mm in diameter. IA produces
Fig. 1—Microstructure of as-received D2 tool steel. The light phase is austenite/ferrite, the dark phase is the M7C3 carbide.
powders 300 to 1400 lm in size. Since the droplet size directly correlates to the cooling rate, it allows the investigation of a large range of cooling rates (~100 to 10,000 K/s) with a single run. Comparisons between results obtained with these two methods allow investigation of the effect of both the undercooling and the cooling rate on the final microstructure of the samples.
II.
Fig. 2—Calculated pseudo-binary phase diagram for D2 tool steel us
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