forming until 1475°C. The alloy is predicted to finish solidi- fying at 1415 °C. Therefore, TiN inclusions could assist the nucleation of δ-ferrite dendrites which later went through a solid state transformation to austenite. It was these austenite dendrites that were then observed in Samples 2 and 3.
Future Work
Based on the observed inclusions, crystallographic analysis and open literature, TiN and TiC inclusions can function as nuclei. Verifying their ability to nucleate and refine the micro- structure of as-cast steels requires rigorous experimentation. Introducing a large number of these particles to a bath of liq- uid steel while it is solidifying would increase the number of dendrites and therefore reduce the grain size of the solid steel.
One experimental technique to confirm the nucleation and refinement potential of TiN or TiC would be to add a powder of these materials into a small cup. A small thermocouple could be placed at the bottom of the cup, and the temperature verses time could be measured after molten steel is added. The resulting cooling curve could be analyzed for the amount of undercooling necessary to initiate solidification. This type of cooling curve analysis has been successfully employed in analyzing nucleation in steels, aluminum alloys, and cast irons.2,5,18
Figure 16 schematically depicts a typical thermal analysis curve and how the amount of undercooling could be calculated from it.
If TiC and TiN are effective nuclei, then the amount of under- cooling should decrease.2
The reason for the decrease is that
an effective solid nuclei that is already present has created an interface between the liquid steel and solid. Normally, a large amount of undercooling is necessary to provide the driving force to create a liquid/solid interface. With a pre-existing in- terface that iron atoms can attach to, the energy required to begin forming solid is reduced decreasing the undercooling needed to initiate solidification. A simple optical microscopy investigation would confirm any reduction in grain size.
Conclusions
Several commercial castings were examined for possible nucleation sites. Through optical and electron microscopy, TiN particles were found near the center of several dendrites in two of the samples. Titanium nitride has a good crystallo- graphic match to δ-ferrite which supports the idea that it acted as a nuclei for some of the primary dendrites. Thermodynamic predictions indicate that δ-ferrite forms before austenite in the steel alloy used to produce those samples. While not detect- ed, TiC particles would also have excellent crystallographic matching with δ-ferrite. Purposely introducing or forming many TiN particles will reduce the grain size of castings lead- ing to improved mechanical properties. Future testing will be required to confirm the improvement in properties with the introduction and formation of small TiN or TiC particles.
24
Acknowledgements
The author would like to acknowledge the financial support of the U.S. Navy’s Office of Naval Research for support- ing this work under award number N000140811052. Special thanks are extended to Acracast, Bernier Cast Metals, and Bay Cast for supplying sample castings. Andre Zahrebelney and Lenard Noel also contributed their time in preparing the samples. Lastly, the author would like to thank Jennie Tuttle for her usual editorial assistance.
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5. Bramfitt B. L., “The Effect of Carbide and Nitride Ad- ditions on the Heterogeneous Nucleation Behavior of Liquid Iron,” Metallurgical Transactions, vol. 1, pp. 1987-1995 (1970).
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