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The solidification structure of remelt ingots of IN718 has been modeled by many authors using a number of different computation techniques. The models have generally been validated by comparing the computed liquidus line with the “pool profile” as outlined by a variety of techniques in industrial ingots. Recent models have also been validated by comparing the dendrite spacings predicted by the computed solidification parameters to those derived from laboratory directional solidification experiments. Both these methods have strong drawbacks. The “pool profile” is almost certainly not the liquidus line in a practical ingot due to supercooling and to momentum transfer from the bulk liquid to the interdendritic liquid in the region where the solid fraction is small. In the case of the dendrite spacings, the nature of the spacings’ dependence on the solidification parameters means that the variations expected in practical ingots are close to the precision with which the spacings can be determined. We therefore lack sufficient means to properly test the computed ingot structures against actual practice. A further test of computed ingot thermal conditions has been used. In this test, the average size of primary NbC precipitates is used as the solidification monitor. Laboratory directional solidification experiments have shown that this parameter is sensitive to the local solidification time and can thus be used to check the computed position of the solidus relative to the computed liquidus. The purpose of the present work is to determine the extent to which this method is valid over the range of compositions which are use for alloy IN718. We find that the method is valid provided that the correct relationship is used for the alloy composition in question. The method, however, is difficult to implement in practice due to the problem of determining exactly which carbides in the microstructure are the appropriate primary particles. This difficulty reduces the method to one which is of the same order of precision as the one which uses dendrite spacings. Nonetheless, the method is a useful additional determination of the solidification parameters.

The Precipitation of Primary Carbides in IN718 and its Relation to Solidification Conditions