T. Morillon, M. A. Dayananda, M. J. M. Krane, Purdue University, West Lafayette, IN

Summary: The solidification of a Ni-Cr-Mo alloy (Haynes 242) during electroslag remelting (ESR) gives rise to significant molybdenum macrosegregation in the final ingots. Such segregation causes the formation of defects including, but not limited to, secondary phase precipitation of a brittle ƒÝ phase in the ƒ× matrix. The aim of the study is to characterize and predict the dissolution or growth of the ƒÝ phase during the heat treatments following ESR. The alloys were solidified, followed by heat treats at 1100¢XC and 650¢XC, the industrial annealing and aging temperatures. The ƒÝ phase was identified using X-ray diffraction and electron microprobe composition measurements, and its evolution during heat treatments was measured by calculating phase fractions after different heat treating times. Diffusion coefficients were measured using a ƒ×ƒ}ƒÝ diffusion couple. A two dimensional diffusion model was developed to predict the dissolution of the secondary phase in a binary two-phase system, based on Fick¡¦s second law and a mass balance at the phase boundaries. Because of the negligible segregation of chromium in this alloy, the diffusion model treated the alloy as a binary Ni-Mo system and used the effective diffusion coefficients within each phase. The model and experimental results matched well when comparing the microsegregation level within the ƒ× phase. The calculation and experiments show an initial increase in the ƒÝ phase fraction in the early stages of the high temperature heat treatment, followed by a slow decrease, but there was not a good agreement for the level and time of the maximum amount of the ƒÝ phase. The discrepancy is attributed to the use of a binary model with no effect of the Cr content on the Mo diffusion and alternate ways of addressing this problem will be discussed. It was shown that a smaller precipitate spacing accelerates the ƒÝ phase growth and dissolution, which could take up to several days in typical as-cast structures.