Effect of cooling rate on dendritic segregation and solidification structure of a Ni-Cr-Co-Mo based alloy

The cooling rate of the center and edge of vacuum induction melting (VIM) or vacuum arc remelting (VAR) ingots exhibit substantial difference, leading to markedly distinct dendritic structures and precipitates. The current lack of precise predictions for dendritic segregation and the distribution of precipitates in ingot makes it difficult to determine the annealing and homogenization heat treatment process. Thus, clarifying the impact of cooling rate on the solidification behavior of alloy is significantly important. The dendritic structure and precipitation characteristics of as-cast C-HRA-3 Ni-Cr-Co-Mo-based heat-resistant alloy were investigated using Thermo-Calc thermodynamic calculations, scanning electron microscopy observations, and electron probe microanalyzer. Based on high temperature observation system, the effects of cooling rate on the dendritic structure, dendritic segregation, and precipitation in this alloy were explored. The results showed that the precipitates in the as-cast C-HRA-3 alloy primarily consist of blocky Ti(C,N) phases, large-sized Ti(C,N)-M₆C-M₂₃C₆ symbiotic phases and M₆C-M₂₃C₆ carbides, and small-sized dispersed M₆C and M₂₃C₆ carbides surronding these symbiotic phases. The primary constituent elements of these precipitates are Mo, Cr, C, and Ti, which predominantly concentrate in the interdendritic regions of the as-cast alloy. There is a clear power-law relationship between the secondary dendrite arm spacing and the cooling rate. The dendritic segregation ratio of Mo, Cr, and Ti exhibits a piecewise functional relationship with the cooling rate, under equiaxed dendritic solidification condition. These predictive models and theoretical analyses were validated using numerical simulations and experimental results from the 200 kg grade VIM electrode.