Unveiling the influential mechanism of nitrogen on the precipitation and dissolution of eutectic phases in cast boron-bearing super austenitic alloy S31254

The severe precipitation of eutectic phases in cast super austenitic alloy poses significant challenges in subsequent homogenization and hot working. In this study, the effects of N on the formation and dissolution behaviors of the eutectic phases in cast B-bearing super austenitic alloy S31254 were investigated using experimental and theoretical analyses. The results indicated that increasing N content from 0.2 to 0.3 wt% prevented the precipitation of δ phase, significantly reduced and refined σ phase, but slightly promoted the formation of Laves and μ phase. As N content increased, the initial formation conditions of the δ and σ phases moved to higher Cr and Mo contents in the residual liquid and lower temperatures, and both Cr and Mo activities decreased to relatively low levels, thus suppressing the precipitation of the two phases. Accordingly, the consumption of Mo for the formation of both δ and σ phases decreased. Meanwhile, B dramatically aggregated into the residual liquid phase and reduced the interface energies at the solidification end. Therefore, B coupled with Mo promoted the formation of Mo-rich Laves and μ phase. With N content increasing from 0.2 to 0.3 wt%, the dissolution activation energy decreased from 393.2 to 196.4 kJ mol⁻¹. The significant reduction of the σ phase amount by increasing N was the main reason for accelerating the dissolution kinetics.