Thermodynamic calculations and validation of the competing precipitation behavior of BN and Nb(C,N) in 9CrMoCo heat-resistant steel

In this study, the thermodynamic conditions were calculated for the formation of nitrides in the liquid phase and precipitation in the solid phase of the 9CrMoCo steel matrix. The mutual dependency of BN and Nb(C,N) was demonstrated through thermodynamic calculations and validated experimentally. The equilibrium distribution coefficients for Nb, N, and B at the solid–liquid interface were considered, and the solidification rate (fs) was introduced into the thermodynamic calculation. BN and NbN met the formation conditions when the fs values were 97.96% and 99.04%, respectively. The formation of Nb-containing nitrides was restrained by the formation of BN inclusions in the molten steel, which was confirmed by observing the elemental constituents of the Nb-rich precipitates. N (0.019%) in the solid solution was separated from the BN inclusions at 1523 K and it participated in the precipitation of Nb(C,N) during the subsequent cooling process. Needle-like Nb(C,N) precipitates and particles of Nb(C0.15N0.85) were formed under slow furnace cooling and austempering at 1223 K, respectively. In addition, the secondary precipitation of BN in austenite was restricted by the formation of the Nb(C,N) precipitate during the cooling process after the dissolution of BN inclusions.