Applications and Thermodynamic Analysis of Equilibrium Solution for Secondary Phases in Ti–N–C Gear Steel System with Nano-Particles

Thermodynamic analysis and computations of the equilibrium solution for the multivariable secondary phase in steels have been conducted, based on the chemical equilibrium. Solid solution precipitation in the Ti–N–C microalloy steel for the secondary phase was analyzed, and its engineering application in the development of carburizing gear steels was investigated. The complete dissolution temperature increases with more C, N, or Ti in gear steels, and the effects of Ti and N are more significant than C. At a given temperature, the amount of [Ti] increases naturally with more Ti element content, while it decreases with more C or N elements content. It declines rapidly in the complete dissolution temperature range in steels until 1100 ◦C. The effective activity coefficient k1 increases with the temperature decreases, and increases with the higher content of Ti or C, while it decreases with the higher N content under the same temperature. Thermodynamic analysis shows that nitrogen has been precipitated largely as TiN micron-particles above 1400 ◦C in gear steels. Then, titanium precipitated mainly as TiC nano-particles, thus this secondary phase can hinder grain coarsening during heat treatment.