Grain Boundary Segregation Behavior and Thermodynamic Analysis Based on CALPHAD Method for B Added High Carbon Steel

The segregation behaviors on the prior austenite grain boundaries for the B added-high C case hardening steel (Fe-0.82C-0.22Si-0.86Mn- 0.02P-1.1Cr-0.21Mo-0.005B (mass%)) were evaluated by three-dimensional atom probe (3DAP). The results revealed the intense segregation of C, Mo, Cr, and B on the grain boundaries. In contrast, it was confirmed that the segregation of P known as a strong segregation element for steel was suppressed. The thermodynamic analysis based on the parallel tangent law by McLean-Hillert was carried out in order to validate the segregation of each element. To evaluate the chemical potentials taking the interaction with multiple elements into account, the CALPHAD (CALculation of PHAse Diagram) method was employed, where liquid phase was adopted to estimate the Gibbs free energy of grain boundaries. The calculation results represented the segregation tendencies obtained from 3DAP. Detailed investigations of the interaction effects of C, B, and Mo on the other elements were also conducted. The results showed the suppression of the P segregation by increasing B content. Therefore, the efficiency of the segregation prediction method that implemented the CALPHAD method for the B added high C steel was demonstrated.