Verification of Thermodynamic Models for Predicting Grain Boundary Segregation of Carbon and Nitrogen in Ferritic Steels

Abstract

The concentrations of C and N segregated at the grain boundaries (GB) in Fe-C, Fe-N binary alloys, and Fe-C-N ternary alloys were calculated using the McLean and the Hillert-Ohtani models. Comparison of the calculated data with experimental values obtained by three-dimensional atom probe tomography (3DAP) revealed that the experimental values were roughly explained by the Hillert-Ohtani model for the binary alloys. The McLean model also can predict similar results when the fraction of available sites in the GB layer is assumed to be saturated at 0.1. On the other hand, the Hall-Petch coefficient (ky) of Fe-C-N, determined from tensile tests, was linearly related to the predicted concentrations of segregated C and N. Proposedly, ky can be predicted from the chemical composition and heat treatment temperature via thermodynamic calculations.