Effect of cementite on grain boundary segregation kinetics in tempered Fe-C-3%Mn martensite

High-strength steels with tempered martensitic microstructures can exhibit low toughness due to grain boundary fracture. One contributing factor is the segregation of Manganese (Mn) at prior austenite grain boundaries (PAGB), leading to decreased grain boundary fracture toughness. Therefore, controlling the amount of Mn segregation is crucial in material design. The analysis of Mn segregation to PAGB during tempering is complex due to the concurrent partitioning of Mn to the cementite particles that precipitate during tempering. The aim of this study is to explore the effect of Carbon (C) content on the amount of Mn segregation during tempering. Two model alloys with the compositions Fe-0.05mass%C-3mass%Mn and Fe-0.3mass%C-3mass%Mn were tempered at 823 K for durations of up to 3.6 × 106 s. Changes in Mn partitioning to cementite and segregation to grain boundaries were assessed using atom probe tomography. The results revealed that the steel with higher C content had a large amount of cementite and reduced Mn segregation at PAGB. Furthermore, it was found that experimental values could be effectively modeled by calculating the Mn concentration of the “boundary phase” which is in “equilibrium” with the interfacial concentration of Mn in cementite.