Influence of post-carburizing heat treatment on the core microstructural evolution and the resulting mechanical properties in case-hardened steel components

The case hardening process (carburizing followed by quenching to room temperature) is one of the heat treatments routinely conducted in various industrial sectors. Depending on the parameters applied during the post-carburizing stage, different microstructural constituents may develop in the interior (core) section of a case-hardened component which substantially influence the overall mechanical properties. In the present work, the effects of different heat treatments following the carburizing stage on the core microstructure evolution, the resulting toughness and hardness properties were investigated. The heat treatments were carried out employing Navy C-ring specimens, the core of which were used for microstructural investigations and fabrication of mini-Charpy specimens. Besides, nanoindentation tests were also carried out in the same region to examine the local hardness of different constituents. Furthermore, the microstructural evolutions were also studied utilizing multiphase field modeling. Based on the obtained results, it can be shown that the increase in fractions of bainite and retained austenite is in direct correlation with the impact toughness improvement, however, the increase in fraction of ferrite and martensite acts inversely. Moreover, despite the differences in fractions of constituents, the level of overall hardness is similar in all the developed microstructures. Additionally, the nanoindentation results revealed that the formation of large fraction of bainitic ferrite leads to high level of scattering in nanohardness of martensite. Phase field simulations show that this is attributed to the degree of carbon partitioning into the adjacent austenitic area where depending on the local morphology and size can be quite different.