Suppression of Austenite Grain Coarsening by Using Nb–Ti Microalloying in High Temperature Carburizing of a Gear Steel

Austenite grain coarsening behavior during pseudo‐carburizing is investigated in a 18CrNiMo7‐6 steel, Nb microalloyed, and Nb–Ti microalloyed 18CrNiMo7‐6 steels, which are designed to evaluate the effects of Nb–Ti microalloying on suppressing austenite grain growth at elevated temperatures appropriate to high temperature carburizing operations. Pseudo‐carburizing heat treatments, without carburizing atmosphere, are performed in the temperature range of 850–1150 °C for 0.5–8 h. Transmission electron microscopy (TEM) incorporated with EDS spectra is used to investigate the morphology and size distributions of precipitates in pseudo‐carburized specimens. The results show that Nb–Ti microalloying has an obvious advantage in restraining austenite grain growth and increasing the grain coarsening temperature. Even in the temperature range of 1000–1150 °C, Nb–Ti microalloyed steel produces fine and uniform grain structures. The fine grain sizes observed in Nb–Ti microalloyed steel are attributed to the pinning effect of Nb–Ti precipitates that hinder austenite grain coarsening. The key reason for the significant difference of grain growth mechanisms between the base steel and Nb–Ti microalloyed steel is because Nb–Ti microalloying changes grain growth kinetic of the base steel. The results indicate that Nb–Ti microalloying can be successfully used to suppress grain growth in gear steels for high temperature carburizing. Nb–Ti microalloying has an obvious advantage in restraining austenite grain growth and increasing the grain coarsening temperature. Even in the temperature range of 1000–1150 °C, Nb–Ti microalloyed steel produces fine and uniform grain structures. Simultaneous control of Nb, Ti, and N additions provides optimal grain growth resistance at higher carburizing temperatures, e.g., up to 1150 °C. The figure shows optical micrographs showing austenite grain structures of Nb–Ti microalloyed steel after pseudo‐carburizing at 1100 °C for 1 h.