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 degrees 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 degrees 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.