Abstract
With the development of new carburizing steels and processing techniques, there have been corresponding advances in the fatigue performance of automotive components, including carburized gears. These advances have led to increased component life and smaller power transfer systems, and combined with higher temperature processing, to decreased production costs. Higher carburizing temperatures associated with vacuum or plasma carburizing have led to requirements for new materials with improved resistance to unstable austenitic grain growth, as several studies have shown that improved fatigue performance is associated with finer prior austenite grain sizes achieved during carburizing. One methodology to produce steels for high temperature carburizing is to use microalloy additions, particularly Nb, along with controlled processing, to achieve precipitate distributions that are effective at the processing temperatures in suppressing unstable austenite grain growth during carburizing. In this paper, the effects of heating rate, rolling history, and processing temperature on the evolution of austenite grain structures in carburizing steels are considered in a study on Ti-modified SAE 8620 steels with Nb additions up to 0.1 wt pct. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions, during the onset of carburizing after annealing at different heating rates, and after annealing for various times at carburizing temperatures up to 1100 degrees C. Heating rate to the carburizing temperature was shown to be an important variable, and suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates, stable at the austenitizing temperature. The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.