Abstract: Austenite grain growth behavior of two gear steels with different contents of Al and N austenitized at 980, 1050 and 1100 ℃ was experimentally studied. Equilibrium dissolution of AlN precipitates and its effect on austenite grain size were studied by means of optical microscope(OM), scanning electron microscope(SEM), transmission electron microscope(TEM) and carbon film replicas. The results show that with the increase of Al and N content, the critical temperature for abnormal growth of austenite grains in the gear steel also increases. When Al content is 0.038% and N content is 0.019%, the austenitizing temperature of the tested steel can be increased to 1050 ℃, at which after holding for 6 h, the austenite grains are still small and uniform, with grain size levels ranging from 7.5 to 8. Through the analysis of AlN precipitates, it is found that with the increase of austenitizing temperature, the AlN precipitates transforms from polygonal and nearly spherical to elongated rod-shaped, and its size continuously increases. When its equivalent radius is greater than 30 nm, the AlN precipitates cannot pin the austenite grain boundaries, resulting in mixed grains. Due to the coarsening and dissolution of the AlN precipitates, the growth rate of the ultimate grain size of austenite becomes faster and faster, and the ultimate pinning force for abnormal growth of austenite grains is between 0.045 and 0.095 J/cm³.

Key words: gear steel, grain growth, mixed grain, AlN precipitates