PINNING OF AUSTENITE GRAIN-BOUNDARIES BY AIN PRECIPITATES AND ABNORMAL GRAIN-GROWTH

The growth behavior of austenite grains in the presence of AlN precipitates varies with the temperature and time of anneal. To study this behavior, two iron alloys, (in weight percent) a 0.1 carbon base chemistry with 0.03Al/0.01N and 0.09Al/0.04N, respectively, were annealed between 1000-degrees-C and 1200-degrees-C for times of up to 180 minutes. Using optical microscopy, as many as 1000 austenite grains per heat-treatment condition were measured. Conditions of suppressed, abnormal, and uniform grain growth were observed. Using an extraction replica technique, the size, shape, and distribution of the AlN particles were determined using transmission electron microscopy (TEM). The largest grain boundary curvatures calculated, using the Hellman-Hillert pinning model, were in close agreement with independent calculations of curvatures using the grain size data. The largest grains in the lognormal size distribution of austenite grains were found to be the ones with the potential to grow to abnormally large sizes.