Dissolution and coarsening of aluminum nitride precipitates in low carbon steel - Distribution, size and morphology

Transmission electron microscopy (TEM) and scanning transmission electron m icroscopy (STEM) have been used to investigate the morphology, spatial dist ribution, particle size distribution and dissolution/growth kinetics of alu minum nitride (ALN) precipitates during reheating of an Al-killed, low carb on steel. The precipitates have hexagonal shapes after long aging times in the ferrite. The shapes change from hexagonal to rectangular after annealin g in the austenite region. In ferrite, the majority of the precipitates are distributed along the ferrite grain boundaries. Networks of AIN precipitat es can be found after annealing in austenite. Ripening of the AIN takes pla ce at isothermal temperatures below about 1150 degrees C, while complete di ssolution of the precipitates occurs at temperatures higher than about 1150 degrees C. A theoretical model has been developed to predict the dissoluti on and coarsening behavior of the precipitate. Satisfactory agreement with the experimental results has been obtained.