A special phase transformation phenomenon in high-nitrogen austenite

A special phenomenon of media temperature phase transformation is observed in high-nitrogen-containing austenite. The fine nuclei of decomposition for m in the austenite grains and grain boundaries firstly, and the finer decom position products can be found in the nucleus. Then the decomposition nucle us grows very slowly while the amount of the new phase nucleus increases co nstantly. Finally, the regions containing the decomposition nucleus are con nected to one another and a high-dispersive morphology is formed. This stru cture is different not only from martensite but also from lower bainite wit h plate or needle structure. The hardness of the decomposition product reac hes to 900 HV, which is higher than that of tempered martensite. The isothe rmal decomposition has a long incubation period. The amount of decompositio n products increases with time and finally reaches to 100%, which is differ ent from the bainite transformation. The observation shows that structural morphology and dynamics of the phase transformation are significantly diffe rent from bainite and martensite. (C) 2001 Elsevier Science B.V. All rights reserved.