Isothermal transformation in Fe-N hypereutectoid alloy

Abstract

In order to understand the mechanism of isothermal transformation of Fe-N alloy, the isothermal transformation microstructure that forms in a wide temperature range below Ae1 was investigated in Fe-2.6 mass%N hypereutectoid alloy by means of the electron back scatter diffraction method in addition to the conventional microstructural observation methods. High-nitrogen austenite fully decomposed to ferrite and Fe4N over the entire temperature range, and the time-temperature-transformation (TTT) diagram had a C shape with a nose temperature around 700 K. The hardness linearly increased with decreasing transformation temperature because the microstructure became finer, but the morphology of the (ferrite + Fe4N) structure changed discontinuously at around 800 K. From the microstructural and crystallographic analyses, it was concluded that the microstructure formed at higher temperature is a lamellar eutectoid structure, braunite, while the other is an upper bainitic structure containing bainitic ferrite formed through a displacive mechanism and Fe4N formed by concentration and ordering of the nitrogen. Since Fe4N is a counterpart of the cementite in Fe-C alloy, the respective structures are similar to pearlite and upper bainite in carbon steel. © 2013 ISIJ.