Interphase precipitation in microalloyed engineering steels and model alloy

Abstract

Interphase precipitation of alloy carbide or carbonitride which occurs in association with the transformation interface between austenite and ferrite and between austenite and pearlite in steels can result in a fine particle dispersion that can contribute significantly to strength levels in a wide range of structural and engineering steels. Interphase precipitation has been found to occur in both proeutectoid and pearlitic ferrite, and various morphological aspects have been studied in a range of low carbon and higher carbon steels. Studies of this precipitation reaction are hindered by the fact that the transformation interface is lost upon cooling to room temperature, either by further transformation or by decomposition of the residual austenite phase to martensite. In the present work this has been avoided by developing a model alloy in which the precipitation reaction occurs, but in which the austenite is stabilised to room temperature, thus allowing a detailed examination of the interfacial region by transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). Weak beam TEM failed to reveal any evidence for VC precipitation in austenite ahead of the interface. Therefore APFIM was also performed on aged specimens to examine the segregation behaviour in the stable austenite ahead of the interface, which is otherwise difficult to study in commercial steels because it decomposes into highly dislocated martensite. These results are discussed in terms of recent interpretations of the mechanism of interphase precipitation.

MST/1711