INTERPHASE PRECIPITATION IN MICROALLOYED ENGINEERING STEELS AND MODELALLOY

Interphase precipitation of alloy carbide or carbonitride which occurs in association with the transformation interface between austenite an d ferrite and between austenite and pearlite in steels can result in a fine particle dispersion that can contribute significantly to strengt h levels in a wide range of structural and engineering steels. Interph ase precipitation has been found to occur in both proeutectoid and pea rlitic ferrite, and various morphological aspects have been studied in a range of low carbon and higher carbon steels. Studies of this preci pitation reaction are hindered by the fact that the transformation int erface is lost upon cooling to room temperature, either by further tra nsformation or by decomposition of the residual austenite phase to mar tensite. In the present work this has been avoided by developing a mod el 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 microsc opy (TEM) and atom probe field ion microscopy (APFIM). Weak beam TEM f ailed to reveal any evidence for VC precipitation in austenite ahead o f 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 ste els because it decomposes into highly dislocated martensite. These res ults are discussed in terms of recent interpretations of the mechanism of interphase precipitation. (C) 1993 The Institute of Materials.