Ledges and carbides in lower bainite in a hypereutectoid steel

Abstract

The ferrite/austenite interfaces and carbides in lower bainite have been observed using transmission electron microscopy (TEM) on isothermally reacted specimens of a hypereutectoid steel. Superledges are found to exist at the broad faces of ferrite plates. The height and the width of the superledges are approximately 5 to 40 nm and 15 to 80 nm, respectively. Multiple arrangements of carbides in lower bainite have been observed. This is inconsistent with the single variant of carbides oriented at an angle to the sheaf axis repeatedly reported in lower bainite. The experimental results show that carbide precipitation occurs in austenite at ferrite/austenite boundaries located in gaps between the ferrite plates and/or between the ferrite subunits. Therefore, the conclusion is incorrect that carbides associated with lower bainite precipitate from wholly supersaturated ferrite. In a word, all of these observations suggest that lower bainite forms by the diffusion-controlled movement of ledges.