Fine Structure and Formation Mechanism of Bainite in Steels

Abstract

Scanning tunneling microscopy (STM), together with light optical microscopy (LOM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), of ultra fine structure of bainitic ferrite (α) shows that subunits inside a bainitic plate consist of sub-subunits, and STM of the surgace relief effect accompanying bainitic structure further confirms the existence of sub-subunits. The multi-layer structural constitution of bainite exhibits the typical self-embedded characteristics, and the fractal multi-dimension has been determined to be 2.74. Moreover, STM shows that an individual relief corresponding to a bainitic plate is composed of a sheaf of miniature reliefs which seem to arise from the formation of bainitic platelets, such as subplates, subunits and/or sub-subunits. The discovery of bainitic sub-subunits is helpful to give insight into transformation mechanism and interpret the complex and variable morphology of bainite. Based on the findings regarding the multi-layer structure of bainite, it is proposed that bainite forms via ledgewise growth which is essentially accompanied by sympathetic nucleation, i.e., sympathetic-nucleation-ledgewise-growth (SNLG). In this paper the SNLG mechanism will be theoretically discussed and practically applied to explain the multi-layer structure of bainite.