Morphology and growth process of bainitic ferrite in steels

Abstract

The morphologies and the formation mechanisms of isothermally transformed Widmanstfitten ferrite and bainite in various steels were investigated. Widmanstfitten ferrite often grew from the grain boundary ferrite allotriomorph formed by a diffusional mechanism in the temperature range of the upper C-curve, whereas, bainite grew directly from an austenite grain boundary showing its own C-curve in a TIT diagram at temperatures between Bs (bainite starting temperature) and Ms (martensite starting temperature). Both structures accompanied well-defined surface reliefs of the invariant plane strain-type, and were in the shape of a lath or a plate consisting of several parallel needle-like ferrite subunits with parallelogram cross sections. The crystallographic properties of Widmanst~itten ferrite and upper bainite were similar to those of lath martensite. Therefore, it was concluded that the difference between bainitic ferrite and Widmanst~tten ferrite existed only in the nucleation events, where both structures grew by the same mechanism.

Effect of laser power and deposition environment on the microstructure and properties of direct laser metal-deposited 12CrNi2 steel

“…According to the research by Jung et al [14], when austenite transformation occurs at high temperature for a longer time associated with lower cooling rates, Widmanstatten ferrite can occur along proeutectoid ferrites at the austenite grain boundaries. Microstructures of top layers of samples, which were deposited by laser power of 900 and 1100 W, also show these phases in Fig.…”