Formation of Widmanstätten ferrite in a 0.036% Nb low carbon steel at temperatures above the Ae3

Abstract

The formation of Widmanstätten ferrite was investigated in a 0.036% Nb microalloyed steel at temperatures above the Ae3 using optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Such strain-induced ferrite appears to form in two consecutives stages: (i) stage I, first observed at strains below 0.5, leads to the presence of Widmanstätten ferrite plates only about 200 nm wide, (ii) stage II, observed at strains >0.5, involves the coalescence of the Widmanstätten plates into grains. Thus the microstructures formed by dynamic transformation are composed of fresh Widmanstätten plates (stage 1) and polygonal grains (stage 2). Over the experimental temperature range of 836-896°C, the ultrafine ferrite plates and grains have areas below 2 μm2 and are difficult to detect using optical microscopy. When austenite is deformed above the Ae3, very fine Widmanstätten plates are formed, which are only about 200 nm wide. On continued deformation, the plates coalesce into polygonal grains. Because of the fine scale of the microstructure, these are difficult to detect using conventional optical microscopy.fx1 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.