Growth of Bainite and Widmanstätten Ferrite

Abstract

The growth processes of bainite and Widmanstätten ferrite have been investigated by means of scanning electron microscopy. The effects of an abrupt change in temperature during the isothermal decomposition of austenite was also examined. The results obtained are as follows:(1) Carbide-free bainitic ferrite laths grow discontinuously from a lower bainite plate by raising the transformation temperature from 280 to 520°C, and the habits of bainite laths are completely different from those of lower bainite.(2) By raising temperature to 510°C during the isothermal transformation at 360°C, carbide-free bainitic ferrite laths grow in a completely continuous fashion from the fine bainite laths, which involve cementite platelets and are often referred to as lower bainite.(3) These facts indicate that bainite should not be classified by cementite dispersion but by ferrite morphology, and that all the bainites with lath-like morphology should be identified as upper bainite.(4) Widmanstätten ferrite plates grow from grain boundary ferrite allotriomorphs at temperatures slightly lower than T0 with accompanying the invariant plane strain-type surface reliefs. Thus it seems likely that Widmanstätten ferrite plates grow displacively from the grain boundary ferrite formed by a diffusional mechanism.