Ferrite Formation Behaviors from B1 Compounds in Steels

Abstract

The effect of chemical composition of B1 compounds on the formation of intragranular ferrite during isothermal austenite-to-ferrite transformation was investigated by embedding single crystalline grains of chemically pure B1 compounds in low-alloy steel. Two kinds of B1 compounds having similar misfit against ferrite, TiO and TiN, were adopted in this study. Isothermal transformation experiments were carried out both above and below the bainitic transformation temperature. The orientation relationships between compound and ferrite and between ferrite and prior austenite were characterized by scanning electron microscope equipped with electron backscatter diffraction analyzer.It was found that the Baker-Nutting (B-N) orientation relationship played a dominant role in ferrite formation from B1 compounds at higher transformation temperature, but some low-index orientation relationships other than the Baker-Nutting orientation relationship were also observed as the transformation temperature decreased, which led to an increase in ferrite formation having the Kurdjumov-Sachs (K-S) orientation relationship with prior austenite. Ferrites from TiO had larger misorientation angles from the exact B-N orientation relationship and thus tended to have the K-S orientation relationship with prior austenite at lower transformation temperatures.