The Isothermal Decomposition of Austenite in Simple Chromium Steels

Abstract

Two high-purity steels containing 12 and 5% chromium with 0.2% carbon have been isothermally transformed in the temperature range 775-600°C. The various ferrite and carbide morphologies encountered in this transformation range have been examined by optical and thin-foil electron microscopy. At the higher temperatures (775-700°C) coarse perlitic-type transformation products are obtained in which the carbide phase is M₂₃C₆ or M₇C₃, depending on the chromium content of the steel. However, in the range 700-600°C, several complex morphologies were found. Fibrous chromium carbides occurred in association with ferrite, but on a finer scale, forming incoherent boundaries with the austenite. Planar structures involving bands of M₂₃C₆ or M₇C₃ (interphase preciptation) were also a striking feature of the structures; these were normally associated with faceted α/γ interfaces, the facets being between 500 and 5000 Å high. Dark-field precipitate spot electron microscopy indicates that the carbide nucleates preferentially on the long planar faces which are less mobile, while the facets which move more rapidly do not nucleate precipitate. The results suggest that the fibrous carbide/ferrite aggregates form curved high-energy interfaces with austenite, while the banded structures are usually associated with low-energy, planar γ/α boundaries in the transformation range 700-600°C. Both structures are frequently observed in the same austenite grain.