AbstractAn examination by transmission electron microscopy has been made of the austenite→ferrite + carbide reaction in an Fe–C–V low-alloy steel over the transformation temperature range 550–850°C. Thin films of interlath retained austenite in the untransformed martensite regions of partially transformed specimens have been used to investigate the crystallographic relationships between parent and product phases. Grain boundary ferrite allotriomorphs nucleate with a rational orientation relationship to at least one, but also occasionally to both, of the two adjacent austenite grains. Interphase precipitation of vanadium carbide can occur in the ferrite irrespective of the austenite–ferrite crystallographic relationship and, in particular, it is shown that the single habit plane adopted by the carbide is not a consequence of a three-phase crystallographic relationship between parent austenite and the product ferrite and carbide phases. It is thought that the precipitate adopts that habit plane which lies at the smallest angle to the plane of the transformation interface, and additional experimental evidence to support this is presented.MST/664