Precipitation of carbides at γ─α boundaries in alloy steels

Austenite containing 1 to 2 wt % vanadium and 0.2 wt % carbon, held in the range 600 to 850°C transforms to an extremely fine dispersion of V4C3 in ferrite. The carbide particles are nucleated on the γ─α phase boundary at successive positions during the transformation, forming sheets of precipitate which are 5 to 30 nm apart, while the individual particles are 3 to 10 nm diameter (interphase precipitation). High resolution electron microscopy has provided details of the morphology and crystallography of the carbide particles and the transformation front. A similar reaction has been studied in an iron +6.3 wt % tungsten + 0.23 wt % carbon alloy in which M6C precipitates on a much coarser scale enabling precise observations to be made on the transformation front. Interphase precipitation has also been examined in an iron + 3.45 wt % molybdenum + 0.22 wt % carbon alloy. A model is proposed for the interphase precipitation which takes into account the observed morphology and crystallography of the carbide phases and of the ferrite.