The influence of high energy density sources on morphological changes in steels was studied by a physical simulation. Strips of carbon steels were subjected to heat cycles including continuous rapid heating to temperatures between 400 and 1200°C and immediate water quenching. The heat cycles were carried out by passing a high intensity electrical current through trapezoidal specimens in a special device allowing to obtain heating rates up to 10000°/s with an excellent temperature control. Real-time temperature recordings were drawn so as to define some characteristic temperatures of continuous austenitization. The evolution of the morphology, initially composed of ferrite and martensite, was examined by means of light and electron (SEM and TEM) microscopy using standard techniques. The results of the examination were related to microhardness measurements. Three distinct stages of the morphological evolution were finally analyzed: - T > AC1 - Beginning of austenite formation in zones of as-tempered martensite, mostly at former austenite grain boundaries. - AC1 << T < AC3 - Development of irregular lath-shaped interface between the growing (austenite) and shrinking (ferrite) phases. - T > AC3 - Final massive transformation of supersaturated ferrite areas.