Kinetics of polymorphic transformation in ferrite-pearlite steel on heating in the intercritical temperature range

However, the processes in structural steel on heating in the intercritical range and their influence on the structure and properties of the metal have not been adequately studied. Taking account of the scientific and practical interest in polymorphic transformation within the intercritical temperature range, it is appropriate to study the transformations in ferrite‐pearlite steel (considered in the present work) and in high-strength steel and to determine the physicomechanical properties of the rolled steel. In the first stage of this work, attention focuses on carbon and low-alloy sheet steel: eU 3 OO , 09 E 2 e , 09 E 2 aiA , and 16 E 2 Ai steel (Tables 1 and 2). We know that the position of the intercritical range and the transformations within it depend not only on the chemical composition of the steel but also on its initial structure and heating conditions [5‐12]. For steel subjected to mechanical or phase hardening, at high heating rates, the critical points A c 1 and A c 3 lie lower than for steel with equilibrium structure. With slower heating, facilitating recovery and recrystallization, the critical points are higher. Therefore, to obtain more general results, the steel is investigated in two structural states: after annealing (heating to 920 ° C, holding for 20 min, cooling in the furnace to 300 ° C and then in air); and after phase hardening (quenching in water from 920 ° C after 20-min holding at 920 ° C). The thickness of the plate samples is equal to the sheet thickness, with a width of 200 mm and a length of 300 mm. The position of the critical points A c 1 and A c 3 and the relative volume of α γ transformation in heat-treatment conditions or in the thermal cycles of welding may be determined by means of a high-speed vacuum dilatometer, for example [13]; however, this method is slow. Therefore, a different dilatometer design is proposed: the rod samples are electrically heated in air, and oxidation and scale formation are eliminated by applying a metallicchrome layer of thickness 4‐8 µ m. To equalize the temperature over the sample length, a working section of stepped form is employed (Fig. 1). Thermal changes in