Thermal Processes in the Heating of Powder Compacts of Metals and Their Compositions II. Thermokinetics of Processes in the Heating of Porous Iron

The thermal processes involved in the recrystallization within porous ultrapure iron compacts heated after cold pressing in a steel die were experimentally studied. The temperatures of recrystallization stages were shown to correspond to the heating behavior of metals and alloys. Depending on the heating rate of compacted powdered iron, various types of thermokinetic behavior were identified, indicating the nonlinear nature of interactions within the deformed powder body. Stages with both nonlinear and linear thermal behavior were present, as evidenced by the competition between several processes leading to the emergence of wave-like transfer of energy, accumulated while powdered iron deformed during cold pressing. The thermokinetic patterns observed in the heating of compacts, especially those made of pure iron, allowed all stages of relaxation in the deformed metal to be identified. The temperature profile in the heating process generally reflects the thermal state of the powdered metal, influenced by the additional release of energy accumulated in deformation. At some stages, heat release led to nonlinear processes, resulting in the occurrence of thermal waves. Both asynchronous and synchronous temperature changes were observed. In asynchronous behavior, nonlinear waves emerged. The superposition of relaxation processes in the generation of thermal waves is possible. At specific heating rates, the maximum amount of energy is simultaneously released in the relaxation and recrystallization processes, as testified by an increase in temperature to 550°C. The relaxation processes involve less energy compared to the recrystallization processes and the transition to the annealed state. In addition, after complete recrystallization, the onset of the sintering process was observed under temperature oscillations with damping.