Graphitization behavior of medium-carbon high-silicon steel and its dependence on temperature and grain size

The graphitization behavior of medium-carbon, high-silicon steel (Fe-0.55C-2.3Si) is investigated with particular focus on the effects of heat-treatment temperature and initial grain size on the graphitization rate and mechanical properties. With the progress of heat treatment, pearlite gradually disappears because of the decomposition of the cementite layers and graphites are formed because of the diffusion and aggregation of carbon atoms; these occurrences lead to a gradual decrease in the hardness of the material. A higher heat-treatment temperature promotes the graphitization behavior, which consequently leads to a decrease in the time taken for completion of graphitization and an increase in the average graphite size. Graphites are dominantly formed at grain boundaries owing to the high energy and disordered atomic arrangements at these boundaries. Further, because grain boundaries act as graphite nucleation sites, a smaller grain size corresponds to faster graphitization as well as the formation of finer and more abundant graphites. The complete graphitization treatment causes a significant reduction in the tensile and compressive strengths of the material and a considerable improvement in its ductility and cold forgeability.