Effect of initial microstructure on graphitization behavior of Fe–0.55C–2.3Si steel

In this study, the effect of the initial microstructure (i.e., ferrite + pearlite or martensite) on the graphitization behavior and microstructural evolution during heat treatment of a hot-rolled medium-carbon, high-silicon steel (Fe–0.55C–2.3Si) is investigated. When a sample consists of ferrite and pearlite, its microstructure gradually changes to ferrite and graphite during heat treatment via the decomposition of pearlite and the formation of graphite particles. In the sample with a martensite microstructure, the phase transformation occurs in the following step: martensite → ferrite + cementite → ferrite + graphite. Although the graphitization process of the martensite sample is more complex than that of the ferrite + pearlite sample, the graphitization rate is substantially faster in the former owing to more abundant grain boundaries and triple junctions that act as nucleation sites for graphite. When the initial microstructure changes from ferrite + pearlite to martensite, the graphite completion time significantly reduces from 16 to 2 h at 650 °C and from 6 to 0.5 h at 700 °C. After complete graphitization, the average size and number density of graphites in the martensite sample are respectively smaller and higher than those in the ferrite + pearlite sample. The rate of hardness decrease (i.e., material softening) during heat treatment is also faster in the martensite sample. These results demonstrate that changing the initial microstructure from ferrite + pearlite to martensite is an effective means to accelerate the graphitization behavior and acquire a graphitized steel with uniformly distributed fine graphite particles.