Effect of Si content and tempering temperature on microstructure and precipitation behavior of graphite particles in Fe-0.58C-1.0Al steel

Si and Al can act as graphitizing elements in high-carbon steel, but an elevated Si content can lead to higher hardness and deterioration of machinability in graphite steel. The technical approach of substituting Al for Si has the potential to offer advantages in achieving simultaneous optimization of graphitization and hardness in high carbon steel. This study investigates Fe-0.58C-1.0Al high-carbon and high-aluminum steel using optical microscopy (OM), field emission scanning electron microscopy (FESEM), and electron probe microanalysis (EPMA). The effects of Si content (0.55wt%~2.67wt%) and tempering temperature (680℃, 715℃) on the tempering structure and the precipitation behavior of graphite and Fe3C were systematically studied. The results show that, at both tempering temperatures, the microstructure of 0.55wt% Si content steel is ferrite + granular Fe3C, and the microstructures of 1.38wt%~2.67wt% Si content steel are ferrite + petaloid graphite + granular Fe3C. With increasing Si content from 1.38 to 2.67wt% at constant tempering temperature, the number density of graphite particles increases, though their average size decreases significantly at 2.67wt% Si. Meanwhile, the number density and average size of Fe3C in four kinds of experimental steels continuously decrease. For 0.55wt% Si steel without graphite precipitation, increasing tempering temperature promote the accumulation and growth of Fe3C. For 1.38wt%~2.67wt% Si steel with graphite precipitation, higher tempering temperatures promote graphite particles growth while accelerating Fe3C decomposition and refinement. Compared with the experimental steels containing 0.55wt% Si, 1.38wt% Si, and 2.67wt% Si, the 1.89wt% Si variant exhibits significantly lower hardness. When tempered at 715℃, Fe-0.58C-1.0Al steel with 1.89wt% Si effectively promotes graphitization behavior and reduces hardness, which is nearly 20 HV lower than previously reported Fe-0.55C-2.33Si steel.