• Increasing Si content from 1.38wt% to 2.67wt% increases graphite particle number density but decreases their average size, while Fe3C particles continuously decrease in both number density and size.
• Higher tempering temperature (715°C vs 680°C) promotes graphite growth and accelerates Fe3C decomposition and refinement in steels with graphite precipitation, but only coarsens Fe3C in Si-lean steel without graphite.
• Fe–0.58C–1.0Al steel with 1.89wt% Si exhibits significantly lower hardness than other Si contents, especially after tempering at 715°C, achieving nearly HV 20 lower hardness than previously reported Fe–0.55C–2.33Si steel.
• The study demonstrates a silicon-saving, aluminum-containing free-cutting steel design that balances graphitization and hardness, offering improved machinability and environmental benefits over traditional S/Pb-based steels.