Austempering Temperature and Isothermal-Holding Effects on the Microstructure and Mechanical–Tribological Response of Si–Mo Alloyed Ferritic Ductile Iron Produced by SSF

This study investigates the influence of austempering temperature and isothermal-holding time on the microstructure, mechanical behavior, and tribological performance of Si–Mo alloyed ferritic ductile iron produced by the solution-strengthened foundry (SSF) route. After austenitization at 900 °C for 90 min, the specimens were salt bath austempered at 300, 350, and 400 °C for 30 and 90 min. The results showed that the austempering parameters influenced the evolution of the ausferritic matrix, retained austenite characteristics, and the resulting mechanical and tribological response. Among the investigated conditions, the specimen austempered at 300 °C for 30 min exhibited the highest ultimate tensile strength (1344.8 MPa ± 32.97) and hardness (52.92 HRC ± 1.64), as well as the lowest specific wear rate. The specimen austempered at 300 °C for 90 min showed the highest impact energy (54.7 J ± 4.443). These findings indicate that lower austempering temperatures favor a finer ausferritic structure, resulting in improved strength and wear resistance, whereas longer holding at the same temperature is more favorable for impact toughness. SEM examination showed that abrasive, adhesive, and oxidative wear mechanisms were present depending on the matrix condition. Overall, the study demonstrates that austempering parameters play a critical role in controlling microstructural evolution and the resulting mechanical–tribological performance.