Effect of Austempering Time and Temperature on the Mechanical and Microstructural Properties of a Niobium-Alloyed Austempered Ductile Iron

This study evaluated the influence of niobium addition and austempering time and temperature on the microstructure and mechanical behavior of ductile iron. Three alloys were produced: unalloyed ductile iron (H1) and two Nb-alloyed ductile iron (H2, 0.11 wt.% Nb and H3, 0.32 wt.% Nb). After austenitizing at 900 °C for 60 min, samples were austempered at 250 °C and 300 °C for 15, 30, 60, and 90 min. The as-cast microstructure of H3 exhibited a higher pearlite fraction (73.31 vol%) and increased carbide content (2.48 vol%), accompanied by reduced nodularity and nodule count. X-ray diffraction analysis revealed that the highest fraction of carbon-rich retained austenite was obtained in H3 after 30 min at 300 °C, reaching 42.48%. Hardness decreased with increasing retained austenite, confirming the inverse relationship between this phase and matrix strengthening. Wear testing showed that H2 presented slightly lower volume loss due to carbide precipitation, with the lowest value recorded after 15 min at 300 °C (1.088 mm3). Tensile tests indicated that ultimate tensile strength and yield strength were superior at 250 °C, with H3 achieving the highest values at 90 min (1353 and 1090 MPa, respectively). Overall, niobium promoted carbide formation and austenite stabilization, modifying the balance between hardness, toughness, and wear resistance in austempered ductile iron.