Study on Strength–Toughness Properties and Microscopic Fracture Mechanism of 1.4 GPa–Grade Quenched‐Partitioned and Austempered Ductile Iron

This study systematically investigates the strength–toughness properties and microscopic fracture mechanism of a 1.4 GPa–grade partitioning‐austempered ductile iron. The results show that the 200 °C partitioning –300 °C austempering process yields excellent comprehensive mechanical properties: a tensile strength of 1386 MPa, an elongation of 12.0%, and an impact energy of 4.9 J. A multiphase microstructure dominated by fine acicular ferrite, fine martensite laths (length: 2.9 ± 0.7 μm, thickness: 0.21 ± 0.1 μm) and stable blocky retained austenite (size: 3.5 ± 1.0 μm 2 ) is obtained. EBSD analysis further reveals that the QP&AT300 specimen exhibits a more uniform strain distribution (average KAM value: 0.85°) and a higher proportion of medium‐angle grain boundaries (21.3% in the range of 15°–50°), which help delay crack propagation and improve toughness. The fracture mode is of a mixed ductile‐brittle type, dominated by fine dimples around graphite nodules with local cleavage steps, indicating better toughness characteristics. By optimizing the Q&P heat treatment process, this study achieves a synergistic enhancement of ultrahigh strength and good toughness in ductile iron, providing an important reference for microstructure control and engineering application of high‐strength ductile iron.