Synergistic effects of quenching and partitioning on the microstructural evolution and thermal fatigue behavior of H13 hot-work tool steel

H13 hot-work tool steel is widely used in high-temperature forming applications, but conventional quenching and tempering often produce a coarse tempered martensitic structure with limited thermal fatigue resistance. In this work, the effects of quenching and partitioning (Q&P) treatment on the microstructure, mechanical properties, and thermal fatigue behavior of H13 steel were systematically investigated by combining hardness and impact testing with XRD, SEM, EBSD, and TEM analyses. The Q&P treatment generated a refined multiphase microstructure composed of primary martensite (M1), secondary martensite (M2), retained austenite (RA), and dispersed carbides. Carbon partitioning from supersaturated martensite into the surrounding austenite effectively stabilized RA, and the retained austenite fraction reached 17.8% under the optimized condition. The quenching temperature strongly affected the M1/M2 ratio and martensite morphology, while the partitioning process mainly regulated carbon redistribution and RA stability. Among the investigated conditions, quenching to 200 °C followed by partitioning at 400 °C for 30 min provided the best balance of properties, with a hardness of 55.9 HRC and an impact absorbed energy of 205 J. Compared with conventionally quenched and tempered steel, Q&P-treated specimens exhibited markedly improved resistance to thermal fatigue cracking, as evidenced by reduced crack length and crack density after cyclic thermal loading. This improvement is attributed to the refined martensitic substructure, the presence of film-like RA, and the more homogeneous distribution of fine carbides, which together alleviate stress concentration, enhance crack deflection, and suppress oxidation-assisted crack propagation.