Evolution mechanisms of microstructure and mechanical properties in spray-formed H13 during isothermal quenching near Ms and subsequent tempering

The production of larger hot-work die steels has become a prevailing industrial trend. This study addressed the challenge of predicting and controlling the complex multiphase microstructures formed through isothermal treatment near the martensite start temperature (Ms), which result from sluggish core cooling during quenching of large-scale hot-work die steels. Using spray-formed H13 steel, we systematically investigated the coupled effects of near-Ms isothermal quenching on microstructure and mechanical properties through dilatometry testing, multi-scale characterization (SEM/EBSD/TEM/XRD), and mechanical testing. The influence mechanisms of subsequent tempering processes were further elucidated. The results demonstrate that quenching at 220°C (below Ms) produces a composite microstructure of nano-structured bainite, tempered martensite, and retained austenite. This microstructure exhibits the finest grain size (1.66 μm) and approximately doubled impact energy (11.3 J/cm 2 ) compared to specimens quenched at 260 or 300°C (above Ms), which exhibited ∼5.7 J/cm 2 . The enhancement primarily originates from grain refinement strengthening, TRIP effect enabled by high-stability retained austenite, and absence of brittle carbide precipitation. During 350°C tempering, specimens isothermally quenched above the Ms point exhibit improved toughness due to dissolution of brittle acicular M 3 C carbides and stabilization of retained austenite. In contrast, 500°C tempering causes drastic toughness deterioration (≈3 J/cm 2 ) through precipitation of semi-coherent acicular M 7 C 3 carbides and aggregated nanoscale carbide precipitation. Tempering at 650°C resulted in complete decomposition of retained austenite, accompanied by a sharp reduction in dislocation density and precipitation of large elliptical M 23 C 6 carbides, resulting in a marked decrease in strength and a significant improvement in toughness.