This study systematically investigates the effects of raw powder particle size distribution and hot isostatic pressing (HIP) temperature on the microstructure and mechanical properties of powder metallurgy tool steel (PM-TS). Three gas-atomized powders with distinct median particle sizes (P1: 24 µm, P2: 82 µm, P3: 192 µm) were consolidated via HIP at 1000 °C, 1050 °C, and 1150 °C, followed by a standardized quenching and tempering heat treatment. Contrary to conventional wisdom, the coarsest powder (P3) consolidated at the lowest HIP temperature (1000 °C) yielded an ultrafine-grained structure in the final heat-treated state, attributed to the accumulation of high deformation stored energy during low-temperature densification. This stored energy, in synergy with the fine ferrite grains, promoted the formation of a refined austenite structure during austenitization, thereby constraining the final martensitic grain size to an ultrafine 0.83 µm. The coarsest powder ( D 50 = 192 µm), after consolidation by HIP at 1000 °C and the standard quench and tempering, exhibited the highest hardness and superior wear resistance, facilitated by a stable, continuous oxide glaze layer during sliding. In contrast, samples from finer powders or higher HIP temperatures showed coarser microstructures and inferior properties. These findings highlight that a synergistic control of powder size and HIP temperature—enabling a “high stored energy path”—is essential for optimizing the microstructure and performance of PM-TS, challenging the traditional emphasis on powder fineness alone.
Wen et al. (Tue,) studied this question.