Effects of the raw particle size and hot isostatic pressing (HIP) temperature on the microstructure and mechanical properties of powder metallurgy tool steel

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.