Mechanical Response and Microstructure Characteristics of Powder Metallurgical High-Speed Steel (ASP 60) Impacted at −195°C and 800°C

Abstract

The dynamic mechanical behaviour of high-alloyed powder metallurgical high-speed steel ASP 60 is investigated using a compressive split-Hopkinson pressure bar at strain rates of 2.5 × 10³ and 4.0 × 10³ s⁻¹ and temperatures of −195°C and 800°C, respectively. The effects of the strain rate and temperature on the microstructure evolution of the impacted specimens are examined using scanning electron microscopy and transmission electron microscopy. A negative strain rate sensitivity is observed at both temperatures. The flow stress, strain rate sensitivity, temperature sensitivity, fracture mechanism, and dislocation substructures are all significantly affected by the strain rate and temperature. The SEM fractographs reveal a brittle fracture mode at −195°C and localized melting at 800°C. The specimens impacted at −195°C exhibit a dislocation multiplication microstructure entangled with fine precipitates, which collectively increase the flow resistance of the sample. However, the microstructures of the specimens impacted at 800°C show a lower density of dislocations and coarse precipitates, resulting in a loss of flow resistance. The flow stress of the ASP 60 specimens shows a linear decrease with the square root of the dislocation density at both temperatures. The rate of decrease in the flow stress is higher under a cryogenic temperature. Hence, the relationship between the dislocation density and the mechanical response is inferred to be temperature-dependent.