Effects of Strain Rate and Temperature on Plastic Deformation Behavior of Molybdenum-Based High-Speed Tool Steel HS-6-5-2

High-speed tool steel HS-6-5-2 used for metal cutting tools and cold forging dies which require the stress-strain relationship during high-speed deformation. However, it is difficult to experimentally obtain the stress-strain relationship at high-strain rate by uniaxial tests for high-strength materials. Therefore, we newly developed an apparatus for performing split Hopkinson pressure bar (SHPB) impact compression tests by using cemented carbide bars as the elastic bars. In this study, the effects of strain rate and temperature on compressive properties of HS-6-5-2 were experimentally investigated by using not only a universal testing machine but also a developed SHPB apparatus. A series of quasi-static and impact tests were conducted under three temperature conditions, 77 K, 197 K and 298 K. In addition, temperature condition of 473 K was conducted only in quasi-static tests. Most of all specimens showed that the flow stress increased with the increase of strain rate except for the impact test at 77 K where the effect of work softening was more significant than the strain rate dependence of strength. By using the theoretical consideration based on the thermal activation theory of dislocation motion, it was found that the deformation mechanism of HS-6-5-2 in a wide range of strain rates and temperatures is quite similar to that of general steels.