Effects of Strain Rate and Temperature on Deformation Behaviour and Microstructural Evolution of Powder Metallurgical High-speed Steel (ASP 60) Part 2 Microstructural Study

The impact behaviour and quasi -static state deformation of powder metallurgical high-speed steel ASP 60 are investigated at strain rates of 10(-3) 10 s-i using a material testing system and 2.5 x10(3) 6.0 x10(3) s(-1) using a split-Hopkinson pressure bar under temperatures ranging from -195 degrees C 1000 degrees C. The results show that the flow stress under dynamic impacts generally increases with decreasing temperature and increasing strain rate. However, a negative strain rate sensitivity occurs at certain strain rates and temperatures due to the high interfacial area of the powder metallurgic structure of ASP 60. The microstructural observations reveal the presence of fine precipitates and dislocation multiplications at 195 25 degrees C and 2.5x10(3) 6.0x10(3) s(-1), which result in a high flow resistance of the deformed material. Under both dynamic impacts and quasi -static deformation, the dislocation density is sensitive to the deformation temperature and strain rate. However, the precipitations in the dynamic impact specimens are finer than those in the quasi -static state specimens. Finally, the relationship between the microstructure and mechanical behaviour of the ASP 60 specimens is well described by the work hardening stress, which increases linearly with the square root of the dislocation density..