The effects of temperature and strain rate on the dynamic flow behaviour of different steels

Abstract

A compressive type split-Hopkinson pressure bar is utilized to compare the impact plastic behaviour of three steels with different levels of carbon content. S15C low carbon steel, S50C medium alloy heat treatable steel (abbreviated hereafter to medium carbon steel) and SKS93 tool steel with a high carbon and low alloy content (abbreviated hereafter to high carbon steel) are tested under strain rates ranging from 1.1 × 10 3 s −1 to 5.5 × 10 3 s −1 and temperatures ranging from 25 to 800 °C. The effects of the carbon content, strain rate and temperature on the mechanical responses of the three steels are evaluated. The microstructures of the impacted specimens are studied using a transmission electron microscope (TEM). It is found that an increased carbon content enhances the dynamic flow resistance of the three steels. Additionally, the flow stress increases with strain and strain rate in every case. A thermal softening effect is identified in the plastic behaviour of the three steels. The activation energy, Δ G *, varies as a function of the strain rate and temperature, but is apparently insensitive to the carbon content level. The present study identifies maximum Δ G * values of 58 kJ/mol for the S15C low carbon steel, 54.9 kJ/mol for the S50C medium carbon steel, and 56.4 kJ/mol for the SKS93 high carbon steel. A Zerilli–Armstrong BCC constitutive model with appropriate coefficients is applied to describe the high strain rate plastic behaviours of the S15C, S50C and SKS93 steels. The errors between the calculated stress and the measured stress are found to be less than 5%. The microstructural observations reveal that the dislocation density and the degree of dislocation tangling increase with increasing strain rate in all three steels. Additionally, the TEM observations indicate that a higher strain rate reduces the size of the dislocation cells. The annihilation of dislocations occurs more readily at elevated temperatures. The square root of the dislocation density increases linearly with the work hardening stress. The current results provide a valuable reference for the application of S15C low carbon steel, S50C medium carbon steel, and SKS93 high carbon steel in high-speed plastic forming processes.