Interpretation of Charpy impact energy characteristics by microstructural evolution of dynamically compressed specimens in three tempered martensitic steels

In this study, Charpy impact energy characteristics of three tempered martensitic steels were evaluated at room and low temperatures by instrumented Charpy impact tests, and were interpreted by fracture initiation and propagation mechanisms in relation with microstructural evolution of dynamically compressed specimens. As the tempering temperature increased, the volume fraction of total carbides increased, while carbides were spheroidized, and the overall Charpy absorbed energy increased at both 25 °C and −50 °C. At −50 °C, the fracture initiation energy largely increased in the tempering temperature range of 400-500 °C, while the propagation energy increased greatly (about 20 times) in the range of 500-600 °C. According to microstructural analyses of dynamically compressed specimens, adiabatic shear bands were formed in the 400 °C- and 500 °C-tempered steels, and worked as preferred fracture propagation paths to critically reduce the fracture propagation energy. In the 600 °C-tempered steel, the deformation energy due to the pendulum impact was effectively absorbed by the combination of fine spheroidized carbides and softened tempered martensitic matrix without forming adiabatic shear bands, thereby leading to the relatively homogeneous deformation in the pendulum-impacted region and the very large increase of fracture propagation energy.