Quantifying damage accumulation during the hot deformation of free-cutting steels using ultra-fast synchrotron tomography

Many different approaches have been proposed to simulate the nucleation and evolution of damage during the hot forming of steels. However, there is a lack of time-resolved, three dimensional quantification of the evolution of damage, a requirement for validation of the kinetic and morphological model predictions. One very significant industrial case is the hot forming of free-cutting steels (FCS), where small additions of heavy metal inclusions are added to enhance the machinability and surface quality of the steel. In this paper, we present the in situ ultra-fast synchrotron X-ray tomographic observations and quantification of FCS during hot deformation, including measurement of applied load. This allowed the correlation of strength to the different stages of the cracking process. The results are augmented with high spatial resolution interrupted studies. The interrupted tomographs enabled the quantification of the volume fraction, equivalent diameter, spatial distribution and orientation of inclusions and damage at various strain levels. The combination of these two studies provides a benchmark experiment for the validation of physically-based finite element models, both directly, and via constitutive equations for the time/temperature dependent effects of dislocation density, damage, strain rate and temperature.