Characterization of the precipitation behavior and resulting mechanical properties of copper-alloyed ferritic steel

From literature, it is well known that Cu precipitates in steels lead to an increase of hardness and quasi-static strength. However, the relation between Cu precipitates and cyclic mechanical properties has been investigated rather limited. Consequently, in the present work the evolution of Cu precipitates in a copper-alloyed low carbon, ferritic steel, aged at 550 °C, 600 °C and 650 °C for various times, and their influence on the resulting static and cyclic material properties were analyzed. Three-dimensional atom probe tomography (3DAPT) provided evidence that the ageing treatment led to the formation of Cu precipitates with different sizes and distribution. The existence of copper precipitates resulted in an increase of the ultimate tensile strength and yield strength of the investigated ferritic steel. Moreover, cyclic indentation tests were performed to investigate the influence of the ageing treatment and, hence, Cu precipitates, on hardness and especially cyclic properties of the material. The present results show a maximum of hardness and cyclic hardening potential at sizes of the precipitates of around 2.2 nm. Additionally, synchrotron X-ray diffraction (SYXRD) measurements were in a first approach proved to be viable for investigating the crystallographic structure of Cu precipitates within ferritic steels. Considering the evolution of Cu precipitates, determined with synchrotron as well as atom probe, a high dependency of quasi-static and cyclic properties on Cu precipitates could be demonstrated.