The mechanism of natural age-strengthening of grey cast iron: PEC-ageing

In this work we introduce the concept of natural ageing via precipitation arising from eutectic cell structure (PEC-ageing). Natural age-strengthening in grey cast iron at room temperature is long recognised yet still poorly understood, despite its beneficial effect on tensile strength and machinability. We investigate its underlying mechanisms by comparing fresh and naturally aged industrial grey cast iron using high-resolution synchrotron Wide-Angle X-ray Scattering (WAXS), complemented by scanning electron microscopy and X-ray energy-dispersive spectroscopy. A strict sample-preparation process was designed to minimise artificial ageing and X-ray attenuation artefacts. We present direct experimental evidence of an X-ray diffraction peak that forms upon natural ageing in grey cast iron. Based on its position in reciprocal space, a plausible candidate for this peak is the iron nitride phase . Spatially resolved analysis further shows that this phase is strongly localised along eutectic cell boundaries, supporting a nitride-related, boundary-mediated precipitation mechanism (PEC-ageing) as the main driver of the ageing phenomenon. These results provide a spatially resolved description of natural ageing in grey cast iron, establish WAXS as a powerful tool to track ageing-related changes in the bulk, and highlight that logistics and sample preparation remain the main practical challenges for future in-situ studies. • We discover a diffraction peak that develops during natural ageing of grey cast iron by using synchrotron Wide-Angle X-ray scattering. • Ageing-related precipitation is found to occur preferentially along eutectic cell boundaries. • The position of this new diffraction peak is consistent with precipitation of an iron nitride phase, with iron nitride (Fe4N) emerging as a plausible candidate. • These findings provide evidence consistent with a mechanistic link between boundary chemistry, precipitation at eutectic cell boundaries, and the natural age-strengthening of grey cast iron.