Vitreous enamel steels (VES) are a class of metal-ceramic composite materials realised with a low carbon steel basement coated by an enamel layer. During the firing phase to adhere the enamel to the metal, several gas bubbles remain entrapped inside the enamel volume modifying its internal structure. In this work high-resolution X-ray computed tomography (micro-CT) was used to investigate these composite materials. The micro-CT reconstructions enabled a detailed investigation of VES minimising the metal artefacts. The tomograms were used to develop finite element models (FEM) of VES by means of a representative volume element (RVE) to evaluate the thermal residual stresses caused by the manufacturing process, as well as the effect of the 3D bubbles distribution on the internal stress patterns after the thermic gradient. The promising results from this study have the potential to inform further research on such composite materials by optimising manufacturing processes for targeted applications.
Vitreous enamel steels are a particular class of composite materials composed by a low carbon steel basement coated by a vitreous enamel layer. Throughout the firing process applied to fix the enamel on the steel substrate, several gas bubbles remain entrapped inside the internal volume of the enamel modifying its internal microstructure. The presence of these bubbles substantially modifies the internal mechanical state of the structure developing residual stresses both among the bubbles and between the enamel-metal surface. However, to date no methods are still available to properly investigate the 3D bubbles morphology, distribution and stress patterns inside these materials. For this reason, in the present study we developed for the first time a high-resolution X-ray computed tomography (micro-CT) protocol able to investigate the vitreous enamel steels full field structure and numerically study their mechanics when the thermal gradient is applied. The micro-CT scans reconstructions allowed the visualisation of the enamel coating structure minimising metal artefacts. Moreover, the scans were postprocessed developing unpreceded 3D reconstructions with which the distribution, the volume and the mean diameter of the bubbles were analysed and defined. Subsequently, full field finite element computational models able to evaluate the thermal residual stresses produced inside the enamel volume were developed. They permitted to investigate the effect of the bubbles distribution on the internal residual stress patterns due to the thermal gradient generated throughout the cooling phase. The promising results from this study have the potential to inform further research on such composite materials by optimising manufacturing processes for targeted applications.
Keywords: FEA; RVE; micro-CT; porous materials; residual thermal stress; vitreous enamel steel.
© 2020 Royal Microscopical Society.