Functionalized Carbon‐Bonded Filters with an Open Porous Alumina Coating: Impact of Time on Interactions and Steel Cleanliness

Abstract

Understanding interactions between filter and molten steel is essential to improve the purity of casted products by filtration. Characteristic, in situ formed layers on the surface of carbon-bonded alumina filters result from these interactions. To comprehend their formation, this study illustrates the time dependency of the layer buildup. Therefore, reactions at the filter/steel interface under quasi static conditions are examined using spark plasma sintering (SPS) equipment. Immersion tests in a steel casting simulator, which provides close-to-reality conditions, complement these investigations. Microstructure and phase analyses reveal that interfacial reactions between filter and steel lead to a thin in situ formed layer on the filter surface. During a "reactive" stage, large polycrystalline alumina structures are formed. Thereby, material is transported both from the carbon-bonded material underneath (i.e., gaseous reaction products) and from the molten steel (i.e., precipitating particles and endogenous inclusions) to the filter/steel interface. The formation of these alumina particles comes to an end as soon as the carbon supply, which triggers the dissolution and precipitation processes at the interface, is cut-off. From that point on, endogenous inclusions are deposited on them ("active" stage). The filters were most efficient during the reactive stage, that is, as long as the interfacial reactions take place.

Filter Functionalization with Carbon Nanotubes and Alumina Nanosheets for Advanced Steel Filtration

“…In addition, many clusters of nano‐sized inclusions with dendritic shape were found on top of this structure. Similar particles were detected by Schmidt et al on alumina‐coated filters after the same contact time . We believe that these particles are the actual endogenous inclusions produced by our treatment before the immersion test.…”

Interactions between Carbon‐Bonded Alumina Filters and Molten Steel: Impact of a Titania‐Doped Filter Coating

“…That layer‐build‐up is quite similar to the ones reported by Storti et al, Bock et al, and Schmidt et al The latter one proposed a scheme to describe the observed structures and to illustrate interactions between alumina‐coated Al 2 O 3 –C filters and molten steel. To avoid any confusion, this article adopts the denotations of Schmidt et al to describe similar structures. Figure presents an extended scheme on the layer build‐up for carbonaceous alumina‐coated Al 2 O 3 –C filters, which covers most of the observations on the here investigated filters.…”

“…As a result of the interfacial reactions between filter and steel melt, a layer build‐up on the filter surface was observed on all investigated samples by electron microscopy. That layer‐build‐up is quite similar to the ones reported by Storti et al, Bock et al, and Schmidt et al The latter one proposed a scheme to describe the observed structures and to illustrate interactions between alumina‐coated Al 2 O 3 –C filters and molten steel. To avoid any confusion, this article adopts the denotations of Schmidt et al to describe similar structures.…”

Formation of Interface Layers between Corundum‐Based Refractory Ceramics with Carbon Additions and Molten 42CrMo4 Steel

“…The amount of the developed CO gas, which was determined semiquantitatively as the pressure obtained from the pressure measuring system in the SPS device and integrated over the process time, increased with increasing carbon content in the functional coatings. This result confirms the observations from the study by Schmidt et al [9] The reaction layer is amorphous (Figure 3b) and consists mainly of O and Fe. The concentrations of Al and Si are lower than in the samples with lower carbon contents in the Al 2 O 3 ─C coating.…”