Carbon-bonded Alumina Filters Research Articles

Lactose/tannin-based calcium aluminate coatings for carbon-bonded alumina foam filters: A novel approach in environment-friendly steel melt filtration

In this paper, a new approach in the development of calcium aluminate (CA) coatings for ceramic foam filters for steel melt filtration is presented. This approach is based on applying an environmentally friendly binder system based on lactose and tannin. The use of such a binder was previously demonstrated for the matrix of carbon-bonded alumina filters. In this research, the influence of the total solid content, the binder content, and the centrifugation speed on the coated ceramic foam filters’ final properties was studied. All of the prepared coating slips were analyzed in terms of their rheology. The most promising slips were applied on carbon-bonded alumina filters, which were prepared using a lactose- and tannin-based binder system. For the selected coating slips, three different centrifugation speeds (600, 1000, and 1400 rpm) were used to optimize the coating layer thickness and the overall amount of the coating material applied to the filter. The coated and thermally treated filters were tested in terms of their cold crushing strength. Furthermore, the microstructure of the filters and mainly the coating layer thickness were studied. It was observed that both the centrifugation speed and the total solid content in the coating slip have a major impact on the filter properties. The most promising results were obtained for a coating containing 72.5 wt% of total solids applied at 1000 rpm centrifugation speed. This batch of coated samples showed a cold crushing strength of more than double the value obtained for the uncoated reference, while having a uniform coating layer with the thickness of ∼177 μm.

Impact of high temperature on the compression behavior of carbon-bonded alumina filters with functionalized coatings

Carbon-bonded alumina is currently applied in metal casting processes due to its excellent thermomechanical performance. Used as ceramic foam filters in steel casting, the surface composition of such material is essential for trapping primary and secondary inclusions in liquid steel, where functionalized coatings are proven to enhance the filtration efficiency considerably. In this paper, compression tests on uncoated and coated Al2O3–C foam filters are performed at high temperatures under argon atmosphere. The macrostructure of the prepared filters before the high temperature compression tests and the microstructure afterwards are examined by computer tomography and scanning electron microscopy, respectively. High temperature compression strength of the studied filters generally exceeds cold crushing strength values thanks to plastification processes. The applied coatings have positive impact on the filter strength. The observed high scatter of the compression strength can be attributed to the inhomogeneity of strut thickness, distribution of the oxide and non-oxide materials within the struts as well as coating thickness.

Aluminum Melt Filtration with Carbon Bonded Alumina Filters.

The wetting behavior was measured for Al2O3-C in contact with AlSi7Mg with a conventional sessile drop test (vacuum, 950 °C and 180 min) and a sessile drop test with a capillary purification unit (vacuum, 730 °C and 30 min). The conventional test yielded contact angles of around 92°, whereas the sessile drop measurement with capillary purification showed a strongly non-wetting behavior with a determined apparent contact angle of the rolling drop of 157°. Filtration tests, which were repeated twice, showed that the Al2O3-C filter possessed a better filtration behavior than the Al2O3 reference filter. For both filtration trials, the PoDFA (porous disc filtration analysis) index of the Al2O3-C filter sample was equal to half of the PoDFA index of the Al2O3 reference filter sample, indicating a significantly improved filtration performance when using Al2O3-C filter. Notable is the observation of a newly formed layer between the aluminum and the Al2O3-C coating. The layer possessed a thickness between 10 µm up to 50 µm and consisted of Al, C, and O, however, with different ratios than the original Al2O3-C coating. Thermodynamic calculations based on parameters of the wetting and filtration trials underline the possible formation of an Al4O4C-layer.

Mechanical and physical characterization of Al2O3–C foam filters produced by distinct processing routes: The importance of the ceramic strut morphology

In this work the influence of the production route and filter dimensions on the mechanical properties of carbon-bonded alumina foam filters was investigated. Features like cold crushing strength and fracture behavior were analyzed and used to compare them. Microfocus computer tomography (μCT) was applied to characterize the geometry of the filters, providing a deep correlation with their fracture behavior. Furthermore, computer-generated filter geometries were used as reference. The results indicated that the centrifugation process is better suited for the production of carbon-bonded alumina filters, and is even more effective when the foam dimensions are increased in the range of those analyzed in this research. Finite element simulations showed the influence of the relative density and strut tapering on the cold crushing strength of filters, providing a correlation between filter structure and its failure mechanism. The attained results provided further insights towards the production of inclusions-free metal parts.

Impact of spinel forming systems (Fe-/Mg-/Mn-Al-O) as functional coating materials for carbon-bonded alumina filters on steel melt filtration

Since solid, non-metallic inclusions influence considerably the quality of casted steel products, carbon-bonded alumina foam filters are used in secondary metallurgical treatments to remove these particles from steel melts. In order to attain a significant improvement of the filtration process, five different carbonaceous spinel compounds from the Fe-/Mg-/Mn-Al-O systems are applied on carbon-bonded alumina filters in this study and investigated with regard of their filtration efficiency. However, these spinel compounds decompose partially during sintering at 1400 °C under reducing atmosphere, wherefore the resulting coatings contain not only spinel compounds, but also oxidic and metallic components. The subsequent interaction with molten steel leads to the development of multicrystal structures on the filter surface, which stem from interfacial reactions between coating materials, molten steel, and inclusions. As a result of this procedure, a reduction of almost 60% alumina inclusions is measured with the aid of an automatic SEM, whereby spinel compounds from the Fe-Mn-Al-O system achieve highest filtration efficiencies.

Innovative carbon-bonded filters based on a new environmental-friendly binder system for steel melt filtration

New carbon-bonded alumina filters for steel melt filtration were developed. The carbonaceous matrix was based on a new, environmental friendly binder system based on lactose and tannin. The filter preparation was analogous to the production of conventional foam filters according to the Schwartzwalder process. The processing as well as the rheology of the slurries was investigated. An addition of n-Si increased the carbon yield and the cold crushing strength (CCS) of the samples. Higher values of CCS were obtained after coating of the filters with alumina. The material was characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The applicability of these new filters was assessed in impingement tests with a steel melt, in which three out of four recipes survived the thermal shock.

Development and testing of carbon-bonded alumina foam filters for continuous casting of steel

Carbon-bonded alumina filters with special design for continuous casting were produced by the replication technique using different coating procedures. The filters were investigated regarding filter strut diameter, porosity and cold crushing strength. The potential filter capacity and the mechanical load expected during the immersion in a continuous casting tundish were calculated. The best compromise between mechanical strength and structural features was achieved by combining the advantages of centrifugation, dip coating and spray coating. This filter type was analyzed by computed tomography and withstood the contact with molten steel in a casting test. After melt contact, the filter material exhibited near-surface decarburization and alumina-based in-situ layer formation. The observed layers were more pronounced in zones with potentially lower melt flow velocity. The results of the study indicated that the produced filters are able to withstand the severe conditions in continuous casting.

Interface reactions of differently coated carbon-bonded alumina filters with an AZ91 magnesium alloy melt

To investigate possible reactions between differently coated carbon-bonded alumina filters and an AZ91 magnesium alloy melt, immersion tests were carried out. Uncoated as well as MgAl2O4-, Al2O3-, nano- (carbon nano tubes/alumina nano sheets) and MgO-C-coated filters were tested. Thermodynamic calculations showed that only magnesia (MgO) and carbon are stable against molten magnesium; alumina (Al2O3) and spinel (MgAl2O4) will be reduced under the formation of magnesia. Optical and scanning electron microscopy as well as EDX analysis were performed near and at the filter-magnesium alloy-interface of the cooled and sectioned filter samples after their immersion into the AZ91 melt. The results of the thermodynamic calculations were confirmed by the experiments. The MgO-C-coated filter was the only one that did not show an in situ-formed layer on its surface after being in contact with the magnesium alloy melt. The alumina- or spinel-containing filter surfaces displayed platelet-like in situ layers after their contact with the molten AZ91. The results of the EDX analysis of these layers suggest their composition of MgO, since notable respective Mg and O contents were detected, as predicted by the calculations.

Carbon-bonded alumina foam filters produced by centrifugation: A route towards improved homogeneity

An alternative route for producing carbon-bonded alumina filters was analyzed and compared with the state of the art, both based on the replica method. The latter comprises a dip coating and subsequent rolling and spraying steps (R+S). The proposed route is based on dip coating and centrifugation (CF-T1). Aiming to investigate the influence of an additional thermal treatment on the ceramic coating homogeneity, a modified centrifugation route was analyzed (CF-T2). Computer tomography (CT) analyses provided a targeted evaluation of the spatial material distribution after each processing step. It was found that log-normal functions fitted well the wall thickness values attained for all investigated filters. Qualitative surface evaluation was conducted using a digital (optical) microscope. The R+S and CF-T1 filters exhibited surface macro-cracks and hollow struts, due to the release of the polyurethane foam template during firing. The CF-T2 presented partly filled struts with a high amount of micro-cracks on the surface, being a potential alternative to increase the filtration efficiency, as the filter surface area was raised. However, in preliminary mechanical tests, the surface micro-cracks seemed to considerably reduce the cold crushing strength of the CF-T2 filters. Based on the CT data, it was possible to conclude that the centrifuged filters presented a more homogeneous material distribution, as a consequence of higher automation level of the process.

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

In order to explore the influence of nano-sized materials on steel filtration, carbon-bonded alumina filters are coated with multi-walled carbon nanotubes and alumina nanosheets. The samples are tested for different times in contact with a steel melt containing endogenous inclusions at 1650 °C. Investigations of the filters after the test reveal the typical layer buildup described in the literature. After 60 s fine endogenous inclusions are found on the top layer. By means of a special automatic SEM, steel samples after the tests are thoroughly analyzed. The population of detected inclusions is classified in terms of size and chemistry. Nano-coated filters show excellent filtration efficiency for 10 s immersion time. Longer times result in worse performance and generation of new silicate inclusions.

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

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.

Short-time performance of MWCNTs-coated Al2O3-C filters in a steel melt

Multi-walled carbon nanotubes (MWCNTs) have been recently applied on the surface of carbon-bonded alumina filters in order to influence the reactions occurring during the first contact with the steel melt. They could potentially accelerate the deposition of endogenous particles, which would in turn result in a better filtration process. In the present work, the performance of MWCNTs-coated carbon-bonded alumina filters was assessed in comparison to uncoated filters, using a special steel casting simulator under controlled atmosphere. Short immersion times were used in order to better compare the behavior of these samples and check the microstructure evolution at the beginning of the filtration process. The microscope investigations indicated a better performance of the MWCNTs-coated filters, both after 10 and 30s of immersion.

Development of multi-walled carbon nanotubes-based coatings on carbon-bonded alumina filters for steel melt filtration

Ceramic foam filters (CFFs) have been used by steel foundries for several years to remove non-metallic inclusions from the metal castings. In order to improve their filtration efficiency, promising approaches which involve “active” and “reactive” coatings applied on the filters have recently been proposed.In this work, a new coating system containing multi-walled carbon nanotubes (MWCNTs) is presented. A special water-based dispersing technique for carbon nanotubes was developed. Xanthan gum was used both as a dispersing agent and as a rheology modifier for the spraying slurries. The synthetic coal tar pitch Carbores® P was added as a binder for the carbon nanotubes. The coatings were cold-applied on 10 pores per inch (ppi) carbon-bonded alumina filters previously manufactured via the Schwartzwalder process. After drying, the samples were heat treated at 800°C. The performance of the coated filters was investigated with the aid of a special steel casting simulator in fully-controlled atmosphere.

Characterization of carbon-bonded alumina filters with active or reactive coatings in a steel casting simulator

Carbon-bonded alumina filter materials—with and without coatings of carbon-bonded MgO and carbon-free Al2O3—were tested in a steel casting simulator in contact with steel melts containing defined endogenous alumina inclusions. For the evaluation of the filtration efficiency of the different filter surfaces, prismatic samples were immersed in the steel melt under an argon atmosphere at 1650°C. The carbon-bonded alumina filter material reacted in situ in the steel melt, forming gaseous alumina sub-oxides. These sub-oxides reacted with the dissolved oxygen of the steel and formed a thin crystalline α-alumina layer on the area in contact with the steel. The secondary alumina layer was formed on the carbon-bonded alumina surface as well as on the alumina-coated carbon-bonded alumina substrate. The newly formed layer reflected the roughness of the substrate, influencing the wettability and, therefore, the deposition behavior of the inclusions on the filter material surfaces tested. Sintered fine endogenous inclusions were identified in various shapes on this thin layer. In the case of carbon-bonded magnesia on a carbon-bonded alumina substrate, carbothermal reduction of the magnesia and subsequent reoxidation to secondary magnesia in the form of a thin layer on the contact area with the steel was observed. Based on the carbothermal reduction of alumina and magnesia, MgAl2O4 spinel whiskers were formed in situ in the hollow filter struts.

Functionalization of carbon-bonded alumina filters through the application of active oxide coatings for steel melt filtration

Abstract

Development of novel carbon bonded filter compositions for steel melt filtration

Carbon-bonded alumina filters for steel melt filtration are increasingly used by steel foundries. Their potential regarding material characteristics and filtration efficiency has not been fully exploited yet. A new filter composition based on the synthetic coal tar pitch Carbores® P, has been explored. Its compatibility with a water-based dispersing system allows additions of carbon black, graphite, and additives to reach a residual carbon content of up to 30wt.%. For determining physical properties and variables of the respective compositions, 10 pores per inch (ppi) foam ceramic filters were manufactured via the Schwartzwalder process. The influence of different binder contents on the processing properties and mechanical characteristics of the system was evaluated. Based on these studies, an increase in cold crushing strength of approx. 30% has been achieved in comparison to current state of the art materials.