- Email: [email protected]
Short-time performance of MWCNTs-coated Al2O3-C filters in a steel melt
Journal of the European Ceramic Society 36 (2016) 857–866
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short-time performance of MWCNTs-coated Al2 O3 -C filters in a steel melt Enrico Storti a,b,∗ , Steffen Dudczig a , Gert Schmidt a , Paolo Colombo b,c , Christos G. Aneziris a a
Technische Universität Bergakademie Freiberg, Institut für Keramik, Glas- und Baustofftechnik, Agricolastraße 17, 09599 Freiberg, Germany Università degli Studi di Padova, Dipartimento di Ingegneria Industriale, Via Marzolo 9, 35131 Padova, Italy c Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16801, USA b
a r t i c l e
i n f o
Article history: Received 16 August 2015 Received in revised form 19 October 2015 Accepted 25 October 2015 Available online 6 November 2015 Keywords: Carbon nanotube Alumina Carbon Ceramic foam filter
a b s t r a c t 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 30 s of immersion. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Non-metallic inclusions in cast metal parts remarkably influence their mechanical properties such as fracture toughness, tensile strength and fatigue resistance. Not only the amount of inclusions, but also their kind, shape and distribution are of great importance especially in applications where high-tensile steel is used. Under load, the stress concentrations are found in the proximity of inclusions, which control the deformation behavior of the product [1]. In order to limit the casting repairs and possibly avoid rejected castings, very high-purity metal melts are thus required. This is achieved by adding a physical refining process, i.e., filtration, after the ladle treatment [2–6]. The thermodynamic fundamentals for the process of refining molten steel from non-metallic inclusions, using the method of filtration with ceramic filters, have been presented by Janiszewski and Kudlinski [7]. The main non-metallic inclusions which need to be removed from steel melts are oxides, carbides, sulfides and nitrides. Exogenous inclusions originate from slag entrainment or lining erosion. Endogenous inclusions, instead, include products of deoxidation
∗ Corresponding author at: Technische Universität Bergakademie Freiberg, Institut für Keramik, Glas- und Baustofftechnik, Agricolastraße 17, 09599 Freiberg, Germany. E-mail address: [email protected] (E. Storti). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.10.036 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
or inclusions precipitated during cooling and solidification. When steel is deoxidized with aluminum, the resulting fine alumina inclusions do not separate completely into the slag phase despite their lower density with respect to steel. After solidification, a variety of different shapes for these particles has been observed, such as spheres, polyhedral, plates, dendrites and clusters [8]. The morphology of non-metallic inclusions may be understood by considering parameters that influence crystal growth, i.e., atomic structure, degree of supersaturation (Al and O activities in the steel bath), presence of impurities and stirring conditions. Dendrites grow along supersaturation gradients, while turbulence in the reaction zone leads to the formation of clusters. Plate-like particles have a large surface-to volume ratio and a trigonal symmetry, which suggests that the corresponding alumina structure is corundum. The plate-like habit has been explained by considering adsorption of impurities on the {1 1 1} face, which hinders the growth in this direction. Polyhedral inclusions originate from spherical inclusions when the equilibrium condition is approached. Depending on the wetting properties of the material, a stage of agglomeration and sintering may follow the growth: it was observed that alumina inclusions, due to their high interfacial energy, readily generate three-dimensional clusters via collision and aggregation [9,10]. Endogenous inclusions are named “primary” if generated immediately after the addition of the deoxidizing agents, while “secondary” and “tertiary” inclusions form respectively during
858
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
cooling and solidification. “Quaternary” inclusions are generated below the solidus temperature [11]. Aneziris et al. have proposed new approaches of surface functionalized filter materials based on so-called “active” as well as “reactive” coatings [12,13]. In the first case, the same chemistry as the chemistry of the primary or secondary inclusions that have to be removed is generated on carbon-bonded filters. On the other hand, “reactive” coatings interact with the dissolved gas in the melt (oxygen in steel melts for instance) and generate inclusions above the liquidus temperature, which are accordingly deposited on the filter. This approach helps reducing the amount of tertiary and also quaternary inclusions [14,15]. Ceramics filters in various shapes such as open porous foams, honeycomb-like structures or “spaghetti” stacks are used today in the steel industry [16]. In particular, the majority of the applied filters are open-cell ceramic foams produced via the replica process [17]. These filters have a high specific surface area and deliver a higher hydraulic resistance than multi-hole filters: both these features favorably affect the micro-mechanism of filtration [18,19]. Moreover, the metal flow is adjusted from a turbulent into a laminar flow, reducing the risk of reoxidation and the formation of cavities. As a result, a better surface quality and shorting casting times are achieved [20]. For the filters carbon-bonded alumina material is commonly used, since it exhibits good thermal shock performance and increased creep resistance in comparison to zirconia, which is the other main material of choice for steel melt filtration. Carbon has a high thermal conductivity (up to 400 W/mK for crystalline graphite) which results in the good thermal shock resistance of the filters. Moreover, an increase in strength with increasing temperature up to 2500 ◦ C is observed for graphite, and the plastic deformation with a following annealing of cracks and defects becomes possible [21]. Recent studies have shown that the carbon-bonded alumina material reacts when immersed in a steel melt, producing a new thin, dense alumina layer (identified as crystalline ␣-alumina at room temperature) on the contact area with the steel. Thermodynamic studies have suggested a partial dissolution of alumina and carbon in the steel at the interface, followed by the reaction of this liquid with some fresh steel and the precipitation of the secondary alumina layer. Thanks to the new metal/alumina interface, the wettability of the steel melt on the filter is suddenly decreased and the chemistry and crystallographic structure of the surface are changed to that of the endogenous inclusions. The new formed layer is consequently quite efficient in removing these particles from the melt [22,23], and the faster this layer is generated the better the trapping efficiency for the inclusions of the filter is. A schematic material buildup on an uncoated carbon-bonded alumina filter surface after a 60-s immersion test is presented in Fig. 1: zone A represents the unreacted carbon-bonded alumina substrate; zone B consists of a decarburized, porous layer where alumina grains have partially sintered; zone C is the thin alumina layer just described; zone D represents a dense collection layer, mainly consisting of small plate-like or polyhedral endogenous inclusions; the final zone E shows entrapped particles in complex shapes, such as dendrites, in a loose assemblage, from which the commonly used “coral-like” term derives [12,24]. Storti et al. have recently added a MWCNTs-based coating on standard carbon-bonded alumina filters by spray coating, with the aim to promote the formation of the thin alumina layer and to improve the filtration efficiency. The carbon nanotubes were removed from the filter during the 60-s immersion test, but no carbon buildup in the steel was detected in the end. An investigation by SEM showed a dense collection zone with plate-like particles directly over the decarburized zone. The authors proposed that the high surface reactivity granted by the MWCNTs accelerated the formation of zone C, which after 60 s was completely covered
Fig. 1. Schematic diagram of the cross-section of a carbon-bonded alumina foam after immersion in steel melt (not true to scale and particle shape) (A) unaltered carbon-bonded alumina material; (B) decarburized zone with residual alumina; (C) thin secondary layer; (D) dense collection zone; (E) loose collection zone. Adapted from [16].
by endogenous inclusions and thus no longer recognizable as an independent layer [25]. In the present study, two different carbon-bonded alumina foams (i.e., uncoated and MWCNTs-coated) were tested in a socalled steel casting simulator, in order to compare their filtration behavior for short times of immersion in a steel melt. In an oxygenfree argon atmosphere, the prismatic filter samples were dipped into steel melts containing artificially-generated endogenous alumina particles, before being removed and allowed to cool down under inert conditions [22]. This procedure allowed the investigation of filter surfaces after a simulated filtration with adsorbed particles but without the presence of solid steel, as in the case under normal industrial trials. In fact, the study of filters from castings is hindered by solid steel filling the porosity after use. 2. Materials and methods The raw materials used for the preparation of the Al2 O3 -C filters were aluminum oxide (Martoxid MR 70, Martinswerk, Germany, 99.80 wt.% Al2 O3 , d90 ≤ 3.0 m), modified coal tar pitch powder
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 2. Heat treatment schedule for the samples. The same treatment was used after application of the MWCNTs coating.
(Carbores® P, Rütgers, Germany, d90 < 0.2 mm—used as a binder as well as a carbon source), fine natural graphite (AF 96/97, Graphit Kropfmühl, Germany, 96.7 wt.% carbon, 99.8 wt.% <40 m), and carbon black powder (Luvomaxx N-991, Lehmann & Voss & Co., Germany, carbon content ≥99.0 wt.%, ash content >0.01 wt.%, primary particle size of 200–500 nm). The additives were ligninsulfonate (C12C, Otto-Dille, Germany—used as wetting agent and temporary binder), Castament VP95L (BASF, Germany—used as a dispersing agent), and Contraspum K1012 (Zschimmer & Schwarz, Germany—used as an antifoam agent). Due to an agglomeration problem during storage, a pre-treatment was performed on our raw aluminum oxide in order to restore the material close to its original grain size distribution. A dry ball milling technique with a bimodal ball size distribution was used, the process being explained in detail by Fruhstorfer et al. [26]: in our case a milling time of 40 min was necessary to destroy the agglomerates in the 4–10 m size; after this deagglomeration step the registered d90 value shifted from 7.6 down to 1.8 m. The carbon-bonded filters were manufactured via the replica process patented by Schwartzwalder in 1963 and using a two steps approach, as explained in detail by Emmel [17,27]. 20 wt.% Carbores® P was used in the starting mixture in order to achieve a strong bonding and good thermo-mechanical properties after the pyrolysis process. The exact composition can be found in [13] under the label “AC5”. The filters were heat treated in retorts filled with calcined petcoke (Müco, Essen, Germany) with particle size distribution of 0.2–2.0 mm, in order to guarantee reducing conditions. The schedule (reported in Fig. 2) used a 1 K/min heating rate, intermediate holding steps of 30 min and a final holding time of 180 min. The raw materials used for the preparation of the coating were MWCNTs (TNM8, Chengdu Organic Chemicals, Chengdu, China, purity >95%, outer diameter >50 nm, length = 10–20 m), and modified coal tar pitch powder (Carbores® P, see above—used as a binder). The additives were Xanthan powder (Rhodopol 50 MD, Erbslöh, Krefel, Germany—used both as a dispersing agent and thickening agent), ammonium-ligninsulfonate and Contraspum K1012 (see above). The carbon nanotubes were not purified before use. The preparation and application of the MWCNTs coating were performed exactly as explained in [25], but in this investigation only the slurry named “C” (i.e., with the highest nanotubes content) was used. It must be noticed that, after application and drying of the coating, the filters were thermal treated once again under reducing conditions (with the schedule in Fig. 2) in order to achieve the bonding of the CNTs and close some residual porosity and cracks.
859
A metal casting simulator located at the Institute of Ceramic, Glass and Construction Materials in Freiberg (Systec, Germany) was used to compare the filtration behavior of our uncoated and coated filters during very short operation times. A 10 ppi prismatic ceramic foam sample based on a template of geometry 125 mm × 20 mm × 20 mm was fixed at a special sample holder in a gas tight sewer port above a melting crucible. The used apparatus has been presented in detail by Aneziris in [13]. In the present work, a so-called “finger test” was performed according to the procedure described by Dudczig et al. [22]. All tests were carried out under fully controlled argon atmosphere. 38 kg of commercially available 42CrMo4 steel (AISI 4142) were melted under Ar in a special hydratable alumina-bonded alumina/alumina–magnesia–spinel crucible not containing any silica, calcia or further additions, in order to prevent unwanted reactions with the crucible material during the experiment. The oxygen content and the temperature of the steel melt were measured in situ with a pO2 /T—sensor system Celox (Heraeus Electro-Nite, Germany) for the duration of each test. To create defined alumina impurities in the steel melt, 0.5 wt.% (related to the steel mass) of an iron oxide mixture were added. The commercial product (Mineralmühle Leun, Germany) consists of 75 wt.% hematite and 25 wt.% magnetite. After reaching a temperature of 1650 ◦ C, the oxide mixture was added directly to the melt and an increase of the dissolved oxygen from around 10 ppm up to around 60 ppm was detected. At this point, the endogenous alumina inclusions were created by adding 0.05 wt.% (related again to the steel mass) of pure aluminum metal to the melt. Accordingly, the dissolved oxygen content fell back close to the starting value. After this step the prismatic sample, without any preheating, was dipped 60 mm into the steel melt through the sewer port and rotated for 10 (or 30) s at 30 rpm inside the melt. The filter was next removed from the melt and cooled down in a chamber under argon in order to prevent oxidation of the carbon, which would surely occur if the hot sample were left in contact with a normal ambient atmosphere. The influence of impurities from refractory materials was almost eliminated by using one crucible per melt. For the optical investigation of the samples after the contact with the steel melt, a digital light microscope (VHX-200 D, Keyence, Germany) was used. The microstructural evaluation of new phases and collected particles on the surface was carried out by means of scanning electron microscopy (SEM XL30, Philips, Germany) in combination with energy dispersive X-ray spectroscopy (EDS).
3. Results and discussion Fig. 3 shows the Al2 O3 -C samples after the immersion test. They all survived the initial thermal shock as well as the rapid cooling (which is also critical), suffering no heavy damage. This could be expected since both kinds of filters were already reported to survive a 60-s immersion test [22,25]. For all filters, the part which was in direct contact with the melt could be detected by the color change from black to gray, which was due to decarburization of the surface, reactions with the molten steel and (as discussed below) following deposition of a new phase. This phase consisted of endogenous alumina inclusions, deliberately generated in the steel through the iron oxide addition and following deoxidation, as explained in Section 2. Digital optical images of the filters (contact zone) are presented in Figs. 4–7 . On all samples, some spherical, bright particles of different sizes were detected: as previously reported by Dudczig et al., these particles consisted of entrapped steel, the peculiar shape due to the low wettability of iron toward the filter material [22]. After 10 s of immersion in the molten steel, the uncoated carbonbonded alumina filter showed a relatively regular surface, with a gray or light gray color and a smooth appearance (Fig. 4). Some
860
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 3. Filter samples after finger test. Top left: uncoated filter, 10 immersion; top right: MWCNTs-coated filter, 10 immersion; bottom left: uncoated filter, 30 immersion; bottom right: MWCNTs-coated filter, 30 immersion.
Fig. 4. Digital optical images of uncoated filter after a 10-s immersion test.
Fig. 5. Digital optical images of MWCNTs-coated filter after a 10-s immersion test.
Fig. 6. Digital optical images of uncoated filter after a 30-s immersion test.
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
861
Fig. 7. Digital optical images of MWCNTs-coated filter after a 30-s immersion test.
Fig. 8. SEM-BSE micrographs of uncoated filter (10 ) with detail of layer E on the side of crack.
macroscopic damage could be detected on the sample in the form of cracks and also missing strut parts. In these particular spots, the filter had a darker tone of gray compared to the rest of the dipped sample, suggesting that the damage was not due to the initial strong thermal shock, but rather to the rapid cooling or to the handling after the end of the finger test. Thin cracks revealed instead a light gray color of the exposed material and implied a contact of these areas with the steel melt. Moreover, some thin lines in light gray color were observed on the filter surface, following the same distribution of cracks at the edges of the struts: these are the weakest spots in filters obtained by the replica technique, where the wall thickness is minimal because of the hollow shape left from the foam degradation. At higher magnifications, small bright spots were detected, mostly in proximity of these cracks or light gray edges. The CNTs-coated filter, after 10 s of contact, is presented in Fig. 5: unlike the uncoated sample, its surface was quite irregular, with some large areas showing a bright phase and a gray, apparently smooth background. At higher magnifications, it was confirmed that the light gray phase (consisting of adsorbed endogenous particles) grew on top of the structure and was not uniformly distributed on the surface of the foam. Assuming that the CNTs presence allowed for a faster interaction with the melt, this distribution of entrapped particles could indicate an uneven distribution of the nanotubes on the surface, as shown by Storti et al. [25]. This sample showed also some damage in the form of small cracks and pores, with the darker substrate material exposed. However, the overall amount and width of cracks seemed to be limited in comparison to those observed on the reference carbon-bonded filter. This was probably related to the different carbon content of the two surfaces. The uncoated filter which was immersed for 30 s showed quite a rough surface with a gray or light gray color (Fig. 6). In this case, only a very limited damage was observed and the collection extent
Fig. 9. Partially developed layer C on uncoated filter (10 ).
seemed much more considerable than after 10 s. In addition, many linear protrusions were detected, particularly located at the weak edges. This suggested some kind of “healing” or sealing of the initial cracks thanks to the interaction with the steel melt and deposition of more inclusions. The MWCNTs-coated sample which underwent the 30-s test seemed as almost undamaged as the previous one, with the same kind of protrusions at the weak edges (Fig. 7). However an apparent (not quantified) higher roughness and overall brighter appearance of this surface suggested a more efficient entrapment of endogenous particles and a better development of the E layer. In fact, the bright appearance of some spots could be ascribed to coral-like particles which, thanks to their complex shapes and their random
862
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 10. SEM-BSE micrograph of material buildup on uncoated filter (10 ), where layer C is visible.
orientation, had a greater ability at reflecting the sidelight used in our investigation (see later). SEM micrographs of the uncoated carbon-bonded alumina filter after 10 s of immersion in the steel melt are presented in Fig. 8. As expected, some initial deposition on this sample was observed. New particles in the shape of dendrites, plates or polyhedra were clearly detected. Such shapes are typical for endogenous inclusions and indicated that the filtration process started very quickly after the first contact with the melt, which is of course the desired
behavior for a molten metal ceramic filter. An EDS analysis confirmed that the entrapped particles consisted of alumina with a low amount of impurities. Interestingly, the coral-like collection (zone E) was mainly detected in small areas located exactly at the sides of cracks. We posit that the steel flux was slowed down and “trapped” for longer times in proximity of such defects, which resulted in a more efficient collection of inclusions. The BSE signal revealed some white spots with a spherical shape, the same shape detected before by optical microscopy: iron and traces of other metals such as Mn and Cr were found by means of EDS, and these particles were hence associated with steel entrapment by the filter. A higher presence of steel spherules on the sample was always detected in the areas with a coral-like structure. The main microstructure detected on the uncoated sample (see Fig. 9) revealed nevertheless an incomplete reaction with the melt. It consisted in fact of spherical alumina particles (which belonged to the substrate) partly sintered together, while the high porosity observed was due to dissolution of the carbon component into the steel melt. A densification was observed in some areas, indicating the progressive formation of the secondary alumina layer C. On this basis, we propose that 10 s of immersion were not enough for an efficient deposition of endogenous inclusions on the uncoated Al2 O3 -C filter. From a cross-sectional point of view (Fig. 10), and in reference to the different characteristic areas of material buildup as explained in Section 1, we could observe the decarburized layer B, consisting of residual alumina grains, present over the unaffected substrate material (A). Next, a thin layer with a thickness of about 200 nm could be noticed between the entrapped inclusions and the decarburized zone (see Fig. 11). Its structure was overall very similar to the one reported by Dudczig. According to Zienert et al., the thin alumina layer was likely produced by the first interaction
Fig. 11. Secondary alumina layer C after finger test. Top left: uncoated filter, 10 immersion; top right: MWCNTs-coated filter, 10 immersion; bottom left: uncoated filter, 30 immersion; bottom right: MWCNTs-coated filter, 30 immersion.
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
863
Fig. 12. SEM micrographs of MWCNTs-coated filter (10 ) with detail of the inclusion collection and zones B and C.
Fig. 13. Material buildup on MWCNTs-coated filter (10 ). Layer C detached from the underlying material.
with the steel melt through a reduction and quick reoxidation of Al2 O3 from the filter surface [23]. Finally, a collection zone with plate-like particles and dendrites with a random orientation was observed (see Fig. 10). Fig. 12 shows the MWCNTs-coated filter after 10 s of immersion: a similar microstructure to the uncoated filter was detected on its surface, with both the dense and the coral-like collection zones. As expected, the presence of the carbon nanotubes on the surface did not hinder the filtration process. On the contrary, in this case layer D, consisting of plate-like particles, seemed better developed and covered the majority of the investigated surface. In fact, the decarbonized filter material could still be recognized under this layer, but only in limited areas. Moreover, the loose coral-like particles had a random distribution and were not detected just in proximity of cracks as in the uncoated sample. Fig. 13 presents the material buildup on the MWCNTs-coated sample: a very thin alumina layer (about 200 nm in thickness), totally comparable to the previous one, was again detected over the substrate. As expected, no carbon nanotubes or MWCNT agglomerates were found on any spot of the sample surface after the test: nanotubes were only present in a very thin layer on the external surface of the Al2 O3 -C foam and reacted rapidly with the steel melt.
SEM micrographs of an uncoated filter after a 30-s immersion in the steel melt are presented in Fig. 14. Compared to the 10-s test, here the dense collection layer clearly appears to have developed on the whole investigated surface, showing clusters of small particles with a low aspect ratio. The sintering process of the inclusions on the filter did not create a single homogenous layer, hence the sample possessed quite a rough surface. The presence of facets allowed us to recognize that the inclusions were found in plate-like or polyhedral shape before forming the aggregates. The decarbonized microstructure (B) and the substrate material (A) were still observed, but only where the top material was broken off because of the preparation process. Conversely, not many coral-like zones were detected over the dense layer, probably because of the late formation of layers C and D. The analysis of a cross-section of the foam (Fig. 15) allowed again to detect the thin secondary alumina layer C that formed between the decarbonized zone and the entrapped particles. Its appearance, composition and thickness were all similar to the results obtained in the 10-s test. It is interesting to observe that the thickness of the secondary alumina layer did not remarkably increase with the testing time (see Fig. 11): this structure was the first responsible for the collection of endogenous inclusions and after its formation only layers D and E could develop and increase in thickness. Along with the plate-like, dendritic and polyhedral particles, another type of structure was detected as part of the collection zone on this sample: a few agglomerates with dimensions of some microns and consisting of fine, partially sintered spherical inclusions (Fig. 16). The BSE detector did not find any remarkable composition difference with the surrounding alumina inclusions. According to Dekkers et al. [9], the shape of these particles suggest that they originated at a very fast rate in a zone of the melt with high O and low Al activity. Finally, some very fine particles with an undefined shape were observed just barely over the alumina inclusions in proximity of steel spherules (Fig. 17). These inclusions showed a slightly different tone of gray under BSE. The chemical composition estimated by EDS analysis revealed the presence of Mn and S, which indicate a deposition of MnS impurities from the melt: MnS has in fact been previously observed to nucleate on oxide particles in the temperature range between 1500 ◦ C and 1200 ◦ C in a low-carbon steel melt [28]. Accordingly, both Dudczig and Storti detected lower S content in the steel after the immersion test of Al2 O3 -C filters [22,25]. Fig. 18 shows the MWCNTs-coated surface after 30 s of immersion. With respect to the uncoated sample, a smoother appearance was noticed where the coral-like collection zone was missing: small
864
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 14. SEM micrographs of uncoated filter (30 ), with detail of layer D.
Fig. 16. Small alumina particles deposited over the dense collection layer D.
Fig. 17. SEM-BSE detail of entrapped particles on uncoated filter (30 ). Apart from steel spherules and alumina platelets, some other inclusions (likely MnS) were detected. Table 1 Composition by EDS of the different zones from an uncoated sample (see Fig. 15) in comparison to the theoretical composition of alumina (wt.%).
Fig. 15. SEM-BSE micrograph of material buildup on uncoated filter (30 ).
plate-like and polyhedral inclusions appeared to have sintered into a denser layer. This difference could be expected, since the formation of the D layer on the uncoated filter was for the most part still in progress after 10 s of immersion in the steel melt. The material buildup from a cross-sectional view on the MWCNTs-coated filter is presented in Fig. 19: the secondary thin alumina layer was observed on this sample also, with the same structure and same average
Element
A
B
C
D+E
Theoretical composition Al2 O3
Al O C Na Si Fe P Mn
24.51 39.12 33.54 0.19 0.18 2.46
34.15 54.97 9.08 0.45 0.34 0.69 0.18 0.16
32.34 55.81 10.02 0.58 0.55 0.70
38.92 49.34 11.27 0.23 – 0.23
52.93 47.07
thickness as for the previous filters. Similarly to the uncoated sample, some agglomerates of fine spherical alumina particles were also detected here. Tables 1 and 2 present the results of the element analysis conducted by means of EDS on the four main microstructures detected on both our samples after 30 s of immersion in the steel melt (see Figs. 15 and 19). As expected, a large amount of carbon was detected in the substrate material (zone A): about 30 wt.% is expected to be the residual carbon content for the AC5 filters after pyrolysis. However, a much higher value was registered for the MWCNTs-coated filter: indeed, after the application of the coating and the second heat treatment, a surface enrichment in carbon was expected. The
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
865
Fig. 18. SEM micrograph of MWCNTs-coated filter (30 ) with detail of layer D.
Table 2 Composition by EDS of the different zones from a MWCNTs-coated sample (see Fig. 19) in comparison to the theoretical composition of alumina (wt.%). Element
A
B
C
D+E
Theoretical composition Al2 O3
Al O C Na Si Fe Mn S
16.70 26.09 56.67 – – 0.55
34.74 42.88 20.10 1.06 0.32 0.22 0.43 0.25
22.54 38.09 37.54 0.37 – 1.46
35.41 47.47 15.19 0.60 – 1.34
52.93 47.07
iron impurity (white spots in zone A) should be attributed to the first melt which came in contact with the filter surface and did not completely recede after the formation of the thin alumina layer C. The surface of the MWCNTs-coated sample showed a higher carbon content not only in the base material, but also in the decarburized zone B, the thin layer C and the entrapment zone. Nevertheless it must be noticed that, since the collected X-rays came from a sample volume with a depth of a few microns, the recorded spectrum for very thin layers such as B and C could be distorted by the underlying signal. Moreover, the instrumental error for atoms lighter than Na can be remarkable (up to ca. 10%) due to the low energy of the incoming X-rays. Iron was detected in all microstructures, as well as other impurities like Na and Si. Traces of P, Mn and S were only found in zone B. With regard to the theoretical composition of alumina, all layers showed a lower Al content. Layers B and C had an excess oxygen but only in the uncoated Al2 O3 -C sample. The large discrepancy with the theoretical composition of alumina was likely due to the error related to oxygen detection, to the presence of other minor oxides and to some chemical inhomogeneity of the scanned areas, which also heavily affects the accuracy of a quantitative EDS analysis. From our results, we could conclude that the surfaces of both samples followed the general buildup scheme, proposed by Dudczig and Zienert for a test time of 60 s [22,23], already after very short immersion times. However, for the 10-s test, the development of the secondary alumina layer C and the following deposition of inclusions were more efficient for the MWCNTs-coated filter. After 30 s of immersion, both filters developed a dense collection layer on the surface, but the densification appeared to be at a more advanced stage on the coated sample. Moreover, optical microscopy analysis indicated a more efficient collection of loose endogenous particles on the sample coated with MWCNTs. The data obtained from this study indicate that the initial response of the foam filter is crucial in the development of the different layers and the efficient removal of
Fig. 19. SEM-BSE micrograph of material buildup on MWCNTs-coated filter (30 ).
endogenous particles from the melt. Since no exogenous particles were produced during the test, nothing can be yet assumed about the filter performance in the collection of these other inclusions. In the future, the material buildup on uncoated and MWCNTscoated filters after different immersion times will be characterized by means of computer tomography, in order to enable a quantification of the different trapping performance of the two types of filters. 4. Conclusions The behavior of two different foam filters in contact with molten steel containing endogenous alumina inclusions was investigated. Uncoated and MWCNTs-coated carbon-bonded alumina filters were immersed for 10 and 30 s in a 42CrMo4 (AISI 4142) steel melt at 1650 ◦ C under an argon atmosphere. Both surfaces developed a thin secondary corundum layer (around 200 nm in thickness) at the contact area with the melt, by means of partial dissolution and reprecipitation of Al2 O3 . This layer was responsible for trapping inclusions with similar chemistry and various shapes, which partially sintered on the surface. A decarbonized zone was observed below the newly formed secondary alumina layer, consisting mainly of residual alumina with the typical shape of the raw material. After 10 s of immersion, the uncoated filter showed a remarkable collection of inclusions only in proximity of cracks, while the characteristic corundum layer was not completely developed on
866
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
the rest of the surface. Thanks to the higher reactivity provided by the MWCNTs, the coated filter presented instead wider and non-preferential zones of collection of inclusions. The two surfaces showed both a dense and a coral-like collection layer after a 30-s test, but optical microscopy suggested a higher presence of loose inclusions on the coated filter. Both samples were slightly damaged by the thermal shock during the 10-s test, but after 30 s thin protrusions at the weak edges locations were observed, suggesting a healing mechanism of cracks by the trapping of inclusions and sintering. No remarkable thickness variation of the secondary alumina layer could be observed with increasing the testing time. In conclusion, the microscope investigations indicated that the MWCNTs-coated filter had a better performance than the uncoated one. Acknowledgements The authors would like to thank Mrs. C. Ludewig for the sample preparation, Mr. D. Thiele and Mr. R. Fricke for support during the test in the steel casting simulator. The studies were carried out with financial support from the German Research Foundation (DFG) within the framework of the Collaborative Research Center SFB 920 “Multi-Functional Filters for Metal Melt Filtration—A Contribution toward Zero Defect Materials”. Enrico Storti is supported by a grant from the National Interuniversity Consortium of Materials Science and Technology (INSTM, Italy). References [1] L. Zhang, B.G. Thomas, Inclusions in continuous casting of steel, Proceedings of the XXIV National Steelmaking Symposium (2003) 138–183. [2] D. Apelian, R. Mutharasan, S. Ali, Removal of inclusions from steel melts by filtration, J. Mater. Sci. 20 (1985) 3501–3514. [3] K. Uemura, M. Takahashi, S. Koyama, M. Nitta, Filtration mechanism of non-metallic inclusions in steel by ceramic loop filter, ISIJ Int. 32 (1) (1992) 150–156. [4] L. Bulkowsi, U. Galisz, H. Kania, Z. Kudlinski, J. Pieprzyca, J. Baranski, Industrial tests of steel filtering process, Arch. Metall. Mater. 57 (1) (2012) 363–369. [5] K. Janiszewski, Refining of liquid steel in a tundish using the method of filtration during its casting in the CC machine, Arch. Metall. Mater. 58 (2) (2013) 513–521. [6] K. Janiszewski, B. Panic, Industrial investigations of the liquid steel filtration, Metalurgija 53 (3) (2014) 339–342. [7] K. Janiszewski, Z. Kudlinski, Removal of liquid non-metallic inclusion from molten steel using the method filtration, Metal (2006) 1–9. [8] R. Dekkers, B. Blanpain, P. Wollants, F. Haers, C. Vercruyssen, B. Gommers, Non-metallic inclusions in aluminium killed steels, Ironmak. Steelmak. 29 (6) (2002) 237–244.
[9] R. Dekkers, B. Blanpain, P. Wollants, Crystal growth in liquid steel during secondary metallurgy, Metall. Mater. Trans. B 34 (2003) 161–171. [10] K. Wasai, K. Mukai, A. Miyanaga, Observation of inclusion in aluminum deoxidized iron, ISIJ Int. 42 (5) (2002) 459–466. [11] S. Ovtchinnikov, S. Kazakov, D. Janke, Formation of oxide inclusions during deoxidation and solidification of high purity steels, Ironmak. Steelmak. 30 (6) (2003) 487–496. [12] C.G. Aneziris, S. Dudczig, J. Hubálková, M. Emmel, G. Schmidt, Alumina coatings on carbon bonded alumina nozzles for active filtration of steel melts, Ceram. Int. 39 (3) (2013) 2835–2843. [13] C.G. Aneziris, S. Dudczig, M. Emmel, H. Berek, G. Schmidt, J. Hubálková, Reactive filters for steel melt filtration, Adv. Eng. Mater. 15 (1-2) (2013) 46–59. [14] C.G. Aneziris, M. Emmel, S. Dudczig, Aktive, keramische Filter. German patent DE 102011109681, (2013). [15] C.G. Aneziris, M. Emmel, S. Dudczig, Keramische, reaktive Filter. German patent DE 102011109684, (2013). [16] O. Dávila-Maldonado, A. Adams, L. Oliveira, B. Alquist, R.D. Morales, Simulation of fluid and inclusions dynamics during filtration operations of ductile iron melts using foam filters, Metall. Mater. Trans. B 39 (6) (2008) 818–839. [17] K. Schwartzwalder, A.V. Somers, Method of making porous ceramic articles, United States patent US 3090094 (1963) May. [18] S. Ali, R. Mutharasan, D. Apelian, Physical refining of steel melts by filtration, Metall. Trans. B 16 (1985) 725–742. ´ J. Bazan, J. Dobrovská, M. Balcar, P. Fila, L. Martínek, Application of [19] K. Stránsky, the theory of physical similarity for the filtration of metallic melts, Mater. Technol. 42 (4) (2008) 175–178. [20] H. Jaunich, B. Pech, C.G. Aneziris, Vorsprung durch Innovation bei der Entwicklung von Speisen und Filter für die Gießereitechnik, Cfi-Ber DKG 80 (5) (2003). [21] H.O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes, 1st edition, Noyes Publications, Weinheim, 1993. [22] S. Dudczig, C.G. Aneziris, M. Emmel, G. Schmidt, J. Hubálková, H. Berek, Characterization of carbon bonded alumina filters with active or reactive coatings in a steel casting simulator, Ceram. Int. 40 (2014) 16727–16742. [23] T. Zienert, S. Dudczig, O. Fabrichnaya, C.G. Aneziris, Interface reactions between liquid iron and alumina-carbon refractory filter materials, Ceram. Int. 41 (2015) 2089–2098. [24] R.B. Tuttle, J.D. Smith, K.D. Peaslee, Casting simulation of calcium titanate and calcium zirconate nozzles for continuous casting of aluminum-killed steels, Metall. Mater. Trans. B 38 (1) (2007) 101–108. [25] E. Storti, M. Emmel, S. Dudczig, P. Colombo, C.G. Aneziris, Development of multi-walled carbon nanotubes-based coatings on carbon-bonded alumina filters for steel melt filtration, J. Eur. Ceram. Soc. 35 (5) (2015) 1569–1580. [26] J. Fruhstorfer, S. Schafföner, C.G. Aneziris, Dry ball mixing and deagglomeration of alumina and zirconia composite fine powders using a bimodal ball size distribution, Ceram. Int. 40 (9) (2014) 15293–15302. [27] M. Emmel, C.G. Aneziris, Development of novel carbon bonded filter compositions for steel melt filtration, Ceram. Int. 38 (6) (2012) 5165–5173. [28] S. Kimura, K. Nakajima, S. Mizoguchi, H. Hasegawa, In-situ observation of the precipitation of manganese sulfide in low-carbon magnesium-killed steel, Metall. Mater. Trans. A 33 (2002) 427–436.
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Short-time performance of MWCNTs-coated Al2 O3 -C filters in a steel melt Enrico Storti a,b,∗ , Steffen Dudczig a , Gert Schmidt a , Paolo Colombo b,c , Christos G. Aneziris a a
Technische Universität Bergakademie Freiberg, Institut für Keramik, Glas- und Baustofftechnik, Agricolastraße 17, 09599 Freiberg, Germany Università degli Studi di Padova, Dipartimento di Ingegneria Industriale, Via Marzolo 9, 35131 Padova, Italy c Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16801, USA b
a r t i c l e
i n f o
Article history: Received 16 August 2015 Received in revised form 19 October 2015 Accepted 25 October 2015 Available online 6 November 2015 Keywords: Carbon nanotube Alumina Carbon Ceramic foam filter
a b s t r a c t 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 30 s of immersion. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Non-metallic inclusions in cast metal parts remarkably influence their mechanical properties such as fracture toughness, tensile strength and fatigue resistance. Not only the amount of inclusions, but also their kind, shape and distribution are of great importance especially in applications where high-tensile steel is used. Under load, the stress concentrations are found in the proximity of inclusions, which control the deformation behavior of the product [1]. In order to limit the casting repairs and possibly avoid rejected castings, very high-purity metal melts are thus required. This is achieved by adding a physical refining process, i.e., filtration, after the ladle treatment [2–6]. The thermodynamic fundamentals for the process of refining molten steel from non-metallic inclusions, using the method of filtration with ceramic filters, have been presented by Janiszewski and Kudlinski [7]. The main non-metallic inclusions which need to be removed from steel melts are oxides, carbides, sulfides and nitrides. Exogenous inclusions originate from slag entrainment or lining erosion. Endogenous inclusions, instead, include products of deoxidation
∗ Corresponding author at: Technische Universität Bergakademie Freiberg, Institut für Keramik, Glas- und Baustofftechnik, Agricolastraße 17, 09599 Freiberg, Germany. E-mail address: [email protected] (E. Storti). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.10.036 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
or inclusions precipitated during cooling and solidification. When steel is deoxidized with aluminum, the resulting fine alumina inclusions do not separate completely into the slag phase despite their lower density with respect to steel. After solidification, a variety of different shapes for these particles has been observed, such as spheres, polyhedral, plates, dendrites and clusters [8]. The morphology of non-metallic inclusions may be understood by considering parameters that influence crystal growth, i.e., atomic structure, degree of supersaturation (Al and O activities in the steel bath), presence of impurities and stirring conditions. Dendrites grow along supersaturation gradients, while turbulence in the reaction zone leads to the formation of clusters. Plate-like particles have a large surface-to volume ratio and a trigonal symmetry, which suggests that the corresponding alumina structure is corundum. The plate-like habit has been explained by considering adsorption of impurities on the {1 1 1} face, which hinders the growth in this direction. Polyhedral inclusions originate from spherical inclusions when the equilibrium condition is approached. Depending on the wetting properties of the material, a stage of agglomeration and sintering may follow the growth: it was observed that alumina inclusions, due to their high interfacial energy, readily generate three-dimensional clusters via collision and aggregation [9,10]. Endogenous inclusions are named “primary” if generated immediately after the addition of the deoxidizing agents, while “secondary” and “tertiary” inclusions form respectively during
858
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
cooling and solidification. “Quaternary” inclusions are generated below the solidus temperature [11]. Aneziris et al. have proposed new approaches of surface functionalized filter materials based on so-called “active” as well as “reactive” coatings [12,13]. In the first case, the same chemistry as the chemistry of the primary or secondary inclusions that have to be removed is generated on carbon-bonded filters. On the other hand, “reactive” coatings interact with the dissolved gas in the melt (oxygen in steel melts for instance) and generate inclusions above the liquidus temperature, which are accordingly deposited on the filter. This approach helps reducing the amount of tertiary and also quaternary inclusions [14,15]. Ceramics filters in various shapes such as open porous foams, honeycomb-like structures or “spaghetti” stacks are used today in the steel industry [16]. In particular, the majority of the applied filters are open-cell ceramic foams produced via the replica process [17]. These filters have a high specific surface area and deliver a higher hydraulic resistance than multi-hole filters: both these features favorably affect the micro-mechanism of filtration [18,19]. Moreover, the metal flow is adjusted from a turbulent into a laminar flow, reducing the risk of reoxidation and the formation of cavities. As a result, a better surface quality and shorting casting times are achieved [20]. For the filters carbon-bonded alumina material is commonly used, since it exhibits good thermal shock performance and increased creep resistance in comparison to zirconia, which is the other main material of choice for steel melt filtration. Carbon has a high thermal conductivity (up to 400 W/mK for crystalline graphite) which results in the good thermal shock resistance of the filters. Moreover, an increase in strength with increasing temperature up to 2500 ◦ C is observed for graphite, and the plastic deformation with a following annealing of cracks and defects becomes possible [21]. Recent studies have shown that the carbon-bonded alumina material reacts when immersed in a steel melt, producing a new thin, dense alumina layer (identified as crystalline ␣-alumina at room temperature) on the contact area with the steel. Thermodynamic studies have suggested a partial dissolution of alumina and carbon in the steel at the interface, followed by the reaction of this liquid with some fresh steel and the precipitation of the secondary alumina layer. Thanks to the new metal/alumina interface, the wettability of the steel melt on the filter is suddenly decreased and the chemistry and crystallographic structure of the surface are changed to that of the endogenous inclusions. The new formed layer is consequently quite efficient in removing these particles from the melt [22,23], and the faster this layer is generated the better the trapping efficiency for the inclusions of the filter is. A schematic material buildup on an uncoated carbon-bonded alumina filter surface after a 60-s immersion test is presented in Fig. 1: zone A represents the unreacted carbon-bonded alumina substrate; zone B consists of a decarburized, porous layer where alumina grains have partially sintered; zone C is the thin alumina layer just described; zone D represents a dense collection layer, mainly consisting of small plate-like or polyhedral endogenous inclusions; the final zone E shows entrapped particles in complex shapes, such as dendrites, in a loose assemblage, from which the commonly used “coral-like” term derives [12,24]. Storti et al. have recently added a MWCNTs-based coating on standard carbon-bonded alumina filters by spray coating, with the aim to promote the formation of the thin alumina layer and to improve the filtration efficiency. The carbon nanotubes were removed from the filter during the 60-s immersion test, but no carbon buildup in the steel was detected in the end. An investigation by SEM showed a dense collection zone with plate-like particles directly over the decarburized zone. The authors proposed that the high surface reactivity granted by the MWCNTs accelerated the formation of zone C, which after 60 s was completely covered
Fig. 1. Schematic diagram of the cross-section of a carbon-bonded alumina foam after immersion in steel melt (not true to scale and particle shape) (A) unaltered carbon-bonded alumina material; (B) decarburized zone with residual alumina; (C) thin secondary layer; (D) dense collection zone; (E) loose collection zone. Adapted from [16].
by endogenous inclusions and thus no longer recognizable as an independent layer [25]. In the present study, two different carbon-bonded alumina foams (i.e., uncoated and MWCNTs-coated) were tested in a socalled steel casting simulator, in order to compare their filtration behavior for short times of immersion in a steel melt. In an oxygenfree argon atmosphere, the prismatic filter samples were dipped into steel melts containing artificially-generated endogenous alumina particles, before being removed and allowed to cool down under inert conditions [22]. This procedure allowed the investigation of filter surfaces after a simulated filtration with adsorbed particles but without the presence of solid steel, as in the case under normal industrial trials. In fact, the study of filters from castings is hindered by solid steel filling the porosity after use. 2. Materials and methods The raw materials used for the preparation of the Al2 O3 -C filters were aluminum oxide (Martoxid MR 70, Martinswerk, Germany, 99.80 wt.% Al2 O3 , d90 ≤ 3.0 m), modified coal tar pitch powder
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 2. Heat treatment schedule for the samples. The same treatment was used after application of the MWCNTs coating.
(Carbores® P, Rütgers, Germany, d90 < 0.2 mm—used as a binder as well as a carbon source), fine natural graphite (AF 96/97, Graphit Kropfmühl, Germany, 96.7 wt.% carbon, 99.8 wt.% <40 m), and carbon black powder (Luvomaxx N-991, Lehmann & Voss & Co., Germany, carbon content ≥99.0 wt.%, ash content >0.01 wt.%, primary particle size of 200–500 nm). The additives were ligninsulfonate (C12C, Otto-Dille, Germany—used as wetting agent and temporary binder), Castament VP95L (BASF, Germany—used as a dispersing agent), and Contraspum K1012 (Zschimmer & Schwarz, Germany—used as an antifoam agent). Due to an agglomeration problem during storage, a pre-treatment was performed on our raw aluminum oxide in order to restore the material close to its original grain size distribution. A dry ball milling technique with a bimodal ball size distribution was used, the process being explained in detail by Fruhstorfer et al. [26]: in our case a milling time of 40 min was necessary to destroy the agglomerates in the 4–10 m size; after this deagglomeration step the registered d90 value shifted from 7.6 down to 1.8 m. The carbon-bonded filters were manufactured via the replica process patented by Schwartzwalder in 1963 and using a two steps approach, as explained in detail by Emmel [17,27]. 20 wt.% Carbores® P was used in the starting mixture in order to achieve a strong bonding and good thermo-mechanical properties after the pyrolysis process. The exact composition can be found in [13] under the label “AC5”. The filters were heat treated in retorts filled with calcined petcoke (Müco, Essen, Germany) with particle size distribution of 0.2–2.0 mm, in order to guarantee reducing conditions. The schedule (reported in Fig. 2) used a 1 K/min heating rate, intermediate holding steps of 30 min and a final holding time of 180 min. The raw materials used for the preparation of the coating were MWCNTs (TNM8, Chengdu Organic Chemicals, Chengdu, China, purity >95%, outer diameter >50 nm, length = 10–20 m), and modified coal tar pitch powder (Carbores® P, see above—used as a binder). The additives were Xanthan powder (Rhodopol 50 MD, Erbslöh, Krefel, Germany—used both as a dispersing agent and thickening agent), ammonium-ligninsulfonate and Contraspum K1012 (see above). The carbon nanotubes were not purified before use. The preparation and application of the MWCNTs coating were performed exactly as explained in [25], but in this investigation only the slurry named “C” (i.e., with the highest nanotubes content) was used. It must be noticed that, after application and drying of the coating, the filters were thermal treated once again under reducing conditions (with the schedule in Fig. 2) in order to achieve the bonding of the CNTs and close some residual porosity and cracks.
859
A metal casting simulator located at the Institute of Ceramic, Glass and Construction Materials in Freiberg (Systec, Germany) was used to compare the filtration behavior of our uncoated and coated filters during very short operation times. A 10 ppi prismatic ceramic foam sample based on a template of geometry 125 mm × 20 mm × 20 mm was fixed at a special sample holder in a gas tight sewer port above a melting crucible. The used apparatus has been presented in detail by Aneziris in [13]. In the present work, a so-called “finger test” was performed according to the procedure described by Dudczig et al. [22]. All tests were carried out under fully controlled argon atmosphere. 38 kg of commercially available 42CrMo4 steel (AISI 4142) were melted under Ar in a special hydratable alumina-bonded alumina/alumina–magnesia–spinel crucible not containing any silica, calcia or further additions, in order to prevent unwanted reactions with the crucible material during the experiment. The oxygen content and the temperature of the steel melt were measured in situ with a pO2 /T—sensor system Celox (Heraeus Electro-Nite, Germany) for the duration of each test. To create defined alumina impurities in the steel melt, 0.5 wt.% (related to the steel mass) of an iron oxide mixture were added. The commercial product (Mineralmühle Leun, Germany) consists of 75 wt.% hematite and 25 wt.% magnetite. After reaching a temperature of 1650 ◦ C, the oxide mixture was added directly to the melt and an increase of the dissolved oxygen from around 10 ppm up to around 60 ppm was detected. At this point, the endogenous alumina inclusions were created by adding 0.05 wt.% (related again to the steel mass) of pure aluminum metal to the melt. Accordingly, the dissolved oxygen content fell back close to the starting value. After this step the prismatic sample, without any preheating, was dipped 60 mm into the steel melt through the sewer port and rotated for 10 (or 30) s at 30 rpm inside the melt. The filter was next removed from the melt and cooled down in a chamber under argon in order to prevent oxidation of the carbon, which would surely occur if the hot sample were left in contact with a normal ambient atmosphere. The influence of impurities from refractory materials was almost eliminated by using one crucible per melt. For the optical investigation of the samples after the contact with the steel melt, a digital light microscope (VHX-200 D, Keyence, Germany) was used. The microstructural evaluation of new phases and collected particles on the surface was carried out by means of scanning electron microscopy (SEM XL30, Philips, Germany) in combination with energy dispersive X-ray spectroscopy (EDS).
3. Results and discussion Fig. 3 shows the Al2 O3 -C samples after the immersion test. They all survived the initial thermal shock as well as the rapid cooling (which is also critical), suffering no heavy damage. This could be expected since both kinds of filters were already reported to survive a 60-s immersion test [22,25]. For all filters, the part which was in direct contact with the melt could be detected by the color change from black to gray, which was due to decarburization of the surface, reactions with the molten steel and (as discussed below) following deposition of a new phase. This phase consisted of endogenous alumina inclusions, deliberately generated in the steel through the iron oxide addition and following deoxidation, as explained in Section 2. Digital optical images of the filters (contact zone) are presented in Figs. 4–7 . On all samples, some spherical, bright particles of different sizes were detected: as previously reported by Dudczig et al., these particles consisted of entrapped steel, the peculiar shape due to the low wettability of iron toward the filter material [22]. After 10 s of immersion in the molten steel, the uncoated carbonbonded alumina filter showed a relatively regular surface, with a gray or light gray color and a smooth appearance (Fig. 4). Some
860
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 3. Filter samples after finger test. Top left: uncoated filter, 10 immersion; top right: MWCNTs-coated filter, 10 immersion; bottom left: uncoated filter, 30 immersion; bottom right: MWCNTs-coated filter, 30 immersion.
Fig. 4. Digital optical images of uncoated filter after a 10-s immersion test.
Fig. 5. Digital optical images of MWCNTs-coated filter after a 10-s immersion test.
Fig. 6. Digital optical images of uncoated filter after a 30-s immersion test.
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
861
Fig. 7. Digital optical images of MWCNTs-coated filter after a 30-s immersion test.
Fig. 8. SEM-BSE micrographs of uncoated filter (10 ) with detail of layer E on the side of crack.
macroscopic damage could be detected on the sample in the form of cracks and also missing strut parts. In these particular spots, the filter had a darker tone of gray compared to the rest of the dipped sample, suggesting that the damage was not due to the initial strong thermal shock, but rather to the rapid cooling or to the handling after the end of the finger test. Thin cracks revealed instead a light gray color of the exposed material and implied a contact of these areas with the steel melt. Moreover, some thin lines in light gray color were observed on the filter surface, following the same distribution of cracks at the edges of the struts: these are the weakest spots in filters obtained by the replica technique, where the wall thickness is minimal because of the hollow shape left from the foam degradation. At higher magnifications, small bright spots were detected, mostly in proximity of these cracks or light gray edges. The CNTs-coated filter, after 10 s of contact, is presented in Fig. 5: unlike the uncoated sample, its surface was quite irregular, with some large areas showing a bright phase and a gray, apparently smooth background. At higher magnifications, it was confirmed that the light gray phase (consisting of adsorbed endogenous particles) grew on top of the structure and was not uniformly distributed on the surface of the foam. Assuming that the CNTs presence allowed for a faster interaction with the melt, this distribution of entrapped particles could indicate an uneven distribution of the nanotubes on the surface, as shown by Storti et al. [25]. This sample showed also some damage in the form of small cracks and pores, with the darker substrate material exposed. However, the overall amount and width of cracks seemed to be limited in comparison to those observed on the reference carbon-bonded filter. This was probably related to the different carbon content of the two surfaces. The uncoated filter which was immersed for 30 s showed quite a rough surface with a gray or light gray color (Fig. 6). In this case, only a very limited damage was observed and the collection extent
Fig. 9. Partially developed layer C on uncoated filter (10 ).
seemed much more considerable than after 10 s. In addition, many linear protrusions were detected, particularly located at the weak edges. This suggested some kind of “healing” or sealing of the initial cracks thanks to the interaction with the steel melt and deposition of more inclusions. The MWCNTs-coated sample which underwent the 30-s test seemed as almost undamaged as the previous one, with the same kind of protrusions at the weak edges (Fig. 7). However an apparent (not quantified) higher roughness and overall brighter appearance of this surface suggested a more efficient entrapment of endogenous particles and a better development of the E layer. In fact, the bright appearance of some spots could be ascribed to coral-like particles which, thanks to their complex shapes and their random
862
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 10. SEM-BSE micrograph of material buildup on uncoated filter (10 ), where layer C is visible.
orientation, had a greater ability at reflecting the sidelight used in our investigation (see later). SEM micrographs of the uncoated carbon-bonded alumina filter after 10 s of immersion in the steel melt are presented in Fig. 8. As expected, some initial deposition on this sample was observed. New particles in the shape of dendrites, plates or polyhedra were clearly detected. Such shapes are typical for endogenous inclusions and indicated that the filtration process started very quickly after the first contact with the melt, which is of course the desired
behavior for a molten metal ceramic filter. An EDS analysis confirmed that the entrapped particles consisted of alumina with a low amount of impurities. Interestingly, the coral-like collection (zone E) was mainly detected in small areas located exactly at the sides of cracks. We posit that the steel flux was slowed down and “trapped” for longer times in proximity of such defects, which resulted in a more efficient collection of inclusions. The BSE signal revealed some white spots with a spherical shape, the same shape detected before by optical microscopy: iron and traces of other metals such as Mn and Cr were found by means of EDS, and these particles were hence associated with steel entrapment by the filter. A higher presence of steel spherules on the sample was always detected in the areas with a coral-like structure. The main microstructure detected on the uncoated sample (see Fig. 9) revealed nevertheless an incomplete reaction with the melt. It consisted in fact of spherical alumina particles (which belonged to the substrate) partly sintered together, while the high porosity observed was due to dissolution of the carbon component into the steel melt. A densification was observed in some areas, indicating the progressive formation of the secondary alumina layer C. On this basis, we propose that 10 s of immersion were not enough for an efficient deposition of endogenous inclusions on the uncoated Al2 O3 -C filter. From a cross-sectional point of view (Fig. 10), and in reference to the different characteristic areas of material buildup as explained in Section 1, we could observe the decarburized layer B, consisting of residual alumina grains, present over the unaffected substrate material (A). Next, a thin layer with a thickness of about 200 nm could be noticed between the entrapped inclusions and the decarburized zone (see Fig. 11). Its structure was overall very similar to the one reported by Dudczig. According to Zienert et al., the thin alumina layer was likely produced by the first interaction
Fig. 11. Secondary alumina layer C after finger test. Top left: uncoated filter, 10 immersion; top right: MWCNTs-coated filter, 10 immersion; bottom left: uncoated filter, 30 immersion; bottom right: MWCNTs-coated filter, 30 immersion.
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
863
Fig. 12. SEM micrographs of MWCNTs-coated filter (10 ) with detail of the inclusion collection and zones B and C.
Fig. 13. Material buildup on MWCNTs-coated filter (10 ). Layer C detached from the underlying material.
with the steel melt through a reduction and quick reoxidation of Al2 O3 from the filter surface [23]. Finally, a collection zone with plate-like particles and dendrites with a random orientation was observed (see Fig. 10). Fig. 12 shows the MWCNTs-coated filter after 10 s of immersion: a similar microstructure to the uncoated filter was detected on its surface, with both the dense and the coral-like collection zones. As expected, the presence of the carbon nanotubes on the surface did not hinder the filtration process. On the contrary, in this case layer D, consisting of plate-like particles, seemed better developed and covered the majority of the investigated surface. In fact, the decarbonized filter material could still be recognized under this layer, but only in limited areas. Moreover, the loose coral-like particles had a random distribution and were not detected just in proximity of cracks as in the uncoated sample. Fig. 13 presents the material buildup on the MWCNTs-coated sample: a very thin alumina layer (about 200 nm in thickness), totally comparable to the previous one, was again detected over the substrate. As expected, no carbon nanotubes or MWCNT agglomerates were found on any spot of the sample surface after the test: nanotubes were only present in a very thin layer on the external surface of the Al2 O3 -C foam and reacted rapidly with the steel melt.
SEM micrographs of an uncoated filter after a 30-s immersion in the steel melt are presented in Fig. 14. Compared to the 10-s test, here the dense collection layer clearly appears to have developed on the whole investigated surface, showing clusters of small particles with a low aspect ratio. The sintering process of the inclusions on the filter did not create a single homogenous layer, hence the sample possessed quite a rough surface. The presence of facets allowed us to recognize that the inclusions were found in plate-like or polyhedral shape before forming the aggregates. The decarbonized microstructure (B) and the substrate material (A) were still observed, but only where the top material was broken off because of the preparation process. Conversely, not many coral-like zones were detected over the dense layer, probably because of the late formation of layers C and D. The analysis of a cross-section of the foam (Fig. 15) allowed again to detect the thin secondary alumina layer C that formed between the decarbonized zone and the entrapped particles. Its appearance, composition and thickness were all similar to the results obtained in the 10-s test. It is interesting to observe that the thickness of the secondary alumina layer did not remarkably increase with the testing time (see Fig. 11): this structure was the first responsible for the collection of endogenous inclusions and after its formation only layers D and E could develop and increase in thickness. Along with the plate-like, dendritic and polyhedral particles, another type of structure was detected as part of the collection zone on this sample: a few agglomerates with dimensions of some microns and consisting of fine, partially sintered spherical inclusions (Fig. 16). The BSE detector did not find any remarkable composition difference with the surrounding alumina inclusions. According to Dekkers et al. [9], the shape of these particles suggest that they originated at a very fast rate in a zone of the melt with high O and low Al activity. Finally, some very fine particles with an undefined shape were observed just barely over the alumina inclusions in proximity of steel spherules (Fig. 17). These inclusions showed a slightly different tone of gray under BSE. The chemical composition estimated by EDS analysis revealed the presence of Mn and S, which indicate a deposition of MnS impurities from the melt: MnS has in fact been previously observed to nucleate on oxide particles in the temperature range between 1500 ◦ C and 1200 ◦ C in a low-carbon steel melt [28]. Accordingly, both Dudczig and Storti detected lower S content in the steel after the immersion test of Al2 O3 -C filters [22,25]. Fig. 18 shows the MWCNTs-coated surface after 30 s of immersion. With respect to the uncoated sample, a smoother appearance was noticed where the coral-like collection zone was missing: small
864
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
Fig. 14. SEM micrographs of uncoated filter (30 ), with detail of layer D.
Fig. 16. Small alumina particles deposited over the dense collection layer D.
Fig. 17. SEM-BSE detail of entrapped particles on uncoated filter (30 ). Apart from steel spherules and alumina platelets, some other inclusions (likely MnS) were detected. Table 1 Composition by EDS of the different zones from an uncoated sample (see Fig. 15) in comparison to the theoretical composition of alumina (wt.%).
Fig. 15. SEM-BSE micrograph of material buildup on uncoated filter (30 ).
plate-like and polyhedral inclusions appeared to have sintered into a denser layer. This difference could be expected, since the formation of the D layer on the uncoated filter was for the most part still in progress after 10 s of immersion in the steel melt. The material buildup from a cross-sectional view on the MWCNTs-coated filter is presented in Fig. 19: the secondary thin alumina layer was observed on this sample also, with the same structure and same average
Element
A
B
C
D+E
Theoretical composition Al2 O3
Al O C Na Si Fe P Mn
24.51 39.12 33.54 0.19 0.18 2.46
34.15 54.97 9.08 0.45 0.34 0.69 0.18 0.16
32.34 55.81 10.02 0.58 0.55 0.70
38.92 49.34 11.27 0.23 – 0.23
52.93 47.07
thickness as for the previous filters. Similarly to the uncoated sample, some agglomerates of fine spherical alumina particles were also detected here. Tables 1 and 2 present the results of the element analysis conducted by means of EDS on the four main microstructures detected on both our samples after 30 s of immersion in the steel melt (see Figs. 15 and 19). As expected, a large amount of carbon was detected in the substrate material (zone A): about 30 wt.% is expected to be the residual carbon content for the AC5 filters after pyrolysis. However, a much higher value was registered for the MWCNTs-coated filter: indeed, after the application of the coating and the second heat treatment, a surface enrichment in carbon was expected. The
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
865
Fig. 18. SEM micrograph of MWCNTs-coated filter (30 ) with detail of layer D.
Table 2 Composition by EDS of the different zones from a MWCNTs-coated sample (see Fig. 19) in comparison to the theoretical composition of alumina (wt.%). Element
A
B
C
D+E
Theoretical composition Al2 O3
Al O C Na Si Fe Mn S
16.70 26.09 56.67 – – 0.55
34.74 42.88 20.10 1.06 0.32 0.22 0.43 0.25
22.54 38.09 37.54 0.37 – 1.46
35.41 47.47 15.19 0.60 – 1.34
52.93 47.07
iron impurity (white spots in zone A) should be attributed to the first melt which came in contact with the filter surface and did not completely recede after the formation of the thin alumina layer C. The surface of the MWCNTs-coated sample showed a higher carbon content not only in the base material, but also in the decarburized zone B, the thin layer C and the entrapment zone. Nevertheless it must be noticed that, since the collected X-rays came from a sample volume with a depth of a few microns, the recorded spectrum for very thin layers such as B and C could be distorted by the underlying signal. Moreover, the instrumental error for atoms lighter than Na can be remarkable (up to ca. 10%) due to the low energy of the incoming X-rays. Iron was detected in all microstructures, as well as other impurities like Na and Si. Traces of P, Mn and S were only found in zone B. With regard to the theoretical composition of alumina, all layers showed a lower Al content. Layers B and C had an excess oxygen but only in the uncoated Al2 O3 -C sample. The large discrepancy with the theoretical composition of alumina was likely due to the error related to oxygen detection, to the presence of other minor oxides and to some chemical inhomogeneity of the scanned areas, which also heavily affects the accuracy of a quantitative EDS analysis. From our results, we could conclude that the surfaces of both samples followed the general buildup scheme, proposed by Dudczig and Zienert for a test time of 60 s [22,23], already after very short immersion times. However, for the 10-s test, the development of the secondary alumina layer C and the following deposition of inclusions were more efficient for the MWCNTs-coated filter. After 30 s of immersion, both filters developed a dense collection layer on the surface, but the densification appeared to be at a more advanced stage on the coated sample. Moreover, optical microscopy analysis indicated a more efficient collection of loose endogenous particles on the sample coated with MWCNTs. The data obtained from this study indicate that the initial response of the foam filter is crucial in the development of the different layers and the efficient removal of
Fig. 19. SEM-BSE micrograph of material buildup on MWCNTs-coated filter (30 ).
endogenous particles from the melt. Since no exogenous particles were produced during the test, nothing can be yet assumed about the filter performance in the collection of these other inclusions. In the future, the material buildup on uncoated and MWCNTscoated filters after different immersion times will be characterized by means of computer tomography, in order to enable a quantification of the different trapping performance of the two types of filters. 4. Conclusions The behavior of two different foam filters in contact with molten steel containing endogenous alumina inclusions was investigated. Uncoated and MWCNTs-coated carbon-bonded alumina filters were immersed for 10 and 30 s in a 42CrMo4 (AISI 4142) steel melt at 1650 ◦ C under an argon atmosphere. Both surfaces developed a thin secondary corundum layer (around 200 nm in thickness) at the contact area with the melt, by means of partial dissolution and reprecipitation of Al2 O3 . This layer was responsible for trapping inclusions with similar chemistry and various shapes, which partially sintered on the surface. A decarbonized zone was observed below the newly formed secondary alumina layer, consisting mainly of residual alumina with the typical shape of the raw material. After 10 s of immersion, the uncoated filter showed a remarkable collection of inclusions only in proximity of cracks, while the characteristic corundum layer was not completely developed on
866
E. Storti et al. / Journal of the European Ceramic Society 36 (2016) 857–866
the rest of the surface. Thanks to the higher reactivity provided by the MWCNTs, the coated filter presented instead wider and non-preferential zones of collection of inclusions. The two surfaces showed both a dense and a coral-like collection layer after a 30-s test, but optical microscopy suggested a higher presence of loose inclusions on the coated filter. Both samples were slightly damaged by the thermal shock during the 10-s test, but after 30 s thin protrusions at the weak edges locations were observed, suggesting a healing mechanism of cracks by the trapping of inclusions and sintering. No remarkable thickness variation of the secondary alumina layer could be observed with increasing the testing time. In conclusion, the microscope investigations indicated that the MWCNTs-coated filter had a better performance than the uncoated one. Acknowledgements The authors would like to thank Mrs. C. Ludewig for the sample preparation, Mr. D. Thiele and Mr. R. Fricke for support during the test in the steel casting simulator. The studies were carried out with financial support from the German Research Foundation (DFG) within the framework of the Collaborative Research Center SFB 920 “Multi-Functional Filters for Metal Melt Filtration—A Contribution toward Zero Defect Materials”. Enrico Storti is supported by a grant from the National Interuniversity Consortium of Materials Science and Technology (INSTM, Italy). References [1] L. Zhang, B.G. Thomas, Inclusions in continuous casting of steel, Proceedings of the XXIV National Steelmaking Symposium (2003) 138–183. [2] D. Apelian, R. Mutharasan, S. Ali, Removal of inclusions from steel melts by filtration, J. Mater. Sci. 20 (1985) 3501–3514. [3] K. Uemura, M. Takahashi, S. Koyama, M. Nitta, Filtration mechanism of non-metallic inclusions in steel by ceramic loop filter, ISIJ Int. 32 (1) (1992) 150–156. [4] L. Bulkowsi, U. Galisz, H. Kania, Z. Kudlinski, J. Pieprzyca, J. Baranski, Industrial tests of steel filtering process, Arch. Metall. Mater. 57 (1) (2012) 363–369. [5] K. Janiszewski, Refining of liquid steel in a tundish using the method of filtration during its casting in the CC machine, Arch. Metall. Mater. 58 (2) (2013) 513–521. [6] K. Janiszewski, B. Panic, Industrial investigations of the liquid steel filtration, Metalurgija 53 (3) (2014) 339–342. [7] K. Janiszewski, Z. Kudlinski, Removal of liquid non-metallic inclusion from molten steel using the method filtration, Metal (2006) 1–9. [8] R. Dekkers, B. Blanpain, P. Wollants, F. Haers, C. Vercruyssen, B. Gommers, Non-metallic inclusions in aluminium killed steels, Ironmak. Steelmak. 29 (6) (2002) 237–244.
[9] R. Dekkers, B. Blanpain, P. Wollants, Crystal growth in liquid steel during secondary metallurgy, Metall. Mater. Trans. B 34 (2003) 161–171. [10] K. Wasai, K. Mukai, A. Miyanaga, Observation of inclusion in aluminum deoxidized iron, ISIJ Int. 42 (5) (2002) 459–466. [11] S. Ovtchinnikov, S. Kazakov, D. Janke, Formation of oxide inclusions during deoxidation and solidification of high purity steels, Ironmak. Steelmak. 30 (6) (2003) 487–496. [12] C.G. Aneziris, S. Dudczig, J. Hubálková, M. Emmel, G. Schmidt, Alumina coatings on carbon bonded alumina nozzles for active filtration of steel melts, Ceram. Int. 39 (3) (2013) 2835–2843. [13] C.G. Aneziris, S. Dudczig, M. Emmel, H. Berek, G. Schmidt, J. Hubálková, Reactive filters for steel melt filtration, Adv. Eng. Mater. 15 (1-2) (2013) 46–59. [14] C.G. Aneziris, M. Emmel, S. Dudczig, Aktive, keramische Filter. German patent DE 102011109681, (2013). [15] C.G. Aneziris, M. Emmel, S. Dudczig, Keramische, reaktive Filter. German patent DE 102011109684, (2013). [16] O. Dávila-Maldonado, A. Adams, L. Oliveira, B. Alquist, R.D. Morales, Simulation of fluid and inclusions dynamics during filtration operations of ductile iron melts using foam filters, Metall. Mater. Trans. B 39 (6) (2008) 818–839. [17] K. Schwartzwalder, A.V. Somers, Method of making porous ceramic articles, United States patent US 3090094 (1963) May. [18] S. Ali, R. Mutharasan, D. Apelian, Physical refining of steel melts by filtration, Metall. Trans. B 16 (1985) 725–742. ´ J. Bazan, J. Dobrovská, M. Balcar, P. Fila, L. Martínek, Application of [19] K. Stránsky, the theory of physical similarity for the filtration of metallic melts, Mater. Technol. 42 (4) (2008) 175–178. [20] H. Jaunich, B. Pech, C.G. Aneziris, Vorsprung durch Innovation bei der Entwicklung von Speisen und Filter für die Gießereitechnik, Cfi-Ber DKG 80 (5) (2003). [21] H.O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerenes, 1st edition, Noyes Publications, Weinheim, 1993. [22] S. Dudczig, C.G. Aneziris, M. Emmel, G. Schmidt, J. Hubálková, H. Berek, Characterization of carbon bonded alumina filters with active or reactive coatings in a steel casting simulator, Ceram. Int. 40 (2014) 16727–16742. [23] T. Zienert, S. Dudczig, O. Fabrichnaya, C.G. Aneziris, Interface reactions between liquid iron and alumina-carbon refractory filter materials, Ceram. Int. 41 (2015) 2089–2098. [24] R.B. Tuttle, J.D. Smith, K.D. Peaslee, Casting simulation of calcium titanate and calcium zirconate nozzles for continuous casting of aluminum-killed steels, Metall. Mater. Trans. B 38 (1) (2007) 101–108. [25] E. Storti, M. Emmel, S. Dudczig, P. Colombo, C.G. Aneziris, Development of multi-walled carbon nanotubes-based coatings on carbon-bonded alumina filters for steel melt filtration, J. Eur. Ceram. Soc. 35 (5) (2015) 1569–1580. [26] J. Fruhstorfer, S. Schafföner, C.G. Aneziris, Dry ball mixing and deagglomeration of alumina and zirconia composite fine powders using a bimodal ball size distribution, Ceram. Int. 40 (9) (2014) 15293–15302. [27] M. Emmel, C.G. Aneziris, Development of novel carbon bonded filter compositions for steel melt filtration, Ceram. Int. 38 (6) (2012) 5165–5173. [28] S. Kimura, K. Nakajima, S. Mizoguchi, H. Hasegawa, In-situ observation of the precipitation of manganese sulfide in low-carbon magnesium-killed steel, Metall. Mater. Trans. A 33 (2002) 427–436.