UDK 621.74:666.76:666.762.14 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 47(6)777(2013) SYNTHESIZING A NEW TYPE OF MULLITE LINING SINTEZA NOVE VRSTE OBLOGE IZ MULITA Zagorka Acimovic1, Anja Terzic2, Ljubiša Andric3, Ljubica Pavlovi}3, Marko Pavlovic1 !University of Belgrade, Faculty of Technology and Metallurgy, Belgrade, Serbia 2Institute for Materials Testing, Belgrade, Serbia 3Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade, Serbia anja.terzic @institutims.rs Prejem rokopisa — received: 2013-03-07; sprejem za objavo - accepted for publication: 2013-03-27 Various possibilities for developing new mullite-based refractory linings that can be applied in a casting process were investigated and are presented in this paper. An optimization of the refractory-lining composition design with the controlled rheological properties was achieved by applying different lining components and altering the lining-production procedure. Mullite was used as a high-temperature filler. A mullite sample was tested with the following methods: X-ray diffraction analysis, differential thermal analysis and scanning-electron microscopy. The particle shape and particle size were analyzed with the program package for an image analysis called OZARIA 2.5. It was proved that an application of this type of lining has a positive effect on the surface quality, structural and mechanical properties of the castings of Fe-C alloys obtained by casting into sand molds, according to the method of expandable patterns (the EPC casting process). Keywords: refractory lining, mullite, quality of casting, EPC casting process V članku so predstavljene različne možnosti za razvoj nove mulitne ognjevzdržne obloge, uporabne pri postopku ulivanja. Optimiranje sestave ognjevzdržne obloge s kontroliranimi reološkimi lastnostmi je bilo doseženo z uporabo različnih sestavin obloge in s spremembo postopka izdelave obloge. Mulit je bil uporabljen kot visokotemperaturno polnilo. Vzorec mulita je bil preiskovan z naslednjimi metodami: z rentgensko difrakcijo, diferenčno termično analizo in vrstično elektronsko mikroskopijo. Oblika in velikost zrn sta bili določeni s programsko opremo za analizo slik OZARIA 2.5. Dokazano je bilo, da uporaba te vrste obloge ugodno vpliva na kvaliteto površine, strukturo in mehanske lastnosti ulitkov iz Fe-C zlitin pri ulivanju v peščene forme po metodi ekspandirane pene (EPC-postopek ulivanja). Ključne besede: ognjevzdržna obloga, mulit, kvaliteta ulitkov, EPC-postopek ulivanja 1 INTRODUCTION The main role of a lining is the formation of an efficient, unbreakable and firm refractory barrier which separates the sandy substrate from the liquid metal flow. For such a role, certain lining properties are required: high refractoriness, suitable gas permeability, simple application, good adhesion to a sandy mold and polymer model, simple adjustment of the lining-layer thickness and high drying rate. These requirements can be successfully fulfilled with an optimization of the lining composition and production technology.1-3 The basic characteristic of the EPC casting process is that the patterns and gating of molds made of polymers stay in the cast until the liquid-metal inflow occurs. The pattern-decomposition kinetics is the function of the liquid-metal temperature, with which the pattern comes in contact. The important factors influencing the pattern decomposition and, consequently, the evaporation process, besides temperature and pattern density, are: type of refractory lining, thickness of the lining layers covering the evaporable pattern, type and size of the sand grains and their granulation, permeability of the sandy model, gating of the mold construction, etc.4-7 Manufacturing the castings with the projected application quality by means of the EPC process has not been investigated enough and, thus, there is a need for a systematic research of the 'triad' including structure/properties/ technology, to which special attention was paid in this paper. The mullite was chosen as the refractory-lining filler due to the following properties: low thermal conductivity (6 W m-1 K-1);8 low coefficient of the linear thermal expansion (5.4 ■ 10-6 °C-1 at 25 °C);9 high thermal-shock resistance (quenching/500) and high maximum-use temperature of 1650 °C;10 flexural strength of 180 MPa; elastic modulus of 151 GPa; compresive strength of 1310 MPa; hardness of 1070 kg mm-2; extreme resistance to liquid-metal absorption;6 no gas production when in contact with liquid metal. Different additive types and various quantities were tested in order to enable the best possible absorption between the additives and the refractory filler particles and, thus, the maintenance of the filler in a dispersed state and prevention of the filler build-up or segregation. 2 EXPERIMENTAL WORK For the synthesis of mullite (3AI2O3 • 2SiO2) the mixture of kaolin and alumina was used with an addition of a mineralizer (1 % of NaF). Alumina was added in order to achieve the mullite stoichiometric ratio of 3 : 2. The crushing of the reactive components and the homo- genization were performed in a planetary ball mill PM 100 with sintered, aluminium-oxide grinding balls. After the homogenization and an addition of the mineralizer, the mixture was wetted with water and, subsequently, pressed in the mold with the 100 N/mm2 pressure and, afterwards, dried. The synthesis of mullite was performed by means of isothermal heating in the laboratory high-temperature 'Netzsch' furnace at the temperature of 1450 °C, with the heating rate of 10 °C/min in air atmosphere. The lining compositions were defined (Table 1) and the lining-component preparation methods were determined. Table 1: Composition of refractory mullite-based linings Tabela 1: Sestava ognjevzdrzne obloge na osnovi mulita Component Refractory lining based on alcohol Refractory lining based on water ararsid14156519 Refractory filler Mullite with the grain size of 35-40 pm, 90-94 % Mullite with the grain size of 40 pm, 93-95 % Binding agent Colophonium (C20H30O2), 2.5 % Bentonite, 2.5%; Bindal H, 0.5%; Na3P3O3, 1-3 % Additive/ suspension Bentone 25, 0.8-1 % Carboxymethyl-cellulose, 0.5-1 % Solvent Alcohol Water Refractory linings were applied to the sandy molds with a brush. During the application of the refractory lining on the polymer model using the technique of immersion into the tank with a lining, the process parameters were: the suspension density of 2 g/cm3, and the suspension temperature of 25 °C. The drying procedure was as follows: for the water-based linings, the duration of drying the first layer was 2 hours and the final layer was dried for 24 h; for the alcohol-based linings burning was used. The thickness of the lining layer on the model after drying was 0.5-1 mm. For casting, Fe-C alloys were used. The casting temperature was 1350 °C. For the production of sandy molds, the mold mixture based on quartz sand was used, with the mean grain size being 0.17 mm, with an addition of bentonite (3 %) and dextrin (0.5 %). To produce the EPC-casting-process molds, dry quartz sand with the mean grain size of 0.25 mm was used and evaporable models were made of polystyrene with the density of 20 kg/m3. The mineral-phase composition of mullite was analyzed by means of X-ray powder diffraction (an XRD-Philips PW-1710 diffractometer). DTA was performed with a Shimadzu DTA-50 apparatus. The microstructure of the samples was characterized with the scanning-electron-microscopy method (SEM) using a JEOL JSM-6390 Lv microscope. Distribution of the refractory filler and bonding agent in a lining suspension was conducted with a polarized-light optical microscope of the passing-light JENAPOL type (Carl Zeiss - Jena). The analysis of the particle size and shape factor was conducted with the PC software OZARIA 2.5. 3 RESULTS AND DISCUSSION In Figure 1 the results of the X-ray structural analysis of mullite powder are shown. The mean grain size of the refractory filler was between 35-40 pm, the grain-shape mean factor was 0.63, which means that the grains are round and suitable for the production of homogeneous linings. A DTA curve for the mullite sample is presented in Figure 2. It can be concluded that mullite has a high refractoriness and, thus, it is suitable for casting Fe-C alloys. In Figure 3, the results of the qualitative mineralo-gical analysis of the filler based on mullite are shown. The analysis shows that the mullite particles are principally of equal size and morphology, but there are also some differences in the particle size. This is favorable Figure 1: X-ray diffractogram of mullite Slika 1: Rentgenski difraktogram mulita >