Development and Characterization of Foundry Refractory Coating And Validation Through Factorial Design of Experiment

Foundry Refractory Coatings are used to improve the surface finish of grey iron castings and reduces the post cleaning and re-work cost to eliminate surface defects. Present research was concentrated on the qualitative and quantitative analyses of commercial coatings through X-ray Diffraction and Energy Dispersive Spectroscopy, respectively. A number of coating compositions were developed by the addition of refractory fillers, binders. The effect of particle size, which is the most important parameter was validated via Design of Experiment protocol. Ultimately, three types of coating formulation were prepared and characterized using X-ray Diffraction. Fe O 2 3 and SiO were determined to the major phases in all coating samples. Particle size of 75 microns was selected as 2 optimum to produce smooth surface finish. With these refractory coatings, the Gray iron castings were produced without inducing any significant effect on the mechanical properties and microstructural features. Finally, based on the experimental results, the formulation of a refractory coating is introduced that satisfied the required performance criteria.

Two types of coatings are extensively used in furnace for 3 hours. The dried FRC powder samples foundries namely water and alcohol based. Most of were designated as B1, B2, and B3 respectively. the foundries preferred alcoholic based coatings due The structural properties of FRC samples A1, A2 to quick drying time and elimination of mold drying and A3 were studied using X'PERT PRO before pouring. The preference of using alcohol PANalytical X-ray diffractometer (XRD) equipped based coating instead of water is also due to the with Cu-Ká1 (ë = 1.540Å) operated at 40 kV, 40 mA. reaction between the moisture and chemical binder For determining the particle morphology and of the core, which may lead to reduce the strength of chemical nature of ingredients, Scanning Electron core materials under tensile and flexural loading as Microscopy (SEM) (S-3700) coupled with Energy reported elsewhere [6]. Harveth et al. [7] also Dispersive Spectroscopy (EDS) was employed to depicted the importance of spirit based and analyzed surface morphology and chemical compared with water-based coatings by comparing compositions of the samples. The variation in the the color change versus moisture content on drying.
structure of coated samples before and after heat Although the importance has given to continual treatment was also evaluated by XRD analyses. improvement to casting shop floor but development The XRD analysis were done on the powder samples in the core making, molding, pouring and cleaning containing all the constituents and mixed in the processes is also necessary. Number of desired amount. Briefly the powder samples were experiments, time, and overall development cost ground in a ball mill for 8 hours to reduce particle can be reduced by using the factorial design. Many size and for uniform mixing of constituents. The researchers applied factorial design for the average particle size was determined by sieve development of various processes in diverse analysis. applications [8][9].
For the development of coating, design of In the developing world, the foundry sector faces experiment was performed using mini tab software many challenges i.e. sustainable supply chain and to select the best combination of coating sudden change in the import polices. Therefore, the formulation. Factorial design of experiment was local development of FRC should be initiated. The used to minimize the total number of experiments to local demand for refractory coating in the foundries attain the best coating combination. Numbers of has increased up to 50% in the last decade.
features influence the process i.e. quantity of However, there is no FRC available that could meet binder, filler and particle size distribution are the intended properties i.e. low rejection of castings studied. To change the amount of each variable is due to improvement in the surface finish and tedious and time consuming so factorial design of minimization of post-casting treatments that could experiment can minimize these difficulties by save money, time and efforts. This research work optimizing all the other parameters. A total 16 aims to develop FRC at low cost that has improved combinations were tested on the basis of particle performance and properties as demanded by size, quantity of binder and fillers as given in Table  various industries.
1. Additives and suspension agents were also 2. Experimental Work: included in such combinations and on the basis of In this study three foundry refractory coating (FRC) binder type, 13 combinations were rejected. Only samples of different origins were collected from three coating formulations were selected after various industries and designated as A1, A2 and A3.
conducting design of experiment that was Fresh samples were in the form of suspensions, developed and practically capable of o which were placed in an oven at 110 C to evaporate implementation in the grey iron foundry [10]. liquid carrier. Keeping in view, the high 2.1 Practical implementation of Coating: temperature of the mold/core, the dried A1, A2 and There are many advanced technologies for metal o A3 samples were heated at 1000 C in a muffle casting but the sand casting process is one of the 2020 most widely used and conventional process. The walls was 80 and 45, respectively. advantage of using sand casting process is the low Three recipes of the FRC were prepared and applied cost of raw materials, application to wide variety of through spraying on the core surface and mold casting with respect to size, composition and the cavity of intricate 85kg casting. Grey iron scrap, possibility of recycling the molding sand. The green coke, limestone and iron-silicon alloy were molding sand was composed of silica sand (86.8%), processed in a 5 ton capacity cupola furnace. The sodium based bentonite clay (10.3%), bituminous molten grey iron was poured into the green sand o coal dust (2.4%), yellow dextrin (0.45%) and water mold at 1400 C in 19 seconds. After complete o as balance. This composition is analogous to the one solidification and cooling to 630 C, the mold was reported by Carnin et al. [11] and actually sent to the casting shakeout section to clean the applicable to wide variety of castings produced in surface. many foundries. Green strength, compactness and 3. Results and Discussion: hardness were determined according to DIN The FRC was applied at the mold/core surfaces and (Deutsches Institut für Normung) standards. The dried in air. These coatings experienced thermal green strength of molding sand was measured to be o excursion at 1400 C when molten metal was poured. 14.2 psi at 50% compaction whereas the minimum The coating filled the pores of the mold surface. The requirement for green strength and compactness is appearance of the surface became uniform and suggested as 11-15 psi and 45-52, respectively [2].
refine. The hardness measured at the bottom of mold and The X-rays Diffraction (XRD) pattern of dried FRC, patterns of these FRC reflected the presence of labeled as A1, A2 and A3 are shown in Fig. 1 [13]. originated in sample A3 (blue curve). The XRD  but only two coating samples are analyzed by EDS signal of carbon in the EDS spectra were related to to estimate the approximate chemical composition.
the graphite that was added to the FRC. The The XRD results of the FRC illustrated the addition of graphite could effectively resist the existence of almost similar phases. The EDS metal penetration and could provide good surface analysis of A1 and A2 FRC samples are presented in finish by forming a stable colloidal suspension in Fig. 3 (a) and (b) respectively. The analysis showed alcohol. Similarly, Hematite was added in the FRC the presence of oxygen and carbon elements in the to reduce the carbon defects in the grey iron casting. heat treated FRC samples. The strong signals of Fe and Si are possibly associated with the hematite The Fig. 4(a) and (b) exhibited the surface may enhance the surface finish of the mold. It has morphology of samples A1 and A2 respectively. The been suggested in the literature that the shape and grain size of the FRC was non-uniform and particle size of the filler particles in the FRC are always shape was irregular. However, the fine filler considered critical for the production of high quality particles could settle in the pores of sand grains and casting of improved surface finish [15].

Vol. XXXXVIII
The cube plot for the quality coating rating (%) was 31.5 that were of very poor quality and high cost. shown in the (Fig.5) contained eight corner points The poor quality was attributed to improper and with different combinations of binder, fillers and higher percentage of binder as sand particles may particle size. Maximum quality (%) was observed at adhere to the casting surface. Similarly, all other 95 that are considering best quality and low cost corner points had different percentages of binder, and the minimum quality (%) had been observed at filler and particle size.  The R =99.09% and adjusted R =98.29% two-way interaction between Binder and Filler as represented the percentage of variation in the well as three interactions between all factors (i.e., quality coating (%) data explained by the particle Particle size, Binder and Filler) effect on quality size, Binder and Filler in the model while Predicted coating (%). But the two-way interaction of particle 2 R value indicated that the model could predict size with Binder and Filler was significantly 96.36% future data. affecting the quality coating (%) because p-values of

Journal of the Pakistan Institute of Chemical Engineers
Vol. XXXXVIII The developed refractory coatings were shown in Both silica and alumina are highly desired (Fig. 6) casting. determined. Defect free casting also has been Practically prediction of casting defects in casting process is important factor in resource management produced by applying Mullite based refractory that could be helpful to save energy, money and coating having particle size 40-45 µm as reported by improvement in the production capacity of the Presetic et al. [15]. foundries. Comparison of the surface finish of the 3.2 Baume Test: three samples after application of refractory coating can be estimated visually as shown in Fig. 7(a), (b) Hydrometer is used to measure the viscosity of the and (c). The sand particles were adhered to the FRC samples before applying on mold and core casting surface both in C1 and C2. The presence of surfaces. The spirit-based coating was mixed sand particles on the surface of castings highlighted thoroughly before measuring the viscosity of the the ineffectiveness of the binder used in coating formulation. This may be due to the washed out of FRC slurry. The viscosity of the FRC was measured FRC during interaction with molten metal at a to be 13. o temperature approximately 1200 C. Additionally, 3.3 Effect of Coating on Microstructure, rough casting could be produced by using the coarse Mechanical Properties and Chemical particle size of the coating constituents. It can be Analysis: seen that FRC (C3) presented a defect free casting having smooth and bright surface finish compared The FRC was applied uniformly sprayed on the to other formulations. mold and core surfaces before pouring of molten 3.1 Particle size Analysis of refractory coating metal. After complete solidification, fettling and The average particle size of the FRC samples was short blasting, the universal component of tractor determined by sieve analysis. Nwaogu et al.[6] transmission case was sectioned, 200 mm UTS bar reported that the average particle size of the coating was drawn from 18 mm section thickness for UTS constituents should be less than 100 microns for testing, mechanical properties, microstructure and better surface finish. The coating constituent had chemical analysis. an average particle size of 140 mesh number that was equal to approx. 106 µm and in the bulk volume 200 mesh number which was around 75 µm was  Table 3 illustrates the chemical composition and mechanical properties of the casting that is the mechanical properties of the grey iron casting with matter of primary concern [20]. desired specifications having good surface finish. In 3.3.1 Microstructure: cast iron C, Si and P were the major elements based The microstructure of Grade 220 (Grade 14) showed on Carbon Equivalent (C.E) and played important uniform distribution of A type flakes of random role during the formation of casting. Casting shows orientation as shown in Fig. 8. Presence of A-flakes brittleness and chilling due to excess percentage of is the requirement of grey iron casting. The %P and %S respectively. The alloying composition microstructure of the cast iron was composed of 90% was controlled during casting to produce BS 1452 pearlite and 10% ferrite. Nwaogu et al.
[5] reported Grade 220 castings. It is clear in Table 3 that FRC that refractory coating had not produced any effect did not affect the chemical composition and on microstructure of casting.