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Development of Grinding Media Balls Usin

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    1 The Third International Conference on Structure, Processing and Properties of Materials 2010, SPPM 2010 24- 26 February 2010, Dhaka, Bangladesh, SPPM 2010 A11 1. INTRODUCTION The technological development of materials is essential in the manufacturing of equipment operating in extreme conditions by incorporating more quality, enabling a longer service life, and lower production costs [1, 2]. In many industrial applications coarse ore is ground in rod or ball mills before being mixed with a liquid to form slurries for economical transfer between processing stages. In those industries a hard and wear resistant material is used to make balls in the media. They are called grinding media ball. The slurries generally contain a high proportion of abrasive particles of varying sizes, due to the nature of the crushing process, creating an erosive environment. In order to minimize wear in critical areas, processing components used for the transfer of slurries are either cast or weld overlaid with wear resistant materials. Very high abrasive wear resistance combined with relatively low production costs make these alloys particularly attractive for applications where grinding, milling and pumping equipment is used to process materials such as ore, coal, and gravel [3–5]. Another important application of these materials is the manufacture of working rolls for hot strip mills [6, 7]. The development of new methods and thermal treatments for cast iron alloys allow new materials to perform better mechanical properties and more resistance to corrosion when in several aggressive industry environments [8, 9]. White chrome cast irons are currently leading the several fields of applications, being mainly employed in mechanical parts submitted to severe wearing. In addition, they are used when high corrosion resistance is required. In the evaluation of new alloys, designed to operate in aggressive media, it became essential to know the transformations undergone during thermal treatments and their influence on the expected mechanical and corrosion properties [10]. High chromium white irons are ferrous based with the main alloying additions being 11-35 wt% chromium and 1.8-7.5 wt% carbon. As we know wear resistance of these alloys stems from the distribution of hard carbides, which form in-situ on cooling from the melt of a casting or molten weld deposit, in a softer ductile matrix. Their as-cast microstructures consist of primary austenite dendrites with eutectic austenite (partially transformed to martensite) and eutectic M 7 C 3  in the interdendritic regions. The chemical composition of the alloys can be varied to produce different proportions of carbides, usually expressed as carbide volume fraction (CVF). In the as-cast condition, the austenitic Cr irons possess but limited, abrasion resistance and fracture toughness. Improvements in abrasion resistance and impact toughness can be achieved by suitable heat treatments to produce controlled martensitic matrix structures, and also by additional alloying to provide harder carbides. In this present work an attempt has been made to develop grinding media balls with the locally available raw materials and techniques. Initially different types of grinding balls, imported from different sources were collected. Their chemical analysis, hardness, microstructure were observed. From these data a composition were chosen to develop the ultimate raw material. Finally the newly developed material was tested for hardness, wear resistance and chemical analysis, whether they match up with imported balls or not. Abstract Almost all cement factories of Bangladesh currently use imported grinding balls and lining materials. This requires time; hassle and foreign currency. In the preliminary stage of this research, some foreign made grinding media balls are collected from different sources. The characterization of these imported grinding balls was done by microstructurally, chemically and mechanically. From compiling all these results, the ultimate compositions of the grinding ball that were to be produced from locally available resources were determined. In this case, high chromium based cast iron wereidentified as a potential material for making the grinding media balls. High chromium cast irons (HCCIs) are excellent wear-resistant materials which are a widely used tool in mineral processing, coal, ceramic and cement industries. The exceptional wear resistance of HCCIs exerts from the high volume fraction of hard chromium carbides. After charge calculation, the commercially available pig iron and ferrochrome were melted in an induction furnace at 1600 o C and the melt was poured in a mould prepared by CO 2  process. Finally the as-cast samples were tested for hardness, impact strength, wear resistance and chemical analysis to verify whether they match up with the imported balls or not. In summary, the results of the developed balls were found to be almost satisfactory with respect of hardness, microstructural and wear rate in comparison to the imported ones. Keywords: Municipal Solid Waste, Incinerator Bottom Ash, Characterization. Development of Grinding Media Balls Using Locally Available Materials T Rahman  1 , A Sharif  1  and K M Shorowordi   2 Department of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh. Institute   of Appropriate Technology, Bangladesh University of Engineering and Technology (BUET), Dhaka-1000, Bangladesh    2 2. EXPERIMENTAL PROCEDURE The research was started from collecting foreign made grinding media ball and identifying the chemical compositions and properties of these imported grinding balls. By compiling all these results the ultimate composition of the grinding ball was formulated. After formulating the chemical composition of the ball, the production route was determined according to the composition and properties required. For melting, a small induction furnace was used. The melt was cast into the desired shapes. The chemical composition was determined by wet chemical analysis. The microstructures were investigated of the prepared samples using standard metallographic technique. The hardness measurement was carried out by using digital Rockwell hardness tester. The wet wear test of grinding media balls was done by using laboratory scale pot mill. 3. RESULTS AND DISCUSSION 3.1 Commercially available media ball collection and characterization For the purpose of developing locally produced grinding media ball two types of commercially available imported balls were collected. The following tests were carried out to characterize them. 3.1.1 Micro study 3.1.2 Chemical analysis 3.1.3 Hardness test 3.1.1 Micro study The micro structure of collected commercially available grinding media balls are shown below.  Figure 1 : Microstructure of foreign -1 (left) and foreign- 2 (right) grinding media ball, 400×, (Etched with 5% Nital) As we can the structures are hypereutectic in category. The carbide volume fraction is lower in the Foreign- 2 ball then the Foreign – 1. So the Cr% is also expected to be lower. 3.1.2 Chemical analysis The chemical composition is given below Table 1: Chemical composition Grinding media ball Grinding media ball %C % Cr % Mn % Si Foreign -1 2.3 26.877 0.3 1.904 Foreign- 2 2.6 11 1.02 - 3.1.3 Hardness The hardness is given below. Table 2 : Hardness of grinding media ball Grinding media ball Average hardness( RC scale) Foreign -1 58.925 Foreign - 2 61 3.2 Development 3.2.1 Composition determination From the chemical analysis of the two main types of industrially used balls, for simplicity a composition with only two elemental variables was formulated for the production of grinding media ball. This is as follows C – 2.3%, Cr- 26%    3 3.2.2 Raw materials characterization Casting technique was chosen as the production route with the desired composition. Samples for pot mill test, hardness test and micro study were made. Locally available pig iron and ferrochrome were used for casting. Composition of the ferrochrome is collected from the vendor. The chemical composition of pig iron was analyzed by wet chemical method and the compositions of the two raw materials are given below. Pig iron - C – 3.85%, Cr – traces Ferrochrome - C – 0.1%, Cr – 60% (Composition is supplied by the vendor) 3.2.3 Pattern and Mould design For casting, the patterns were made first. Then the whole casting system (mould) was designed. In this case CO 2 process was used to produce the mould. In the figure below, the whole unit for mould making and also the patterns are shown. Figure 2 : Schematic diagram of the casting unit and the patterns  3.2.4 Charge calculation for casting The calculation for the casting is given below. Target Composition: C – 2.3%, Cr- 26% and the total casting wt. = 4 kg C% = 2.3 %, carbon wt. = 2.3×4/100= 0.092 kg Cr % = 26%, Chromium wt = 26× 4/100= 1.04 kg The balance is done mostly by iron. Now for 1.04 kg Cr Ferrochrome is needed = 100×1.04/60 = 1.73 kg So, 1.73 kg Ferrochrome gives = 0.1×1.73/100 = 0.00173 kg Carbon The rest= (0 .092 – 0.00173) = 0.09027 kg which has to be supplied by pig iron. So, pig iron needed = 100×0.09027/3.85 = 2.34kg So, finally for 4kg casting the raw materials were Pig iron = 2.3 kg, Ferro Chrome = 1.7kg 3.2.5 Casting For casting an induction furnace was used. Pig iron bar was cut to suitable pieces and Ferrochrome was ground finely. These were taken in a crucible to melt in the induction furnace. The temperature was about 1600°C. The mould was previously prepared by CO 2 process. The liquid metal was poured in the mould and set for some time to solidify and cool down. After solidification the parts were taken out, sand was washed, samples were separated from the excess parts and finally they were taken to the grinding machine. For pot mill test the sample was prepared as a spherical ball of 30mm dia and for hardness sample, 10mm×10mm×55mm bars were made. The casting unit Patterns    4 Figure 3 : Samples of different shapes, impact bar (left), wear pin (middle) and pot mill ball (right) 3.3 Characterization of the developed Sample The following tests were carried out to characterize the newly developed material. 3.3.1 Chemical analysis 3.3.2 Micro study 3.3.3 Hardness test 3.3.4 Pot mill test 3.3.1 Chemical analysis The target composition of the casting was 26% chromium and 2.3% carbon. The avarage compositions of the cast samples was shown below. Table 3 : Chmemical composition of the cast product % C % Cr % Si 4.789   23.23   1.9  The Si and Cr content was found to be closed to the desired one. As for carbon, it was assumed that the additional C was picked up from the ferrochrome that was commercially procured. Although as per vendor the C content in the ferrochrome supposed to be around 0.1%. 3.3.2 Microstructure Analysis The following microstructures were observed in 23.23% Cr and 4.789% C casting. 100× 400X  Figure 4 : Microstructure of the developed sample, etched with 5% Nital   Here we can see that the microstructure is hypereutectic type with iron-carbide phase (white phase) and pearlite (dark phase) with other carbide precipitates in the pearlite.

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