High-Temperature Furnace Element - Lab Report Example

HIGH TEMPERATURE FURNACE ELEMENT Name: Course: Instructor: Institution: City: Date: Table of Contents A Brief Description 3 Property Requirements for the Application 3 Assessment of Candidate Materials 4 Comparison of Selected Candidate Materials 6 Selected Preferred Material 7 Manufacturing Processing Route 7 Environmental Impact Assessment 8 The Material Microstructure 9 Summary of Material and Manufacturing Process 10 References 10 A Brief Description A heating element may be defined as being one that employs the mechanism of resistive heating to convert electrical power into thermal power i.e. heat. The basic functionality of a heating element involves the application of resistance to an electric current within the heating element. This process leads to the heating of that element. Heating elements are categorized into three main classes i.e. metal heating elements, ceramic heating elements, and combinational heating elements. This classification is usually based on the type of material used in the heating element. Whereas metallic heating elements are made of such metals as kanthal, nichrome, and cupronickel, ceramic heating elements are mainly made of such materials as molybdenum disilicide and PTC ceramic elements. Composite heating elements, on the other hand, encompass both metals and ceramics. Some applications of heating elements, however, require materials that not only can withstand high temperature conditions, but ones that also have premium physical and mechanical characteristics. That is where high temperature furnace elements come in. Typically, these are heating elements whose materials have superior characteristics that can withstand high temperatures. Property Requirements for the Application In order to achieve superior functionality requirements as a high temperature furnace element, it must meet certain obligatory characteristic requirements. First, high temperature furnace elements are required to exhibit high thermal conductivity properties. This property is essential in ensuring that there is thermal uniformity all through the element. Secondly, these elements are preferred to be of high mechanical durability. Ideally, this helps to ensure that the product withstands brittleness even at the most extreme of temperatures. The ultimate benefit of this property is that the element does not become prone to cracking especially when exposed to mechanical stress. Thirdly, high temperature furnace elements are required to have superior electrical conductivity so as to ensure the passage of electrical current before being converted into thermal energy. These elements should also be highly ductile and tough as well. This property helps them be easily drawn into wires especially in instances where furnaces that are wound. Additionally, ductility is also fundamental in that it allows the element to be rolled into sheets for particular furnace designs that require such shapes. Fifth, high temperature heating elements should use materials whose coefficient of thermal expansion is very low. This is because of the likely focussing problem particularly during heating. Finally, but most importantly, high temperature furnace elements should be able to remain stable even at the highest of temperatures of up to 25000C. It is an important characteristic because these elements are more often than not used under high temperatures. Assessment of Candidate Materials There is a variety of materials whose characteristics perfectly match those required by the high temperature furnace elements. Due to the aforementioned property requirements for these elements, materials primarily made of metals or ceramics can serve excellently well. One such material is nichrome, which is essentially an alloy of nickel metal and chromium. Its application in high temperature furnace elements is basically as a result of its stability at extremely high temperatures. Nichrome also offers high resistance to oxidation. Properties of this material are largely dependent on those of the constituent metals. The most standardized ratio in which these constituent metals i.e. nickel and chromium are compounded is 80% nickel and 20% chromium, though this can be varied to meet the wanted requirements. Kanthal is another commonly used material in high temperature furnace elements. Kanthal is simply an alloy of iron, chromium, and aluminium, with its chemical formula being written as FeCrAl. Just like nichrome, kanthal is well known for its ability to withstand very high temperatures. In addition, this alloy has moderate electrical resistance. Kanthal has high resistance to three main destructive processes i.e. oxidation, thermal shock, and carburization. This is an important characteristic for a material used for such functions as this. Kanthal achieves this extremely high resistance mainly as a result of its superior mechanical characteristics. This alloy also forms an oxide layer which essentially prevents the heating element from the effects of wear and tear. Molybdenum disilicide is also another candidate material for high temperature furnace elements. As a refractory ceramic, this material has characteristics whose application is majorly in high temperature furnace elements. It derives its name from the fact it is a silicide of molybdenum. One of the properties of molybdenum disilicide is its moderate mass to volume ratio. With a melting point of 2,0500C, this material is quite relevant in applications where tolerance to high temperature is critical. Similar to the other material candidates, molybdenum disilicide is also resistant to oxidation especially at high temperatures where it forms a silicon (IV) oxide layer around it. The final candidate material for high temperature furnace elements is the PTC ceramic material. It derives its name from its material characteristics i.e. Positive Thermal Coefficient (PTC) of resistance. This implies that its resistance is directly proportional to temperature so that any increase in temperature makes the material more resistant. From the assessment of the four candidate materials discussed above, it is prudent to note that a material’s resistance and heat (current) are important parameters for consideration in high temperature furnace elements. A mathematical analogy can better explain this. According to Ohm’s Law, This implies that current, I, and resistance, R, are inversely proportional, which also implies that at extremely high current, there’s negligibly small resistance and vice versa. The power generated or consumed when electric power flows is given by: In other words, the heating element dissipates power equals to the product of the square of current and resistance. The analogy, particularly in the second equation, leads to the conclusion that whereas high resistance is important for a high temperature furnace element, a balance should be considered so that the resistance is not so high as to eliminate current literally. Comparison of Selected Candidate Materials Taking into account the mathematical analogy above, it significant to point out that despite their high temperature tolerability, not every material is fit for high temperature furnace elements. It is, therefore, important to narrow down on a few materials based on the superiority of their properties. Of the four candidate materials discussed above, a comparison of the best two i.e. the two alloys – nichrome and kanthal. The first comparison is in terms of affordability. Even though nichrome and kanthal both serve a similar purpose, kanthal is relatively cheaper than its nichrome counterpart. In instances where cost is a factor worth considering in a high temperature furnace element, therefore, a mechanical engineer would prefer using kanthal to nichrome. Secondly, kanthal has far much more durability than nichrome. This is can be attributed to the fact that in comparison to kanthal, nichrome seemingly gets hotter rapidly, and as a result, wears and tears faster. On the contrary, however, kanthal withstands higher temperatures than nichrome. Thirdly, kanthal has a slightly higher resistance than nichrome. This typically means that the heat generated by nichrome is more than that generated by kanthal. Finally, kanthal heats up much slower than nichrome. In fact, this characteristic is what contributes significantly to the assertion that nichrome is less durable than kanthal. Selected Preferred Material Narrowing down even further from the two selected candidate materials i.e. nichrome and kanthal, it is only undoubtedly logical to settle on kanthal (an alloy of iron, chromium, and aluminium) as the preferred material for a high temperature furnace element. To begin with, high temperature furnace element made of kanthal heats up slowly, and as a result, it does not suffer from extreme wear and tear. In as far as affordability is concerned; kanthal is also cheaper than nichrome. For that reason, it would be preferred as well because of economic reasons. In conclusion, therefore, it suffices to settle on kanthal as the preferred material for high temperature furnace elements. Manufacturing Processing Route The manufacturing process of an alloy of iron, chromium, and aluminium – kanthal – used for high temperature furnace elements entails a number of mini-processes. The very first step is melting of the individual constituent metals separately. This is usually carried out so as to make easy the process of ‘mixing’ of these constituents while they are still in molten state. The second step in the manufacturing process is hot – rolling, a process in which the combination of the three elements is rolled at very high temperatures. This is done after the combination has been allowed to cool down and solidify. It is recommended that the temperature used here is higher than that of a kanthal alloy so as to prevent it from recrystallizing. Thirdly, acid pickling is then done. This process involves the use of an acidic solution such as hydrofluoric and nitric acids. The main purpose of acid pickling is to clean the alloy chemically with a view to removing a thin surface layer on it. In order to relieve it of internal pressures, the alloy is then annealed. Through annealing, it is first heated, before being allowed to cool systematically. Annealing makes the alloy even stronger and tougher. The final step in the manufacturing processing route of kanthal before packaging is quality control where the end product is tested for the set quality assessments. Environmental Impact Assessment The manufacturing process of kanthal as a material for high temperature furnace elements may cause some detrimental effects on the environment, especially if not handled in the right manner. The design process of the manufacturing process of kanthal is the better placed stages in which a mechanical engineer should consider assessing the possible environmental impact of the expected end product. At this stage, the likely environmental impacts of the preferred material, i.e. kanthal, should be properly analyzed. In fact, such analysis may play an important role too in the selection of the preferred material. The mining processes for the constituent elements for the manufacture of kanthal may end up harming the environment in one way or another. In order, therefore, to prevent the destruction of the ecological habitat, it would be prudent to consider such aspects as soil erosion, discharge of trace metals in large water bodies, protection of contamination of the groundwater, and forest cover. Finally, and perhaps most importantly, environmental considerations during material processing would be of key interest. Such considerations include packaging of the material, minimization of energy use during processing, and the minimization of release of waste products during the manufacturing process. The Material Microstructure The FeCrAl alloy is a three-layered structural material whose inner layer is continuously growing. Its outer layer, though, is outwardly growing, whereas the middle layer is static. The typical size of kanthal alloy is in the range of 25 mm to 48 mm, which is generally the size of a grain. For temperatures beyond 25000C the FeCrAl alloy is prone to recrystallization. The ultimate impact created is a microstructure that is long and flat. Its dimensions are in units of µms as illustrated in Fig. 1 below: Figure 1: A hot-rolled plate microstructure of kenthal Summary of Material and Manufacturing Process In summary, it is significant to point out that kanthal is just but a trademark for a conglomeration of alloys made of three elements i.e. iron, chromium, and aluminium. The symbol of this compound is given as FeCrAl. The alloy has the most applications in instances where high temperature requirements are necessary. It is also important to take note of the fact that kanthal is usually available in various forms, which include tubular, bar, ribbon, and strip. The manufacturing process of FeCrAl can be summarized as a five-step process. These steps are melting, hot-melting, acid pickling, annealing, quality control, and packaging respectively. It can be seen that the alloy is basically ready after the very first process – melting – though, the subsequent processes are essential in modifying the end product. References Bulten-Kanthal. (1972). The Kanthal super handbook: electric heating and resistance material. Hallstahammar, Sweden, Bulten-Kanthal. Carter, C. B., & Norton, M. G. (2013). Ceramic materials science and engineering. New York, NY, Springer. http://dx.doi.org/10.1007/978-1-4614-3523-5 Kaplan, R. S. (1989). Kanthal (A). Boston, MA, Harvard Business School. Motzfeldt, K. (2013). High temperature experiments in chemistry and materials science. http://catalogimages.wiley.com/images/db/jimages/9781118457696.jpg National Research Council (U.S.). (1970). High-temperature oxidation-resistant coatings; coatings for protection from oxidation of superalloys, refractory metals, and graphite. Washington, National Academy of Sciences. Woldman, N. E., & FRICK, J. P. (2000). Woldman's engineering alloys. Materials Park, ASM International. Read More