Around the year 1900, W.C. Heraeus, a German-based firm developed the first commercial platinum furnace. In the year 1902, the company marketed a platinum ribbon-bound furnace that could reach a temperature of 1500ºC in 5 minutes, operate at 1500ºC for several hours and could attain a temperature of 1700ºC for brief time periods. For the past nearly 200 years, since that furnace was first developed, resistive electric furnaces have seen numerous advancements in insulation, controls and heating materials.
There are a wide range of materials offered that can be utilized as heating elements for electric resistance furnaces. The materials include ceramic metal-based materials, metallic alloys and carbon or graphite materials. Certain cermets can be obtained in a wire form, a cermet being a robust alloy of a heat-resistant compound. This article focuses on traditional wire metallic alloys, the commonly present ones include tungsten, iron-chrome-aluminum, nickel-chrome, molybdenum, tungsten, platinum, tantalum and platinum-rhodium alloys. These alloys can be grouped into two classes, one that are workable when oxygen is present and the other that should be provided adequate protection from oxygen. The class of alloys that need to be protected from oxygen include tantalum, tungsten and molybdenum.
When temperatures increase, the atmosphere plays an important role as materials react in different ways to different compounds. It is possible that a system that functions perfectly at a particular temperature in air may fail rapidly if used at an identical temperature but a different atmosphere. The element’s service life is also an essential service parameter. It is important to find out whether you need the element to work for some weeks, a few months or years. For any particular element, the higher the working temperature, the shorter its lifespan. In order to have a long life, the heating element must have a minimal head temperature with reference to the temperature of the furnace. This is possible when the wait loading is considerably lowered. It is essential to note that when the element loading is reduced, more elements need to be added to satisfy the heat load requirements of the furnace.
Types of Materials Used as Heating Elements
The different materials discussed below include the following:
The most convenient and economic alloy to be used here is iron-chrome aluminum alloys. These also have a minimal operating temperature in an oxidizing atmosphere. Metallic alloy materials are quite rugged when subjected to thermal or mechanical shock. Their resistance does not change with respect to service life and element temperature. Using these two factors one can obtain a product that can be easily controlled, providing a convenient and cost-effective power supply. In doing so, the overall project capital costs are considerably reduced proving that this group of materials is really worth using. A large number of these alloys are offered in strip, wire, tube and rod forms. Standard element configurations include coils in ceramic tubes or grooves, a free radiating design termed as an ROB or a sinuous loop element or as a section of a packaged system in which the alloy is either implanted or mounted on a ceramic or insulation panel on floors, roofs or walls of the furnace.
Nickel-chrome alloys are probably the oldest electrical heating materials and are used widely even now. They exhibit properties of ductility, hot strength and form stability. The three commonly used compositions utilized in heat applications include the following:
- ASTM “A” grade (80% nickel, 20% chromium)
- ASTM “C” grade (60% nickel,26% chromium, balance iron)
- ASTM “D” grade (35% nickel, 20% chromium, balance iron)
Another alloy has been recently introduced that has a mix of 70% nickel and 30% chromium. Among these alloys, the 70/30 material has the highest maximum element temperature of 1250°C in air, and a maximum chamber temperature of 1150°C. The main reason for its introduction was to resist “Green rot”. Green rot may be defined as an intergranular oxidation of chromium that occurs in other ASTM grades when utilized in either endothermic or exothermic atmospheres in a temperature range of 1500 to 1800°F.
Iron-chrome-aluminum alloys have a standard mix of 72.5% iron, 22% chrome and 5.5% aluminum. The higher grades obtained from conventional melt technologies have temperatures restricted to 1300ºC for the chamber and 1400°C for the element. Several other grades are also offered where the amount of aluminum has been reduced and the balance comprises iron. The operating temperature and resistance are high and the density is low when compared to nickel-chrome alloys. This ensures a cost-effective, long-lasting heating element. Some disadvantages include low hot strength, lower ductility and embrittlement on use.
Iron-Chrome-Aluminum PM Grades
Recently, iron-chrome-aluminum alloys are used with powder metal (PM) technology in their manufacturing process. First a high-grade iron-chrome-aluminum alloy obtained from conventional melt technology is crushed into a powder form and then subjected to compression to form a billet. The billet is formed by a hot isostatic press or, sometimes, a cold isostatic pressing operation. From this billet the final strip, wire or tube product is obtained. Although the process is expensive and complicated, the hot strength and end-use temperature show a drastic increase.
The wide variety of materials that can be utilized as heating elements for electrical resistance furnaces have been discussed in detail. The materials include ceramic metal-based materials, metallic alloys and carbon or graphite materials.
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This information has been sourced, reviewed and adapted from materials provided by Thermcraft.
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