Using crucibles
Crucibles of a certain composition with special coatings have been developed for optimal performance of various tasks. Traditionally, attempts have been made to find one universal type of crucibles that will be suitable for any type of furnaces. Nevertheless, this assumption has never been justified, despite the fact that under certain circumstances it could correspond to reality.
In truth, small foundries that work with bronze, brass or aluminum often perform all operations in the same crucible, while observing a certain sequence. First they melt the metal with the highest melting point, for example, nickel-plated silver or bronze, then they melt brass and, last of all, aluminum.
Most foundries, however, use different types of crucibles – each specifically for a specific metal.
With the invention of silicon-carbide crucibles on a carbon bundle in the USA in the twenties of the last century and, subsequently, in the 60s - 70s in Europe, it was assumed that this is a universal crucible. But still, the slogan of those years that, "SiC(silicon carbide) crucibles are universal, forget about crucibles on a graphite bundle" was nevertheless erroneous.
In some cases, such as melting in an induction furnace or during the production of zinc dust (powder), zinc oxide, graphite crucibles on a clay bundle hold the primacy in use.Also, it is preferable to use clay crucibles on a graphite bundle at aluminum casting enterprises, because these crucibles are cheaper, which affects the cost of production. Another reason for using such crucibles is the fact that casting in a clay crucible on a graphite bundle leaves less than a ton. n. "difficult places" than if melting took place in crucibles made of silicon carbide. For this reason, the culling of final products is reduced.
As a result, all manufacturers now offer both types of crucibles. AUG GUNDLACH manufactures and sells under the MARS CRUSIBLES brand crucibles of both types of various modifications, as well as additional accessories to them: stands, gutters, pyrometer tubes, nozzles, limiters, etc.
Induction furnaces for non-ferrous metals mainly have an average frequency (400-3000Hz). In such conditions, crucibles on a clay bundle spend less energy on heating than crucibles made of silicon carbide. However, both types of crucibles can be used in furnaces with a low frequency (50-400Hz).
Unlike the heating rules for induction furnaces, in which it is recommended to first heat up empty crucibles in the furnace, it is currently even recommended to fill a new crucible with metal, especially metal blocks arranged vertically along the walls of the crucible. This arrangement avoids the temperature difference in the center of the crucible walls.
For the production of zinc powder (dust) and zinc oxide, it is preferable to use graphite crucibles on a clay bundle, since the coating of these crucibles reacts less with zinc dust, which provides convenience when cleaning the crucible after use. Crucibles made of silicon carbide are preferred for melting and casting zinc, because they have protection against oxidation, which provides better protection against oxygen exposure at low temperatures (450-500 deg. Celsius). This also applies to operations with the most aggressive impact on the crucible, such as the processing of anodes and cathodes.
The above-mentioned coating of silicon carbide crucibles is another reason for their use when working with aluminum, both when heated with an open flame and when heated in electric furnaces. Nevertheless, with proper use and maintenance, graphite crucibles show the same high performance.
Previously, only clay graphite crucibles were used for melting brass. The main reasons for this were a much lower degree of slag formation and the convenience of cleaning the crucible. Later, special silicon carbide crucibles X, XO appeared, which can also be used for melting brass. With proper and regular care of such crucibles, the advantage of using them is their higher thermal conductivity. For melting copper at high temperatures, it is recommended to use crucibles made of silicon carbide.
Crucibles of both types can be used for melting cast iron. Given the high temperatures and increased wear of the crucibles when working with cast iron, the service life of the crucibles will be lower than when working with non-ferrous metals. Thus, the use of crucibles made of silicon carbide or on a clay bundle for melting iron is rather an exception.
A few years ago, for environmental reasons, as well as to improve the quality of products AUG. GUNDLACH decided to reconsider the use of tar and tar as a binder for crucibles. These materials have been replaced with synthetic resins. They have more stable properties, higher binding qualities and contribute to less environmental pollution.
Crucibles made of silicon carbide with a similar bundle are marked with the letters “X” or “XO” and they are accordingly suitable for use at high temperatures.
All MARS crucibles have a protective coating. For aluminum melting, the crucibles can be coated with a special additional coating. Unfortunately, there is no universal coating that will be optimal for both high and low temperatures. To choose the right crucible, it is necessary to know what working material will be used, as well as the type of furnace.
Used materials of the company Mars-tiegel