In large-scale casting processes, different kinds of casting defects can occur in all chemically bound mold materials. Their causes lie in not only the smelting area but also the mold material and coating areas. In some cases, casting defects can be successfully avoided with small measures such as adding mold material additives, replacing the binder or coating systems, changing the casting parameters or, at a later point in time, performing additional work on the casting. One casting surface defect that can occur takes the form of white deposits or film, and removal of the defect leaves a pitted surface (Fig. 1). This has been a problem in metalcasting facilities for some years now and is extremely costly and time-consuming to resolve.
This casting defect takes the form of white deposits or film (Figs. 2 and 3a) and is the result of a chemical reaction that takes place in the edge zone between the liquid metal and the heated coating and mold material. The resulting reaction products comprise fibrous silicon-, oxygen- and carbon-containing inclusions and amorphous silicon oxide SiOx. The theory states that SiOx forms along with silicon, carbon and other elements due to the reduction of silicon dioxide SiO2. This casting defect can arise on coated and uncoated mold surfaces. A coating is a special substance that is applied onto the surface of the mold and core surface and has the effect of separating the iron from the mold material in order to create a flawless and smooth casting surface.
The resulting layers, which are chemical in origin, are brittle, porous and easy to remove. Once removed, they leave behind a rough, pitted surface (Fig. 3b). As a result, the pitted surface on the finished casting has to be filled or ground, which can be time-consuming and lead to increased costs. The casting defect also affects the structure. In the case of iron with nodular graphite (nodular casting), for example, graphite degradation to a depth of 0.5–1 mm normally can be observed. Graphite degradation describes a degraded graphite layer, in the form of a lamellar graphite seam, on the surface of the nodular casting.
This casting defect usually arises on chemically bound furan and phenolic resin mold materials, in iron with nodular graphite and in cast iron with lamellar graphite. The furan resin process is a self-curing process with organic binding agents. The furan resin mold materials are cured through the addition of para toluene sulfonic or phosphoric acids at room temperature. The binders pyrolize when the molds made from furan resin mold materials are cast. Pyrolysis refers to the thermochemical decomposition of organic compounds.
Gasification and combustion occur due to the effects of heat, without the addition of oxygen. The carbon reacts with free oxygen and forms toxic gases such as CO. The remaining carbon stays in the system and continues to undergo reaction. Here, the carbon can reduce the SiO2 content, leading to the formation of SiO. The SiO compound is stable at high temperatures and, at room temperature, can disproportionate in line with the Boudouard equilibrium of the carbon : 2SiO ↔ Si + SiO2.