Foundry Burn In More Depth
Most defects in metal casting are caused by the use of an improper molding mixture, giving rise to problems such as burn on, hot spots, and others(1).
Burn on is caused by molten metal penetrating shallowly into the sand mold, typically occurring when the mold becomes hot enough to allow partial decomposition of the binder, allowing molten metal to flow into the sand(2). When a metal is liquid for a slightly lengthened period of time, burn on can also occur. The phenomenon usually takes place in corners, next to thick casting sections and on thin cores(3). It is not known how much time is required to produce such an effect.
Another term for the most serious cases of burn on is ‘penetration’. This is where localised overheating (“hot spots”) causes a deeper flow into the mold(4). Overall though, both require machining post-casting to remove these surface defects.
Closely related to burn on is “burn in”, more correctly known as fusion. The net effect of burn in is similar to burn on, but differs in the size and distribution of the defects. A further related burn on phenomenon is the sintering of the clay and silicate components in a sand, if present, allowing the formation of iron silicates in the case of an iron/steel casting. The sintering and melting of these compounds allows molten metal to penetrate further into the sand mold(5)
Burn on and burn in are only some examples of surface defects caused by the interaction of molten metal and molding sand. Other defects include those caused by the transition of elements from molding sand to the metal and vice versa (for example silicon and phosphorus from the sand and manganese and iron from the metal) causing a chemical change at the surface(6), and potentially physical changes to the casting microstructure and thus the surface and bulk properties of the final product.
Wetting is an often cited issue in casting, and is a contributing factor in foundry burn on. The manifestations of wetting are akin to many other burn on phenomena - liquid metal adheres to sand and/or oxides present, which means that the casted product is not smooth or of even thickness upon release from the mold - it will be peppered with burrs and other defects at irregular intervals. As with other surface defects, the effects of wetting need to be removed by machining by hand or another post-casting processing manifold. The increased time and labour costs can be detrimental to a foundry’s bottom line by meaningfully decreasing production efficiencies(7).
The science behind wetting is complex and involves a detailed understanding of surface science. Briefly, however, wetting is the ability of a liquid and solid to maintain contact as a result of surface interactions. If an interaction is described as ‘strongly wetting’ then it has a good solid-liquid interaction. Conversely, should an interaction be described as ‘poorly wetting’ then the degree of solid-liquid interaction is low. Materials that can reduce the likelihood of strong wetting interactions occurring are ideally suited to situations where burn on could become an issue - it stands to reason that if there’s a poor interaction between solid and liquid (sand and/or mold and metal) then there will be fewer or even potentially zero surface defects. It should be noted that a mold that has been subject to wetting cannot be reused in its current state - a further addition to costs. A detailed discussion on wettability is beyond the scope of this article.
Coal Dust
Coal dust is the product produced when coal is finely ground. Typically, it will be of a higher quality of coal such as anthracite and not lignite. Higher quality coals have higher proportions of pure carbon, and thus burn much more cleanly without the release of toxic gases such as those associated with the burning of bituminous coal. As early as 1945, studies in Britain showed that industrial conditions where coal had been used in a foundry environment were poor for health - not least because of the evolution of toxic gases(8). The use of higher quality coals reduces the amount of harmful gases produced by virtue of having lower sulfur and bituminous materials present.
Coal dust is an inexpensive product and is used widely in the smelting and casting industries, as part of refractories, for example. It is a crucial additive to sand a greensand castings owing to its ability to reduce/prevent defects associated with binding of metals to sands.
Preventing Foundry Burn
Briefly, to prevent foundry burn; by increasing the amount of carbonaceous material in the casting mold, the amount of coke and lustrous carbon increases. Upon heating, these pyrolyse (not combust*) and provide a barrier layer of gas and a layer of thin carbon between the molten metal and the sand casting mold. Even a small layer is sufficient to prevent significant amounts of burn on by not permitting the molten metal to come in contact with the sand. It was previously thought that the formation of the gaseous envelope was the main factor limiting burn-on-type surface defects, but the realisation of a carbon layer formation has largely confined the gaseous pocket to a secondary preventative measure.
For a conventional iron casting, when a sand or greensand mold with sufficient coal dust content is used, the hydrocarbons in that coal immediately pyrolyse due to the significant heat brought about by the molten metal. A thin film of solid carbon is rapidly deposited at the liquid-sand interface, this prevents metal penetration into the sand and vice versa and affords an excellent surface finish, with no protruding metal burrs. This non-wetting behaviour is unexpected as carbon is soluble in many metals, but is explained by a solid carbon layer (on the metal) contacting a solid carbon later (on the sand).
Specifications for idealised carbon types have been proposed. Sources of coal dust that are predominantly anthracite, have a good coking capacity, have no more than 30% volatiles by weight, with lower than 0.8% by weight sulfur and have low ash contents are preferred(9). Patent literature from as early as the late 1960s suggests that higher quality grades of coal were being used from then onwards as a substitute for broad spectrum coal dust. Patent authors suggested that anthracite at a grind size of 0.3 mm in quantities of up to 3% by weight would be sufficient to replace coal dust(10). The same authors continued to claim that anthracite, when used, produced a greater proportion of lustrous carbon. This lustrous carbon is the same as the “thin film” deposited at the metal-sand/mold surface, as mentioned earlier.
Pyrolysis is not limited to carbonaceous materials in the mold. Binders can also break down in this method, and depending on the components of the binder, they may enhance or worsen the surface finish. Urethane and other organic binders will break down in a similar way to coal dust and provide no surface defects, and in certain cases, superior surface finishes. On the other hand, binders that contain some furans, sulfonic or phosphoric acids can pyrolyse into surface defect-causing materials. Phosphoric acid vapour can react with iron oxide or chromite (in the case of greensand) and form iron phosphate, which can interact with components in a ferrous metal being cast. Sulfonic acids can react with many components in the sand under high temperature conditions, eventually forming sulfonates and finally sulfides. These sulfides can cause damage to cast materials. With regards to these chemical burn on-type effects, sufficient coal dust in the sand can prevent these via the aforementioned methods.
Coal dust in a sand casting mixture is associated with pressure increases(11). Such increases are seemingly dramatic (just 5% of coal dust in a greensand mixture will result in a pressure increase twice that of greensand alone) but are well within tolerances and there is little risk to the mold or sand box. Naturally, a pressure increase is to be expected due to the evolution of gases from combustion of the coal dust, in addition to the evaporation of water from the sand. Because of its small atomic radius, the only potential issue with elevated pressure is if there is hydrogen present. Hydrogen could penetrate the metal(12).
As mentioned previously, another type of burn on is the formation of iron silicates when silica reacts with iron oxide, causing sand grains to fuse and meld into each other, depositing on the casted surface, which are difficult to remove(13). Prevention of iron oxide formation is key to alleviating this phenomenon, and is also achieved by the pyrolysis of coal dust producing a reducing atmosphere in which oxidation of iron cannot occur(14). Similar effects are experienced with lower grade chromite, which often contains small amounts of silica. Reducing atmospheres are aided by the production of hydrogen gas from the pyrolysis of coal dust, and other materials, and this can aid in preventing the formation of oxides and silicates(15).
In addition to preventing foundry burn, the addition of coal dust to a sand mixture is said to moderately increase the compressive strength of the sand, likely due to good associations forming with clay(16).
Overall, it can be said with confidence that the use of coal dust increases the overall quality of the casting by preventing burn on, burn in and other interaction processes. Additionally, it should not be forgotten that anthracite can behave as a refractory material in its own right (a material that is highly tolerant to physical or chemical change when exposed to high temperatures) and part of anthracite’s appeal in the foundry castings setting is related to this property. Anthracite as a refractory material is discussed in depth elsewhere on this website.
Operating Considerations
As with all hydrocarbons that are liable to combust, an amount of pollutants will be produced. If the coal dust used is a powdered higher grade of coal, such as anthracite, it will emit significantly fewer hazardous pollutants (17) than lower grade bituminous coal would(18). The lowest grade coals (even lower than bituminous) contain less pure carbon and more sulfur and resinous materials. Lignite, for example, has a carbon content of between 20 and 35% - rendering its use only for power generation as it is too poor a quality for virtually anything else. There is such a thing as too much coal dust - gas holes, misruns and the formation of a blue skin on the casting are all possible outcomes. In addition, high levels of coal dust in the green sand can cause decreases in permeability and enhanced moisture requirements. Typical amounts of coal dust used in sand casting molds rarely exceed 5%, as more than this reduces the permeability of the sand mixture and requires more binder content(19).
Adding to the aforementioned advantages of coal dust to prevent surface defects, coal dust increases the refractoriness of the green sand mold, and as such the mold can withstand higher temperature, with coal dust having a fusion temperature in excess of 1,600 °C.
Calcined anthracite has also been shown to have utility in anti-burn on applications as a source of carbon, where it behaves in much a similar way to conventional anthracite but is suited better to foundries operating with significantly higher casting temperatures(20). Perhaps counterintuitively, however, calcined anthracite containing fillers and moulding sand additives did not have any better stability properties compared to their non-calcined counterparts, or that of conventional lower grade coals.
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