Wetting: What Is It And Why Is It Such A Concern?
The modern foundry producer is interested in the production of the best quality product without compromise - wetting can be a cause of some problems in the metals casting process.
Introduction
Wetting is the phenomenon whereby a liquid is able to maintain contact with a solid surface, being brought about by intermolecular interactions. In the modern foundry, the major problem caused by wetting is in impacts on surface finish of a casted material. The result of this is rarely detrimental to the casted material, but it almost always means that extra work (and therefore time and money) is required to remove the surface defects, usually via machining by hand. Naturally, foundry owners and operators are keen to reduce the likelihood of wetting occurring, therefore minimising the chances of post-casting processing needing to be done. Surface defects typically take the form of ‘burrs’, i.e. where some molten metal has come into contact with the sand and cooled in an irregular fashion. Sand can also penetrate the metal.(1,2,3)
In the case of a liquid being present, at foundry temperatures it will boil and evolve gas. This creates space for molten metal and sand to interact, in the case of sand based metal castings. A surface interaction may be described as ‘strongly wetting’ when there is good solid-liquid interaction. ’Poorly wetting’ refers to a weak solid-liquid interaction.
Additives may be added to reduce the likelihood of wetting occurring, in addition to ensuring adequate and appropriate moisture control. Surface defects are much more likely to occur when there are strong wetting interactions (a detailed discussion of surface chemistry is beyond the scope of this article), which predominantly arise from Van der Waal’s intermolecular forces and hydrogen bonding.
Strategies To Negate Or Avoid Wetting
Prevention is better than cure. In virtually all cases, foundrymen will incorporate material into their casting moulds to negate the impact of wetting, or better still prevent it outright. These additives are often organic materials, with various starchy plant residues, oils and noxious organic chemicals finding broad uses over the years - all of which were examples of carbonaceous materials. Virtually any carbon rich material can be used providing it will incorporate well through the sane. One of the stand out additives to prevent wetting, however, is coal dust/anthracite. Being relatively cheap, easy to work with and completely combustible make it an attractive option for a wide variety of casting types.
Simply put, increasing the amount of carbonaceous material in a sand mould will result in more pyrolysis of that material. Pyrolysis is incomplete combustion, i.e. combustion in the absence of oxygen. For organic materials, this means the production of carbon (which tends to deposit itself as a layer on both the metal and sand) and hydrogen (which acts as a gaseous barrier to metal and/or sand penetration, and has other impacts on oxide formation, see later). Compounds well suited to act as this carbonaceous material are those that have a high coking capacity, a relatively low component of volatile organics (no more than 30% by weight), low ash content and a very low sulfur content (no more than 0.8% by weight). Grind size of the carbonaceous additive must be suitable for full incorporation through the sand mould, with early reports suggesting a particle size of no more than 0.3 mm(4).
As wetting causes the formation of small channels in the sand, molen metal can seep into it and this is the origin of many surface defects, such as in ‘burn on’ and ‘burn in’ via surface changes at the casting interface, leading to uneven casted surfaces. Effects as simple as molten metal adhering to the sand are common signs that wetting has occurred. Silicon and phosphorus in the sand can exchange with iron and manganese in the molten metal which can cause a fundamental material and chemical change - wholly preventable if wetting is prevented by the addition of a carefully selected carbonaceous material(5).
Fundamentally, wetting is a common issue in metal casting, but is also a preventable one. It can be as simple as incorporating a small amount of a material such as coal dust (anthracite) into the sand, which pyrolysis cleanly, and will prevent wetting and the subsequent associated surface defect phenomena, preventing the need for post-casting treatment by hand(6).
Coal Dust
Coal dust is powdered anthracite. It is a high quality coal product milled to a fine specification, being low in volatile organic compounds, low in ash and low in sulfur - in quantities consistently below those specified above. Anthracite is the highest quality coal, it is not bituminous like lignite, and contains the highest proportion of pure carbon, meaning that when combusted or pyrolysed, fewer hazardous pollutants are emitted compared to lower grade coals(7,8).
It is perhaps completely logical that coal dust replaced classical carbonaceous materials such as bituminous coal, as performance would be expected to be the same. It outperforms it. Due to the evolution of toxic gases such as benzene, toluene and xylene from poor quality coals(9), local atmospheric conditions around foundries were found to be poor in the 1940s. Higher quality coals such as anthracite dramatically reduce such gas evolution. It has been reported that anthracite produces more of the protective thin film carbon per unit mass than other types of coal - to be expected as there are less contaminants, therefore more carbon.
Despite the higher quality, anthracite remains an inexpensive material and finds broad use in a variety of settings. Its clean pyrolysis profile makes it the ideal choice (and heavily favoured by industry) for green sand castings, where it is responsible for fewer wetting related incidences.
Noteworthy too is the fact that dried, small grain size and therefore free flowing coal dust will mix easily and disperse evenly throughout the sand. More resinous or bituminous carbon sources such as lignite will not disperse as evenly because of their ‘sticky’ nature leading to clumping, therefore running the risk of having areas in the sand close to the molten metal interface with no carbon material at all - therefore offering zero protection and opening the door to wetting phenomena and the subsequent surface defects it will cause.
Preventing Foundry Burn
Highly related to wetting is foundry burn, which manifests itself in a similar way and requires virtually identical post-casting processing to remove the effects. Foundry burn is fairly common in iron and steel production - it occurs when iron silicates form on the surface of the metal being casted, when silica (from the sand mould) and iron oxide (from the iron) react. This causes grains of sand to fuse and deposit on the casted surface, being difficult to remove and requiring machining, akin to wetting(10). As such, foundrymen desire to prevent the formation of iron oxide in the first instance. Just like in wetting prevention, pyrolysis of anthracite/coal dust produces a highly effective reducing atmosphere which inhibits completely the oxidation of iron, therefore preventing the formation of iron oxide(11). This principle can be applied too in greensand castings, where the chromite refractory often contains small to moderate amounts of silica. Again, the hydrogen gas production resulting from pyrolysis gives rise to the reducing atmosphere and so prevents oxide formation(12). Further protection against burn on is provided as the pyrolysis process causes a thin film of pure carbon to be deposited at the metal surface, which prevents metal penetration into the sand. The reverse is also true; sand cannot penetrate the metal. Broadly speaking, this can be thought of as a non-wetting-like behaviour.
Safety
As mentioned, anthracite is non-toxic. Pressure build up is the only concern, as the pyrolysis of the coal dust causes expulsion of gas(13) with a greensand mixture containing only 5% of coal dust can lead to pressures double that of solely greensand. Some penetration of the metal is to be expected by evolved hydrogen(14), but this will be negated somewhat by the thin carbon layer formed (see earlier). Overall, the pressure build up and hydrogen evolution are well within tolerances and design capability of modern sand-based casting setups.
Summary
- Wetting is a phenomenon whereby a liquid - usually water - is able to maintain contact with a solid
- In the case of foundry work and casting, wetting directly causes the formation of surface defects, which later have to be removed manually, adding cost and time
- Preventing wetting is usually done via the addition of an organic material to the casting mould
- Coal dust (anthracite) is the leading choice for wetting prevention owing to its excellent performance, ease of handling and non-toxic nature - all while being very cost effective
- The incomplete combustion of the coal dust (pyrolysis) produces a reducing environment which prevents certain types of surface defects, while simultaneously thin layers of carbon are formed which prevent other defect types
- Anthracite is non-toxic and will not result in the evolution of toxic gases
- Foundry burn is a related phenomenon, and can also be largely prevented through the use of coal dust
References
1 A. Josan and C. P. Bretotean, Using special additions to preparation of the moulding mixture for casting steel parts of drive wheel type, in: International Conference on Applied Sciences 2014 (ICAS2014), Hunedoara, Romania, 2014
2 B. E. Brooks and C. Beckermann, Production of Burn-on and Mold Penetration in Steel Casting using Simulation, in: 60th SFSA Technical and Operating Conference, Chicago, 2006
3 Analysis of Casting Defects Committee, Analysis of Casting Defects, American Foundrymen’s Society, Des Plaines, United States
4 US Patent US3666706A, 1969
5 M. Holtzer et al., Microstructure and Properties of Ductile Iron and Compacted Graphite Iron Castings, Springer, Cambridge, 2015
6 B. Drevet (ed) Wettability at High Temperatures; Pergamon Materials Series Volume 3, Elsevier, Amsterdam, 1999
7 G. Thiel and S. R. Giese, Am. Foundry Soc. Trans., 2005, 113, 471
8 J. Wang and F. S. Cannon, Study of pyrolysis of carbonaceous additives in green sand foundries, in Seattle: The International Carbon Conference, Seattle, 2007
9 G. F. Keatinge and N. M. Potter, Br. J. Ind. Med., 1945, 2, 125
10 A. Petro et al., Am. Foundry Soc. Trans., 1980, 88, 683
11 H. W. Duetert et al., Am. Foundry Soc. Trans., 1970, 78, 145
12 D. T. Peterson et al., Am. Foundry Soc. Trans., 1980, 88, 503
13 J. Mocek and J. Samsonowicz, Arch. Found. Eng., 2011, 11, 87
14 A. Campbell, Complete Casting Handbook (2nd ed.), Butterworth Heinemann, London, 2015
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