Mold coating

An improvement in the process for casting metal in sulphur-containing molds is disclosed. The mold is washed before casting with a compound, preferably barium dioxide. The compound reacts with gaseous sulphur compounds produced by the mold, thus lowering the diffusion of sulphur into the casting, and thereby minimizing graphite deterioration at and near the surface of the casting.

My invention relates to a mold coating such as that conventionally used on 
iron and steel molds. More particularly, it relates to a coating or mold 
wash used on foundry molds made from sands and binders that contain 
sulphur bearing materials such as toluene sulphonic acid or benzine 
sulphonic acid catalysts. 
It is by now well know by those skilled in the art that the structure of a 
gray iron casting at the surface is often quite different from the 
structure at some point away from the casting surface. This difference in 
structure is due to the influence of the mold itself. Thus, in gray iron 
castings it has become recognized that a ferritic surface layer often will 
be found in a casting that is normally pearlitic. 
In nodular iron castings, it is common to find a layer of flake graphite at 
the surface of a casting although the structure contains nodular graphite 
away from the surface. Such a flake graphite surface layer may be 0.025" 
or more deep and is a matter of concern on certain castings where complete 
structural integrity is important. Where the casting is made in a molding 
material containing sulphur, the problem of graphite deterioration at the 
casting surface is especially severe. In heavy section casting in 
particular, this graphite deterioration may lead to scrapping of the 
casting due to low impact strength or leakage under fluid pressure. 
Molding material containing sulphur may be molding sand containing coal 
dust or pitch, but in most cases, as in no-bake or chemically setting 
molding materials, the sulphur is present due to the use of sulphonic acid 
catalysts, such as benzene-sulphonic acid or toluene sulphonic acid. 
It is well known that the action of heat on molds of this kind causes the 
evolution of sulphur bearing gases such as sulphur dioxide and hydrogen 
sulphide. These gases pollute the atmosphere and evidently combine with 
molten metal, especially where the metal contacts the sand. In a material 
such as nodular iron, the sulphur gases increase the local sulphur content 
of the metal to the point where nodular graphite is converted into flake 
graphite. 
It has been reasoned that the application of a non-sulphur containing 
refractory wash on the mold would prevent direct contact of the metal and 
the sand mold and that this would alleviate the sulphur pick-up problem. 
Unfortunately, this is not so, probably because the sulphur from mold to 
metal transfer mechanism is gaseous in nature. Thus, because all washes 
are permeable, gases such as sulphur dioxide will contact the metal 
surface even though the metal is not in direct contact with the mold 
itself. 
My invention is based on the discovery that certain ingredients applied to 
the mold surface underneath the refractory wash, or incorporated in the 
wash itself, can act as a barrier to trap sulphur bearing gases such as 
sulphur dioxide. This prevents direct sulphur absorption by the molten 
metal and, therefore, prevents structural deterioration due to this 
sulphur. 
I have found that the salts of barium, and especially barium chloride and 
barium dioxide, are effective in acting as barriers to prevent the 
migration of sulphur from mold to metal. 
It is only natural that some of these barium salts may not be compatible 
with the other properties required in a mold wash. Also, they may be in 
themselves toxic in nature as, for example, the chlorides, making their 
general use for this purpose less desirable. I have found that the double 
oxides of barium are advantageous as sulphur migration barriers without 
having any noticeably adverse effect on the refractory wash material or on 
the surface properties of the mold. 
The particular effectiveness of barium dioxide is perhaps due to the strong 
tendency to form barium sulphate when exposed to sulphur bearing gases. 
This barium sulphate, which presumably is formed, is reasonably stable 
under the action of the heat. 
In accordance with the foregoing, one object of this invention is to 
produce a coating that will negate the harmful effects of sulphur 
containing gases derived from the action of heat from the metal on the 
molds or cores that contain sulphur. 
Another object is to reduce surface defects on nodular irons and other cast 
irons that are affected by sulphur pick-up. 
Another object is to reduce or eliminate the surface deterioration of 
nodular graphite castings or the formation of flake or other forms of 
graphite at the surface of nodular graphite castings. 
Another object is to reduce the occurrence of dross defects in nodular iron 
castings or other ferrous castings which are prone to the occurrence of 
sulphide - silicate drosses. 
Still other objects will be apparent from the specification and the 
drawings in which FIGS. 1-5 illustrate the effects of surface 
contamination by sulphur and are described in detail in the specification. 
The process of my invention is best illustrated by means of example. 
I prepared a test mold made of sulphur-free sodium silicate and containing 
two rectangular internal cores. This mold was filled with nodular iron and 
after it had solidified and cooled, the casting was sectioned and examined 
microstructurally at and away from the surface adjacent to the cores. Such 
a test set-up allowed the use of different core materials as well as 
different refractory coatings applied to the surface of the cores.

I have found, also, that the barium compound may be used quite effectively 
by first washing the mold or core surface with a 10-40% aqueous solution 
and drying this, and then applying a conventional mold wash such as zircon 
wash on top of this barium compound wash. 
Alternatively, the barium wash may be applied after the conventional wash. 
The use of a double wash procedure, such as this, obviates the need for 
mixing the barium oxide in the wash itself and places the barium content 
where it will do the most good. By doing this, the barium content may be 
confined to critical castings where surface deterioration of graphite must 
be avoided. 
I have found also that the application of barium containing washes or 
underwashes to molds and cores decreases substantially the amount of 
sulphur dioxide that is formed. Sulphur dioxide is an oxidizing gas that 
can combine with magnesium in nodular iron to form drosses or slaggy 
materials. The use of these barium washes, therefore, reduces the 
likelihood of forming dross on the surface or near the surface of nodular 
iron castings. 
In the case of steel castings, I find that the use of refractory coatings 
containing barium dioxide reduces and even eliminates the pick-up of 
sulphur in the casting. This is an important feature where such steel 
castings carry a low sulphur requirement for impact strength at sub-zero 
temperatures, as would be the case for armor castings and other 
transmotive castings and castings used in the petroleum or nuclear 
industries. 
I have tried other alkaline earth materials, such as barium oxide, calcium 
oxide, calcium chloride, strontium chloride and magnesium chloride, as 
sulphur retardant materials and find them to have some limited beneficial 
effect on reducing sulphur contamination of the metal. In general, I find 
that barium, and especially barium dioxide, is particularly effective and 
this is, therefore, the preferred material of my invention. 
I believe that the addition of sulphur retardant chemicals to the mold 
washes to prevent migration of sulphur from the mold to the metal is a 
completely novel concept and it is entirely possible that many chemicals 
other than barium and alkaline earth components could be used for this 
purpose. In fact, it may be expected that any compound capable of reacting 
with sulphur-bearing gases to form a solid sulphur compound, which 
diffuses into the interior of the metal only a very limited extent, will 
be useful in this process. 
I have described this invention with a certain degree of particularity, but 
it is understood that modifications and variations may be resorted to 
without departing from the spirit and scope of the invention. Such 
variations and modifications apparent to those skilled in the art are 
considered to be within the purview and scope of the invention and 
appended claims.