Coating composition for the production of mold coatings on lost molds or cores for iron and steel casting

Description

[0001] The present invention relates to a sizing composition for the production of mold coatings on lost molds or on cores for iron and steel casting, the use of an alkali metal or alkaline earth metal salt of carbonic acid and / or diphosphoric acid for the production of mold coatings on lost molds or on cores for iron and steel casting, a method for producing a casting, and a mold made from a chemically bonded base material for producing a casting.

[0002] Most products of the iron and steel industry, as well as the non-ferrous metals industry, undergo casting processes for their initial shaping. In this process, molten materials, ferrous or non-ferrous metals, are transformed into shaped objects with specific properties. To form these castings, sometimes very complex molds must first be created to hold the molten metal. These molds are divided into expendable molds, which are destroyed after each casting, and permanent molds, which can produce a large number of castings. Expensive molds are usually made of a refractory, granular molding material that is hardened with a curable binder.

[0003] Molds are negatives; they contain the cavity to be poured, which will produce the casting. The internal contours of the future casting are formed by cores. During mold making, the cavity is formed into the molding material using a model of the casting. The internal contours are represented by cores, which are formed in a separate core box.

[0004] Both organic and inorganic binders can be used to produce molds, and these can be cured using either cold or hot processes. Cold processes are those in which curing occurs essentially at room temperature without heating the molding material mixture. Curing in these processes usually occurs through a chemical reaction, which can be triggered, for example, by passing a gaseous catalyst through the molding material mixture to be cured, or by adding a liquid catalyst to the mixture. In hot processes, the molding material mixture is heated to a sufficiently high temperature after shaping, for example, to drive off the solvent contained in the binder or to initiate a chemical reaction that cures the binder through cross-linking.

[0005] In the production of molds for large castings, such as engine blocks for marine diesel engines or large machine parts like rotor hubs for wind turbines, so-called "no-bake" binders are typically used. In the "no-bake" process, the refractory molding material (e.g., sand) is often first coated with a catalyst (hardener), then the binder is added and evenly distributed over the already catalyst-coated granules of the refractory molding material by mixing. This process frequently utilizes continuous mixers. The resulting molding material mixture can then be formed into a molded part. Because the binder and catalyst are evenly distributed throughout the molding material mixture, even large molded parts cure largely uniformly.

[0006] Spheroidal graphite cast iron (SGI) is an iron-carbon alloy in which the carbon is predominantly present in spheroidal (globular) form. SGI possesses steel-like material properties. The globular form of the graphite results in high strength combined with excellent permanent ductility (elongation). Low pearlite content improves machinability and extends tool life. SGI is used, for example, in the automotive industry, mechanical engineering, shipbuilding, pressure vessels, and wind power. SGI grades are described in DIN EN 1563, Foundry - Spheroidal graphite cast iron.

[0007] Lamellar graphite cast iron (GJL) also possesses excellent practical properties and, in many cases, presents similar challenges to skilled workers as GJS. The essential differences between GJL and GJS that are relevant to practical application are well known to those skilled in the art.

[0008] In GJS and GJL castings, casting defects known as "pitted surfaces" occur.

[0009] These defects are widespread, pockmarked roughenings and depressions on the casting surface or in the casting skin. They are covered with a white to slightly bluish coating. Therefore, the defect is also referred to as "white coating." The coating consists primarily of (fibrous) silicon oxides. When the castings are blasted, the coating is removed, leaving only the pitted surface.

[0010] The defect occurs in GJS and GJL castings produced using molds and cores made from a chemically bonded molding material, which in turn consists of quartz sand and an acid-cured binder. The defect is particularly prevalent in acid-cured furan resin molding materials and other acid-cured, cold-curing processes, such as the phenolic resin process. The highest susceptibility to the defect was observed in regenerated furan resin sands with a loss on ignition between 3 and 4.5%.

[0011] The defect also occurs when using cores and molds produced using the Croning process, as well as the clay-bonded molding material process.

[0012] A detailed description of the error pattern can be found in the following literature: 1) M. Schrod, H.J. Wojtas, Surface defects especially in GJS, 7th Molding Materials Days, Duisburg, February 2008 2) HG. Levelink, FPMA Julien, Properties of regenerated furan resin sand, Giesserei 68 (1981) 340 3) S. Hasse, Casting and structural defects, Schiele & Schön, Berlin, 2nd edition, 2003, 343

[0013] Document DE 20 2015 105 368 U1 discloses the use of a basic composition comprising a basic component consisting of one or more bases and a carrier liquid for the basic component as an infiltration agent for the molding material of a casting mold comprising quartz sand and an acid-cured binder, and for suppressing the formation of pitted surfaces during the casting process.

[0014] WO 2004 / 071738 A1 discloses a coating composition comprising a water-soluble salt as a filler. It also describes the use of this coating composition as a sizing for salt cores for use in foundry technology.

[0015] Document WO 2009 / 007093 A2 discloses a sizing composition comprising ceramic materials and at least one binder selected from the group consisting of water glass-based binders and refractory binders in combination with fluoride-containing components. The water glass used is produced by a reaction of carbonates with quartz sand, whereby carbon dioxide is released and the carbonates are decomposed during the reaction.

[0016] DD 63 853 A1 discloses a coating for permanent molds for casting light and non-ferrous metals. The described coating contains an alkali and / or alkaline earth salt, preferably sodium carbonate, sodium sulfate, or magnesium sulfate, which is infusible and non-decomposing at casting temperature.

[0017] EP 2 853 320 A1 describes a mold and core made of coated molding sand for metal casting. A first layer is applied to the surface of the molding sand grains; this first layer is hardened and consists of water glass and / or phosphate glass. Phosphate glasses are glasses that predominantly contain phosphorus pentoxide as a glass former.

[0018] The white deposits occur predominantly on thick-walled parts, i.e., on medium to heavy castings. Smaller, compact castings with a high modulus are also affected. On the casting, the defect usually appears in thermally highly stressed zones, such as radii, but can also spread from there over larger areas. Below the surface of the affected zones, the graphite is partially degenerated.

[0019] This casting defect leads to increased rework in the foundry and can currently only be countered by increasing the allowance on the affected surfaces. In extreme cases, pitted surfaces lead to the rejection of the casting.

[0020] Due to the roughening and indentations on the casting surface, the affected areas must first be painstakingly ground before they are subjected to ultrasonic testing or crack testing for quality control of the casting.

[0021] In the process for manufacturing GJS and GJL castings using cold-curing molding materials with expendable molds, there is a need for a consistent process that enables the reproducible and cost-effective production of castings with good surface finishes. Therefore, solutions are required that prevent pitting even in specific areas of the casting surface.

[0022] In S. Hasse's "Gussing and Structural Defects," Schiele & Schön, Berlin, 2nd edition, 2003, p. 343, several measures are already described that can at least reduce casting defects. However, none of the described measures are yet satisfactory, as they lead, for example, to an increase in sand waste, thus increasing the cost of the casting process and are therefore either not economically viable or technically difficult to implement.

[0023] In "Contribution to the Formation Mechanism of the Casting Surface Defect 'White Coating' and Development of Proposed Solutions for its Prevention, Dissertation E. Potaturina, TU Bergakademie Freiberg, March 2014," penetrating coatings are described. These penetrating coatings contain manganese(IV) oxide (pyrolusite). Manganese(IV) oxide (pyrolusite) is insoluble in water and alcohols and is amphoteric. The use of impregnating coatings with manganese dioxide has the disadvantage that the manganese dioxide reduces the refractoriness of the molding material. There is a risk of casting defects forming. Furthermore, as described by E. Potaturina, these penetrating coatings can, at best, only reduce the casting defect "pitted surfaces."

[0024] Penetrating coatings are typically used in casting processes; these coatings penetrate the molding material and fill the pores with their refractory components. They are particularly suitable for preventing casting defects such as penetration and erosion. These coatings regularly contain significant amounts of refractory materials and may also include inorganic or organic binders. Commercially available coatings typically penetrate the molding material upon application.

[0025] A primary objective of the present invention was to find a way to suppress or significantly reduce the formation of pitted surfaces (white deposits) during the casting process. This should, for example, make it possible to manufacture GJS and GJL castings with standard allowances.

[0026] An important aspect of the present invention was that no toxic chemicals are used to solve the primary problem and that the emission of toxic compounds during casting is minimized or completely avoided.

[0027] Ideally, it should remain possible to use the processes and resources typically available in the foundry for core and mold production. It should also be preferable to treat only the critical areas of the molds and cores for the production of GJS and GJL castings; this would reduce production costs. Furthermore, the process should minimize the impact on the molding material cycle to avoid costly compensatory measures or the addition of new sand. It should also be possible to avoid or achieve at least one, preferably several or all, of the following points: Avoidance of scrap, avoidance of rework, avoidance of increased machining allowances, achievement of required surface finishes, avoidance of sintering of the casting surfaces, reduction of testing effort (e.g., in ultrasonic testing / crack testing), improvement of the mechanical properties of the finished casting.

[0028] This problem is surprisingly solved by a sizing composition for producing mold coatings on lost molds or on cores for iron and steel casting according to claim 5.

[0029] Surprisingly, our own investigations have shown that alkali metal or alkaline earth metal salts of carbonic acid and alkali metal or alkaline earth metal salts of diphosphoric acid, either individually or in combination, can significantly reduce or completely prevent the casting defect "white coating". Furthermore, the alkali metal or alkaline earth metal salts of carbonic acid and the alkali metal or alkaline earth metal salts of diphosphoric acid are toxicologically harmless.

[0030] For the purposes of this invention, a refractory material is understood to be a material that is resistant to temperatures typically used in iron and steel casting between 1300 and 1600 °C, i.e., that retains its chemical composition even under oxidizing conditions prevalent in the casting industry and preferably also retains its physical properties.

[0031] In a particularly preferred embodiment of the sizing composition according to the invention, it is a covering sizing and not an infiltration sizing.

[0032] A coating layer according to the invention is a coating layer that penetrates less than 2 mm into the molding material or preferably does not penetrate the molding material at all and typically comes into direct contact with the melt. After application to the molding material, coating layers form a covering layer on the molding material.

[0033] Infiltration coating as defined in the invention is a coating that penetrates at least 2-4 mm into the molding material.

[0034] According to the invention, preferred sizing compositions are those wherein the alkali metal or alkaline earth metal salt of carbonic acid is a primary or secondary carbonate and / or the alkali metal or alkaline earth metal salt of diphosphoric acid is a dihydrogen diphosphate.

[0035] A secondary carbonate has the general formula MHCO 3 and is a hydrogen carbonate, and a secondary carbonate has the general formula M 2 CO 3 or MCO 3, where M stands for an alkali metal or alkaline earth metal.

[0036] Our own investigations have shown that the hydrogen carbonates or the dihydrogen diphosphates are particularly preferred according to the invention, since they lead to particularly good casting results in the sizing compositions according to the invention when the sizing composition is used to produce mold coatings on lost molds or on cores for iron and steel casting.

[0037] According to the invention, preferred sizing compositions are those in which the alkali metal or alkaline earth metal salt of carbonic acid is a lithium, sodium, potassium, calcium, magnesium, barium or strontium salt, the sodium salt being particularly preferred.

[0038] According to the invention, preferred sizing compositions are those wherein the alkali metal or alkaline earth metal salt of carbonic acid is sodium carbonate or sodium hydrogen carbonate.

[0039] Surprisingly, it has been found that sodium bicarbonate and sodium carbonate exhibit particularly good properties. Sodium bicarbonate is used as a dietary supplement, e.g., in baking powder, and is therefore toxicologically harmless. Sodium carbonate is also used as a dietary supplement and is commonly known as soda. Furthermore, both compounds are available in large quantities and are inexpensive. In addition, sizing compositions according to the invention, which contain sodium bicarbonate or sodium carbonate, exhibit a very strong reduction or even a complete elimination of the casting defect "white coating".

[0040] According to the invention, preferred sizing compositions are comprising 0.1 to 50 wt.%, preferably 0.5 to 30 wt.%, particularly preferably 3 to 15 wt.% of the alkali metal or alkaline earth metal salt of carbonic acid, based on the total weight of the sizing composition.

[0041] According to the invention, preferred sizing compositions are comprising 0.1 to 50 wt.%, preferably 0.5 to 30 wt.%, particularly preferably 3 to 15 wt.% of the alkali metal or alkaline earth metal salt of diphosphoric acid, based on the total weight of the sizing composition.

[0042] According to the invention, sizing compositions are preferably having a molar ratio between the alkali metal or alkaline earth metal salt of carbonic acid and the alkali metal or alkaline earth metal salt of diphosphoric acid in the range of 1 : 26 to 10 : 2.6, preferably in the range of 1 : 13 to 5 : 2.6, and particularly preferably in the range of 1 : 5.2 to 1 : 1.3.

[0043] Our own investigations have shown that the content of alkali metal or alkaline earth metal salts of carbonic acid or diphosphoric acid influences the quality of the casting. At too low a concentration, the alkali metal or alkaline earth metal salts of carbonic acid or diphosphoric acid have no or very little effect, while at too high a concentration, the effect is no longer improved.

[0044] According to the invention, preferred sizing compositions are those wherein the alkali metal or alkaline earth metal salt of diphosphoric acid is disodium dihydrogen diphosphate.

[0045] Surprisingly, it has been found that disodium dihydrogen diphosphate exhibits particularly good properties. Disodium dihydrogen diphosphate is used as a food supplement, e.g., in baking powder, and is therefore toxicologically harmless. Furthermore, it is available in large quantities and is inexpensive. In addition, sizing compositions according to the invention containing disodium dihydrogen diphosphate exhibit a very strong reduction or even a complete elimination of the casting defect "white coating".

[0046] Our own investigations have surprisingly shown that a mixture of an alkali metal or alkaline earth metal salt of carbonic acid and an alkali metal or alkaline earth metal salt of diphosphoric acid leads to an almost complete elimination of the casting defect "white coating." These mixtures exhibit a synergistic effect that cannot be achieved with the individual compounds in this form. Furthermore, our own investigations have shown that the use of a mixture of an alkali metal or alkaline earth metal salt of carbonic acid and an alkali metal or alkaline earth metal salt of diphosphoric acid also significantly reduces or completely prevents sintering of the casting surfaces.

[0047] According to the invention, sizing compositions containing sodium carbonate and / or sodium hydrogen carbonate as alkali metal or alkaline earth metal salts of carbonic acid and disodium dihydrogen diphosphate as alkali metal or alkaline earth metal salts of diphosphoric acid are particularly preferred.

[0048] In particular, the mixture of a) sodium bicarbonate and / or sodium carbonate and b) disodium dihydrogen diphosphate shows in our own investigations a complete or almost complete avoidance of the casting defect "white coating" and a prevention of sintering of the casting surface.

[0049] According to the invention, sizing compositions are preferred which have a weight ratio between a) sodium hydrogen carbonate and / or sodium carbonate and b) disodium dihydrogen diphosphate in the range of 1 : 10 to 10 : 1, preferably in the range of 1 : 5 to 5 : 1, particularly preferably in the range of 1 : 2 to 2 : 1.

[0050] According to the invention, preferred sizing compositions are those comprising a mixture of an alkali metal or alkaline earth metal salt of carbonic acid and an alkali metal or alkaline earth metal salt of diphosphoric acid, wherein the sizing composition has a pH value of 7 to 9.9, preferably a pH value of 7.5 to 9.5.

[0051] In an alternative embodiment of a non-inventive sizing composition, the sizing comprises an alkali metal or alkaline earth metal salt of carbonic acid, but no alkali metal or alkaline earth metal salt of diphosphoric acid, and has a pH value of 9 to 10, preferably a pH value of 9.5 to 11.5.

[0052] In an alternative embodiment of a non-inventive sizing composition, the sizing comprises an alkali metal or alkaline earth metal salt of carbonic acid, but no alkali metal or alkaline earth metal salt of diphosphoric acid, and has a pH value of 9 to 12, preferably a pH value of 9.5 to 11.5.

[0053] In an alternative embodiment of a non-inventive sizing composition, the sizing comprises an alkali metal or alkaline earth metal salt of diphosphoric acid, but no alkali metal or alkaline earth metal salt of carbonic acid, and has a pH value of 5 to 8, preferably a pH value of 5.5 to 7.5.

[0054] According to the invention, preferred sizing compositions are those wherein the carrier liquid is water or contains water.

[0055] Water as a carrier fluid has the particular advantage of being readily available, inexpensive, and toxicologically harmless. Furthermore, the alkali metal or alkaline earth metal salts of carbonic acid (and especially sodium bicarbonate or sodium carbonate) and the alkali metal or alkaline earth metal salts of diphosphoric acid (and especially disodium dihydrogen diphosphate) are very soluble in water.

[0056] Preferred according to the invention are sizing compositions wherein the carrier liquid is or contains an alcohol, preferably an alcohol selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, n-butanol, iso-butanol, sec-butanol, tert-butanol, pentan-1-ol, pentan-2-ol, pentan-3-ol, 2-methyl-butan-1-ol, 2-methyl-butan-2-ol, 3-methyl-butan-1-ol, 3-methyl-butan-2-ol, 2,2-dimethyl-propan-1-ol, hexan-1-ol, hexan-2-ol, hexan-3-ol, 2-methylpentan-1-ol, 3-methylpentan-1-ol, 4-methylpentan-1-ol, 2-methylpentan-2-ol, 3-Methylpentan-2-ol, 4-Methylpentan-2-ol, 2-Methylpentan-3-ol, 3-Methylpentan-3-ol, 2,2-Dimethylbutan-1-ol, 2,3-Dimethylbutan-1-ol, 3,3-Dimethylbutan-1-ol, 2,3-Dimethylbutan-2-ol, 3,3-Dimethylbutan-2-ol, 2-Ethylbutan-1-ol and mixtures thereof, particularly preferred are ethanol, 1-propanol, 2-propanol and mixtures thereof.

[0057] According to the invention, preferred sizing compositions are those in which the carrier liquid is a mixture of one or more organic solvents and water, the organic solvent preferably being an alcohol. Our own experiments have shown that water, and in particular the preferred alcohols, exhibit particularly good practical properties. They evaporate sufficiently quickly, are physiologically harmless or only slightly harmful, and can be used in foundries without extensive safety precautions.

[0058] According to the invention, sizing compositions are preferred, wherein the solids content of the sizing composition is 5 to 85 wt.%, preferably 10 to 80 wt.%, and particularly preferably 30 to 70 wt.%.

[0059] According to the invention, sizing compositions are preferred in which the sizing composition contains 10 to 85 wt.% refractories, based on the solids content of the sizing composition.

[0060] According to the invention, preferred sizing compositions are those in which the refractory materials are selected from the group consisting of quartz, aluminum oxide (preferably corundum), zirconium dioxide, aluminum silicates, mullite, zircon sands, zircon silicates, olivine, talc, mica, coke, feldspar, diatomite, calcined kaolins, kaolinite, metakaolinite, magnesium silicates, iron oxide, bauxite, graphite, and mixtures thereof, preferably magnesium silicates (especially talc), zircon silicates and mixtures thereof.

[0061] According to the invention, preferred sizing compositions are those comprising additionally color pigments, rheological additives, adjusting agents and / or thickeners.

[0062] Particularly preferred according to the invention are sizing compositions for the production of mold coatings on lost molds or on cores for iron and steel casting, consisting of or comprising a) 3 to 15 wt.% sodium bicarbonate and / or sodium carbonate and 3 to 15 wt.% disodium dihydrogen diphosphate, each based on the total weight of the sizing composition; b) a carrier liquid, preferably selected from the group consisting of water, ethanol, 1-propanol, 2-propanol and mixtures thereof; c) refractories selected from the group consisting of quartz, aluminum oxide (preferably corundum), zirconium dioxide, aluminum silicates, mullite, zircon sands, zircon silicates, olivine, talc, mica, coke, feldspar, diatomite, calcined kaolins, kaolinite, metakaolinite, magnesium silicates, iron oxide, bauxite, graphite, and mixtures thereof, preferably consisting of magnesium silicates (in particular talc), zircon silicates and mixtures thereof. wherein the sizing composition has a pH value of 7 to 9.9, preferably a pH value of 7.5 to 9.5 and wherein the weight ratio between a) sodium bicarbonate or sodium carbonate and b) disodium dihydrogen diphosphate is preferably in the range of 1 : 10 to 10 : 1, more preferably in the range of 1 : 5 to 5 : 1, and particularly preferably in the range of 1:2 to 2:1.

[0063] The use of an alkali metal or alkaline earth metal salt of carbonic acid and / or an alkali metal or alkaline earth metal salt of diphosphoric acid for the production of mold coatings on lost molds or on cores for iron and steel casting is also described.

[0064] Another aspect of the present invention is the use of a sizing composition according to claim 1.

[0065] According to the invention, the use is to prevent or reduce the formation of white deposits on the surface of castings during the casting process.

[0066] Another aspect of the present invention relates to the use of an alkali metal or alkaline earth metal salt of carbonic acid and an alkali metal or alkaline earth metal salt of diphosphoric acid as an additive in coatings for the foundry industry according to claim 13.

[0067] According to the invention, the use of a) sodium carbonate and / or b) sodium hydrogen carbonate and c) disodium dihydrogen diphosphate as an additive in coatings for the foundry industry is particularly preferred.

[0068] According to the invention, the use of a) a mixture of is particularly preferred. ai) Disodium dihydrogen diphosphate and a.ii) Sodium hydrogen carbonate and / or sodium carbonate as an additive in coatings for the foundry industry.

[0069] According to the invention, the use of a) sodium carbonate and / or b) sodium hydrogen carbonate and c) disodium dihydrogen diphosphate as additives in coatings for the foundry industry is also preferred, resulting in a coating composition according to the invention.

[0070] Another aspect of the present invention relates to a method for producing a casting according to claim 2.

[0071] A preferred method according to the invention is one for producing a casting in which, prior to coating at least one area of ​​the mold with the coating composition (step (ii) of the method according to the invention), a penetrating coating is applied to at least one area of ​​the mold. The penetrating coating contains aluminum oxide and / or kaolin.

[0072] Particularly preferably 50 to 75 wt.% aluminium oxide and / or 5 to 10 wt.% kaolin, each based on the total weight of the penetration coating.

[0073] A method for producing a casting is also preferred according to the invention, in which a first coating is applied to at least one area of ​​the mold before coating it with the coating composition (step (ii) of the method according to the invention), wherein this first coating does not comprise an alkali metal or alkaline earth metal salt of carbonic acid and / or an alkali metal or alkaline earth metal salt of diphosphoric acid. It is preferred that the applied first coating is dried after application. The first coating contains mullite and / or graphite, particularly preferably 50 to 75 wt.% mullite and / or 1 to 5 wt.% graphite, each based on the total weight of the coating.

[0074] In a particularly preferred embodiment of the inventive method for producing a casting, prior to coating at least one area of ​​the mold with the coating composition (step (ii) of the inventive method), a penetrating coating is applied to the at least one area of ​​the mold, and subsequently a first covering coating is applied to the at least one area of ​​the mold and dried, wherein this first covering coating does not comprise an alkali metal or alkaline earth metal salt of carbonic acid and / or an alkali metal or alkaline earth metal salt of diphosphoric acid.

[0075] A method for producing a casting is therefore particularly preferred according to the invention, comprising the following steps: (a) Providing or manufacturing a mold from chemically bonded mold base material, preferably comprising one or more acid-cured binders and quartz sand; (b) coating at least one area of ​​the mold with a penetration coating; (c) coating at least one part of the area of ​​the mold coated in step (b) with a topcoat, wherein this first topcoat does not comprise an alkali metal or alkaline earth metal salt of carbonic acid and / or an alkali metal or alkaline earth metal salt of diphosphoric acid, and drying the applied first topcoat; (d) coating at least one part of the area of ​​the mold coated in step (c) with the coating composition and drying the coated coating composition; (e) filling the mold with a molten iron-carbon alloy; (f) allowing the iron-carbon alloy to solidify in the mold, resulting in a casting.

[0076] Another aspect of the present invention relates to a casting mold according to claim 4.

[0077] A preferred casting mold according to the invention is one which is additionally coated with a penetrating sizing, wherein the penetrating sizing is penetrated into the chemically bonded mold base material below the sizing composition according to the invention.

[0078] A casting mold is also preferred according to the invention, which additionally contains a coating, wherein this coating does not comprise an alkali metal or alkaline earth metal salt of carbonic acid and / or an alkali metal or alkaline earth metal salt of diphosphoric acid, and wherein this coating forms a layer between the coating composition according to the invention and the chemically bonded mold base material.

[0079] Within the scope of the present invention as defined by the claims, several of the aspects designated as preferred above are preferably implemented simultaneously; in particular, the combinations of such aspects and the corresponding features resulting from the appended claims are preferred.

[0080] The present invention is explained in more detail below by means of examples and a figure.

[0081] Figure 1 The photograph shows the casting produced in Example 8 with an area marked "X". The part of the casting marked "X" comes from the part of the mold that was coated with a sizing composition according to the invention in Example 8. The surface of the casting is free of defects and shows no "white coating", sintering, or surface scarring. Examples: Example 1:

[0082] A mold was produced for a casting (ANZ E 101) made of EN-GJS 400-15 with a casting weight of 19,200 kg and a finished casting weight of 15,800 kg. The mold features several radii that are subjected to high thermal stress during casting. The iron-to-sand mass ratio was approximately 1:1. Mechanically regenerated furan sand with a mean grain size of 0.33 mm was used as the molding material. The binder used was SRV-1 from Hüttenes-Albertus, Düsseldorf, and the activator was 7809, also from Hüttenes-Albertus, Düsseldorf.

[0083] A sizing composition (pH value 10.6) based on highly refractory magnesium silicates and oxide ceramic components and water, containing 9 wt% sodium carbonate, was applied to a specially marked area (in this example area "A") of the mold.

[0084] After casting at a temperature of approximately 1320 °C, the casting was inspected. In area "A" treated with the coating, the casting showed no formation of a "white coating". A "white coating" was observed in the untreated area of ​​the casting. Example 2:

[0085] Example 2 was carried out analogously to Example 1, however, a sizing composition (pH value 5.5) based on highly refractory magnesium silicates and oxide ceramic components and water was applied, containing 9 wt% disodium dihydrogen diphosphate.

[0086] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating, the casting showed no formation of a "white coating." A "white coating" was observed in the untreated area of ​​the casting. Example 3:

[0087] Example 3 was carried out analogously to Example 1, however, a sizing composition according to the invention (pH value 9.0) based on highly refractory magnesium silicates and oxide ceramic components and water was applied, containing 4.5 wt.% disodium dihydrogen diphosphate and 4.5 wt.% sodium carbonate.

[0088] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating composition according to the invention, the casting showed no formation of a "white coating". A "white coating" was observed in the untreated area of ​​the casting. Example 4:

[0089] Example 4 was carried out analogously to Example 1, however, a sizing composition (pH value 5.5) based on highly refractory magnesium silicates and oxide ceramic components and water was applied, containing 9 wt% disodium dihydrogen diphosphate.

[0090] Before applying the sizing composition, the mold was pretreated with a penetrating sizing coating Foseco PDI 1658 / 1.

[0091] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating, the casting showed a significant reduction in the formation of "white deposits." "White deposits" were observed in the untreated area of ​​the casting. Example 5:

[0092] Example 5 was carried out analogously to Example 1, however, a sizing composition (pH value 10.6) based on highly refractory magnesium silicates and oxide ceramic components and water, containing 9 wt% sodium carbonate, was applied.

[0093] Before applying the sizing composition, the mold was pretreated with a penetrating sizing coating Foseco PDI 1658 / 1.

[0094] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating, the casting showed a significant reduction in the formation of "white deposits." "White deposits" were observed in the untreated area of ​​the casting. Example 6:

[0095] Example 6 was carried out analogously to Example 1, however, a sizing composition (pH value 5.5) based on highly refractory magnesium silicates and oxide ceramic components and water was applied, containing 9 wt% disodium dihydrogen diphosphate.

[0096] Before applying the sizing composition, the mold was pretreated with a penetrating sizing Foseco PDI 1658 / 1 and then with a first covering sizing Foseco PDI 1514 / 4 and subsequently dried.

[0097] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating, the casting showed a significant reduction in the formation of "white deposits." "White deposits" were observed in the untreated area of ​​the casting. Example 7:

[0098] Example 7 was carried out analogously to Example 1, however, a sizing composition (pH value 10.6) based on highly refractory magnesium silicates and oxide ceramic components and water, containing 9 wt% sodium carbonate, was applied.

[0099] Before applying the sizing composition, the mold was pretreated with a penetrating sizing Foseco PDI 1658 / 1 and then with a first covering sizing Foseco PDI 1514 / 4 and subsequently dried.

[0100] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating, the casting showed a significant reduction in the formation of "white deposits." "White deposits" were observed in the untreated area of ​​the casting. Example 8:

[0101] Example 8 was carried out analogously to Example 1, however, a sizing composition according to the invention (pH value 9.0) based on highly refractory magnesium silicates and oxide ceramic components and water was applied, containing 4.5 wt.% disodium dihydrogen diphosphate and 4.5 wt.% sodium carbonate.

[0102] Prior to the application of the sizing composition according to the invention, the mold was pretreated with a penetrating sizing Foseco PDI 1658 / 1 and subsequently with a first covering sizing Foseco PDI 1514 / 4 and then dried.

[0103] After casting at a temperature of approximately 1320 °C, the casting was inspected. In the area treated with the coating composition according to the invention, the casting showed no formation of a "white coating" and no surface sintering. The surface of the casting in the treated area was completely free of defects; no reworking of the treated area was necessary. A "white coating" was observed in the untreated area of ​​the casting.

[0104] A photograph of the produced casting surface is in Figure 1 shown (area "X").

Claims

1. Use of a wash composition comprising a) 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of carbonic acid and 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of diphosphoric acid; or 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal hydrogen carbonate salt or alkaline earth metal hydrogen carbonate salt; or 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal dihydrogen diphosphate salt or alkaline earth metal dihydrogen diphosphate salt, b) a carrier liquid and c) refractories, as a covering wash for producing mould coatings on expendable moulds or on cores for the casting of iron and steel, wherein the covering wash penetrates less than 2 mm into the mould material, forms a covering layer on the mould material after application to the mould material, and comes into direct contact with the melt, wherein the use is as a covering wash for coating expendable moulds or on cores for the casting of iron and steel, which comprise silica sand and an acid-cured binder, for avoiding or reducing the formation of white film on the surface of castings during the casting operation.

2. Process for producing a casting, comprising the following steps: (i) providing or producing a casting mould composed of chemically bound base mould material comprising one or more acid-cured binders and silica sand, (ii) coating at least one region of the casting mould with a wash composition comprising a) 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of carbonic acid and 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of diphosphoric acid; or 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal hydrogen carbonate salt or alkaline earth metal hydrogen carbonate salt; or 3% to 50% by weight, based on the total weight of the wash composition, of an alkali metal dihydrogen diphosphate salt or alkaline earth metal dihydrogen diphosphate salt, b) a carrier liquid and c) refractories and drying the coated wash composition, (iii) filling the casting mould with a molten iron-carbon alloy, (iv) allowing the iron-carbon alloy to solidify in the casting mould so as to result in a casting, wherein prior to coating the at least one region of the casting mould with the wash composition in step (ii) a penetrating wash comprising aluminium oxide and / or kaolin, or a first covering wash is applied to the at least one region of the casting mould, wherein said first covering wash does not contain any alkali metal salt or alkaline earth metal salt of carbonic acid or any alkali metal salt or alkaline earth metal salt of diphosphoric acid, wherein the first covering wash - contains mullite and / or - contains graphite.

3. Process according to Claim 2, wherein the first covering wash - contains 50% to 75% by weight of mullite, based on the total weight of the covering wash, and / or - contains 1% to 5% by weight of graphite, based on the total weight of the covering wash.

4. Casting mould composed of a chemically bound base mould material for producing a casting, wherein the chemically bound base mould material comprises one or more acid-cured binders and silica sand, wherein at least one region of the casting mould has been coated with a wash composition comprising a) 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of carbonic acid and 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of diphosphoric acid; or 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal dihydrogen diphosphate salt or alkaline earth metal dihydrogen diphosphate salt, b) a carrier liquid and c) refractories, wherein the casting mould additionally contains a covering wash, wherein said covering wash does not contain any alkali metal salt or alkaline earth metal salt of carbonic acid or any alkali metal salt or alkaline earth metal salt of diphosphoric acid, and wherein said covering wash forms a layer between the wash composition and the chemically bound base mould material.

5. Wash composition for producing mould coatings on expendable moulds or on cores for the casting of iron and steel, comprising a) 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of carbonic acid and 0.5% to 50% by weight, based on the total weight of the wash composition, of an alkali metal salt or alkaline earth metal salt of diphosphoric acid, b) a carrier liquid and c) refractories.

6. Wash composition according to Claim 5, wherein the wash composition has a molar ratio between the alkali metal salt or alkaline earth metal salt of carbonic acid and the alkali metal salt or alkaline earth metal salt of diphosphoric acid in the range from 1:26 to 10:2.6, preferably in the range from 1:13 to 5:2.6, particularly preferably in the range from 1:5.2 to 1:1.3.

7. Wash composition according to either of Claims 5 and 6, wherein the wash composition has a pH of from 7 to 9.9, preferably has a pH of from 7.5 to 9.5 and / or the alkali metal salt or alkaline earth metal salt of diphosphoric acid is a dihydrogen diphosphate.

8. Wash composition according to any of Claims 5 to 7, wherein (i) the alkali metal salt or alkaline earth metal salt of carbonic acid is sodium hydrogen carbonate or sodium carbonate and / or (ii) the alkali metal salt or alkaline earth metal salt of diphosphoric acid is disodium dihydrogen diphosphate.

9. Wash composition according to any of Claims 5 to 8, comprising (a) 0.5% to 30% by weight, particularly preferably 3% to 15% by weight, of the alkali metal salt or alkaline earth metal salt of carbonic acid, based on the total weight of the wash composition, and (b) 0.5% to 30% by weight, particularly preferably 3% to 15% by weight, of the alkali metal salt or alkaline earth metal salt of diphosphoric acid, based on the total weight of the wash composition.

10. Wash composition according to any of Claims 5 to 9, wherein the carrier liquid a) is water or contains water and / or b) is or contains one or more organic solvents, wherein one or all of the organic solvents preferably is alcohol or are alcohols.

11. Wash composition according to any of Claims 5 to 10, wherein the carrier liquid is an alcohol or contains alcohol, preferably an alcohol selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, n-butanol, isobutanol, sec-butanol, tert-butanol, pentan-1-ol, pentan-2-ol, pentan-3-ol, 2-methylbutan-1-ol, 2-methylbutan-2-ol, 3-methylbutan-1-ol, 3-methylbutan-2-ol, 2,2-dimethylpropan-1-ol, hexan-1-ol, hexan-2-ol, hexan-3-ol, 2-methylpentan-1-ol, 3-methylpentan-1-ol, 4-methylpentan-1-ol, 2-methylpentan-2-ol, 3-methylpentan-2-ol, 4-methylpentan-2-ol, 2-methylpentan-3-ol, 3-methylpentan-3-ol, 2,2-dimethylbutan-1-ol, 2,3-dimethylbutan-1-ol, 3,3-dimethylbutan-1-ol, 2,3-dimethylbutan-2-ol, 3,3-dimethylbutan-2-ol, 2-ethylbutan-1-ol and mixtures thereof, with particular preference given to ethanol, 1-propanol, 2-propanol and mixtures thereof.

12. Wash composition according to any of Claims 5 to 11, comprising a) 3% to 15% by weight of sodium hydrogen carbonate and / or sodium carbonate and 3% to 15% by weight of disodium dihydrogen diphosphate, in each case based on the total weight of the wash composition, b) a carrier liquid, preferably selected from the group consisting of water, ethanol, 1-propanol, 2-propanol and mixtures thereof, c) refractories, selected from the group consisting of silica, aluminium oxide, preferably corundum, zirconium dioxide, aluminium silicates, mullite, zircon sands, zirconium silicates, olivine, talc, mica, coke, feldspar, diatomite, calcined kaolins, kaolinite, metakaolinite, magnesium silicates, iron oxide, bauxite, graphite and mixtures thereof, preferably consisting of magnesium silicates, preferably talc, zirconium silicates and mixtures thereof, wherein the wash composition has a pH of from 7 to 9.9, preferably has a pH of from 7.5 to 9.5, and wherein the weight ratio between aa) sodium hydrogen carbonate or sodium carbonate and bb) disodium dihydrogen diphosphate is preferably in the range from 1:10 to 10:1, more preferably is in the range from 1:5 to 5:1, particularly preferably is in the range from 1:2 to 2:1.

13. Use of a mixture of an alkali metal salt or alkaline earth metal salt of carbonic acid and an alkali metal salt or alkaline earth metal salt of diphosphoric acid as additive in washes for the foundry industry.