MXPA06002032A - Slurries containing iron compounds used in the casting of metals - Google Patents

Abstract

Slurries containing tri-or polyvalent cations are used for investment casting and making molds as well as core coatings. These slurries are aqueous or alcohol-based containing a mineral component and a polyanionic binder, where at least one of the continuous phase or mineral component contains trivalent or polyvalent cations or cations that are converted into trivalent or polyvalent cations during use. The cationic component is selected from a group consisting of at least one of the metals iron or aluminum. The polyanionic component contains at least one of the group consisting of colloidalsilica or one or more water-soluble organic polymers.

Description

PASTAS CONTAINING IRON COMPOUNDS USED IN THE METALS FOUNDRY DESCRIPTION OF INVESTMENT CASTING TECHNIQUES Investment casting is a process for making finely detailed parts, which involve making a model of the article to be melted in a material such as wax or expanded polystyrene foam that can be melted at temperatures between 50 and 150 ° C, and then form a coating, the shell, around the model by repeated applications of a paste containing a mineral component and a binder, the latter typically being based on colloidal silica. The shell construction process typically begins with the fabrication of a thin shell consisting of a finely divided aggregate, typically of particle size below 75μm. This primary shell is reinforced by the application of additional layers of coating on it and by more applications of pastes that may contain thicker aggregates, after which the whole is heated in two stages. The first stage dries the shell and melts the model of wax or foam underneath. For this reason the process is also known as the process of loss wax or foam loss. In the process of wax loss, the wax ends up leaving the dry "green" shell that is then heated to a temperature of 600 ° C or more to produce a strong ceramic shell inside which the metal is poured to burn any residue waxy. The process for making the mold by casting by inversion of lost foam is similar but the shell is simpler in construction and thinner, being held in place by a bed of compacted sand. In contrast to the wax loss process the model is made of a foam polymer, usually expanded polystyrene, which shrinks when the shell is dry but remains inside it instead of finishing as is the case with the lost wax process. The foam residues are then burned when the molten metal is poured into the mold. It is extremely important that the paste is capable of forming a strong green shell that can be handled without breaking and contains no components that can include gases that can break the shell during heating. Similarly, it is important that the final shell is strong enough to withstand the stresses incurred during the melting of the metal. Silicic acid sol is one of the very few binders that can supply these properties and the only binder of meaning used for the purpose. However, it is extremely sensitive to chemical impurities and requires that other components used with it be extremely pure. As an example, it is recommended that only deionized water be used in the manufacture of pastes. A similar situation belongs to the mineral component, one of the most common being a chemically pure aluminum silicate made from the fusion of pure alumina and pure silica together and then crushing and sifting the resulting product. Other minerals used for shell fabrication include zirconium (zirconium silicate), muilite (natural aluminum silicate) and molten powdered silica. All these combined requirements make investment casting a more expensive process that is used only for the most demanding parties. The shortage of suitable minerals discards or at least severely limits the use of a number of potentially valuable processes that could help reduce costs. One of these, microwave heating, has been the subject of considerable interest, since it heats pasta more quickly than convective heating, allowing the shell to be dried and the wax or foam to be melted more quickly. For this reason, the industry has also been studying the use of components, particularly modeling waxes, that can absorb microwave energy and thus are heated to their melting point more quickly in a microwave instead of waxes standard in a typical convection oven. While efficient in this respect, these waxes, continually containing small droplets of water emulsified within the wax, are more difficult and costly to recover.

Even more significant are the savings that can be achieved if it is possible to use minerals to make the breastplates that could be heated to the green state and then heated when subjected to an electromagnetic field, for example by microwave or induction methods. Such minerals are described in the provisional US patent application. UU 60 / 496,675 and include compounds of transition metal elements (iron, cobalt and nickel) in the state of bivalence, such as norite (iron calcium aluminosilicate) ilmenite (iron titanate), chromite (iron chromite), olivine (ferruginous magnesium silicate), magnetite (iron sesquioxide), hyperite (iron magnesium aluminosilicate). Attempts to use these materials in conventional pastes for investment casting or mold coating with polyanionic binders such as colloidal silica purposes fail due to gelation of the binder.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY Light metal casting shells This invention describes how ferrous metal containing pulps including those that can be heated by electromagnetic fields (EMF), can be stabilized and used to make shells that are more cost effective and versatile that the state of the products of the matter. The fact that pulps containing ferrous materials and polyanionic binders are unstable prescribes their use for investment casting. This phenomenon is not adequately explained in the literature, although suppliers of products used for these purposes establish maximum levels of iron content, typically less than 200 ppm. It is therefore unexpected that the stability of the pastes having iron contents larger than this can be used by the resource of adding at least 0.2% by weight of a reducing agent such as sodium hypophosphite to the binder system. A similar effect was also found in the addition of at least 0.2% of ex. diamonium hydrogen phosphate and particularly trisodium phosphate. It was also found that mixtures of stabilizers such as these could be used with a similar effect. As an example, the following paste can be used as a first coating for casting by investment of aluminum loss wax: 1 Remet Inc. brand 2 Azko Nobel brand 3 Ciba Geigy brand Aluminum calcium silicate aluminosilicate as mined in Rekefjord, Norway Also as an example, the following paste can be used for secondary coatings: 1 Remet Inc. brand 2 Azko Nobel brand 3 Ciba Geigy brand 4 Iron calcium aluminosilicate as mined in Rekefjord, Norway In all cases it is foreseeable that the trisodium orthophosphate is dissolved in the binder before the ore is added. A thicker norl, for example 35/50 mesh, can then be bathed on this secondary coating to supply a finished layer consisting of 87.5% bypaste and 12.5% by thicker norite. There are a significant number of advantages for this system over conventional systems. First, the superior thermal conductivity of the iron cores allows them to be dried and dewatered in standard equipment (autoclave) in less than half the time required by the armor made by conventional systems, thus increasing the efficiency of the process and reducing their energy needs. Second, the drying and dewaxing can be carried out even faster by induction or microwave heating in a circulating air oven, by which process the residual wax can be removed before heating. Third, these cuirasses can be heated by induction or microwave methods much faster and with much lower power requirements than in a conventional hot air oven. Fourth, tap water can be used to make trisodium or tripotassium phosphate-containing pastes, without the concurrent inesiability that can affect the state of the matter's pastes.

This represents a considerable saving, given that iron foundries using conventional systems are obliged to use deionized water, which may need to be purchased at a price that reflects both manufacturing and transport costs. Fifth, the specific use of trisodium or tripotassium phosphate produces pastes that have fairly stable pH values.

This is not the case for conventional pastes that often require an adjustment with caustic soda or potassium, with all that this implies in terms of monitoring and work requirements. The shells made in this way are suitable for investment casting of aluminum and the casting of other alloys at temperatures below 1, 200 ° C.

It is possible that the addition of chelating agents, reducers or phosphates together with ferrous substances, including those that may be present in tap water, work by counteracting the harmful effects of trivalent cations by reducing them to divalent cations or converting them into compounds so insoluble that they interact in a minimal way with the polyanionic agglutinates. Divalent calions such as Fe2 + do not destabilize these binders but do so fast when they are oxidized to the trivalent state.

Investment Cast Beads for Ferrous Metals The method of the invention can also be used to make ceramic shells for cast iron by the process of lost foam or steel casting by loss foam processes or loss wax, using a mineral such as olivine containing less than 8% iron. Where systems sensitive to induction or microwave energy fields are desired, the elemental carbon can be added to the second coating for example in the mixing range of 25% finely divided graphite carbon and 75% olivine having an iron content maximum of 8%. Other minerals that do not form low-melting compounds with the silica binder, such as zirconium and calcium orthosilicate, can be used instead of olivine. The shell should then be reheated in an oxygen-free environment.

TECHNIQUES OF THE PREVIOUS TECHNIQUE FOR THE MANUFACTURE OF COATINGS FOR MOLDS OF IRON AND CAST IRON NUCLEOS Molds and cores are often coated to improve the surface finish of the coating or to help prevent metal penetration. A coating is a paste containing a finely ground mineral, often zircon, and a substance such as bentonite that is capable of providing some bond strength even at casting temperatures. It is important that these coating pastes remain stable for many months, since they are typically supplied as freshly made products that are stored until they are used by the iron smelter. While iron smelters have, for many years, used alcohol-based coatings that allow the continuous phase to be burned without the need to be dried, recent environmental legislation has decreed that the use of organic volatile liquid should be restricted. Considerable efforts are now being made to replace alcohol with water-based coatings and colloidal silica binders are in many ways ideal for this application, since they maintain their integrity during casting and are not so stubborn. They are, however, hardly used because of the poor storage stability and the fact that they are restricted to a limited range of minerals.

Coatings for molds and cores Continuing with the premises described herein as a possible cause for the stability limitations of coatings based on colloidal silica, the formulations given in the sub-section entitled "Light metal casting tools" and "Hearts cast iron investment for earlier ferrous metals have proven themselves to be suitable for use as mold and core coatings. The following systems are examples of satisfactory products, showing little change after storage for 6 months at 30 ° C: Brand by Remet Inc. 2Azko Nobel brand 3Ciba Geigy brand 4Aluminum calcium iron silicate as mined in Rekefjord, Norway Remet Inc. Brand 2 Azko Nobel Brand 3 Ciba Geigy Brand 4 A mineral in the family mined in Rekefjord, Norway 1 Remet Inc. Brand 2 Azko Nobel Brand 3 Ciba Geigy Brand 4 Ferrous Magnesium Silicate The purpose of polymer latex is to provide a so-called green force so that the coating does not break during drying or movement of the mold or core. Coatings made with colloidal silica binders have an exceptionally good hot strength and are highly refractory. It is not possible to manufacture stable water-based coatings using minerals such as those mentioned earlier, June with a colloidal silica or a similar polyanionic binder, unless the method of this invention is followed. One particular advantage of being able to use ferrous minerals such as Norite in the manufacture of water-based coatings, is that they can be dried rapidly by exposure to an oscillating electromagnetic pulse, such as a microwave or preferably an induction field. While the norite is suitable for metals such as aluminum, more refractory minerals need to be used in coatings used in cast iron and especially steel. Some of these ex. iron chromite and magnetite, are sufficiently refractory to be used in cast iron either alone or mixed with for example anorthosis. However, temperatures experienced in steel casting can cause the silica in the binder to react with ferrous minerals to form less refractory compounds such as fayelite (iron orthosilicate) or gruenerite (iron metasilicate), which can rule out the use of minerals containing more than 8% or something of ferrous compounds. However, the addition of 3-30% or more of graphitic carbon to a mineral with low iron content will usually generate sufficient heat in an oscillating electromagnetic field. The foregoing description should be considered as illustrative only of the principles of this invention. Numerous applications of the present invention will easily occur to limit the invention to the preferred embodiments described above. Preferably, all suitable and equivalent modifications can be appealed, falling within the scope of the invention.

Claims (10)

1. CLAIMS 1. Pastes containing tri or polyvalent cations used for investment casting and making core molds and coatings.
2. 2. Alcohol-based and aqueous pastes according to claim 1, containing a mineral component and a polyanionic agglutinin, wherein at least one of the continuous phase or the mineral component contains trivalent or polyvalent cations or cations that are converted to polyvalent or trivalent cations during its use.
3. 3. Pastes according to claim 2, characterized in that the cationic component is selected from the group consisting of at least one of the metals, iron or aluminum.
4. 4. Pastes according to claim 2, characterized in that the polyanionic component contains at least one of the group consisting of colloidal silica or one or more water-soluble organic polymers.
5. 5. The pastes according to claim 1 made by the incorporation of at least 0.01% by weight of mineral or water of an agent that converts said cationic components into the paste in substances that are incapable of flocculating in polyanionic substances.
6. 6. The pastes according to claim 2 made by the incorporation of at least 0.01% by weight of mineral or water of an agent that converts said cationic components into the paste in substances that are incapable of flocculating in polyanionic substances.
7. 7. The pastes according to claim 3 made by the incorporation of at least 0.01% by weight of mineral or water of an agent that converts said cationic components into the paste in substances that are incapable of flocculating in polyanionic substances.
8. 8. The pastes according to claim 4 made by the incorporation of at least 0.01% by weight of mineral or water of an agent that converts said cationic components into the paste in substances that are incapable of flocculating in polyanionic substances. The agents according to claim 5 which contain at least one agent selected from the group consisting of a chelating agent or a reducing agent or a water soluble phosphate. 10. The pastes according to claim 1 containing minerals or carbon that can be heated in an electromagnetic field selected from the group consisting of microwave, induction or radiofrequency radiation. eleven . The pastes according to claim 2 containing minerals or carbon that can be heated in an electromagnetic field selected from the group consisting of microwave, induction or radiofrequency radiation. The pastes according to claim 3 containing minerals or carbon that can be heated in an electromagnetic field selected from the group consisting of microwave, induction or radiofrequency radiation. The pastes according to claim 4 containing minerals or carbon that can be heated in an electromagnetic field selected from the group consisting of microwave, induction or radiofrequency radiation. 14. The pastes according to claim 5 containing minerals or carbon that can be heated in an electromagnetic field selected from the group consisting of microwave, induction or radiofrequency radiation. 15. The pastes according to claim 6 containing minerals or carbon that can be heated in an electromagnetic field selected from the group consisting of microwave, induction or radiofrequency radiation.