FR2748409A1 - NOVEL INSULATING COATING FOR METALLIC MOLD AND METHOD OF MAKING SUCH COATING - Google Patents

Abstract

The invention relates to a novel insulating coating for a metal mold intended for the casting of ferrous metals, in particular by centrifugal casting, comprising at least one primary coating applied to the surface of the mold and intended to remain there permanently and a secondary coating applied. on the primary coating and completely or partially renewed at each casting operation. This coating is remarkable in that it comprises or consists essentially of metakoalin. The invention also relates to the process for producing this coating. The invention finds its application in the metallurgical industry, more especially in the manufacture of metal parts intended for the automobile industry.

Description

The invention relates to a novel insulating coating for mold

  metal, especially for the casting of ferrous metals. The invention finds application in the metallurgical industry, especially in the manufacture of metal parts for the automotive industry. In the technique of casting in a metal mold, the application of a coating (also called potting) on the mold is absolutely essential. It has a dual function: - Chemically isolate the mold from the liquid metal during casting, so as to avoid adhesion and allow subsequent separation mold-metal part; ensure a thermal break between the mold and the workpiece. If this function is not fulfilled, the metal mold will, during successive flows, undergo thermal stresses such that its destruction will occur prematurely. The thermal break must not, however, be such that the cooling of the room is compromised and thus the solidification occurs late. There is therefore a delicate balance to achieve. For the manufacture of metal parts, for example cast iron, several casting techniques are used, including static casting and centrifugal casting. In static casting, it is sought to ensure rapid cooling of the part of the part in contact with the metal mold, so as to demold quickly, even if the part is not fully solidified. This prevents excessive heating of the metal mold. In this case, it uses a poteyage which, while allowing easy mold-room separation, is relatively conductive. The most commonly used is carbon black from the incomplete combustion of acetylene. It is deposited at each casting operation by

  a burner set in reducing phase.

  In centrifugal casting, intended to obtain hollow cylindrical bodies, it is essential that the part is completely solidified before the

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  demoulding, which assumes a long enough contact time between mold

metal and piece.

  In this case, according to the prior art, the metal mold is coated with an insulating poteyage, based on silica, in the form of a coating.

single or multiple layer.

  The latter material, silica, is used in the prior art, as a basic constituent of the poteyage, in its various forms 1) crystallized: quartz, cristobolite, tridymite;

  - amorphs: tripoli, kieselghur, diatomite, fused silica.

  Practically, because of their low thermal conductivity, the silica products have a high insulating power and are widely used preferably, especially since their cost is low and their

important availability.

  Their implementation is carried out very generally by spraying a ceramic slip on the hot shell of the mold, rotated. This slip consists of a suspension of silica flour in water previously added with products likely to improve the rheological properties, generally gelling agents,

minerals or organic.

  In this way, the metal mold is coated with a uniform refractory layer. However, according to the prior art, during centrifugal casting operations of ferrous metals to manufacture metal parts, for example cast cast iron GL cast pearlitic structures casting, it appeared that most of the refractory material remained adhered to the metal part and that we had to renew the coating, at least partially, at each casting operation.

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  A known improvement of the prior art has therefore consisted in producing the coating in several layers, a first layer (or primary coating) applied directly to the surface of the mold and intended to remain there permanently, and a second layer (or secondary coating), applied to the previous layer and

  renewed totally or partially at each casting operation.

  By the very fact of the rotation of the mold which creates large air movements, it can not be avoided that a fraction of the poteyage does not escape to the outside environment, despite the use of capturing devices. In this way, there is an important potential danger in the environment for staff assigned to workstations, including

  included during cleaning and finishing operations.

  However, the silicogenic effects of the crystallized silica products are known, the finest particles and, in particular, those with a diameter of less than five (5) microns being recognized as having

the degree of maximum harm.

  The use of amorphous silica, known to have little silicosis, does not make it possible to overcome this disadvantage because the use made of it has the effect of bringing them to high temperature and thus to transforming them into silica.

  crystallized, with the same risks of harmfulness.

  The invention therefore aims to eliminate any use of silica products as insulating coating for a metal mold, and to substitute a non-silicogenic refractory product with sufficient insulating power. According to the present invention, an insulating coating for a metal mold intended for the casting of ferrous metals, in particular by centrifugal casting, comprising at least one primary coating applied to the surface of the mold and intended to remain there permanently and to a secondary coating, applied to the primary coating and renewed totally or partially at each casting operation, is remarkable in that said secondary coating comprises metakaolin.

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  Secondary coating is often called in the literature

maintenance coating.

  By metakaolin is meant hereinafter, the material obtained from natural kaolin, which is previously purified in order to remove any trace of impurities of the quartz or mica type and which is calcined at a temperature of between 750 and 950 C, so to

  completely eliminate the water of constitution.

  According to one embodiment of the invention, the secondary coatings and

  primary consist mainly of metakaolin.

  The following description will provide a better understanding of the

  different variants or preferred embodiments of the invention.

  From natural kaolin, which was purified and then calcined for about 1 hour, a powder having a particle size distribution between 0.5 and 20 microns with a medium grain was obtained.

centered on 2 microns.

  This product has the advantage of being refractory, because of its chemical constitution, to be inert, due to lack of water constitution, to be insulating, because of its particle size distribution and to be medically neutral, because of a free silica content of less than

0.5%.

  From this product, a mixture based on metakaolin and sodium silicate, used as inorganic binder, was prepared and sprayed on the bare metal shell of a mold, previously heated to between 300 and 400 ° C, to provide an adequate primary coating, a

  thickness between about 50 and 100 microns.

  From this same product, a suspension of meta-kolin powder in water, with a concentration in weight of between 25 and 35%, was also produced. To improve the rheological properties of this suspension, it is possible to add a small amount of sodium bentonite,

  with a concentration by weight of between 0.1 and 1%.

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  After application by spraying, on the primary coating obtained previously, a secondary coating is thus produced

  suitable, with a thickness between about 50 and 100 microns.

  The mold, thus coated, has been used several times, for the casting of cast iron, to obtain metal parts, especially for

automobile industry.

  The secondary coating, after demolding of the metal part, remains largely adherent to said part, while a minority fraction remains adherent to the primary coating. Between each casting operation, a new coat was applied by spraying.

  reconstitute the secondary coating.

  The primary coating, strongly adhering to the shell, remained permanently in place after more than one hundred (100)

casting operations.

  Examination of the metallic part thus produced (thermal engine cylinder liners) in gray cast iron showed a pearlitic structure in the raw state of casting, entirely free of cementite. The presence of cementite would have been deduced by the appearance of a rim soaked on the part of the piece in contact with the metal mold. Such

  defect would have been deemed unacceptable, and led to the rejection of such a piece.

  It is obvious to those skilled in the art that many variants of the invention can be imagined, from what is described above without doing creative work, and therefore without leaving

  of the scope of the invention as defined by the claims below

3o appended.

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Claims (6)

  1 - Insulating coating for a metal mold intended for the casting of ferrous metals, in particular by centrifugal casting, comprising at least one primary coating applied to the surface of the mold and intended to remain there permanently and a secondary coating, applied on the primary coating and renewed totally or partially at each casting operation, characterized in that said coating secondary includes metakoalin.   2 - insulating coating for metal mold, according to claim 1, characterized in that said secondary coating is constituted essentially by metakaolin.   3 - insulating coating for metal mold, according to claim 1 or 2, characterized in that the primary coating is also constituted by metakaolin.   4 - Process for producing an insulating coating for a metal mold, according to claim 3, characterized in that: a) a metakaolin powder is prepared from kaolin which is purified and then calcined between 750 and 950 C for about one (1) hour; b) a mixture of this metakaolin powder and sodium silicate having substantially the same particle size is produced, and this mixture is sprayed on the bare surface and previously heated said metal mold between 300 and 400 C, so as to obtain a primary coating , generally between 50 and 100 microns thick; c) a suspension of metakaolin in water with a concentration by weight of between 25% and 35% is carried out, then this suspension is sprayed onto the primary coating, so as to obtain a secondary coating, generally of a thickness between 50 and 100 microns. 7 2748409   - Metal mold coated with an insulating coating according to one of the Claims 1 to 3.   6 - Metal mold coated with an insulating coating, obtained by carrying out the method according to claim 4. 7 - Using a metal mold coated with an insulating coating,   according to one of claims 5 or 6, for the casting of ferrous metals.   l) 8 - Use of a metal mold coated with an insulating coating,   according to one of claims 5 or 6, for the manufacture of parts   metal, by casting ferrous metals in said mold.   9 - Use of a metal mold coated with an insulating coating,   according to one of claims 5 or 6, for the manufacture of parts   metal products, intended for the automotive industry, in particular   cast iron with raw pearlitic structures.