EP0806258B1 - Novel isolating coating for metallic mould and process of preparation of such coating - Google Patents

Description

The invention relates to a new insulating coating for molds. metallic, intended in particular for the casting of ferrous metals. The invention finds its application in the metallurgical industry, more especially in the manufacture of metal parts for industry automobile.

Document EP-A-0.014.055 describes a fibrous refractory product free of magnet formed by kneading of a composition comprising aluminosilicate fiber and kaolin-type clay, addition of water, shaping the mixture to form an article, drying the article and then baking the article to a temperature which transforms clay into the amorphous metakaolin phase. Such a product can find application in deflectors, tanks or tubular elements.

Document US-A-4,600,437 describes an inorganic material comprising an alkaline salt of the acid silicic and metakaolin and exhibiting, after solidification, good water resistance properties, said material being able to be used as coating on a metal or an organic substance.

Document JP-A-7215772 describes a refractory material containing from 0.1 to 20% metakaolin in weight of a composition containing refractory magnesia in the raw state and its applications for coating steel fabrication containers.

• isoler chimiquement le moule du métal liquide lors de la coulée, de manière à éviter toute adhérence et permettre la séparation ultérieure moule-pièce métallique ;
• assurer une rupture thermique entre le moule et la pièce. Si cette fonction n'est pas remplie, le moule métallique va, au cours des coulées successives, subir des contraintes thermiques telles que sa destruction interviendra prématurément. La rupture thermique ne doit pas toutefois être telle que le refroidissement de la pièce soit compromis et qu'ainsi la solidification intervienne tardivement. Il y a donc un équilibre délicat à réaliser.

In the metal mold casting technique, the application of a coating (also called poteyage) on the mold is absolutely essential. It performs a dual function

• chemically isolate the mold from the liquid metal during casting, so as to avoid any adhesion and allow subsequent separation of the mold from the metal part;
• ensure a thermal break between the mold and the part. If this function is not fulfilled, the metal mold will, during successive castings, undergo thermal stresses such that its destruction will take place prematurely. The thermal rupture must not however be such that the cooling of the part is compromised and that thus solidification occurs late. There is therefore a delicate balance to be struck.

For the production of metal parts, for example cast iron, we uses several casting techniques, including static casting and casting centrifugal.

In static casting, we seek to ensure rapid cooling of the part of the part in contact with the metal mold, so that be able to unmold quickly, even if the part is not completely solidified. This prevents excessive heating of the mold metallic.

In this case, a poteyage is used which, while allowing a easy mold-part separation, is relatively conductive. most typically used is carbon black from combustion incomplete 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 demoulding, which requires a fairly long contact time between molds metallic and part:

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

• cristallisées : quartz, cristobolite, tridymite ;
• amorphes : tripoli, kieselgunr, diatomite, silice fondue.

This last material, silica, is used in the prior art, as a basic constituent of poteyage, in its different forms

• crystallized: quartz, cristobolite, tridymite;
• amorphous: tripoli, kieselgunr, diatomite, fused silica.

In practice, due to their low thermal conductivity, the silica products have great insulating power and are widely preferably used, especially since their cost is low and their significant availability.

Their implementation is generally carried out by spraying a ceramic slip on the hot mold shell, set rotation.

This slip consists of a suspension of silica flour in water previously added with products likely to improve the rheological properties, in general of gelling agents, mineral or organic.

In this way, the metal mold is coated with a refractory layer uniform.

However, according to the prior art, during casting operations centrifugal of ferrous metals to make metallic parts, for example example in lamellar graphite (GL) cast iron centrifugally cast with pearlitic structures raw casting it turned out that most of the material refractory remained adherent to the metal part and which therefore had to renew the coating, at least partially, with each operation of casting.

A known improvement in the prior art has therefore consisted in: make the coating in several layers, a first layer (or primary coating), applied directly to the mold surface and intended to remain there permanently, and a second layer (or secondary coating), applied to the previous layer and totally or partially renewed at each casting operation.

By the very fact of the rotation of the mold which creates important air movements, we cannot avoid that a fraction of the coating escapes to the outside environment, despite the use of devices of capture. In this way, a danger is created in the ambient environment significant potential for staff assigned to workstations, including included during parts cleaning and finishing operations.

However, we know the silicogenic effects of silica products crystallized, the finest particles and, in particular, those of diameter less than five (5) microns, being recognized as having the maximum degree of harmfulness.

The use of amorphous silica, reputed to be little silicogenic, does not allow to overcome this drawback because the use which is made of them has the effect of bring to high temperature and therefore transform them into crystallized silica, with the same harmful risks.

The invention therefore aims to eliminate any use of silica as an insulating coating for a metal mold, and to replace it a non-silicogenic refractory product having an insulating power sufficient.

According to the present invention, an insulating coating for mold metal intended for the casting of ferrous metals, in particular by casting centrifugal, comprising at least one primary coating applied to the mold surface and intended to remain there permanently and 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.

The secondary coating is often called in the literature maintenance coating.

By metakaolin, we mean in the remainder of this document, 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 between 750 and 950 ° C, so that completely eliminate the water of constitution.

According to one embodiment of the invention, the secondary coatings and primary consist essentially of metakaolin.

The following description will help you better understand the different preferred variants or embodiments of the invention.

From natural kaolin, which has been purified, then calcined for about 1 hour, a powder was obtained having a distribution particle size between 0.5 and 20 microns with a medium grain centered on 2 micron.

This product has the advantage of being refractory, due to its chemical constitution, to be inert, due to the absence of water from constitution, to be insulating, due to its particle size distribution and to be medically neutral, due to a free silica content of less than 0.5%.

From this product, a mixture based on metakaolin was produced. and sodium silicate having substantially the same particle size, used as a mineral binder, which has been sprayed on the bare metal shell of a mold, previously heated between 300 and 400 ° C, to constitute an adequate primary coating, of a thickness between about 50 and 100 microns.

From this same product, we also produced a suspension metakaolin powder in water, with a concentration by weight between 25 and 35%. To improve the rheological properties of this suspension, we can add a small amount of sodium bentonite, with a concentration by weight of between 0.1 and 1%.

After spray application, on the primary coating previously obtained, a secondary coating is thus produced adequate, with a thickness between about 50 and 100 microns.

The mold, thus coated, was used several times, for the casting of cast iron, in order to obtain metal parts, in particular for automobile industry.

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

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

Examination of the metal part thus produced (cylinder liners engine) in gray lamellar cast iron to show a structure pearlitic in the raw state of casting, entirely free of cementite. The presence of cementite would have been deduced by the appearance of a border soaked on the part of the part in contact with the metal mold. Such defect would have been considered unacceptable, and led to the discard such a part.

It is obvious to those skilled in the art that many variants of the invention could be imagined, from what is described above without doing creative work for it, and therefore without going out of the scope of the invention as defined by the claims below attached.

Claims (8)

1. Process for obtaining an insulating coating for a metal mould intended for casting ferrous metals, especially by centrifugal casting, comprising the application to the surface of the mould of at least one primary coating, intended to remain permanently thereon, and the application of a secondary coating to the primary coating, the said secondary coating being completely or partially replenished at each casting operation, and characterized in that the said secondary coating comprises metakaolin.
2. Process according to Claim 1, characterized in that the said secondary coating essentially consists of metakaolin.
3. Process according to Claim 1 or 2, characterized in that the primary coating also consists essentially of metakaolin.
4. Process for producing an insulating coating for a metal mould, according to Claim 3, characterized in that:

   a) a metakaolin powder is prepared from kaolin that is purified, and then it is calcined between 750 and 950°C for about 1 hour;

   b) a mixture of this metakaolin powder and sodium silicate, having approximately the same particle size, is produced and this mixture is sprayed onto the bare surface of the said metal mould, the said surface having been preheated to between 300 and 400°C, so as to obtain a primary coating with a thickness of generally between 50 and 100 microns; and

   c) a suspension of metakaolin in water is produced with a concentration by weight of between 25% and 35%, and then this suspension is sprayed onto the primary coating so as to obtain a secondary coating, with a thickness of generally between 50 and 100 microns.
5. Metal mould coated with an insulating coating obtained by implementing the process according to any one of Claims 1 to 4.
6. Use of a metal mould according to Claim 5 for the casting of ferrous metals.
7. Use of a metal mould according to Claim 5 for the manufacture of metal castings, by casting ferrous metals in the said mould.
8. Use of a metal mould according to Claim 5 for the manufacture of metal castings intended for the motor-vehicle industry, especially those made of cast iron having perlitic as-cast structures.