EP0145576A2 - Additive for maintaining the working order of molten-salt baths for the treatment of metal pieces - Google Patents

Abstract

1. A make-up charge which is intended to compensate for the losses of a bath of molten salts for the treatment of metal components, for maintenance of said bath, said losses being due on the one hand to entrainment of the molten salts by the components when they come out of the bath and on the other hand degeneration of an active component into a less active degraded component which, in its ultimate form of degeneration in which it has become totally inactive, is referred to as a degradation product, said degradation product being capable of becoming the active component again by reaction with at least one regenerating agent introduced into the bath for that purpose, the charge being characterised in that it is formed by compressed pellets produced by compacting of a mixture of at least two particular materials, one of said materials being formed by a replenishing product having the same composition as the degradation product and another material being formed by said regenerating agent.

Description

The molten salt baths for the treatment of metallic parts generally comprise at least one active compound, for example cyanate for nitriding, carbide for cementation, etc.

Such treatments by baths of molten salts are aimed in particular, in whole or in part, at improving the resistance to wear, seizure, corrosion and fatigue of metal parts.

These baths have been around for a long time and are still used with interest. Examples of nitriding baths are illustrated, inter alia, by French patents 1,073,922, 1,255,191, 2,171,993 and 2,271,307 and its addition 2,307,051.

But molten salt baths, whatever their type, suffer losses during their service.

These losses are due, on the one hand, to the entrainment of molten salts by the parts leaving the bath, on the other hand, to the degeneration of the active compound by reaction with the treated parts and, if necessary, with the atmosphere, oxidation most often. The active compound is thus transformed little by little into a degraded compound less and less active containing more and more a completely degenerate and inactive product, in general carbonate, designated in what follows by product of degradation.

• - d'une part, un sel dit "sel de base" de composition sensiblement identique à celle du bain fondu au moment où on l'introduit et qui sert principalement à rétablir le niveau du bain ;
• - d'autre part, un sel dit "régénérant", qui peut être un composé unique ou un mélange de plusieurs composés et qui permet, par réaction dans le bain avec le produit dégradé,de ralentir ou d'inverser le phénomène de dégradation du bain, en reconstituant le composé actif.

To compensate for these losses from the bath and thus ensure the maintenance of the latter, additions of two kinds are generally introduced into the bath:

• - On the one hand, a salt called "base salt" of composition substantially identical to that of the molten bath at the time when it is introduced and which mainly serves to restore the level of the bath;
• - on the other hand, a so-called "regenerating" salt, which can be a single compound or a mixture of several compounds and which makes it possible, by reaction in the bath with the degraded product, to slow down or reverse the phenomenon of degradation of the bath, by reconstituting the active compound.

• - le sel de base est un composé complexe, par conséquent coûteux ;
• - le fait de se servir de deux sels différents, de base et de régénération, oblige à des contrôles fréquents, parfois bi-quotidiens, de la composition du bain ;
• - il y a rarement adéquation parfaite entre les exigences du maintien de niveau et celles du maintien d'analyse, de sorte que du sel est perdu ;
• - parfois, la réaction de régénération est trop vive et provoque une effervescence du bain inacceptable ;
• - parfois, chaque opération de régénération donne suite à un changement trop brusque du pouvoir opérant du bain, pouvoir nitrurant, pouvoir carburant, etc.. et la couche créée sur les pièces par le traitement ne présente, ni la compacité ni l'épaisseur, fidèles et constantes, requises pour les besoins des industries mécaniques.

This way of proceeding has various drawbacks:

• - base salt is a complex compound, therefore expensive;
• - the fact of using two different salts, base and regeneration, requires frequent checks, sometimes twice-daily, of the composition of the bath;
• - there is rarely a perfect match between the requirements for level maintenance and those for analysis maintenance, so that salt is lost;
• - sometimes, the regeneration reaction is too strong and causes an unacceptable effervescence of the bath;
• - sometimes, each regeneration operation gives rise to an abrupt change in the operating power of the bath, nitriding power, fuel power, etc. and the layer created on the parts by the treatment has neither compactness nor thickness, faithful and constant, required for the needs of the mechanical industries.

The subject of the present invention is a feedstock intended to compensate for the losses of a bath of molten salts for the treatment of metal parts with a view to the maintenance of this bath, these losses being due, on the one hand, to the entrainment of molten salts by the parts leaving the bath, on the other hand to the transformation of an active compound into degradation product, this degradation product being capable of becoming again the active compound by reaction with a regenerant admitted into the bath this effect, this filler providing, for the first time, the possibility of maintaining a bath of molten salts for the treatment of metal parts in the form of a single total addition, of allowing its introduction into the bath without gassing harmful and to maintain the active power of the bath faithful and constant, said charge being made from inexpensive materials which are commonly found in commerce.

According to the invention, this feedstock is in particular characterized in that it consists of tablets obtained by compacting a mixture of at least two particulate materials, one of these materials being formed from a restocking product of the same composition. as the degradation product, and another material being formed of said regenerant.

Such a charge has multiple advantages.

It allows, first of all, with a unique global addition, to satisfy both and in a perfectly precise way the requirements of maintaining the level of the bath and those of maintaining the quality of the bath. It is sufficient to provide in advance, in suitable proportions, the respective quantities, on the one hand, of restocking product of the same composition as the degradation product and, on the other hand, of regenerating, so that the bath is maintained in perfect conditions.

In addition, as the replenishment material is none other than the imitation degradation product, this material is generally inexpensive.

When the filler is introduced into the bath, the fact that the regenerant is premixed with the simulated degradation product, makes the action of the regenerant in the bath progressive, which avoids any risk of effervescence and allows excellent stability.

It will be appreciated that the overall charge according to the invention therefore has the particularity of bringing together, for the maintenance of the bath, two constituents, the first of which corresponds to the ultimate product of degeneration of the bath, while the other, in an original manner, has a regeneration mission which goes beyond the sole needs of the bath, since it is called upon to regenerate not only the bath salts that have become inactive, but also the first constituent itself, admitted at the same time as it in the bath.

According to another characteristic, the regenerant is present in the tablet in the form of at least one granule, and preferably in the form of a multitude of granules, coated with the restocking product.

Thanks to this arrangement, when the regenerant is admitted into the bath, it is protected from too rapid contact with the bath thanks to the gangue of imitation degradation product which is allowed to melt slowly and this is extremely progressive and favorable that the regenerant is allowed to act in the bath. Consequently, we obtain, at the same time as the restoration of the level of the bath, an effective regeneration and spread over time, in excellent conditions.

In a preferred embodiment, each regenerant granule coated with the restocking product in the tablet is preformed under high compactness before coating and compacting of the tablet.

Thus the regenerant is localized in the tablet in depth and in a precise manner, which allows an effective and faithful mode of action.

According to another characteristic, with respect to the proportions which correspond to the reaction between the degradation product and the regenerant, the tablet has a slight excess of regenerant, this excess being by weight, of the order of 1% to 5% and of preferably around 2 ° ô. In this way, the charge acts in the bath in a balanced manner, since the regenerant is in a quantity suitable for ensuring the regeneration both of the imitation degradation product which is admitted with it into the bath, and of the degradation product which s is already there.

In the case, for example, of a treatment by nitriding, the active compound comprises a cyanate and the tablets advantageously comprise granules of urea and of urea condensation products, coated with carbonate.

The granules advantageously have a dimension of the order of a millimeter, while the tablet preferably has a dimension of the order of several centimeters, which allows great convenience in use.

A nonlimiting example of a feedstock according to the invention is described below.

A bath of molten salts, working at around 560 ° C., is provided for a treatment by nitriding of metal parts in order to improve their resistance to wear, seizure, corrosion and fatigue.

The active principle of this bath is based on yanate.

This bath suffers losses which are due; on the one hand, the entrainment of molten salts by the parts leaving the bath, on the other hand, the degeneration of the active compound cyanate into a less active degraded compound which in its ultimate form of degeneration where it has become totally inactive is a product carbonate degradation. When the carbonate content increases, the nitriding potential of the bath decreases, the melting point of the bath rises and correspondingly, the viscosity of the bath increases, so that the quality of the treatment is not constant.

For the maintenance of the bath, a filler charge according to the invention is used which alone and overall makes it possible to compensate for all the losses of the bath.

This charge consists of tablets obtained by compacting a mixture of at least two particulate materials, one of these materials being formed from a restocking product consisting of a similar degradation product, and another material being formed from the regenerant. This regenerant, which can itself be a mixture of several compounds, is present in the tablet in the form of at least one granule, and preferably of a multitude of granules coated with the restocking product.

More particularly, the feedstock is presented as the mixture of the imitation product totally degraded by oxidation, in this case alkali carbonates, and of the regenerant, in this case urea, or at least one of its condensation products. , or advantageously a mixture of urea and at least one of its condensation products.

Initially, the regenerant is pressed in the form of very compact granules, having a dimension of the order of a millimeter. Then these granules are mixed with carbonates, these being brought in the form of powder.

In the case, for example, where the regenerant is urea, its reaction with carbonates can be written as follows:

We therefore see that the strict mixture should be 2 kg of urea and 1 kg of carbonates in CO ₃ ^{2- form} ; but the charge according to the invention advantageously has 1 to 5% and preferably 2% more regenerant than in the formula and its composition is actually 1 kg of alkali carbonates in the form of CO ₃ ^2- and 2.02 kg of urea.

Quite advantageously, a mixture of urea and at least one of its condensation products is used as regenerator. By acting in this way and with respect to pure urea, the gassing of ammonia gas and water vapor is limited, according to a reaction of the same type as that described above, which concretely results in the formation of the surface of the parts of layers much more compact, that is to say more resistant to wear. However, this beneficial effect is only obtained if a precise proportion is compacted between the respective amounts of urea and of its condensation product (s). In this case, the weight ratio between the amount of urea and that of its condensation products should be between 1.5 and 2.

In a second step, the mixture according to the proportions of the invention of the regenerant granules and of the powder of the restocking product is compacted under a pressure of several tons per square centimeter, in the form of tablets of great compactness, having a dimension of the order of several centimeters. Said compaction pressure must be greater than 2 tonnes per square centimeter and it is advantageously of the order of 5.

Experience shows that the feedstock according to the invention is then completely capable of ensuring, by itself, perfect maintenance of the bath of molten nitriding salts.

This charge, made of current products, is inexpensive. As the reaction between the regenerant and the carbonates has a very small surface, this reaction is regular. The effervescence is avoided and the absence of variations in the nitriding effect of the bath is noted.

In addition, there is a significant saving of energy, because the reaction of the regenerant on the carbonates is very exothermic and provides calories in situ.

The granules of the regenerant can be uniformly distributed in the restocking product, but, preferably, they are concentrated in a central region of the tablet, which allows better progressiveness in the mode of action of the regenerant in the bath of molten salts.

Of course, the invention is not limited to the example describes but embraces all variants in the realization of its various elements and in its applications, within the scope of the claims. In particular, the filler according to the invention can be used, not only with baths of molten nitriding salts, but also with all other suitable baths, in particular cementation baths, the active compound of which is a carbide degenerating into carbonate.

Claims (13)

, 1) Filler charge intended to compensate for the losses of a bath of molten salts for the treatment of metal parts, with a view to the maintenance of this bath, these losses being due, on the one hand, to the entrainment of salts melted by the parts leaving the bath, on the other hand, on the degeneration of an active compound into a less active degraded compound which, in its ultimate form of degeneration where it has become totally inactive, is said to be a product of degradation, this degradation product capable of once again becoming the active compound by reaction with at least one regenerant admitted to the bath for this purpose, charge characterized in that it consists of tablets obtained by compacting a mixture of at least two materials particulate, one of these materials being formed of a restocking product having the same composition as the degradation product and another material being formed of said regenerant. 2) Filler according to claim l, characterized in that the regenerant is present in the tablet in the form of at least one granule coated with the restocking product. 3) Filler according to claim 1 or claim 2, characterized in that the regenerant is present in the tablet in the form of a multitude of granules coated with the restocking product. 4) Filler according to claim 3, characterized in that the regenerant granules are uniformly distributed in the restocking product. 5) Filler according to claim 3, characterized in that the regenerant granules are concentrated in a central region of the tablet. 6) Filler according to any one of claims 1 to 5, characterized in that each regenerant granule coated with the restocking product in the tablet is preformed under high compactness before coating and compacting of the tablet. 7) Filler according to any one of claims 1 to 6, characterized in that the compaction of the compri mé is produced under a pressure of several tonnes per square centimeter. 8) Filler according to claim 7, characterized in that the compaction pressure is greater than 2 tonnes per square centimeter and preferably of the order of 5 tonnes per square centimeter. 9) Filler according to any one of claims 1 to 8, characterized in that each granule has a dimension of the order of a millimeter, while the tablet has a dimension of the order of several centimeters. 10) Filler according to any one of claims l to 9, characterized in that compared to the proportions which correspond to the reaction between the degradation product and the regenerant, the tablet has a slight excess of regenerant. 11) Filler according to claim 10, characterized in that this excess is, by weight, of the order of 1% to 5% and preferably close to 2%. 12) Filler according to any one of claims 1 to 11, in which the active compound comprises a cyanate, characterized in that the tablets comprise granules of urea and / or of its condensation products coated in carbonate . 13) Filler according to claim 12, wherein the regenerant consists of a mixture of urea and one or more condensation products of urea, characterized in that the weight ratio between the amount of urea and that of its or its condensation products is between 1.5 and 2.