EP0015813A1 - Process for boronizing articles made of metal or cermet, and articles provided with a boronized surface - Google Patents

Abstract

The invention relates to a method and a device for boriding metal or cermet parts and parts botured on the surface. The parts are placed in an enclosure 3 between 850 and 1150 ° C. and they are subjected, in the presence of boron carbide, to a gas stream of trifluorinated boroxole (BOF) 3. Boron carbide is advantageously powdery and out of contact with the parts to be borided.

Description

The subject of the invention is a process for treating parts of material from the group consisting of alloys of metals of the iron family (Fe, Ni, Co) and by cermets, in which the parts are brought to an operating temperature of of the order of 850 to 1150 ° C. in the presence of a solid boronizing agent and the boronization is activated by simultaneously subjecting the parts to the action of contact of the current of a gaseous fluorinated agent under operating conditions of pressure and temperature defined. The subject of the invention is also a device for implementing the method and parts boronated on the surface.

We know from French Patent No. 2,018,609 and its equivalent US 3,673,005, a process of the type mentioned above for the boriding of steels, in which the activator is a fluoroborate mixed with the boriding agent in the presence of borax and optionally added with a diluting agent consisting of alumina. The whole reaction takes place in the solid phase and makes it possible to obtain a coating in which two phases are observed, one of FeB, the other of Fe ₂ B. But the different crystal structures of these two phases create harmful tensions. good resistance to cooling, especially since the FeB phase is more fragile, hence the risk of chipping of the coating. It is observed, on the other hand, that the parts borided by this known process keep traces of powder stuck due to the appearance of a molten phase, so that they must be subjected to an additional treatment to remove the powder. more or less sintered, more or less adherent on their surface. In addition, the activating agent, which is consumed, being in the treatment bed, it must be regenerated, for example by quarters, with a new powder after each treatment operation.

It is known, moreover, that the same process is applicable with the same advantages and disadvantages to cermets, in particular to tungsten carbide or titanium taken from a cobalt matrix. Reference is made, for example, to the article GL ZHUNKOVSKII et al. Boronizing of cobalt and some cobalt base alloys. Soviet powder metal- lurgy - 11, (1972) p. 888-90, and in article O. KNOTEK et al. Surface layers on cobalt base alloys by boron diffusion. Thin solid films, 45 (1977) p. 331-9.

The object of the invention is to propose a new very economical process and a new device making it possible to avoid the abovementioned drawbacks, in particular by obtaining a single-phase layer as regards carbon steels, and by obtaining clean parts without powder adhesion in all cases.

These aims are achieved, according to the invention, by a process of the type described at the start, thanks to the fact that the gaseous fluorinated agent contains trifluorinated boroxole (BOF) ₃ . This activating agent has many advantages which will appear below.

According to the invention, it is advantageous to use as starting gas boron trifluoride BF ₃ or a gas mixture containing BF ₃ and, according to a preferred embodiment, the gaseous fluorinated agent containing trifluorinated boroxole is produced by making pass the starting gas through a pulverulent mass of mineral oxides free of cationic impurities, such as simple or complex oxides of silicon, aluminum and magnesium, for example a silica sand, brought to a temperature at least equal to 450 ° C.

dans laquelle MO est l'oxyde simple ou complexe.In this way, there are no more disadvantages due to the internal consumption of the activating agent, since it is brought from the outside. In this way, too, and depending on the speed of passage of the fluorinated activating agent through the pulverulent mass of oxides, a moderation of the action of the effluent gaseous agent of said mass is observed. If, as will be the most economical according to the invention, the agent introduced into the mass brought to at least 450 ° C. is boron trifluoride, the effluent will contain trifluorinated boroxole according to the reaction,

in which MO is the simple or complex oxide.

In all cases, it is a great simplification according to the invention to separate the borating agent which is very little consumed, a moderating agent also very little consumed and the activating agent.

It is advantageous to bring the fluorinated activating agent at adjustable pressure, and preferably at a pressure close to atmospheric pressure, in contact with the parts to be borided.

According to one embodiment, the fluorinated agent is diluted in a neutral carrier gas.

According to one embodiment, the boriding agent can be, not only B ₄ C _l but any boron carbide B _n C, in which n is between 4 and 10. It is also according to an advantageous characteristic of the invention that it will be possible to choose to enrich or deplete in B ¹⁰ the boron of the solid boronizing agent and / or of the gaseous fluorinating agent of activation or starting gas. In this way, it will be possible to obtain parts with more or less high effective cross section for neutron shutdown by enriching with B ¹⁰ with high effective cross section or with B ¹¹ very transparent to neutrons.

According to a preferred embodiment of the invention, the solid boronizing agent and the parts to be boronized are subjected to the contact action of the stream of gaseous fluorinated agent outside mutual contact. This embodiment is decisive for enabling the production of clean parts free of more or less sintered powder. This embodiment therefore operates in the gas phase, as will be explained below, hence saving and ease of implementation.

To this end, it is advantageous that the solid boriding agent present with the parts to be borided is interposed in the stream of the gaseous fluorinated agent upstream from the parts to be borided. This embodiment will allow the parts to be borided to be placed directly in a treatment enclosure in order to expose them to the gaseous phase alone. treatment. However, for very small parts, they can however be placed in a bed constituted by a granular or powdery inert mass, such as silicon carbide.

Although, according to the preferred embodiment, the solid boronizing agent and the parts to be boronized are out of mutual contact, it is however possible that the solid boronizing agent is arranged in the form of pulverulent solid material constituting treatment bed for the parts to be boronized, as it is known per se.

It is advantageous to recycle, at least partially, the gaseous fluorinating activating agent.

• - une première enceinte de traitement de boruration,
• - des moyens de chauffage de ladite première enceinte à une température de l'ordre de 850 à 1150°C,
• - une seconde enceinte pour une masse pulvérulente ou granuleuse d'oxydes minéraux,
• - des moyens de chauffage de ladite seconde enceinte à au moins 450°C environ,
• - des moyens d'amenée d'un gaz fluoré dans ladite seconde enceinte,
• - un passage de transfert de l'effluent gazeux fluoré de la seconde enceinte à la première enceinte,
• - des moyens d'évacuation de l'effluent gazeux fluoré de ladite première enceinte.

A device particularly suitable for implementing the invention comprises:

• - a first boronization treatment enclosure,
• means for heating said first enclosure to a temperature of the order of 850 to 1150 ° C,
• - a second enclosure for a powdery or granular mass of mineral oxides,
• means for heating said second enclosure to at least approximately 450 ° C.,
• means for supplying a fluorinated gas into said second enclosure,
• a passage for the transfer of the fluorinated gaseous effluent from the second enclosure to the first enclosure,
• - means for discharging the fluorinated gaseous effluent from said first enclosure.

The invention also relates to the parts of carbon steels having undergone a boriding treatment on the surface to a thickness of about 20 to 200 μm covered with a single-phase layer of Fe ₂ B crystals of acicular formation.

• - La figure 1 est un schéma d'ensemble d'une installation selon l'invention pour la mise en oeuvre du procédé selon l'invention,
• - les figures 2, 3 et 4 sont des coupes partielles à plus grande échelle, de la partie du réacteur de la figure 1 contenant les deux enceintes décrites plus loin,
• - les figures 5 à 11 sont des coupes micrographiques d'aciers borurés par le procédé de l'invention,
• - la figure 12 est une vue en coupe, à plus grande échelle, d'une variante pour le réacteur compris dans le schéma de la figure 1.

Other characteristics and advantages will emerge from the description, which will be given below, only by way of examples, of embodiments of the invention. For this purpose, reference is made to the drawings and attached micrographs, in which:

• FIG. 1 is an overall diagram of an installation according to the invention for implementing the method according to the invention,
• FIGS. 2, 3 and 4 are partial sections on a larger scale, of the part of the reactor of FIG. 1 containing the two enclosures described below,
• FIGS. 5 to 11 are micrographic sections of boron steels by the process of the invention,
• FIG. 12 is a sectional view, on a larger scale, of a variant for the reactor included in the diagram of FIG. 1.

An installation according to the invention comprises a reactor 1 made of refractory steel. In this reactor are arranged, from top to bottom, two enclosures 2 and 3 simply separated by a retaining grid 4 disposed at the bottom of the enclosure 2. The bottom enclosure 3 is intended to contain the parts to be boronized 6. L the upper enclosure 2 is intended to contain a pulverulent mass of mineral oxides 7. The reactor 1 is in an oven 8, the temperature of which is regulated, in a manner known per se, by means of a thermocouple 9.

A pipe 10, controlled by a valve 11 is inserted into the upper wall of the reactor 1, so as to open into the enclosure 2. The enclosure 3 and the reactor 1 are closed, at the bottom, by porous parcels, respectively 12 and 13, the porous wall 13 being closed, on its other face, by a pipe 14 for discharging gaseous effluents.

The valve 11 is connected, for the supply of gas, to two gaseous sources, respectively a source 15 of compressed boron trifluoride and a source 16 of diluent inert gas, such as argon or nitrogen. These two sources 15 and 16 are connected to the valve 11 through two flow meters 17 and 18 discharging on a pipe com mune 19.

For its part, the pipe 14 arrives on a valve 20 connected to a pressure gauge 21 and to a washer assembly 22 by a pipe 23. At the outlet of the pipe 24, it is possible to add a distribution valve 25 between a pipe d discharge 26 and a recycling pipe 27, which brings part of the gas effluent to valve 11, which is then a mixing valve.

In the embodiment of FIG. 1, provision has been made for the lower enclosure 3 to contain the boriding agent 5 in the form of a bed coating the parts, as is known per se. But according to the embodiment of Figure 2, the lower enclosure 3 does not contain any powdery or granular bed. In this case, the parts 6 and the solid boronizing agent are separated from each other, the agent is arranged in the form of sintered elements 30 suspended in the lower enclosure 3.

But, according to the invention, the preferred embodiment is that of Figures 3 and 4 which differs from the previous one, by the presence of a retaining grid 31 disposed in the upper part of the lower enclosure 3 for an interposed bed solid borating agent 33 in powder form with a particle size of 1 to 2 μm on the path of the gaseous activating agent supplied through the powdery mass of mineral oxides 7. The embodiment of FIG. 3 is suitable for small parts which can be coated with powdered silicon carbide 34, as an inert agent. In FIG. 4, we simply omitted to place the bed of silicon carbide to deposit the part or parts 6 directly in the enclosure 3.

In the installation of FIG. 1, a boronizing agent of known type consisting of a powder of B ₄ C at a particle size of 1 to 100 μm mixed with a powder of silicon carbide of 100 has been placed in enclosure 3. µm in mass proportion from 2/98 to 100/0. In enclosure 2 we put a pure silica sand washed with acids 90% of which passes through a 2 mm sieve. After placing the parts to be treated in the bed of enclosure 3, the enclosure is scanned with a neutral gas, nitrogen or argon, at the same time as the temperature is raised. Then the BF ₃ gas, possibly diluted, is sent when the temperature reaches approximately 500 to 950 ° C. The latter is chosen as the boronization temperature. The duration of the passage of the activating gas varies from half to the entire time this stay of the parts at 950 ° C., said residence time having been approximately 5 hours. Simultaneously, the temperature of the silica bed 7 was brought to about 850 ° C.

Examples 1 and 2 It was tested with a mass proportion B ₄ C / SiC of 20/80 two carbon steels at 0.1% and 0.35% respectively designated, XC 10 and XC 35, according to AFNOR designation. After cooling, the parts were examined in the laboratory.

We could see (see the micrographic sections in Figures 5 and 6) that, in both cases, the parts were covered with a single-phase layer A of 170 µm of Fe ₂ B crystals oriented with formation of teeth penetrating well into the metal C by constituting an acicular formation there. A layer B of only 10 μm of undirected FeB / Fe ₂ B crystals covered the layer of Fe ₂ B and was therefore not likely to cause harmful tensions therein, since, as in the known processes, this layer can be eliminated by simple sandblasting, or even preserved as such as being eliminated in use if we can accept parts with a mat appearance.

We thus obtained useful layers of 170 μm, practically single-phase, whereas with the known process, all other things being equal, we obtained layers of 200 μm but two-phase with two layers of different strongly oriented phases of FeB and Fe ₂ B in a proportion 1/2 to 1/3.

Then, according to the preferred embodiment of the invention, the installation of FIG. 1 was modified as shown in FIGS. 2, 3 and 4.

Example 3 In the embodiment of FIG. 2, a piece of carbon steel 6 was placed in the presence, but without contact, of hot sintered pieces 30 made of β boron, B ₄ C and B ₁₀ C. We sent BF ₃ through the sand bed 7 of enclosure 2 for 18 h while maintaining the temperature of enclosure 3 at 1000 ° C. FIG. 7 shows a micrographic section of the steel thus borided.

Example 4 In the embodiment of FIG. 3, two parts, one made of carbon steel, the other of chromium-nickel 18/10, were placed in the SiC bed of enclosure 3. BF ₃ was surrounded by the sand bed 7 of enclosure 2 for 2.5 h while maintaining the temperature of enclosure 3 at 1020 ° C. FIG. 8 shows a micrographic section of the carbon steel thus boronized and in FIG. 9 a section of the chrome-nickel steel thus boronized.

Examples 5 and 6 In the embodiment of FIG. 4, gold treated a piece 6 of carbon steel having received two saw cuts of 0.5 mm on its edge. BF ₃ was sent through the sand bed 7 of enclosure 2 for 2 h at 1000 ° C. FIG. 10 shows a micrographic section of the external surface of the workpiece and in FIG. 11 a micrographic section of the surface of the notch with a saw.

Each of these operations led to the boriding of the steel parts present in the reactor. The thickness of the compact layer (Fe ₂ B alone) is fairly small in the case of the method of Example 3, ₁₅ to 20 μm approximately. The metallographic study of the parts thus treated provides information on the morphology of these layers. In the case of the method of Example 4, they are identical to those already observed in Examples 1 and 2. The layer is not actually flat (FIG. 9), note the stopping of boriding at certain joints of grains when these are parallel to the surface or have an angle of up to about 120 ° with it. FIG. 7 shows the appearance of the borated layer obtained in the case of the reactor of Example 3. The progression of the dendrites is not perpendicular to the surface but has been disturbed by the presence of a phase which appears as perlite after cooling. The boronization speed therefore has an important influence on the progression of the boronized layer in the matrix and the growth direction [001] is not absolute.

With regard to the part having received saw cuts, it can be seen that this part is boronised (FIGS. 10 and 11), not only on the two external faces (90 to 120 μm), but also on the internal faces defined by the saw cuts. A micrograph of these internal faces shows a borated layer of variable thickness and of discontinuous acicular character which is explained by the only intervention of a gas phase.

We draw from these tests, the conclusion that boruration in the gas phase being perfectly satisfactory, it becomes possible industrially in the reactor to separate the enclosure of the generator from borated gaseous agents (BF ₃ + Si0 ₂ , B ₄ C), parts metal to boron which can be conveniently arranged in a SiC bed or if necessary, left naked.

It can be seen that the invention has made it possible to develop an original process making it possible to boron all steels up to tool steels with total reliability. The previous processes led to poor quality parts on mild steels (formation of two layers FeB + Fe ₂ B), the flexibility of the process of the invention combined with the use of an activation moderator (Si0 ₂ ) allows, including in industrial conditions, to produce parts of consistent and satisfactory quality. Mechanical tests have shown that the behavior of the layers obtained on tool steel was of very good quality. As with the known methods, boronization of stainless steel to chromium-nickel 18.10 remains of weak effect.

In addition, from a purely industrial point of view, the advantages of the process are considerable: simplicity, flexibility, economy of labor (not sticking from powder to parts) and total reliability based on numerous full-scale tests. The cost of the operation is divided by approximately three as for consumable materials and handling is reduced to a minimum.

The preceding operating conditions are the preferred conditions, but it has been possible to obtain valid results with Al ₂ O ₃ and MgO, it being noted however that these two oxides lead to a fairly high activity of the effluent used as activating gas agent. , which then contains boric anhydride B ₂ 0 ₃ . The moderating role of Si0 ₂ is ultimately the most favorable, which makes it preferable.

The durations, percentages and particle sizes given in the previous description are not limiting. They can be varied as a function of the desired greater or lesser speed of formation and thickness of the layer. Some of these factors have only a slight influence, such as, for example, the particle size of B ₄ C and of SiC.

The applicant has also been able to observe good results with boron carbides other than B ₄ C, such as borides B _n C, in which n is between 4 and 10.

It will not depart from the scope of the invention to supply activating gas to several boronization enclosures 3 from a single enclosure 2.

As regards the application of the invention to cermets, tests were carried out on tungsten carbide tools containing variable proportions of cobalt (or nickel, or iron) with the installation of FIG. 1. With a BF ₃ flow rate of 1 to 5 1 / h and by setting the treatment temperature between 800 and 1100 ° C, borided parts are obtained.

At 950 ° C the main phase detected by X-ray diffraction is CoB; the mixed boride W ₂ CoB ₂ also seems to be present, on the other hand W ₂ B ₅ is absent. Depending on the temperature, various mixed borides (W-Co) can form.

Tests of machining by carriage of different materials (carbon not graphite, Nickel stainless steel - chromium 18-10, high speed steel, ceramics, ...) were carried out. It has been observed that the borated tool shows a wear resistance much higher than that of the untreated tool and that the test on high speed steel shows that the borated or non-borated tools deteriorate fairly quickly; however, the cut obtained with the borided tool is straightforward (the non-borided inserts do not allow the cut).

In Figure 12, there is shown a particularly simple embodiment of a reactor for implementing the method of the invention. The lower part of the reactor constitutes the enclosure 3 closed by a watertight cover 40 with seal 41 cooled with water. The enclosure 2 is produced in the form of a container which can be fitted into the reactor before fitting the cover 40. The bottom of the enclosure 2 comprises the grid 4 for retaining the sand and letting the activation gas pass and a grid 31 for retain boron carbide, preferably powdery. A pipe 10 fixed to the enclosure 2 crosses the cover to bring BF ₃ through the sand of the enclosure2. The cover is crossed by a central chimney 14 which also crosses, in a sealed manner, the enclosure 2 to end near the bottom of the reactor under a grid 12 for retaining the parts to be borided. The temperature probe 9 can be placed in the chimney 14.

Claims (18)

1) Method of treatment by boriding of parts of material from the group consisting of alloys of metals of the iron family (Fe, Ni, Co) and by cermets, in which the parts are brought to an operating temperature of the order from 850 to 1150 ° C in the presence of a solid boronizing agent and boriding is activated by simultaneously submitting the parts to the action of contact of the current of a gaseous fluorinated agent under defined operating conditions of pressure and temperature, characterized in that the gaseous fluorine-containing agent contains rifluorinated boroxole ( ^BOF ) ₃ . 2) Process according to claim 1, in which BF ₃ or a gaseous mixture containing BF ₃ is used as the starting gas, characterized in that the gaseous fluorinated agent is produced containing trifluorinated boroxole by passing the starting gas to through a powdery mass of inorganic oxides free from cationic impurities, such simple oxide or silicon complex, aluminum and magnesium, heated to a temperature at least equal to ⁴ 50 ° C. 3) Method according to claim 2, characterized in that the pulverulent mass of mineral oxides consists of silica sand. 4) Method according to any one of claims 1 to 3, characterized in that the gaseous fluorinating agent of activation is brought into contact with the parts with adjustable flow. 5) Method according to any one of claims 1 to 4, characterized in that the gaseous fluorinating activating agent is brought into contact with the parts at a pressure close to atmospheric pressure. 6) Method according to any one of claims 1 to 5, characterized in that the fluorinated agent activation gases and / or the starting gas are diluted in a neutral carrier gas. 7) Method according to claim ₂ , characterized in that the pulverulent mass of mineral oxides is at a substantially homogeneous particle size between 0.1 and 2 mm approximately. 8) Method according to claim 1, characterized in that the solid boronating agent is a boron carbide B _n C, in which n is between 4 and 10. 9) Method according to claim 1, characterized in that the boron of the solid boronizing agent and / or of the gaseous fluorinating agent of activation or starting gas is enriched or depleted in B ¹⁰ . 10) Method according to claim 1, characterized in that the solid boronizing agent and the parts to be boronized are subjected to the contact action of the stream of gaseous fluorinated agent out of mutual contact. 11) Method according to claim 1, characterized in that the solid boriding agent present with the parts to be borided is interposed in the current of the gaseous fluorinated agent upstream of the parts to be borided. 12) Method according to claim 1, characterized in that the parts to be borided are arranged in a bed constituted by a granular or pulverulent inert mass, such as silicon carbide, to maintain the parts to be borided there, as is known in oneself. 13) Method according to claim 1, characterized in that the solid boriding agent is arranged in the form of pulverulent solid material constituting treatment bed for the parts to be borided, as it is known per se. 14) Method according to any one of claims 1 to 13, characterized in that at least partially the fluorinated gaseous activating agent is recycled. 15) Device for implementing the method according to any one of claims 2 to 14 comprising a first enclosure for boriding treatment and means for heating this enclosure to a temperature of the order of 850 to 1150 ° C, characterized in that it comprises: - a second enclosure for a powdery or granular mass of mineral oxides, means for heating said second enclosure to at least approximately 450 ° C., means for supplying a fluorinated gas into said second enclosure, a passage for the transfer of the fluorinated gaseous effluent from the second enclosure to the first enclosure, - means for discharging the fluorinated gaseous effluent from said first enclosure. 16) Device according to claim 15, characterized in that it comprises, in the zone bordering on the two said enclosures, a retaining means, permeable to gases, for an interposed bed of sintered or pulverulent solid boriding agent. 17) Device according to any one of claims 15 and 16, characterized in that the means for discharging the gaseous effluent from said first enclosure is a chimney passing through the two said superimposed enclosures. 18) Parts of carbon steels having undergone a boriding treatment on the surface over a thickness of approximately 20 to 200 μm, characterized in that they are covered with a single-phase layer of Fe ₂ B crystals of acicular formation.

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