US2874070A

US2874070A - Method for the formation of diffusion superficial alloys, in particular chromium alloys

Info

Publication number: US2874070A
Application number: US465657A
Authority: US
Inventors: Galmiche Philippe
Original assignee: Office National dEtudes et de Recherches Aerospatiales ONERA
Current assignee: Office National dEtudes et de Recherches Aerospatiales ONERA
Priority date: 1951-05-16
Filing date: 1954-10-29
Publication date: 1959-02-17
Anticipated expiration: 1976-02-17

Description

2,874,070 ION SUPERFICIAL UM ALLOlgS Feb. 17, 1959 P. GALMICHE OR THE FORMATION'OF DIFFUS LOYS, IN PARTICULAR CHROMI Sheets-Sheet 1 METHOD F 'AL Filed Oct; 29. 1954 INVENTUR PHILIPPE GALNICHE BY 6 a ATTORNEY nmurwrwmq L av P. GALMICHE 2,874,070 N SUPERFICIAL ALL RMATION OF DIFFUSIO PARTICULAR CHROMIUM OYS v 5 Sheets-Sheet 2 Feb. 17, 1959 FOR THE F0 LLOYS, IN

METHOD A Fil ed Oct. 29, 1954 /NVENTDR PHILIPPE BALMICH E M 'ATTDPINEY 17, 1959 F GALMICHE 2,874,070

METHOD FOR THE FORMATION OF DIFFUSION SUPERFICIAL ALLOYS, IN PARTICULAR CHROMIUM ALLOYS Filed 001;. 29, 1954 5 Sheets-Sheet 3 INVENTDR PHI IPPE CALMICHE y/ AT TDHNEY I Feb.'17, 1959 Filed 001;. 29. 1954 P. GALMICHE METHOD FOR THE FORMATION OF DIFFUSION SUPERF'ICIAL ALLOYS, IN PARTICULAR CHROMIUM ALLOYS 5 Sheets-Sheet 4 f w i w 9 r l ET 1 W I I L l.l A (D Y gnu m nu 5 \J um um 1 H \\H v \\H 22 INVENTDR PHILIPPE GALMIEHE EzY 2,874,070 IFFUSION SUPERFICIAL Feb. 17, 1959 I P. GALMICHE I METHOD FOR THE FORMATIONOF D ALLOYS, IN PA M ALLOYS RTICIJLAR- CHROMIU 5 Sheets-Shae Filed Oct. 29, 1954 Fig];

INVENTUR PHILIPPE BALM/CHE ATTUHNFY United States Patent METHOD FOR THE FORMATION OF DIFFU- SION SUPERFICIAL ALLOYS, IN PARTICU- LAR CHROMIUM ALLOYS Philippe Galmiche, Paris, France, assignor to Office National dEtudes et de Recherches Aeronautiques, 'Chatillon-sous-Baqneux, France, a society of France Application October 29, 1954, Serial No. 465,657 Claims priority, application France May 16, 1951 so Claims. or. 117-107 addition metals.

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, superficial hardness, when the pieces to be treated are of small size.

On the contrary, when these pieces have an important mass and/ or a high thermal conductivity, it may happen, in particular at the beginning of the treatment, that condensations of the halide or halides of the addition metal or metals are formed on the pieces to be treated.

The object of the present invention is to provide methods and apparatus for the formation of such alloys which are better adapted to meet the requirements of practice than those known up to the present time.

The invention relates to the case where the diifusion alloy is obtained by subjecting the piece to be treated to the action of at least one halide of an addition metal, obtained from a suitable cementing substance, and its essential feature consists in preventing condensation of said. halide on the piece in treatment and also, if

- necessary, on anyregeneration metal as may be in contact with said piece. a

Preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example, and in which: i

Fig. 1 is a diagrammatic vertical section of -a furnace for the formation of a superficial alloy according to my invention;

Figs. 2, 3, 4, 5 and 6 show other furnaces according to my invention;

Fig. 7 shows, on an enlarged scale and in vertical section, a reaction case to be used in the furnace shown by Fig. 6;

Fig. 8 diagrammatically shows a continuous chromizing furnace for carrying out my invention;

Fig. 9 shows, on an enlarged scale and in elevational view partly in section, a reaction case intended to be circulated through the furnace of Fig. 8;

.Figs. 10 and 11 are diagrammatical views of two other furnaces for carrying out my invention.

In what follows, it will be supposed, as a rule, that it is desired to form on ferrous pieces 1 superficial diffusion alloys including chromium.

It is known that intermetallic diffusion alloys may be obtained by the action on the pieces to be treated of halides of the addition metals and in particular of the fluorides of these metals, intermetallic ditfusion taking place either in a purely gaseous phase, that is to say with the pieces wholly out of contact both with the cementing mixture which supplies said metals and with any metal which may be used for regenerating active vapors, or according to what is called the semi-contact process, i. e. with the pieces in contact with a regeneration metal in the divided state, the vapors of halide or halides of the addition metals being caused to pass through this mass of regeneration metal.

Such methods make it possible to obtain perfectly bright surfaces, with an' even protective layer of good These condensations result, .at the end of the treatment, in the formation of dull zones on pieces 1, and, in the case of the semi-contact process, in the fact that the regeneration metal sticks to the surface of said pieces.

It was found that most of these detrimental effects take place during the period of heating up of the reaction chamber to the temperature of operation. 7

I will therefore first examine the application-of the present invention to this period.

According to this invention, and in order to avoid the drawbacks above mentioned, I proceed in such manner that, at least from the time when the halide of the addition metal has, in the vicinity of the pieces to be treated, a vapor tension capable of giving rise to detrimental condensations on the pieces in course of treatment, the surface temperature of said pieces is not more than 200 C. below the temperature of the cementing mixture and is preferably higher than said temperature.

For the sake of simplicity, it will be hereinafter supposed that the treatment is performed with a single halide.

In a general manner, I may, according to the present invention, either cause the halide vapors to condense on a filter layer advantageously constituted by a metal in a divided state and of high conductivity, so that the heating of pieces 1 up to the desired temperature can take place without contact with halide vapors at a higher temperature, or subject the pieces to be treated to a. preliminary heating, or again keep the cementing mixture at a temperature lower than the surface temperature of said pieces In the first case, I may dispose, between the cementing mixture and the ,first pieces to be treated, a sufficient layer of a metallic material in big lumps, on which condensation takes place, before the halide can reach the insufficiently hot pieces to be treated. 7

- I might also, when the treatment is to be applied simultaneously both to light pieces (for instance clockwork elements) and to heavier pieces, instead of providing a special filter layer, place the heavier pieces in regions of the chamber of treatment at a greater distance from the reserves of cementing mixture than the regions where the lighter pieces are located.

In this case, preferably, I make use of the I gaseous phase process for the lighter pieces and of the semicontact process for the bigger pieces which suffer less risk of being deformed in contact with the lumps of regenerating metal.

The preheating solution is advantageously employed, in case of a purely gaseous phase treatment,.by subjecting the pieces to be treated to an independent preheating in a neutral or reducing atmosphere.

On the contrary, when the treatment is carried out with the semi-contact process, it will be advantageous to proceed to the preheating, in suitable vessels and in a neutral or reducing atmosphere, of both the pieces to be treated and the metallic reserve of the addition metals. As for the third solution (suitable adjustment of the temperature of the cementing mixture), it may be applied, either by providing, between the heating wall and the cementing mixture, an intermediate screen of low thermal conductivity, or by heating the'chamber of treatment in such'manner that the pieces of greater mass are heated first, the cementing mixture being brought to high temperature only when all risks of condensation of the halide have disappeared, or again by limiting the rate of heating of the cementing mixture by addition of water or a halogen acid in aqueous solution, the amount thereof being higher than that necessary for the formation of the desired amount of halide of the addition metal, the latent heat of vaporization of water limiting the vapor tension of the halide until the diffusion equilibrium has been reached.

Fig. 1 shows, merely by way of example, an apparatus for applying some of the features above stated.

The cementing mixture is disposed at A at the lower part of a removable case 2. The sweeping gases (hydrogen, ammonia, either cracked or not, for instance) flow in through a conduit 3 and out through a conduit 4, these conduits being fixed while case 2 is interchangeable.

The lighter pieces are treated in a purely gaseous phase in a perforated cage 15, whereas the heavier pieces are embedded in a reserve'B of chromium or ferro-chromium in the form of big lumps (semi-contact process) in which may be incorporated some neutral ammonium fluoride to produce a sweeping gas stream, and possibly molybdenum.

Furthermore, the pieces of higher thermal capacity are located at a greater distance from cementing mixture A than the lighter pieces, a layer C of ferro-chromium in the divided state being possibly provided to promote, between said cementing mixture and the lighter pieces, condensation of the halide during the heating period.

On the other hand, the heating elements 16 act upon the side wall and the upper wall of case 2 but not on the bottom thereof, which supports the cementing mixture A, which is favorable for the proper preliminary heating of pieces 1. In order to obtain a preheating of the heavier pieces 1, I may also distribute the heating elements 16 in several groups which are separately fed wtih current, the elements which correspond to the portion of the heating case where are located the heavier pieces being then fed first.

If it is merely desired to chromize, in a purely gaseous phase, relatively small pieces, for instance clockwork pieces, it may be sufficient to preheat these pieces in a neutral atmosphere before subjecting them to the action of the chromium halide (preferably fluoride) vapors.

For this purpose, it is advantageous to have recourse to the apparatus illustrated by Fig. 2, which diagram-. matically shows a vertical tubular furnace of a refractory alloy, the inner diameter of which may be relatively small, for instance averaging 10 centimeters and inside which are provided three distinct zones of treatment, to wit, going from top to bottom, a cooling zone I of relatively great length, a preheating zone II, heated, for instance electrically, to a temperature ranging from 1100 to 1200 C. by heating elements 16a, and a chromizing zone III heated to a temperature ranging from 1150 to 1300 by heating elements 16b which also heat the bottom of the tubular furnace.

Sweeping may be performed by means of a mixture (heavier than air) of argon and cracked (or noncracked) ammonia, this mixture flowing in through conduit 3a preferably at the junction between zones I and II whereas it flows out through the top of the furnace.

A reserve of chromium or ferro-chromium B is provided on the periphery of zone III. Through a tube 17, there is introduced successively ammonium fluoride to drive out the air at the beginning of the treatment, then chromium fluoride in small amounts during the whole of the operation. In view of its high density, chromium fluoride will remain in zone 'III and will mix but little with the sweeping gases of zones II and I.

The operation is very simple. Pieces 1 are preheated in zone II for some minutes, then lowered into the chromizing zone III where they stay for a time ranging from some minutes to one hour (position shown in solid lines).

They are then cooled in zone I for the necessary time (position shown in dotted lines).

I will now describe, still by way of example, a furnace, of general U shape, for chromizing, without risks of halide condensation, pieces 1 treated according to the semicontact process and in semi-continuous manner.

This furnace, diagrammatically illustrated by Fig. 3, essentially includes three portions, to wit: a preheating zone constituted by a vertical branch 18 of the furnace, into which the cases are introduced and where they are heated at a temperature of about 1000 C.; a horizontal chromizing zone 19, heated at about 1050, through which the cases are passed; and a cooling zone, constituted by the other vertical branch 20 of the furnace, serving to the exit of the cases.

The cases which areintroduced into the furnace are constituted by cylindrical boxes. The top 21 and the bottom 22 of these cases project slightly from the periphery of the side walls and their diameter is nearly equal to the inner diameter of the vertical branches of thefurnace in order to constitute therewith a relatively fluidtight joint. The sidewalls of the cylinders are provided with holes and contain the pieces to be treated, in contact with chromium B in the divided state containing a small amount of neutral ammonium fluoride (a product which gives oil? a large volume of reducing gas and practically does not react with chromium). The case includes, at its lower part, a housing in which may be placed a container filled with the cementing mixture A. The total height of the case is greater than the height of the horizontal branch of the furnace so that, as soon as a case is introduced into the preheating zone.18, the cementing mixture is already in the chromizing zone 19.

The twovertical branches of the furnace may be closed by covers 23a and 23b. 7

At one of the ends of the horizontal portion 19 of the furnace, there is advantageously provided an orifice through which can be slidably displaced a bar 24 serving to circulate the cases.

The vertical branch 20 of the furnace, which serves to the cooling of the pieces, is separated from the heated part by a heat insulating joint 25 and may be closed at its lower part by a gate of a refractory material 26.

The .furnace is swept by a gas (ammonia for instance) entering both the chromizing zone, at the exit thereof, and the cooling zone. The inflow of this gas into these two furnace portions is controlled by

valves

27 and 28.

The operation of the furnace includes three distinct periods, to wit preheating, chromizing and cooling. The preheating period includes the following operations:

Cover 23a is removed;

Case a, which was resting on case b, is lifted in branch Case b being thus released and the end case n1 in pant 19 .having been, lifted into branch 20, cases [2, c, etc., are pushed forward;

Case a is then lowered into the position previously occupied by case b and a new case is introduced into branch 18 to occupy the position previously occupied by a;

Cover 23a is placed back.

The case introduced into the preheating zone is in a reducing atmosphere and is brought to the desired temperature without coming into contact with chromium fluoride, which is evolved exclusively in the chromizingzone.

The chromizing period takes place as follows:

The cases, introduced into the horizontal portion 19of the furnace are heated to a temperature ranging from 1050 to .1100. Gas sweeping produces in the furnace a slight overpressurepreventing the, inflow of oxidizing gases.

Finally, the cooling period includes the following operations:

when the cementing vnarilyv by the action ithecase of the semiacontact process) also before the lifted into the cooling Owing to this furnace construction, it is possible to perform chromizing in a semi-continuous manner.

For a charge of about 50 kgs., preheating requires about 45 minutes, chromizing 2 hours and a quarter of an hour, and cooling about 45 minutes. It therefore suffices to provide a furnace in which one case is in heating position, three cases in chromizing position and one in cooling position. v

In the preceding description, I have indicated various steps 'to be taken to avoid halide condensation on the pieces to be treated during the period for which the reaction chamber is heated up to the reaction temperature.

If size in a purely gaseous phase, these steps lead to excellent results.

On the contrary, when the treatment is of the semicontact type (the pieces being in contact with chromium intended toregenerate the halide), these steps are sometimes insuflicient and during the period of cooling of the reaction chamber some spots may appear upon pieces 1 and, what is more important, the chromium used for regeneration may be soiled. When this chromium is next used for another operation, the soiled spots thereof will melt, spread and in turn spoil the new pieces treated.

This drawback may be avoided in one of the following manners.

A first solution consists in introducing only the strictly necessary amounts of the addition halide or halides or of the elements which, in cementing mixture A, give rise to these halides, so that, at the end of the treatment, the atmosphere contains just the amount imposed by the laws governing diffusion. The addition halide or halides,

,or the elements which form them, may then be introthe course of the diffusion opduced either gradually in eration or wholly at the beginning thereof.

It should be noted that the reaction casesintroduced into the furnace must be sufficiently fluidtight mixture has been introduced in controlled amount at the beginning of the operation. In order to obtain a gradual and limited disengagement of the active vapors, in the particular case of chromizing ferrous pieces, one may have recourse for instance to the following steps: either to drop on chromium-located out of contact with the pieces anamount of ammonium fluoride carefully determined in accordance with the needs; or to constitute cement A by a controlled amount of chromium fluoride obtained prelimion chromium of a metallic fluoride such as ammonium of liquid hydrofluoric acid, or again to subject chromium located out of contact with the pieces to the action of fluorine or hydrofluoric acid in controlled amount, diluted ;if necessary in an inert or reducing gas, or again to subject a reserve of chromium placed out of contact with the. pieces to the action of an auxiliary fluoride which, in a reducing atmosphere, disengages hydrofluoric acid orproduces an exchange reaction in the presence of -chromium, thus disengaging chromium fluoride and abandoning its metal in contact with chromium.

I. A second solution consists in cooling down the cementing mixture A before the pieces 1 to be treated and (in regeneration metal. B.

thetreatment is performed on pieces of medium taming, at the lower part thereof, the cementmg mixture instance at temperatures averaging 1000 A third solution consists in introducing into the reaction chamber, advantageously in the vicinty of the cey nienting mixture, at the end of the treatment, a body of which is at a temperature lower than that of the at mosphere of the furnace, which body may either have a high thermal inertia or be kept at the desired temperature by circulation of a cooling 'iluid. This body may be introduced either gradually or as a whole. The operation may be stopped and started again a plurality of times. Thus, the excess of halides contained in the reaction cases when the temperature is lowered at the end of the treatment will condense on these cold points or inthe vicinity thereof. i Y According. to still another feature of my invention, when a portion of the reaction chamber is brought (for instance according to the above mentioned third solution) to a temperature lower than the zone of the furnace where are located the pieces and the regeneration metal, this portion is surrounded, preferably completely, by a layer constituted exclusively by lumps of a conductor metal, preferably the addition metal, which are therefore at a temperature intermediate between that of said cooled portion and that of the pieces and the regeneration metal, i. e. are colder than said piecesand said regeneration metal.

This arrangement makes it halide condensations on the metal of this layer. the regeneration metal which surrounds the pieces to be treated is not soiled and can be utilized during a subsequent operation without risks of direct contacts between the said pieces and the halide in the solid or liquid state. It should be noted that this last mentioned arrangement makes it possible to obtain, at the end of anoperation, a metal (that contained in the second chamber) on which the halide has concentrated, this metal constituting, in the course of the next operation, a reserve of halides of the addition metal (for instance of chromium fluoride) having a large area of gaseous disengagement owing to which the atmosphere of the furnace is quickly homogenized. I

I will now describe, by way of example, some-furnaces for carrying out my chromizing method as above set forth. c

Fig. 4 shows a furnace the lower portion I of which is kept at a temperature lower than that of the, other portions 11. These last mentioned portions may be for C., the temperature of the lower portion decreasing, near the bottom, from 1000 C. toabout 500 C. V

Advantageously, I provide a reducing gas sweeping, for instance by means of hydrogen, the sweeping gas being fed to the furnace through a conduit 3 located in the top portion of the furnace.

In order to carry out the diffusion treatment in this furnace, I make use of a fiuidtight reaction case 2 conpossible to localize the A (for instance a mixture giving off chromium fluoride),

- the pieces 1 to be treated and the regeneration chromium ,60 fluoride in the presence of water or B being located above thiscementing mixture,

The treatment takes place as follows: At the beginning, case 2 is brought into the position illustrated by the drawing and pieces 1, same as chromium B, undergo a preheating with respect to the cement A located in the portion of thefurnace at lower temperature. Case 2 is then lifted to level h so that the whole is ata substantially homogeneous temperature, cement A, which is nearer to zone I of the furnace, being however at a temperature slightly lower than thatof pieces 1. This phase corresponds to the chromizing treatment proper. Once this treatment is finished, a precooling action (in order to prevent'halideJcondensations on pieces 1 andchro- Thus,

tion chromium B (this corresponding to the second solution above mentioned). Finally, general cooling is obtained by removing the reaction case 2 from the furnace (position shown in dot-and-dash lines).

Fig. 5 shows a vertical furnace including a central apertured shaft 19 for the introduction, at any time, of the reactive elements and, in particular, of a cementing mixture A containing the chromium halide, this shaft further serving, at the end of the operation, to introduce a cold tube (third solution above mentioned). The pieces 1 to be treated are disposed in baskets 20, in contact with lumps of regeneration chromium B.

Advantageously, shaft 19 is surrounded by an apertured sleeve 21 and the annular space between said shaft and said sleeve is filled with lumps of chromium (or ferro-chromium) P which cause any possible condensation to take place thereon. Thus, no halide condensation takes place at the end of the Operation on pieces 1 or the regeneration chromium B in contact with said pieces.

This furnace is then advantageously completed by heat insulation fins 22 rigid with the cover 23 of the furnace and by a plug 24 for stopping shaft 19.

Figs. 6 and 7 show another embodiment of a vertical furnace in which are introduced ,fluidtight autonomous cases 25 containing each its own reserve of cementing mixture, said cases being stacked one above the other, and a hollow tube 19a extending therethrough for the introduction of the element intended to localize halide condensations (a cooled tube for instance).

Every case 25 contains, in addition to pieces 1 and regeneration chromium B which surrounds them, one or several apertured vessels 26 containing a reserve of cementing mixture A surrounded by pieces of chromium of ferro-chromium F playing the same part as the pieces of chromium located, in the case of Fig. 5, between shaft 19 and sleeve 21. i

Fig. 8 diagrammatically shows a continuous chromizing furnace including a main element 27 in which the temperature is substantially homogeneous, said element being located between two

exchangers

28 and 2? serving respectively to preheating (at the beginning of the operation) and precooling (at the end).

Through these various elements extends a passage 39 in which circulate fluidtight cases 31 containing the pieces to be treated, these cases entering on the side of the preheatingelement 28 and being removed on the side of the cooling element 29. 1

Every fiuidtight case 31 contains, in addition to the pieces 1 to be treated and the regeneration chromium B which surrounds said pieces, a reserve of cementing mixture A located in the rear portiorrof the case and close to the walls thereof, said reserve of cementing mixture A being separated from pieces 1 and the regeneration chromium B by a reserve of lumps of chromium F for causing condensation to take place thereon, this reserve extending preferably around the walls of case 31 and being separated from the regeneration chromium B by ,apertured partitions 32.

With such a furnace, the cycle of. operations is as follows: Case 31 is first introduced into exchanger 28,

where it undergoes a first heating, which concerns the whole case. It is then pushed further into passage 30 until it partly engages heating element 27, pieces 1 and the regeneration chromium B then undergoing a preheating. It is then fully engaged into element 27 for the chromizing process proper. It is then wholly removed from element 27 and brought .to excharlgerzi where precooling takes place, the products closest to the wall of case 3i (cementing mixture A and chromium reserve F) being cooled more quickly and .more intensively than pieces 1 and-the regeneration chromium B. Finally, case 31 is withdrawn'from passage 30 to undergo the general cooljug phase.

Advantageously, the cooling liquid arrives at the level of theportion of the passage (where it passes through exchanger 29) which is in contact with the portion of case 31 where is located cementing mixture A.

According to still another feature of my invention, particularly illustrated by Figs. 10 and 11, I establish every reaction casing 2 in such manner that it is fluidtight with the exception of a leakage zone f intended to make it possible to balance the internal and external pressures, which leakagezone is advantageously located in the portion of the casing which is most remote from the cementation mixture.

I then provide, around the reaction casing or casings, a neutral or reducing atmosphere, for instance containing a high percentage of hydrogen, at least at the end of the treatment.

Preferably, according to my invention, during the heating up period, I preheat the mass constituted by the body or bodies to be treated and the regeneration metal, for instance chromium, in such manner that, on the one hand, the coldest points of said mass are brought to a temperature which is at least C. and advantageously at least C. above the temperature of the portion of the furnace where is located the halogen containing reserve, and, on the other hand, that the hottest point of said mass is at a temperature not more than 600 C. (and advantageously 500 C.) above the temperature of the portion of the furnace where is located the cementation mixture. It should be noted that such an intensive preheating is suflicient to ensure a perfect finish of the articles, even if no special step is taken to deacidify the cementation mixture. As a matter of fact, in this case there is no risk, in view of the temperature range at which preheating is performed, of substantial amounts of solid or liquid chromium fluoride being formed by the action of hydrofluoric acid on the regeneration chromium.

Consequently, it is even possible in these conditions to make use of a cementation mixture which does not contain chromium but only a reserve capable of forming a halide (preferably a fluoride) of another metal, such a reserve forming vapors of the halide of this metal which will'react with the regeneration chromium to form vapors of chromium halide which serve to perform the desired chromi zing'of the articles.

Such a cement, which may be called an indirect cement, may be constituted, for instance, in one of the following ways:

A first method consists in mixing a little volatile halide (for instance nickel fluoride) and a little volatile oxide (for instance tungstic .oxide), 'this mixture gradually evolving a halide vapor (tungsten fluoride) which itself yields, by contact with the regeneration "chromium, chromium fluoride. I might associate in the same manner aluminum or chromium fluoride with tungstic anhydride or moiybdic anhydride.

Another method consists in making use of products capable of gradually generating halogen ions, for instance hypo-halogenites having the general formula MOX (for instance CaO C1 which, when heated, gradually yield metal oxides (MO) and evolve the halo- It is then advantageous, in order to increase the area of exchange between the hot gases andthe mass to be preheated, to distribute the articles P, the regeneration chromium R and the cementation mixture C in several independent casings or boxesl, for instance in two boxes as shown by Fig. 10, each box then constituting an independent treatment unit.

It may be advantageous to give special shapes to these boxes (for instance annular boxes or boxes provided with passages) so as to increase the area of exchange.

Figs. 1 0 and 11 show, by way of example, two examfples offurnaces for operating according to my invention.

' (about of the charge).

' not disturb the balance of the cated remain on the outside.

' containers 2 may be introduced by means of an elevator hydrofluoric acid) in which concentrated liquid hydrofluoric acid is poured onto fine particles of chromium to -which has been added a small amount of ammonium fluoride (about one hundred grams of commercial concentrated hydrofluoric acid for three hundred grams of chromium in particles of the size of rice grains, with the addition of from two to ten grams of ammonium fluoride).

The temperature is slightly raised and the reaction begins at a temperature of 25-30 C., accelerating when the mixture comes to a temperature of 50-80 C. It goes on for some minutes after which the temperature is raised to 200-600 C. to eliminate the excess of hydrofluoric acid and to avoid a subsequent active hydrolysis of the chromium fluoride that is formed (case where it is necessary subsequently to add a small amount of water to the cementation mixture in order to decarburize the articles).

I then add to the mixture thus obtained from 1 to 3% of tungstic acid in order to form a homogenizing halide and from 1 to 3% of ammonium fluoride in order to eliminate the oxidizing atmosphere in the furnace. A

small jet of gaseous ammonia into the mixture completes deacidification thereof. It is then possible, if necessary,

to add a small amount of water or ammonium carbonate to decarburize the articles superficially without any practical risk of hydrolysis of the reserve of chromium fluoride.

In every treatment container of five litres, there is placed about fifty grams of the cementation mixture A thus formed, and this cementation mixture is covered with a layer 36 of trapping chromium in rather big lumps The articles 1 to be treated and the regeneration chromium B are then introduced in the respective propor- -tions of 60% and 40% (approximately) of the total charge. Hydrogen is circulated around the containers (or ammonia which is cracked at the treatment temperature) which gas will hardly penetrate into containers 2 until the end of the cooling thereof and which thus does chromizing atmosphere. For the preheating period, the containers are incompletely introduced into the furnace, so that the portions of said containers where the cementation mixture A is 10- It should be noted that the portion of the containers which is introduced into the furnace will be heated more efliciently as it is surrounded by a gas (hydrogen) which is a good conductor of heat.

At the end of the operation, the boxes or containers are brought back into the position where they were placed to undergo preheating. The portion of every i containerwhich is provided with a leakage zone is still in a hydrogen atmosphere which is the only gas to be able the casing during cooling. I thus obtain a precooling of the cementation mixture and of the trapping chromium, such a precooling achieving condensation of the vapors on elements (trapping chromium and cementation mixture) other than the articles to be to penetrate into treated and the regeneration chromium which must notbe soiled.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efiicient embodiments of my invention, it should be well 10 understood that I do not wish to be limited thereto as there mightbe changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

. This application is a continuation-in-part of my U. S. applications Ser. No. 287,989, of May 15, 1952 and Ser. No. 351,680, of April 28, 1953, both now abandoned.

What I claim is:

l. A method of forming a superficial alloy of chromium on a metal body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising heating a cementation mixture capable of forming chromium fluoride vapors on heating, heating said metal body to reach and maintain a temperature at which chromium from said vapors will diffuse into the surface of said metal body, bringing said vapors in contact with, while maintaining said cementation mixture out of contact with, said metal body surface, and maintaining the relative temperatures of said vapors and said metal body surface during the heating of said metal body to reach said diffusion temperature so as to prevent condensation of chromium fluoride on said metal body.

2. A method of forming a superficial alloy of chromium on a plurality of metal bodies of relatively small and large sizes and having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising positioning in a chamber a cementation mixture capable of forming chromium fluoride vapors on heating, positioning said larger metal bodies in said chamber spaced from said cementation mixture, positioning said smaller metal bodies in said chamber spaced from said cementation mixture and between said larger metal bodies and said cementation mixture, heating said chamber to form said chromium fluoride vapors and to reach and maintain a temperature at which chromium from said vapors will diffuse into the surfaces of said metal bodies,

bringing said vapors in contact with said larger metal bodies by passing over said smaller metal bodies whereby said vapors do not reach contact with said larger metal bodies until the temperature of said larger metal bodies has reached the point where said vapors will not condense thereon.

3. A method as claimed in claim 2 additionally comprising the positioning of a regeneration mass of chromium in contact with said larger metal bodies.

4. A method of forming a superficial alloy of chromi um on a metal body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising placing in a chamber a cementation mixture capable of forming chromium fluoride vapors on heating, heating said chamber to the temperature at which chromium from said vapors will diffuse into the surface of said metal body, preheating said metal body to a temperature above that where said vapors would condense thereon, and introducing said preheated metal body into said chamber spaced from said cementation mixture whereby said vapors in said chamber are diffused into said body.

5. A method as claimed in claim 4 additionally comprising the positioning of a regeneration mass of chromium in contact with said metal body.

6. A method of forming a superficial alloy of chromium on a metal body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising positioning said metal body in a chamber, positioning in said chamber spaced from said body a cementation mixture capable of forming chromium fluoride vapors on heating, heating said chamber to form said chromium fluoride vapors and to reach and maintain a temperature at which chromium from said vapors will diffuse into the surface of said metal body surface, controlling the heating of said cementation mixture so as to maintain the temperature of said fluoride 1 1 vapors formed below the point where there will be substantial condensation of said vapors on said metal body during the'heating of said cha'mber to reach said diffusion temperature.

7. A method as claimed in .claim 6 additionally comprising maintaining such controlled relationship of the temperature of said fluoride vapors and of said metal body during the period of cooling down of said chamber.

8. A method as claimed in claim 6 wherein the composition of said cementation mixture is so regulated as to insure that the amount of fluoride vapors formed therefrom shall not substantially exceed the proportion required to complete the diffusion reaction.

9. A method as claimed in claim 6 additionally comprising, at the end of said diffusion, cooling said cementation mixture at a rate in relation to the temperature of said metal body so as to maintain the temperature relationship between said vapors and said metal body such that no substantial condensation of said vapors on said metal body will occur.

10. A method as claimed in claim 6 additionally comprising, at the end of said diffusion, introducing a cooling body within said chamber nearer to said cementation mixture than said metal body, said cooling body having a substantially lower temperature than the atmosphere of said chamber.

11. A method of forming a superficial alloy of chromium on a metal body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising positioning said metal body in a chamber, positioning in said chamber spaced from said body a cementation mixture capable of forming chromium halide vapors on heating, positioning a regeneration mass of chromium in contact with said metal body, heating said chamber to form said chromium halide vapors and to reach and maintain a temperature at which chromium from said vapors will ditiuse into the surface of said metal body, bringing said vapors in contact with said metal body surface, and maintaining the relative temperatures of said vapors and the surfaces of said metal body and said regeneration mass during the heating of said metal body to reach said difiiusion temperature so as to prevent condensation of chromium hal de .on said surfaces of said metal body and regeneration mass.

12. A method as claimed in claim 11 wherein the temperature of said surfaces are maintained above 200 C. below the temperature of said cementation mixture.

13. A method as claimed in claim 11 wherein said relative temperatures are maintained by maintaining the cementation mixture at a temperature below where said chromium halide will be substantially vaporized while heating said metal body and said regeneration mass to reach said diffusion temperature.

"14. A method as claimed in claim 11 wherein said relative temperatures are maintained while reaching said diffusion temperature by' maintaining the coldest point of said metal body and regeneration mass at least 100 C. above the temperature of the portion of said chamber occupied by said cementation mixture.

15. A method as claimed in claim 14 wherein the hottest point of said metal body and said regeneration mass is below 500 C. above the temperature of the portion of said chamber occupied by said cementation mixture.

16. A method as claimed in claim 14 wherein said cementation mixture includes no chromium and contains a halogen compound of another metal capable of forming said other metal halide vapors which will react with said chromium regeneration mass to form chromium halide vapors.

17. A method as claimedin claim 11 additionally comprising circulating a reducing gas which is a good heat conductor around said chamber and allowing penetration of said gas to a limited degree into said chamber through a leak passage in the .wall of said chamber;

18. A method as claimed in claim 17 wherein said gas includes a high percentage of hydrogen. i

19. A method as claimedlin claim 18 additionally comprising positioning'lof said cementation mixture at the portion most remote from said leakage passage.

20. A method as claimed in .claim 11 wherein-the relative temperatures of said vapors and surfaces are maintained by controlling the heating ofsaid cementation mixture so as to maintain the temperature of said halide vapors formed below the point where there will be substantial condensation of said vapors on said metal body and regeneration mass.

21. A method as claimed in claim 20 additionally comprising maintaining such controlled relationship of the temperature of said fluoride vapors and of said metal body and said regeneration mass during the periodof cooling down of said chamber.

22. Ajmethod as claimed in claim 20 wherein the composition of said cementation mixture is so regulated as to insure that the amount of halide vapors formed therefrom shall not substantially exceed the proportion required to complete the diifusion reaction.

23. A method as claimed in claim20 additionally comprising, at the end of said diffusion, cooling said cementation mixture at a rate in relation to the temperature of said metal body and regeneration mass so as to maintain a temperature relationship between said vapors and said metal body and said regeneration mass such that no substantial condensation of said vapors on said metal body and said regeneration mass will occur.

24. A method as claimed in claim 20 additionally comprising, at: the end of said diffusion, introducing a cooling body within saidcharnber nearer to said cementation mixture than said metal body and said regeneration mass, said cooling body having asubstantially lower temperature than the atmosphere of said chamber.

25. A method as claimed in claim 20 additionally comprising positioning a metal reserve between said cementation mixture and said metal body and said regeneration mass, and, at the end of said diffusion, introducing a cooling body into said metal reserve, said cooling body having a substantially lower temperature than the atmosphere of said chamber, said metal reserve serving to condense available vapors when diffusion does not occur.

26. A method as claimed in claim 25 wherein said metal reserve has a thickness such that the portion thereof most remote from said cooling body is at a temperature just sufiiciently high to prevent condensation of said vapors thereon.

27. A method as claimed in claim 20 wherein said cementation mixture is heated in the presence of sufficient water to maintain the vapor tension of said halide below the point where any substantial condensation thereof will occur on said metal body and said regeneration mass While reaching said diffusion temperature.

28. A method as claimed in claim 27 wherein said water is incorporated in an aqueous solution of a halogen acid.

29. A method of forming a superficial alloy of chromium on a metal body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising positioning said metal body in a chamber, positioning in said chamber spaced from said body a cementation mixture capable of forming chromium fluoride vapors on heating, heating said chamber to form said chromium fluoride vapors and to reach and maintain a temperature at which chromium from said vapors will diffuse into the surface of said metal body, bringing said vapors in contact with said metal body surface, and preventing effective movement ofsaid vapors into that portion of said chamber occupied by said metal body during the heating of said metal body to reach said diffusiontemperature.

30. A method as claimed in claim 29 additionally co pr g th pos t on ng Q a r ene a io mass 9 chromium in (:ofitact with, and in that portion of said 2,274,671 chamber occupied by said metal. 2,604,395 2,612,442 References Cited in the file of this patent UNITED STATES PATENTS 5 1,902,503 Howe Mar. 21, 1933 160,812 2,257,668 Becker et a1. Sept. 30, 1941 14 Daeves Mar. 3, 1942 Gonser et a1 July 22, 1952 Goetzel Sept. 30, 1952 FOREIGN PATENTS Australia Aug. 30, 1951

Claims

1. A METHOD OF FORMING A SUPERFICIAL ALLOY OF CHROMIUM ON A METAL BODY HAVING AS A PRINCIPAL COMPONENT A METAL SELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL AND COBALT COMPRISING HEATING A CEMENTATION MIXTURE CAPABLE OF FORMING CHROMIUM FLUORIDE VAPORS ON HEATING, HEATING SAID METAL BODY TO REACH AND MAINTAIN A TEMPERATURE AT WHICH CHROMIUM FROM SAID VAPORS WILL DIFFUSE INTO THE SURFACE OF SAID METAL BODY, BRINGING SAID VAPORS IN CONTACT WITH, WHILE MAINTAINING SAID CEMENTATION MIXTURE OUT OF CONTACT WITH, SAID METAL BODY SURFACE, AND MAINTAINING THE RELATIVE TEMPERATURES OF SAID VAPORS AND SAID METAL BODY SURFACE DURING THE HEATING SAID METAL BODY TO REACH SAID DIFFUSION TEMPERATURE SO AS TO PREVENT CONDENSATION OF CHROMIUM FLUORIDE ON SAID METAL BODY.