US2816048A

US2816048A - Process of forming superficial alloys of chromium on metal bodies

Info

Publication number: US2816048A
Application number: US572455A
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: 1949-08-05
Filing date: 1956-03-19
Publication date: 1957-12-10
Anticipated expiration: 1974-12-10

Description

Dec. l0, 1957 P. GALMICHE 1 2,816,048v

- PRocEss oF Femme SUPERFIGIAL ALLoYs Y OF CHROMIUM 0Nv METAL BODIES Fileduarch 19, 195e /NVENTDR Phill' pf Galmcbe- ATTDHNE United States Patent O inociss's or Borrarme suPEnFiCIAL ALLoYs or CHROMIUM. oN METAL nomas Philippe Galmiche, Paris, France, assigner to Olce National dEtudes` et `de `Recherches Aeronautiques, `Chatillon-sous-Bagrieux (Seine), France, a society of France AppicatinMrch 19, 1956,'Serial No. 572,455 Claims priority, application France ugust'S, 1949l 1 1s' claimsi '(cl. 117-407) The :present invention relates to an improved `process of forming superficial alloys-ofchromium 'on metal bodies andfmetalpwdersconsisting of or rhaving as a principal constituent a metal with which chromium -Will become lloyed under conditions promoting inter-metallic dif- Afusionof :the two metals.` The invention-has particular application-in thfe formation of supercial valloys of "chrov mium with? metal bodies' having asa principal'constituent a metal selected frornthev-group consisting of iron,lnickel and cobalt. -Other applications of `theinvention will be hereinafter described.

@The invention -in itsbroader aspects consistsin ysubl .(vapor tension and boilingip'oint) andlits thermodynamic ,cgnstantsf are well adapted to a balanced'exchange reaction inethe gaseous state, at the temperatures necessary `forfthe-diliusion of chromium in the metal under treatfeature" the step -ofg` transporting .the chromiumijluoride ,Y

vapors from the pointof `their generation, which may'be within or `outside Vthe chamber wherein the metal bodies lare disposed. for treatment, Vto the treatment 'zone and maintaining the temperature in such treatment vzone below the boiling point of thej fluoride but at a point high enough to insure aconcentrationof the vapors sulicientto pro:- mote the intermetallic diffusion. In this way a `positive control of the vapor concentration without loss of vapors to the system or building up of undue. pressures l'within the treatment ,chamber is insured.

Another feature of my invention relates moreV particuv larly towmethods in.-which thepiecesto be treated are exposed tothe action of a fluoride of` the addition metal in the gaseous form and consists inproviding,.in the reaction chamber, a reserve mass .of .the addition metal (or one ofitscompounds) intended directly to regenerate thefactive vapor.` This mass may be `above the-.pieces to be treated or in contact therewith.

Still another feature consists inperforminganelectrolyticlpolishinguafter the thermal treatment through which a superiicial. alloy of diffusion was formed. on -the pieces to be treated. I `Preferred embodiments of my invention will `be hereinafter described with reference to the accompanying drawings, given merely yby way of example and infwhich the only ligurediagrammatically shows an-embodiment of an'apparatus for carrying out a method according to my' invention.

" My inventionwill be describedmore particularly as' applied-'to Athe case ofa method of forming asuperlicial l chromium alloy on steel pieces 1.

2,816,048 latented Dee. 1o, 1957 1 According to my invention, I make use of chromium fluoride a's chromium carrier.

Preferably, pieces 1 are directly exposed to chromium fluoride vapors formed and maintained during the operation, by the action of ammonium uoride on chromium `or a Chromium compound.

Said vapors may also be obtained'by heating chromium iluoride prepared in advance and separated substantially from the substances which have served to its preparation.

"For instance, I may use commercial chromium fluoride, irrespective of the' fact that this uoride may contain some-amount of acid.

Advantageously I add to this chromium uoride some amount of the mixture of ammonium uoride and chromium or a chromium compound. Both such a mixture andc'liromiur'n liuoride formed inadvance will be' called cementation product.

It "will be possible, Yby addition of complementary re- -Vagentsgsueh 'for ins-tance as alumina, zirconia, aluminum,

silicon-or zirconium, to obtain mixed cementations.

" On the other` hand, I advantageously work in a reducing atmosphere, `for instance of hydrogen.

Chromium'transfer is then conditioned by the follow- 'ing'reaetionss First, there-is a disengagement of chromium and formation of iron fluoride.Vv

This'iron liuorideis immediately reduced by hydrogen `according 'to` the following reaction: ,(21)

. Nlhechoice of chromium liuoride as a vehicle for the addition metal has numerous advantages.

As a matterfof fact, the physical constantsof this-halide It should be noted that the use of a iiuorine compound `leads vto' the formation vof .an iron iluoride which, at the beginning of the ,operation and prior to the iron reduction [see Reaction 2l constitutes a thin temporary protective layer which eliminates all risks of pitting of the treated pieees,contrary to what takes place in particular with methodsmaking use ofa chlorine compound which yields yolatileiron chloride, thus causing pitting which impairs the surfaces ofpieces treated in agaseous medium.

Y Another `advantage of the chromium uoride process lies in the fact that it is not hygroscopic so that when the dilusion chromium is supplied by means of a cementat1on product containing or constituted by chromium fluoride, :this cementation product can be handled after the operation without giving rise to the formation of a stable hydrated halide as is the case when the carrier element that is adopted is chromium chloride which releases vits water# of Acrystallization at a high temperature and causes the'as yet unreacted metallic chromium in the treatment container to be oxidized.

A Finally, ammonium iiuoride is well adaptedto the application of a particular feature of the invention according to which-water is added to the cementation mixture in such manner as to permit initial attack on chromium in the liquid state.

` .It should be pointed out here that, in particular when itis desired to operate in the presence of water, it will be possible eventually to substitute, either wholly or partly, hydrouoric-acd in solution for the ammonium iiuoride entering into the composition of the cementation mixture.

`I thus get the advantage of a much higher coeflicient of utilization of the ammonium fluoride, that is to say of a higher amount of chromium fluoride being formed. In the course of heating up, water is eliminated -and volatilizing of the excess of ammonium fluoride vdrives off the last traces of steam so that, at the end of the treatment, there is observed no oxidizing of the chromium present in the cementation mixture still remaining in the reaction chamber. Furthermore, owing to this addition of water, the fumes of ammonium fluoride which are formed at the beginning of the treatment are very substantially reduced.

Therefore, when using a cementation mixture I preferably adopt, as the inorganic fluorine compound of the cementation mixture, ammonium fluoride (either in the neutral or in the acid state) which may be either merely Ymoistened, or introduced in the form of an aqueous solution, or again mixed with hydrouoric acid. p

Ammonium fluoride is the preferred source for the fluorine ion in this invention but hydrofluoric acid alone or various fluorine compounds, such as nickel fluoride or another suitable metallic fluoride, may be utilized. The primary consideration is that the iluorine compound .reacts with chromium or a chromium compound to form chromium fluoride. Although ammonium fluoride has been disclosed herein as producing chromium fluoride in situ, i. e., in the immediate vicinity of the metal to be treated, it will be understood that the invention may be practiced by utilizing chromium fluoride derived from other sources. For example, solid or liquid chromium fluoride may be positioned in the chamber in place of the cementation mixture and vaporized by heating. Of course, this chromium fluoride in the solid or liquid state must not be in contact with the surface to be chromized. On the other hand, chromium fluoride vapors may be generated and fed directly into the chamber from an outside source.

I will now indicate, by way of example, the reactions that take place during the formation of chromium fluoride from a cementation mixture which contains water.

During the attack in a liquid medium, the hydrofluoric acid resulting from hydrolysis of ammonium fluoride reacts upon chromium with the formation of chromium fluoride and hydrogen.

This reaction takes place chiefly at about 100 C. during the heating up of the reaction chamber which accordingly remains for a time at constant temperature.

Ammonium fluoride then volatilizes, driving off the last traces of steam and partly Idissociating itself so as to form hydrofluoric acid which, upon reacting with chromium, yields chromium fluoride through the dry process in a non-oxidizing atmosphere. Partial dissociation of ammonium fluoride takes place according to the following reaction:

.and the hydrofluoric acid reacts upon chromium according to the reaction:

Concerning the temperature of treatment, it will range from 600 to l200 C. according to the nature of the metal that is being treated and to the duration of the operation, which duration may range from a fraction of an hour to some tens of hours. For ferrous metals, it will be of advantage to maintain the temperature at from 1050 to ll C. Below about 600 there will not be any appreciable diffusion of chromium into iron, nickel or cobalt. It is preferred to operate below the boiling point of chromium fluoride (about 1300" C.) because the vapor pressure above the boiling point requires operation under higher pressures or, alternatively, provision for recirculation of the vapors.

When preparing a cementation mixture for use in practicing the process, chromium or the chromium compound (ferro-chrome for instance), after it has been broken into pieces or powdered, is mixed with ammonium fluoride or another suitable fluorine compound and with a substance intended to avoid coalescence, that is to say agglomeraton by sintering of chromium.

Although I may consider utilizing, as such a substance, an inert or substantially inert matter such as kaolin, magnesia, hematite, I may also use a compound which participates in the reaction, which then makes it possible to obtain a mixed cementation intended, for instance. either to improve the resistance to oxidation (case of the addition of alumina), or to insure a deeper penetration of chromium into ferrous metals containing a high proportion of carbon (case of the addition of zirconia).

Hereinafter, I will designate by the expression cementation agent the total mixture of chromium, ammonium fluoride and addition products. y

The decomposition of the cementation mixture gives a non-oxidizing atmosphere.

It is then possible, either to make use merely of this internal production, in which case it will be necessary to provide valves for the evacuation of the excess of gas without inflow of air, or artificially to supply the reaction chamber with a reducing or inert gas.

Concerning now the apparatus for carrying out such a method, I may make use, to constitute it, of the construction illustrated by the drawing and according to which;

There is provided at A, at the bottom of a cementation casing 2, preferably of vertical cylindrical shape, the

cementation agent as above defined;

Pieces 1 are separated from said agent by means of a partitioning 3, for instance horizontal, these pieces being disposed above layer A, which prevents their being in contact with the cementation agent.

Preferably, I provide, at the top of the cementation casing 2, a reserve B of chromium or ferro-chrome contained in a kind of basket 4 and serving to facilitate a direct regeneration of the active vapor.

The atmosphere in casing 2 is preferably a reducing atmosphere, for instance of hydrogen supplied from the outside through a conduit 5.

The chemical process during heating is then as follows:

The chromium fluoride vapor reacts with iron according to Equation l, the chromium that is evolved being diffused into the pieces undergoing treatment, whereas the iron fluoride is immediately reduced according to Reaction 2 and regenerates iron which therefore does not leave the treated piece; the hydrofluoric acid which is disengaged by the reduction of iron fluoride enters into contact with the reserve B of chromium placedr at the top of the cementation casing 2 and a further amount of chromium fluoride is formed which, being heavier than hydrofluoric acid, drops back onto the pieces in treat ment and yields thereto the chromium it contains.

rIhus a to and fro movement of the carrier fluoride takes place between the cementation agent and the chromium reserve B, every passage on the pieces to be treated corresponding to a disengagement of chromium.

Furthermore, if alumina, zirconia, aluminum, silicon or zirconium is provided in the cementation agent, a portion of the chromium compound reacts on these cle-l ments to form the corresponding fluoride compound which cooperates in the cementation process.

This method makes it possible to obtain, in particularly economic conditions, perfectly smooth and bright layers comparable with the best electrolytic deposits and involving no trace of adhesion of the cementation agent. The proportions of chromium are high in all cases and may exceed 50% at the surface; by way of indication, the average proportion of aluminum in the layers reaches about 2% in the case of mixed chromium and aluminum cementation. A

As the treatment 'takes place ina vapor medium; it is possible successfully to treat pieces of complicated shape, involving recesses or holes.

By way of example, on mild steel containing 0.1 percent of carbon, it is" possible tob'tai t- 1-100' G5.- a depth of penetration of 0.2 millimeter in an operation of a duration of four hours, at l000 C. a penetration of 0.1 mm. in the same time. The hardness of the layer approximates that of iron-chromium alloys, i. e. isclose to 200 Vickers. The layers are lxible and ductile and therefore have good characteristis 'f rsistance to wear and tear; they have a high resistance to corrosion by nitric acid and solutions of sodium chloride; at the same time, they have a good resistance to dry oxidation during very long times up to temperatures above 900-l000 C.; their regularity and their homogeneity are perfect.

On steels containing a higher amount of carbon, the

thickness that is obtained is smaller but the hardness higher; by way of example, with steel containing 0.5% of carbon, there is obtained a layer of 0.03 mm. in four hours at l000 C., the hardness averaging 800 Vickers. If an initial decarburizing of the steel on cast iron is performed, for instance by heating in moist hydrogen, it is possible to obtain, on steels containing a high amount of carbon, thicknesses equivalent to those obtained on mild steels.

Another embodiment yof my invention consists in heating pieces 1 in contact with chromium or ferro-chromium broken into pieces and below which the cementation mixture is placed.

It should be noted that, in all cases, the treatment might be conducted in an ammonia atmosphere or, still better, be followed by a nitriding treatment by ammonia at temperatures ranging from 600 to l000 C. This would make it possible to obtain layers of an extremely high hardness, as high as 1500 Vickers, having on the other hand good friction and wearing properties and also a high resistance to corrosion and dry oxidation.

The treatment above described may be applied in the same conditions to other metals or alloys such as nickel, cobalt, high nickel-molybdenum iron-alloys known under the name Hastelloy, either sintered or in powder form.

Finally, it will always be possible, according to a feature of the invention applicable to all thermal chromium coating method in a gaseous medium but more particularly advantageous in the case of iiuoride cementation, to subject the pieces that have been chromium coated through such methods to a subsequent anodic polishing treatment which permits of obtaining, in addition to the brightening inherent in such polishing treatments, elimination of the sigma state of the ferro-chromium, which has a tendency to appear, in certain conditions, at the outer limit of the diffusion layers, such a state having the drawback of lowering the resistance of the surface to said corrosion.

For this purpose, it will be advantageous to have recourse to an anodic polishing process making use of an electrolytic bath containing perchloric acid and acetic acid.

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

This is a continuation-in-part application of my copending application, Serial No. 175,502, filed Iuly 24, 1950, now abandoned.

I claim:

1. A process of forming a supercial alloy of chromium on a body having as a principal component a metal selected from the group consisting of iron, nickel and cobait comprising heating a cementation mixture containing in the above desaid heating treatment;Y

acrobate chromium and an inorganic fluorine compound capable of reacting. therevs'fithl to form chromium fluoride, bsaid heating, being carried out at a'- t'emperature to produce vapors of sai'd chromium lluoride, and bringing: said chromium fluoride vapors inthe presence or' hydrogen into contact with said metal'body at a tenrperatnreabove 600' C. while maintainirigsaid cementation mixture out of contact with said metal body.

2. A process as claimedin`l claim l whereinsaid cementation mixturey containsV water at the beginning of 3. A process of forming a superficial alloy of chromium on a body having as a principal component a nieta-l selected from the group consisting of iron, nickel and cohalt comprising positioning said body in a chamber, positioning in said chamber spaced from said body a cementation mixture including chromium and an inorganic fluorine compound capable of reacting therewith to form chromium fluoride, heating said chamber within the range of 600 to 1300u C. to form said chromium iluoride and produce vapors thereof, and bringing said vapors in the presence of hydrogen into contact with said body.

4. A process as claimed in claim 3 wherein said inorganic uorine compound is hydrouoric acid.

5. A process as claimed in claim 3 wherein said inorganic iluorine compound is ammonium liuoride.

6. A process as claimed in claim 5 wherein said metal is iron.

7. A process as claimed in claim 3 wherein a further mass of chromium is positioned in said chamber and spaced above said mixture.

8. A process as claimed in claim 7 wherein said further mass of chromium is spaced above said body.

9. A process as claimed in claim 7 wherein said further mass of chromium is in contact with said body.

10. A process of forming a superficial alloy on a body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising positioning said body in a chamber, positioning in said chamber spaced from said body a cementation mixture including chromium, an inorganic fiuorine compoundcapable of reacting therewith to form chromium iiuoride and a material selected from the group consisting of alumina, aluminum, zirconia, zirconium and silicon, heating said chamber within the range of 600 to 1300" C. to react said mixture components and produce vapors, and bringing said vapors in the presence o hydrogen into contact with said body, whereby an alloy is formed on said body.

1l. A process as claimed in claim l0 wherein said metal is iron and said fluorine compound is ammonium fluoride. l

l2. A process as claimed in claim l0 wherein said second compound is alumina.

13. A process of forming a superficial alloy of chromium on a surface of a body having as a principal component a metal selected from the group consisting of iron. nickel and cobalt comprising bringing chromium iluoride in the state of vapor in the presence of hydrogen into contact with the whole of said surface while maintaining a temperature of above 600 C.

14. A process as claimed in claim 13 whrein said temperature is maintained below the boiling point of chromium fluoride.

15. A process of forming a superficial alloy of chromium on a body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt comprising positioning said body in a chamber, heating chromium uoride prepared in advance and separated substantially from the substances which have served to its preparation to produce chromium uoride vapors, and bringing said chromium uoride vapors in the presence of hydrogen into contact with said metal body at a temperature above 600 C. while maintaining said chromium fluoride out of contact with said metal body.

2,816,048 7 16. A process as claimed in claim 15 wherein a furto 1300 C. to form chromium fluoride vapors, and bringther mass of chromium is positioned in said chamber.

ing said vapors in the presence of hydrogen into Contact with said body.

I References Cited in the file of this patent UNITED STATES PATENTS cobalt comprising posltlonng Said body 1n a Chamber ggg? "Ig/[113e positioning in said chamber spaced from Sald body a 2219004 Daeve Qt-1- n se i 30 1941 cementation product essentially cOnStlUfSd by CbfOmlum 10 2,257,668 B k S t 1 u S pt' 30 194' Hum-ide, heating said Chamber Within the fange 0f 600 ec ere a ep g l

Claims

13. A PROCESS OF FORMING A SUPERFICIAL ALLOY OF CHROMIUM ON A SURFACE OF A BODY HAVING AS A PRINCIPAL COMPONEBT A METAL SELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL AND COBALT COMPRISING BRINGING CHROMIUM FLUORIDE IN THE STATE OF VAPOR IN THE PRESENCE OF HYDROGEN INTO CONTACT WITH THE WHOLE OF SAID SURFACE WHILE MAINTAINING A TEMPERATURE OF ABOVE 600*C.