Heat treatment of metals

United States Patent Office Patented Oct. 4, 1966 not taken. In order to limit these drawbacks, it has already been suggested to perform these operations in an atmosphere of hydrogen, but, despite this precaution, superficial oxidation remains important because a stream 5 of hydrogen, even purified, always contains traces of oxygen which are dangerous because they combine with some highly oxidizable elements contained in the alloy in treatment (Ti, Al, Cr, etc.).

In particular it has already been proposed to treat 70 turbine blades made of refractory alloys of compositions such as above stated, by taking the rough-shaped articles obtained after forging and subjecting them to a thermal treatment for bringing into solution some components thereof at a temperature ranging from 900 to 1300° C., 15 and preferably from 1050 to 1100° C., for instance for ten hours at 1080° C. Then the articles are subjected to a controlled cooling after which they are machined.

In order to remedy the structural modification resulting from the machining operation, the articles are again 20 heated at a temperature equal, or close to, the above mentioned initial thermal treatment temperature, but for a very short time, for instance averaging from 8 to 10 minutes at 1080° C. in a bath of boric anhydride containing a small amount of cryolite, or in an atmosphere of 25 hydrogen as pure as possible. This short complementary homogenizing treatment after a machining or other cold working operation is called “skin-annealing.”

Skin-annealing, in the above specified conditions, involves some drawbacks, such as the following ones:

(a) The results obtained by such a treatment are not uniform and, without any prior indications, often the articles are partly covered at the end of the treatment with a layer of oxide (chromium and/or nickel oxide) which can be eliminated only by a mechanical treatment which reintroduces the structural modifications which were intended to be eliminated by the skin-annealing operation.

(b) The articles are subjected to an electrolytic polishing intended to permit of eliminating those which have 40 undesirable inclusions; indirectly, this polishing operation permits of eliminating the new layer where undesirable structural modifications have taken place, but on the other hand it has the very serious drawback of weakening the surface protection of the articles.

(c) The fact that the skin-annealing operation is carried out in a bath of boron salt causes boron to be introduced into the surface layers of the article, and this small amount of boron acts as a flux on the metal oxides, which is particularly undesirable.

(d) Articles having undergone the skin-annealing operation may become particularly sensitive to the type of corrosion which produces cracks in the metal of the articles, this kind of corrosion constituting a danger even for articles working at temperatures where there is only 55 a negligible corrosion by oxidation and scaling.

(e) Articles having undergone a skin-annealing treatment generally have an unsatisfactory surface, even after cleaning thereof.

It is proposed elsewhere to perform such heating treat60 ments of refractory articles at high temperatures (for instance 1050°-1100° C.) in a neutral or reducing atmosphere containing at least one metal halide of high boiling point the metal of which diffuses into the article in treatment at this temperature. But such metal diffu⁶⁵ sions caused differences between the composition of the superficial layers of the article and that of the mass thereof and there are cases where such differences are objectionable.

My invention relates to treatments including at least one operation during which the metal article (as defined hereinbefore) is treated at temperatures higher than 700°

3,276,903

HEAT TREATMENT OF METALS

Philippe Galmiche, Paris, France, assignor to Office National d’Etudes et de Recherches Aerospatiales, called O.N.E.R.A., Chatillon-sur-Bagneux (Seine), France, a society of France

No Drawing. Continuation of application Ser. No. 42,200, July 12,1960. This application Mar. 10,1965, Ser. No. 438,788

Claims priority, application France, July 11, 1952, 631,962, Patent 1,060,225; Feb. 4, 1953, 642,305, Patent 1,074,096; Mar. 12,1953, 644,414, Patent 1,075,620; Feb. 16, 1955, 54,951, Patent 1,060,225; May 13,1955, 691,668, Patent 1,134,763

Claims. (Cl. 117—107.2)

This application is a continuation of my copending application Serial No. 42,200, filed July 12, 1960, now abandoned, which was itself a continuation-in-part of my copending applications Serial Nos. 367,286, filed July 10, 1953, now abandoned, and 565,286, filed February 13, 1956, now abandoned.

The present invention relates to methods for the heat treatment, at temperatures higher than 700° C., of articles made of alloys containing chromium and at least one metal of the group consisting of nickel and cobalt and which may also contain iron.

The object of this invention is to permit of carrying out the heat treatment without any oxidizing of the article being treated.

The essential feature of my invention consists in conducting said heat treatment in a non-oxidizing atmosphere essentially constituted by at least one fluoride having a boiling point greater than 700° C. under conditions preventing diffusion of chromium into said article.

A first embodiment of my invention is concerned with the treatment of ferrous pieces made of stainless steels (i.e., of chromium, nickel and iron alloys) for brightening the surface thereof. For this purpose, according to my invention, such pieces are heated, at temperatures above 700° C. in a fluorine containing atmosphere. Such a brightening reheating of these ferrous pieces is performed in a particularly easy way by placing the pieces of bulk in a diluting body such as talcum containing ammonium fluoride. The brightening of ferrous pieces which have been initially oxidized may be explained by the volatilizing of the oxide in the state of iron fluoride at temperatures averaging 900° C. with formation of water which is gradually eliminated. Subsequently, the fluoride vapor thus formed is reduced and the metal deposits on the piece which is thus covered with an iron film.

A second embodiment of my invention is concerned with the treatment of metal articles made of refractory alloys having as main component, or components, nickel or/and cobalt, and further containing chromium and possibly at least one of the following components, to wit: titanium, aluminum, molybdenum, tungsten, zirconium, iron, manganese, cerium and thorium. Such alloys are known in the art under the name of Nimonics (Nimonic 80, Nimonic 90, Nimonic 95), Waspalloys, etc.

The treatment of such articles generally requires a shaping in the hot state, such as forging, stamping, rolling, extruding, etc., which is generally carried out in the presence of air. This treatment involves a considerable superficial oxidizing of the alloys which reduces the percentage of protective elements in the superficial layers of the article and which necessitates costly scouring operations. If the articles are to be machined after this shaping operation, this drawback is less important, but however the formation of oxides between the successive steps of the shaping operation involves risks of producing undesirable inclusions in the articles if special precautions are

3,276,903

C. in a neutral or reducing atmosphere containing at least one fluoride of a metal component of the alloy of which the article is made having a boiling point higher than 700° C., and according to my invention, the conditions in which the operation is carried out and the com- 5 position of said atmosphere are so chosen as practically to prevent any superficial metal diffusion into the article.

The expression “treatment operations during which the article is heated at temperatures higher than 700° C.” includes, for instance, shaping treatments for refractory jq and stainless alloys, thermal treatments and, in particular, skin-annealing operations of refractory alloys.

For the sake of clarity, in the following description, I will call “inert” fluoride any fluoride which, in the conditions of the treatment operation, does not cause the 15 formation of a superficial diffusion alloy into the article in treatment. Therefore the term “inert” fluoride: applies to the fluorides of the main components of the alloy, that is to say nickel and/or cobalt, and also to ammonium and hydrogen fluorides. In opposition with this, an “ac- 20 tive” fluoride is a metal fluoride which, in the conditions of the operation, causes its metal to diffuse to a substantial degree into the article to form in the superficial layers thereof, an alloy with the metal of the article. 25

My invention is concerned with the case where it suffices to add to the neutral or reducing protective atmosphere in the treatment container one or several inert fluorides. As a matter of fact, it was found that some gas turbine elements working at relatively moderate tem- 30 perature, but subjected to very strong vibrations, give much more satisfactory results if no difference of composition and/or of structure is created between the mass of the element and the superficial layers thereof. The only condition to be complied with in this case is that the 35 treatment, for instance the skin-annealing treatment, must not result in a loss of chromium in the superficial layers of the elements.

In such cases, the protective atmosphere may be mainly constituted, for temperatures above 700° C., by one or 40 several inert metal fluorides, and it must not include any metal fluoride capable of producing a diffusion of its metal into the article. As above stated, the metals of said inert fluorides must be metals entering into the composition of the alloy, and in particular nickel and cobalt. 45

By way of indication, it is pointed out that aluminum fluoride causes diffusion of aluminum into the superficial layers of a refractory metal article, but to such a low degree that this fluoride may be classified either as an active fluoride or as an inert one. 50

The fluoride vapors which are to form the protective atmosphere in the treatment container are advantageously obtained by placing in said container elements which will be called “sources” of such a fluoride and which are located out of contact with the articles to be 55 treated (said articles being, for instance, separated by grids from said sources).

When the container is heated, such a source gives off, either directly or indirectly, the desired metal fluoride vapors. It may be constituted either by a so-called 60 cementation product, requiring only the action of heat thereon to give off said metal fluoride vapors, or by a metal mass or a compound capable of giving off said metal fluoride vapors under the combined actions of heat and of other fluoride vapors evolved in another region 65 of the container.

In the case of a metal mass, this metal is that of the fluoride to be obtained and the fluoride vapors acting thereon may be vapors of hydrofluoric acid. Such a metal is for instance used in addition to a cementation 70 product located in another region of the treatment container. When said container is heated, the cementation product gives off vapors of the desired fluoride; These vapors, upon coming into contact with the metal article, decompose into the metal of the fluoride, which diffuses 75 into the superficial layers of the article, and vapors of hydrofluoric acid, which come to attack the metal of the above mentioned mass, thus regenerating vapors of the metal fluoride. These vapors in turn come into contact with the article and decompose as above stated, and so on. In view of this operation, the metal mass is called “regeneration” mass.

By way of example, a cementation product may be constituted by the metal fluoride itself in the solid state (which disengages vapors under the effect of heat), or by a mixture of components which, under the effect of heat, react upon each other to produce such vapors. I may also make use of metal powders which, when attacked by an ammonium fluoride or a hydrogen fluoride (or hydrofluoric acid), give off the desired fluoride vapors.

I may also use a metal mass placed in the treatment container in the vicinity of the article, but out of contact therewith, and which is attacked by suitably formed fluoride vapors whereby the desired metal fluoride is formed on the surface of said mass, this metal halide being vaporized by the action of heat to give off the fluoride vapors of the protective atmosphere.

A certain amount of ammonium fluoride, introduced in the container, may serve to sweep the atmosphere thereof at the beginning of the treatment.

As above stated, the “treatment operation during which the article is treated at temperatures higher than 700° C.” may be a skin-annealing treatment. According to my invention, this treatment is preferably performed in a purely gaseous phase, that is to say without direct contact of the article with the source or sources of fluoride or fluorides. Advantageously, the article is separated from said sources by grids of a shape conforming as much as possible to that of the article and advantageously made of a metal (or of an alloy containing a metal) which is a main component of the alloy of the article.

In view of the good protection of the article against oxidation during the skin-annealing operation, this operation is advantageously carried out in two successive steps taking place at different temperatures. The temperature of the first step is higher than that at which skin-annealing is generally conducted, so as to permit of more efficiently bringing into solution some compounds which contribute to the structural hardening of the alloy, such as carbides and nitrides (temperature ranging from 1150 to 1200° C. for Nimonic 90 and Nimonic 95 alloys). The temperature during the second step is lower than that during the first step by about one hundred degrees, so as to obtain a dispersed precipitation of said components (temperature ranging from 1050 to 1100° C. for the above mentioned alloys), while maintaining in solution other purely metallic structural hardening compounds, such as Ni₃TiAl, which precipitate in controlled fashion during the final annealing.

As a rule, it is advantageous to heat up as quickly as possible the treatment container and the parts present therein (at least for temperatures higher than that at which precipitation can begin) so that the heating at homogenizing temperature takes place after not too long a period during which the conditions are not well defined and dangerous from a structural point of view. Also, all the portions of the article must be constantly at the same temperature, or approximately so.

For this purpose, the skin-annealing treatment is preferably performed in boxes which are small or at least one dimension of which is small (flat boxes or boxes limiting a volume in the form of an annular cylindrical space).

This heating is advantageously performed in powerful furnaces in an atmosphere of hydrogen so as to obtain a conductivity of heat as high as possible.

According to the nature of the alloy and to the shape and mass of the articles, cooling of the boxes is accelerated by circulation of air or by immersion in a liquid. In

3,276,903 any case, it is advantageous to cool down the bottom of the container so as to trap the fluoride vapors and to prevent their condensation on the articles in treatment.

When, according to my invention, skin-annealing is conducted in an inert protective atmosphere, it is ad- 5 vantageous to provide in this atmosphere one or several of the following fluoride compounds: nickel fluoride, cobalt fluoride, ammonium fluoride, hydrofluoric acid. However, precautions must be taken to obtain a protective atmosphere which lasts during the whole time of the treatment. As a matter of fact, due to their high volatility, ammonium fluoride and hydrofluoric acid can exert their protective function only if they are being evolved continuously or if they are accompanied by a halide having a high boiling point and of a substantial vapor tension. 15 This is the case in particular of nickel fluoride (boiling point 1025° C.) which may be obtained from the corresponding ammonium or hydrogen fluorides.

If the thermal treatments which are considered require temperatures higher than 1200° C., I may make use of 20 other fluorides having a higher boiling point, such as cobalt fluoride (boiling point 1300° C.).

I obtain very good results by performing the skinannealing treatment of Nimonic articles in the presence of nickel fluoride (which may be formed by heating a fine 25 nickel powder with ammonium fluoride) with a small amount of ammonium fluoride which drives off the air in the container at the beginning of the heating operation, or with ammonium fluoride alone if its vapors are made to form nickel fluoride by attacking a mass of nickel placed 30 in the treatment container close to the article but out of contact therewith. This mass may be constituted by small lumps of nickel of suitable size or by nickel grids surrounding the articles. This formation of nickel fluoride during the skin-annealing operation may even be obtained 35 by reaction of ammonium fluoride or hydrofluoric acid on the metal of the Nimonic article itself, which forms on said article a protective layer of nickel fluoride which volatilizes at high temperature.

In the course of a special operation, I may reduce the 40 proportion of titanium and aluminum from the superficial layers of Nimonic articles, before the skin-annealing operation, by performing a controlled oxidation for some hours at 700-750° C., without subsequent scouring. The effect of this operation is superficially to soften the alloy by lowering the proportion of titanium and aluminum ⁴⁵therein.

, When the atmosphere does not contain ammonium fluoride, the slight oxidation which takes place during the period before the vaporizing of the nickel automatically ₅₀produces such a superficial softening.

According to my invention, it is necessary to avoid the formation of undesirable fluorides inside the treatment container. As a rule, such undesirable fluorides are fluorides of metals which are not included in the composition ₅₅of the alloy and which might give rise to the formation of undesirable superficial diffusion alloys.

Thus, it is necessary to prevent the introduction or the formation of iron, vanadium, and boron fluorides. Such fluorides diffuse superficially into nickel and/or cobalt in θο a fluoride vapor atmosphere, in the same manner as chromium diffuses into iron articles. It is therefore preferable to avoid the presence, inside the treatment container, of iron or any alloy incorporating iron, because the protective nickel or cobalt fluorides which are normally in- 65 eluded in the atmosphere of the container might give, by reaction with iron, corresponding iron fluorides which would soil the superficial layers of the article. Therefore the treatment container will be preferably made of the main component metal or one of the main component 70 metals of the alloy of which the article is made, with the exception of chromium, or, alternately, this container will be coated or lined with such a metal or metals. If for some reasons, for instance of economy or mechanical resistance, it is however desired to use containers or ac- 75 cessories which contain iron, any noxious effects on the alloy may be avoided by surrounding the iron parts or the article to be treated with means (such as nickel grids for instance) through which the iron fluoride vapors must pass before reaching the articles to be treated.

I will now describe some examples of treatments made, according to my invention, in cementation boxes provided with partly gastight covers, said boxes being heated in an electric furnace, preferably in an atmosphere of hydrogen or containing hydrogen, for instance of cracked ammonia. Heating up is conducted as quickly as possible, and in the case of a furnace where the temperature would rise slowly from 700 to 1000° C., it would be necessary to increase the duration of the skin-annealing operation. The treatment boxes I used were made of iron but their inner walls were lined with nickel sheets. When the boxes were withdrawn from the furnace, their bottom was placed in a vessel containing water so as to condense any fluorides carried along by them and they were covered with a hood into which a stream of hydrogen was supplied so as to prevent the inflow of air during the cooling period. Finally the whole box was placed under a fan in order to obtain a quick cooling of the treatment container, at least down to the temperature of 700° C.

Example 1

Turbine blades made of Nimonic 95 were skin-annealed with the use of a cementation substance consisting of nickel fluoride and ammonium fluoride obtained by preliminary heating of ammonium fluoride and nickel powder. The articles were placed without separation in the boxes and treated for 1 hour at 1060° C. After this treatment, the articles were of a very bright light grey.

Analogous results were obtained by placing in the boxes a certain amount of ammonium fluoride, the articles being placed in nickel grids surrounding them, whereby the protective nickel fluoride was formed by the vapors of ammonium fluoride acting on said grids.

Example 2

Two turbine blades made of Nimonic 95 were skinannealed by being placed in a diluting mass consisting of thorium or zirconium oxide. The metal fluoride to be formed in the container was obtained from acid fluoride of ammonium to which may be added a small amount of nickel fluoride. In order to compensate for the relatively slow heating in this case, the duration of the heating at the temperature of 1080° C. was 40 minutes.

Example 3

A turbine wheel made of Nimonic 80 was skin-annealed under conditions similar to those of the preceding example, with the exception that the diluting substance is titanium oxide. Due to the size of the article, and therefore of the treatment box (350 mm. of diameter and 175 mm. of height), the thermal treatment was carried out as follows:

Heating up, from 700 to 1050° C._____1 h. 30 minutes.

Heating at 1050° C.-----------------40 minutes.

Cooling, from 1050 to 700° C.________35 minutes.

During the cooling operation, the box was placed under a hood fed with hydrogen, and cooled by atomized water.

Example 4

Gas turbine blades of an alloy of nickel-cobalt-chromium-titanium-aluminum and molybdenum were skin-annealed for 20 minutes at a temperature of 1080° C. without any material chromizing in a reducing atmosphere consisting of directly cracked ammonia in a closed partly gastight box of ordinary steel which contains at the bottom thereof a mixture of chromium or ferrochromium (in granular form) with ammonium fluoride either acid or neutral, the amount of this fluoride being about 2 grams per liter. The articles in the box are kept out of contact with the above mentioned mixture by a nickel grid. In

3,276,903 order to prevent diffusion of chromium into the pieces, said pieces are surrounded by an envelope of nickel wire gauze (mesh of about 1.2 mm.; thickness of wire 0.3 inm.; two layers of gauze).

Example 5

The pieces to be treated are the same as in the preceding example and the conditions of treatment are similar but, in this case, in order to prevent diffusion of chromium into the pieces, instead of enveloping these pieces in a ₁₀nickel wire gauze, the mixture intended to produce the fluoride vapors, instead of consisting of ammonium fluoride and chromium is a mixture of neutral ammonium fluoride with nickel powder and iron powder.

Example 6⁴³

Turbine combustion chambers made of stainless steel containing 20% nickel and 25% chromium had been assembled by welding individual elements. To eliminate stresses in the assembly and to deoxidize the welded zones, 20 the chambers were placed in a closed partly gastight box of ordinary steel which contained, at the bottom thereof, a mixture of chromium or ferrochromium (in granular form) with ammonium fluoride (either acid or neutral), the amount of this fluoride being about 2 grams per liter, 25 the chambers being separated from the mixture by a nickel grid. The box was heated in a reducing atmosphere consisting of directly cracked ammonia, for about one hour at a temperature of 1100-1120° C., during which the mixture of chromium and ammonium fluoride evolved gg chromium fluoride vapors. In order to prevent diffusion of chromium from these vapors into the articles in treatment, said articles were surrounded, either individually or as a whole, by an envelope of nickel wire gauze (mesh of about 1.2 mm.; thickness of wire 0.3 mm.; two layers 35 of gauze). This nickel envelope serves, by causing chromium to diffuse into its surface, to prevent chromizing of the articles in treatment. After the annealing operation, the box is cooled outside of the treatment oven in an atmosphere of reducing gas. 40

Example 7

Similar stainless steel articles were treated as in Example 6 except diffusion of chromium into the articles was prevented without nickel wire gauze but by produc- _4ging the fluoride vapors from a mixture of neutral ammonium fluoride with nickel powder and iron powders. The articles after treatment have not the appearance of polished chromium but a light gray bright silvered appearance. 50

Example 8

Sheets of stainless steel containing 25% of chromium and 20% of nickel were annealed between two rolling operations to improve the aptitude of the sheets to being stamped either in the hot or cold states. The sheets were 55 placed in a partly gastight closed box of ordinary steel which contains, at the bottom thereof, a mixture of nickel and iron (in powder form) with ammonium fluoride, the amount of this fluoride being about 2.5 grams per liter. The sheets are placed vertically in the box with small 60 amounts of ammonium fluoride placed in the intervals between said sheets, these sheets resting upon a nickel grid which keeps them out of contact with the above mentioned mixture. The box was heated in a reducing atmosphere consisting of directly cracked ammonium for about 65 half an hour at a temperature of 1080-1100° C., after which the boxes were cooled on the outside of the annealing oven in a reducing gas atmosphere. The sheets obtained after this treatment have an exceptional aptitude to being stamped either in the cold or hot state. This is 70 due to the fact that the annealing operation has been performed without any possibility of oxidizing and also that the surface of the sheet has been softened (elimination of any inclusion or of carbon in this surface over a depth of some tenths of a micron). 75