METHOD FOR THE MANUFACTURE OF A COMPOSITE REFRACTORY METALLIC ELEMENT WITH POROUS COATING
United States Patent 3713206
The casing of a turboreactor is constituted of a refractory alloy based on ickel and includes an abradable porous metallic coating constituted by a nickel felt. The coating is bonded by brazing to the casing by applying the coating on the casing by a pressure ring with a larger coefficient of expansion and by effecting the brazing in a fluorinated-hydrogenated atmosphere at a temperature in the region of the fusion temperature of the solder. The assembly is then subjected to a chromization or chromaluminization treatment in a halogenated-hydrogenated atmosphere (free of fluorine) at a temperature of 800° to 1100°C.
US Patent References:
Method of bonding a liner within a bore
Crook - July 1947 - 2424878
Process of fluxing and joining metal parts
Edson - July 1951 - 2561565
Process of joining metal parts
Moore - February 1952 - 2585819
High temperature metallic joint
Feaster - July 1960 - 2945295
Braze bonding of concentric tubes and shells and the like
Word et al. - March 1962 - 3025596
Method of bonding a liner within a bore
Crook - July 1947 - 2424878
Process of fluxing and joining metal parts
Edson - July 1951 - 2561565
Process of joining metal parts
Moore - February 1952 - 2585819
High temperature metallic joint
Feaster - July 1960 - 2945295
Braze bonding of concentric tubes and shells and the like
Word et al. - March 1962 - 3025596
Inventors:
Galmiche, Philippe M. (Clamart, FR)
Pelissier, Jean H. (Verrieres-le-Buisson, FR)
Spinat, Roland R. (Bretigny-sur-Orge, FR)
Pelissier, Jean H. (Verrieres-le-Buisson, FR)
Spinat, Roland R. (Bretigny-sur-Orge, FR)
Application Number:
05/044508
Publication Date:
01/30/1973
Filing Date:
06/08/1970
View Patent Images:
Export Citation:
Assignee:
Office National d'Etudes et de Recherches Aerospatiales (Chatillon-sous-Bagneux, FR)
Societe Nationale d'Etude et de Construction de Moteurs d'Aviation (Paris, FR)
Societe Nationale d'Etude et de Construction de Moteurs d'Aviation (Paris, FR)
Primary Class:
Other Classes:
228/219
International Classes:
B23K35/00; F01D5/28; F01D11/12; F01D11/08; B23K35/38; B23K31/02
Field of Search:
29/494,497.5,473.5
US Patent References:
| 3417460 | Methods of brazing | December 1968 | Golmiche |
Primary Examiner:
Overholser, Spencer J.
Assistant Examiner:
Shore, Ronald J.
Claims:
We claim
1. Method for the manufacture of a composite refractory metallic element comprising a compact metallic support bearing a porous metallic coating, said method, to ensure the firm bonding of said porous coating to said support, including the successive steps of
2. Method according to claim 1, wherein the element is subjected to a treatment of chromization or of chromaluminization utilizing a halogenated atmosphere free of fluorine said treatment being effected for from several hours to several tens of hours at a temperature of 800° to 1100°C in a container partially gas-tight to the gases used, from the beginning of the rise in temperature to the end of cooling, in an enclosure containing a hydrogenated atmosphere, by means of which there is obtained, by reason of the application of chromium or of chromium and of aluminum, an improvement of the characteristics in depth of the porous coating, of those of the zone connecting the coating and the compact support, as well as an increase in the refractory quality of the brazing filler metal layer when coating and said support have been effectively bonded by brazing.
3. Method according to claim 1, wherein during the brazing operation, the pressure of application of the porous coating against the compact support is of the order of several grams per cm2.
4. Method according to claim 2, wherein during the brazing operation, the pressure of application of the porous coating against the compact support is of the order of several grams per cm2.
5. Method according to claim 4, wherein said halogenated carrier is a member selected from the group consisting of a bromide, a chloride and a mixture of a bromide and a chloride.
6. Method according to claim 1, for the manufacture of a composite refractory metallic element of which the compact outer support has the shape of a ring, wherein the means to create the pressure for the application of the porous coating against the above said support are constituted by a clamping ring constituted of a metal or alloy having a coefficient of expansion greater than that of the outer ring, said clamping ring being arranged inside the ring constituting the porous coating.
7. Method according to claim 1, wherein said brazing metal is nickel.
1. Method for the manufacture of a composite refractory metallic element comprising a compact metallic support bearing a porous metallic coating, said method, to ensure the firm bonding of said porous coating to said support, including the successive steps of
2. Method according to claim 1, wherein the element is subjected to a treatment of chromization or of chromaluminization utilizing a halogenated atmosphere free of fluorine said treatment being effected for from several hours to several tens of hours at a temperature of 800° to 1100°C in a container partially gas-tight to the gases used, from the beginning of the rise in temperature to the end of cooling, in an enclosure containing a hydrogenated atmosphere, by means of which there is obtained, by reason of the application of chromium or of chromium and of aluminum, an improvement of the characteristics in depth of the porous coating, of those of the zone connecting the coating and the compact support, as well as an increase in the refractory quality of the brazing filler metal layer when coating and said support have been effectively bonded by brazing.
3. Method according to claim 1, wherein during the brazing operation, the pressure of application of the porous coating against the compact support is of the order of several grams per cm2.
4. Method according to claim 2, wherein during the brazing operation, the pressure of application of the porous coating against the compact support is of the order of several grams per cm2.
5. Method according to claim 4, wherein said halogenated carrier is a member selected from the group consisting of a bromide, a chloride and a mixture of a bromide and a chloride.
6. Method according to claim 1, for the manufacture of a composite refractory metallic element of which the compact outer support has the shape of a ring, wherein the means to create the pressure for the application of the porous coating against the above said support are constituted by a clamping ring constituted of a metal or alloy having a coefficient of expansion greater than that of the outer ring, said clamping ring being arranged inside the ring constituting the porous coating.
7. Method according to claim 1, wherein said brazing metal is nickel.
Description:
The invention relates, on the one hand, to methods for the manufacture of composite refractory metallic elements comprising a porous metallic coating with communicating pores fixed on a compact refractory metallic support, and, on the other hand, to the corresponding composite refractory metallic elements.
It relates more particularly, because it is in this case that its application seems to offer most advantage, but not exclusively, among these methods and elements, to those in which the porous coating is intended to play the role of an abradable sealing joint in a rotary machine, and, more particularly again, to those for which the porous coating concerned is intended to line the inner wall of the casing of a gas turbine and to be progressively worn by the contact of the ends of the blades of the said turbine by reason, in particular, of the slow but inevitable elongation of the said blades under the effects of flow caused by the centrifugal forces at the operating temperatures of the engine.
A first feature of the invention, relative to the methods of the type concerned and intended to ensure, in the refractory metallic element with a porous coating, the rigid bonding of the porous metallic coating and of the compact metallic support intended to receive the said coating, consists, in interposing between the said porous coating and the said compact support a filler metal brazing suited to the nature of these two materials and containing at least one metal of the group iron-cobalt-nickel (advantageously nickel), in then subjecting the composite element thus constituted to an application pressure causing compression of the brazing filler metal between the porous coating and the compact support, and finally in effecting the brazing operation proper in a partially gas-tight container, by subjecting the brazing, filler metal through the porous coating, to the action of a fluorinated atmosphere at a temperature close to the fusion temperature of the brazing filler metal so that the latter effectively wets the contact surfaces without in practice penetrating into the porous coating, this brazing operation being conducted in a hydrogenated atmosphere including herein its initial phase of bringing up to temperature and final phase of cooling, due to which the bond effected by brazing is very good, even if the application pressure is relatively slight (of the order of several grams per cm 2 ), and the porous coating remains free of any inner contamination and obstruction by the brazing filler metal whatever the relative position of the said coating with respect to the compact support during the brazing operation.
A second feature of the invention, relative to the improvement of a composite refractory metallic element of the type concerned of which the porous coating contains nickel, consists, once the porous coating is fastened to the compact support, especially by a brazing operation according to the first feature, in subjecting the abovesaid element to a treatment of chromization or of chroma-luminization utilizing a halogenated atmosphere free of fluorine (possibly iodine but preferably chlorine and/or bromine by reason of the particularly high vapor pressures at high temperatures of chlorides and bromides), the abovesaid treatment being effected during several hours to several tenths of hours at a temperature of 800 to 1100°C in a partially gas-tight container and arranged, from the beginning of the rise in temperature to the end of cooling, in an enclosure in which a hydrogenated atmosphere exists, due to which there is obtained, by reason of the application of chromium or of chromium and aluminum, an improvement of the characteristics in depth of the porous coating, of those of the junction zone of the said coating and of the compact support, as well as an increase in the refractory quality of the brazing layer when the abovesaid coating and the abovesaid support have effectively been joined by brazing, especially according to the first feature of the invention.
The advantage presented, not only by the application of one or other of these two features, but also their successive application with regard to a same composite refractory metalic element, will easily be seen, which will then be able, after having being placed under treatment conditions (porous coating applied against the compact support with the interposition of brazing filler metal), to be subjected successively to treatments conforming respectively to the two abovesaid features.
The application alone or in combination of the features which have just been considered is particularly advantageous when, on one hand, the compact support is constituted of stainless steel or of refractory alloy based on nickel or cobalt, and, on the other hand, the porous coating has a total porosity at least equal to 25 percent and contains an appreciable proportion of nickel and/or of cobalt and/or of iron.
In fact, for such an application, the utilization of the first feature enables the production in improved manner (increased resistance to tearing-off) of the desired bond between the porous coating and the compact support, and the later utilization of the second feature enables the transformation in depth of the porous coating by chromization or chromaluminization by thus rendering it unoxidizable, of improving the behavior to oxidation and of raising the fusion point, hence of improving the refractory characteristics, of the brazing, and even of transforming the superficial layers of the compact support by softening of the constituent refractory material of the said layers.
Although the application (singly or combined) of these features can be envisaged for a large variety of materials intended to be exposed in operation to mechanical and/or thermal fatigues and to corrosive effects(filters, fluid-tight labyrinths,soft friction abradable joints for hinges or bearings,etc.),a more particularly advantageous field of application would appear to be that of casings with an abradable internal coating for stages of gas turbines, which casings can be constituted by composite elements (envelope and inner coating) having, either the form of one-piece rings, or the form of sectors subsequently assembled together in an outer support.
There will be now described in more detailed manner preferred embodiments of the invention, which are in no way limiting, by referring to the accompanying drawing in which:
FIG. 1 shows, in diagrammatic manner and in longitudinal section, a gas turbine casing element with abradable porous inner coating, and said element being constructed according to the invention, and
FIG. 2 shows, in diagrammatic manner and in transverse section, an assembly used for applying against one another the inner coating and the outer envelope of the abovesaid casing element during the process of the manufacture of the latter.
The composite turbine casing element (especially for a turboreactor turbine) shown in FIG. 1 comprises essentially a refractory metallic envelope 1 and an abradable porous metallic coating 2 firmly bonded to the abovesaid envelope 1 in the manner which will be specified below, the abovesaid coating enveloping with a slight play (initially and in the cold) the periphery of the vanes A of the ring of vanes situated at the level of this composite element of the casing.
The envelope 1 is advantageously constituted of a refractory alloy based on nickel, on cobalt, or on chromium, and there may be mentioned, as refractory alloys of this type, those denoted by the trade names HASTELLOY X, MULTIMET, RENE 41, HS 25, T.D. NICKEL, T.D. NICKEL-CHROME and ACIER 25-20.
As for the abradable porous metallic coating 2, it is preferably constituted by a nickel felt generally having the form of strips of a thickness of two to three millimeters and a porosity of about 80 percent, felts of this type being marketed by the American firm GENERAL ELECTRIC and by the French firm METAFRAM.
According to one of the essential features of the invention, the fixing of the coating 2 on the inner wall of the envelope 1 is effected,
by applying with a slight pressure (of the order of several grams, for example ten, per square centimeter) the coating 2 against the envelope 1 with the interposition, between these two elements, of a brazing filler metal layer 3 containing phosphorus and at least one metal of the group iron - cobalt - nickel, and which is preferably a nickel-based brazing, metal, for example a powdered brazing metal for example brazing by powder, known commercially by the name NICROBRAZ 50, based on nickel containing chromium (13 percent) and phosphorus (10 percent), the latter brazing material having a fusion point of 890°C and its positioning, in the form of a thick layer of 50 to 250 microns, being done preferably from a suspension of the said brazing material in a lacquer or by deposition by a plasma torch,
and by effecting a brazing operation by exposing the layer of solder brazing material 3 to the effect of a fluorinated atmosphere acting through the porous coating 2. This operation is conducted at a temperature (of the order of 900° to 920°C in the case envisaged of the NICROBRAZ 50 solder) hardly higher than the fusion temperature of the brazing material, so that there is obtained the desirable wetting of the surfaces to be united without producing appreciable penetration of the brazing material into the porous coating 2. The elements subjected to this brazing operation are arranged in partially gas-tight containers, at the bottom of which is placed a mixture of particles of chromium and of ammonium fluoride, exposed to a hydrogenated protective atmosphere during the rise in temperature, the treatment proper (several hours) and the cooling.
After this brazing operation, the assemblies obtained have a uniform pearly gray appearance and excellent resistance (distinctly greater than that of the porous coating considered alone) to tearing-off forces tending to separate the compact envelope 1 from the porous coating 2 free in depth of brazing material, these good results being obtained whatever the relative positions of the said coating with respect to the abovesaid envelope.
Moreover, given the slight application pressures brought into play, the initial porosity and thickness of the porous coating 2 are not appreciably affected by the brazing operation which has just been considered.
It is then advantageous to improve further the properties of the composite refractory metallic element thus obtained (porous coating 2 brazed on the compact envelope 1) by subjecting the said composite element, preferably in the same hydrogenated atmosphere enclosure and in the same types of partially gas-tight containers, to a subsequent treatment according to the second essential feature of the invention, i.e., a treatment of chromization or of chromaluminization utilizing a halogenated atmosphere employing, as halogen, chlorine and/or bromine.
This subsequent treatment has the effect, on the one hand, of conferring on the porous coating 2, throughout its mass, i.e., on all the walls and its multiple communicating cells, an unoxidizable character, on the other hand, of reinforcing the behavior to oxidation of the brazing zone and of raising its fusion point, and, finally, on the other hand, of also improving the behavior to oxidation of the superficial layers of the subadjacent envelope to the abovesaid brazing zone.
There will first of all be examined the case where a simple treatment of chromization follows, this treatment being capable of being effected, either by arranging the composite elements to be treated in contact with chromium in small granules in the presence of a halogenated carrier mixture and of chromium powder placed at the bottom of the treatment container in a cup preferably covered with a nickel grid, or by arranging the abovesaid composite elements in contact with an intimate mixture of chromium in ultra-fine powder and of magnesia or of alumina supplemented by a halogenated carrier.
In the two cases, the assembly constituted by the parts and the chromization charge is placed in partially gas-tight containers, which are then heated (for several hours between 850° and 1000°C, and preferably for eight hours in the neighborhood of 950°C), then cooled, under a hydrogenated protective atmosphere.
The halogenides of chromium having the best penetrating power in the case of the treatment of porous materials are the bromide and the chloride of chromium by reason of the particularly high vapor pressures at high temperature of these two halogenides. The halogenides of chromium are formed in situ on the chromization treatment from a reaction between chromium and the additional halogenated carrier, in this instance ammonium bromide and/or chloride which are added initially in amounts of the order of 1 to 2 percent by weight in the chromization charges.
Such a chromization treatment ensures a complete transformation of the porous nickel into stainless porous nickel-chromium of pearly gray appearance and, in addition, it improves the strength characteristics of the composite element and of the brazing zone. Moreover, the enrichment in chromium of the brazed zones increases appreciably the fusion temperature and the resistance to oxidation of the said zones.
The average content of chromium of the chromized porous nickel coatings reaches about 35 percent, i.e., the materials obtained have a chemical composition close to those of the best nickel-chromium refractory alloys.
Finally, the plasticity of the porous coating thus chromized is substantially increased, which enables possible envisaging of a later trueing of assemblies which have been subjected to deformations during the operation of bonding or of protection.
If necessary, the chromization treatment could be followed by a selective homogenization and oxidation annealing, by heating for several hours towards 850° - 950°C in an atmosphere of non-purified electrolytic hydrogen or of wet hydrogen, this complementary treatment giving rise to the formation of a very fine layer of pure chromium oxide on the surface of each grain of chromized nickel, such a layer substantially improving the behavior of the treated material relative to oxidation at high temperature.
As regards now the treatment of protection by chromaluminization, which treatment is intended to transform the porous nickel into unoxidizable porous nickel-aluminum-chromium, it is preferably effected by having recourse, for the application of chromium and aluminum, to ultra-fine and homogeneous pre-alloyed powders, of which each grain is constituted by an alloy of chromium and aluminium, such powders enabling permanent assurance of saturation of the treatment atmosphere of halogenide carriers, which leads to a much accentuated penetrating power.
The composite elements to be treated are arranged in partially gas-tight containers in contact with an intimate mixture of pre-alloyed ultra-fine powder of chromium-aluminum and alumina supplemented with a bromine and/or chlorine halogenated carrier (especially ammonium bromide and/or chloride), the containers thus equipped being heated (for several hours between 850° and 1000°C, and preferably for 8 hours in the region of 950°C), then cooled, under a hydrogenated protective atmosphere.
The porous nickel is completely transformed into unoxidizable porous nickel-aluminum-chromium of bluish gray appearance and the resistance characteristics of the composite elements and of the bonding zones are improved as in the case of simple chromization.
The average contents of aluminum and of chromium of the porous material tranformed by chromaluminization are respectively of the order of 20 and 5 percent, and the average hardness of the transformed porous material is very much greater than that of the initial porous material.
In conclusion, there will now be given a specific and complete example of a treatment according to the invention applied to a turbine casing element, especially of a turboreactor turbine casing, with indication of an assembly enabling the creation of the application pressure required by the treatment.
It is assumed, for this specific example, that the turbine casing element comprises, as shown in FIG. 2, on the one hand, an outer support ring 1 constituted of a refractory alloy based on nickel, and, on the other hand, an abradable porous metallic coating 2, also in the form of a ring, constituted essentially of nickel and having a porosity of the order of 80 percent, a layer 3 of NICROBRAZ 50 solder being interposed between these two rings.
To obtain the desired application pressure, during treatment, there is provided, inside the porous ring 2, a clamping ring 4 constituted of a metal or alloy having a greater coefficient of expansion than that of the outer support ring 1, the positioning of this clamping ring 4 being able to be effected by previously removing a sector 4a of this ring, by subjecting the said ring to constriction enabling its positioning in the ring of porous coating 2, and by re-establishing the continuity of the clamping ring 4 by the re-introduction of the sector 4a which then plays the role of a gap-filling insert.
As constituent material of the clamping ring 4, there may be adopted, in the case where the constituent refractory alloy of the outer support ring 1 (for example based on nickel) has a coefficient of expansion at 20°C less than 15.10 - 6 , a stainless steel Z 10 CNT 18 according to the French nomenclature (American nomenclature AISI 321), and in the case where the constituent refractory alloy of the outer support ring 1 (for example refractory steel Z6 NCT 25 according to the French nomenclature or A 286 according to the American nomenclature) has a coefficient of expansion at 20°C greater than 15.10 - 6 , a stainless steel Z 50 NMC 12 according to the French nomenclature corresponding to the American standard AMS 56-24.
In order to effect the brazing of the coating ring 2 on the support ring 1, the assembly is then placed in a muffle itself arranged in a hydrogen oven, the atmosphere of the muffle communicating with that of the oven and the abovesaid muffle containing, out of contact with the above said assembly, powder of chromium and of ammonium fluoride.
The brazing treatment is then effected at a temperature of 900° to 920°C for several hours (4 to 5 hours for example).
The assembly thus treated is then disengaged, the clamping ring 4 is withdrawn and the one-piece element constituted by the support ring 1 and the coating 2 bonded by brazing material is washed.
This monobloc assembly is then subjected to a chromaluminization treatment effected in the same muffle and the same hydrogen furnace as the brazing treatment.
The monobloc element is immersed in a reactive mass filling the muffle, the said reactive mass being constituted by a mixture of pre-alloyed powder of chromium and aluminum, and alumina, supplemented with a slight quantity of ultra-fine powder of magnesothermic chromium and ammonium bromide, the ratio by weight of chromium and aluminum in this mixture being of the order of 10.
The assembly is then heated to a temperature of the order of 950°C for about 12 hours, after which the one-piece element is maintained for about 2 hours at a temperature of the order of 800°C to eliminate the halogenides which can remain in the porous coating ring 2.
The monobloc element thus obtained has then practically the same dimensions as following the brazing treatment and the chromaluminization treatment does not cause any internal stress in the abovesaid monobloc element.
It relates more particularly, because it is in this case that its application seems to offer most advantage, but not exclusively, among these methods and elements, to those in which the porous coating is intended to play the role of an abradable sealing joint in a rotary machine, and, more particularly again, to those for which the porous coating concerned is intended to line the inner wall of the casing of a gas turbine and to be progressively worn by the contact of the ends of the blades of the said turbine by reason, in particular, of the slow but inevitable elongation of the said blades under the effects of flow caused by the centrifugal forces at the operating temperatures of the engine.
A first feature of the invention, relative to the methods of the type concerned and intended to ensure, in the refractory metallic element with a porous coating, the rigid bonding of the porous metallic coating and of the compact metallic support intended to receive the said coating, consists, in interposing between the said porous coating and the said compact support a filler metal brazing suited to the nature of these two materials and containing at least one metal of the group iron-cobalt-nickel (advantageously nickel), in then subjecting the composite element thus constituted to an application pressure causing compression of the brazing filler metal between the porous coating and the compact support, and finally in effecting the brazing operation proper in a partially gas-tight container, by subjecting the brazing, filler metal through the porous coating, to the action of a fluorinated atmosphere at a temperature close to the fusion temperature of the brazing filler metal so that the latter effectively wets the contact surfaces without in practice penetrating into the porous coating, this brazing operation being conducted in a hydrogenated atmosphere including herein its initial phase of bringing up to temperature and final phase of cooling, due to which the bond effected by brazing is very good, even if the application pressure is relatively slight (of the order of several grams per cm 2 ), and the porous coating remains free of any inner contamination and obstruction by the brazing filler metal whatever the relative position of the said coating with respect to the compact support during the brazing operation.
A second feature of the invention, relative to the improvement of a composite refractory metallic element of the type concerned of which the porous coating contains nickel, consists, once the porous coating is fastened to the compact support, especially by a brazing operation according to the first feature, in subjecting the abovesaid element to a treatment of chromization or of chroma-luminization utilizing a halogenated atmosphere free of fluorine (possibly iodine but preferably chlorine and/or bromine by reason of the particularly high vapor pressures at high temperatures of chlorides and bromides), the abovesaid treatment being effected during several hours to several tenths of hours at a temperature of 800 to 1100°C in a partially gas-tight container and arranged, from the beginning of the rise in temperature to the end of cooling, in an enclosure in which a hydrogenated atmosphere exists, due to which there is obtained, by reason of the application of chromium or of chromium and aluminum, an improvement of the characteristics in depth of the porous coating, of those of the junction zone of the said coating and of the compact support, as well as an increase in the refractory quality of the brazing layer when the abovesaid coating and the abovesaid support have effectively been joined by brazing, especially according to the first feature of the invention.
The advantage presented, not only by the application of one or other of these two features, but also their successive application with regard to a same composite refractory metalic element, will easily be seen, which will then be able, after having being placed under treatment conditions (porous coating applied against the compact support with the interposition of brazing filler metal), to be subjected successively to treatments conforming respectively to the two abovesaid features.
The application alone or in combination of the features which have just been considered is particularly advantageous when, on one hand, the compact support is constituted of stainless steel or of refractory alloy based on nickel or cobalt, and, on the other hand, the porous coating has a total porosity at least equal to 25 percent and contains an appreciable proportion of nickel and/or of cobalt and/or of iron.
In fact, for such an application, the utilization of the first feature enables the production in improved manner (increased resistance to tearing-off) of the desired bond between the porous coating and the compact support, and the later utilization of the second feature enables the transformation in depth of the porous coating by chromization or chromaluminization by thus rendering it unoxidizable, of improving the behavior to oxidation and of raising the fusion point, hence of improving the refractory characteristics, of the brazing, and even of transforming the superficial layers of the compact support by softening of the constituent refractory material of the said layers.
Although the application (singly or combined) of these features can be envisaged for a large variety of materials intended to be exposed in operation to mechanical and/or thermal fatigues and to corrosive effects(filters, fluid-tight labyrinths,soft friction abradable joints for hinges or bearings,etc.),a more particularly advantageous field of application would appear to be that of casings with an abradable internal coating for stages of gas turbines, which casings can be constituted by composite elements (envelope and inner coating) having, either the form of one-piece rings, or the form of sectors subsequently assembled together in an outer support.
There will be now described in more detailed manner preferred embodiments of the invention, which are in no way limiting, by referring to the accompanying drawing in which:
FIG. 1 shows, in diagrammatic manner and in longitudinal section, a gas turbine casing element with abradable porous inner coating, and said element being constructed according to the invention, and
FIG. 2 shows, in diagrammatic manner and in transverse section, an assembly used for applying against one another the inner coating and the outer envelope of the abovesaid casing element during the process of the manufacture of the latter.
The composite turbine casing element (especially for a turboreactor turbine) shown in FIG. 1 comprises essentially a refractory metallic envelope 1 and an abradable porous metallic coating 2 firmly bonded to the abovesaid envelope 1 in the manner which will be specified below, the abovesaid coating enveloping with a slight play (initially and in the cold) the periphery of the vanes A of the ring of vanes situated at the level of this composite element of the casing.
The envelope 1 is advantageously constituted of a refractory alloy based on nickel, on cobalt, or on chromium, and there may be mentioned, as refractory alloys of this type, those denoted by the trade names HASTELLOY X, MULTIMET, RENE 41, HS 25, T.D. NICKEL, T.D. NICKEL-CHROME and ACIER 25-20.
As for the abradable porous metallic coating 2, it is preferably constituted by a nickel felt generally having the form of strips of a thickness of two to three millimeters and a porosity of about 80 percent, felts of this type being marketed by the American firm GENERAL ELECTRIC and by the French firm METAFRAM.
According to one of the essential features of the invention, the fixing of the coating 2 on the inner wall of the envelope 1 is effected,
by applying with a slight pressure (of the order of several grams, for example ten, per square centimeter) the coating 2 against the envelope 1 with the interposition, between these two elements, of a brazing filler metal layer 3 containing phosphorus and at least one metal of the group iron - cobalt - nickel, and which is preferably a nickel-based brazing, metal, for example a powdered brazing metal for example brazing by powder, known commercially by the name NICROBRAZ 50, based on nickel containing chromium (13 percent) and phosphorus (10 percent), the latter brazing material having a fusion point of 890°C and its positioning, in the form of a thick layer of 50 to 250 microns, being done preferably from a suspension of the said brazing material in a lacquer or by deposition by a plasma torch,
and by effecting a brazing operation by exposing the layer of solder brazing material 3 to the effect of a fluorinated atmosphere acting through the porous coating 2. This operation is conducted at a temperature (of the order of 900° to 920°C in the case envisaged of the NICROBRAZ 50 solder) hardly higher than the fusion temperature of the brazing material, so that there is obtained the desirable wetting of the surfaces to be united without producing appreciable penetration of the brazing material into the porous coating 2. The elements subjected to this brazing operation are arranged in partially gas-tight containers, at the bottom of which is placed a mixture of particles of chromium and of ammonium fluoride, exposed to a hydrogenated protective atmosphere during the rise in temperature, the treatment proper (several hours) and the cooling.
After this brazing operation, the assemblies obtained have a uniform pearly gray appearance and excellent resistance (distinctly greater than that of the porous coating considered alone) to tearing-off forces tending to separate the compact envelope 1 from the porous coating 2 free in depth of brazing material, these good results being obtained whatever the relative positions of the said coating with respect to the abovesaid envelope.
Moreover, given the slight application pressures brought into play, the initial porosity and thickness of the porous coating 2 are not appreciably affected by the brazing operation which has just been considered.
It is then advantageous to improve further the properties of the composite refractory metallic element thus obtained (porous coating 2 brazed on the compact envelope 1) by subjecting the said composite element, preferably in the same hydrogenated atmosphere enclosure and in the same types of partially gas-tight containers, to a subsequent treatment according to the second essential feature of the invention, i.e., a treatment of chromization or of chromaluminization utilizing a halogenated atmosphere employing, as halogen, chlorine and/or bromine.
This subsequent treatment has the effect, on the one hand, of conferring on the porous coating 2, throughout its mass, i.e., on all the walls and its multiple communicating cells, an unoxidizable character, on the other hand, of reinforcing the behavior to oxidation of the brazing zone and of raising its fusion point, and, finally, on the other hand, of also improving the behavior to oxidation of the superficial layers of the subadjacent envelope to the abovesaid brazing zone.
There will first of all be examined the case where a simple treatment of chromization follows, this treatment being capable of being effected, either by arranging the composite elements to be treated in contact with chromium in small granules in the presence of a halogenated carrier mixture and of chromium powder placed at the bottom of the treatment container in a cup preferably covered with a nickel grid, or by arranging the abovesaid composite elements in contact with an intimate mixture of chromium in ultra-fine powder and of magnesia or of alumina supplemented by a halogenated carrier.
In the two cases, the assembly constituted by the parts and the chromization charge is placed in partially gas-tight containers, which are then heated (for several hours between 850° and 1000°C, and preferably for eight hours in the neighborhood of 950°C), then cooled, under a hydrogenated protective atmosphere.
The halogenides of chromium having the best penetrating power in the case of the treatment of porous materials are the bromide and the chloride of chromium by reason of the particularly high vapor pressures at high temperature of these two halogenides. The halogenides of chromium are formed in situ on the chromization treatment from a reaction between chromium and the additional halogenated carrier, in this instance ammonium bromide and/or chloride which are added initially in amounts of the order of 1 to 2 percent by weight in the chromization charges.
Such a chromization treatment ensures a complete transformation of the porous nickel into stainless porous nickel-chromium of pearly gray appearance and, in addition, it improves the strength characteristics of the composite element and of the brazing zone. Moreover, the enrichment in chromium of the brazed zones increases appreciably the fusion temperature and the resistance to oxidation of the said zones.
The average content of chromium of the chromized porous nickel coatings reaches about 35 percent, i.e., the materials obtained have a chemical composition close to those of the best nickel-chromium refractory alloys.
Finally, the plasticity of the porous coating thus chromized is substantially increased, which enables possible envisaging of a later trueing of assemblies which have been subjected to deformations during the operation of bonding or of protection.
If necessary, the chromization treatment could be followed by a selective homogenization and oxidation annealing, by heating for several hours towards 850° - 950°C in an atmosphere of non-purified electrolytic hydrogen or of wet hydrogen, this complementary treatment giving rise to the formation of a very fine layer of pure chromium oxide on the surface of each grain of chromized nickel, such a layer substantially improving the behavior of the treated material relative to oxidation at high temperature.
As regards now the treatment of protection by chromaluminization, which treatment is intended to transform the porous nickel into unoxidizable porous nickel-aluminum-chromium, it is preferably effected by having recourse, for the application of chromium and aluminum, to ultra-fine and homogeneous pre-alloyed powders, of which each grain is constituted by an alloy of chromium and aluminium, such powders enabling permanent assurance of saturation of the treatment atmosphere of halogenide carriers, which leads to a much accentuated penetrating power.
The composite elements to be treated are arranged in partially gas-tight containers in contact with an intimate mixture of pre-alloyed ultra-fine powder of chromium-aluminum and alumina supplemented with a bromine and/or chlorine halogenated carrier (especially ammonium bromide and/or chloride), the containers thus equipped being heated (for several hours between 850° and 1000°C, and preferably for 8 hours in the region of 950°C), then cooled, under a hydrogenated protective atmosphere.
The porous nickel is completely transformed into unoxidizable porous nickel-aluminum-chromium of bluish gray appearance and the resistance characteristics of the composite elements and of the bonding zones are improved as in the case of simple chromization.
The average contents of aluminum and of chromium of the porous material tranformed by chromaluminization are respectively of the order of 20 and 5 percent, and the average hardness of the transformed porous material is very much greater than that of the initial porous material.
In conclusion, there will now be given a specific and complete example of a treatment according to the invention applied to a turbine casing element, especially of a turboreactor turbine casing, with indication of an assembly enabling the creation of the application pressure required by the treatment.
It is assumed, for this specific example, that the turbine casing element comprises, as shown in FIG. 2, on the one hand, an outer support ring 1 constituted of a refractory alloy based on nickel, and, on the other hand, an abradable porous metallic coating 2, also in the form of a ring, constituted essentially of nickel and having a porosity of the order of 80 percent, a layer 3 of NICROBRAZ 50 solder being interposed between these two rings.
To obtain the desired application pressure, during treatment, there is provided, inside the porous ring 2, a clamping ring 4 constituted of a metal or alloy having a greater coefficient of expansion than that of the outer support ring 1, the positioning of this clamping ring 4 being able to be effected by previously removing a sector 4a of this ring, by subjecting the said ring to constriction enabling its positioning in the ring of porous coating 2, and by re-establishing the continuity of the clamping ring 4 by the re-introduction of the sector 4a which then plays the role of a gap-filling insert.
As constituent material of the clamping ring 4, there may be adopted, in the case where the constituent refractory alloy of the outer support ring 1 (for example based on nickel) has a coefficient of expansion at 20°C less than 15.10 - 6 , a stainless steel Z 10 CNT 18 according to the French nomenclature (American nomenclature AISI 321), and in the case where the constituent refractory alloy of the outer support ring 1 (for example refractory steel Z6 NCT 25 according to the French nomenclature or A 286 according to the American nomenclature) has a coefficient of expansion at 20°C greater than 15.10 - 6 , a stainless steel Z 50 NMC 12 according to the French nomenclature corresponding to the American standard AMS 56-24.
In order to effect the brazing of the coating ring 2 on the support ring 1, the assembly is then placed in a muffle itself arranged in a hydrogen oven, the atmosphere of the muffle communicating with that of the oven and the abovesaid muffle containing, out of contact with the above said assembly, powder of chromium and of ammonium fluoride.
The brazing treatment is then effected at a temperature of 900° to 920°C for several hours (4 to 5 hours for example).
The assembly thus treated is then disengaged, the clamping ring 4 is withdrawn and the one-piece element constituted by the support ring 1 and the coating 2 bonded by brazing material is washed.
This monobloc assembly is then subjected to a chromaluminization treatment effected in the same muffle and the same hydrogen furnace as the brazing treatment.
The monobloc element is immersed in a reactive mass filling the muffle, the said reactive mass being constituted by a mixture of pre-alloyed powder of chromium and aluminum, and alumina, supplemented with a slight quantity of ultra-fine powder of magnesothermic chromium and ammonium bromide, the ratio by weight of chromium and aluminum in this mixture being of the order of 10.
The assembly is then heated to a temperature of the order of 950°C for about 12 hours, after which the one-piece element is maintained for about 2 hours at a temperature of the order of 800°C to eliminate the halogenides which can remain in the porous coating ring 2.
The monobloc element thus obtained has then practically the same dimensions as following the brazing treatment and the chromaluminization treatment does not cause any internal stress in the abovesaid monobloc element.