MXPA99012034A - Pulsed-vapor phase aluminide process for high temperature oxidation-resistant coating applications - Google Patents

Abstract

An improved process for applying aluminide coatings to superalloy components (10) used in gas turbine applications resulting in a more uniform coating with less hazardous waste by-products. The process involves the steps of placing the superalloy components (10) into a retort (20) with an aluminum-containing source (30), evacuating air from the retort (20) and introducing an inert gas (50), heating the retort (20) to a preselected temperature, while maintaining the preselected temperature purging the inert gas (50) from the retort by introducing hydrogen gas (60), while maintaining the preselected temperature, pulsing, by reducing the retort pressure to a preselected pressure below atmospheric pressure, followed by introducing a halide-containing gas (42) to react with the aluminum-containing source (30) to create an aluminum-rich vapor that deposits aluminum on the components, then reintroducing hydrogen gas (60) into the retort (20) to purge the gases within the retort (20);and coo ling the retort (20).

Description

PROCESS OF ALUMINÜRO IN PHASE OF STEAM DRIVEN FOR APPLICATIONS OF COATING RESISTANT TO HIGH TEMPERATURE OXIDATION 5 BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a process for applying aluminide coatings to components of superalloy used in gas turbine applications, and more particularly, to an improved vapor phase aluminide application process, to coat nickel-based and cobalt-based superalloy components used in the turbine section of the engines gas turbine, DISCUSSION OF THE PREVIOUS TECHNIQUE The aluminide coatings are applied to provide protection of the superalloy turbine components of gas turbine engines, to protect to the substrate material by many different processes. One such process is set forth in United States Patent No. 3,837,901 ('901) now expired, to Seybolt, incorporated herein by reference, and assigned to the assignee of the present invention. In the '901 patent, an aluminide coating is applied embedding the components of the turbine in a bed of powders that have ildHÜÉMÉlilÉWÉailllilÉiÉlt aluminum as a source. In general, the active powders stipulated in this patent were iron-aluminum compounds mixed with inert alumina, and the powders were activated by passing a halide gas through the carrier, while the bed was heated to a temperature on the scale from approximately 899 ° C to approximately 1093 ° C Subsequent improvements in the process have included modifications to aluminum powder sources, modifications in powder sizes, and improvements in the supply systems of gas These processes have generally become known as "package processes", or vapor phase aluminum aluminiuide processes. These processes have several shortcomings. First, because they involve producing coatings that require specific compositions, they must be carefully mixed, such that the coating compositions can be obtained. However, once exhausted, the mixed metal powders are not easily recyclable, can not be filled, and present a waste problem. A second problem associated with these package processes is that the The measurement and movement, as well as the disposal of the powders, are intense in manpower. Third, the process, although producing a good protective coating, produces a coating of variable thickness that is not easily controlled. Finally, as the number of temperature demands on the gas turbines, have been added ~ *. ^ * **** ~ *,. cooling passages to the components of the turbine. Powders from package processes often clog these channels, and removal of these powders from these fine cooling passages is an additional problem. What is desired is a coating method that produces a good quality aluminide coating, while eliminating the problems associated with the prior art package processes.

BRIEF COMPENDI OF THE INVENTION The present invention provides an improved process for applying aluminide coatings to superalloy components used in gas turbine applications. Compared with the prior vapor phase aluminide application techniques available for applying these aluminide coatings, the processes of the present invention provide an aluminide coating having a more uniform coating thickness, while maintaining the advantage of the coatings relatively slender typically associated with the application processes of aluminide in vapor phase. An additional advantage of the present invention is that the process is less intense in labor and more suitable for the environment, because no heavy dust is involved, eliminating the need to move these powders • AAA ^^ or to dispose of these powders. The granules used in the present invention are easier to segregate and reprocess, if necessary. According to the present invention, an improved process for applying aluminide coatings to the superalloy components used in gas turbine applications comprises a series of steps, the first of which is to place the components of the superalloy in a retort with a source that contains aluminum. The air is then evacuated from the retort by introducing an inert gas into the retort. The retort is then heated, usually by placing the retort in an oven, at a previously selected temperature. While maintaining the previously selected temperature, the inert gas in the retort is purged by introducing hydrogen gas. The hydrogen gas in the retort is then reduced to a previously selected pressure below atmospheric pressure, imposing a partial vacuum, while maintaining the constant temperature. Next, a gas containing halide is introduced into the retort. This gas reacts with the aluminum source in the retort at the previously selected temperature, creating a gas vapor rich in aluminum. The aluminum-rich gas vapor passes over the surface of the superalloy substrate, interacting with it to deposit a thin, substantially uniform coating, until a thin coating is obtained. Hydrogen gas is then reintroduced into the retort to purge the gases from the retort. The process of introducing and purging the halide-containing gas in the retort can be repeated to uniformly increase the thickness of the coating as desired. After the desired thickness is reached, the pressure of the gases in the retort is again reduced below atmospheric pressure, the inert gases are introduced into the retort, and the retort is cooled. Accordingly, it can be seen that an advantage of the present invention is that a uniform, and yet thicker, aluminide coating can be achieved with the process of the present invention, if desired. Another advantage of the present invention is that, because it is not required to use powders as a source of aluminum or as a filler material, the tendency for the cooling orifices to become clogged with the powders of the typical turbine components is eliminated. , such as the blades. Finally, because no dust is required, the labor-intensive powder preparation process, which involves precise weighting and powder mixing, can be eliminated. Other features and advantages of the present invention will become clearer from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram of the apparatus used to practice the process of the present invention. Figure 2 is a flow chart of the process used to produce the coating of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 depicts a schematic of one embodiment of the apparatus used to practice the process of the present invention. The process of the present invention can be fully understood by reference to this schematic drawing, and of which Figure 2 is a flow diagram of the process used to produce the coating of the present invention. Whenever possible, the same reference numbers will be used throughout all the Figures to refer to the same parts. In accordance with the present invention, the turbine components, typically the vanes 10, are placed in a retort after being cleaned. These components are usually made of nickel-based and cobalt-based superalloy compositions. If the blades 10 are newly manufactured, they are normally cleaned by degreasing. However, the present invention can also be used to recoat the components of the turbine removed from service. These components must first be separated from any previous coatings by well-known separation processes, usually by sandblasting, with a fine alumina powder, although acid separation followed by fluoride ion cleaning is also used. A retort 20 is filled with a source containing aluminum, which acts as a medium. This medium, in a preferred embodiment, is shown as thick granules 30, which are preferred if the vanes 10 are to be "packed" inside the medium. The coarse granules act to prevent clogging of fine passages of air, which is a common problem when using fine powders as the medium. In the best mode of practicing the present invention, the vanes were placed in a plurality of coating boxes 15. The boxes 15 were placed in a retort 20, which was then placed in an oven (not shown). In the best mode, the granules 30 were placed in proximity to the blades 10, but not in physical contact with the blades 10. The best mode of practicing the present invention is shown in Figure 1. The configuration of the granules and the blades it can be made in any of a number of ways, such as by incorporating a camera inside a coating box 15 to contain the granules 30, such as the vertical chambers shown in Figure 1. However, the blades 10 they can be suspended on a bed of granules 30 in each box 15, or they can be supported on platforms in each box 15, which raise the blades 10 on the granules 30. In a preferred embodiment, the size of the granules varies from about 4. millimeters (0.15 inches) to approximately 16 millimeters (0.63 inches). In another variation, the blades 10 can be suspended on the medium inside the retort 20, and the medium need not be of coarse granules. Typically, components that undergo a cooling process will not be suspended simultaneously above the package nor will they be packed in the medium, because it would be difficult to control the thickness of the coating applied to both the packaged and suspended vanes 10. In the preferred embodiment, the blades 10 are placed in coating boxes out of contact with the medium, and reference will be made to these blades 10, it being understood that all other aspects of the invention can be made using blades packed in granules, or blades 10 suspended on the the retort 20. After the blades 10 are placed inside the retort 20 with the aluminum-containing medium, which in a preferred embodiment are cobalt-aluminum granules, then the retort 20 is sealed and placed in an oven, not shown in Figure 1. The system includes an internal gas distribution system 22. It will be understood by those skilled in the art, that it can be used r any source of heating retort 20, such as heat sources by convection or by induction, and that the temperature inside the retort 20 is controlled by thermocouples. The gas distribution system 22 is connected to an inlet gas system 40 and an outlet gas system 70, and the retort is sealed. The inlet gas system 40 includes a supply of hydrogen fluoride activator gas 42, an inert carrier supply 50, and a supply of hydrogen gas 60. Associated with each gas volume, there is a pressure regulator or mass flow regulator 44, 52, 62, respectively. A pressure regulator or a mass flow regulator can be use interchangeably, because, in a given system, if the pressure flow is known, the mass flow can be calculated, and vice versa. A supply line 46, 54, 64 connects the respective gas supplied to the valves 48, 56, 65, which in turn are connected to the inlet gas line 66, which is connects to an inlet valve 68. Each of the valves is operated by a controller (not shown), which opens or closes the valves, such that the required gas can flow as established by the gas regulators 44, 52 and 62, from gas supplies to line 66, and to the gas distribution system in the retort.

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The exhaust gas system 70 is comprised of an outlet line 72, an outlet valve 74, and an outlet vacuum pump 76. Both the outlet valve 74 and the outlet vacuum pump 76 are operated by the controller (not shown), which maintains control over the inlet gas 40 and outlet 70 systems, such that the process of the present invention can be performed. After the sealed retort 20 is placed inside the oven, the air is evacuated by removing a partial vacuum from the retort, by activating the outlet gas system 70, opening the valve 74, and activating the vacuum pump 76. After that the air has been evacuated, the pump 76 is turned off, and inert gas, preferably argon, flows from the supply 50, through the valves 56, 68, to the retort, while the valves 48 and 65 remain closed . The argon purges the system for a previously selected first time at a previously selected first flow rate, in the best mode for at least about 30 minutes at a flow rate of 5.66-8.49 m3 / hour. Times and flow rates are not critical, and different times and flow rates can be used, as long as the air is purged successfully. Following the purge of the air, the retort is heated to a first temperature previously selected in the furnace, while maintaining an inert gas atmosphere.

- «- 'In the best mode, the gas flow was reduced to 2.83-5.66 m3 / hour, while it was heated to a temperature in the range of 926 ° C to approximately 1204 ° C, and preferably on the scale of 1051 + 12 ° C. Upon reaching the first previously selected temperature, the flow of inert gas is stopped, and gas is introduced from the hydrogen supply 60 to the system, at a previously selected third flow rate, for a second time previously selected, with the object from purge the inert gas from the system. This is done by closing valve 56 and opening valve 65. In the best mode, hydrogen flowed at a rate of 5.66-8.49 m3 / hour for at least 30 minutes, while maintaining the first temperature previously selected. The times and flow rates are not critical, and different times and flow rates can be used, provided that the inert gas is purged successfully. The system is now in impulses. While maintaining the first temperature previously selected, At about 1051 ° C in a preferred embodiment, the pressure inside the retort 20 is reduced to a first previously selected reduced pressure, evacuating the hydrogen gas. This pressure should be at least about 680 Torr, and preferably less than 600 Torr.

In the best mode, this pressure was approximately 550 Torr.

However, lower pressures are acceptable, and are even preferable if they can be achieved. The evacuation is done by closing the gas valves 64, as well as 56, if open, and activating the pump 76. Preferably, valve 68 is also closed. This reduced pressure is maintained for a previously selected third time, preferably approximately one minute. While the first previously selected temperature is maintained, then activating gas containing halide is introduced into the retort, which is below atmospheric pressure. In the best mode, the gas containing halide was hydrogen fluoride, HF. In addition, the HF is introduced into the retort with a carrier gas at a previously selected fourth flow rate, for a previously selected fourth time. Although the carrier gas may be an inert gas, in the best mode, it was hydrogen, and the ratio of the hydrogen gas to the activating gas was on the scale of (3-10): 1, and more preferably on the scale of 7: 1. This is done by providing a flow of activating gas of approximately 1.4 m 3 / hour, and a flow of hydrogen gas of approximately 10 m 3 / hour. The valves 48 and 65 are opened, and the carrier gas and the activating gas are mixed. In the best mode, the gas pressure is allowed to accumulate to approximately 3.5-4.2 kg / cm2, and the gases flow through the gas distribution system 22, and upwards around ^^^^ u ^ the granules 30. As the gases flow over and through the granules containing aluminum, the HF reacts with the granules, and a gas containing aluminum is formed, which then deposits a coating uniformly on the blades as the gas passes over and through the blades. This flow continues for approximately 30 minutes in the best mode, but longer or shorter times are permissible. A coating thickness in the range of about 25.4 microns to 127 microns is preferred, and in the best mode, a coating thickness in the range of 50.8 microns to 101.6 microns was usually achieved after 3 to 4 cycles or pulses. The closure of valve 48 now stops the flow of halide containing gases. The system is now purged, supplying hydrogen gas at a fifth selected flow velocity, preferably at approximately 1.1-1.7 m3 / hour, for a previously selected fifth time. This can be done by closing either or both valves 65 and 68, thereby stopping the flow of hydrogen, readjusting the regulator 62, and reopening the valves 65, 68, or simply reducing the flow of hydrogen by utilizing of the regulator 62. Although sufficient time and pressure to purge the halide containing gas from the system are acceptable, in the best mode, the flow was maintained for approximately 30 to 60 minutes. If a thicker coating is desired, the impulse process is repeated by stopping the flow of halide-containing gases, reducing the pressure of the retort to less than atmospheric pressure, re-entering the halide-containing gas, followed by purging of the system with hydrogen. When the desired coating thickness is reached, the impulse process can be stopped. As noted above, in the best mode for practicing the present invention, a coating thickness of 50.8 microns to 101.6 microns was achieved in 3 to 4 cycles or "pulses". After the pulse is finished, the pressure of the retort is again reduced to a second previously selected pressure below the atmospheric pressure for a previously selected sixth time. This is done by closing the inlet valve 68 and activating the pump 66. The pressure must be reduced to at least 680 Torr, preferably 600 Torr, and lower pressures are not preferable. In the best mode for practicing the present invention, a pressure of 550 Torr was reached. This pressure was maintained for approximately one minute. Hydrogen gas is then reintroduced into the system at a previously selected sixth flow rate, for a previously selected seventh time. As before, the flow velocity and time are not critical, as long as they are sufficient to purge the system of any remaining halide gases. In the best mode, a flow velocity of approximately 11.3 m3 / hour was established, by adjusting the regulator 62, and opening the valves 65, 68 for approximately 30 minutes. The retort was then allowed to cool to a second previously selected temperature, in the preferred embodiment, at about 760 ° C. At this point, inert gas, argon, was re-introduced into the retort at a previously selected seventh flow rate, closing valve 65 and opening valve 56. With the argon purging the system, the retort was cooled to a third previously selected temperature, of approximately 121 ° C. In the best mode, upon reaching 121 ° C, the system was pumped to below 680 Torr, preferably below 600 Torr, for about 20 to 30 minutes, and more preferably up to 50 Torr for 20 minutes, to remove any residual gases . Then the retort was refilled with argon. The coated parts were then removed from the retort 20. The step of removing the waste gases in the best mode at 121 ° C is optional. The blades coated by the process of the present invention have a more uniform coating thickness, because the impulse effect of the gases in the retort resulted in a more even distribution of the gases around the part. Each impulse cycle takes an hour to an hour and a half. In addition, the cobalt / aluminum granules can be reused to coat additional parts. If a package is used, to prevent clogging of the cooling holes, the fine parts can be sifted by passing the granule medium through 5 meshes. The fine parts can then be reprocessed. Although the present invention has been described in connection with specific examples and embodiments, those skilled in the art will recognize that the present invention may have variations and modifications within its scope. These 10 examples and embodiments are intended as being typical of, rather than by way of limitation on, the scope of the present invention, as presented in the appended claims. i? r? | -a > | - ??? iattWt ^; M? ilii? i ??? É? É 11? m n I ~ ~ ¿? -? - * - - - - * -

Claims (20)

1. CLAIMS 1. An improved process for applying coatings 5 of aluminide to superalloy components used in gas turbine applications, which comprises the steps of: placing the superalloy components in a retort (20) with a source containing aluminum (30); evacuate the air from the retort (20), and introduce an inert gas (50); heating the retort (20) to a previously selected temperature; while maintaining the previously selected temperature, purge the inert gas (50) from the retort (20), by introducing hydrogen gas (60); while maintaining the previously selected temperature, boost by reducing the pressure of the retort to a preselected pressure below 20 atmospheric pressure, followed by the introduction of a gas containing halide (42) to react with the source containing aluminum (30), in order to create a rich aluminum vapor that deposits aluminum on the components, and then reintroduce the hydrogen gas (60) into the retort (20) to purge the gases inside the retort; SMM. ^ 1 »- ^ - ^^ cool the retort (20). The process of claim 1, wherein the step of boosting includes maintaining the previously selected temperature within the range of about 926 ° C to 1204 ° C, and reducing the pressure to below at least about 680 Torr before introducing the gas containing halide (42). 3. An improved process for applying aluminide coatings to superalloy components used in gas turbine applications, which comprises the steps of: cleaning superalloy components; placing a source containing aluminum (30) in a retort (20); placing the components in the retort (20) with the source containing aluminum (30); then seal the retort (20); evacuate the air from the retort (20) by reducing the gas pressure below the atmospheric pressure, followed by purging the retort with an inert gas (50) at a first flow rate previously selected during a previously selected first time; heating the retort (20) to a first previously selected temperature, while inert gas (50) is provided at a second previously selected flow rate; - ** • '• * > * "" - 'upon reaching the first previously selected temperature, stop the flow of the inert gas (50), and introduce hydrogen gas (60) at a third previously selected flow rate during a previously selected second time, to purge the gas inert (50) of the retort (20); then while maintaining the previously selected temperature, boost by reducing the gas pressure in the retort to a previously selected first pressure below atmospheric pressure, and maintain the pressure for a previously selected third time, then introduce a gas containing halide (42) at a fourth flow rate previously selected in the retort (20) for a previously selected fourth time, to cause the halide-containing gas (42) to react with the aluminum-containing source (30), creating a vapor containing aluminum that flows uniformly over the components, causing deposition of a coating on the surfaces of the components, followed by halting the flow of halide-containing gas (42), and introducing hydrogen gas (60) into the retort (20) at a fifth flow rate previously selected for a fifth time previously selected; repeating the step of boosting by reducing the pressure of the retort, flowing a halide-containing gas (42), and stopping the flow of the halide-containing gas (42) until a thickness of the previously selected coating is reached; then reduce the pressure of the retort to a second pressure previously selected under atmospheric pressure, and maintain for a sixth time previously selected; then introduce hydrogen (60) at a sixth previously selected flow rate for a seventh time previously selected, then cool the retort (20) to a second previously selected temperature; then introduce a flow of inert gas (50) into the retort (20); and 15 cooling the retort (20). The process of claim 3, wherein the first time previously selected is about 30 minutes, the inert gas (50) is argon, and the first flow rate previously selected is about 5.6 to 8.5 20 m3 / hour. The process of claim 3, wherein the first temperature previously selected is in the range of about 926 ° C to about 1204 ° C, and the second flow rate previously selected is from 25 approximately 2.8 to 8.5 m3 / hour. «ÉMHMlÉHHiííÉfl 6. The process of claim 5, wherein the first temperature previously selected is on the scale of about 1051 ° C + 12 ° C, and the second flow rate previously selected is about 2.8 to 5.6. 5 m3 / hour. 7. The process of claim 3, wherein the third previously selected flow rate of hydrogen gas (60) is 5.6 to 8.5 m3 / hour, and the second time previously selected is about 30. minutes 8. The process of claim 3, wherein the step of boosting includes reducing the gas pressure to a pressure below at least 680 Torr. 9. The process of claim 8, wherein the step of boosting includes reducing the gas pressure to a 15 pressure below at least approximately 600 Torr. The process of claim 8, wherein the step of boosting includes reducing the gas pressure to approximately 550 Torr. The process of claim 8, wherein the step of boosting includes maintaining the reduced pressure for a pre-selected third time of about one minute. The process of claim 3, wherein the step of boosting further includes introducing an activating gas that 25 contains halide (42) at a flow rate of üitf? aÉiiÉÉGá nM ua ... i, «" > » .! .. about 11.3 m3 / hour, wherein the halide containing activating gas is comprised of HF flowing at a rate of 1.4 m3 / hour, and a carrier gas flowing at a rate of 10 m3 / hour. 13. The process of claim 12, wherein the halide-containing activating gas (42) is comprised of HF and hydrogen (60) in the ratio of HF to hydrogen (60) from about 1 to about 3-7. The process of claim 3, wherein the driving step includes a previously selected fourth time of about 30 minutes. The process of claim 3, wherein the step of boosting includes a fifth previously selected flow rate of hydrogen (60), to about 15 1.1-1.7 m3 / hour, for a previously selected fifth time of approximately 30 to 60 minutes. 16. The process of claim 3, wherein each step of driving is performed in a time from about one to about one and a half hours. 17. The process of claim 3, wherein the second pressure previously selected is below at least about 680 Torr, and the sixth time previously selected is at least about one minute. 18. The process of claim 3, wherein the 25th previously selected flow rate of hydrogen i? t? i? ííff? a? i ?? íT ^ ilfi ^ p ^ r -tr ------ 'J- ^ e, ~ ^ - * - > - < - - * - * '< > - - «> - -It is approximately 11.3 m3 / hour, and the seventh time previously selected is approximately 30 minutes. 19. The process of claim 3, wherein the second previously selected temperature is about 760 ° C. 20. An improved process for applying aluminide coatings to blades (10) having substrates comprised of superalloys selected from the group consisting of nickel and cobalt, which comprises the steps of: cleaning the blades (10); place cobalt-aluminum granules (30), with a size of approximately 4 millimeters to approximately 16 millimeters, in a retort (20); place the blades (10) in proximity to the cobalt-aluminum granules (30) in the retort (20); then seal the retort (20); evacuate the air from the retort (20), reducing the gas pressure below at least about 680 Torr, followed by purging the retort (20) with argon gas (50) at a flow rate of about 5.6 to 8.5 m3 / hour, for about 30 minutes; heating the retort (20) to a temperature in the range of about 1051 ° C + 12 ° C, while providing argon (50) at a flow rate of about 2.8 to 5.6 m3 / hour; upon reaching the temperature of about 1051 ° C + 12 ° C, stop the flow of argon (50) and introduce hydrogen gas (60) at a flow rate of about 5.6 to 8.5 m3 / hour, for about 30 minutes, to purging argon from the retort (20); then while maintaining the temperature of approximately 1051 ° C + 12 ° C, boost by reducing the gas pressure in the retort (20), at a pressure on the scale of approximately 550-600 Torr, and maintain the pressure for about one minute, then introduce a combination of hydrogen gas (60) and hydrogen fluoride gas (42) in the proportions of approximately 7: 1, at a flow rate of approximately 11.3 m3 / hour in the retort (20). ) for about 30 minutes, to cause the gas to react with the source containing aluminum (30), creating a vapor containing aluminum that flows uniformly on the blades (10), causing the deposit of a coating on the surfaces of the blades (10), followed by stopping the gas flow and introducing hydrogen gas (60) into the retort (20), at a flow rate of about 1.1 to 1.7 m3 / hour, for about 30 to 60 minutes; Repeat the step of boosting by reducing the pressure of the retort, flowing the combination of hydrogen gas (60) and fluoride gas from hydrogen (42), and stop the gas flow until a coating thickness is reached in the range of about 25.4 microns to 101.6 microns; then reduce the pressure of the retort to a pressure on the scale of 500 to 600 Torr, and maintain for at least about one minute; then introduce hydrogen (60) at a flow rate of approximately 11.3 m3 / hour, for a time of about 30 minutes; then cooling the retort (20) to a temperature of about 760 ° C; then introduce a flow of argon (50) into the retort (20); cooling the retort (20) to about 121 ° C, followed by reduction of the argon pressure (50) in the retort (20) to below at least about 600 Torr, for at least about 20 minutes; and cooling the retort (20) to room temperature, and stirring the blades (10). 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