METHOD FOR STRIPPING A TURBOMACHINE PART

Abstract

The invention relates to a method for pickling a part (1) of a turbomachine, comprising the following steps: - Positioning the part in a closed chamber (2), - Injection of a gaseous mixture (3) into the chamber (2), the gaseous mixture (3) comprising a halogenated gas, - Heating of the chamber (2), the method being characterized in that: - the gaseous mixture further comprises dihydrogen, - the heating step is carried out at a temperature above 1000°C and - the gaseous mixture injection step (3) is carried out by circulating through the chamber (2) a flow of gaseous mixture (3) having a flow rate of between 6 and 15 times the volume of the chamber (2) per hour.

Description

TITLE: METHOD FOR PICKLING A TURBOMACHINE PART GENERAL TECHNICAL FIELD AND PREVIOUS ART The invention relates to the general field of surface treatment processes, and more particularly to surface stripping processes for turbomachine parts. A turbomachine typically includes at least one flow channel through which an airflow passes, which is compressed by one or more compressors before entering a combustion chamber where the air is mixed with fuel and then ignited. The mixture of burnt gases then rotates one or more turbines which in turn rotate the compressor(s), the gas flow then being ejected. Turbine parts, exposed to very high temperatures, are generally treated or coated with refractory materials or alloys to limit their degradation. For example, it is known to coat such parts with one or more layers of aluminum alloys, called aluminides, for example titanium aluminides, and / or with one or more layers of oxides, for example molybdenum oxides, or ceramics, forming a thermal barrier on the surface of the part. When it is necessary to repair such parts, coating stripping operations are required in order to rehabilitate the base material composing the part, called the substrate. These stripping steps typically include several stages: - at least one sandblasting or chemical bath stage to remove the thermal barrier, - at least one chemical bath stage to remove aluminide coatings, - at least one additional sandblasting stage of the part after the chemical bath to remove any remaining residue. However, these operations do not eliminate oxides or contaminants incorporated into cracks or defects on the surfaces of the parts. In order to clean these crevices of any oxides or corrosion, an additional step is carried out in which a thermochemical operation is performed on the part, classically in a high-temperature oven and under a fluorinated atmosphere (classically called FIC, from the English Fluorine Ion Clearing). However, such operations pose risks to the parts due to the potential danger of chemical attack on the substrate material by the chemicals used. Sandblasting operations, which are mechanical abrasion operations, naturally attack the substrate. Furthermore, the search for weight reduction in order to minimize the total weight of turbomachines leads to turbomachine parts having increasingly thinner walls, which limits the margin of substrate consumption during such stripping processes. The sequence of these operations also creates a challenge in the quality of the treatment. Indeed, if a step is not carried out perfectly and leaves areas untreated, the subsequent treatment operations will also be degraded, and the part may then be rendered unusable, because repeating the process to remove the remaining coating areas would attack the substrate too deeply. On the other hand, the chemical baths used contain substances or components that are dangerous for operators, such as hydrofluoric acids which are classically used in aluminide pickling baths. GENERAL PRESENTATION OF THE INVENTION One aim of the invention is to simplify the process of stripping the surfaces of turbomachine parts. Another objective of the invention is to reduce the risk of substrate degradation during the pickling process of turbomachine parts. Another aim of the invention is to limit the use of products that are hazardous to operators. To this end, the invention proposes a method for pickling a turbomachine part, comprising the following steps: - Positioning the part in a closed chamber, - Injection of a gaseous mixture into the chamber, the gaseous mixture comprising a halogenated gas, - Heating of the chamber, the method being characterized in that: - the gaseous mixture also includes dihydrogen, - the heating stage is carried out at a temperature above 1000°C and The gas mixture injection step is carried out by circulating a flow of gas mixture through the enclosure at a rate of between 6 and 15 times the volume of the enclosure per hour. Such a process makes it possible in a single thermochemical treatment phase to remove the layers of the aluminide-based coating and the layers of the metal oxide-based coating, which makes it possible to strip the part and reveal the substrate without requiring a chemical bath or sandblasting step to remove this type of layer. The stripping process is therefore simplified and made safer, as the substrate is not degraded during the process and operators are not brought into contact with hazardous products. Such a process is advantageously complemented by the following characteristics, taken alone or in combination: - the gaseous mixture contains fluorine; a halogenated element allowing a higher reaction rate than when using other halogenated elements; - the temperature of the heating stage is above 1030°C; this increases the efficiency of the pickling and cleaning process, in particular by increasing the reaction kinetics; - the gas mixture also includes an inert gas, for example argon; this allows the reactive gases to be transported and contributes to the homogenization of the gas mixture within the furnace enclosure; - a concentration of halogenated gas in the gas mixture is between 4% and 12%, preferably between 6% and 8%; this allows control of the quantity of reactive gas introduced into the enclosure and thus control of the reaction on the surface of the parts, in particular by controlling the diffusion rate of the species; - the flow rate of the gas mixture is between 8 and 12 times the volume of the enclosure per hour; this allows the necessary and sufficient quantity of active gas to be supplied to carry out an effective reaction over the entire part in the enclosure; - the total pressure in the enclosure is approximately equal to atmospheric pressure; - a total pressure in the enclosure is lower than atmospheric pressure; this saves time, requires less gas and is more efficient because the gases can penetrate more quickly and very efficiently into crevices, cracks and cavities; -The process consists of a succession of steps: - Sandblasting of the part (1), - Positioning of the sandblasted part (1) in the closed chamber (2), - Injection of the gas mixture (3) into the chamber (2) and - Heating of the chamber (2). PRESENTATION OF THE FIGURES [Fig. 1] Figure 1 is a schematic representation of the implementation of a stripping process for a part according to the invention. Other features and advantages of the invention will become apparent from the following description, which is purely illustrative and not limiting, and should be read in conjunction with the single attached figure, which is a diagram representing a device for implementing a stripping process for a part according to the invention. DESCRIPTION OF ONE OR MORE IMPLEMENTATION AND REALIZATION METHODS The invention relates to a method for pickling a turbomachine part 1, characterized in that it comprises the following steps: - Positioning the part 1 in a closed chamber 2, - Injection of a gas mixture 3 into the chamber 2, the gas mixture 3 comprising at least one halogenated gas, - Heating of the chamber 2. The method is characterized in that: - the gas mixture further comprises dihydrogen, - the heating stage is carried out at a temperature above 1000°C, and - the gas mixture injection stage 3 is carried out by circulating through enclosure 2 a flow of gas mixture with a flow rate between 6 and 15 times the volume of enclosure 2 per hour. The invention is advantageously applicable to a part 1 comprising a coating 4 comprising at least one aluminide layer 41 comprising one or more species of aluminides, or at least one oxide layer 42 comprising one or more species of metal oxides, or a combination of such layers. During this process, the halogenated gas(s) react with the aluminide layers 41 and the oxide layers 42 according to the following reactions: For aluminide species: HX(g) + ScAl(s) -> AlX3(g) + H2 + Sc(s) For oxide species: HX(g) + MxOy(s) -> MxX (g) + H2O(g) With X a halogen species, H hydrogen, M a metal, O oxygen, Al aluminum, Sc a transition metal. For example, X could be fluorine, chromium, bromine, or iodine, and Sc could be nickel, cobalt, titanium, or any other transition metal. Preferably, the halogen species X contains fluorine, for example in the form of hydrofluoric acid. Fluorine exhibits high reactivity and allows for a faster reaction than with the use of other halogenated elements. It is understood that this type of reaction is an illustrative example, and the process can be applied to any aluminide or modified aluminide compound, and not only nickel aluminide. Examples include nickel aluminides, platinum-modified aluminides, cobalt aluminides, titanium aluminides, ... The layers of the aluminide-based coating 41 and the layers of the metal oxide-based coating 42 are therefore removed in a thermochemical treatment phase, which makes it possible to strip the part 1 and reveal the substrate 5 without requiring a chemical bath or sandblasting step to remove this type of layer. The stripping process is therefore simplified and made safer, as the substrate 5 is not degraded during the process and the operators are not brought into contact with hazardous products. The enclosure 2 is continuously traversed by a flow of gaseous mixture 3 which has a flow rate representing between 6 and 15 times the volume of the enclosure 2 per hour, preferably between 8 and 12 times the volume of the enclosure 2 per hour. This allows the necessary and sufficient quantity of active gas to be supplied to carry out an effective reaction over the entire part in the enclosure (in terms of volume or surface area). The gas mixture flow rate 3 can be adapted according to the quantity of parts 1 to be treated, or the total surface area to be stripped. As an example, the flow rate of halogenated gas can be between 6L / min and 10L / min, and the flow rate of dihydrogen can be between 130 L / min and 160 L / min for an enclosure with a volume of approximately 1 m3 in which 45 parts are placed. The heating phase includes a temperature rise, a temperature maintenance plateau, and a cooling phase. The temperature maintenance period can last between 2 hours and 10 hours, preferably between 3.5 hours and 5.5 hours (meaning 3 hours and 30 minutes or 5 hours and 30 minutes). The holding temperature is above 1000°C, preferably above 1030°C, for example between 1035°C and 1055°C. Such temperature ranges increase the efficiency of the pickling and cleaning process compared to simple oxide cleaning, as is the case in standard FIC processes. Indeed, the kinetics of the reactions involved, whether with the oxides or with the NiAl or NiAIPt alloys of the coating to be stripped, are temperature-dependent. Advantageously, a sandblasting step can be carried out prior to the thermochemical treatment. This makes it possible to remove combustion residues, for example mill scale, formed on the surface of the coating 4 during the operation of the turbomachine, as well as any ceramic thermal barrier layers 43 and the passivating layers 44 that cover them, for example layers comprising calcium-magnesium-aluminosilicate. The sandblasting step thus reveals the aluminide layers 41 and the oxide layers 42 of the coating, which will be removed in a thermochemical cycle phase. The sandblasting step carried out upstream therefore does not represent a danger to substrate 5. After the preliminary sandblasting, one or more parts 1 are placed in a closed enclosure 2, preferably on a grid, allowing better circulation of the gas mixture 3 along the entire surface of the part(s) 1, which improves the treatment. The gas mixture 3 is then injected into chamber 2. Optionally, gas mixture 3 further comprises one or a combination of components from the following: - hydrofluoric acid HF, - hydrochloric acid HCl, - hydrobromic acid HBr, - hydroiodic acid Hl. Gas mixture 3 may also advantageously include dihydrogen. Optionally, but advantageously, gas mixture 3 also includes an inert gas, for example helium, neon, argon, krypton, xenon or radon, or a combination of these elements. This allows the reactive gases to be transported and contributes to the homogenization of the gas mixture 3 in the chamber 2 of the furnace. The concentration of halogenated gas in gas mixture 3 is advantageously between 4% and 12%, preferably between 6% and 8%. This allows control of the quantity of reactive gas introduced into the chamber, thus optimizing the surface reaction of the parts. This allows the process to be optimized so as to prevent reactions from becoming too slow if concentrations are low, or conversely prevent high concentrations from leading to saturation of the atmosphere which would impair the efficiency of the reactions and lead to a risk of contamination of the base material (fluoride, chlorine, etc.). The concentration of halogenated gases has an influence on the diffusion rate of reactive species, along with temperature. Optionally, the supply of gas mixture 3 to chamber 2 follows a sequential cycle, and presents: - an injection phase, during which the gaseous mixture 3 is injected into the chamber 2, - a treatment phase during which the gas mixture 3 is maintained in the enclosure 2 during heating so as to react with the coating 4, and - a purging phase during which the treatment reagents are evacuated with the gas mixture 3 contained in the enclosure 2. Following the purging phase, a new injection phase is carried out and a feeding cycle begins again until the thermochemical treatment is completed. Thermochemical treatment can be carried out at atmospheric pressure, or preferably under reduced pressure (or low pressure, i.e., below 300 mbar). Treatment under reduced pressure saves time, requires less gas, and is more effective because the gases can penetrate more quickly and efficiently into cracks, fissures, and cavities, even if they are too narrow and deep.

Claims

Demands

1. A method for pickling a turbomachine part (1), comprising the following steps: - Positioning the part in a closed enclosure (2), - Injecting a gas mixture (3) into the enclosure (2), the gas mixture (3) comprising a halogenated gas, - Heating the enclosure (2), the method being characterized in that: - the gas mixture further comprises dihydrogen, - the heating step is carried out at a temperature above 1000°C and - the gas mixture injection step (3) is carried out by circulating through the enclosure (2) a flow of gas mixture (3) having a flow rate between 6 and 15 times the volume of the enclosure (2) per hour.

2. Pickling process according to claim 1, wherein the gas mixture comprises fluorine.

3. Pickling process according to any one of claims 1 or 2, wherein the temperature of the heating step is greater than 1030°C.

4. A pickling process according to any one of claims 1 to 3, wherein the gas mixture (3) further comprises an inert gas, for example argon.

5. Pickling process according to any one of claims 1 to 4, wherein a concentration of the halogenated gas in the gas mixture (3) is between 4% and 12%, preferably between 6% and 8%.

6. A pickling method according to any one of claims 1 to 5, wherein the flow rate of the gas mixture stream (3) is between 8 and 12 times the volume of the enclosure (2) per hour.

7. A stripping method according to any one of claims 1 to 6, wherein a total pressure in the enclosure (2) is equal to atmospheric pressure.

8. A pickling method according to any one of claims 1 to 6, wherein a total pressure in the enclosure (2) is less than atmospheric pressure.

9. A stripping method according to any one of claims 1 to 8, said method comprising only the steps of: - Sandblasting the part (1), - Positioning the sandblasted part (1) in the closed enclosure (2), - Injection of the gaseous mixture (3) into the enclosure (2) and - Heating of the enclosure (2).

Images