IES80923B2 - Method of cleaning metal components - Google Patents

Description

The invention relates to a process of cleaning metal components, and is particularly concerned with a method for repairing nickel based superalloys, for io example in the repair of cracks or other defects in turbine components.

BACKGROUND ART Nickel based high temperature superalloys are used in the manufacture of 15 aircraft, and land-based, gas turbine engines. These components include, but are not restricted to, turbine blades, nozzle guide vanes, turbine discs, exhaust struts, combustors, heat shields, and the like. In the operation of the engines these components are subjected to severe stresses caused by extreme thermal cycles and, in many cases to collision with foreign objects drawn into the turbine. This causes the development of cracks or other defects in the components. These defects may also occur during the casting of the components.

It is a standard procedure in the aircraft industry to periodically inspect the engine and to remove any component parts which contain defects. Because of the high cost of superalloy turbine components the component parts are salvaged and repaired where possible.

It is known to repair cracks and other defects by a repair process which involves thoroughly cleaning the part to be repaired which involves thermochemically removing oxides and other contaminants from the surfaces of the parts in particular from the interior of the cracks or other gaps to be repaired.

When the part has been cleaned a filler metal composition is deposited in the crack or other defect and the part is then subjected to a thermal brazing cycle to fuse the metal brazing composition and to cause it to diffuse into the metal part to be repaired.

It is most important that all of the surfaces of the crack are thoroughly cleaned as otherwise the braze metal will not wet the crack surfaces and a good bond is not achieved. The conventional process is to thermochemically clean the Components by placing the components in an oven and subjecting them to hydrogen cleaning followed by halide cleaning, for example, using hydrogen fluoride.

For example, U.S. Patent 4,098,450 discloses a method for cleaning and repairing superalloy parts using fluoride ions to contact oxides in cracks and crevices to convert the oxide to gaseous fluoride.

EP 0 097 607 discloses repair methods in the gas turbine engine art, in which oxides are removed from parts to be repaired by mechanical means, or, through the use of hydrogen prior to application of the brazing alloy.

Turbine blades and nozzle guide vanes made from nickel or cobalt based superalloy are frequently coated with oxidation or corrosion resistant coatings, the most common being aluminide coatings formed by the diffusion of aluminium into the surface of the material.

In known methods, prior to the repair of cracks in these components by widegap brazing, the coatings are stripped, normally by immersion in acid solutions and by grit blasting. The components are then subjected to vacuum cleaning and/or halide (hydrogen fluoride) cleaning in order to remove surface oxide layers particularly from the inside of cracks that are to be repaired. In the case of vacuum the components are heated to a high temperature (typically 1100-1200°C) in a high vacuum (typically 10-4 to 10-5 mBar) and under these conditions many metallic oxides are reduced to their metals. However, certain oxides (for example alumina and titania) are not reduced therefore hydrogen fluoride cleaning is used. In this case the components are heated •-J 5 in a hydrogen atmosphere at typically 975°C and hydrogen fluoride is introduced. This reacts with the alumina and titania and produces fluorides of aluminium and titania. These fluorides are gaseous at this temperature and are removed from the component leaving the surfaces of the component clean. 10 However, in production conditions, certain problems arise in the known methods. It is often difficult to strip the last traces of the aluminide coating and/or small patches of aluminide coating are overlooked by the production operators and are not stripped properly. During the subsequent hydrogen fluoride cleaning there is then sufficient aluminium present to consume all of the hydrogen fluoride and the surface oxides are not reduced. The component is usually visible dirty and the acid stripping and halide cleaning process have then to be repeated. 15 It is also known that aluminide coatings can be stripped by gaseous hydrogen chloride. However, the process is not in common use because there is a danger of intergranular attack of the base metal either in areas where there is no coating or indeed after all the coating has been removed. 20 OBJECT OF THE INVENTION 25 It is an object of the invention to overcome the disadvantages associated with known methods of cleaning cracks in superalloy components and to provide an improved cleaning process. SUMMARY OF THE INVENTION Ά 30 * In a first embodiment, the invention provides a process for cleaning cracks and other surface defects in components made from heat resistant nickel or cobalt based superalloy materials, prior to the repair of the crack by brazing or the like, which comprises heating the components in a gaseous atmosphere at an elevated temperature, characterised in that the gaseous atmosphere comprises a mixture of hydrogen, hydrogen fluoride, and hydrogen chloride gases.

Suitably, the process is carried out at temperatures in the range of950°C to 1000°C, typically about 975°C. The gaseous atmosphere preferably comprises upto 20% hydrogen flouride, hydrogen chloride in the ratio 1-2% hydrogen chloride/ hydrogen flouride, and the balance comprising hydrogen.

Some embodiments of the invention are hereinafter described with reference to the accompanying drawings, wherein :Figure 1 is an elevation of cleaning apparatus for use in the process of the invention; In accordance with the invention, superalloy components containing cracks are first subjected to a chemical cleaning process. This is a conventional process which comprises cleaning the component by immersing it in acid or alkaline baths.

For example, the components may typically be cleaned by immersion in a series of tanks containing the following proprietary solutions:1. Turco4181 2. Ardox 1871 3. Turco4338 4. Turco 4409 for the purpose of removing most of the oxide material adhering to the components.

The components may also be immersed in a solution containing nitric acid, acetic acid, phosphoric acid and ferric chloride for the purpose of removing sulphide and oxy-sulphide deposits.

Following the chemical cleaning of the surfaces of the component as described above, the component is then placed in a cleaning apparatus of the invention. One embodiment of such an apparatus is illustrated in Figure 1 of the drawings. The apparatus comprises a retort 10 having a lid 11. The retort contains a metal rack 12 containing a plurality of shelves 13 on which the components 15 to be treated are arranged vertically in tiers. A tube 14 is disposed vertically and substantially centrally of the retort 10. The tube is adapted to convey a cleaning gas into the retort and a plurality of apertures 16 are positioned along the tube 14..

The retort 10 is purged with hydrogen gas and heated to a temperature in the range 950-1000°C (typically 975°C) in well known manner, such as in an electric furnace having electric resistive elements 21. A gaseous source of flouride ions, such as hydrogen flouride, is conveyed through tube 14 to the interior of the retort. This gas is admixed with hydrogen chloride, Preferably, the gas mixture contained with the retort 10 comprises from hydrogen chloride in the ratio 1-2% hydrogen chloride/ hydrogen flouride, about 15% hydrogen fluoride, the balance being hydrogen. The gas mixture emanates from the tube 14 as jets and the jets of gas are directed by the aperture 16 into the interior of the components 15. Thus the gas impinges directly onto the inner surfaces of cracks or other surface defects in the interior of the components.

The thermochemical cleaning process is carried out for a duration of between 15 minutes and 60 minutes. At reaction temperature within the retort, any oxides deposited on the surfaces of cracks are converted to gaseous metal fluorides which are flushed away by the flow of the gaseous stream. To assist this flushing it is possible to change between hydrogen gas and the gas capable of releasing fluoride ions two, three or more times using a series of valves 20 on the gas supply. It has been found that by this method the interior surfaces of even very deep cracks are thoroughly cleaned of oxides and other contaminants and are capable of being wetted by molten braze metal to form a homogenous bond between the braze and the parent alloy.

Table 1 Table 1 shows a known halide cleaning process broken down into stages argon purge, heat up cleaning cycle etc.

Process Stage Temperature C Duration Min Argon Litres/Min Hydrogen Litres/Min HF Litres/Min 1. Ambient 30 100 2. Heat to 975 As Necessary 100 3. 975 30 128 20 4. 975 15 128 5. 975 30 128 20 6. Heat to 1020 C As Necessary 128 7. 1020 C 15 128 8. Cool to ambient As necessary Note: Stages 4 and 5 may be repeated for contaminated components After the part has been cleaned as described above, the cracks are filled with a 10 suitable braze material, in well known manner.

Table 2 shows the same halide cleaning process but incorporating the method of the invention.

Table 2 Process Stage Temperature C Duration Min Argon Litres/Min Hydrogen Litres/Min HF Litres/Min HCI Litres/Min 1. Ambient 30 100 Ί 2. Heat to 975 As Necessary 100 3. 975 30 128 20 0.3 4. 975 15 128 5. 975 30 128 20 0.3 6. Heat to 1020 C As necessary 128 7. 1020 C 15 128 8. Cool to ambient As necessary Example The cleaning of a Rolls Royce RB211 524 G/ H high pressure and intermediate pressure nozzle guide vanes manufactured in MarM 002 material.

MarM 002 material has a composition 9% Cr, 10% Co, 5.5% Al, 1.5% Ti, % W, 2.5% Ta, and the balance Ni. By immersing the guide vane in a solution of 10% nitric acid in water also containing 50 gram/litre sulphamic acid, it is possible to strip an aluminide coating. The extent of stripping is checked by heat tinting whereby the components are grit blasted and heated in air at 700°C for one hour. A light surface oxidation occurs and the colour of the oxide film gives a good indication of the presence of residual coating. A blue colour indicates base metal, a slate grey indicates thick coating and a yellow colour indicates thin residual coating.

After this initial process the vanes are cleaned using a halide process. Using the process of the prior art, vanes that are substantially blue but with areas of yellow colour indicating residual coating to the extent of20-30% of the surface area have been treated. When these components are processed according to table 1 they are not cleaned and emerge with a smutty appearance where there was residual coating and metallographic sectioning shows that oxide in cracks is not removed. When processed according to table 2, they emerged visible clean having a bright silveiy δ appearance and metallographic sectioning shows that the oxide in the cracks is removed and also that no intergranular attack has occurred. This has been tried on over 100 production runs, each containing typically 30-40 components. Each run has been successfully cleaned with no components dirty because of residual coating.

Claims (5)

Claims

1. A process for cleaning cracks and other surface defects in components made from metallic based superalloys, prior to the repair of the crack by brazing and the like, comprising the steps of: a) placing the component in a furnace retort b) heating the furnace to an elevated temperature, and while at said elevated temperature, c) introducing a gaseous atmosphere into said retort, and characterised in that the gaseous atmosphere comprises a mixture of hydrogen, a gaseous source of flouride ions and a gaseous source of chloride ions.

2. The process as claimed in claim 1, wherein the gaseous source of flouride ions is hydrogen flouride and the gaseous source of chloride ions is hydrogen chloride

3. The process as claimed in any preceding claim, wherein the gaseous atmosphere comprises upto 20% hydrogen flouride, hydrogen chloride substantially in the ratio 1 to 2% hydrogen chloride/ hydrogen flouride and the balance comprising hydrogen.

4. The process as claimed in any preceding claim, wherein the elevated temperature is substantially in the range 950°C to 1000°C, and preferably around 975°C.

5. The process as claimed in any preceding claim wherein the metallic based superalloys are selected from nickel and/ or cobalt based superalloys.