IE64961B1 - A vapour chromising process and composition - Google Patents

Description

A VAPOUR CHROMISING PROCESS AND COMPOSITION TECHNICAL FIELD The invention relates to a vapour chromising process and composition, and is particularly concerned with the production of diffusion coatings on nickel based superalloys, for example in the repair of turbine components.

BACKGROUND ART Nickel based high temperature superalloys are used in the 15 manufacture of aircraft, and land-based, gas turbine engines. These components comprise 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.

Many superalloy components are coated with oxidation-resistant chromium or aluminium coatings. Very often it is found when inspecting turbine components that the coating has deteriorated and needs replacement. Further, when the components are repaired as described above it may also be desirable to replace the coating using a chromising or aluminising process.

Surface chromising was developed as a production technique between 1939-1945 in Germany as the result of a shortage of chromium 4 9 61 - 2 metal preventing the manufacture of stainless steels. With the rapid growth in the use of gas turbine aircraft engines in the following decades the principle of metal transfer using a halide carrier has been extensively applied both to chromising and aluminising. An example of a process for repairing aluminide-coated gas turbine engine components is described in U.S. Patent No. 3,544,348.

The thermochemistry of chromising is such that somewhat higher temperatures or longer processing times are required to produce a given thickness of chromising compared to aluminising. Pack chromising is frequently used where high chromium contents (> 50%) and thick coatings (several thousands of an inch) are required. This process, which is also called pack cementation, involves embedding the part in a pack of dry chromising powder, an inert filler, and an activator, and heating the pack to an elevated temperature.

A problem with pack chromising is the tendency to form alpha chromium, a separate chromium rich phase at the surface rather than a solid solution of chromium in the base material. This can be prevented by physically separating the components from the powder. One way of doing this is to coat the component in a slurry of alumina through which the chromising vapour will pass.

Vapour chromising using the out of pack principle is the most suitable means of producing coatings with intermediate chromium levels (20-30%) and thickness in the region of one thousandth of an inch. These coatings find extensive application on aircraft gas turbines. Because the components do not contact the powder the risk of alpha chromium formation is extremely small.

Another problem associated with the chromising of super alloy components, both in pack chromising processes and out-of pack processes is the control of the processing temperature at which the chromising takes place.

The selection of the process temperature can be critical because operation of the process within certain temperature ranges can be detrimental to certain alloys. - 3 In manufacturing nickel based superalloy components, the component is cast from molten metal, cooled, and the subjected to a two-stage heat treatment which involves solutionising and aging, or solutionising and precipitating. This comprises heating the nickel to a solutionising temperature at which the various elements which have been added to strengthen the alloy go into solution in the basic nickel. At the end of this treatment the metal is cooled rapidly and is then subjected to an aging or precipitation heat treatment by reheating to a lower temperature than the solutionising temperature. This precipitation process must be carried out at a temperature sufficiently low for the solid solution to be supersaturated, but sufficiently high for atomic movement to take place, so that precipitation of the excess of solute will occur with the production of an alloy consisting of small particles of a solute-rich phase embedded in a matrix of solid solution. It is important to maintain the precipitation temperature within carefully controlled parameters because if the precipitation temperature is increased the size of the precipitates also increases. It is also known that for some materials, coarse precipitates do not readily go back into solution during subsequent solutionising heat treatments.

In carrying out a chromising process it is important that the operating temperature is not in the range where this form of undesirable coarse precipitation occurs. It must therefore be carried out below this temperature (although it can still be above a temperature where small particles precipitation occurs) or above this temperature up to the solutionising temperature. For example in the case of a nickel based superalloy known as Mar M 002 the temperature range at which coarse precipitates will form is approximately 1040°C - 1070°. Thus with this alloy it is important that the chromising process is carried out either in the range 1070°C - 1100°C (the latter temperature being the solutionising temperature) or below 1040°C down to for example 900°C.

Other factors which must be taken into consideration in deciding upon the operating temperature of the chromising process is the desired thickness of the chromium coating. It is desirable that the chromium content of the coated component should amount to at least 20%. The - 4 upper limit depends upon the requirement of the engine manufacturer, normally it is the range of 20-30% and up to 50% in the case of land-based gas turbines. It is also important that the coating should have a predetermined thickness, preferably in the range 0.0006 to 0.0013. The thickness achieved is temperature dependent. If the temperature is too low the coating may not have a desired thickness whereas if the temperature is too high the coating may exceed the thickness desired.

DISCLOSURE OF INVENTION It is an object of the invention to overcome the aforesaid difficulties and to provide a chromising process and composition which will operate successfully over a range of temperatures to suit the metallurgy of the base material being coated and yet provide a coating of desired thickness.

The invention is based on our discovery that by utilizing a particular halide or mixture of halides in specified amounts as carriers we can operate the chromising process throughout a wide temperature range and still produce a coating of the desired thickness.

Accordingly, the invention provides a process for producing a high-temperature corrosion-resistant protective coating on nickel-based superalloy components which comprises diffusing into the surface of the components one or more metals of the chromium group of elements by means of a pack, or out-of-pack, chromising process wherein the process is carried out at a temperature in the range 900 - 1100°C in the presence of ammonium chloride and one or more of ammonium fluoride, ammonium bifluoride, and chromium chloride. The coating obtained has a thickness in the range of 0.0006 to 0.0013.

The particular operating temperature range is chosen to suit the metallurgy of the base material, and is above the solutionising temperature, or below the precipitating temperature. Such temperatures are well known to the person skilled in the art. - 5 Preferably, where chromium chloride is used the halides are present in the following amounts: Ammonium Chloride: 0.25% to 2.0% by weight, Ammonium Fluoride: 0 to 2.0% by weight, Ammonium Bifluoride: 0 to 2.0% by weight, Chromium Chloride: 0.25% to 2.0% by weight.

Examples of halide compositions which may be used in the process of the invention are as follows:Halide 1.0% Ammonium Chloride 0.5% Ammonium Biflouride 0.5% Ammonium Chloride 0.5% Ammonium Fluoride 0.5% Ammonium Chloride 0.5% Chromium Chloride 0.5% Ammonium Fluoride Operating Temperature range 1070-1080°C 969-1000°C 1000°C-1035°C Thickness of Coating obtained 0.0008-0.0011 0.0007-0012 0.0008-0.00H BRIEF DESCRIPTION OF DRAWING The invention is further illustrated with reference to the accompanying drawings wherein Figure 1 is a perspective view of a rack for supporting components to be coated in an out-of-pack chromising process of invention; Figure 2 is perspective view of a chromising retort in which which the rack of Figure 1 is contained and which the process is carried out; - 6 Figure 3 is a photomicrograph of an airfoil section treated by the process of the invention; Figure 4 is a photomicrograph showing the metallography of a blade root section treated by a process according to the invention; Figure 5 shows sectional views of various components showing thicknesses of coatings obtained by the process of the invention.

Referring to the drawings, there is described a process for producing a high-temperature corrosion-resistant protective coating on nickel-based superalloy components which have been removed from an aircraft turbine engine. The components are first cleaned, for example by abrasive blasting using alumina grit of a mesh size in the range 90-400 B.S. 410, preferably 120-220 B.S. 410. Additionally, the components may be vapour degreased using a solvent such as 1-1-1 trichlorethane.

The cleaned components are suspended on a support rack of the kind shown in Figure 1. The rack is contained within a cylindical chromising retort as illustrated in Figure 2. The retort contains a layer of a chromising powder composition which is approximately 1 deep. The components are suspended above the powder layer. The retorts containing the components, are stacked up to three high within an outer pit furnace retort. This arrangement has the capacity of approximately 400 turbine blades per furnace.

The chromising powder composition will vary depending upon the metallurgy of the particular component to be coated and the thickness of coating desired. Suitably, the composition is within the following ranges: Chromium 10 to 30%, by wt, preferably about 20%, by wt.

Ammonium chloride 0.25% to 2.0% by wt.

Alumina Balance (allowing for other halides) Depending upon the metallurgy of the component the composition - 7 0 to 2.0% by weight to 2.0% by weight 0.25% to 2.0% by weight. also contains one or more of the following halides: Ammonium Ammonium Chromium Fluoride Bifluoride Chloride The pack is heated to a temperature in the range 900° 1100°C, the temperature being selected to suit the metallurgy of the base material. The process is preferably carried out in an inert atmosphere, for example in the presence of argon, but it may also be carried out in a hydrogen atmosphere. The process is permitted to proceed for a period sufficient to permit growth of the coating to a desired thickness, usually for a period between 2-10 hours. The coated components are then removed from the retorts and subjected to a subsequent heat treatment depending upon the chromising temperature and the heat treatment requirements of the base material.

The following non-limiting examples are illustrative of the invention.

EXAMPLE 1 COATING OF MAR M 002 MATERIAL COMPONENTS Turbine engine components made from a superalloy, known as Mar M002, are coated by a process as described above. The composition of Mar M002 alloy is as follows: Balance Nickel 9.5-10.5% Cobalt 8.5- 9.5% Chromium 5.0- 6.0% Aluminium 2.25-2.75% Tantalum 1.25-1.75% T itanium 9.5-10.5% Tungsten The components were supported on a rack as shown in Figure 1 and - 8 the rack was placed in a retort as shown in Figure 2. The components were chromised using an out-of-pack process. The pack composition comprised: Chromium 20%, by wt. Ammonium Chloride 1.0%, by wt. Ammonium Fluoride 0.5% by wt. Alumina Balance The chromising process is carried out at a temperature between 970°C - 1000° for 8 hours. The chromised parts were given a subsequent heat treatment by holding them at a solutionising temperature of 1100°C for 1 to 2 hours, and then aging at 870° for 16 hours.

EXAMPLE 2 COATING OF RENE 77 and RENE 80 MATERIALS Two CF-50 LP blades in R77 material, an airfoil segment from an LP vane in R77 material and a CF6 HP1 blade in R80 material were coated using the vapour chromising process of the invention. A rack was used as described above in relation to Figures 1 and 2. The process therefore was an out-of-pack process.

The pack composition was as follows: Chromium 25% Ammonium Chloride 0.5% Chromium Chloride 0.5% Ammonium Fluoride 0.5% Alumina Balance The process was carried out at 1025°C for 6 hours.

After coating the components had a bright silver appearance that is characteristic of the process. An airfoil section and a blade root section were prepared for metallography (see Figs 3 and 4). - 9 The photomicrographs of Figs. 3 and 4 show a non-etching chromium rich coating free from contamination and oxide/grit entrapment. The coating thickness distributions are given in Fig. 5 and they show the following average thickness: RENE 77 RENE 80 0.00089 0.00085 Figure 5a shows an R77 vane airfoil section, Figure 5b shows an R77 LP turbine blade airfoil section, Figure 5c shows an R77 LP turbine blade root section, and Figure 5d shows a HP turbine blade root section. The coating thickness distributions are shown in 0.0001 inch units.

No alpha chromium was found in any of the metallographic sections. Chemical analysis of the outer 15% of the coating thickness was carried out using a JEOL JXA 8600 scanning electron microscope with a Link energy dispersive analyzer equipment. The instrument was precalibrated with standards for the elements found to be present and was programmed to correct for interactions between elements. The following results were obtained: RENE 77 RENE 80 wt% Cr wt% Cr - 10 CLAIMS

Claims (12)

1. A process for producing a high-temperature corrosion-resistant protective coating on nickel-based superalloy components comprising turbine blades, nozzle guide vanes, turbine discs, exhaust struts, combustors and heat shields, which process comprises diffusing into the surface of the components one or more metals of the chromium group of elements by means of a pack, or out-of-pack, chromising process wherein the process is carried out at a temperature in the range 900 1100°C in the presence of ammonium chloride and one or more of ammonium fluoride, ammonium bifluoride, and chromium chloride.

2. A process as claimed in claim 1 or claim 2, wherein the halides are present in the following amounts: Ammonium Chloride: Ammonium Fluoride: Ammonium Bifluoride: Chromium Chloride: 0.25% to 2.0% by weight, 0 to 2.0% by weight, 0 to 2.0% by weight, 0.25% to 2.0% by weight.

3. A process as claimed in claim 2, wherein the process is carried out using a pack composition containing chromium in an amount between 10 to 30%, by weight, ammonium chloride in an amount of between 0.25% to 2.0% by weight, one or more of ammonium fluoride, ammonium bifluoride, and chromium chloride in an amount as stated in claim 2, and the balance comprising alumina.

4. A process as claimed in any of the preceding claims wherein the pack composition is as follows: ammonium chloride ammonium bifluoride Chromium Alumina 1% by wt. 0.5% by wt. 10 to 30% by wt. Balance and the process temperature is between 1070 to 1080°C to produce a coating having a thickness in the range 0.0008 - 0.0011

5. A process as claimed in any of the preceding claims wherein the pack composition is as follows: ammonium chloride 0.5% by wt. ammonium fluoride 0.5% by wt. Chromium 10-30% by wt. Alumina Balance and the process temperature is between 969 to 1000°C to produce a coating having a thickness in the range 0.0007 - 0012. 6. A process as claimed in any of the preceding claims wherein the pack composition is as follows: ammonium chloride 0.5% by wt. chromium chloride 0.5% by wt. ammonium fluoride 0.5% by wt. Chromium 10 to 30% by wt. Alumina Balance and the process temperature is between 1000 to 1035°C to produce a coating having a thickness in the range 0.0008 - 0011.

6. 7. A process as carried out in any of the preceding claims in which the components to be coated are suspended above a layer of the pack chromising composition in a retort.

7. 8. Use in a vapour chromising process in the production of high-temperature corrosion-resistant protective coatings on nickel-based superalloy components as claimed in claim 1 of a composition comprising ammonium chloride and one of more of ammonium fluoride, ammonium bifluoride, and chromium chloride.

8. 9. Use as claimed in claim 8, wherein the halides are present in the amounts stated in claim 2.

9. 10. Use as claimed in claim 8, wherein the composition is as stated in any of claims 3 to 6. - 12

10. 11. A chromising process substantially as with reference to example 1 or example 2.

11.

12. A chromising composition substantially as with reference to example 1 or example 2. hereinbefore described hereinbefore described TOMKINS & CO. SIFCO RESEARCH S DEVELOPMENT LIMITED