Cleaning and liquid contact with solids – Processes – For metallic – siliceous – or calcareous basework – including...
Reexamination Certificate
1994-01-31
2001-04-17
Vincent, Sean (Department: 1731)
Cleaning and liquid contact with solids
Processes
For metallic, siliceous, or calcareous basework, including...
C134S007000, C427S292000, C427S328000, C427S331000
Reexamination Certificate
active
06217668
ABSTRACT:
This invention relates to the refurbishing of superalloy or heat resistant steel parts which have been corroded by hot gases. Such parts includes blades from stationary gas turbines as well as from marine—and aeroengines as well as exhaust valves in diesel engines and similar parts.
Parts subjected in operation to hot gases are usually made of base materials like superalloys or heat resistant steels, to which base materials protective coatings may be applied. Typical of such parts are the blade and vanes of stationary gas turbines made from superalloys which generally operate at a temperature up to 1000° C., in particular within a temperature range between 650° C. and 900° C.
The term superalloy is well known in the art and is used to describe an alloy developed for service at elevated temperatures where severe mechanical stressing is encountered and where surface stability frequently is required.
All these superalloys usually consist of various formulations made from the following elements, namely iron, nickel, cobalt and chromium as well as lesser amounts of tungsten, molybdenum, tantalum, niobium, titanium and aluminum, Nickel-chromium, iron-chromium and cobalt-chromium alloys containing minor quantities of the other elements are representatives of such superalloys. For example, such superalloys may contain, by weight, approximately 12-35% chromium and up to 80% nickel together with additives in minor amounts such as titanium, tungsten, tantalum and aluminum. Representative alloys of this type are those identified as In 738 Lc and In 939 as well as Udimet 500. These designations are known in the art.
Such parts as those referred to above may also be made of heat resistant steel. By heat resistant steel is meant an alloy based on iron with alloying elements present to improve the anti-scaling resistance of the alloy surface to high temperature oxidation. These alloying elements generally include chromium, aluminum, silicon and nickel.
Parts made of such a superalloy or of heat resistant steel may be provided with protective coatings such as diffused chromium by chromising or diffused aluminum by aluminizing or with overlay coatings of any desired composition deposited by plasma spraying or physical vapour deposition, for instance.
Even such parts with protective coatings are subject to corrosion on their exposed surfaces and may have to be refurbished in order to keep them useful for a sufficiently long service life.
Thus, turbine blades generally have to be refurbished after certain periods during their service life, which may be up to 100,000 hours.
Corrosion on gas turbine components and the like at high temperatures results from contaminants in the fuel and/or air; furthermore, oxidation may also occur at high temperatures. Depending on the conditions of operation, an oxide layer of varying thickness may form on the surface of the part, e.g., the turbine blade. Also, and very significantly, sulphur can penetrate into the base material, especially along the grain boundaries, to form sulphides deep in the material. Also, internal oxides and nitrides may form within the metal near the surface.
Refurbishing or reconditioning involves the removal of all corrosion products derived from the base material and/or the coating, optionally followed by the application of a new protective coating on the newly exposed surface of the blade.
With regard to the types of corrosion described above, it is necessary when removing all the corrosion products to remove all the deep inclusions, such as sulphides, because if these inclusions were allowed to remain, there would be a risk that during subsequent heat treatment and further operation they might diffuse into the base material—especially in the case of thin-walled components—and thus endanger its mechanical integrity. Also, there is a danger that the application of a new coating might be disturbed or made impossible.
In the present invention relating to a turbine blade or the like made of superalloy or heat resistant steel and optionally provided with a protective coating the surface of the corroded part is removed or stripped by a combination of mechanical treatment (e.g. abrasive blasting) and chemical treatment (e.g. etching with acids or other suitable agents). More recently, a high temperature treatment with fluoride chemicals which generate hydrogen fluoride as the active species has proved useful. In this treatment, aluminum and titanium oxides and nitrides which are otherwise highly resistant are converted into gaseous fluorides which in their turn are easily removed. This treatment is in particular widely used in preparation of components for repair welding and brazing.
There are, however, problems associated with the use of fluorine compounds. The first problem is environmental both within the workplace and elsewhere. The second problem is that the treatment has the disadvantage that it has no effect on sulphur occlusions, so that the grain boundary sulphides mentioned above cannot be removed by such treatment. Accordingly, it is necessary to grind the affected areas by hand which can lead to uncontrolled removal of material.
In an article entitled “Refurbishment Procedures for Stationary Gas Turbine Blades” by Bürgel et al (Bürgel, Koromzay, Redecker: “Refurbishment Procedures for Stationary Gas Turbine Blades” from proceedings of a conference on “Life Assessment and Repair”, edited by Viswanathan and Allen, Phoenix, Ariz., Apr. 17-19, 1990) reference is made to an aluminizing treatment of as received service-exposed blades prior to stripping in order to make stripping of the coating easier by chemical means. The aluminum coating is applied by a pack cementation process, as normally used to apply aluminum diffusion coatings. This procedure is said to imply a high temperature treatment which leads to an enhanced inward diffusion of elements of the residual coating. It is also said that almost the whole wall thickness of the cooled blades is influenced at the leading edge and that microstructure deteriorations which are definitely not due to service exposure of the blades occur. The treatment is said to be a negative example of what can happen during stripping.
U.S. Pat. No. 4,339,282 discloses a method and composition for removing aluminide coatings from nickel superalloys, which nickel superalloys may in particular form turbine blades. Removal of an aluminide coating is accordingly done by etching with a special composition which avoids attacking the nickel superalloy. Besides a brief statement that a coating to be removed may be deteriorated, there is no specific disclosure on corrosion problems and the efficient removal of products of corrosion from a nickel superalloy substrate.
In accordance with the foregoing, it is the primary object of the invention that a corroded surface of a superalloy or heat resistant steel part may be removed effectively by deposition of an aluminide coating on the component, the depth of the coating being such as to enclose all the products of corrosion, and removal of the aluminide coating, whereby the products of corrosion are removed as well.
The inventive process for the refurbishing of a corroded superalloy or heat resistant steel part having a surface with products of corrosion comprises cleaning the surface, subsequently applying an aluminide coating on said surface and removing said aluminide coating together with the products of corrosion.
By this method, substantially all the products of corrosion, including grain boundary sulphides, can be removed.
It has been found by contrast to the teaching of the document by Bürgel et al cited above that the aluminization of the surface of a part which has become corroded by hot gases can be carried out to give the advantages described above if the surface is cleaned before aluminizing and the aluminizing is carried out as explained herein.
After removal of the aluminide coating the part may be recoated with a protective coating, for example by diffusion, in particular by chromising, plasma spraying or physical vapour deposition.
In another aspect
Czech Norbert
Kempster Adrian
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Vincent Sean
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