Patent Application
20100147481
The invention relates generally to casted articles, and more specifically to cleaning the casted articles, for example, turbine engine components.
Liquid metal cooling is often used to form high-gradient castings of superalloy components in advanced gas turbines, as well as other industrial parts. During the cooling process, some of the molten metal used to cool the casting can breach the casting container and be deposited as a contaminant on the surface of the casted article. The casted article is typically subjected to a series of thermal fabrication and heat treatment cycles before becoming a useful casted article. The metal contaminant, if present, can diffuse below the surface of the casted item during the thermal processing cycles, and precipitously affect the surface quality and bulk properties of the finished article.
Accordingly, there is a need for methods of effectively removing such metallic contaminants. It would also be desirable if the processes did not result in the formation of an unacceptable amount of hazardous fumes. The processes should also exhibit some degree of selectivity. For example, the process should effectively remove the metallic contaminant while substantially preserving the casted article.
Embodiments of the invention are directed towards a method for manufacturing a casted article.
According to one embodiment of the invention, a method for manufacturing a casted article is disclosed. The method includes steps of forming a casted article by a liquid metal cooled directional solidification process, removing a metallic material from a surface of the casted article and inspecting the surface of the casted article. The surface of the casted article is inspected for the presence of the metallic material by exposing the surface to a visualization reagent.
In another embodiment, a method comprises filing a mold with a molten metal and immersing the mold into a cooling metallic liquid progressively to cause a solidification interface to pass through the molten metal. The method further includes recovering a casted article from the mold and removing a metallic material from a surface of the casted article. Furthermore, the method includes inspecting for the presence of the metallic material on the surface of the casted article by exposing the surface to a visualization reagent.
Another embodiment of the invention is a system, comprising a heating furnace at a temperature above the liquidus temperature of a metal within a mold, a liquid cooling bath comprising a cooling metallic liquid and a bath having an aqueous composition. The aqueous composition removes a metallic material from a surface of a casted article. The system further includes a unit to expose at least a portion of the casted article to a visualization reagent, and a visualization aid for observing a response of the visualization reagent.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
As used herein, the term “superalloy” refers to a nickel, cobalt or iron-based heat resistant alloy that has superior strength and oxidation resistance at high temperatures. Nickel and cobalt-based alloys are favored for high-performance applications. The superalloy can contain chromium to impart surface stability and one or more minor constituents such as molybdenum, tungsten, titanium, iron or aluminum for strengthening purposes. Physical properties of the superalloy make the superalloy particularly useful for manufacturing a gas turbine component.
During a representative casting process, a casted article gets contaminated by the deposition of metal contaminants on the surface of the casted article. Various methods or techniques can be used to strip the deposited metal contaminant from the surface of the casted article. Even after effective cleaning or striping processes, there is a possibility of some residual metal contaminants to be present on the surface. As fully cleaned articles or components are desired, it is necessary to determine if the surface of the article is cleaned or still contains the metal contaminants. To ensure quality of the surface of the casted article, an inspection test of the surface is performed to detect the presence of any metal contaminants on the surface, according to an embodiment of the present invention. A visualization reagent can be used for the detection of the residual metal present on the surface. The response of the visualization reagent indicates the presence or absence of any metal contaminant on the surface. The testing of the surface, in this way, further assures the effectiveness of the cleaning process used.
As used herein, the term “visualization reagent” refers to a chemical reagent that gives an optical response on reacting with a metallic material. The optical response of the chemical reagent can typically be observed in the visible region of the electromagnetic radiation.
According to an embodiment of the invention, a method of making a casted article is described with the help of a flow diagram 10 as illustrated in
Referring now to
The casted article may include a metal, or a metal alloy. In one embodiment, the casted article includes iron, cobalt, nickel, aluminum, chromium, titanium, and mixtures or alloys, for example, stainless steel. In a preferred embodiment, the casted article includes a superalloy.
The mold 48 is chosen according to the desired shape of the casted article and may differ from the shape as shown in
The liquid metal cooling bath 54, in the illustrated embodiment of
According to an embodiment of the invention, a metallic material or a metal contaminant is deposited on a surface of a casted article as a result of the ingression of liquid metal during a casting process. The ingression of liquid metal may occur when a mold containing the casted article cracks while the mold is still immersed in a liquid metal cooling bath. When the mold cools and develops cracks while still in the liquid metal cooling bath, the liquid metal can flow along the cracks in the mold and eventually make contact with the surface of the casted article inside the mold. The liquid metal can react with the interior of the mold while flowing through the mold cracks and with the article material while in contact with surfaces of the casted article. For this reason, some elements in the mold and in the casted article may also be present in the metal contaminant due to the interaction of the ingressed liquid metal with the mold and casted article materials.
As used herein, the “metallic material” is a material containing metal or metal alloys. The metallic material is deposited on the surface of the casted article in excess of any amount of the material which amount may be present in the casted article. In a non-limiting example, the metallic materials may include at least one element selected from the group consisting of tin, iron, cobalt, nickel, aluminum, chromium, titanium, and combinations thereof, for example stainless steel. The metallic material may further include other modifying constituents co-deposited with the metal or metal alloy, such as silicon, zirconium, yttrium, oxygen or combinations thereof.
The thickness of the metallic material deposited on the surface will depend on various factors, such as the type of the article being cast, the casting process employed, the materials being employed, and the like. In one embodiment, the metallic material may have a thickness between about 2 microns and about 2000 microns. In another embodiment, the metallic material may have a thickness between about 5 microns and about 1000 microns. In yet another embodiment, the metallic material may have a thickness between about 10 microns and about 500 microns.
The cleaning method 14 of the casted article, in the illustrated embodiment of
The cleaning method 14 of the article has a very desirable degree of selectivity. In other words, the metallic material can be effectively removed from the surface of the casted article, without adversely affecting or damaging the article. This is very advantageous for preserving the structural integrity and dimensions of the casted article. Moreover, the treatment composition described herein is relatively benign, from an environmental standpoint, as compared to mineral acid-based compositions.
As used herein, the term “removing the metallic material” is meant to refer to the severe degradation of the metallic material, leaving at most only a metallic material residue. The residue weakly adheres to the underlying surface. The residue can be removed by a subsequent, conventional technique such as “de-smutting,” as discussed below.
In an embodiment,
Immersion time and bath temperature will depend on various factors such as the type of metallic material being removed, the aqueous composition being used in the bath, and the equipment capabilities. In one embodiment, the treated article 72 is free of any residual metallic material. In another embodiment, the treated article 72 contains the residual metallic material 76 on a surface 74.
Usually, the bath 70 filled with the aqueous composition 68 is maintained at a temperature in the range of about room temperature to about 100 degrees Celsius, while the article 49 is immersed therein. In preferred embodiments, the temperature is maintained in the range of about 45 degrees Celsius to about 95 degrees Celsius. The immersion time in the bath may vary considerably. It is usually in the range of about 10 minutes to about 72 hours, and preferably, from about 1 hour to about 20 hours. Longer immersion times may compensate for lower bath temperature.
In one embodiment, the aqueous composition 68 or the treatment composition includes an acid having the formula HxAF6. In this formula, A is selected from the group consisting of Si, Ge, Ti, Zr, Al, and Ga. The subscript x is a quantity from 1 to 6, and more typically, from 1 to 3. Materials of this type are available commercially, or can be prepared without undue effort. The HxAF6 compound, sometimes referred to herein as the “primary acid”, is preferably H2SiF6 or H2ZrF6, or mixtures thereof. In some embodiments, H2SiF6 is especially preferred. The compound H2SiF6 is referred to by several names, such as “fluosilicic acid,” “hydrofluosilicic acid,” “fluorosilicic acid,” and “hexafluorosilicic acid.”
Precursors to the HxAF6 acid may also be used. As used herein, a “precursor” refers to any compound or group of compounds that can be combined to form the acid or its dianion AF6−2, or which can be transformed into the acid or its dianion under reactive conditions, for example, the action of heat, agitation, catalysts, and the like. Thus, the acid can be formed in situ in a reaction vessel, for example.
As one illustration, the precursor may be a metal salt, inorganic salt, or an organic salt in which the dianion is ionically bound. Non-limiting examples include salts of Ag, Na, Ni, K, and NH4+, as well as organic salts, such as a quaternary ammonium salt. Dissociation of the salts in an aqueous solution yields the acid. In the case of H2SiF6, a convenient salt, which can be employed, is Na2SiF6.
In one embodiment, H2SiF6 can be formed in situ, for example, by the reaction of a silicon-containing compound with a fluorine-containing compound. An exemplary silicon-containing compound is SiO2, while an exemplary fluorine-containing compound is hydrofluoric acid, that is aqueous hydrogen fluoride.
When used as a single acid, the HxAF6 acid can be somewhat effective for removing chromide deposition. The preferred level of acid employed will depend on various factors, such as the type and amount of deposition being removed; the location of the metallic material on the article; the type of the article; the thermal history of the article and deposition, for example, the level of interdiffusion; the technique by which the article is being exposed to the treatment composition as described above; the time and temperature used for treatment; and the stability of the acid in solution.
In general, the HxAF6 acid is present in the treatment composition at a level in the range of about 0.05 M to about 5 M, where M represents molarity. Molarity can be readily translated into weight or volume percentages, for ease in preparing the solutions. Usually, the level is in the range of about 0.2 M to about 3.5 M. In the case of H2SiF6, a preferred concentration range is often in the range of about 0.2 M to about 2.2 M. Longer treatment times and/or higher treatment temperatures, described below, may compensate for lower levels of the acid, and vice versa. Adjustment of the amount of HxAF6 acid, and of other components described below, can readily be made by observing the effect of particular compositions on the deposition removal from the article.
In preferred embodiments, the treatment composition also includes at least one additional acid or “second acid” or precursor thereof. The additional or “second” acid is preferably a phosphorous-containing compound, or nitric acid. Non-limiting examples of the phosphorous compounds include phosphoric acid and phosphorous acid, as well as mixtures thereof. In general, the phosphorous compounds are commercially available, as is nitric acid. These compounds can also be synthesized by well-known techniques.
Those skilled in the art can select the most appropriate additional acid, based on observed effectiveness and other factors, such as availability, compatibility with the primary acid, cost, and environmental considerations. Moreover, a precursor of the acid may be used, such as a salt, as described above in reference to the primary acid. For most embodiments, the preferred additional acid is a phosphorous compound, with phosphoric acid being especially preferred.
The present inventors do not wish to be bound to any particular theory in regard to the unexpected efficacy of the phosphorous compounds and nitric acid. However, they appear to provide the acidic capacity to rapidly oxidize the metal in the metallic material. This in turn appears to induce the metallic material to become solubilized, and to readily detach from the casted article surface region.
The amount of additional acid employed, that is, the phosphorous compound or nitric acid, will depend on the acid itself, as well as the identity of the primary acid, and on many of the factors set forth above. Phosphorous compounds are usually present in the composition at a level in the range of about 0.1 M to about 20 M. In some preferred embodiments, for example, in the case of phosphoric acid, the preferred range is from about 0.5 M to about 5 M. Furthermore, some preferred embodiments contemplate a range of about 2 M to about 4 M.
When present as the additional acid, nitric acid is present at a level, which will minimize degradation of casted articles being treated according to this invention. Usually that level will be no greater than about 1.2 M. In preferred embodiments, the range will be from about 0.3 M to about 1 M.
In some embodiments, the treatment composition 68 includes a minor amount of a third acid. This constituent is usually a strong acid, having a pH of less than about 3.5 in pure water. Thus, the third acid can be nitric acid, when the second acid is a phosphorous compound. Non-limiting examples of other strong mineral acids are sulfuric acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid, hydriodic acid, perchloric acid, alkyl sulfonic acids, and mixtures of any of the foregoing. The strong acid appears to be especially useful for removing portions of the metallic material, which metallic material may have diffused into the casted article.
In one embodiment, the third acid comprises hydrochloric acid, nitric acid, or mixtures thereof. In a preferred embodiment, the third acid is hydrochloric acid. Typically, the acid is advantageously supplied and used in aqueous form, for example, 35-38 percent hydrochloric acid in water.
The amount of third acid employed will depend on the identity of the primary acid and the second acid, and on many of the factors set forth above. To minimize degradation of the article, the third acid is preferably present at the levels described above, in regard to nitric acid. Thus, the concentration of the acid in the treatment composition is usually no greater than about 1.2 M, and preferably in the range of about 0.3 M to about 1 M.
The aqueous composition 68 of the present invention may further include various other additives, which additives serve a variety of functions. Non-limiting examples of these additives are inhibitors, dispersants, surfactants, chelating agents, wetting agents, deflocculants, stabilizers, anti-settling agents, reducing agents, and anti-foam agents. Those of ordinary skill in the art are familiar with specific types of such additives, and effective levels of usage. An example of an inhibitor for the composition is a relatively weak acid like acetic acid. Such a material tends to lower the activity of the primary acid in the composition. This is desirable in some instances, for example, to decrease the potential for pitting of the surfaces of some types of casted articles, if contacted with the treatment composition.
Treatment of the casted article 62 in the aqueous composition 68 severely degrades the integrity of the metallic material 66 being removed. The degraded metallic material is referred to herein as “smut” or “metallic material residue.” The metallic material residue often continues to weakly adhere to an underlying layer of the casted article. Consequently, the treatment is usually followed by a post-stripping step, often referred to as a “de-smutting” operation. Such a step is known in the art, and described in various references. De-smutting may be performed in the form of a gentle abrasion step that minimizes damage to the casted article or the underlying layer. As one example, grit-blasting can be carried out by directing a pressurized air stream containing aluminum oxide particles across the article surface. The air pressure is usually less than about 100 psi. The grit-blasting is carried out for a time period sufficient to remove the degraded deposition. The duration of grit-blasting in this embodiment will depend on various factors, such as the thickness and specific composition of the smut layer; the size and type of grit media, and the like. The process is typically carried out for about 30 seconds to about 3 minutes.
Other known techniques for abrading the surface may be used in lieu of grit-blasting. For example, the article surface can be manually scrubbed with a fiber pad, such as a pad with polymeric, metallic, or ceramic fibers. Alternatively, the article surface can be polished, for example, with a flexible wheel or belt in which alumina or silicon carbide particles have been embedded. Liquid abrasive materials may alternatively be used on the wheels or belts. These alternative techniques should be controlled in a manner that maintains a contact force against the article surface that is no greater than the force used in the grit-blasting technique discussed above.
Other techniques, or combinations of techniques, can be employed in place of abrasion, to remove the degraded metallic material. Examples include laser ablation of the article surface, or tumbling of the casted article, including water tumbling. Alternatively, the degraded material could be scraped off the article surface. As still another alternative, sound waves, for example, ultrasonic waves, could be directed against the surface, causing vibrations to shake loose the degraded material. For each of these alternative techniques, those skilled in the art would be familiar with operating adjustments which are made to control the relevant force applied against the surface of the casted article, as in the case of the abrasion technique, to minimize damage to the article being preserved. The article is sometimes rinsed after this step, for example, using water or a combination of water and a wetting agent.
The treated casted article 72, in the illustrated embodiment of
According to one embodiment of the invention, the cleaning method 14 further includes a method 26 of inspecting a cleaned casted article, as illustrated in
Suitable techniques to treat the surface with the extraction reagent may include swiping the surface with the extraction reagent, pouring the extraction reagent over the surface or the like. In one embodiment, the extraction reagent includes an acid. Suitable acids as the extraction reagent may include, but not limited to, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid or a combination thereof.
A variety of visualization reagents can be used for the detection of the metallic material. Suitable visualization reagents may include, but not limited to, a calorimetric reagent or a chemical indicator, a fluorescent dye or an optically active material. The “optically active material” as used herein may refer to a material, which absorb, scatter or deflect light. The response is observed on exposure of the reagent to the surface of the casted article. The visualization reagent may result in a calorimetric response that is a change in color, absorbance, scatter or fluorescence response corresponding to the visualization reagent used.
In a preferred embodiment, a calorimetric reagent is used for the detection of the metallic material on the surface. Change in color may depend on the type of the metallic material present on the surface and the calorimetric reagent used for the detection. Various calorimetric reagents can be used for detecting different metals. For example, pyrocatechol violet (PV) is a sensitive reagent for calorimetric determination of tin, iron, aluminum, chromium and yttrium. Other calorimetric reagents sensitive to tin may include galleon, pyrogallol red, bromopyrogallol red, or phenylfluorone.
Pyrocatechol violet is a sulfone phthalein dye made from condensing two moles of pyrocatechol with one mole of o-sulfobenzoic acid anhydride. Pyrocatechol violet has four hydroxyl groups, two sulphone oxygens and a quinone oxygen. Chemical structure of pyrocatechol violet is as follows
All of the functional groups are in conjugation with a delocalized aromatic system that imparts color to the compound by absorbing light in the characteristic visible wavelengths. Pyrocatechol violet has color indicator properties and forms complexes of blue to blue-violet colors with metal ions.
In an exemplary embodiment, a method of inspection or detection of a metallic material on a cleaned surface of a casted article by using pyrocatechol violet is illustrated in a flow chart 80 of
In some embodiments, the detection of a specific metal present on the surface can also be achieved by using a masking agent. The masking agent can be applied to the surface before exposing the surface to the visualization reagent. The masking agent allows the specific metal to be exposed to the visualization reagent and does not allow other metallic materials to be exposed to and react with the visualization reagent. For example, suitable masking agents that allow only tin to be inspected and mask other metals, are lactic acid and ascorbic acid. These acids selectively reduce metals except tin. Other masking agent may be N-(2-hydroxyethyl) ethylenedinitrilotriacetic acid (HEDTA) or ethylenediaminetetraacetic acid (EDTA), which selectively chelates other metals.
Various techniques can be used to expose the surface to the visualization reagent. A unit can be used to expose at least a portion of the casted article to a visualization reagent. Suitable units may include, but are not limited to, a cotton swab, a block paper, a spray, a bath, a glass rod, a glass pipe, or a cloth.
Similarly, various techniques can be used to observe the response on exposing the visualization reagent to the surface. The response of the visualization reagent may be observed with or without the use of a visualization aid. In one embodiment, the response may be observed by the human eye. In another embodiment, the visualization aid may be an optical system such as a light source, an optical detector, an optical differentiator or a combination thereof. The optical differentiator may include a filter, a prism, a spectrometer, an interferometer or the like. For example, the emission of a fluorescent dye may be observed under UV excitation.
In exemplary embodiments, some preferred techniques for using a calorimetric reagent are illustrated in
The example that follows is merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention.
The following Example 1 illustrates a directional solidification process conducted utilizing a tin metal cooling bath. In this process, an article, for example, as turbine blade is first cast in a mold that is made from Ni based superalloy. The superalloy article comprises 7.5 weight percent cobalt, 7.0 weight percent chromium, 6.2 weight percent aluminum, 6.5 weight percent tantalum, 1.5 weight percent molybdenum, 5.0 weight percent tungsten, 3.0 weight percent rhenium, trace amounts of hafnium, yttrium, boron and carbon, with the balance being nickel. The mold and casting are lowered into a bath of molten tin at a rate of 0.5 cm/minute. The temperature of the molten tin is maintained at about 300 degree Celsius, approximately 50 degree Celsius above the melting temperature of the pure tin. The thermal gradient measured in the cast part is 98 degree C./cm.
A deposit of metal contaminant having a thickness of about 500 microns is located on the surface of the casted superalloy article. The metal contaminant is primarily comprised of tin, due to the ingression of liquid tin during the casting process. The ingression of liquid tin occurs in a liquid-tin-cooled directional solidification process when the mold containing the casted superalloy article cracks while the mold is still immersed in the liquid tin bath. As the mold cools, it develops cracks while still in the liquid tin bath. The liquid tin flows along the cracks in the mold and makes contact with the surfaces of the casted article inside the mold. The liquid tin can react with the interior of the mold while flowing through the mold cracks and with the superalloy while in contact with the casted article surface. As a result, some elements in the casting mold and in the casted article may also be present in the metal contaminant.
The contaminated casted article is immersed in a bath of an aqueous composition comprising commercially available grades of approximately 71 volume percent hydrofluosilicic acid (H2SiF6), 24 volume percent phosphoric acid (H3PO4), and 5 volume percent hydrochloric acid (HCl). The bath is maintained at a temperature of 80 degrees Celsius, while the article is immersed in the aqueous composition for 4 hours. The metal contaminant is substantially removed by the aqueous acid composition after the approximately 1-hour immersion. The treated surface of the article is inspected to detect any residual tin present on the surface. The surface is first treated with concentrated hydrochloric acid. The hydrochloric acid used is 10-volume percent acid in water. The tin present on the surface reacts with HCl to form tin chloride. A cotton swab dipped into a 10−3 M solution of PV is applied on the acidified surface. The swab turns pink due to formation of protonated pyrocatechol violet (PV).
The contaminated casted article is immersed in a bath of an aqueous composition comprising commercially available grades of approximately 71 volume percent hydrofluosilicic acid (H2SiF6), 24 volume percent phosphoric acid (H3PO4), and 5 volume percent hydrochloric acid (HCl). The bath is maintained at a temperature of 80 degrees Celsius, while the article is immersed in the aqueous composition for 4 hours. The metal contaminant is substantially removed by the aqueous acid composition after the approximately 4-hour immersion. The treated surface of the article is inspected to detect any residual tin present on the surface. The surface is first treated with concentrated hydrochloric acid. The hydrochloric acid used is 10-volume percent acid in water. Any tin present on the surface reacts with HCl to form tin chloride. A cotton swab dipped into a 10−3 M solution of PV is applied on the acidified surface. The swab turns blue-violet due to formation of tin complex with pyrocatechol violet (PV).
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
This application is a continuation in part of U.S. patent application Ser. No. 12/334582, entitled “A PROCESS FOR REMOVING METALLIC MATERIAL FROM CASTED SUBSTRATES AND RELATED COMPOSITIONS,” filed on Dec. 15, 2008, which is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12334582 | Dec 2008 | US |
| Child | 12494727 | US |
