Patent Application
20140287143
The present invention relates to coating of turbine components, such as turbine blades, where a region exposed to relatively high operating temperature is aluminized and another region exposed to relatively lower operating temperatures is masked to prevent aluminizing while concurrently being enriched in Cr and/or retaining a pre-existing Cr content.
Gas turbine engine superalloy turbine blades and/or turbine vanes are coated in the airfoil region and sometimes in the platform region and even the shank of the root region with a simple or Pt-modified diffusion aluminide coating to provide a bond coat for thermal barrier ceramic coating, protection against deterioration by high temperature oxidation, or mild salt promoted corrosion processes that occur at the operating temperature experienced during use. Formation of the diffusion aluminide coating is accompanied by dimensional growth which can be tolerated in the those regions of the turbine blade/vane.
However, the fir tree region or other attachment region of the superalloy turbine blade or vane cannot tolerate such dimensional growth since it may exceed the dimensional tolerance of fitting/mating surfaces leading to assembly problems and possible mechanical failure in highly stressed attachment regions, e.g. fir tree roots. Chromizing of the fir tree region or other attachment region portion concurrently with aluminizing of the other regions of the turbine blade/vane has been attempted to protect the fir tree region or other attachment region from lower temperature corrosion without experiencing unwanted dimensional growth there. In one known method, a first mask comprising chromising composition is arranged on the selected region of the superalloy turbine component and an aluminising mask is arranged on the chromising composition.
The chromising composition comprises chromium powder, ferrochrome powder or other chromium containing powder, an inert refractory diluent powder, and a halide activator mixed with binder to form a slurry that is applied to the region to be coated. The first mask is covered by a second mask comprising an aluminizing mask, which can be a slurry coating or alternatively a particle-filled making box. The second mask comprises nickel powder, nickel oxide powder or nickel alloy powder, refractory powder such as alumina, and an inorganic resin binder.
The present invention provides a mask for use in aluminizing of a superalloy turbine component, such as a turbine blade, where a region exposed to relatively high operating temperature is aluminized to form a diffusion aluminide coating and another region exposed to relatively lower operating temperatures is masked to prevent aluminizing while concurrently providing Cr enrichment and/or retention of a pre-existing Cr-content from the superalloy chemistry itself or from a previous chromizing operation.
One embodiment of the invention provides a Cr-modified mask that comprises intentionally-added Cr-containing powder, nickel-containing powder, and refractory powder such as alumina wherein the Cr-containing powder is present in the mask in an amount that provides a Cr chemical activity that is greater than the Cr chemical activity of the turbine component superalloy to be coated or a pre-existing Cr enrichment. For Cr enrichment without alpha Cr layer formation, the Cr content of the Cr-containing powder typically does not exceed about 25 weight % based on the weight of the mask. For purposes of illustration and not limitation, for coating CMSX-4® superalloy having nominally 6.5 weight % Cr, the mask will have a Cr content greater than 10 weight % and typically less than about 25 weight %. The mask is useful for CVD or above-the-pack aluminizing at a temperature of about 1050 C or less for a time of about 8 hours or less.
In one method embodiment of the invention, the turbine component to be coated is positioned in a coating chamber in a manner that at least a portion of the root region is covered by the mask and other regions to be aluminized is/are exposed to a gaseous aluminizing atmosphere in the chamber to form a diffusion aluminide coating on those regions. Concurrently, the masked portion is enriched in Cr, or an existing Cr content there is retained. For example, the coating temperature and coating time can be about 1050 C or for a time of about 8 hours or less.
Another embodiment of the invention provides a multi-mask system having an inner mask and outer mask on the inner mask. The inner mask comprises substantially pure Cr powder or Cr-containing alloy powder in direct contact with the surface to be coated. The outer mask comprises the Cr-modified mask described above. The multi-mask system is useful for CVD or above-the-pack aluminizing at a relatively higher temperature above about 1050 C for a time greater than about 8 hours.
In another method embodiment of the invention, the turbine component to be coated is positioned in a coating chamber in a manner that at least a portion of the root region is covered by the inner mask and the outer mask on the inner mask and other regions to be aluminized is/are exposed to an aluminizing atmosphere in the chamber to form a diffusion aluminde on those regions. Concurrently, the masked portion is enriched in Cr, or an existing Cr content there is retained. The coating temperature and coating time can be above about 1050 C for a time greater than about 8 hours.
Advantages and other features of the invention will become more apparent from the following detailed description taken with the following drawings.
One embodiment of the invention provides a Cr-modified mask for use in aluminizing of a turbine component region at a relatively lower temperature and shorter time. For purposes of illustration and not limitation, the Cr-modified mask 200,
The Cr-modified mask comprises a powder composition that includes intentionally-added Cr-containing powder together with Ni-containing powder, and refractory powder such as alumina or other refractory materials. The Cr-containing powder can comprise a metallic Cr powder (e.g. −325 mesh powder) and/or a Cr-containing alloy powder (e.g. 30 weight % Cr-balance Ni powder) of similar particle size. The Ni-containing powder can comprise metallic Ni powder, a Ni alloy powder, and/or nickel oxide powder.
In an illustrative embodiment of the invention, the mask can comprise a commercially available M1 maskant available from Akron Paint and Varnish, Akron, Ohio (also known as APV Engineered Coatings) to which the Cr-containing powder is added and mixed. An exemplary maskant useful in practicing the invention into which Cr-containing powder (nominal particle size of about 5 to about 10 microns) can be mixed can comprise alumina powder (nominal particle size 0.5 to 15 microns) and a nickel alloy powder wherein the nickel alloy powder is present in an amount of about 15 to about 35 volume %, preferably about 22 to about 27 volume %, and the balance is the alumina powder and wherein the nickel alloy powder (nominal particle size of 1 to 10 microns) comprises about 15 to about 20 weight % Al and 0 to about 4 weight % Cr, and balance Ni, preferably 16 to 17 weight % Al and 1.5 to 2.5 weight % Cr and balance Ni.
The Cr-containing powder is provided in the M1 maskant in an amount that provides a Cr chemical activity that is greater than the Cr chemical activity of the turbine component alloy to be coated or of a pre-existing Cr enrichment from a previous chromizing operation wherein the more Cr in the turbine component alloy, the more Cr that is used in the mask to increase the Cr surface enrichment of the alloy. The Cr content of the Cr-containing powder is controlled to this end to drive Cr into the surface of the component alloy to form a Cr-enriched surface layer on the superalloy, or to maintain a pre-existing Cr enrichment at the surface layer of the superalloy by supplying Cr to a pre-existing Cr-enriched surface layer formed by a prior chromizing operation to counteract loss of Cr which occurs during the aluminizing operation when the aforementioned commercially available M1 maskant is used without modification. For Cr surface enrichment without alpha Cr layer formation, the Cr content of the Cr-containing powder typically does not exceed about 25 to about 30 weight % based on the weight of the mask. Higher than 25 weight % of pure Cr can be used, but the resulting Cr content of the surface enrichment will reach saturation (the α-Cr phase) at less than 25 weight % Cr. As a result, Cr contents of the mask of about 25 to about 30 weight % can produce a thin continuous to a thick amount of alpha Cr layer on the alloy with Cr enrichment beneath the alpha Cr layer of the alloy (substrate). Using a Cr—Ni or Cr—Fe alloy powder may require greater than 25 weight % Cr to reach formation of the α-Cr phase layer. For purposes of illustration and not limitation, for coating CMSX-4® superalloy having nominally 6.5 weight Cr, the Cr-modified mask will have a Cr content greater than 10 weight % and less than about 25 weight %. The Cr-modified mask is useful alone for masking a selected region of the turbine component for gas phase aluminizing such as by CVD (chemical vapor deposition) or by above-the-pack aluminizing at a temperature of about 1050° C. or less for a time of about 8 hours or less.
In one method embodiment of the invention, a turbine component to be coated is positioned in a coating chamber to form a diffusion aluminide coating on one region while another region is covered by the Cr-modified mask. For purposes of illustration and not limitation, referring to
To this end, the turbine blade is shown with its root end located in a masking box B having the Cr-modified powder mask 200 pursuant to the invention therein while leaving the airfoil region 10 and the upper surface of the platform region 12 exposed to the gaseous aluminizing atmosphere. In
After the aluminizing operation, the turbine blade is removed from the masking box B and residual mask material is cleaned off, taking care not damage the Cr enriched surface and/or the pre-existing Cr enriched surface which is retained as a result of appropriate selection of the Cr content of the mask.
Although
Another embodiment of the invention provides a multi-mask system having an inner mask 100 and outer mask 200 on the inner mask for use in aluminizing a turbine component region at relatively higher temperature of greater than about 1050° C. for times of more than about 8 hours. The inner (first) mask 100 comprises substantially pure Cr powder (e.g. −325 mesh Cr powder) or Cr-containing alloy powder (e.g. 30 weight % Cr-balance Ni powder) of similar particle size in direct contact with the surface to be coated. The first mask does not include an intentionally-added activator in it. Typically, the Cr-containing powder is mixed with a binder comprising water and polyvinyl alcohol to provide a slurry that can be applied to the region to be masked by dipping, brushing, spraying and other application techniques.
The outer (second) mask 200 comprises the Cr-modified mask 200 described above for the single mask system or other maskant.
In another method embodiment of the invention, a turbine component to be coated is positioned in a coating chamber to form a diffusion aluminide coating on one region while another region is covered by the two part mask system. For purposes of illustration and not limitation, referring to
To this end, the turbine blade is shown with its masked root end located in a masking box B having the Cr-modified mask therein while leaving the airfoil region 10 and the upper surface of the platform region 12 exposed to the gaseous aluminizing atmosphere. In
After the aluminizing operation, the turbine blade is removed from the masking box B and residual mask material is cleaned off taking care not damage the Cr enriched surface and/or any pre-existing Cr enriched surface which is retained as a result of appropriate selection of the Cr content of the mask.
Although the invention has been described in connection with certain illustrative embodiments, those skilled in the art will appreciate that modifications and changes can be made therein with the scope of the invention as set forth in the appended claims.
This application claims benefit and priority of U.S. provisional application Ser. No. 61/851,746 filed Mar. 13, 2013, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61851746 | Mar 2013 | US |
