U.S. patent application number 09/737089 was filed with the patent office on 2002-06-20 for salvaged castings and methods of salvaging castings with defective cast cooling bumps.
Invention is credited to Abuaf, Nesim, Hasz, Wayne Charles, Johnson, Robert Alan, Lee, Ching-Pang, Schaeffer, Jon Conrad.
Application Number | 20020076571 09/737089 |
Document ID | / |
Family ID | 24962530 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020076571 |
Kind Code |
A1 |
Johnson, Robert Alan ; et
al. |
June 20, 2002 |
SALVAGED CASTINGS AND METHODS OF SALVAGING CASTINGS WITH DEFECTIVE
CAST COOLING BUMPS
Abstract
Castings for gas turbine parts exposed on one side to a
high-temperature fluid medium have cast-in bumps on an opposite
cooling surface side to enhance heat transfer. Areas on the cooling
surface having defectively cast bumps, i.e., missing or partially
formed bumps during casting, are coated with a braze alloy and
cooling enhancement material to salvage the part.
Inventors: |
Johnson, Robert Alan;
(Simpsonville, SC) ; Schaeffer, Jon Conrad;
(Greenville, SC) ; Lee, Ching-Pang; (Cincinnati,
OH) ; Abuaf, Nesim; (Lincoln City, OR) ; Hasz,
Wayne Charles; (Pownal, VT) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
8th Floor
1100 North Glebe Rd.
Arlington
VA
22201-4714
US
|
Family ID: |
24962530 |
Appl. No.: |
09/737089 |
Filed: |
December 14, 2000 |
Current U.S.
Class: |
428/553 ;
29/889.1; 428/600 |
Current CPC
Class: |
Y10T 428/12389 20150115;
B23K 35/3033 20130101; Y10T 428/12063 20150115; B23K 1/0008
20130101; B23K 35/0233 20130101; Y10T 428/12201 20150115; Y10T
428/12104 20150115; Y10T 29/49618 20150115; Y10T 29/49318 20150115;
B23P 6/007 20130101; B23K 2101/001 20180801; B32B 15/01 20130101;
B23K 2101/14 20180801; Y10T 428/12396 20150115 |
Class at
Publication: |
428/553 ;
428/600; 29/889.1 |
International
Class: |
B32B 015/16 |
Goverment Interests
[0001] This invention was made with Government support under
Contract No. DE-FC21-95MC31176 awarded by the Department of Energy.
The Government has certain rights in this invention.
Claims
What is claimed is:
1. A method of salvaging a casting having cast cooling bumps
projecting from a surface thereof wherein at least one area of said
surface has defectively cast bumps manifested by one or more
missing or partially cast bumps, comprising the steps of:
identifying the defectively cast area; and applying a coating on
said defectively cast area to form an overlying coated surface
forming a coated surface area in excess of the uncoated defective
surface area to afford enhanced heat transfer across the casting
relative to the heat transfer across the casting without applying
the coating.
2. A method according to claim 1 including applying the coating
solely to the defectively cast area.
3. A method according to claim 1 wherein the coating comprises a
braze alloy and cooling enhancement material, and including the
further step of fusing the braze alloy onto the defectively cast
area to bond the cooling enhancement material thereto.
4. A method according to claim 1 wherein said coating includes a
brazing sheet having a braze alloy and a binder, said coating
further including cooling enhancement material having metal
particles.
5. A method according to claim 1 wherein said coating includes a
braze sheet having a braze alloy without a binder.
6. A method of salvaging a casting having cast cooling bumps
projecting from a surface thereof wherein at least one area of said
surface has defectively cast bumps manifested by one or more
missing or partially cast bumps, comprising the steps of:
identifying the defectively cast area; providing a brazing sheet
having cooling enhancement material; and fusing the brazing sheet
to said defectively cast area such that said cooling enhancement
material is bonded thereto.
7. A method according to claim 6 including fusing the brazing sheet
to said defectively cast area such that the cooling enhancement
material forms protuberances projecting therefrom.
8. A method according to claim 6 wherein said brazing sheet
comprises a braze tape having first and second surfaces on opposite
sides thereof, said cooling enhancement material being applied to
said second surface of said tape and fusing the tape to said
defectively cast area with said first surface of said tape being
applied thereto.
9. A method according to claim 6 including applying the brazing
sheet solely to the defectively cast area.
10. A cast body having a cooling surface and an opposite surface
for exposure to a high-temperature fluid medium; said cooling
surface having a plurality of cast cooling bumps projecting
therefrom and at least one area thereof having defectively cast
bumps manifested by one or more missing or partially cast bumps; a
coating overlying said defectively cast area forming a coated
surface having an area in excess of the defectively cast area to
afford enhanced heat transfer across the casting relative to the
heat transfer across the coating without the coating.
11. A cast body according to claim 10 wherein said coating overlies
substantially solely the defectively cast area.
12. A cast body according to claim 10 wherein said coating includes
a braze alloy and particulate cooling enhancement material.
13. A cast body according to claim 10 wherein said coating includes
a brazing sheet having a braze alloy and a binder, said coating
further including cooling enhancement material having metal
particles.
Description
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a turbine casting having
cast-in cooling bumps along a surface to provide improved heat
transfer between a cooling medium and the opposite side of the
casting. Particularly, the present invention relates to methods for
salvaging castings with defective cast cooling bumps by applying a
coating to areas of the defectively cast bumps to improve their
heat transfer characteristics. The present invention also relates
to the salvaged castings.
[0003] Various techniques have been devised to maintain the
temperature of gas turbine components below critical levels. For
example, a cooling medium such as coolant air from the turbine
compressor or steam is often directed to the component along one or
more component surfaces. Such flow is understood in the art as
backside flow, where the cooling medium is directed at a surface of
the component not directly exposed to high temperature gases of
combustion. Enhanced heat transfer is also accomplished by
providing cast cooling bumps along the backside flow surface. For
example, cast cooling bumps may be provided in a gas turbine on the
inside surfaces of the stage 1 and stage 2 nozzles. It will be
appreciated that the outer surfaces of the nozzles are exposed to
the hot gases and are subject to very high temperatures on the hot
gas path exposed side thereof. A cooling medium such as steam or
air flows through various cavities within the nozzles along the
interior nozzle surfaces to provide backside cooling flow. The
cast-in bumps on the interior surfaces of the nozzle have a
generally hill-like shape and are spaced from one another to
provide a coolant side surface area larger than that of the
baseline smooth surface area.
[0004] In certain gas turbine components, for example, nozzles, the
cast-in cooling bumps are sometimes defective. By defective cast
bumps is meant that one or more bumps are missing from the surface
of the cast part or the bump is only partially formed. These
defects occur as a result of manufacturing process limitations.
When the parts are cast and inspected, defective areas can be
identified and the parts are sometimes scrapped. This results in a
significant financial loss. Accordingly, there is a need to provide
a method for salvaging cast parts of a turbine that have defective
cast cooling bumps.
BRIEF SUMMARY OF THE INVENTION
[0005] In accordance with a preferred embodiment of the present
invention, there is provided methods of salvaging a casting having
cast cooling bumps projecting from a surface thereof wherein one or
more areas of that surface have defectively cast bumps manifested
by one or more missing or partially cast bumps. To accomplish the
foregoing, the surface area or areas manifested by one or more
missing or cast bumps are first identified by visual inspection or
thermography. Once identified, the area or areas are cleaned and
the defective bumps removed, e.g., by grinding or grit blasting.
Thus, partially formed bumps may be ground down to the surface area
between the bumps, or the smooth area or areas with partially
formed bumps may simply be roughened. After cleaning, cooling
enhancement material is applied to the surface area(s) manifesting
defectively cast bumps and the spaces between the defectively cast
bumps. Preferably, a coating containing particles, e.g., metal
particles, is applied to the defective area. For example, a green
braze tape coated with a metallic powder is set in intimate contact
with the defective area and brazed thereto. The size of the
metallic powder particles is selected to provide heat transfer
enhancement on the local defective surface area. The alloy of the
metallic powder particles is chosen to withstand the part operating
conditions while providing a high thermal conductivity. The braze
alloy must also withstand the part operating temperature while not
diminishing other part properties, i.e., LCF. By applying the
coating in the local area of the defectively cast bumps, the part
can be salvaged and utilized, notwithstanding the lack of bumps or
partial bump formation in one or more areas of the backside
surface. It will be appreciated that this salvage or repair
technique can be used on most or all of the gas turbine parts
having cast bumps for enhancing heat transfer, such as shrouds,
certain stator nozzles, buckets and the like.
[0006] In a preferred embodiment according to the present
invention, there is provided a method of salvaging a casting having
cast cooling bumps projecting from a surface thereof wherein at
least one area of the surface has defectively cast bumps manifested
by one or more missing or partially cast bumps, comprising the
steps of identifying the defectively cast area and applying a
coating on the defectively cast area to form an overlying coated
surface forming a coated surface area in excess of the uncoated
defective surface area to afford enhanced heat transfer across the
casting relative to the heat transfer across the casting without
applying the coating.
[0007] In a further preferred embodiment according to the present
invention, there is provided a method of salvaging a casting having
cast cooling bumps projecting from a surface thereof wherein at
least one area of the surface has defectively cast bumps manifested
by one or more missing or partially cast bumps, comprising the
steps of identifying the defectively cast area, providing a brazing
sheet having cooling enhancement material and fusing the brazing
sheet to the defectively cast area such that the cooling
enhancement material is bonded thereto.
[0008] In a still further preferred embodiment according to the
present invention, there is provided a cast body having a cooling
surface and an opposite surface for exposure to a high-temperature
fluid medium, the cooling surface having a plurality of cast
cooling bumps projecting therefrom and at least one area thereof
having defectively cast bumps manifested by one or more missing or
partially cast bumps, a coating overlying the defectively cast area
forming a coated surface having an area in excess of the
defectively cast area to afford enhanced heat transfer across the
casting relative to the heat transfer across the coating without
the coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a fragmentary cross-sectional view of a casting
having cooling enhancement bumps along a surface thereof;
[0010] FIG. 2 is a plan view of the surface illustrating an area of
defectively cast bumps; and
[0011] FIG. 3 is a view similar to FIG. 1 illustrating the
defective area coated with a cooling enhancement material.
DETAILED DESCRIPTION OF THE INVENTION
[0012] Referring to FIG. 1, there is illustrated a metal casting 10
having a plurality of bumps 12 raised along one side of the casting
10. As an illustrative example, the casting 10 may comprise the
wall of a nozzle, bucket or a shroud for a gas turbine. It will be
appreciated that in both cases one surface 14 of the casting 10 is
exposed to a high-temperature fluid such as hot gases of combustion
flowing through a hot gas path. The opposite side contains a series
of cast-in, generally hill-like or shaped cooling bumps arrayed
along the cooling surface side of the casting to afford enhanced
heat transfer. It will be appreciated that the bumps can be formed
in many configurations such as semi-spheres, short pins,
cylindrical or rectilinear and that the term "bumps" as used herein
is not limited to any particular configuration, provided the bumps
afford an increased surface area to enhance heat transfer across
the casting.
[0013] As sometimes occurs, the bumps 12 are defective. That is,
the bumps in certain areas along the cooling side surface 18 of the
casting 10 are missing or only partially formed during the casting
process. For example, as illustrated in FIG. 2, the cooling side
surface 18 has a plurality of bumps 12 which are intended to be
arranged in a patterned array of rows and columns. From a review of
FIG. 2, however, it will be appreciated that certain bumps, e.g.,
bumps 19, are only partially formed or are missing from the rows of
regularly spaced bumps 12. The partially cast bumps may extend only
partly to their full height or have irregular configurations, or
both. When significant areas of the casting are found to be
defectively formed during the casting process, the parts are
typically scrapped. In accordance with the present invention, the
defectively cast parts are salvaged by applying a coating of heat
transfer enhancement material to the defective areas to improve the
local heat transfer.
[0014] To accomplish the foregoing, the defective area or areas of
the casting are first identified. This can be done visually, by FPI
inspection, thermography or even X-ray. Once identified, the
defective area is cleaned, removing some or all of the defective
bumps. For example, grinders or grit blasting may be applied to the
defective areas, depending upon their accessibility. In nozzles,
the openings to the nozzles are sufficiently large to insert a
grinding head and thus remove poorly cast bumps or roughen the
surface of the area which is defectively clear of bumps.
Alternatively, the defectively formed bumps may remain in the
defective area after cleaning. Subsequent to cleaning the defective
areas, braze microturbulators are added to the defective locations
and brazed on top of those areas or on top of the defective bumps
and between the bumps. The application of these microturbulators
significantly enhances the heat transfer of the local surface area
and, for salvaging parts, they are applied only to the defective
area or areas.
[0015] According to embodiments of the present invention, a layer
of material containing at least a braze alloy component and a
cooling enhancement material is utilized to provide cooling
enhancement locally on the defective surface of the casting. As
used herein, the term "layer" of material is used to denote a
single layer or several discrete sub-layers that are sandwiched
together. A "layer" of material may have several phases, including
a matrix phase having a discrete phase dispersed therein, and
several phases defined by sub-layers. The layer of material may be
in the form of a free-standing sheet containing at least the
cooling enhancement material and the braze alloy component. As used
herein, "cooling enhancement material" is a material that, upon
fusing to a substrate, forms a plurality of protuberances that
extend beyond the surface of the substrate. These plurality of
protuberances together define a "surface area enhancement," which
appears as a roughened surface that is effective to increase heat
transfer to the treated substrate. According to several embodiments
of the present invention, the cooling enhancement material
comprises a particulate phase comprised of discrete particles
bonded to the substrate in the defective areas thereof. The
particulate phase of discrete particles may be formed from a coarse
powder, described in more detail below with respect to embodiments
herein. While not intended to be bound by any theory of operation,
it is believed that the cooling enhancement of the defective areas
is a function of the increased surface area with the cooling
enhancement material applied to the defective area(s) of the
cast-in bumps as well as turbulation caused by the bumps and
applied cooling enhancement material.
[0016] In one embodiment of the invention, the layer of material is
a brazing patch or sheet, particularly a green braze tape. Such
tapes are commercially available. In an embodiment, the green braze
tape is formed from a slurry of metal powder and binder in a liquid
medium such as water or an organic liquid. The liquid medium may
function as a solvent for the binder. The metal powder is often
referred to as the "braze alloy." In a second embodiment, a braze
foil is used, i.e., a thin sheet of braze alloy with no binder.
[0017] The composition of the braze alloy is preferably similar to
that of the cast part or substrate. For example, if the substrate
is a nickel-based superalloy, the braze alloy can contain a similar
nickel-based superalloy composition. In the alternative,
nickel-based braze alloys or cobalt-based braze alloys are usually
used with cobalt-based superalloys. Nickel- or cobalt-based
compositions generally denote compositions wherein nickel or cobalt
is the single greatest element in the composition. The braze alloy
composition may also contain silicon, boron, phosphorous or
combinations thereof, which serve as melting point suppressants. It
is noted that other types of braze alloys can be used, such as
precious metal compositions containing silver, gold, or palladium,
mixtures thereof, in combination with other metals, such as copper,
manganese, nickel, chrome, silicon, and boron. Mixtures that
include at least one of the braze alloy elements are also possible.
Exemplary braze alloys include by weight percent, 2.9 boron, 92.6
nickel, 4.5 tin; 3.0 boron, 7.0 chromium, 3.0 iron, 83.0 nickel,
and 4.0 silicon; 19.0 chromium, 71.0 nickel, and 10.0 silicon; 1.8
boron, 94.7 nickel, and 3.5 silicon.
[0018] A variety of materials are generally used as binders in the
slurry for forming the green braze tape. Non-limiting examples
include water-based organic materials, such as polyethylene oxide
and various acrylics. Solvent-based binders can also be used.
Additional organic solvent (e.g., acetone, toluene, or various
xylenes) or water may be added to the slurry to adjust
viscosity.
[0019] The slurry is usually tape cast onto a removable support
sheet, such as a plastic sheet formed of a material such as
Mylar.RTM.. A doctor-blade apparatus can be used for tape-casting.
Substantially all of the volatile material in the slurry is then
allowed to evaporate. The resulting braze alloy tape usually has a
thickness in the range of about 1 micron to about 250 microns, and
preferably, in the range of about 25 microns to about 125
microns.
[0020] Braze tapes containing the above-mentioned braze alloy and
binder are commercially available. An example of a commercial
product is the Amdry line of braze tapes, available from Sulzer
Metco. An exemplary grade is Amdry.RTM.100.
[0021] The cooling enhancement material that is applied to the
green braze tape is typically a coarse powder, being formed of
particles having a size sufficient to form protuberances that
function to increase heat transfer of the treated component. In
many embodiments, the size of the particles is determined in large
part by the desired degree of surface roughness and surface area
(and consequently, heat transfer) that will be provided by the
protuberances. Surface roughness is characterized herein by the
centerline average roughness value "Ra," as well as the average
peak-to-valley distance "Rz" in a designated area as measured by
optical profilometry. According to an embodiment, Ra is greater
than about 0.1 mils, such as greater than about 1.0 mils, and
preferably greater than about 2.0 mils. Ra is typically less than
about 25 mils, more typically less than about 10 mils. Similarly,
according to an embodiment, Rz is greater than about 1 mil, such as
greater than about 5 mils. Rz is typically less than about 100
mils, more typically less than about 50 mils. As used herein, the
term "particles" may include fibers, which have a high aspect
ratio, such as greater than 1:1. In one embodiment, the average
size of the cooling enhancement powder particles is in the range of
about 125 to about 4000 microns, such as about 150 to about 2050
microns. In a preferred embodiment, the average size of the powder
particles is in the range of about 180 microns to about 600
microns.
[0022] The cooling enhancement material is often formed of a
material similar to that of the substrate metal, which is in turn
similar to that of the braze alloy. The cooling enhancement powder,
however, must have a higher melting point or softening point than
that of the braze alloy such that the powder remains largely intact
through the fusing operation. Usually, the powder comprises at
least one element selected from the group consisting of nickel,
cobalt, aluminum, chromium, silicon, iron, and copper. The powder
can be formed of a superalloy bond coat composition for thermal
barrier coating (TBC) systems, such as a superalloy composition of
the formula MCrAIY, where "M" can be various metals or combinations
of metals, such as Fe, Ni, or Co. The MCrAIY materials generally
have a composition range of about 17.0-23.0% chromium; about
4.5-12.5% aluminum; and about 0.1-1.2% yttrium; with M constituting
the balance.
[0023] However, it should be emphasized that an important advantage
of the present process relates to the ability to change the surface
"chemistry" of selected portions of the substrate by changing the
composition of the cooling enhancement material. For example, the
use of oxidation-resistant or corrosion-resistant metal alloys for
such material will result in a turbulated surface that exhibits
those desirable properties. As another illustration, the thermal
conductivity of the cooling enhancement material, which affects the
heat transfer, can be increased by using a material with a high
thermal conductivity, such as nickel aluminide which has a thermal
conductivity on the order of 450 Btu.multidot.in/ft.sup.2.m-
ultidot.hF. In one embodiment, the cooling enhancement powder is
formed of a material having a thermal conductivity greater than
about 60 Btu.multidot.in/ft.sup.2.multidot.hF, preferably greater
than about 80 Btu.multidot.in/ft.sup.2.multidot.hF, such as greater
than about 130 Btu.multidot.in/ft.sup.2.multidot.hF. In contrast,
prior art casting techniques for producing turbulation usually
employ only the base metal material for the protuberances, thereby
limiting flexibility in selecting the characteristics of the
turbulated surface.
[0024] The powder can be randomly applied to the braze sheet by a
variety of techniques, such as sprinkling, pouring, blowing,
roll-depositing, and the like. The choice of deposition technique
will depend in part on the desired arrangement of powder particles,
to provide the desired pattern of protuberances. As an example,
metered portions of the powder might be sprinkled onto the tape
surface through a sieve in those instances where the desired
pattern-density of the protuberances is relatively low.
[0025] Usually, an adhesive is applied to the surface of the braze
tape prior to the application of the cooling enhancement powder
thereon. Any braze adhesive can be used, so long as it is capable
of completely volatilizing during the subsequent fusing step.
Illustrative examples of adhesives include polyethylene oxide and
acrylic materials. Commercial examples of braze adhesives include
"4B Braze Binder," available from Cotronics Corporation. The
adhesive can be applied by various techniques. For example,
liquid-like adhesives can be sprayed or coated onto the surface. A
thin mat or film with double-sided adhesion could alternatively be
used, such as 3M Company's 467 Adhesive Tape.
[0026] In one embodiment, prior to being brazed, the powder
particles are shifted on the tape surface to provide the desired
alignment that would be most suitable for heat transfer. For
example, acicular particles, including fibers, having an elongated
shape may be physically aligned so that their longest dimension
extends substantially perpendicular to the surface of the brazing
sheet contacting the substrate. The alignment of the powder may be
carried out by various other techniques as well. For example, a
magnetic or electrostatic source may be used to achieve the desired
orientation. In yet another embodiment, individual particles or
clusters of particles are coated with braze alloy, and such coated
particles are placed on an adhesive sheet for application to a
substrate. The adhesive sheet can be formed of any suitable
adhesive, provided that it is substantially completely burned-out
during the fusing operation. Suitable adhesives are discussed
above.
[0027] In some embodiments, the cooling enhancement powder is
patterned on the surface of the braze sheet. Various techniques
exist for patterning. In one embodiment, the powder is applied to
the substrate surface through a screen, by a screen printing
technique. The screen would have apertures of a pre-selected size
and arrangement, depending on the desired shape and size of the
protuberances. Alternatively, the braze adhesive is applied through
the screen and onto the sheet. Removal of the screen results in a
patterned adhesive layer. When the powder is applied to the sheet,
it will adhere to the areas that contain the adhesive. By use of a
screen, a pattern may be defined having a plurality of "clusters"
of particles, wherein the clusters are generally spaced apart from
each other by a pitch corresponding to the spacing of the openings
in the screen. The excess powder can easily be removed, leaving the
desired pattern of particles. As another alternative, a "cookie
cutter" technique may be employed, wherein the braze tape is first
cut to define a desired turbulation pattern, followed by removal of
the excess braze tape. The powder can then be applied to the
patterned tape. In yet another embodiment, particles of the
turbulation material are coated with braze alloy, and the coated
particles are adhered onto an adhesive sheet that volatilizes
during the fusing step. Here, the adhesive sheet provides a simple
means for attachment of the cooling enhancement material to the
substrate prior to fusing, but generally plays no role in the
final, fused article.
[0028] In another embodiment, the turbulation powder is mixed with
the other components of the green braze tape, such as braze alloy
powder, binder and solvent, during formation of the green braze
tape, rather than providing the powder on a surface of the already
formed tape. The powder in turn forms a dispersed particulate phase
within the green braze tape.
[0029] To apply the braze tape to the defective area(s), the tape
is sized to the defective area. The tape is then attached to the
defective surfaces of the casting 10 where turbulation, i.e.,
enhanced heat transfer, is desired. A simple means of attachment is
used in some embodiments. The green braze tape can be placed on the
defective surface, and then contacted with a solvent that partially
dissolves and plasticizes the binder, causing the tape to conform
and adhere to the defective surface, i.e., the tape flows to match
the contours of the defective cast bumps or the surface area clear
of the bumps. As an example, toluene, acetone or another organic
solvent could be sprayed or brushed onto the braze tape after the
tape is placed on the defective surface. Where a braze foil system
is used, the foil can be spot-welded to the part.
[0030] Following application of the braze tape to the defective
area, the cooling enhancement material is fused to the substrate.
The fusing step can be carried out by various techniques, such as
brazing and welding. Generally, fusing is carried out by brazing,
which includes any method of joining metals that involves the use
of a filler metal or alloy. Thus, it should also be clear that
braze tapes and braze foils can be used in fusing processes other
than "brazing." Brazing temperatures depend in part on the type of
braze alloy used, and are typically in the range of about
525.degree. C. to about 1650.degree. C. In the case of nickel-based
braze alloys, braze temperatures are usually in the range of about
800.degree. C. to about 1260.degree. C.
[0031] When possible, brazing is often carried out in a vacuum
furnace. The amount of vacuum will depend in part on the
composition of the braze alloy. Usually, the vacuum will be in the
range of about 10.sup.-1 torr to about 10.sup.-8 torr, achieved by
evacuating ambient air from a vacuum chamber to the desired level.
In the case of cooling enhancement material being applied to an
area which does not lend itself to the use of a furnace, a torch or
other localized heating means can be used. For example, a torch
with an inert atmosphere cover gas shield or flux could be directed
at the brazing surface. Specific, illustrative types of heating
techniques for this purpose include the use of gas welding torches
(e.g., oxy-acetylene, oxy-hydrogen, air-acetylene, air-hydrogen);
RF (radio frequency) welding; TIG (tungsten inert-gas) welding;
electron-beam welding; resistance welding; and the use of IR
(infrared) lamps.
[0032] The fusing step fuses the brazing sheet to the defective
surface area. When the braze material cools, it forms a
metallurgical bond at the surface, with the turbulation material
mechanically retained within the solidified braze matrix
material.
[0033] In the embodiments described above, the structure of the
component after-fusing includes a solidified braze film that forms
a portion of the outer surface of the component, and protuberances
that extend beyond that surface. The protuberances are generally
made up of a particulate phase comprised of discrete particles. The
particles may be arranged in a monolayer, which generally has
little or no stacking of particles, or alternatively, clusters of
particles in which some particles may be stacked on each other.
Thus, after fusing, the treated component has an outer surface
defined by the film of braze alloy, which has a particulate phase
embedded therein. The film of braze alloy may form a continuous
matrix phase. As used herein, "continuous" matrix phase denotes an
uninterrupted film along the treated region of the substrate,
between particles or clusters of particles. Alternatively, the film
of braze alloy may not be continuous, but rather, be only locally
present to bond individual particles to the substrate. In this
case, the film of braze alloy is present in the form of localized
fillets, surrounding discrete particles or clusters of particles.
In either case, thin portions of the film may extend so as to coat
or partially coat particles of the powder.
[0034] As an illustrative example of the use of a rough coating of
the foregoing-described type in areas of defectively cast bumps to
enhance heat transfer, and referring to FIG. 3, there is
illustrated an element 10 forming a cast part of the gas turbine.
The element 10 comprises a wall, such as a nozzle or shroud wall,
separating a high temperature region 20 and a cooling region 22
from one another. Thus, element 10 has a hot side 20 having a hot
side surface 24 and a cooling region 22, e.g., a coolant side
surface 26. Cast-in bumps 28, generally in the nature of
cylindrical projections 28, are illustrated on the coolant side
surface 26 and provide a surface area larger than the surface area
of the coolant side surface 26 without the bumps 28 to afford
increased heat transfer values. Additionally, as illustrated in
FIG. 3, defective surface areas include partially formed bumps 30
and/or areas 32 where bumps 28, though intended, were not cast. It
will be appreciated that such defective areas do not obtain the
benefits of enhanced heat transfer resulting from properly cast-in
bumps 30.
[0035] In accordance with a preferred embodiment of the present
invention, a surface coating 34 is applied only on the defective
areas, i.e., areas 30 and 32. The coating may be of the type as
previously described, e.g., comprises a braze alloy and a roughness
producing cooling enhancement material. The material 34 in the
coating preferably comprises metallic particles 36 bonded to the
defective surface areas. With the material and the coating, the
surface area ratio, i.e., the surface area with the coating and
cooling enhancement material divided by the defective surface area
without the material and coating is in excess of the first surface
area ratio and affords enhanced heat transfer values. Thus, the
local heat transfer enhancement value of the surface coated with
the coating and protuberances fused to the surface is greater than
the heat transfer value of the defective surface area(s) without
the coating. It will be appreciated that the coating may be applied
in accordance with any of the techniques described previously to
form a brazed alloy coating that forms a continuous matrix phase
and a discrete particulate phase comprised of cooling enhancement.
The articles may be randomly arranged or arranged in a
predetermined pattern, as discussed.
[0036] From the foregoing description, it will be appreciated that
the surface areas of parts which have defectively cast-in bumps may
be effectively repaired to produce enhanced heat transfer
characteristics. The cast parts, which previously contained
defective bumps, need not, with the advent of the present
invention, be scrapped. Rather, the parts can be salvaged and used
without the resulting economic loss.
[0037] While the invention has been described in connection with
what is presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *