Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to the treatment of metal
by diffusion coating.
Among the objects of the present invention is the pro-
vision of improved coating and treating processes and improved pro-
ducts thus formed. A particular object is to protect metals against
corrosion or oxidation at elevated temperatures. Another object
is the provision of a novel catalyst material.
In one particular aspect the present invention provides
a coated article comprising: a superalloy substrate, a first coating
thereon consisting essentially of an aluminide of the basis metal
of the substrate, and an MCrAlY coating thereover, where M is
; selected from the group consisting o~ nickel, cobalt, and iron.
In another aspect the present invention provides a coated
article comprising: a superalloy substrate the basis metal of which
~- is an element selected from the group consisting of nickel, cobalt
or iron, a ~irst protective coating thereon the principal protective
element of which consists essentially of a ductile aluminide of the
`~ basis metal; and an MCrAlY overlay coating over the first coating,
where M is selected from the group consisting of nickel, cobalt and
: 20 iron.
- In yet another aspect the present invention provides a
coated gas turbine engine component which comprises: a nickel-base
-~ superalloy substrate, a ductile nickel aluminide coating formed at
- the substrate surface, and a NiCoCrAlY coating over the aluminide
coating.
In a further aspect the present invention provides a
. :-
~- coated gas turbine engine component which comprises: a nickel-base
superalloy substrate, a ductile nickel aluminide layer on the
substrate surace, and a CoCrAlY coating over the aluminide layer.
'. ~ jc/~
7~4
The foregoing as well as additional objects of the pre-
sent invention will be more fully understood from the following
description of several of its exemplifications, reference being
made to the accompanying drawings wherein:
Fig. 1 is a sectional view of a packed retort for carry-
ing out a differential coating process of the present invention, and
Fig. 2 is a cross-section along line 2-2 of Fig. 1 of a
workpiece packed in the retort.
It is known that superalloy articles such as turbine vanes
and blades as well as burner rings in the hot section of jet en-
gines can be diffusion coated with chromium and then diffusion
coated with aluminum to improve their resistance to corrosion and
oxidation at temperatures as high as 1100C.
One very effective technique for chromizing a superalloy
workpiece in preparation for the aluminizing is as follows:
EXAM2LE 1
A group of B-1900 jet engine blades was packed in a cup-
shaped retort 4 inches high in an NH4Cl-energized dif-
fusion coating pack having 14% powdered chromium and 15%
powdered Ni3Al. The remainder of the pack was alumina,
but can be any other inert material. The energizer
content was 1/2% by weight of the total of the other
pack ingredients. Chromizing was conducted in a hydrogen-
bathed atmosphere, as in U.S. Patent 3,764,371, with the
- retort loosely covered, holding a 1925 F temperature for
-2a-
iC/ji.~
;,i~
/~
qO4
20 hours, giving a very uniform chromized case about
~ 0.7 mils deep, essentially free of oxide inclusions and
¦ without the formation of alpha-chromium phase.
! In the event the Ni3Al content of the pack is omitted or is
less than about 3% by weight, a substantial amount of oxide lnclusions are formed
in the case, and these may cause the case to spall off under the influence of
repeated thermal shock treatment, particularly if their number increases to
form a continuous layer of inclusions. Such inclusions tend to form in the
chromium diffusion case of any superalloy containing aluminum and/or titanium.
The number of such inclusions formed diminishes sharply when the Ni3Al content
of the pack is at least 3% by weight, and reaches a minimum when the Ni3Al
content ls about 6%. As much as about 20% Ni3Al can be contained in the pack
so that there is considerable tolerance for it and a wide concentration range
for its use. It is preferred to use 8 to 15% of Ni3Al so as not to require
accurate measuring and also to make it unnecessary to add make-up Ni3Al after
each use of the chromi~ing pack.
In addition to reducing oxide illclusions, the Ni3Al behaves
like an inert diluent in the pack since it does not interfere significantly
with the chromizing. Thus the chromium content of the pack can be as low as
10% and as high as 40%, regardless of the Ni3Al content.
The formation of oxide inclusions during chromizing is also
reduced when the chromizing takes place in an evacuated atmosphere as described
for example in U.S. Patent 3,290,126 granted December 6, 1966. In. an evacuated
atmosphere the chromium content of the pack should be relatively high, e.g.
from about 25 to about 60% by weight to keep the chromiæing time from exceeding
30 hours, and the energizer should be a non-volatile halide. ;
The foregoing reduction in oxide inclusion and alph~-
chromium phase formation is also obtained when other nickel-base
.
-3-
104104
super alloys are substituted for the B-1900 alloy o~ ~xample 1.
jSuch alternative superalloys include any al:Loy having 50 to 75%
: nickel and a little aluminum or titanium. Also the Ni3Al can be
replaced by intermetallics ranging from ~i3.sAl to ~i2Al with
equivalent resultq.
When chromizing the foregoing superalloys at atmospheric
jpressure or at somewhat above atmospheric pressure there is a ten-
¦dency to form a~pha phase chromium on the chromized superalloy
workpiece even when the chromium pick-up is as low as 1 to 3 milli
Igrarns per square centimeter of surface. Such alpha phase forma-
¦tion is helpful in that after a subsequent aluminizing coated .
¦me:~ers have greater resistance to corrosion, but the alpha chro-
¦mium tends to be brittle and does not provide a good surface for
Ireceiving vapor-deposited top coatings such as that described in
¦U.S.Patent 3,676,085 granted July 11,1972. By using a cup-shaped
Iretort effectively not over 5 inches in height, it has been dis-
¦covered that the formation of alpha chromium phase is prevented~
¦Retort cups taller than 5 inches can be effectively used without
¦alpha chromium formation by perforating the ~ide wall of the retor
¦at a level within 5 inches of its bottom. The perforations can be
¦ 1/8 inch diameter holes dri~led through the retort wall to provide
venting about 1 to 2 square inches in cross-sectional area for
every pound of diffusion coating pack. Small holes such as those
1/8 inch in diameter generally do not permit any significant amount
of the pack to spi~l out through them,but larger size holes can be
used and covered by a wire screen when the retort is being loaded.
It is pre~erred to maintain an effective retort height
¦ of at least two inches, as by providing the ~oregoing vent~ng at
least two inches up from the bottom of the retort. It should also
be noted that such venting i5 not to the air but to the spa~e
that surrounds the inner retort. That qpace is bathed by a stream
. ..
. ',.
. ' . .
.'. . '~ , ~ . .
Y~: ,. 1
. ~
. ~ .: - : :
. . ~ , . : .
~¦ o~ hyd~ogen, b~t can ~h ~e~ hed by a tr m ~ lne gaa
like argon, during the chromizing. In general this reduction in
alpha phasc formation is obtained with anychromium-dif~usion packl
but it does not require the presence of any of the foregoingnicke~ .
aluminide intermetallics in the pacX. However the pxesence of 3~ ¦
or more of such intermetallic in the pack will even Eurther re
duce the tendency to form alpha chromium. Modifying E~ample 1 by
~-¦ replacing its retort with an unperforated retort cup 10 incheR
¦high will provide a chromized case about 1.5 mils thick with a
substantial content of alpha phase chromium and suitable for sub
¦sequent aluminizing to make an excellent product that without
further treatment has unusually good sulfidation resistance. .
The diffusion aluminizing that follows the diffusion
chromizing can be either an inhibited oran uninhibited aluminizing .
The uninhibited aluminizing is conducted with no morethan a slightj
amount of chromium, or none at all, present in the aluminizingpacX.
A chromium content about half that of the aluminum, by weight, in-¦
hibits the aluminizing by greatly reducing the aluminum coating
¦rate and is described for instance in U.S. Patent 3,257,230. As
pointed out in that patent, laxger proportions of chromium to
aluminum can also be used in the inhibited aluminizing, and propor-
tions greater than 3:1 by weight cause some of the chromium to
diffuse into the aluminized case along with the aluminum.
Chromium-inhibited aluminizing is however particularly
desiriible as a top coating over a platinum diffusion or electr~-
plated coating on nickel-base superalloys, and in such a combina-
tion provides greater sulfidation resistance at high temperatures
than the use of the uninhibited aluminizing in such a combinatio~
as describea in U.S. Patent 3,677,789 granted July 18, 1972. The
¦sare ad~ont e is obtained when other platinum metals, partic~larl~
I 5. '
~- ' .
' 1!
104~704
~palladium, are used in place of ~latinum. Suitable examples of
;;chromium-inhibited aluminizing ~r~ described in Canadian Pate~t
¦1806,618 issued February 18, 1969, as well as in U. 5. Patent
¦¦3,257,2300 The nickel-base superalloys are also described in
I,those patents and generally are those high temperature alloys
¦Iwhich contain at least about 50yo nickel, and about 6 to 25%
j,chromium. ¦
¦I The following coating illustrates this coating combina-
tion. ~ ' . ,
' EXAMPLE 2
; A jet engine (hot section) blade of B-l900 alloy
and electroplated with a 0.0003 inch thick layer of
platinum was subjected to a hydrogen-bathed pack
di~fusion coating at 1890F for 12 hours, in a
¦ previously used pack consisting of, by weight:
magnefioth~rm;c cbromium po~er 45%
~j alumina S-325 mesh) 45~
aluminum powder (-325 mesh) 10%
¦ activated with 1/2% NH4Cl. ,
¦ ' The thus treated blade had a 0.003 inch thick
i dif~usion case and shows exceptional sulfidation
!¦ resistance.
Other types of very finely divided chromium less t~an
i, 10 microns in size can be used in place of the magnesothermic
!~ powder in the ~oregoing example.
j An aluminum dif~usion coating can also be used to pre-
pare catalytic nickel. Thus a foil 5 mils t~ick of pure nickel
!i
. .
: .1il, , I .
' 6.
' " , . i
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.... . . ~ . .
.. . .
~ 4~7~4
;~ can be aluminized in an ammomium chloride-energized pack consisting of 20%
alumlnum and 80% alumina, using a coatlng temperature of 1100F for 10 hours.
The coated surface contains at least about 30% aluminum, and when subjected to
treatment with 10% aqueous caustic soda at 20 to 40C loses most of its
aluminum to the caustic soda, leaving a highly active nickel surface that
s effectively catalyzes hydrogenation. The caustic stops reacting when the aluminum
content of the surface is sufficiently depleted, and the thus treated surface
should, until ready for use, be kept under water or other protective fluid to
keep it from heating up as a result of contact with the air. The resulting
foil in an effective catalyst for hydrogenating soybean oil for example, using
the continuous flow technique as described on pages 522 and 523 of l'Unit Processes
in Organic Synthesis", P~H, Groggins, editor~in~chief, fourth edition~ published~ 1952 by ~cGraw-Hill Book Company, A catalyst contact time of about 15 seco~nds
i. at a temperature of 130~C and a hydrogen pressure of lOO~atmospheres effects
substantial hydrogenation.
Nickel wool, or nickel-plated iron wool or foil, can be
aluminlzed instead of nickel foil to provide the catalytic nickel surface.
The dissolving of the aluminum from the surface can be effected with any caustic~i including caustic potash and should be carried out at a temperature below the
boiling point of the caustic solution used. The aluminized nickel can be stored
as such for many months, until just before catalytic use, the aluminum being
then dissolved to provide freshly formed catalyst.
. Diffusion coatings can also be applied so that some portions
of a workpiece contain a thinner coating than other portions, Thus roots or
hollow interiors of turbine blades can be arranged to be diffusion coated at
the same time the remainder of the blade is diffusion coated, but with less
- coating than the remainder of the blade, The following example is typical:
jl/~ 7-
.
.,
;`
: l
l -
. ~ ~041qO4
' EXAMPL~ 3
¦¦ A set of hollow first stage 1:urbinc~ blades oE ,
¦ B-l900 alloy had thc~ir hollow intcriors filled with
the following aluminizing pack: ~
Inside PaCk ~'
~; ¦ 45% chromium ~ ' '
, l~yo aluminum
I I Balance alumina plus 1/2% ~H4Cl
` ¦ The blades so filled were packed in an aluminizing pack
, I containing: ''
j ' Outer Pack
`.': 2~Yo chromium
. - . 11,6 aluminum ,
'.' 1.4/a silicon
'' Balance alumina plus 1/2% ~4Cl
" All ingredients were -200 mesh. A retort so packed was
th~n suDjected to a hydrogen-bathed coating heat at
1800F fo~ 5 hours, and after-clean-up,the blades
showed a *.3 milligram per square centimeter picX-up
of aluminum on their interior surfaces, with a 10.2
¦ milligram per square centimeter aluminum pick-up on
~, ¦ their ~xterior surfaces. Similar results are obtained
I whether or not the foregoing packs are given a break-in
,~ ¦ pre-firing. ,
In the same way the roots of blades or buttresses of van~ !5
- !, or trailing edges of both blades and vanes can be given coatings
thinner than the remainder o~ the blades or vanes., Reducin~3 th~ !
- chromium oontent of the internal pack to 2~o incre~ses the internal ~ `
- coating weight. An increase in outer coating is obtained by !j
~, ;' reducing the chromiu~. conten~ of the outer pack or incr~asillg i~
.'~
" ,j aluminum or silicon conten~.
; t! convers~.~y ~ increasing the chr mium ccntent of the
inner pack to 60Yo ~urtller ~1iminishcs the internal co~ting wei~ht.
:~ ;
.:;
'
~ 4 !L~
i Without the chromium in the outer pack, the silicon in that
~` pack only slightly diminishes the magnitude of the aluminum it deposits, and
without the silicon the changes in chromium content of the outer pack have
much less effect, The combination oE the chromium, silicon and alu~inum provides. the coating control when the aluminu~ content of the pack is as little as 3~ and
as much as 20%, with the chromium content greater than, preferably about 1.5 to
3 times, that of the aluminum, and the silicon content about 10 to 20~ that of
- the aluminum. The coating temperatures can vary from about 1600~, preferably ;
1700F, to about 2200F, and the workpieces can be any metal that is not melted
at the coating temperature, such as any nickel- or cobalt-based superalloy, DS
nickel, DS nichrome, chromium-containing iron, and type 300 and 400 stainless
steels.
~mitting the chromium or the silicon or both the chromium
i and the silicon, from the outer pack greatly increases the rate at which the
~ aluminum deposits on the surface of the workpiece.
i Nickel can also be used in the diffusion coatlng pack ln
place of chromium and/or silicon to inhibit the rate at which an aluminum
i diffusion coating forms.
The B l900 alloy turbine blades are preferably heat treated
at 1975F for four hours followed by rapid cooling at least as fast as air
cooling to below 20~F, with a subsequent aging at 1650F for ten hours and a
final rapid cooling, in order to develop their best mechanical properties. -
These heat treating steps can be carried out during the diffusion treatment to
differentially coat, by using the snugly fitting containers and procedure
described in United States Patent No. 3,824,122 granted ~uly 16, 1974.
.
30 ~ 9_
'.
,: ~; ; , . - - . . : .... . . , . .. ., ,. ,, ., . .. . - . .. . . . . . .. .
1041704
Another technique for simultaneously applying two di-
i fferent diffusion coatings is to use dif~erent energi7ers. Thi~
is illustrated by the following example:
¦ EXAMPLE 4
The same B-l9oo blades of Example 3 had their interiors
~illed with the following diffusion coating pacX:
nside Pack
18.5% ~i3~1
18.5% ~lumina
47% ~o
' 15.5% Cr .
.5% ~4Cl
The thus filled blades were packed in the following pack:
Outer Pack
18.5% Ni3Al
l~h Alumlna
. d,6.5~ co -
15% Cr
2% ~H4I
Using a 2000F coating temperature for ten hours in ahydrogen-
bathed atmosphere produced an internal coating which was
essentially a chromized case containing a negligible amount
of cobalt. On the other hand the outer coating was a case
that contained more cobalt than chromium and, after analuminun L
top coat, provided a somewhat greater resistance to hiyh tem-
perat~re oxidation. The two cases had approximately the same ,
thickness. It will be noted that the Ni3Al in these formu-
lations acted as inert diluent and can be replaced by other
nickel aluminides as pointed out above, or by alumina where
the formation o~ oxide inclusion ~5 not objectionable orwhen
the chromizing is e~ected under subatmospheric pressure.
.' . . .
. .
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'
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I 1~417~4
I ..
7 1 Mixing the two energizers ~4Cl and N~4I or their
¦ equivalents) enables the application of diffusion coatings of
intenmediate composition. Thus a mixt~re o~ 0.5% NH4Cl and 0.5%
I2, both by weight, provides a coating containing almost as much
cobalt as chromium. NH~r can be used as energizer in place of
¦chloride, the bromide acting very much like the choride. Other
¦volatilizable compounds of chlorine, bromine and iodine can be
used as energizers with similar resuits so long as there is suf-
ficient chromium and cobalt in the pack to provide the coatings. .
At least aoout l~/o of each o these metals by weight or of all
¦the metal content of the pack is all that is needed, and it is
¦preferred to have at least about 15% inert filler by weight; with-
out filler the pack tends to sinter together at temperatures of
¦2000F or higher.
¦ The wall of the blades of Example 4 does a good job of
¦keeping the diffusion coating atmosphere on the outside of each
¦blade from a~fecting the diffusion coating a~mospheres in th~ -~-
¦interiors of the blades. Where the different coatings of Example
4 are to be applied to adjacent portions of the outer surfaces,
these portions can be effectively separated by a metal wall
separating one pac~ from the other.
Where ~he pack on one side of s~ch a separating wall has
a tendency to vent its activator vapors into the pack on the other
side of the wall, as can happen with the foregoing hollow b1ades
~hen the opening into their hollow interiors is so located that
it is submerged in the external pack, it is preferred to have more
activator presen~ in the external pack than in the Lnternal pack, ~;~
. ' ' ' . ~''
. ~11.' , ' , . ~.
. .
,! . ~ 1 .
1041704
and to have a very small amount of activator in the internal
¦¦pack, ~or example 1/4 to 1/2% by weight o~ the pacX. Even such
I¦ a small amount produces substantial excess vapor on heat-up and
~uch vapor is vented out the opening ~or the hollow interiors.
The effect of such vapors in contaminating the activator vapors
, generated in ~he external pacX is reduced by keeping the activatorj
content low in the internal pack, and swamping any vapors vented
into the external pack by a larger activator content in the ex-
ternal pack as well as by the use of much more external pack than .
internal pack.
As in the case of simple diffusion coating packs, the
inside and outer packs of Example 4 can be reused. It is desir-
able for such reuse to replenish the packs for so much of their
contents as have been consumed in a coating operation. Thi9 .`
keeps tne pack ~airly uniform in composition so it is not neces-
sary to make many adjustments for such reuse or even for repeated
reuse. ~nasmuch as the activator is fairly thoroughly driven off
during any diffusion coating operation, an inside pa~k can be
used as an outer pack or vice versa, the amount and nature o the
act;vator being selected to match the nature of the reuse rather
than the past history of the pack. If it is no troublP to adjust
the coating conditions for reuse without replenishment, this ca~
also be done.
The pack of Example 1 can also be similarly reused with
. or without replenishment.
~ he foregoing chromium and cobalt-chromium coatings are
particularly suited for application at temperatures of at least ¦
1700F to protect nicXcl dase superalloys against high eemperaturel
I'
11~. ` -I
: 1 . ", 1,,
:~.: ,~ :.- : .. .. ..
~ .qO4
! oxidation and sulfidation, in which event it is preferred to apply over these
¦ coa~ings a difusion coating of aluminum or a coating of aluminum-chromium
mixtures such as those described in U.S. Patents 3,528,861 granted September 15,
, 1970, and 3,676,085. For these purposes the differential coatings are preferably
applied with the use of a retort effectively not over five inches high.
The following additional examples i~how modified forms of
differential diffusion coating: ~v
1 EXAMPLE ~5
~I Jet engine hot section blades composed of PWD-1422 and with
hollow interiors, were coated so the outer air foil surface had a heavy alumini~ed ;
case and the root a thin aluminized case, with the interiors uncoated. This
alloy has the following composition:
-, Chromium 9%
Cobalt 10%
Titanium 2%
Colombium 1%
~' - . ,' .
iAluminum 5% :
Tungsten 12.5%
' Carbon 0,15%
~3 20 Boron 0,015%
~- Zirconium,05% -
i Hafniumabout 1%
Nickel Balance
r' To make sure the blades were clean their external and
., , :~
internal surfaces were first solvent cleaned in trichloroethylene, then dry
~, blasted with 220 grit aluminum oxide propelled by air at a pressure of 30 psig.
, Any residual abraslve was then blown off
., .
., .
12-
,j
. 104~704
the blades. The interiors of the blades were then filled with the
¦~masking composition made up of equal parts by weight of Ni3Al and
'lalumina to which mixture is added metaLlic chromium so that it~ ¦
concentration is 1.6% by weight, all ingredients being minus 240
,mesh. $he bladeswere then pac~ed in individual retort arrangements.
The outer air foil section of each blade was packed in a
llclosely fitting pre-aluminized plain carbon steel tube with the
¦ followingheavy coating pack composition (all percentages by weight):
¦ 2~ chromium powder the particles of which range in size
¦ from about 1 to about 10 microns
11% -250 mesh aluminum-silicon alloy containing approximately .
¦ 88% aluminum and 12% silicon
¦ 68.5% 325 mesh aluminum oxide
0.5% ammonium chloride
~he packing was as iIlustrated in the figures where each blade is
¦ shown at 10,its air foil section at 12, its root at 14, the masking
pack at 15,~he opening through which the masking pacX is inserted
¦at 24,the pre-alumLnized steel tube at 16,and the heavy coating
¦pacX at 18. It was then placed in a large retort 20 and a number
¦ o additional blades similarly preparedwere placed alongside it in
¦ that retort. Over this assembly in the retort there was poured
¦the following light coating pack 22 (all percentages by weight~:
¦ 45% of the same chromium powder used in the heavy coating
pack
¦ 15% 325 mesh aluminum powder
¦ 39.5% 325 mesh aluminum oxide
¦ 0.5% ammonium chlorid~
¦Be~ore the pacXing each of the packs was broken in by heating to
¦1600F or higher for 5 hours, after which the ammonium chloride ¦ .
¦content of the packs was return~d to its original value by sup
¦¦plemental add tion~
I'
'13.
.,
~~ r
- - :
. : . .,~
1al4~0~
; A number of retorts 20 were then piled up within an outer
retort as described in U,S, Patent 3,764,371, and heated by a surrounding
. . ~
I furnace under a hydrogen atmosphere to 1650F plus or minus 25F, which
I temperature was held for four hours. The assembly was then cooled under
hydrogen, the hydrogen subsequently flushed out and the retorts opened and
unloaded. The individual blades still with their air foil sections packed in
tube 16, were then rPmoved from the outer pack, a process which is easily
accomplished inasmuch as the relatively low treatment temperature does not
cause the pack particles to adhere toge~her very tightly.~ The individual
blades were then withdrawn from the individual tubes, and the masking mixtures
in the hollows of the blades were finally poured out through the same air-
cooling openings 24 used for introducing that mixture. With the help of a
blast of air all residual packing and masking powder was blown off and the
blades thus cleaned next placed in a furnace where they were heated under dry
hydrogen to 1975E at which temperature they were held for four hours~ following' which they were rapidly cooled down with the help of a hydrogen flush to about - ~-
300F over a one hour period, They were then heated ln air, argon or hydrogen ~ :
or other inert atmosphere at 1650F for ten hours to complete their preparation
for use. The average case depth for the outer air-foil surface was 3.6 mils
and the average case depth for the roots was 1.8 mils.
.
Essentially the same results are obtained when the work-
pieces are completely packed in individual snugly fitting retort tubes as
described in aforementioned United States Patent No. 3,824,122 subjected to ~-
- the heat treatment sequence while still in those tubes and during the coating
step, as also described in that Patent~
~ ' j 1 / .' ~' 'J -14-
:
4~4
When coating with a diffusion coating pack in which the
metal content is aluminum, or a mixture of aluminum and silicon, a prior
break-in heat with such pack is not neecled,
Using the manipulative technique of Example 5 or the
~' alternative technique described in aforementioned U.S. Patent No. 3,824,122,
the process of Example 5 can be modified so the interior surface of the blade
is also coated, by substituting for the masking pack the light coating pack
: used around the root. Three different coatings can be simultaneouæly applied
by using the chromi~ing packs of Example 4 against the root and outer air foil
- 10 surface of a hollow blade, while aluminiæing its interior surface. Thus the
inside pack of Example 4-can be applied to the root, the same pack but with its
. - .
- NH4Cl replaced by an equal quantity of NH4I used against the outer air foil
r ~ .
surface, and the lighter aluminizing pack of Example 5 packed in the hollow
interior of the blade. The blade thus coated is particularly suited to receive
on its outer air foil surface and on its root surface the top coatings of U.S.
Patents 3,528,861 or 3,676,085.
, Alternatively the root surface is masked and the interior
- surface of the air foil given the light aluminum coating while the external
surface of the air foil the heavy coating, A still further alternative is to
subJect the external surface of the air foil to the coating treatment described
in U.S. Patents 3,528,861 or 3,676,085 while the internal surface of the air
foil is masked and the root sub~ected to the light coating of Example 5. I
desired the coating of U.S. Patents 3,528,861 or 3,676,085 can be applied in
` ~ this combination after the diffusion aluminizing of the root, and directly to
the external surfaces of the air foil, or after those external surEaces have
been given a heavy or light aluminizing.
:: . .
^ '' .
. '. ~,
~; 30
. . . .. .
1 ' .
~ ~ :
:
¦ The following is an additional example of differential
coating:
EXAMPLE 6
A row of j.et eng~ne vanes made of cobalt-base superalloy
X-40 is packed in an ~ 800 retort with their external
. ¦ airfoil surface embedded in the following powder pack mix- .
ture (by weight): :
Aluminum 10~
~ Chromium (very fine particles) 28% .-.
-~- Alumina 61.5%
` . ~4C1 ' 0-5% ...
The pack mixture had been prefired, a treatment that drove ,
. off essentially ~all the original ~4~1, and an additional
:. quantity of fresh ~4Cl mixed with the prefired material . .
after it had cooled down. .
. The packing was effected by aligning the vanes so the
`. buttresses at each vane end were ~n the left and right o~ the
.. . . row of vanes. The powder pack beyond the buttresses was
~; then sucked away by a vacuum cleaner wi-th a small nozzle,
.. leaving the far surfaces o~ the buttresses uncovered.
Against these uncovered surfaces is then poured and
. tamped down the following powder pack mixture ~y weight):
:: Aluminum 10
. Chromium (very f ~ e particlesj 45%
.. ¦ Alumina 44.5%
. ~H4C1 0.5%
':~: This pack ~ixture had also been prefired and had had it~
, N~4Cl replenished. The flnal assembly is then subjected to .
'`:,, .
, . . ~ 16. ' ,
'''-''' '
.'',' ', ' . .
:. , . ~
.',,',~ ". . , .
~ ~ ~ ' " i,. ' i
~!- iO41704
diffusion coating conditions in a glass-sealed retoxt assem-
bly at 2050F for twenty hours. After cooling to 200F the
glass seal is broken and the retort emptied. The vanes are
cleaned with a light blasting by very fine glass micro-
spheres blown by a stream of air from a 10 psig ~upply, and
are beige-colored, showing that they are coated all over. .
~owever the coating on the end faces of the butresses mea-
sures about 2.5 mils in case depth, whereas the airfoil
surface coating measures about 3.5 mils in case dept~.
Limiting the diffusion coating treatment so that the
heating of the packed retort is at 1950F for sixteen ;lours
-produces an outer buttress coating case of about 1.5 mils and
¦¦an airSoil oo ~ng case of about 2.5 ~L1s.
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1041704
il When pack diffusion coating the interior of a hollow
¦Ivane or blade whose ~xterior has been coated, or when pack I -
diffusion coating a relati~ely small portion of a workpiece I :
¦Isurface, as for instance to touch up a defective spot having an
jarea up to about 10~/o of a prior coating in the manner shown in
~'Figs. 2 and 3 of U. S. Patent 3,801,357 granted April 2~ 1974, it
.is not necessar,v to apply to the remaining surfaces a pack that
masXs by some kind of gettering action such as the action of ~i3Al
l on aluminum. ~owever to Xeep the remaining surfaces from excessive
deterioration they can be packed in an essentially inert pack
jcontaining 2 de~letion-reducing amount of the critically diff~sible
¦~me.al ingredients in those sur~aces. To this end an aluminized
Isuperalloy surface being touched up in one area by further
'laluminizing, can havè its remainder packed in a pacX of powdered
alumina or other inert material, containing by weight about 1~4% 1 ¦
; finely divided aluminum metal and about 1% 4inely divided ¦ .
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¦Ichromium metal, or an equivalent amount of a~ aluminum-chromium
l! alloy. .The aluminum content of the pack can range rom about
1~ . ,
l/12% to about 2% by weight, and the chromium contert can also have
: jlthe same ra~ge. The use o aluminum without chxomium in the inert.
! pack, is also help~ul but is not as desirable. . . ¦ .
~; ¦i . . Similarly,coating the ;.nterior of a blade whose exterior~
:~jihas already been gi.ven the cobalt-chromium-àluminum-yttrium,
S~,t eS
coating o~ Patent 3,676,085 applied over an alumini~ed or
~ ch~omized case, is pre~erably carried out with the previously
: , '. coatei~ exterior of ~ho workpiece surface cmbedded in a pack of
.. : powdurcd al~mina or ot~r inert material, containing about 1-1~2
. cobalt, abo~uJ 1/2','~ chromium, and a~out 1/4,' alumi~um, all
c~. ulated by wcight~
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Preferred concentration ranges for these three metals in such
pacX are:
Cobalt i/2 to 11~%
Chromium 1/4 to 3 . 5%
Alumin~m 1/8 to ~æ
Although the ~obalt-chromium-aluminum-yttrium coating being pro-
tectea also contains a small but critical amount of yttrium, that
yttrium does not appear to be a critically diffusible metal
whose depletion needs a safeguard such as the presence of a little !
yttrium metal in the inert pack. The cobalt, chromium an~
aluminum particles present in the inert pack can be either of.
the individual metals, or alloys of any two or all three of these
metals. I~ general aluminum, chromium, nickel and cobalt are the
critical diffusible metals found in protective coatings on supe~-
alloys. The masking pack of Example 5 is also such a depletion-
preventing pack. -
~ ha foregoing inert packs containing depletion-reaucing
ingredients can be reused. When reused; their metal ingredients
will generally be inter-alloyed as a result of the 2rior u~e.
In ~ome cases depletion of di*fusible ~aterial rom a
workpiece surace is beneficial. As noted in U.S. Patent
3,647,517 granted ~arch 7, 1972, aluminide coatings dif~used onto
the sur*ace of cobalt-base superalloy workpieces are generally
quite brittle, so that the protection provided by those aluminide
coatings lea-~es something to be desired. Howe~er pretreating tbe
workpieces so as to effect substantial diffusion depletion from
¦those surfaces then causes an aluminîde coating subsequently .
iapplied to be much less brittle.
¦ The ~ollowing ~s a typical example:
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A group of Mar-~_509~ jet engine turbine vanes was packed
j in a plain carbon steel retort in a powder pack of equal parts by weight 325
mesh alumina and finely divided nickel the particles of which are about 40
microns in size, The pack is activated with about ~% by weight am~onium chlorideand the retort 90 packed is heated in hydrogen to a ~emperature of 2,000DF for
20 hours, The hydrogen atmosphere was provided as shown in U.S. Patent
3,764,371.
At the completion of the heating the retort was permitted tD ~ ;
cool and the cooled vanes removed from the pack. These vanes showed a weight
loss of about 35 milligrams per square centimeter over their entire surface,
and a typical cross section of a vane showed on microscopic examination a
significant number of voids adjacent the surface that was in contact with the
pack.
- The resulting vanes were then given a chro~ium-inhibited ;~
aluminum diffusion coating from a diffuslon coating pack in accordance with
Example Z, but with the maximum heating temperature at 2050F maintained for
20 hours. The final vanes had an aluminized case approximately 6 mils deep
which exhibited unusually high resistance to impact damage. The same
Z0 aluminizing carried out on a non-depleted Mar-M-509 vane provides an aluminized
case only about 3 mils thick and very brittle.
- The Mar-M-509 alloy is a well known cohalt-base super-alloy
and its composition is given in U.S. Patent 3,647,517. Other cobalt-base alloys
such as the additional five listed in Table 1 of the last~mentioned patent- alsolend themse-lves to this improved procedure for coating with an impact-resistance
protective aluminide case. In each instance the depletion should provide a
weight loss from about 3 to about 75 milligrams per square centimeter of ~urface.
No scale is formed on the ~Jorkpiece surface as a result of the depleting step,
and the scale removal operation referred to in U.S, Patent 3,647,517 is not needed.
.
1041704
1 Instead of nick~l alone as the mctallic in~redi~nt of
j the deplcting pack, alloys of nickel with alumin~ for exampl~ I
¦¦can be used, although a proportion of aluminum larger than that i~ !
! ~i3Al is not desircd. The nickel or nickel alloy can also be
¦Ireplaced by cobalt, and any o~ these metals can be present in the
¦jdepleting pack in a proportion of from a~out 10 to about 9~/O by
¦Iwei~ht, the remainder of the pack being either alumina or any other
inert diluent such as magnesia, to keep the metal particles from
ilsintering together. It is preferred that the metal particles be~ i
~no greater than about 200 microns in siz~. ¦
The retort can be of steel, stainless steel or nickel-
¦base alloys, and its composition does not seem to affect the
process so long as it does not contain low melting metals such as
zinc.
The pack activator can be any halide dif usion acti~ator
includin~ ammonium iodide, ammonium bromide, ammoni~um bifluoride,
elemental iodide or bromine, etc., and its content can be as low
as 1/8th of 1% of the pack by weight. The depleting tempera~ure
~o which the cobalt-base superalloy or pieces are subjected in
~contact with the pac~ can be as low as 1600F or as high as ~
¦2200F, and the depleting times can be as little as two hours to
!l as much as 100 hours, the longer times being used with the lower
!¦ temperatures and vice versa.
Ii Instead of hydrogen atmosphere during the depletion,
the atmosphere can be of inert gas such as argon. The activator
provides a halide vapox upon heat-up and such vapor accelerates
the depletion in much the sa~e manner as it acc~lerates ~he
~diffusion coating of wor~pecesO
The more impact-resistant aluminiæed cohalt-base super-
i alloy vanes and the like made in the foregoing mannex ~re I :
particulaxly d~sirab1O for use in jct engines such as those in
airc~aft w~erQ th~sQ articlQ, are suhject to impact d~ma~e, a~d
~ak~ long livod first staq~ hot soction va~nes.
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~ickel base superalloys also show the foregoing .
depletion when subjected to the foregoing diffusion depletion
. action, but protective diffusion aluminized cases on nickel
base superalloys are not nearly as brittle as those on cobalt
. base superalloys, so that the aluminizing of the depleted nickel
:~ base superalloys provides a case with only a little more impact
! resistance as compared with the aluminizing of untreated nickel .
~ ¦ ba e ~ per 1oy~.
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Obviously many modification9 and variations of the
present invention are possible in the light of the above
teachings. It is therefore to be understood that within the
scope of the appended claims the invention may be practiced
o~herwise ~ ~ as ~p~cl~ically ~es=r ~ ~d.
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