U.S. patent number 3,779,719 [Application Number 05/094,991] was granted by the patent office on 1973-12-18 for diffusion coating of jet engine components and like structures.
This patent grant is currently assigned to Chromalloy American Corporation. Invention is credited to Eugene V. Clark, Maurice R. Commanday, William J. Martin, Peter J. Plambeck.
United States Patent |
3,779,719 |
Clark , et al. |
December 18, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
DIFFUSION COATING OF JET ENGINE COMPONENTS AND LIKE STRUCTURES
Abstract
Greatly increased thermal fatigue cracking resistance of high
temperature resistant base metals such as cobalt and nickel and
their alloys used to fabricate jet engine components and like parts
is realized by diffusion coating the base metal with chromium,
silicon and aluminum in particular proportions to diffuse into the
base metal to a uniform depth and distribution between and across
intergranular boundaries in the base metal, enabling slowed
initiation of thermal fatigue cracking in the diffusion coating as
well as reduced corrosion in the base metal.
Inventors: |
Clark; Eugene V. (Northridge,
CA), Martin; William J. (Huntington Beach, CA), Plambeck;
Peter J. (Torrance, CA), Commanday; Maurice R. (Palos
Verdes Estates, CA) |
Assignee: |
Chromalloy American Corporation
(New York, NY)
|
Family
ID: |
22248400 |
Appl.
No.: |
05/094,991 |
Filed: |
December 3, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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731631 |
May 23, 1968 |
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Current U.S.
Class: |
428/652; 427/253;
428/941; 148/527; 428/678 |
Current CPC
Class: |
C23C
10/58 (20130101); C23C 10/56 (20130101); Y10T
428/12931 (20150115); Y10T 428/1275 (20150115); Y10S
428/941 (20130101) |
Current International
Class: |
C23C
10/00 (20060101); C23C 10/56 (20060101); C23C
10/58 (20060101); B32b 015/00 (); B32b 015/02 ();
C23c 009/02 () |
Field of
Search: |
;148/31.5,34
;29/183.5,197 ;117/107,17P,170.2,71 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lovell; Charles N.
Parent Case Text
REFERENCE TO RELATED APPLICATION
This application is a continuation in part of our co-pending
application Ser. No. 731,631 entitled "DIFFUSED COATING OF HIGH
TEMPERATURE RESISTANT ALLOYS," filed May 23, 1968, now abandoned.
Claims
We claim:
1. Jet engine component or like structure having improved
resistance to thermal fatigue cracking comprising a high
temperature resistant base metal selected from cobalt, nickel and
alloys in which cobalt or nickel is the largest single ingredient
and a highly chemically resistant diffusion coating over at least a
portion of the base metal and consisting essentially of 10 to 60
weight percent aluminum, 3 to 30 weight percent chromium, and from
0.6 to 1.4 parts by weight silicon per part of chromium but not
less than 2.5 weight percent silicon, said coating being diffused
into the base metal to a uniform depth of between 0.5 and 20 mils
and between and across intergranular boundaries in the base
metal.
2. Structure according to claim 1 in which intergranular spaces in
the base metal beyond the uniform coating depth are free of
aluminum introduced by the coating procedure.
3. Structure according to claim 1 in which said base metal in
nickel.
4. Structure according to claim 1 in which said base metal is
cobalt.
5. Structure according to claim 1 in which said base metal
comprises nickel as the largest single ingredient.
6. Structure according to claim 1 in which said base metal
comprises cobalt as the largest single ingredient.
7. Structure according to claim 1 in which said diffusion coating
contains from 5 to 35 weight per cent silicon.
8. Structure according to claim 7 in which the crystalline
arrangement of the diffusion coating layer is stable in air at
1,750.degree.F.
9. Structure according to claim 1 in which said diffusion coating
contains from 10 to 30 weight per cent aluminum.
10. The method of improving thermal fatigue cracking resistance in
jet engine components or like structure comprising a high
temperature resistant base metal selected from cobalt, nickel and
alloys in which cobalt or nickel is the largest single ingredient
which includes diffusing from 3 to 30 weight percent chromium and
10 to 30 weight percent aluminum into the structure surface at
surface temperatures above about 1,750.degree.C and for a time
sufficient to form a diffusion coating from 0.5 mil to 20 mils in
depth, and controlling the pattern of aluminum distribution through
the diffusion coating to eliminate voids in the coating and
intergranular incursions of aluminum into the base metal by
incorporating silicon in the diffusion coating in an amount between
5 and 35 weight percent and in a ratio between 0.6 and 1.4 parts by
weight of silicon per part of chromium.
11. Method according to claim 10 in which aluminum, chromium and
silicon are sequentially diffused into the structure surface.
12. Method according to claim 10 in which aluminum, chromium and
silicon are simultaneously diffused into the structure surface.
13. Method according to claim 10 in which the structure surface is
heated to not higher than 1,900.degree.F during diffusion of the
coating.
14. Method according to claim 10 in which the structure is immersed
into a diffusion pack composition containing 0.3 to 10 weight per
cent aluminum, 3 to 40 weight per cent chromium and 5 to 35 weight
per cent silicon for said diffusion.
15. Structure according to claim 1 in which said diffusion coating
contains from 8 to 20 weight percent silicon.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention b 731,631 entitled
This invention has to do with improvements in thermal fatigue
cracking resistance through diffusion coating of fabricated high
temperature resistant metal and alloy parts to substantially
increase the service life of such components and parts, in general
through a more highly uniform diffusion and distribution of coating
metals into the part base metal which may be nickel, cobalt or an
alloy in which nickel or cobalt is the largest single
ingredient.
While it is not widely recognized, it is nonetheless true that many
of the technological advances and scientific feats which continue
apace in modern times are largely dependent on progressive
betterment of metal. The advent and commercial acceptance of jet
aircraft transportation, for example, has been made possible by the
development of alloys able to withstand heat and chemical stresses
within a turbine engine. Further advances in this field, e.g., to
more powerful and/or smaller power plants will be made or not
depending on the availability of materials capable of meeting even
more stringent demands during their operating lifetime.
2. Prior Art
High temperature resistant alloys so called super-alloys, have been
refined through application, development and research to a high
state of suitability for their intended usages, e.g. as turbine
blades and nozzles for jet engines. Because of deficiencies in
chemical resistance of the super-alloys, it has been the practice
to surface modify the fabricated alloy component or part with a
more chemically resistant, albeit perhaps less temperature
resistant, metal composition. Temperature resistance herein refers
to mechanical strength or resistance to mechanical deterioration at
elevated temperatures, while chemical resistance refers to
resistance to chemical deterioration, or to corrosion. These
factors are, of course, highly interrelated, since one may be the
cause or occasion of the other. Longer service life is, of course,
the objective. Failure of the protective coating, undetected, may
result in irreparable damage to the alloy component or part,
particularly now that routine turbine engine inspection periods in
some instances are 5,000 hours or more.
It is common practice to diffuse aluminum and chromium into the
surface of a cobalt or nickel alloy component or part to form a
corrosion resisting coating. Such coatings have been shown to have
service lives between 1,000 and 2,500 hours per mil of thickness on
turbine blades in jet engines.
While such diffusion coatings have been a great advance in the art,
the service life of the coated parts is desirably even further
lengthened. It has been observed that thermal fatigue cracking
failure of the components, such as turbine blades and vanes may
occur at thin edges of the part and at relatively regularly spaced
intervals along the thin edge. Investigation of this failure
phenomenon has revealed that the character of the diffusion coating
in terms of uniformity of depth and distribution of coating
components is of extreme importance to thermal fatigue cracking
resistance and must be considered along with the anti-corrosion
properties of the coating itself. For example, studies of nickel
and cobalt base alloys which have been diffusion coated with
diffusion packs comprising aluminum with or without chromium have
revealed that the aluminum is usually maldistributed. Aluminum
diffuses preferentially along grain boundaries which are commonly
columnar in configuration and essentially normal to the thin
trailing edge of the part of the diffusion coated surface of the
superalloy part by entering the spaces between grains of the base
metal, herein referred to as intergranular spaces, which lie
angularly disposed to the advance of the diffusion coating. Not
only does the incursive presence of aluminum in these intergranular
spaces set up stresses which tend to propagate thermal fatigue
and/or cracking corrosion, but the subsequent diffusion of aluminum
from the grain boundaries leaves voids which may serve as
initiation sites for fatigue and/or corrosion. Thus, it will be
seen that the maldistribution of aluminum greatly foreshortens
service life of diffusion coated superalloys, by both creating
fatigue and corrosion attack initiation locations and by setting up
paths of stress for the rapid propagation of cracks and/or
corrosion into the base metal. The latter effect is pernicious
since the component or part may be damaged beyond recovery and need
to be replaced, unlike instances where only coating or surface
failure is encountered.
SUMMARY OF THE INVENTION
Accordingly, it is a major objective of the present invention to
provide nickel or cobalt metal or alloy structures for jet engines
and like purposes having coatings affording greatly improved
thermal fatigue cracking resistance and corrosion resistance for
extended service life, up to 5,000 hours, and more, per mil and
method of producing such structures.
It has now been discovered that increased service life in jet
engine component structures is realized from oxidation, halide and
sulfidation corrosion resistant coatings of aluminum and chromium,
diffused into the cobalt or nickel base metal, which have silicon
additionally diffused thereinto as hereinafter described, by virtue
of the increased uniformity of the coating and the substantial
elimination of voids in the coating or intergranular concentrations
of aluminum in the base metal. Further, the structures exhibit
greatly improved resistance to thermal fatigue cracking.
The achievement of longer periods of corrosion resistance and
fatigue cracking resistance by virtue of the incorporation of
silicon into an aluminum, chromium diffusion layer in a cobalt,
nickel or nickel or cobalt base alloy is highly surprising since
silicon coatings are glass brittle. The result is achieved because
of the improved distribution of the aluminum in the coating caused
by the presence of the hereinafter defined proportions of silicon.
Primary among advantages flowing from the use of silicon according
to the invention is a desirable alteration toward uniformity in the
wearing characteristics of the protective diffusion layer and away
from the void formation previously experienced with aluminum,
chromium diffusion coatings, which in the past has led to premature
failures, again owing to the uniform distribution of the aluminum,
avoiding localized absences and concentrations particularly in
intergranular spaces. In fact, in the structures according to the
invention, the intergranular spaces in the base metal beyond the
metallographically defined coating are free of aluminum introduced
by the coating procedure. Additionally, the diffusion layer is of
demonstrably finer grain, productive of better wear character. It
is also found that the new coated alloys have a blue coloration at
invention levels of silicon content, which color is a convenient
indication of adequate levels of silicon input into the alloy. Also
highly useful is a variation in wearing behavior of the
silicon-containing diffusion layer. This variation comprises a
readily perceptible roughening of the surface occurring at
approximately 60 percent of the service life of the protective
coating. Thus worn coatings which could fail prior to the next
regular inspection, can be timely replaced, avoiding destruction fo
the base metal and the structure itself.
In specific terms, the invention provides a jet engine component or
like structure having improved resistance to thermal fatigue
cracking comprising a high temperature resistant base metal having
a highly chemically resistant diffusion coating layer comprising
diffused, dispersed and well distributed chromium and aluminum,
together with silicon sufficient to maintain the dispersed
distribution fo the aluminum throughout the surface coating. The
corrosion resistant surface layer formed on the nickel or cobalt
based base metal, is generally from 0.5 to 10 mils up to 20 mils in
depth and desirably has a crystalline configuration stable at
1,750.degree.F. The silicon is distributed, by diffusion,
sequentially or simultaneously with the other metal components,
through the structure coating layer usually in a weight ratio,
relative to the chromium of 0.6 to 1.4. Chromium and aluminum
content may vary widely with accordingly different results. Useful
results may be obtained generally with diffusion coatings
containing in the range of 3 to 30 weight per cent chromium, 10 to
30 weight per cent aluminum and 5 to 35 weight per cent silicon and
in the above ratio to the chromium present. Diffusion coating or
surface impregnation of the alloy typically produces small
variations in composition with depth. In the present invention, the
outer portion of the surface layer may be relatively rich in
silicon in which case the blue coloration is quite pronounced.
The invention further contemplates a method imparting thermal
fatigue cracking resistance to these alloy structures which
includes diffusing aluminum and chromium into the alloy surface in
a manner providing a balanced distribution thereof i.e., a
distribution free of grain boundary localized concentrations of
aluminum and incorporating silicon into the alloy surface, to
maintain the balanced distribution of aluminum, either
sequentially, independently or simultaneously with the aluminum and
chromium diffusion, each of which may themselves be diffused in a
separate step. The diffusion is typically carried out in a manner
such that the alloy surface is heated to between 1,750.degree. and
1,900.degree.F in contact with a diffusion pack of suitable
composition, with regard to diffusion time and temperature, and
typically comprising in the range of 0.3 to 10 weight per cent
aluminum, 3 to 40 weight per cent chromium and 5 to 35 weight per
cent silicon; an activator such as halogen or halide and an inert
filler such as a polyvalent metal oxide.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Structure base metals which may be provided with thermal fatigue
improving, corrosion resistant coatings in accordance with the
invention include the high temperature resistant cobalt and nickel
and cobalt and nickel based alloys. The terms "cobalt-based" and
"nickel-based" herein refer to alloys in which cobalt or nickel,
respectively, is the largest single ingredient, in weight per cent,
although this is not necessarily a major weight portion of the
entire alloy. Thus, for example, suitable cobalt base alloys
include those composed by weight of cobalt (35-80 percent) and
tungsten 0-25 percent, chromium 0-40 percent, iron 0-20 percent,
and/or carbon 0-4 percent. Typical cobalt base alloys are given in
the Table below. Among suitable nickel base alloys are those
composed by weight of nickel 35-99.5, chromium 0-25, iron 0-20,
manganese 0-2, molydenum 0-20, cobalt 0-25, tungsten 0-5 as well as
0-20 of platinum, palladium, vanadium, aluminum, titanium,
tantalum, columbium, boron and zirconium. Typical nickel base
alloys are given in the Table below, in weight per cent.
##SPC1##
Preferred alloys for heat resistance contain from 50 to 70 weight
per cent of the base metal and appreciable amounts of metals such
as tungsten and molybdenum. These alloys contain no more than 10
weight per cent tantalum. The coatings taught herein are
particularly effective with these alloys. While not wishing
volatile be bound to any particular theory of operation, it is base
metals the addition of silicon in the recommended proportions to an
aluminum, chromium diffusion coating on these alloys interferes
with the conventional intermetallic formations i.e., formations of
nickel aluminides and cobalt aluminides and facilitates
incorporation (in addition to silicon and aluminum) of a
significant amount of chromium in the coating. This coating then
acts in a synergestic manner. During use exposure of the coating, a
tightly adherent, substantially impervious oxide layer is developed
on the diffusion coating, probably through spinel formations. It
has been observed that localized etching or corrosion of the
present surface coatings does not soon occur, indicating the
substantial absence of concentrations of easily degraded nickel
aluminides, which can be formed once the initial lattice
distribution of the aluminum in the surface layer is broken down.
It is believed too, that the silicon helps limit concentrating
redistribution of the aluminum, even after disruption of the
initial lattice arrangement, further contributing to improved wear
characteristics. Concentration of aluminum in intergranular spaces
is avoided along with the usual concomitant holes in the coating
layer which are initiation sites for corrosion and fatigue attack.
Metals such as tantalum which form volatile oxides at prospective
normal use temperatures do not provide useful base metals since the
absence of inherent chemical resistance such as is found in cobalt
and nickel base metals precludes practically usable structures
since a flaw free coating is rarely achieved and any flaw can cause
catastrophic failure in a tantalum base part, through progressive
oxidation and vaporization of the oxide.
The surface incorporation of aluminum, chromium and silicon into
the base alloy metal is accomplished by diffusion from a pack. Pack
diffusion is a well established metal surface treating technique
and basically comprises heating one or more of the to-be-diffused
metals in surface contact with the metal parts to be surface
modified at elevated temperatures and usually for relatively
extended periods in a suitable container such as a metal box.
Conventionally, and in the application of the diffusion metals of
this invention, an inert diluent is present in the box as is an
activator or transport compound. Diffusion is carried out in a
nonoxygen containing atmosphere.
The pack ingredients are relatively fine powders and may include,
as the inert diluent any of the refractory materials available in
powdered form, preferably about 50 to 350 U.S. mesh, e.g., various
aluminum compounds including clays and aluminum oxides as well as
zirconia and magnesia and other polyvalent metal oxides. The
activator is generally a halogen or halogen percursor compound.
Thus fluorine, chlorine, bromine and iodine per se and in salt
form, particularly alkali and alkaline earth metal and ammonium
salt forms from which they are readily releasable are useful as
activators.
The metal components of the pack composition may be and preferably
are elemental forms of the aluminum, chromium and silicon, suitably
reduced to U.S. mesh sizes of 60-350 mesh, but may be other
compounds similarly size reduced which released these elements
e.g., transition metal compounds thereof particularly the
ferro-compounds such as ferro-silicon.
The pack composition is widely variable and dependent on reaction
conditions used. In a broad sense, a pack of diluent, activator and
one metal diffusant in a wide range of proportions is suitable.
Where all diffusant metals are applied sequentially, the pack may
contain from 2% or less up to 70 percent by weight or more of the
metal to be diffused, a trace amount of activator e.g., 0.1-3
percent by weight and the balance diluent such as aluminum oxide.
Thus, useful pack compositions can contain 0-70 percent by weight
of the diffusant metals provided at least one such metal is present
in an amount of 2 percent by weight or greater. Where two or three
of the diffusant metals are simultaneously incorporated in the
alloy part surface, the pack composition will be adjusted
appropriately. Pack compositions thus may comprise by weight
silicon 8-35 percent, chromium 3-40 percent, aluminum 0.3-7
percent, activator 0.1-3 percent and diluent, the balance.
In preferred practice, the pack composition in finely divided form,
less than 100 mesh, is thoroughly mixed and fired in a treatment
retort at 1800.degree.F for 8 to 12 hours. After this initial "burn
out" cycle, pack additions of activator may be made followed by
packing parts to be treated in the composition disposed in a
diffusion retort and placed in a furnace for heating at above
1,750.degree.F for 8 to 12 and preferably 10 hours, and optimally
not above 1,900.degree.F for achieving most desirable crystalline
patterns in the surface layer.
The heated parts are noted to have a definite blue color which has
been found to be indicative of a surface sufficiently rich in
silicon to have extraordinary resistance to sulfur-salt corrosion
at elevated temperatures.
Surface layer compositions containing by weight 10-60 percent and
preferably 10-30 percent aluminum, 3-30 percent and preferably 4-20
percent chromium and 2.5-70 percent and preferably 8-20 percent
silicon may be obtained by variation of pack composition and
diffusion time and temperature, as will be apparent to those
skilled in the art.
In the present coatings the maintenance of a silicon/chromium ratio
between 0.6 and 1.4 has been found to confer substantial
performance improvements, specifically maximum service life through
reduced corrosion and enhanced thermal fatigue cracking
resistance.
The invention will be further described by the following Examples
in which all parts and percentages are by weight.
EXAMPLE 1
A pack having the following composition:
Silicon -- 35 percent
Chromium -- 40 percent
Aluminum -- 4 percent
Halogen activator -- 0.2 percent
Aluminum oxide q.s. to -- 100 percent
and having an average particle size less than 100 mesh was
thoroughly mixed, introduced into a treatment retort and fired at
1800.degree.F for 10 hours. The compound was then shifted and there
was incorporated therein an additional 0.2 percent halogen
activator. This composition was packed around nickel and
cobalt-based parts to be coated in a metal box having provision for
air exclusion. These parts contained between 50 and 70 percent of
nickel or cobalt respectively. After retorting in a furnace at
1,800.degree.F for 10 hours the parts were removed from the pack
and cooled. The blue color was striking. Analysis showed 23 percent
aluminum, 6 percent silicon and 4.5 percent chromium.
EXAMPLE 2
The procedure of Example 1 was duplicated in treating parts of an
alloy of nickel which contained less than 10 parts each of
tungsten, aluminum, cobalt, molybdenum, titanium and tantalum and
trace amounts of boron and zirconium. The composition of the
coating was the same as in Example 1.
Control I
The procedure of Example 1 is followed using pure tantalum parts,
with the same coating composition as in the Example.
Testing
The diffusion coated parts were tested in an erosion test rig which
exposed them to combustion gases of an oil burner fed jet fuel,
artificial sea water and sulfur, to simulate on an accelerated
basis the corrosive environment of a gas turbine engine burning
high sulfur fuel in a marine environment. In a series of tests the
coated parts of Examples 1 and 2 were found to withstand exposures
of thirty to fifty hours per mil of coating thickness. Since the
erosion rig test is about 100 times more rigorous than actual use
conditions, part life is thus 3,000 to 5,000 hours per mil of
coating thickness. The Control I parts experience catastrophic
failure after only a few hours, as the tantalum oxide
volatilizes.
The mode of erosion too is interesting in that wear is generally
uniform across the part surface and not localized. Moreover, at
about 60 percent wear a detectable roughening of the surface
occurs, which may be used as a guide to determine desirability of
recoating a particular part; to avoid part failure or loss of
repairability during subsequent service.
Control II
The procedure of Example 1 is duplicated but omitting the silicon
from the pack composition. On testing in the erosion rig pitting
due to localized etching occurs in as little as 10 hours followed
by rapid failure as the alloy part is exposed through the coating
to the corrosive gases.
The parts obtained in Example 1 were subjected to thermocycling to
evaluate thermal fatigue cracking resistance. In this test the
parts are subjected to rapid thermal cycling at temperatures up to
1,900.degree.F and back to ambient temperature to produce thermal
stresses in the parts. Typically the parts of Example 1 are free of
cracking at up to between 900 and 1,000 cycles. Parts of the same
base metal and diffusion coated, but without use of silicon
(Control II) show cracking at only 225 to 250 cycles.
Control III
The procedure of Example 2 was duplicated but employing a pack
composition as follows:
Silicon -- 8.5 percent
Chromium -- 3.0 percent
Aluminum -- 0.5 percent
Halogen activator of Example 2 -- 0.2 percent
Aluminum oxide q.s. to -- 100 percent
Following diffusion coating elemental analysis of the coating
showed aluminum 19 percent, chromium 4.5 percent and silicon 9.5
percent. The ratio of silicon to chromium was above 2 well beyond
the highest recommended ratio of 1.4. Testing of the coated parts
in the erosion rig shows a service life of 10-12 hours per mil
which is well below the life obtained with the desirable Si/Cr
ratios mentioned above. Nonetheless the coating wears evenly so
that early pitting and other localized failures are not a
predominant factor indicating that the silicon at these higher
levels relative to chromium is operating to improve the performance
characteristics of the coating, although not to the same extent as
preferred Si/Cr ratios. Use in the above pack of 1.7 percent
aluminum and 11.6 percent chromium will provide coatings with
shorter service life due apparently to a nonlinearity in the
relation of service life to aluminum over the range including 1.7
percent.
Control IV
The procedure of Example 2 is duplicated, but with quite low
amounts of silicon in the pack composition such as might result
from a silicon supply only from a silicide coating on an alloy part
or from silicon associated as an impurity in one of the pack
components. A dissusion coating containing between 1.5 and 2
percent silicon by elemental analysis is obtained. Testing in the
erosion rig shows localized etching and resultant premature
failure, i.e., only a few hours per mil, typical of
aluminum-chromium products containing no silicon.
The present invention is useful in the formation of various
structures intended to be used in high temperature, highly
corrosive environments. Thus, turbine engine parts such as nozzle
guide vanes, blades or buckets, fuel nozzle covers, engine shrouds
and valves for steam turbines are typical structures.
* * * * *