Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
COATING SYSTEM FOR HIGH TEMPERATURE
STAINLESS STEEL
(i) Field of the Invention
The present invention relates to coating systems for the generation of a
protective
surface for carbon steel and stainless steel and, more particularly, relates
to the provision
of metal alloy coatings on the internal wall surfaces of high-temperature
stainless steel
tubes and fittings to produce a surface that provides corrosion resistance
erosion
resistance, and reduces the formation of catalytic coking in hydrocarbon
processing such
as in olefin production and in direct reduction of ores. The protective system
also has
application on carbon steels. For example, in downhole oil and gas
applications, the
protective system enhance erosion and corrosion properties compared to carbon
steel
commonly used.
(ii) Description of the Related Art
Stainless steels are a group of alloys based on iron, nickel and chromium as
the
major constituents, with additives that can include carbon, tungsten, niobium,
titanium,
molybdenum, manganese, and silicon to achieve specific structures and
properties. The
major types are known as martensitic, ferritic, duplex and austenitic steels.
Austenitic
stainless steel generally is used where both high strength and high corrosion
resistance is
required. One group of such steels is known collectively as high temperature
alloys
(HTAs) and is used in industrial processes that operate at elevated
temperatures generally
above 650 C and extending to the temperature limits of ferrous metallurgy at
about
1150 C. The major austenitic alloys used have a composition of iron, nickel or
chromium in the range of 18 to 42 wt.% chromium, 18 to 48 wt.% nickel, balance
iron
and other alloying additives. Typically, high chromium stainless steels have
about 31 to
38 wt% chromium and low chromium stainless steels have about 20 to 25 wt%
chromium.
The bulk composition of HTAs is engineered towards physical properties such as
creep resistance and strength, and chemical properties of the surface such as
corrosion
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resistance. Corrosion takes many forms depending on the operating environment
and
includes carburization, oxidation and sulfidation. Protection of the bulk
alloy is often
provided by the surface being enriched in chromium oxide (chromia). The
specific
compositions of the alloys used represent an optimization of physical
properties (bulk)
and chemical properties (surface). The ability of addressing the chemical
properties of
the surface through a surface alloy, and physical properties through the bulk
composition, would provide great opportunities for improving materials
performance in
many severe service industrial environments.
Surface alloying can be carried out using a variety of coating processes to
deliver
the right combination of materials to the component's surface at an
appropriate rate.
These materials would need to be alloyed with the bulk matrix in a controlled
manner
that results in a microstructure capable of providing the pre-engineered or
desired
benefits. This would require control of the relative interdiffusion of all
constituents and
the overall phase evolution. Once formed, the surface alloy can be activated
and
reactivated, as required, by a reactive gas thermal treatment. Since both the
surface
alloying and the surface activation require considerable mobility of atomic
constituents
at temperatures greater than 700 C, HTA products can benefit most from the
procedure
due to their designed ability of operating at elevated temperatures. The
procedure can
also be used on products designed for lower operating temperatures, but may
require a
post heat treatment after surface alloying and activation to reestablish
physical
properties.
Surface alloys or coating systems can be engineered to provide a full range of
benefits to the end user, starting with a commercial base alloy chemical
composition and
tailoring the coating system to meet specific performance requirements. Some
of the
properties that can be engineered into such systems include: superior hot gas
corrosion
resistance (carburization, oxidation, sulfidation); controlled catalytic
activity; and hot
erosion resistance.
Two metal oxides are mainly used to protect alloys at high temperatures,
namely
chromia and alumina, or a mixture of the two. The compositions of stainless
steels for
high temperature use are tailored to provide a balance between good mechanical
properties and good resistance to oxidation and corrosion. Alloy compositions
which
can provide an alumina scale are favoured when good oxidation resistance is
required,
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whereas compositions capable of forming a chromia scale are selected for
resistance to
hot corrosive conditions. Unfortunately, the addition of high levels of
aluminum and
chromium to the bulk alloy is not compatible with retaining good mechanical
properties
and coatings containing aluminum and/or chromium normally are applied onto the
bulk
alloy to provide the desired surface oxide.
One of the most severe industrial processes from a materials perspective is
the
manufacture of olefins such as ethylene by hydrocarbon steam pyrolysis
(cracking).
Hydrocarbon feedstock such as ethane, propane, butane or naphtha is mixed with
steam
and passed through a furnace coil made from welded tubes and fittings. The
coil is
heated on the outerwall and the heat is conducted to the innerwall surface
leading to the
pyrolysis of the hydrocarbon feed to produce the desired product mix at
temperatures in
the range of 850 to 1150 C. An undesirable side effect of the process is the
buildup of
coke (carbon) on the innerwall surface of the coil. There are two major types
of coke:
catalytic coke (or filamentous coke) that grows in long threads when promoted
by a
catalyst such as nickel or iron, and amorphous coke that forms in the gas
phase and
plates out from the gas stream. In light feedstock cracking, catalytic coke
can account
for 80 to 90% of the deposit and provides a large surface area for collecting
amorphous
coke.
The coke can act as a thermal insulator, requiring a continuous increase in
the
tube outerwall temperature to maintain throughput. A point is reached when the
coke
buildup is so severe that the tube skin temperature cannot be raised any
further and the
furnace coil is taken offline to remove the coke by burning it off (decoking).
The
decoking operation typically lasts for 24 to 96 hours and is necessary once
every 10 to 90
days for light feedstock furnaces and considerably longer for heavy feedstock
operations.
During a decoke period, there is no marketable production which represents a
major
economic loss. Additionally, the decoke process degrades tubes at an
accelerated rate,
leading to a shortened lifetime. In addition to inefficiencies introduced to
the operation,
the formation of coke also leads to accelerated carburization, other forms of
corrosion,
and erosion of the tube innerwall. The carburization results from the
diffusion of carbon
into the steel forming brittle carbide phases. This process leads to volume
expansion and
the embrittlement results in loss of strength and possible crack initiation.
With
increasing carburization, the alloy's ability of providing some coking
resistance through
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the formation of a chromium based scale deteriorates. At normal operating
temperatures,
half of the wall thickness of some steel tube alloys can be carburized in as
little as two
years of service. Typical tube lifetimes range from 3 to 6 years.
It has been demonstrated that aluminized steels, silica coated steels, and
steel
surfaces enriched in manganese oxides or chromium oxides are beneficial in
reducing
catalytic coke formation. AlonizingTM, or aluminizing, involves the diffusion
of
aluminum into the alloy surface by pack cementation, a chemical vapour
deposition
technique. The coating provides an alumina scale which is effective in
reducing catalytic
coke formation and protecting from oxidation and other forms of corrosion. The
coating
is not stable at temperatures such as those used in ethylene furnaces, and
also is brittle,
exhibiting a tendency to spall or diffuse into the base alloy matrix.
Generally, pack
cementation is limited to the deposition of one or two elements, the co-
deposition of
multiple elements being extremely difficult. Commercially, it is generally
limited to the
deposition of only a few elements, mainly aluminum. Some work has been carried
out
on the codeposition of two elements, for example chromium and silicon. Another
approach to the application of aluminum diffusion coatings to an alloy
substrate is
disclosed in U. S. Patent 5,403,629 issued to P. Adam et al. This patent
details a process
for the vapour deposition of a metallic interlayer on the surface of a metal
component,
for example by sputtering. An aluminum diffusion coating is thereafter
deposited on the
interlayer.
Alternative diffusion coatings have also been explored. In an article in
"Processing and Properties" entitled "The Effect of Time at Temperature on
Silicon-Titanium Diffusion Coating on IN738 Base Alloy" by M. C. Meelu and M.
H.
Lorretto, there is disclosed the evaluation of a Si-Ti coating, which had been
applied by
pack cementation at high temperatures over prolonged time periods.
The benefits of aluminising an MCrA1X coating on superalloys for improved
oxidation and corrosion resistance have been previously well documented.
European
Patent EP 897996, for example, describes the improvement of high temperature
oxidation resistance of an MCrAlY on a superalloy by the application of an
aluminide
top coat using chemical vapour deposition techniques. Similarly, U.S. Patent
3,874,901
describes a coating system for superalloys including the deposition of an
aluminum
overlay onto an MCrAlY using electron beam-physical vapour deposition to
improve the
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hot corrosion and oxidation resistance of the coating by both enriching the
near-surface
of the MCrAlY with aluminum and by sealing structural defects in the overlay.
Both of
these systems relate to improvement of oxidation and/or hot corrosion
resistance
imparted to superalloys by the deposition of an MCrAlY thereon. These
references do
not relate to improvement of anticoking properties or corrosion resistance of
high
temperature stainless steel alloys used in the petrochemical industries.
A major difficulty in seeking an effective coating is the propensity of many
applied coatings to fail to adhere to the tube alloy substrate under the
specified high
temperature operating conditions in hydrocarbon pyrolysis furnaces.
Additionally, the
coatings lack the necessary resistance to any or all of thermal stability,
thermal shock,
hot erosion, carburization, oxidation and sulfidation. A commercially viable
product for
olefins manufacture by hydrocarbon steam pyrolysis and for direct reduction of
iron ores
must be capable of providing the necessary coking and carburization resistance
over an
extended operating life while exhibiting thermal stability, hot erosion
resistance and
thermal shock resistance.
Downhole oil and gas drilling, production and casing tube strings and tools
conventionally are fabricated from carbon steels which are prone to corrosion
and to
erosion under hostile subterranean environments. There accordingly is a need
for
protective surface coatings on such carbon steel components.
Plasma transferred arc surfacing (PTAS), as disclosed for example in U.S.
Patents 4,878,953 and 5,624,717, is a technique used to apply coatings of
different
compositions and thickness onto conducting substrates. The material is fed in
powder or
wire form to a torch that generates an arc between a cathode and the work-
piece. The arc
generates plasma that heats up both the powder or wire and the surface of the
substrate,
melting them and creating a liquid puddle, which on solidification creates a
welded
coating. By varying the feed rate of material, the speed of the torch, its
distance to the
substrate and the current that flows through the arc, it is possible to
control thickness,
microstructure, density and other properties of the coating (P. Harris and
B.L. Smith,
Metal Construction 15 (1983) 661-666). The technique has been used in several
fields to
prevent high temperature corrosion, including surfacing MCrAIYs on top of
nickel based
superalloys (G.A. Saltzman, P. Sahoo, Proc. IV National Thermal Spray
Conference,
1991, pp 541-548), as well as surfacing high-chromium nickel based coatings on
exhaust
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valves and other parts of internal combustion engines cylinders (Danish Patent
165,125,
U.S. Patent 5,958,332).
A process entitled Controlled Composition Reaction sintering Process for
Production of MCrAlY coatings disclosed in Technical Report AFML-TR-76-91 by
Air
Force Materials Laboratory and evaluated in a report entitled Development and
Evaluation of Process for Deposition of Ni/Co-Cr-A1Y (McrA1Y) Coatings for Gas
Turbine Components disclosed in Technical Report AFML-TR-79-4097 by Air Force
Materials Laboratory performed by the Solar Division of International
Harvester
Company Research Laboratory, San Diego, California, has been used to produce a
MCrAlY type coating on super-alloys. Gas turbine blades were coated with
atomized
MCrY-alloy using slurry containing an organic binder. The coated turbine
blades were
than embedded in a pack consisting of aluminum oxide (A1203), aluminum powder
(Al),
and ammonium chloride (NH4Cl). The pack was heated in a controlled atmosphere
under controlled time and temperature conditions to produce MCrAIY-coatings
that
resembled coatings deposited by a standard PVD process. The major problem with
this
process when applied to gas turbines is that the thickness of the coating
varies and is
difficult to control. In addition, the Al is added to the coating via pack
aluminizing CVD
process, which is environmentally unfriendly.
Summary of the Invention
It is therefore a principal object of the present invention to impart
beneficial
properties to carbon steel and stainless steel through surface alloying to
substantially
improve the surface properties such as by eliminating or reducing the
catalytic formation
of coke on the internal surfaces of tubing, piping, fittings and other
ancillary furnace
hardware by minimizing the number of sites for catalytic coke formation and by
improving the quality of alumina scale on surface alloy coatings deposited on
such
steels. The alloy coatings of the invention are particularly suited for use on
high
temperature stainless steel for the manufacture of olefins by hydrocarbon
steam
pyrolysis, typified by ethylene production, the manufacture of other
hydrocarbon-based
products in the petrochemical industries, and in the direct reduction of ores
such as
typified by the direct reduction of iron oxide ores to metallic iron in carbon-
containing
atmospheres.
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When high temperature stainless steel tubes used in ethylene furnaces were
coated with this material, an improvement on the anti-coking, anti-
carburization and
resistance to hot erosion properties of the tubes were observed.
It is another object of the invention to increase the carburization resistance
of
HTAs used for tubing, piping, fittings and ancillary furnace hardware whilst
in service.
It is a further object of the invention to augment the longevity of the
improved
performance benefits derived from the surface alloying under commercial
conditions by
providing thermal stability, hot erosion resistance and thermal shock
resistance.
In accordance with the present invention there are provided four embodiments
of
surface alloy structures generatable from the formation of a MCrA1X alloy
coating
directly on a high temperature stainless steel alloy substrate or onto an
intermediary
interlayer, followed by heat treatment to establish the coating microstructure
and to
metallurgically bond the coatings to the substrate. When tubes used in
ethylene furnaces
were treated in this way, an improvement on the anti-coking, anti-
carburization and
resistance to hot erosion properties of the tubes were observed.
The first embodiment of surface alloy structure of the invention comprises the
application of a MCrA1X (where M = nickel, cobalt, iron or a mixture thereof
and X =
yttrium, hafnium, zirconium, lanthanum, scandium or combination thereof)
coating
material onto a high temperature stainless steel alloy substrate and an
appropriate heat
treatment of the MCrA1X coating and the substrate.
The second embodiment of surface alloy structure of the invention comprises
depositing a layer of aluminum, or aluminum alloyed with silicon, or with
silicon and at
least one of chromium and titanium, on the said MCrA1X coating and heat
treating the
composite of aluminum or aluminum alloy, MCrA1X coating and substrate to
establish
the coating microstructure.
The third embodiment of surface alloy structure of the invention comprises
depositing an interlayer onto the high temperature stainless steel alloy
substrate beneath
the MCrA1X coating. The nitrogen and carbon contents of standard high
temperature
stainless steel alloys can lead to the formation of undesirable brittle
nitride and carbide
layers at the coating/substrate interface. The deposition of an interlayer,
containing a
stable nitride former, beneath the MCrA1X coating will act to disperse nitride
precipitates. This is more desirable than a continuous nitride layer. The
interlayer will
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also act to disperse carbide precipitates. Again this is more desirable than a
continuous
carbide layer at the coating/substrate interface. The interlayer will also
increase the
adherence of the McrA1X coating to the substrate and decreases the level of
surface
preparation necessary for coating deposition. The MCrA1X alloy is deposited
onto the
interlayer, an aluminum layer is deposited onto the MCrA1X coating, and the
coating
system is heat-treated to diffuse aluminum into the MCrA1X coating and to
metallurgically bond the layers together and to the substrate and to achieve a
desired
metallurgical microstructure.
The fourth embodiment of surface alloy structure of the invention comprises
depositing of blended powder composition to produce a desired MCrA1XT-alloy
via a
reactive sintering process directly onto the substrate surface.
Each of the above embodiments optionally is pre-oxidized to form a protective
outer layer of predominantly y- alumina. The y- alumina layer is highly
effective at
reducing or eliminating catalytic coke formation. These surface alloys are
compatible
with high temperature commercial processes at temperatures of up to 1150 C
such as
encountered in olefin manufacturing by hydrocarbon steam pyrolysis typified by
ethylene production.
In its broad aspect, the method of the invention for providing a protective
and
inert coating to carbon steel and high temperature stainless steel substrates
at
temperatures up to 11500C comprises the formation onto a steel substrate of a
continuous coating of a MCrA1X alloy, where M = nickel, cobalt or iron or
mixture
thereof and X = yttrium, hafnium, zirconium, lanthanum, scandium or
combination
thereof, having 0 to 40 wt% chromium, about 3 to 30 wt% aluminum and up to
about 5.0
wt%, preferably up to about 3 wt%, and more preferably, about 0.25 to 1.5 wt%
of
yttrium, hafnium, zirconium, lanthanum, scandium or combination thereof, the
balance
M. The MCrAIX alloy may be deposited by a variety of methods including but not
limited to thermal spray, plasma transferred arc, physical vapour deposition
and slurry
coating techniques. Preferably, the overlay and substrate are heat-treated at
a soak
temperature in the range of about 1000 to 1200 C for at least 10 minutes, more
preferably about 20 minutes to 24 hours.
The coating is deposited in a thickness of about 50 to 500 gm, preferably in a
thickness about 120 to 250 m, and most preferably about 150 m, such as by
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magnetron sputtering physical vapour deposition onto the substrate at a
temperature in
the range of about 200 to 1000 C, preferably at about 450 C, and the coating
and
substrate heated to a desired soak temperature. Preferably, the MCrA1X is
NiCrAlY and
has, by weight, about 12 to 22% chromium, about 8 to 15% aluminum and about
0.8 to
1% yttrium, the balance nickel.
The high temperature stainless steel substrate comprises, by weight, 18 to 38%
chromium, 18 to 48% nickel, the balance iron and alloying additives, and
preferably is a
high chromium stainless steel having 31 to 38 wt% chromium or a low chromium
steel
having 20 to 25 wt% chromium.
In accordance with another embodiment of the invention, a high temperature
stainless steel substrate having a continuous coating of said MCrAIX alloy
having a
thickness of about 120 to 250 m is aluminized by depositing a layer of
aluminum
thereon, and the coating with aluminum thereon and the substrate are heat-
treated at a
soak temperature in the range of about 1000 to 1200 C for about 20 minutes to
24 hours
in an oxygen-free atmosphere to metallurgically bond the coating to the
substrate and to
establish a multiphased microstructure. The aluminum layer preferably is
deposited on
the coating in a thickness up to about 50% of the thickness of the coating,
preferably up
to about 20% of thickness of the coating, such as by magnetron sputtering
physical
vapour deposition at a temperature in the range of about 200 to 500 C,
preferably at
about 300 C, and the coating with aluminum layer and substrate heated to the
soak
temperature.
In accordance with a further embodiment of the method of the invention, a
continuous interlayer is first deposited on the substrate before the coating
to disperse the
formation of nitride or carbide layers at the coating/substrate interface. An
effective
interlayer is comprised of about 35 to 45 wt% aluminum, a total of about 5 to
20 wt% of
at least one of titanium or chromium, and 40 to 55 wt% silicon, preferably
about 35 to 40
wt% aluminum, about 5 to 15 wt% titanium and about 50 to 55 wt% silicon, is
deposited
onto a high temperature stainless steel substrate as described in U.S. Patent
No.
6,093,260 issued July 25, 2000, a continuous MCrA1X alloy coating is deposited
onto the
diffusion coating, and an aluminum layer is deposited onto the MCrAIX alloy
coating.
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The interlayer preferably is deposited by physical vapour deposition at a
temperature in the range of 400 to 600 C or 800 to 900 C, preferably at either
450 or
850 C. Thermal spray deposition techniques also may be used. The interlayer is
then
heated to a soak temperature at a rate of temperature rise of at least 5
Celsius
degrees/minute, preferably at a rate of 10 to 20 Celsius degrees/minute, to
establish the
coating microstructure. The MCrA1X coating, and preferably an aluminum layer,
are
deposited such as by physical vapour deposition onto the interlayer and then
heat-treated
to establish the multiphased microstructure and to metallurgically bond the
coating
system.
The systems subsequently can be heated in an oxygen-containing atmosphere at a
temperature above about 1000 C, preferably in the range of above 1000 C to
1160 C, in
a consecutive step or in a separate later step for a time effective to form a
surface layer of
oo- alumina thereon.
The interlayer is deposited in a thickness of about 20 to 100 m and
preferably in
a thickness of about 20 to 60 m. The interlayer is heat-treated at a soak
temperature in
the range of about 1030 to 1150 C for a time effective to form a diffusion
barrier
between the base alloy and enrichment pool containing intermetallics of
silicon and one
or more of titanium or aluminum and the base alloying elements. Preferably,
the
interlayer contains about 6 to 10 wt% silicon, 0 to 5 wt% aluminum, 0 to 4 wt%
titanium
and about 25 to 50 wt% chromium, the balance iron and nickel and any base
alloying
elements.
An alternative process for creating a similar coating system is the deposition
of
the interlayer, MCrAIX alloy coating, and optionally the aluminum layer in
sequence,
and heat-treating in an inert atmosphere at a soak temperature in the range of
about 1030
to 1160 C to establish the microstructure and to bond the coatings.
It is therefore a further object of the present invention to optionally
provide a
surface alloy MCrAIX coating on HTAs by a single process step without a
separate heat
treatment.
In its broad aspect, this embodiment of a method of the invention for
providing a
protective and inert coating to high temperature stainless steels comprises
providing a
protective and inert coating on high temperature stainless steel comprising
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metallurgically bonding a continuous coating of a MCrA1X alloy, where M =
nickel,
cobalt or iron or mixture thereof and X = yttrium, hafnium, zirconium,
lanthanum,
scandium or combination thereof, having about 0 to 40 wt% chromium, preferably
about
to 25 wt% chromium, about 3 to 30 wt% aluminum, preferably about 4 to 20 wt%
5 aluminum, and up to about 5 wt% X, preferably up to about 3 wt% X, more
preferably
0.25 to 1.5 wt% X, the balance M, by plasma transferred arc deposition of the
coating
onto a high temperature stainless steel substrate. The coating is deposited in
a thickness
of about 20 m to 6000 gm, preferably 50 to 2000 gm, more preferably 80 to 500
m
onto the substrate.
10 The MCrA1X may be FeCrAlY having 0 to 25 wt% chromium, about 3 to 40
wt% aluminum, up to about 3 wt% yttrium, and the balance substantially iron.
The MCrA1X preferably is NiCrAlY and has, by weight, about 12 to 25%
chromium, about 4 to 15% aluminum and about 0.5 to 1.5% yttrium, the balance
nickel.
In accordance with this preferred embodiment of the invention, the deposition
of
a dense, anti-coking NiCrAIY alloy coating in a single step on a HTA tube by
plasma
transferred arc deposition produces a gradual metallurgical bond between the
alloy
coating and substrate. The desired final thickness of the coating is between
about 0.02
and 6 mm thick. The preferred thickness is in the range of 80 to 500 m in
order to keep
powder costs reasonable and to not unduly decrease the inner diameter of the
tube.
The NiCrAIY alloy coating provides a source of aluminum to provide an oo-
alumina based layer at the surface thereof by introducing an oxygen-containing
gas such
as air at a temperature above about 1000 C upon heating of the substrate and
coating in a
gaseous oxidizing atmosphere such as air at a temperature above 1000 C in a
separate
step, or during commercial use by the introduction of or presence of an
oxygen-containing gas at operating temperatures above about 1000 C. The more
complete the alumina scale the better the anticoking and anti-corrosion
performance.
Enhanced properties can be therefore sometimes be achieved by a further
aluminizing
step.
In accordance with another embodiment of the invention, however, the high
temperature stainless steel substrate having a continuous coating of said
MCrA1X alloy
with a thickness of about 50 to 2000 gm, preferably about 80 to 500 m, may be
aluminized by depositing a layer of aluminum, or aluminum alloyed with up to
about 60
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wt%, preferably up to about 15 wt%, silicon, or at least about 20 wt% aluminum
alloyed
with up to about 60 wt% silicon and up to a total of about 30 wt% of at least
one of
chromium and titanium, on the plasma transfer arc coating in a thickness up to
about
50% of the coating thickness, preferably about 20% of the coating thickness,
such as by
thermal spray or magnetron sputtering physical vapour deposition. The system
can be
heated in an oxygen-containing atmosphere in a consecutive step or in a
separate later
step for a time effective to form a surface layer of y- alumina thereon.
It is still a further object of the present invention to provide an MCrA1X
alloy
additionally having silicon and/or a T element selected from the group
consisting of
tantalum, titanium, platinum, palladium, rhenium, molybdenum, tungsten,
niobium, or
combination thereof, to enhance the coating properties.
In this aspect of the invention, a MCrA1XSiT alloy is provided in which M
nickel, cobalt, iron or mixture thereof, X = yttrium, hafnium, zirconium,
lanthanum,
scandium, or mixture thereof, and T = tantalum, titanium, platinum, palladium,
rhenium,
molybdenum, tungsten, niobium, or combination thereof, having about 0 to 40
wt%
chromium, about 3 to 30 wt% aluminum, up to about 5 wt% X, 0 to 40 wt%
silicon, and
up to about 10 wt% T, the balance M. Preferably the MCrA1XSiT alloy has about
10 to
wt% chromium, 4 to 20 wt% aluminum, up to 3 wt% X, up to 35 wt% silicon and up
to 10 wt% T. More preferably, the X is present in amount of 0.25 to 1.5 wt%,
silicon is
20 present in amount up to 15 wt% and the T is present in amount of 0.5 to 8.0
wt%, most
preferably T in the amount of 0.5 to 5.0 wt%.
In a preferred embodiment, a MCrA1XSi alloy coating comprising 22 wt% Cr, 10
wt% Al, 1 wt% Y and 3 wt% Si, the balance Ni, promoted a Cr-carbide layer at
the
coating/substrate interface which functioned as a diffusion barrier effective
to retain
25 aluminum within the coating. The presence of the silicon in the MCrAIX
coating also
improved a Cr-based scale produced by the overlay coating.
And a still further object of the invention is the application of a blended
powder
slurry composition to a substrate to produce a desired MCrA1X or MCrAlXSiT.
In accordance with a preferred embodiment of this aspect of the invention, a
mixture of two or more powders of the constituents of a MCrA1X or MCrAIXSiT
are
blended with an effective amount of a binder to adherently coat a workpiece,
and the
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workpiece with MCrA1X or MCrA1XSiT coating is heated to a temperature for
reaction
sintering of the coating and adherent bonding of the coating onto the
workpiece.
This method of the invention for providing a protective and inert coating to
carbon steel and stainless steel at temperatures up to 1150 C can comprise
depositing
onto a steel substrate and metallurgically bonding thereto a continuous
overlay coating of
a MCrA1XSi alloy, where M = nickel, cobalt or iron or mixture thereof and X =
yttrium,
hafnium, zirconium, lanthanum or combination thereof, having about 0 to 25 wt%
chromium, about 3 to 40 wt% aluminum, about 0 to 35 wt% silicon, and up to
about 5.0
wt%, preferably about 0.25 to 1.5 wt% of yttrium, hafnium, zirconium,
lanthanum,
scandium or combination thereof, the balance being a mininum of 40 wt% M. The
overlay coating may be deposited by a variety of methods including but not
limited to
physical vapour deposition (PVD), thermal spray, plasma transferred arc, and
slurry
coating techniques with reaction sintering occurring simultaneously with
deposition or
following deposition. In the case where reaction sintering does not occur
during
deposition, the overlay coating and substrate are heat-treated subsequently at
a soak
temperature in the range of about 500 to 1200 C for at least about 10 minutes
to initiate
reaction sintering.
The inclusion of silicon in the blended powder produces lower melting point
constituents during the reaction sintering process, thereby allowing the
molten alloy to
wet the surface of the substrate and to produce an effective diffusion bond
between the
coating and the substrate. The silicon additions also are believed to prevent
the
formation of brittle carbides at the coating/substrate interface. At silicon
concentration
of 6 wt% or higher, the silicon dissolves chromium carbides formed in the
substrate and
re-precipitate these randomly as the silicon concentration falls below 6 wt%
due to
silicon diffusion into the substrate.
It is preferred to pre-react certain of the constituents with each other, such
as by
atomizing chromium, aluminum and silicon to form a CrAlSi powder prior to
blending
with nickel, NiCr, or NiAI powders, or combination thereof. Pre-reacting of
powders
retards the rate of exothermic reaction of the powders and reduces the amount
of heat
evolved during reaction sintering. The coated workpiece is heated to a
temperature of at
least about 500 C to 1100 C to initiate reaction sintering of the coating on
the
workpiece substrate and the temperature is increased up to 12000C to provide a
CA 02614962 2007-11-26
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continuous impermeable coating bonded to the substrate without a sharp
dividing line
between the coating and the substrate and to provide random distribution of
aluminum
nitrides at the coating/substrate interface.
In accordance with another embodiment of the present invention, the coating is
deposited in a thickness of about 50 to 6000 gm, preferably in a thickness
about 120 to
500 m, more preferably 150 to 350 m, where the MCrA1XSi is NiCrAlYSi blended
powder and has up to 20 wt% chromium, about 4 to 20 wt% aluminum, about 5 to
20
wt% silicon, and about 0.5 to 1.5 wt% yttrium, the balance being a minimum of
40 wt%
nickel.
The high temperature stainless steel substrate comprises, by weight, 18 to 38%
chromium, 18 to 48% nickel, the balance iron and alloying additives, and
preferably is a
high chromium stainless steel having 31 to 38 wt% chromium or a low chromium
steel
having 20 to 25 wt% chromium. For ethylene furnace applications the workpiece
substrate preferably is high temperature stainless steel.
In accordance with another embodiment of the invention, a high temperature
stainless steel substrate, continuously surface alloyed with MCrA1XSi alloy by
reaction
sintering within a thickness of about 150 to 500 m is aluminized by
depositing a surface
layer of aluminum, aluminum alloy containing up to 60 wt%, preferably up to 15
wt%,
silicon, or aluminum alloy containing up to 60 wt% silicon, a total of up to
30 wt% of at
least one of chromium and titanium, the balance at least about 20 wt%
aluminum,
thereon and heat-treating at a soak temperature in the range of about 1000 to
1160 C for
at least about 10 minutes preferably in an oxygen-free atmosphere to establish
a
multiphased microstructure. The aluminum or aluminum alloy surface layer
preferably
is deposited on the overlay in a thickness up to about 50%, preferably up to
about 20%,
of the MCrA1XSi thickness such as by magnetron sputtering physical vapour
deposition
at a temperature in the range of about 200 to 500 C, preferably at about 300
C, and the
surface alloyed substrate with aluminum overlayer is heated to the soak
temperature.
The systems subsequently can be heated in an oxygen-containing atmosphere at a
temperature above about 1000 C, preferably in the range of above 1000 C to
1160 C, in
a consecutive step or in a separate later step for a time effective to form a
surface layer of
oo- alumina thereon.
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Brief Description of the Drawin2s
The process of the invention and the products produced thereby will be
described
with reference to the accompanying drawings in which:
Figure 1 is a photomicrograph of a cross-section NiCrAlY coating deposited on
a
stainless steel substrate;
Figure 2 is a photomicrogaph of the NiCrAlY coating shown in Figure 1 after
heat-treatment;
Figure 3 is a photomicrograph of a cross-section of a NiCrAlY coating with an
aluminum layer deposited thereon;
Figure 4 is a photomicrograph of the NiCrAlY coating with aluminum layer after
heat-treatment;
Figure 5 is a photomicrograph of a diffusion coating deposited on a stainless
steel
substrate with a NiCrAlY coating deposited on the diffusion coating and
an aluminum layer deposited on the overlay coating;
Figure 6 is a photomicrograph of the composite coating shown in Figure 5 after
heat-treatment;
Figure 7 is a photomicrograph of an interface between NiCrAlY coating
deposited
by plasma transferred arc on a HTA alloy 900B;
Figure 8 is a photomicrogaph of a NiCrAlY top surface after 500 hours of aging
in
air at 1150 C;
Figure 9 is a photomicrograph of a bulk microstructure after 500 hours of
aging in
air at 1150 C;
Figure 10 is a photomicrograph of an interface between NiCrAlY coating and a
low
chromium stainless steel after 500 hours aging in air at 1150 C;
Figure 11 is a photomicrograph of an interface between NiCrAlY coating and a
high
chromium stainless steel after 500 hours aging in air at 1150 C; and
Figure 12 is a photomicrograph of an interface between diffusion coating on a
stainless steel substrate with a NiCrAlY coating thereon after
heat-treatment.
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Description of the Preferred Embodiment
A first embodiment of the present invention will be described with reference
to
Figures 1 and 2 of the drawings. A continuous coating of MCrA1X is shown
deposited
onto and metallurgically and adherently bonded to a substrate of a high
temperature
austenitc stainless steel. The MCrA1X alloy of the invention in which M is a
metal
selected from the group consisting of iron, nickel and cobalt or mixture
thereof and X is
an element selected from the group consisting of yttrium, hafnium, zirconium,
lanthanum
and scandium or combination thereof comprises, by weight, about 10 to 25%
chromium,
about 8 to 15% aluminum and up to about 3%, preferably about 0.25 to 1.5%,
yttrium,
hafnium, zirconium, lanthanum, scandium, or combination thereof, the balance
iron,
nickel or cobalt. The inclusion of these elements decreases oxide growth rate,
increasing
the mechanical strength of the oxide scale and functioning as sulphur-getters.
The high
temperature stainless steel substrate has a composition of iron, nickel or
chromium in the
range, by weight, of 18 to 42% chromium, 18 to 48% nickel, the balance iron
and other
alloying additives, and typically is a high chromium stainless steel having
about 31 to
38% chromium or a low chromium stainless steel having about 20 to 25%
chromium.
The substrates to which the MCrA1X coating is applied typically are high
chromium or low chromium stainless steel centrifugally cast or wrought tubes
or fittings
such as used in an ethylene furnace and the coating is applied to the inside
surfaces of
such products. It has been found that application of the coating by magnetron
sputtering
physical vapour deposition permits application of a continuous, uniformly
thick and
dense coating throughout the length of the inside surfaces of the tubes and
the fittings.
The coating is deposited onto the substrate at a temperature in the range of
about
200 to 1000 C, preferably at about 450 C, as a continuous layer in a
substantially
uniform thickness of about 50 to 350 m, preferably about 150 m, by the
magnetron
sputtering. The coating is heated to a soak temperature in the range of 1000
to 1200 C
for about 20 minutes to 24 hours in an oxygen-free atmosphere to
metallurgically
adherently bond the coating to the substrate and to develop a multiphased
microstructure.
The coating provides a source of aluminum to provide an 00- alumina based
layer
at the surface thereof by introducing an oxygen-containing gas such as air at
a
temperature above about 1000 C at the termination of the heat soak as a
consecutive
step, upon heating of the substrate and coating in a gaseous oxidizing
atmosphere such as
CA 02614962 2007-11-26
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air at a temperature above 1000 C in a separate step, or during commercial use
by the
introduction of or presence of an oxygen-containing gas at operating
temperatures above
about 1000 C.
A second embodiment of the invention will now be described with reference to
Figures 3 and 4. The coating of MCrA1X is deposited such as by magnetron
sputtering
on the high temperature stainless steel in a substantially uniform thickness
of about 50 to
350 gm as described above, preferably about 150 m. A uniform layer of
aluminum,
aluminum alloy containing up to about 60 wt%, preferably up to about 15 wt%,
silicon,
or aluminum alloy containing up to 60 wt% silicon, a total of up to 30 wt% of
at least
one of chromium and titanium, the balance at least about 20 wt% alumium, is
deposited
onto the MCrA1X coating such as by magnetron sputtering at a temperature in
the range
of about 200 to 500 C, preferably at about 300 C, in an amount of up to about
50% of
the thickness of the coating, preferably up to about 20% of the thickness of
the coating.
The substrate, coating and aluminum layer are heated to a soak temperature in
the range
of about 1000 to 1200 C for at least about 10 minutes, preferably for about 20
minutes to
24 hours in an oxygen-free atmosphere such as a vacuum to metallurgically bond
the
coating to the substrate and to establish the multiphased microstructure and
to diffuse the
aluminum layer into the coating and then preferably sequentially heated in an
oxidizing
atmosphere of an oxygen-containing gas for at least 20 minutes, preferably 20
minutes to
4 hours to oxidize the aluminum-rich layer and form a uniformly thick and
adherent oo-
alumina based layer thereon. Oxidation of the aluminum layer can be effected
in a
subsequent and separate stage upon heating of the composite coating in an
oxidizing
atmosphere to a temperature typically above about 1000 C, for production of
the oo-
alumina layer preferably in the range of about 1000 - 1150 C, or oxidation can
occur
during commercial operation in an oxidizing atmosphere at a temperature above
about
1000 C.
The presence of the aluminum layer on the coating supplements the source of
aluminum in the MCrA1X coating to maintain an effective continuous alumina
layer
during commercial operation. The diffusion of aluminum into the coating heals
minor
structural defects in the coating, whilst the enrichment of the surface of the
coating near
CA 02614962 2007-11-26
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the surface with aluminum modifies the oxide growth mechanism, decreasing the
number of catalytic sites (such as Ni-oxide) in the protective alumina scale.
A third embodiment of the invention will now described with reference to
Figures 5 and 6. In accordance with this embodiment of the method of the
invention, a
continuous aluminum-containing interlayer comprised of about 35 to 45 wt%
aluminum,
a total of about 5 to 20 wt% of at least one of titanium or chromium, and 40
to 55 wt%
silicon, preferably about 35 to 40 wt% aluminum, about a total of about 5 to
15 wt% of
at least one of titanium or chromium, and about 50 to 55 wt% silicon, are
deposited onto
a high temperature stainless steel base alloy substrate as described in U.S.
Patent No.
6,093,260 issued July 25, 2000, a continuous MCrA1X alloy coating is deposited
onto the
interlayer, and an aluminum or aluminum alloy which include nickel aluminides
is
deposited onto the overlay alloy coating.
In this embodiment, the aluminum within the interlayer combines with the
nitrogen in the substrate to form a dispersion of aluminum nitride
precipitate, thereby
permitting scavenging of nitrogen emanating from the substrate.
The interlayer preferably is deposited by physical vapour deposition at a
temperature in the range of 400 to 600 C or 800 to 900 C, preferably at
either 450 or
8500C. The interlayer is then heated to a soak temperature at a rate of
temperature rise
of at least 5 Celsius degrees/minute, preferably at a rate of 10 to 20 Celsius
degrees/minute, to establish the coating microstructure. The MCrA1X coating,
and
preferably an aluminum layer, are deposited by physical vapour deposition onto
the
interlayer and then heat-treated to establish the multiphased microstructure
and to
metallurgically bond the coatings.
The interlayer is deposited in a thickness of about 20 to 100 gm and
preferably in
a thickness of about 20 to 60 m. The interlayer, MCrAIX coating with aluminum
layer
and substrate base alloy are heat treated at a soak temperature in the range
of about 1030
to 1180 C for a time effective to form an interlayer between the base alloy
and coating
containing intermetallics of silicon and one or more of titanium or aluminum
and the
base alloying elements. Preferably, the interlayer after heat treatment
contains about 6 to
10 wt% silicon, 0 to 5 wt% aluminum, 0 to 4 wt% titanium and about 25 to 50
wt%
chromium, the balance iron and nickel and any base alloying elements.
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The interlayer requires precise heat treatment to form an adequately
stratified and
adherent final coating. Coatings comprising for example 10 wt.% titanium, 40
wt.%
aluminum and 50 wt.% silicon are deposited in the temperature range 400 to
500 C and
preferably at about 450 C using sputtering as the deposition process. It is
possible to
deposit the coating at temperature of up to 1000 C, but unless subsequent
thermal
processing is done in the same furnace, there is little incentive to coat at
these higher
temperatures. During the treatment, the rate of temperature rise must be at
least 5 C per
minute, typically in the range of 10 C to 20 C per minute, from about 500 C to
within
5 C of the maximum temperature.
At temperatures between about 1130 C and 1150 C, a final segregation of the
coating into layers occurs. The final microstructure obtained is strongly
dependent on
the temperature, but not significantly dependent on the time spent at these
temperatures,
within the time range of at least about 10 minutes, preferably about 20
minutes to two
hours. However, a different and less desirable microstructure results if the
time at the
final temperature is too short, for example, for less than 10 minutes. At the
lower end of
this temperature range at 1130 C, void formation is still probable. The
optimum
temperature range for the final temperature soak is 1135 C to 1145 C for at
least about
minutes, preferably about 30 minutes to 2 hours. At higher temperatures, the
diffusion barrier that is formed becomes unstable, and at 1150 C, is quickly
destroyed by
20 inward diffusion of silicon. Above this temperature, aluminum diffusion
downward also
occurs, leaving an aluminum content below 5 percent. However, up to 1160 C,
the
aluminum content is still sufficient for the dispersion of nitrides.
If the stainless steel substrate is a wrought or cast low chromium base alloy
substrate containing 20 to 25 wt.% chromium, the temperature ramp rate should
be the
same as for the high chromium base alloy substrate at about 10 C to 20 C per
minute,
but the preferred soak temperature is within the range 1030 to 1160 C. In this
embodiment, the chromium silicide-containing diffusion barrier does not form
due to the
low chromium concentration in the base alloy. Alloys with 20 to 25 wt.%
chromium
content include the Inco 800TM series alloys, for example 88HTM, 800HTTM and
803TM
CA 02614962 2007-11-26
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alloys. The required minimum temperature ramp rate is not dependent on the
base alloy
composition.
A further embodiment of the invention will now be described with respect to
Figure 12. In accordance with this embodiment of the method of the invention,
an
interlayer of NiCr alloy or TiAlSi alloy was deposited by magnetron sputtering
in a
thickness of about 20 to 100 m onto a high temperature stainless steel
substrate, a
MCrA1X coating was deposited onto the interlayer in one or two continuous
coatings,
and the resulting coated substrate heat treated at a temperature in the range
of 1000 to
1180 C to metallurgically bond the coatings.
These embodiments of the method of the invention will now be described with
respect to the following non-limitative examples.
Tubes and coupons of 25Cr 35Ni (800H, 803, HPM, HK4M) and selected 35Cr
45Ni alloys were coated with MCrA1X embodiments of the invention using a
magnetron
sputtering physical vapour deposition technique. Coated samples were heat
treated at
high temperatures in a vacuum in order to improve the interface adhesion of
the coating
to the substrate by metallurgical bonding and to develop a fine-grained
metallurgical
structure, and then in an oxidizing atmosphere with an aluminum surface
coating on the
coating to develop an oxide outerlayer surface with anti-coking properties.
The top
surface layer of aluminum was deposited using the same coating technique of
magnetron
sputtering to enhance the aluminum content of the MCrAIX coating and to
improve the
coating's ability to regenerate the protective oxide surface layer while
healing minor
structural defects in the coating to decrease the number of catalytic sites in
the alumina
scale.
When coating certain centrifugally cast 25Cr - 35Ni/35Cr - 45Ni alloys, an
aluminum-bearing diffusion coating typified by a TiAlSi alloy was deposited on
the
substrate to function as an interlayer between the substrate and the
aluminized coating to
protect the coating from the outward diffusion of nitrogen from the cast
alloy.
Alternatively, an interlayer of a NiCr alloy can be used to disperse nitride
formation.
The coated and heat-treated samples were characterized for uniformity,
metallurgical bond, microstructure, thickness and composition by standard
laboratory
techniques using optical microscope and scanning electron microscope with
energy
dispersive spectroscopy. Tests to evaluate thermal stability, resistance to
oxidation,
CA 02614962 2007-11-26
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carburization, thermal shock, thermal fatigue and creep resistance were
carried out and
documented.
Small test coupons and 3-foot long tubes were tested in a simulated pyrolysis
test
rig. A residence time of 0.4 - 3 seconds was used at a temperature in the
range of 800 -
9500C. Run lengths varied from 1 to 8 hours. The performance of the coated
tubes and
coupon samples was compared with uncoated high-temperature alloys, ceramics
and
pure nickel.
Coated tubes having a length of 3 feet with OD of 5/8 inch were tested in a
simulated pyrolysis test rig. The performance of the coated tubes were
compared with an
uncoated high temperature alloy and a quartz tube.
The coatings were uniformly deposited on the inner wall surface of the tubes
and
heat treated in accordance with the methods of the invention. Comparisons of
the coated
products with uncoated tubes and fittings were made on the basis of coking
rates,
carburization, ability to metallurgically adhere to the surfaces of
commercially produced
high chromium/nickel centrifugally cast tubes and wrought tubes under thermal
shock
and thermal cycling conditions, and resistance to hot erosion.
Example 1
With reference to Figures 1 and 2, a NiCrAlY coating 10 containing, by weight,
22% chromium, 10% aluminum and 1% yttrium, the balance nickel, was deposited
on an
Incoloy 800HTM stainless steel substrate 12 by magnetron sputtering at 450 C
to provide
an average coating thickness of 150 m. The NiCrAIY coating and substrate were
heat-treated in a vacuum at a rate of temperature rise of 15 Celsius
degrees/minute to
1100 C and held for 1 hour at 1100 C to produce a nickel aluminide precipitate
phase
14 in an alloy matrix illustrated in Figure 2.
The resulting coating was subjected to carburization for 70 16-hour cycles in
a
CO/H2 atmosphere at 1080 C. The coating displayed good carburization
resistance.
The coating was shown to maintain thermal stability for 1000 hours at 1150 C.
The
coating displayed superior mechanical properties (as compared to the diffusion
coating),
whilst stress-rupture testing indicated no significant adverse effects on
substrate
properties.
CA 02614962 2007-11-26
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Example 2
An aluminized NiCrAlY coating 16 containing, by weight, 22% chromium, 10%
aluminum and 1% yttrium, the balance nickel, was deposited on a Sandvik 800HTM
stainless steel substrate 18 by magnetron sputtering at 450 C, to provide a
coating
thickness of 150 to 200 m. An aluminum layer 22 was magnetron sputtered onto
the
coating at 450 C, to give an average aluminum coating thickness of about 40
m, shown
in Figure 3.
The aluminized NiCrAIY coating and substrate were heat-treated in a vacuum at
a rate of temperature rise of 15 Celsius degrees/minute and held for 1 hour at
1100 C to
produce nickel aluminide phase 22 and an underlying nickel aluminide
precipitate phase
in an alloy matrix 26 adjacent stainless steel substrate 18. The aluminized
coating was
oxidized in air for 1 hour at a temperature at about 1050 C to produce an y-
alumina
surface layer 28.
The resulting coating was shown to have good carburization resistance,
withstanding 45 (+) 16 hour cycles in a CO/H2 atmosphere. The coating
maintained
thermal stability for 500 hours at 1150 C. The coating was subjected to 1000
thermal
cycles at 1100 C and exhibited excellent coking resistance, similar to that of
an inert
ceramic.
Example 3
With reference to Figure 5, a diffusion coating 30 containing, by weight, 10%
chromium, 40% aluminum and 50% silicon was deposited by magnetron sputtering
at a
temperature of 850 C, to give an average thickness of 40 m, onto Manoir XTMTM
stainless steel substrate 32. A NiCrAlY coating 34 containing, by weight, 22%
chromium, 10% aluminum and 1% yttrium, the balance nickel, was deposited onto
the
diffusion coating by magnetron sputtering at approximately 850 C, to give an
average
coating thickness of 150 gm. An aluminum layer 36 was applied onto the NiCrAlY
coating 34 using magnetron sputtering at 450 C, to give an average aluminum
coating
thickness of 20 gm.
The aluminized NiCrAIY coating on the diffusion coating was heat-treated in a
vacuum at a rate of temperature rise of 15 Celsius degrees/minute and held for
1 hour at
CA 02614962 2007-11-26
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1150 C to yield a diffusion barrier 40 on the substrate 32 and an enrichment
pool 42
adjacent the diffusion barrier 40, shown in Figure 6. A nickel aluminide phase
44 is
formed by the inward diffusion of the aluminum layer into the upper portion of
the
NiCrAY coating 46. Nickel aluminide phase 44 developed y- alumina based layer
48 on
the surface thereof as a result of adding air at the end of the vacuum heat-
treatment.
The resulting coating was held at 1150 C for 500 hours to evaluate thermal
stability and was also subjected to thermal shock tests. The coating exhibited
good
thermal stability and good resistance to thermal shock.
Example 4
A NiCr alloy, by weight 50% Cr, 50% Ni, was deposited onto a Kubota
KHR35CW alloy by magnetron sputtering at 450 C to provide an average thickness
of
40 m. A NiCrAlY coating, by weight 22% Cr, 10% Al, 1% Y balance Ni, was
deposited by magnetron sputtering at 450 C to provide an average thickness of
60 m,
followed by a second NiCrAlY coating, by weight 18% Cr, 15% Al, 1% Y balance
Ni,
deposited by magnetron sputtering at 450 C to provide an average thickness of
80 m.
The resultant coating and substrate were heat treated in vacuum for 1 hour at
1150 C.
The heat-treated coating was isothermally oxidized in laboratory air for 192
hours, after which the coating was still in relatively good condition. The
coating also
displayed good mechanical properties, as compared to a NiCrAIY without the
NiCr
layer.
Example 5
A TiAlSi alloy, by weight 20% Ti, 20% Al balance Si, was deposited onto a
Manior XM alloy by magnetron sputtering at 850 C to give an average thickness
of 40
m. A NiCrAlY alloy comprising, by weight, 22% Cr, 10% Al, 1% Y balance Ni, was
then deposited by magnetron sputtering at 850 C to give an average thickness
of 160
m. The subsequent coating was heat-treated in vacuum for one hour at 1150 C to
provide the interlayer illustrated in Figure 12.
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The resultant coating was isothermally oxidized in laboratory air and was
shown
to provide protection for up to 480 hours, whilst successfully dispersing
damaging nitride
phases.
Example 6
With reference now to Figures 7 - 11, a continuous coating of MCrA1X was
deposited onto and metallurgically and adherently bonded to a substrate of a
high
temperature austenitic stainless steel by a plasma transferred arc process.
The MCrA1X
alloy of the invention in which M is a metal selected from the group
consisting of iron,
nickel and cobalt or mixture thereof and X is an element selected from the
group
consisting of yttrium, hafnium, zirconium, lanthanum, scandium and combination
thereof
comprises, by weight, about 0 to 40%, preferably about 10 to 25%, chromium,
about 3 to
30%, preferably about 4 to 20%, aluminum, and up to about 5%, preferably up to
3%,
and more preferably about 0.5 to 1.5%, yttrium, hafnium, zirconium, lanthanum,
and/or
scandium, the balance iron, nickel or cobalt or combination thereof.
The substrates to which the MCrA1X overlay coating is applied typically are
high
chromium or low chromium stainless steel centrifugally cast or wrought tubes
or fittings
and it has been found that application of the coating by plasma transferred
arc process
deposition permits application of a continuous, uniformly thick and dense
coating
throughout the length of the inside surfaces of the tubes and the fittings.
A preferred MCrA1X is NiCrAlY which comprises, by weight, about 12 to 25%
chromium, about 4 to 15% aluminum, about 0.5 to 1.5% yttrium, and the balance
substantially nickel.
The deposition process for the NiCrAIY coating involves the application of a
powder raw material of the MCrA1X composition, by a plasma transferred arc
process
with the base alloy substrate forming part of the electric circuit. In the
said process a
plasma arc melts both the powder and the base alloy; argon being used as a
carrier and
shrouding gas to prevent oxidation. The process parameters are controlled
during
deposition to yield a melt puddle that will yield a coating with a desired
thickness. By
melting part of the substrate alloy, some dilution occurs which affects the
final
composition of the coating. It also produces a desired transition zone between
the base
alloy and the coating, which accommodates, in a scattered fashion, the
carbides and
, CA 02614962 2007-11-26
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nitrides formed due to the diffusion of carbon and nitrogen at the high
temperatures at
which ethylene furnaces operate. This significantly reduces the risk of
spallation of the
coatings.
The coating thus produced is dense, forms an alumina scale when exposed to air
at high temperatures, and is tightly adhered to tube. The plasma transferred
arc process
can eliminate a separate aluminizing step. Also, the material transfer method
is highly
efficient and between 80 to 90% of the raw material is incorporated into the
coating,
compared to between 25 and 30% with magnetron sputtering.
Two high temperature alloy stainless steel materials were used as substrates;
one
a H46M alloy the other one 900 B alloy. The coating was obtained from a
NiCrAlY
powder with a nominal composition in weight percentage of Al 10, Cr 22, Y 1,
Ni
balance, with impurities comprising less than 1 wt%. The size distribution of
the powder
was as +45 microns - 106 microns. It was fed to the gun at a rate of 30 grams
per minute
using 100 amps and 50 volts across the arc.
The coating was dense and continuous, over 4 mm thick, with a smooth interface
as shown in Figure 7. No defects spanning from the base alloy to the coating
surface
were observed but some bubbles could be detected near the outer surface of the
coating.
The composition reflected the fact that part of the alloy was melted, so the
NiCrAIY got
mixed and diluted with the elements present in the HTA. In both cases the
aluminum
content was between 5 to 7 wt%. The sample deposited on H46M had however less
iron,
more nickel and chromium than the sample deposited on 900B. Some other
elements
present in the base alloy such as silicon, niobium and manganese diffused into
the
coating but none amounted to more than 1 wt% on the welded layer. No heat
treatment
was given to these samples prior to their examination.
The samples were aged in air at 1150 C for up to 500 hours. After each aging
period the samples were taken out of the oven and dipped in water to assess
the thermal
shock resistance of the ensemble. None of the samples spalled or cracked after
such
treatments. The bulk microstructure did not drastically change after any aging
time, as
indicated in Figures 8 and 9. However, at the free surfaces and at the
interface new
structures developed. A 10 microns thick alumina layer was formed on the outer
surface
which proved to drastically reduce the formation of catalytic coke in coated
HTA alloys.
In voids and other inner defects, a core of mixed oxides (Cr-Al-Ni-Y 0) was
precipitated
CA 02614962 2007-11-26
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inside an alumina skin. The attack by oxygen extended several microns inside
the
coating. At the interface a large amount of nitrides, basically A1N,
developed; these
crystals grew in a dispersed manner as shown in Figures 10 and 11. The number
of
nitrides was larger in the sample prepared on the high chromium H46M alloy,
probably
due to a larger amount of nitrogen dissolved in the alloy. Even in this case,
the nitrides
did not agglomerate in a straight or continuous manner, hence reducing the
possibility of
a mechanical failure. This avoids the need for deposition of an interlayer
whose main
purpose was to absorb the nitrogen coming from the tube. The amount of
aluminum in
the bulk was reduced to just above 5 wt% after 500 hours at aging at 1150 C,
part of the
original aluminum having diffused into the base alloy.
This embodiment of the method of this invention provides a number of important
advantages. NiCrAlY powders are applied by plasma transferred arc to substrate
alloys
and the resulting interface layer is dense, continuous and smooth and forms an
adherent
metallurgical bond with the HTA substrate. Any precipitated nitrides and
carbides are
dispersed in and in proximity to the interface layer, obviating the need for
heat treatment
of the coating or the provision of a separate interlayer. Enough aluminum is
available in
the coating to form an alumina surface scale. After 500 hours of aging in air
at 1150 C
and thermal shock tests, the composition and bulk structure changed only
slightly.
Nitrides formed near the interface layer, however, these are dispersed and
will not result
in coating delamination. The surface region showed evidence of oxidation,
however, the
attack was shallow and sufficient aluminum remained to maintain the protective
alumina
scale. The surface alloy of the invention on HTAs has particular utility in
the coating of
reactor tubes for use in high temperature corrosive environments such as
furnaces for the
production of ethylene.
The additive silicon can be present in the amount of 0 to about 40 wt%,
preferably 3 to 15 wt%. The additive T can be present in an amount of 0 to 10
wt%,
preferably 0.1 to 5 wt%, and more preferably 0.5 to 3 wt%. A preferred
additive T is
titanium, tantalum, platinum or palladium, tungsten, molybdenum, niobium or
rhenium
in an MCrAIX comprised of about 12 to 25 wt% chromium, about 4 to 15 wt%
aluminum, about 0.5 to 1.5 wt% yttrium, the balance nickel. The addition of
silicon to
the MCrAIX coating improves the resistance to both hot corrosion and
oxidation. The
addition of tantalum and tungsten in Cr-based coatings imparts improved
resistance to
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sulphidation and oxidation. The presence of molybdenum to an aluminum-forming
alloy
improves the quality of the Cr-based oxide scale which forms once aluminum has
been
deleted from the coating alloy. The inclusion of titanium in the MCrA1X alloy
composition improves the coatings resistance to hot corrosion, particularly
resistance to
sulphide and/or halide bearing compounds. Niobium additions strengthen the
coating,
altering the coating thermal expansion coefficient to match the thermal
expansion of the
substrate. The presence of palladium, platinum or rhenium provides a superior,
slower
growing alumina scale. A preferred composition is MCrA1XSi comprising 22 wt%
Cr,
wt% Al, 1 wt% Y, 3 wt% Si, the balance nickel.
10 The thickness of the MCrA1XSi or MCrA1XT overlay coating may vary from 20
to 6000 m, preferably 50 to 2000 m, and more preferably 80 to 500 m in
thickness.
A surface layer of aluminum, aluminum alloy containing up to 50 wt%,
preferably up to 15 wt%, of silicon, or aluminum alloy containing up to 60 wt%
silicon,
a total of up to 30 wt% of at least one of chromium and titanium, the balance
at least
about 20 wt% aluminum, may be deposited onto the MCrA1XSi or MCrA1XT coating
in
an amount up to 50% of the thickness of the coating. A preferred top layer is
a layer of
aluminum or aluminum alloy having a thickness up to 20% of the thickness of
the
MCrAlSi or MCrA1XT overlay coating.
An industrial embodiment of the coating of the invention is a coking and
corrosion resistant reactor tube for use in high temperature environments
comprising an
elongated tube of a high temperature stainless steel and a continuous coating
metallurgically bonded on the inner surface of the elongated tube comprising a
MCrA1XT alloy wherein M is nickel, cobalt, iron or a mixture thereof, X is
yttrium,
hafnium, zirconium, lanthanum, scandium or combination thereof, and T is
silicon,
tantalum, titanium, platinum, palladium, rhenium, molybdenum, tungsten,
niobium or
combination thereof, and comprising, by weight, about 10 to 25% chromium,
about 4 to
20% aluminum, up to about 3 wt% X, and up to about 8 wt% T, the balance M,
deposited by one of several methods including physical vapor deposition,
plasma thermal
spray or plasma transferred arc surfacing, or applied by a binder coating, and
wherein the
MCrA1XT coating has a thickness of about 20 m to 6000 m.
It has been found that a MCrA1XSi coating silicon is present in an amount of 3
to
wt% can be applied to a substrate of carbon steel or low-grade or high-
temperature
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stainless steels such as tubes and fittings by adding a blended powder of two
or more of
the MCrA1XSi constituents to an organic binder to form a slurry and coating
the
substrate with the slurry. The coated substrate is dried and heated in a
vacuum furnace
for evaporation of the organic binder and for reaction sintering of the
coating with the
substrate for adhesion of the coating to the substrate.
A preferred slurry composition comprises at least two powder constituents of
MCrA1XSi of which M is nickel. The powder is blended and is added to an
organic
binder such as an acrylic binder dissolved in an organic solvent. The nickel
has a
relatively smaller average size of 2 to 10 m, compared to the average size of
50 to 150
m for the remaining constituent or constituents, and has an irregular shape
compared to
the rounded or spherical shape of the remaining constituent or constituents.
The size and
shape variations permit the particles to interlock and to remain on the
substrate once the
organic binder has evaporated, to be described.
The inclusion of up to 40 wt% silicon in the blended powder lowers the melting
point of the coating to about 900 to 1150 C. At silicon concentration of 6 wt%
or
higher, the silicon dissolves chromium carbides formed in the substrate and re-
precipitate
these randomly as the silicon concentration falls below 6 wt% due to silicon
diffusion
into the substrate.
In accordance with this aspect of the invention, there are provided five
embodiments of surface alloy structures generatable from the deposition of two
or more
powders of the constituents of a MCrA1XSi alloy, and the heating of the
workpiece with
the coating in a vacuum or an oxygen-free atmosphere to a temperature for
reaction
sintering of the MCrA1XSi alloy and diffusion bonding of the alloy to the
substrate.
In a first embodiment of the invention two or more powders of the constituents
of
MCrAlYSi alloy are blended together and isostatic pressed onto the workpiece
surface.
The workpiece with the pressed overlay coating is heated in a vacuum or in an
oxygen-free atmosphere until the reaction sintering takes place. In reaction
sintering, it
is necessary to balance the chemical activity of the components in order to
avoid a
violent reaction. When coatings are being produced the reaction should also
occur at a
temperature where adhesion of the coatings to the substrate will take place.
An example
of an uncontrolled reaction is given by the formation of NiAl intermetallic
from Ni and
Al powders. The reaction between Ni and Al starts at 800 to 900 C. The
temperature
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rises rapidly to - 16000C, producing molten droplets of NiAl on a relatively
cold
substrate surface. The droplets quickly solidify and do not react with the
substrate
because of the low substrate temperature and high chemical stability of NiAl.
In
accordance with the present invention, the activity of the powder is
controlled in order to
avoid a violent reaction between powders. Some of the constituents, such as Si
and Al,
are pre-reacted to lower their activity. For example, atomized CrAlSi powder
can be
blended with a combination of Ni, NiAI and NiCr powders. This reduces the
amount of
heat evolved during the reaction and the reaction occurs at higher
temperatures. At
elevated temperatures the coating reacts with the substrate surface producing
an excellent
coating/substrate bond. The addition of Si to the coating is necessary to
produce low
melting point liquids (900-1000oC) with Fe and Ni. These liquids wet the
surface of the
substrate and produce bonding between the coating and the substrate. The Si
additions
are also used to prevent the formation of brittle carbides at the
coating/substrate
interface. At initial concentrations of 6 wt% or higher, Si dissolves the
chromium
carbides found in the substrate and re-precipitates them randomly as the Si
concentration
falls below 6% Si due to diffusion into the substrate.
In a second embodiment of the invention, two or more powders of the
constituents of MCrAIYSi-alloy are blended together and deposited as a coating
onto the
workpiece surface by thermal spray, chemical vapour deposition or by magnetron
sputtering from a previously thermal sprayed cathode. The workpiece with
coating is
then heated in a vacuum or in an oxygen-free atmosphere until the reaction
sintering
takes place.
In a third embodiment of the invention, two or more powders of the
constituents
of MCrAIYSi-alloy are blended together and deposited onto the workpiece
surface by
plasma transferred arc process, which performs the reaction sintering process
simultaneously with the deposition.
In a fourth embodiment of the invention, two or more powders of the
constituents
of MCrA1YSi-alloy are blended with an effective amount of an organic binder if
necessary, and mixed with a solvent combined with a viscous transporting agent
in order
to be deposited as slurry onto the workpiece surface. The workpiece, with the
overlay
slurry coating is dried prior to heating in vacuum or in oxygen free
atmosphere until the
reaction sintering takes place.
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One of the advantages of the reaction sintering process is that a sharp
dividing
line between the coating and the substrate is not formed. Not only does it
result in better
bonding between the coating and the substrate but in the case of MCrAIY alloys
on a
nitrogen containing substrate it will result in a random distribution of
brittle aluminum
nitrides. In an MCrAIY coating deposited by the PVD process these nitrides can
form
brittle layers at the coating/substrate interface resulting in coating
delamination.
The coating provides a source of aluminum to provide an y-alumina based layer
at the surface thereof by introducing an oxygen-containing gas such as air at
a
temperature above about 1000 C at the termination of the heat soak as a
consecutive
step, upon heating of the substrate and coating in a gaseous oxidizing
atmosphere such as
air at a temperature above 1000 C in a separate step, or during commercial use
by the
introduction of or presence of an oxygen-containing gas at operating
temperatures above
about 1000 C.
The fifth embodiment of surface alloy structure of the invention comprises
depositing a layer of aluminum on top of the said MCrA1XSi surface alloy
structure and
heat treating the composite of aluminum and MCrA1XSi surface alloyed substrate
to
establish the desired coating microstructure.
It will be understood, of course, that modifications can be made in the
embodiments of the invention illustrated and described herein without
departing from the
scope and purview of the invention as defined by the appended claims.
