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
the
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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 MCrAIX coating on superalloys for improved
oxidation and corrosion resistance have been previously well documented.
European
Patent EP 897996, for example, describes the improvement ofhigh temperature
oxidation
resistance of an MCrAIY 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
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an MCrAlY using electron beam-physical vapour deposition to improve the 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 MCrAlYs on top of nickel based
superalloys
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(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
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-AIY (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 (AIZ03), aluminum powder
(Al),
and ammonium chloride (NH4C1). The pack was heated in a controlled atmosphere
under
controlled time and temperature conditions to produce MCrA1Y-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 byminimizing 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
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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.
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, MCrAIX 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
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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
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 -alumina. The -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 1150 C 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 MCrA1X 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
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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 ,um, preferably in
a
thickness about 120 to 250 gm, and most preferably about 150 ,unl, such as by
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 1VICrA1X is NiCrAIY and has, by
weight,
about 12 to 22% chromium, about 8 to 15 1o aluminunl and about 0.8 to 1%
yttrium, the
balance nickel.
The high ternperature stainless steel substrate comprises, by weight, 18 to
38%
chromium, 18 to 48% nickel, the balar-ce 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 arrother 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 4m 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 furthei- embodiment of the method of the invention, a
continuous interlayer is frrst 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'% alumii-tum, 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 LT. S.
Patent No. 6,093,260
issued July 25, 2000 incorporated herein by reference, a continuous MCrA1X
alloy
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coating is deposited onto the diffusion coating, and an aluminum layer is
deposited onto
the MCrAIX alloy coating.
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
- 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
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, MCrA1X 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
25 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 MCrA1X 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
metallurgically
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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 10 to
25 wt%
chromium, about 3 to 30 wt% aluminum, preferably about 4 to 20 wt% 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
,um to 6000
,um, preferably 50 to 2000 ,um, more preferably 80 to 500 m onto the
substrate.
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 NiCrAIY 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 NiCrAlY 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 gm in order to
keep powder
costs reasonable and to not unduly decrease the inner diameter of the tube.
The NiCrAlY alloy coating provides a source of aluminum to provide an 20
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 m, preferably about 80 to 500 ,um, may
be
aluminized by depositing a layer of aluminum, or aluminum alloyed with up to
about 60
wt%, preferably up to about 15 wt%, silicon, or at least about 20 wt% aluminum
alloyed
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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 - 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
present in amount up to 15 wt% and the T is present in amount of 0.5 to 8.0
wt%, most
20 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
aluminum within the coating. The presence of the silicon in the MCrA1X coating
also
25 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 MCrA1XSiT
are
blended with an effective amount of a binder to adherently coat a workpiece,
and the
workpiece with MCrA1X or MCrA1XSiT coating is heated to a temperature for
reaction
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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 1200 C to provide a
continuous
impermeable coating bonded to the substrate without a sharp dividing line
between the
CA 02357407 2001-09-14
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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 ,um, preferably in a thickness
about 120 to
500 ,um, more preferably 150 to 350 ,um, 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 ,um 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
-alumina thereon.
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Brief Description of the Drawings
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 NiCrAIY 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 NiCrAIY 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 MCrAIX 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 ,um, preferably about 150 ,um, 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 -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
CA 02357407 2001-09-14
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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.
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
m 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 -
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 -
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
the surface with aluminum modifies the oxide growth mechanism, decreasing the
number
CA 02357407 2001-09-14
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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
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
5 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 co-pending
application
Serial No. 08/839,831, 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
850 C. 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 MCrAIX 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 kcm 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.
The interlayer requires precise heat treatment to form an adequately
stratified and
adherent final coating. Coatings comprising for example 10 wt.% titanium, 40
wt.%
CA 02357407 2001-09-14
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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 anci 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 20
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 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 rainp 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 fonn 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 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 TiAISi alloy was deposited by magnetron sputtering
in a
CA 02357407 2001-09-14
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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 MCrA1X 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,
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 -
950 C. Run lengths varied from 1 to 8 hours. The performance of the coated
tubes and
CA 02357407 2001-09-14
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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 NiCrAIY 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,um. The NiCrAlY 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.
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 ,um. An aluminum layer 22 was magnetron sputtered onto
the
CA 02357407 2001-09-14
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coating at 450 C, to give an average aluminum coating thickness of about 40
,um, 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 -
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 ,um, 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 ,um. 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 m.
The aluminized NiCrAlY 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
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 -alumina based layer
48 on
the surface thereof as a result of adding air at the end of the vacuum heat-
treatment.
CA 02357407 2001-09-14
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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 ,um,
followed by
a second NiCrAIY 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 ,um. 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 NiCrAlY 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'%0 Cr, 10% Al, 1% Y balance Ni,
was
then deposited by magnetron sputtering at 850 C to give an average thickness
of 160,um.
The subsequent coating was heat-treated in vacuum for one hour at 1150 C to
provide the
interlayer illustrated in Figure 12.
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
CA 02357407 2001-09-14
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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 MCrAIX 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
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,
CA 02357407 2001-09-14
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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 fonnation 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
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
CA 02357407 2001-09-14
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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 MCrA1X
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.McrA1X
coating
improves the resistance to both hot corrosion and oxidation. The addition of
tantalum and
tungsten in Cr-based coatings imparts improved resistance to 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 McrAIX alloy composition iniproves the
coatings
resistance to hot corrosion, particularly resistance to sulphide and/or halide
bearing
compounds. Niobium additions strengthen the coating, altering the coating
thermal
CA 02357407 2001-09-14
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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, 10 wt% Al, 1 wt% Y, 3
wt%
Si, the balance nickel.
The thickness of the MCrAIXSi or MCrA1XT overlay coating may vary from 20
to 6000 m, preferably 50 to 2000 ,um, and more preferably 80 to 500 ,um 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, byweight, 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,um to 6000 m.
It has been found that a MCrA1XSi coating silicon is present in an amount of 3
to
40 wt% can be applied to a substrate of carbon steel or low-grade or high-
temperature
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
CA 02357407 2001-09-14
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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 NiAI intermetallic from Ni and Al
powders. The
reaction between Ni and Al starts at 800 to 900 C. The temperature rises
rapidly to -
1600 C, producing molten droplets of NiAI 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 ofNiAl. In accordance with the present
invention,
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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 (900-1000
C) 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.
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 MCrAlY alloys
on a
nitrogen containing substrate it will result in a random distribution of
brittle aluminum
CA 02357407 2001-09-14
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nitrides. In an MCrAlY 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 -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 MCrAIXSi 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.
