Note: Descriptions are shown in the official language in which they were submitted.
CA 02409880 2002-10-25
EROSION-RESISTANT COATINGS FOR STEEL TUBES
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a method of coating a steel pipe or tube and,
more particularly, relates to a method of providing a protective, erosion-
resistant
coating of a metal composite on a carbon or low alloy steel pipe or tube.
(ii) Description of the Related Art
Tubular goods used in oil and gas production or slurry transportation are
subjected to severe erosion due to abrasive particles present in the fluid
streams. To
protect these structures many materials solutions are sought. A viable
solution is
based on surface coatings. Providing erosion resistant coatings inside such
tubular
structures pose practical difficulties. The techniques of coating large
diameter tubes
with erosion resistant coatings are well known in the art. However, depositing
erosion resistant coating on the inner surfaces of smaller diameter long tubes
encounters many practical difficulties.
Various weld overlay methods can be used to deposit erosion resistant
coatings on conducting substrates. 'l~he selection depends on the iocanon anu
orientation of the surface to be covered, substrate and coating material
types,
thickness, dilution, speed and economics. Most commonly used welding methods
to
deposit erosion-resistant coatings on the inner surfaces of tubular goods
include gas
tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding
and
gas metal arc welding (GMAW), also known as metal inert gas (MIG) welding. The
feed material can be in the powder or wire form with an inert gas providing
the shield
against oxidation. In the GTAW process, the arc is formed between a tungsten
electrode and the substrate that melts the wire or powder fed in between them
whereas
a consumable wire is used as the electrode in the GMAW.
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The main limitation of existing techniques is their inability to deposit
coatings
on inner surfaces of small diameter tube or pipe with longer lengths.
Plasma transferred arc (PTA), 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 torch and the
substrate
work-piece. The arc generates plasma in a plasma plume 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 valves and
other
parts of internal combustion engines cylinders (Danish Patent 165,125, U.S.
Patent
5,958,332).
This technique has been proposed for coating internal surfaces of tubular
goods used in oil and gas production or abrasive slurry transportation.
Key limitations of known PTA process are the inability to deposit thin layers
due to large waviness of the deposits, necessitating larger machining
allowance and
hence thick deposits to obtain smooth surfaces. Excess dilution from the
substrate on
one hand or lack of bonding on the other hand often results in poor coating.
It is accordingly a principal object of the present invention to provide a
method for coating long lengths of steel pipe and tubing, particularly carbon
and low
alloy steels, with an inexpensive, dense, continuous and smooth protective
coating
substantially free of defects.
CA 02409880 2002-10-25
It is another object to provide a erosion-resistant coating within long
lengths of
steel pipe and tubing suitable for use in the erosive environments of oil-and-
gas fields.
A further object of the present invention is the provision of a thin erosion
resistant coating metallurgically bonded to the interior of pipes and tubes by
plasma
transferred arc deposition, or by slurry coating or thermal spraying and
sintering.
Summary of the Invention
In its broad aspect, the method of the invention of providing a protecting
coating on a steel substrate comprises metallurgically bonding one or more
continuous coatings of a WC.'-MX composite where M = one of nickel, cobalt,
chromium, iron or combination thereof and X == one of silicon, boron or
combination
thereof, having about 50 to 95% WC, 5 to 50 wt°ro M and 0 to about 20
wt% X, by
plasma transferred arc deposition of the coating onto the steel substrate, by
slurry
coating or thermal spraying and sintering, or weld overlays such as by gas
tungsten
arc welding or gas metal arc welding. The steel substrate preferably is a
plain carbon
or low alloy steel and comprises the inner surface of a pipe or tube. The
composite
coating has a thickness of 0.1 to 10 mm, preferably 1.0 to 7.0 mm, and most
preferably 3 to 6 mm.
A preferred WC-MX composite comprises 50 to 95 wt% WC, 5 to 30 wt% Ni
or Co, and 5 to 20 wt% Cr and incidental impurities.
The preferred method comprises preparing the steel substrate by boring,
honing, bright finishing, grit blasting, grinding, chemical pickling or
electro-polishing
the steel substrate prior to deposition of the coating. The preparation of the
tube
surface prior to deposition determines coating microstructure with acceptable
level of
porosity. Pre-heating the steel pipe or tube at a temperature in the range of
100 to
800°C, preferably 250 to 600°C, is effective to avoid cracking
and to enhance wetting
and bonding of the coating to the substrate. The coated pipe or tube
preferably is heat
treated at a temperature in the range of 800 to 1 l00°C for a time
effective to restore
CA 02409880 2002-10-25
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pre-coating strength, ductility and toughness of the substrate and is smoothed
by
boring, honing, extruding, drawing, roll-forming, grit blasting, grinding or
electro-
polishing. A second thin coating of the WC-MX composite having a thickness of
about 0.1 to 1.0 mm deposited by plasma transferred arc onto a first
continuous thin
layer of the WC-MX composite previously deposited by plasma transferred arc
provides a smoother coating.
In accordance with another aspect of the invention, the method comprises
providing a protective coating on an inner steel substrate of a carbon or low-
alloy
steel pipe or tube comprising roughening the steel substrate by wet or dry
grit
blasting, knurling or abrasive cleaning and depositing by slurry coating or
thermal
spraying a WC-MX composite coating powder on the substrate, where M = one of
nickel, cobalt, iron, chromium or combination thereof and X = one of silicon,
boron,
or combination thereof, having about SO to 93 wt% WC, about 5 to 50 wt% M,
about
0.8 to about 20 wt% Si, preferably 0.8 to 5 wt% Si, 0 to about 8 wt% B,
preferably 0.8
to about 5 wt°io B, and heat treating the coating at a temperature in
the range of 600 to
1200°C, preferably in the range of about 950 to 1150°C, for
sintering and
metallurgically bonding the coating to the substrate.
A preferred WC-MX composite in which M = one of nickel, cobalt or
combination thereof comprises 50 to 93.4 wt% WC, 5 to 50 wt% M, 0.8 to 8 wt%
Si,
0.8 to 5 wt% B, and incidental impurities.
Pipe or tube coating produced according to the method of the invention
preferably has a length of 5 to 50 feet, more preferably 7 to 20 feet. The
coating has a
thickness of 0.1 to 10 mm, preferably 1.0 to 7.() mm, has a sound
metallurgically bond
with the steel substrate, and has a dense microstructure particularly suitable
for pipe
or tubing used in slurry transportation such as tar sand mixtures.
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Brief Description of the Drawing
Figure 1 is a photograph of a microstructure of a coating/alloy interface of
an
WC-Co coating on a carbon steel tube according to the present
invention
Description of the Preferred Embodiments
A first embodiment of the present invention will be described with reference
to Figure 1 of the drawings. A continuous coating of a WC-MX composite is
shown
deposited onto and metallurgicaly bonded to a substrate of a carbon steel
tube. The
WC-MX composite of the invention in which M is a metal selected from the group
consisting of nickel, cobalt, chromium and iron or mixture thereof and X is an
element selected from the group consisting of silicon, boron or combination
thereof,
has about 50 to 95 w% WC, about 5 to 50 wt% M, and 0 to about 20 wt% X.
Preferred WC-MX composites are nickel base composites such as Eutectic
CastolinTM powder 6503 having a general composition of 50 to 95 wt% WC and 5
to
SO wt% Ni alloy.
Steel substrates to be coated by the method of the invention, particularly
internal surfaces of pipes and tubes used for oil and gas production,
slurry/chemical
transportation and the like typically are formed of carbon steels and low-
alloy steels.
The inner surface to be coated usually is rough as produced and covered with
millscale and rust and must be; cleaned in order to receive a thin, level,
dense coating
free of imperfections and defects such as porosity and pin-holes. The inner
bore
surface of a pipe or tube can be prepared by processes such as boring, honing,
bright
finishing, grit blasting, grinding, chemical pickling or electro-polishing
prior to
deposition. The pipe or tube is then pre-heated to a temperature in the range
of 100 to
800°C, preferably 250 to 6()0°C, to avoid cracking and to
enhance wetting and
bonding of the coating on the substrate.
CA 02409880 2002-10-25
(~
In a preferred embodiment, a powder of the metal alloy to be coated on the
interior of the carbon or low-alloy steel pipe or tube is fed from a hopper at
a
predetermined rate via an elongated stainless steel tube to a plasma
transferred arc
torch head inserted into the tube to be coated which is rotated on its
longitudinal axis.
The transferred arc between the inner surface of the tube and the torch head
provides
the heat energy in a plasma plume needed to melt the powder and a thin layer
of the
tube substrate, forming a mixture of the molten metal in a molten pool. This
mixing
of molten metal leads to metallurgical bonding at the interface of the coating
and the
substrate. As the tube is rotated, the molten pool moves away from the plasma
plume
and solidifies. The rate of solidification, which can be controlled by post
heating and
by the dwell time of the plasma plume, is important to maintain the level of
dilution
of the coating by the substrate to less than 50°/>, preferably less
than 10% dilution.
The torch is cooled by circulating water from a cooler. The power input is
controlled
by controlling the plasma current and voltage, in addition to pre-heating
temperature,
powder flow rate, rotational speed and step-over distance.
Once the coating process is completed, the tube is cooled down to room
temperature in a controlled manner. Then the tube is subjected to a standard
heat
treatment cycle appropriate to the substrate-coating system, involving
austenitizing at
a temperature in the range of 800 to I 100°C,, fast cooling by
quenching in a suitable
medium such as water, oil and polymer mixture, and tempering at a temperature
in the
range of 200 to 750°C to obtain the required level of coating hardness
and,to restore
pre-coating strength, ductility and toughness of the steel substrate.
The inner exposed surface of the coating is rough and is finished smooth such
as by machining, for example, by grinding or honing to a depth of 0.20 to 1.00
mm to
render the inner surface smooth. The surface can be further finished by grit
or shot
blasting.
The metal composite of the coating preferably is deposited in a continuous
layer having a thickness of 0.1 to 10 mm, preferably 1.0 to 7.0 mm, and more
preferably a thin layer of 3.0 to 6.0 mm. A deterrent to the use of plasma
transferred
arc deposition has been the high cost of the coating material. It has been
found that a
CA 02409880 2002-10-25
dense, uniform coating less than 3 mm in thickness metallurgically bonded to
the
substrate providing an inexpensive and erosion-resistant dense coating in long
pipes
and tubes up to a length of 50 feet, preferably in a range of 7 to 20 feet can
be effected
by plasma transferred arc deposition. A second thin coating of the WC-MX
composite having a thickness of about 0.5 to 3 mm deposited by plasma
transferred
arc onto a first continuous thin layer of the WC-MX composite previously
deposited
by plasma transferred arc provides a uniformly thick coating.
The coating may be deposited onto the steel surface by a variety of methods
including but not limited to physical vapour deposition (PVD), plasma arc-
based
techniques, thermal spray, slurry coating techniques with reactive sintering
occurring
simultaneously with deposition or following deposition, and weld overlay
methods
such as provided by TIG and MIG welding. In the case where reactive sintering
does
not occur during deposition, the overlay coating and substrate are heat-
treated
subsequently at a soak temperature in the range of about 600 to 1200°C,
preferably
about 950 to 1150°C for at least about 10 minutes to initiate reactive
sintering.
The WC-MX composite coating can be applied to a substrate of carbon steel
or low-alloy steel such as tubes and fittings by adding a blended powder of
two or
more of the WC-MX constituents to an effective amount of an organic binder, if
necessary, and mixed with a solvent combined with a viscous transporting agent
to
form a slurry and coating the substrate with the slurry. The coated substrate
is dried
and heated in a vacuum furnace or in an oxygen-free atmosphere for evaporation
of
the organic binder and for reactive 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
WC and MX of which M is nickel and X is silicon, boron or combination thereof.
The powder is blended and is added to an organic binder. A portion of the
nickel has
a relatively smaller average size of 2 to 10 Vim, compared to the average size
of 50 to
150 ~m for the remaining constituent or constituents. Some or all of the
powder
preferably has an angular, irregular or spikey shape compared to the rounded
or
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spherical shape of the remaining constituent or constituents for improved
adhesion to
the substrate prior to heat-treatment.
The inclusion of silicon in the blended powder produces lower melting point
constituents during the reaction sintering process, thereby allowing the
molten
composite to wet the surface of the substrate and to produce an effective
metallurgical
bond between the coating and substrate. The coated workpiece is heated to a
temperature of at least about 600°C to 1200°C, preferably about
950 to 1150°C, to
initiate reaction sintering of the coating on the workpiece substrate and held
at the
soak temperature for at least 10 minutes, more preferably about 20 minutes to
24
hours, to provide a continuous impermeable coating metallurgically bonded to
the
substrate.
The coated and heat-treated samples were characterized for uniformity,
metallurgical bond, microstructure density, thickness and composition by
standard
laboratory techniques using optical microscope and scanning electron
microscope
with energy dispersive spectroscopy.
The method of the invention and the products produced thereby will now be
discussed with reference to the following non-limitative example.
Example 1
WC-Co alloy powder (Eutectic Castolin 6503) comprising 60 wt% WC and 40
wt% Ni-Si-B alloy was deposited on the inner surface of a carbon steel tube
(ASTM
A 106) using plasma transferred arc deposition. The current used was 93A and
voltage was 26V. The powder was fed at a rate of 10 gpm. The rotational speed
of
the 3.Oinch inside diameter tube was 0.3 rpm and the step over distance was
0.25 inch.
The microstructure shown in the microphotograph of Figure 1 has a tight
metallurgical bond between substrate 10 and coating 12. The coating appears to
be
dense.
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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.
