Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR HARDFACING A METAL BODY
FIELD OF THE INVENTION
The present invention relates generally to wear-resistant hardfacings for
movable parts. More particularly, the present invention relates to hardfacings
for
rotors of progressing cavity pumps/motors.
BACKGROUND OF THE INVENTION
Progressing cavity pumps have been used in water wells for many
years. More recently, such pumps have been found to be well suited for the
pumping of viscous or thick fluids such as crude oil laden with sand.
Progressing
cavity pumps include a stator which is attached to a production tubing and a
rotor
which is attached to the bottom end of a pump drive string and is made of
metallic material, usually high strength steel.
Progressing cavity motors are used to provide rotary power sections for
use in horizontal and directional drilling. Progressing cavity motors include
a
stator which is connected with a drillpipe and a rotor which is attached to a
drill
bit. Drilling fluid is forced down the drillpipe causing rotation of the rotor
and
operation of the motor to rotate the drill bit.
The rotor is usually electro-plated with chrome to resist abrasion.
However, the corrosive and abrasive properties of the fluids produced in oil
wells
or utilized for drilling fluid frequently cause increased wear and premature
failure
of the rotor. Since it is important for efficient operation of the pump/motor
that a
high pressure differential be maintained across the rotor, only small
variations in
the rotor's dimensions are tolerable. This means that excessively worn rotors
must be replaced immediately. However, replacement of the rotor requires
pulling the whole pump/motor drive string from the well which is costly,
especially
in the deep oil well applications which are common for progressing cavity
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pumps/motors. Consequently, rotors with increased wear resistance and, thus, a
longer service life are desired to decrease well drilling and operating costs.
Various hardfacing methods have been used in the past to increase the
wear resistance of metal surfaces.
A number of progressing cavity pump/motor manufacturers chrome
electroplate the rotors to increase wear resistance. Chrome electroplating
does
provide increased wear resistance, but is susceptible to corrosion in the
harsh
environment of downhole production and drilling.
Another way of increasing wear resistance is to deposit a coating or
layer of material onto the rotor by thermal spraying. Conventional flame
spraying
uses a relatively low flame temperature and particle velocity (such as less
than
about 40 m/s), and results in coatings with high porosity and permeability as
well
as low bond strength. Nevertheless, it allows the spraying of a layer with
much
smaller thickness variations, overcoming the problem of uncontrollable
thickness
variations experienced with other thermal spraying techniques.
US 3,310,423 to Ingham makes reference to conventional flame
spraying usually requiring severe mechanical roughening for example by sand or
grit blasting or machine roughening which forms key-like cavities, and that a
light
sand-blasting is insufficient. Ingham also makes reference to fusing after
flame
spraying to increase the density and bond.
US 4,004,042 to Fairbairn teaches a method for coating with a mixture of
tungsten carbide powder and nickel chrome boron powder. The surface is
cleaned by grit blasting using aluminum oxide particles, for example. Then the
coating is applied by a "stream of energy" such as provided by a plasma
generating gun, covered with a protective film (using boric acid or boric
oxide)
and fused at elevated temperature.
US 4,013,453 to Patel teaches flame spraying a powder containing WC
to obtain a wear resistant coating, including the step of fusing the coating
after
deposition by bringing a torch tip within about 1" of the coating until the
coating
melts and bonds metallurgically to the substrate.
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US 4,161,555 to Appleman teaches a flame spraying process for
materials requiring fusion, in which a particular method of fusing is taught
using a
removable siliceous film.
US 4,241,110 to Ueda et al. teaches a rotor blade having a coating of at
least one coat each of Ni-Cr-B-Si alloy and WC applied by spraying and fusing.
The surface to be treated is cleaned by grid blasting or the like. Then,
powders of
a Ni-Cr-B-Si alloy and WC are fed in succession into and melted or heated by a
flame, e.g. an oxyacetylene flame. For greater joining strength, the coats
formed
by spraying are heated, e.g. by an oxyacetylene flame up to the melting point
of
the alloy to fuse the particles solidly onto the surface.
US 4,517,726 to Yokoshima et al. teaches shot blasting to clean the
surface to be coated, followed by plasma spraying in a method of producing a
seal ring.
US 5,455,078 to Kanzaki teaches a method of coating an aluminum or
aluminum alloy valve lifter with iron, including the steps of primary blasting
to
form a rough surface having larger irregularities (using preferably grit),
secondary
blasting to form smaller irregularities, and forming a coating layer of wear
resistant material on the surface (preferably by thermal spraying) thereby
increasing the adhesion strength of the coated layer.
US 5,395,221 to Tucker et al. teaches a progressive cavity pump/motor
rotor with a coating of metal carbide with a metal alloy using thermal spray
processes which include detonation gun deposition, oxy-fuel flame spraying,
high
velocity oxy-fuel deposition, and plasma spray, and also teaches that the
coating
particle size must be less than the size of the particles in the fluid or the
fluid
particles will abrade and wear off the coating particles. Tucker et al.
teaches the
use of a sealant to address the porosity challenge.
US 6,425,745 to Lavin teaches a surface treatment for helically profiled
rotors, such as progressing cavity pump/motor rotors by high velocity oxygen
fuel
(HVOF) spraying of a WC/ceramic composite coating onto the surface of a rotor
by traversing the axis of the rotor while the rotor is rotated in synchronism
to
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maintain the proper position of the spray relative to the surface of the
rotor, to
give a desired coating thickness profile.
In general, conventional flame spraying techniques result in coatings
with high porosity and permeability as well as low bond strength, although
they
do allow the spraying of a layer of sufficiently consistent thickness.
Thickness
variations on the other hand are a major problem with other coating
techniques,
such as high velocity oxygen fuel (HVOF) or detonation gun (D-gun) coating.
Furthermore, those coating techniques cannot always be used to produce a
sufficiently thick coating. In order to prevent failure of the coating during
use, the
thickness of the coating must be equal to at least 50% of the diameter of any
particles to which the coating is exposed during use. Moreover, sufficiently
thick
coatings, even if achievable are subject to pitting and spalling during use,
due to
insufficient bond strength with the underlying metal layer.
It is, therefore, desirable to provide a method for hardfacing a rotor for a
progressing cavity pump/motor which overcomes the problems associated with
conventional flame spraying and chrome coatings.
SUMMARY OF THE INVENTION
It is an object of the present invention to obviate or mitigate at least one
disadvantage of previous methods for hardfacing and of rotors for progressing
cavity pumps/motors.
In conventional hardcoating processes using flame spraying, the coating
is fused to reduce the high porosity and permeability of the sprayed-on layer.
The
underlying substrate may be roughened prior to flame spraying to provide
increased bond strength. The result is a lower porosity, lower permeability
coating with a stronger bond to the substrate. However, sprayed-on and fused
coatings are still subject to pitting and spalling upon flexing and impact,
even
when the surface to be coated is roughened prior to application of the
hardcoating.
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It has now been surprisingly found by the applicant that the bond
strength of the coating with the underlying substrate can be significantly
increased and pitting and spalling substantially prevented, even on impact and
flexing, if the surface is not only roughened prior to hardcoating, but if the
surface
roughness is coordinated with the coating thickness. In particular, superior
bond
strength is achieved when the substrate is roughened prior to spray coating by
grit blasting to achieve a surface roughness, the depth of which is adjusted
to at
least 40% of the intended coating thickness. The result is a sufficiently deep
inter-penetration of the coating and the substrate to achieve a superior bond
strength, even for relatively thick coatings. The inter-penetration of the
substrate
and the coating to such a large degree also results in a bond strength of the
coating which is substantially equal to the strength of the substrate. The
maximum surface roughness is preferably 90% of the coating thickness, to avoid
exposure of the substrate upon polishing of the coating or premature exposure
of
the substrate during use. For example, for a coating of 10mil thickness, the
surface roughness should be at least 4mil and at most 9mil.
The concept of surface roughness or rugosity is well understood by
the art skilled person and is a measurement of the small-scale irregularities
in a
physical surface. The term "surface roughness" generally refers to the depth
of a
surface profile or texture measured as the peak to valley height of individual
surface features in the surface profile or texture. The term "surface
roughness"
as used herein refers to the depth of the surface profile generated on a
smooth
surface by roughening.
Coatings providing the desired wear resistance and improved corrosion
resistance are selected to increase service life.
In a first aspect, the present invention provides a method of hardfacing a
metal body, with the steps of flame spraying a metallic coating material onto
a
surface of the metal body to produce a metallic coating having a coating
thickness and fusing the metallic coating to provide a hardfacing layer, and
the
additional step of roughening the surface of the metal body prior to the flame
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spraying step for generating a surface roughness of at least 40% and at most
90% of the coating thickness.
In a second aspect, the invention provides a method of hardfacing a
metal body with a coating layer having an intended coating thickness,
comprising
the steps of roughening a surface of the metal body to a surface roughness,
the
depth of which equals at least 40% and at most 90% of the intended coating
thickness, flame spraying a metallic coating material onto the roughened
surface
of the metal body until the intended coating thickness is achieved and fusing
the
layer of metallic material to provide a hardfacing layer.
In a third aspect, the invention provides a rotor for a progressing cavity
pump/motor, comprising a metallic rotor body having a surface, and a layer of
hardfacing on the surface, the hardfacing consisting of flame sprayed and
fused
metallic material applied at a coating thickness, the surface of the rotor
body
having a surface roughness with irregular protrusions for providing a
mechanical
bond between the rotor body and the hardfacing, the surface roughness being
40-90% of the coating thickness.
Preferably, the surface roughness is between 50% and 90%, more
preferably between 60% and 90%, most preferably between 70% and 90% of the
intended coating thickness.
Preferably, the step of roughening the surface of the metal body is
achieved by grit blasting. The grit is preferably selected to have a hardness
at
least equal to that of the metal body. The grit hardness is preferably between
about 20 and 50 Rockwell. The step of grit blasting preferably is carried out
at an
air pressure between about 80 and 150 psi. The minimum surface roughness is
preferably 6 mil, most preferably at least 8mil.
The step of roughening the surface of the metal body preferably creates
a multiplicity of jagged irregular projections and indentations, substantially
covering the surface of the metal body.
Preferably, the hardfacing method includes additional steps prior to the
step of flame spraying the metallic material onto the metal body, namely, the
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steps of predicting the approximate expected grain size of an abrading
substance, to be encountered by the metal body when the metal body is placed
into service, and selecting a grain size for the metal carbide powder that is
finer
than the expected grain size of the abrading substance.
The layer of metallic material is preferably fused by inductive heating to
produce the hardfacing layer. The layer of metallic material is preferably
applied
at a thickness of at least 9 mil and is preferably applied at a substantially
uniform
thickness. The metallic material is preferably selected from the group
consisting
of chromium, molybdenum and nickel and alloys thereof. Most preferably, the
metallic material includes a NiCr alloy. The metallic material preferably
includes
between about 30 wt. % and 80 wt. % metal carbide powder. The metal carbide
powder is preferably selected from the group consisting of the carbides of
tungsten, titanium, tantalum, columbium, vanadium and molybdenum. Most
preferably, the metal carbide powder includes tungsten carbide.
In a further aspect, the present invention provides a downhole
progressing cavity pump/motor, including a pump rotor in accordance with the
invention and a pump stator.
Other aspects and features of the present invention will become
apparent to those ordinarily skilled in the art upon review of the following
description of specific embodiments of the invention in conjunction with the
accompanying drawings.
BRIEF DESCRiPTION OF THE DRAWINGS
Embodiments of the present invention will now be described, by way of
example only, with reference to the attached drawings, wherein:
Fig. 1 shows the principal components of a progressing cavity
pump/motor;
Fig. 2 schematically illustrates a roughened surface of the progressing
cavity pump/motor; and
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Fig. 3 schematically illustrates a hardfacing layer applied to the
progressing cavity pump/motor.
DETAILED DESCRIPTION
The present invention in general is directed to a method of hardfacing a
metal body which includes the steps of flame spraying a metallic coating
material
onto a surface of the metal body to produce a metallic coating having a
coating
thickness and fusing the metallic coating to provide a hardfacing layer,
wherein
the method also includes the step of roughening the surface of the metal body
prior to the flame spraying for generating a surface roughness of at least 40%
and at most 90% of the coating thickness.
In the preferred embodiment, the hardfacing in accordance with the
present invention is applied to the rotor of a progressing cavity pump/motor
10 as
shown in FIG. 1 by roughening the surface of the rotor body, flame spraying a
metallic coating material onto the roughened surface to achieve a metallic
coating of a selected coating fluidness, and fusing the metallic coating. In
the
roughening step, the surface of the rotor body is roughened for generating a
surface rougheness of at least 40% of the thickness of the metallic coating to
be
subsequently applied.
Progressing cavity pumps/motors include a helical rotor 12 made of
ferrous metal, usually high strength steel, and a stator having a generally
double
helical rotor receiving bore 15 of twice the pitch length. The dimensions of
the
rotor and stator are coordinated such that the rotor tightly fits into the
bore 15 and
a number of individual pockets or cavities 13 are formed which are inwardly
defined by the rotor 12 and outwardly by the stator 14. Upon rotation of the
rotor
12 in the operating direction, the cavities 13 and their contents are pushed
spirally about the axis of the stator 14 to the output end of the pump. The
seal
between the cavities is made possible by an interference fit between the rotor
and the elastomeric material of the stator 14. The rotor 12 and stator 14 are
at all
times in tight contact in the areas between the cavities which results in the
wear
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of both components and in particular the rotor, especially when sand-laden and
corrosive liquids are pumped as is often the case in deep oil well
applications.
In the preferred embodiment, the surface of the rotor 12 is mechanically
roughened by grit blasting to provide increased bond strength. The grit
blasting
involves impinging the rotor 12 with steel grit formed of angular particles,
delivered upon the surface of the rotor 12 through the use of pneumatics (such
as through the use of air or an inert or other substance), or other methods
known
to those skilled in the art of surface blasting.
The grit is selected to have a hardness greater than or equal to the
hardness of the rotor 12. The grit blasting forms a multiplicity of jagged or
irregular projections and indentations, substantially covering the surface of
the
rotor body. The roughness of the grit blasted surface is adjusted to be at
least
40% of the intended thickness of the metallic coating. To achieve superior
bond
strength, the grit blasting is preferably carried out under conditions which
will
generate a surface roughness of 40-90% of the intended coating thickness,
preferably 60-90% and most preferably 70-90%. Although surface roughening by
shot blasting is known as well, such roughening is not preferred for the
present
invention. Shot blasting produces rounded indentations. As a result, the
mechanical connection between the metallic coating and the rotor may not be
sufficient to guarantee a long service life for the rotor. Grit blasting
produces
jagged and irregular projections and indentations in the rotor body surface.
Thus,
roughening by grit blasting produces superior bonding strength between the
rotor
and the metallic coating due to the mechanical interlocking connection between
the metallic coating and the rotor body.
After the surface of the rotor 12 is roughened, it may be cleaned to
remove any grit blasting residue, for example by pneumatic cleaning.
A metallic material layer is flame-sprayed onto the roughened surface of
the rotor, or onto a bond coating on the rotor, by way of a flame spray gun.
Flame
spray coating processes and apparatus are well known in the art. In brief, the
flame spray process uses a chemical combustion reaction (flame) from oxygen
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and a fuel (such as acetylene or hydrogen) to produce a heat source which
creates a gas stream. The coating material to be flame sprayed is fed into the
flame in the form of a wire or a powder. The powder is heated by the flame to
a
molten or plastic condition and projected onto the base metal part to be
coated
by a compressed gas (such as air). Upon impact, a bond is formed at the
interface between the molten or plastic powder and the base metal part.
The metallic material may be chromium (Cr), molybdenum, nickel (Ni) or
alloys thereof. In the preferred embodiment, the metallic material is a NiCr
alloy.
The metallic material may be applied in a single layer, or may be applied in a
plurality of layers to form a coating of the metallic material on the rotor
body. The
average thickness of the layer of metallic material can be about 9 mils to
about
100 mils. This can be accomplished in a single layer or single pass. Flame
spraying generally provides a substantially uniform coating thickness.
The metallic material may further include between about 30 wt. % and
80 wt. % metal carbide powder. The metal carbide may be carbides of tungsten,
titanium, tantalum, columbium, vanadium, and molybdenum. In the preferred
embodiment, the metal carbide is tungsten carbide (WC).
As an example, a typical progressing cavity pump/motor rotor may be
hardfaced in accordance with the present invention, as follows. First, the
surface
is roughened by grit blasting with grit having a hardness of 30 Rockwell,
using an
air pressure of 130 psi. Then, a layer of NiCr with 40% WC is applied using
flame
spraying.
In contrast with a plasma gun type thermal spray, the flame spraying of
the present invention provides that the WC is not plasticized as it would be
with
plasma spray, but instead is cemented in place on the rotor body by the
metallic
material (e.g. NiCr). In a typical downhole application, there is some
expectation
or prediction of what abrading substances may be encountered, for example
sand is a well known and expected abrading substance. Typically, the metal
carbide powder (e.g. WC) is selected to have a finer grain size than that of
the
expected abrading substance, so that the hardness of the metal carbide will
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resist abrasion by the abrading material. If the abrading substance is finer
than
the metal carbide, the metallic material (e.g. NiCr) will be abraded and the
metal
carbide will delaminate or fall off.
After application, the surface of the metallic material is fused to reduce
porosity and permeability of the coating. Fusing involves bringing the surface
of
the metallic material almost to, but just short of, its melting point.
Typically this is
done through electric induction heating. However, those skilled in the art
recognize there are many ways to fuse the bond, and are skilled in the
recognition of the appropriate temperature. Typically fusing is done manually
with
visual recognition of the appropriate level or degree of fusion.
Example I
For an intended metallic coating thickness of 9 mils, the surface of a
progressing cavity pump was roughened by grit blasting and subsequently
hardfaced by flame spraying a WC containing metallic material onto the
roughened surface. Typically, only that portion of the rotor surface which
comes
into contact with the fluids to be pumped is roughened and provided with a
hardfacing layer. The grit used was steel grit with a hardness of 30 Rockwell
and
was blown at the rotor using an air pressure of 85 psi.
Example II
A metallic coating thickness of 15 mils was applied and all other
conditions were identical to those described in Example I.
Example III
A metallic coating of 15 to 20 mils was applied to a surface which was
roughened by grit blasting to a surface roughness of 10 to 12 mils. The grit
had a
hardness of 40 Rockwell and was blown at the rotor using an air pressure of
95 psi.
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Example IV
A metallic coating of 25 mils was applied to a rotor surface roughened by
grit blasting with grit of about 50 Rockwell and at an air pressure of 105 psi
to
achieve a surface roughness of 14 mils. Thicker coatings can be equally
applied
to a surface having a surface roughness of 14 mils.
Although the hardfacing method and progressing cavity pump/motor
rotor of the present invention was described in detail only for the
application of a
metallic material such as an alloy of NiCr, a person skilled in the art will
readily
appreciate that other metallic materials can be used such as chrome,
molybdenum and nickel, especially chrome/molybdenum and nickel/chromium
alloys. Similarly, although described in detail only for the application of a
metal
carbide, such as WC, a person skilled in the art will readily appreciate that
other
metal carbides can be used, such as the carbides of tungsten, tantalum,
titanium,
columbium, vanadium and molybdenum. Furthermore, any conventional fusing
process adapted to fuse the coating material and the rotor can be used for the
application of the top layer.
The above-described embodiments of the present invention are intended
to be examples only. Alterations, modifications and variations may be effected
to
the particular embodiments by those of skill in the art without departing from
the
scope of the invention, which is defined solely by the claims appended hereto.
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