Note: Descriptions are shown in the official language in which they were submitted.
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WEAR AND CORROSION RESISTANT COATINGS
AND METHOD FOR PRODUCING THE SAME
Description
This application is related to application
465,339, entitled, "High Strength, Wear and
Corrosion Resistant Coatings and Method for
Producing the Same," and application number 465,338,
entitled "Wear and Corrosion Resistant Coatings
Applied at High Deposition Rates".
Technical Field
The present invention relates to wear and
corrosion resistant coatings and to a method for
producing such coatings. More particularly, the
invention relates to a new family of W-Co-Cr-C
coatings having improved strength and wear
resistance.
Background Art
Coatings of W-Co-Cr-C are used in those
applications where both superior wear and corrosion
resistance are required. A typical composition for
these coatings comprises about 8 to lo weight
percent cobalt, about 3 to 4 weight percent
chromium, about 4.5 to 5.5 weight percent carbon and
the balance tungsten, These coatings can be
successfully applied to various substrates, e.g.,
iron base alloys substrates, using known thermal
spray techniques. Such techniques include, for
example, detonation gun (D-Gun) deposition as
disclosed in US. Patent Nos. 2,714,563 and
2,950,867, plasma arc spray as disclosed in US.
Patent Nos. 2,858,411 and 3,016,447 and other
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so-called "high velocity" plasma or "hypersonic"
combustion spray processes.
Although coatings of W-Co-Cr-C have been
employed successfully in many industrial
applications over the past decade or more, there is
an ever increasing demand for even better coatings
having superior wear resistance. In the textile
industry, for example, there is a need for special
coatings of the type for use on crimper rolls
subjected to extraordinary conditions of abrasive
wear.
As is generally known, these coatings
derive their wear resistance from the presence of
complex carbides of W, Co and Cr. It is also known
that the wear resistance of the coating usually
increases with any increase in the volume fractions
of carbides. Therefore, it has been previously
thought by those skilled in the art that a
relatively high carbon content is necessary in
order to obtain optimum wear resistance.
Summary of the Invention
It has now been surprisingly discovered in
accordance with the present invention that reducing
the carbon content of the W-Co-Cr-C coatings
described above to about 4.0 weight percent or less
with the proper proportions of Co and Or actually
increases the wear resistance contrary to the
teachings of the prior art. It has been found,
however, that when too low a carbon content is
employed, i.e., less than about 3.0 weight percent,
then the resulting coatings are difficult, if not
impossible to grind to a smooth finish. A coating
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composition in accordance with the resent invention
consists essentially of from about 6.5 to about 9.0
weight percent cobalt, from about 2.0 to about 4.0
weight percent chromium, from about I to about 4.0
weight percent carbon and the balance tungsten.
Description of the Preferred Embodiments
The coatings of the present invention can
be applied to a substrate using any conventional
thermal spray technique. The preferred method of
applying the coating is by detonation gun (D-Gun)
deposition. A typical D-Gun consists essentially of
a water-cooled barrel which is several feet long
with an inside diameter of abut 1 inch. In
operation, a mixture of oxygen and a fuel gas, e.g.,
acetylene, in a specified ratio (usually abut 1:1)
is fed into the barrel along with a charge of powder
to be coated. The gas is then ignited and the
detonation wave accelerates the powder to about 2400
ft.~sec. (730 m/sec.) while heating the powder close
to or above its melting point. After the powder
exits the barrel, a pulse of nitrogen purges the
barrel and resides the system for the next
detonation. The cycle is then repeated many times a
second.
The D-Gun deposits a circle of coating on
the substrate with each detonation. The circles of
coating are about 1 inch (25 mm) in diameter and a
few ten thousandths of an inch (microns) thick
Each circle of coating is composed of many
overlapping microscopic splats corresponding to the
individual powder particles. The overlapping splats
interlock and mechanically bond to each other and to
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the substrate without substantially alloying at the
interface thereof. The placement of the circles and
the coating deposition are closely controlled to
build up a smooth coating of uniform thickness and
to minimize substrate heating and establishment of
residual stresses in the applied coating.
The powder use din producing the coating of
the present invention may be essentially the same
powder composition as heretofore employed in
depositing W-Co-Cr-C coatings of the prior art.
However, in this instance, the oxy-fuel gas ratio
employed in the D-Gun process is increased from a
value of about 1.0 to a value of between about 1.1
and 1.2. Under these coercions, changes during the
coating process result in the desired coating
composition. It is also possible to use other
operating conditions with a D-Gun and still obtain
the desired coating composition if the powder
composition is adjusted accordingly. Moreover,
other powder compositions may be used with other
thermal spray coating devices to compensate for
changes in composition during deposition and obtain
the desired coating composition of this invention.
The powders used in the D-Gun for applying
a coating according to the present invention are
preferably sistered powders. However, other forms
of powder such as cast and crushed powder can also
be used. Generally, the size of the powder should
be about -325 mesh. Powders produced by other
methods of manufacture and with other size
distributions may be used according to the present
invention with other thermal spray deposition
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techniques if they are more suited to a particular
spray device and/or size.
As indicated above, it is preferred to use
essentially the same powder composition as
heretofore employed in depositing W-Co-Cr-C coatings
of the prior art. This powder composition consists
essentially of about 10 weight percent cobalt, about
4 weight percent chromium, about 5 weight percent
carbon and the balance tungsten. With this powder,
the feed rate of both oxygen and fuel gas (e.g.,
acetylene) should be adjusted to provide an oxy-fuel
gas ratio of between about 1.1 and 1.2. This ratio
is higher than that usually used heretofore with the
same powder composition and provides an oxidizing
mixture which reduces the carbon content of the
applied coating.
- At oxy-fuel gas ratio close to about 1.1,
the conventional powder composition using the D-Gun
process will produce coatings having a carbon
content of about I weight percent. Conversely, at
oxy-fuel gas mixtures close to about 1.2, this same
powder will produce coatings having a lower carbon
content of about 3.1 weight percent.
Alternatively, the coating of the present
invention can be applied to a substrate by plasma
arc spray or other thermal spray techniques. In the
plasma arc spray process, an electric arc is
established between a non-consumable electrode and a
second non-consumable electrode spaced therefrom. A
gas is passed in contact with the non-consumable
electrode such that it contains the arc. The
arc-containing gas is constricted by a nozzle and
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results in a high thermal content effluent,
Powdered coating material is injected into the high
thermal content effluent from the nozzle and is
deposited onto the surface to be coated. This
process, which is described in US. Patent No.
2,858,411, swooper, produces a deposited coating which
is sound, dense and adherent to the substrate. The
applied coating also consists of irregularly shaped
microscopic splats or leaves which are interlocked
and mechanically bonded to one another and also to
the substrate.
In those cases where the plasma arc spray
process is used to apply the coatings of the present
invention, powders fed to the arc torch may have
essentially the same composition as the applied
coating itself. With some plasma arc or other
thermal spray equipment, however, some change in
composition is to be expected and, in such cases,
the powder composition may be adjusted accordingly
to achieve the coating composition of the present
invention.
The coatings of the present invention may
be applied to almost any type of substrate, e.g.,
metallic substrates such as iron or steel or
non-metallic substrates such as carbon, graphite and
polymers, for instance. Some examples of substrate
material used in various environments and admirably
suited as substrates for the coatings of the present
invention include, for example, steel, stainless
steel, iron base alloys, nickel, nickel base alloys,
cobalt, cobalt base alloys chromium, chromium base
alloys, titanium, titanium base alloys, aluminum,
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aluminum base alloys, copper, copper base alloys,
refractory metals, and refractory-metal base alloys.
Although the composition of the coatings of
the proselyte invention may vary within the ranges
indicated above, the preferred coating composition
consists essentially of from about 7.0 to 8.5 weight
percent cobalt, from about 2.5 to 3.5 weight percent
chromium, prom about 3.0 to 4.0 weight percent
carbon and the balance tungsten. Such coatings are
ideally suited for industrial valves, mechanical
seals, bushings and the like. They are also ideally
suited for use in the textile industry as crimper
rolls, for example.
The micro structure of the coatings of the
present invention are very complex and not
completely understood. However, it is believed that
the major portion of the coatings consist
essentially of a mixture of WE and ~W,Cr,Co)2C
with other metal carbides and possibly metallic
phases. Despite the lower volume fraction of
carbides present as compared to similar coatings of
the prior art, the coatings of the present invention
surprisingly exhibit improved wear resistance
without sacrificing other desirably characteristics
such as hardness, toughness, etc. Typical hardness
values for coatings of the present invention exceed
about 900 DPH300.
The following examples will serve to
further illustrate the practice of the present
invention.
EXAMPLE I
Specimens of ASSAY 1018 steel were cleaned
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and prepared for coating as follows. The surface on
one side ox each specimen was ground smooth and
parallel to the opposite side. The surface was then
grit based with 60 mesh AYE to a surface
roughness of about 120 micro-inch RUMS. All the
specimens were then coated according to the prior
art using a detonation gun (D-Gun) and a sistered
powder of the following composition: 10 weight
percent Co, 4 weight percent Or, 5.2 weight percent
C and the balance W. The size of the powders was
about -325 mesh. Acetylene was used as the fuel
gas. The oxy-fuel gas ratio was 0.98.
A chemical analysis of the coating showed
the following composition: 8 weight percent Co, 3.2
weight percent Or, 4.7 weight percent C and the
balance W. The chemical analysis was carried out
principally by two methods. Carbon was analyzed by
a combustion analysis technique using a Logo Carbon
Analyzer and volumetric determination of gaseous
output. Cobalt and chromium were analyzed by first
fusing the sample in Noah and separating the
cobalt and chromium, then determining the amount of
each potentiometrically.
Abrasive wear properties of the applied
coating were determined using the standard dry
sand/rubber wheel abrasion test described in ASTM
Standard G65-80, Procedure A. In this test, the
specimen was loaded by means of a lever arm against
a rotating wheel with a chlorobutyl rubber rim
around the well. An abrasive (i.e., 50-70 mesh
Ottawa Silica Sand) was introduced between the
specimen and the rubber wheel. The wheel was
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rotated in the direction of abrasive flow. The test
specimen was weighed before, after and periodically
during the test, and its weight loss was recorded.
Because ox the wide differences in the densities of
different materials tested, the mass loss is
normally converted to volume loss to evaluate the
relative ranking of materials. The average volume
loss for these specimens (conventional ~-Co-Cr-C
coating) was 1.7mm3 per 1000 revolutions.
The hardness of these specimens was also
measured by standard methods. The average hardness
was found to be 1100 DPH300. The specimens were
also easily ground to a smooth finish using the
normal method of finishing wear resistant coatings
with a diamond grinding wheel and an indeed of
0.0005 inch per pass.
EXAMPLE II
Specimens of ASSAY 101~ steel were prepared
in the same manner as described in Example I. The
specimen surfaces were then coated using a D-Gun and
the same sistered powder, i.e., 10 weight percent
Co, 4 weight percent Or, 5.2 weight percent C and
the balance W. The powder size was also identical,
i.e., -325 mesh. Acetylene was also used as the
fuel gas. In this instance, however, the oxy-fuel
gas ratio in the D-Gun was 1.1 according to the
present invention.
A chemical analysis of the coating showed
the following composition: 7.6 weight percent Co,
2.9 weight percent Or, 3.5 weight percent C and the
balance W.
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Abrasive wear tests were also carried out
using the ASTM Standard G65-80 Procedure A The
average volume loss for to specimens was 1.1 mm3
per 1000 revolutions. This represents a significant
improvement in wear resistance over the specimens of
Example I.
The hardness of the specimens was also
measured and found to be 1150 DPH300. The
specimens were also easily ground to a smooth finish
using the normal method as in Example I.
EXAMPLE III
Specimens of ASSAY 1018 steel were prepared
in the same manner as described in Example I. The
specimen surfaces were then coated with a D-Gun and
the same wintered powder, i.e., 10 weight percent
Co, 4 weight percent Or, 5.2 weight percent C and
the balance W. The powder size was also identical,
i.e., -325 mesh. Acetylene was also used as the
fuel gas. However, the oxy-fuel gas ratio used in
this instance was 1.2 according to the present
invention.
A chemical analysis of the coating showed
the following composition: 7.8 weight percent Co,
2.9 weight percent Or, 3.1 weight percent C and the
balance W.
Abrasive wear tests were also carried out
on one specimen using the ASTM Standard G65-80. The
volume loss for this specimen was 1.1 mm3 per 1000
revolutions. This also represents a significant
improvement in wear resistance over the specimens of
Example I.
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The hardness of the specimen was also
measured and found to be 1080 DPH30~. The
specimen was ground to a smooth but somewhat rougher
finish using the normal method as in Example I.
EXAMPLE IV
Specimens of ASSAY 1018 steel were prepared
in the same manner as described in Example I. The
specimen surfaces were then coated with a D-Gun and
the same sistered powder, i e., 10 weight percent
Co, 4 weight percent Or, 5.2 weight percent C and
the balance W. The powder size was also identical,
i.e., -325 mesh. Acetylene was also used as the
fuel gas. However, the oxy-fuel gas ratio used in
this instance was 1.3.
A chemical analysis of the coating showed
the following composition: 7.6 weight percent Co,
2.7 weight percent Or, 2.6 weight percent C and the
balance W.
The hardness of this type of coating is
about 1125 DPH300. Abrasive wear tests were
carried out on this coating as in Example I, II and
III with a volume loss of 1.5 mm3 per revolution.
However, attempts to grind the coating to a smooth-
finish were unsuccessful using the normal method as
described in Example I.
EXAMPLE V
Specimens of ASSAY 1018 steel were prepared
in the same manner as described in Example I. The
specimen surfaces were then coated using a plasma
spray torch and the same sistered powder, i.e., 10
weight percent Co, 4 weight percent Or, 5.2 weight
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percent C and the balance W. The powder size was
also -325 mesh.
A chemical analysis of the coating showed
the following composition: 9.2 weight percent Co,
3.5 weight percent Or, 5.0 weight percent C and the
balance W. The cobalt and carbon content of this
coating was higher than that of the coatings of the
present invention.
Abrasive wear tests were also carried out
using the ASTM Standard G65-80, Procedure A. The
average volume loss for the coated specimen was 9.3
mm3 per loo revolutions. The wear properties of
this coating were poor even when compared against
the wear properties of the conventional D-Gun
coatings of Example I. This is to be expected in
the case of plasma spray coatings which do not wear
as well as D-Gun coatings.
The hardness of the specimen was also
measured and found to be 687 DPH300.
EXAMPLE VI
Specimens of ASSAY 1018 steel were prepared
in the same manner as described in Example I. The
specimen surfaces were then coated using a plasma
spray torch and a sistered powder of the following
composition: 10.9 weight percent Co, 4.3 weight
percent Or, 3.8 weight percent C and the balance W.
The powder size of -325 mesh.
A chemical analysis of the coating showed
the following composition: 8.6 weight percent Co,
3.6 weight percent Or, 3.4 weight percent C and the
balance W. This coating composition was within the
scope of the present invention.
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Abrasive wear tests were also carried out
using the ASTM Standard G65-80, Procedure A. The
average volume loss for the coating specimen was 4.1
mm per 1000 revolutions. The wear rate for this
coating was less than half the wear rate for the
plasma spray coating of the previous example using a
conventional powder.
The hardness of the coated specimen was
also measured and found to be 830 DPH300.
Although the powder and coating
compositions have been defined herein with certain
specific ranges for each of the essential
components, it will be understood that minor amounts
of various impurities may also be present. Iron is
usually the principal impurity in the coating
resulting from grinding operations and may be
present in amounts up to about 1.5 and in some cases
2.0 weight percent of the composition.
Although the foregoing examples include
only D-Gun and plasma spray coatings, it will be
understood that other thermal spray techniques such
as "high velocity" plasma, "hypersonic" combustion
spray processes or various other detonation devices
may be used to produce coatings of the present
invention.
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