U.S. patent application number 09/800193 was filed with the patent office on 2002-09-12 for thermal fatigue and shock-resistant material for earth-boring bits.
Invention is credited to Liang, Dah-Ben.
Application Number | 20020124688 09/800193 |
Document ID | / |
Family ID | 26753616 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020124688 |
Kind Code |
A1 |
Liang, Dah-Ben |
September 12, 2002 |
Thermal fatigue and shock-resistant material for earth-boring
bits
Abstract
Thermal fatigue and shock resistant materials have been
disclosed. Such materials have a thermal conductivity exceeding a
minimal value as determined by
K.sub.min=0.00102X.sup.2-0.03076X+0.5464, where K.sub.min is
minimal thermal conductivity in the units of
cal/cm.multidot.s.multido- t..degree. K, and X is cobalt weight
percentage. Cemented tungsten carbide with coarse tungsten carbide
grains and a low cobalt content meet this criterion. The thermal
conductivity of this type of cemented tungsten carbide may be
further enhanced by using tungsten carbide of coarser grains and
higher purity. By adjusting the tungsten carbide grain size and the
cobalt content, a desired toughness and hardness may be achieved
while still maintaining a relatively high thermal conductivity.
Such materials have applications in forming inserts and other
cutting elements.
Inventors: |
Liang, Dah-Ben; (The
Woodlands, TX) |
Correspondence
Address: |
ROSENTHAL & OSHA L.L.P.
1221 MCKINNEY AVENUE
SUITE 2800
HOUSTON
TX
77010
US
|
Family ID: |
26753616 |
Appl. No.: |
09/800193 |
Filed: |
March 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09800193 |
Mar 6, 2001 |
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09231748 |
Jan 15, 1999 |
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6197084 |
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60072666 |
Jan 27, 1998 |
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Current U.S.
Class: |
75/240 ;
175/57 |
Current CPC
Class: |
E21B 10/52 20130101;
C22C 29/08 20130101 |
Class at
Publication: |
75/240 ;
175/57 |
International
Class: |
C22C 029/08; E21B
007/00 |
Claims
What is claimed is:
1. An earth-boring bit comprising: a cutting element formed of a
composition including tungsten carbide and cobalt, the composition
having a thermal conductivity exceeding a value K.sub.min as
determined by the following equation:
K.sub.min=0.00102X.sup.2-0.03076X+0.5464, where X is a cobalt
content by weight, and K.sub.min is in the units of
cal/cm.multidot.s.multidot..degree. K.
2. A rock bit, comprising: a bit body, a rolling cone rotatably
mounted on the bit body, the rolling cone having a cone surface
with an insert press-fit therein, and; the insert formed of a
composition including tungsten carbide and cobalt, the composition
having a thermal conductivity exceeding a value K.sub.min as
determined by the following equation:
K.sub.min=0.00102X.sup.2-0.03076X+0.5464, where X is a cobalt
content by weight, and K.sub.min is in the units of
cal/cm.multidot.s.multidot..degree. K.
3. A cutting element, comprising: a composition including tungsten
carbide and cobalt, the composition having a thermal conductivity
exceeding a value K.sub.min as determined by the following
equation: K.sub.min=0.00102X.sup.2-0.03076X+0.5464, where X is a
cobalt content by weight, and K.sub.min is in the units of
cal/cm.multidot.s.multidot..degr- ee. K.
4. A method of boring an earth formation, comprising: using an
earth-boring bit having a cutting element formed of a composition
including tungsten carbide and cobalt, the composition having a
thermal conductivity exceeding a value K.sub.min as determined by
the following equation: K.sub.min=0.00102X.sup.2-0.03076X+0.5464,
where X is a cobalt content by weight, and K.sub.min is in the
units of cal/cm.multidot.s.multidot..degree. K.
5. A method of boring an earth formation, comprising: using a rock
bit having a rolling cone with an insert press-fit therein, the
insert being formed of a composition including tungsten carbide and
cobalt, the composition having a thermal conductivity exceeding a
value K.sub.min as determined by the following equation:
K.sub.min=0.00102X.sup.2-0.03076X+0- .5464, where X is a cobalt
content by weight, and K.sub.min is in the units of
cal/cm.multidot.s.multidot..degree. K.
6. A method of boring an earth formation, comprising: using an
insert as a cutting element, the insert being formed of a
composition including tungsten carbide and cobalt, the composition
having a thermal conductivity exceeding a value K.sub.min as
determined by the following equation:
K.sub.min=0.00102X.sup.2-0.03076X+0.5464, where X is a cobalt
content by weight, and K.sub.min is in the units of
cal/cm.multidot.s.multidot..degree. K.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority from U.S.
Provisional Application No. 60/072,666, entitled, "Thermal Fatigue
and Shock Resistant Material for Earth-Boring Bits" filed Jan. 27,
1998.
FIELD OF THE INVENTION
[0002] The invention relates to cutting elements formed of
wear-resistant material for use in earth-boring bits and more
particularly to cemented tungsten carbide.
BACKGROUND OF THE INVENTION
[0003] In drilling oil and gas wells or mineral mines, earth-boring
drill bits are commonly used. Typically, an earth-boring drill bit
is mounted on the lower end of a drill string and is rotated by
rotating the drill string at the surface. With weight applied to
the drill string, the rotating drill bit engages an earthen
formation and proceeds to form a borehole along a predetermined
path toward a target zone.
[0004] A rock bit, typically used in drilling oil and gas wells,
generally includes one or more rotatable cones (also referred as to
"rolling cones") that perform their cutting function through the
rolling and sliding movement of the cones acting against the
formation. The cones roll and slide upon the bottom of the borehole
as the bit is rotated, thereby engaging and disintegrating the
formation material in its path. A borehole is formed as the gouging
and scraping or crushing and chipping action of the rolling cones
removes chips of formation material that are then carried upward
and out of the borehole by circulation of a liquid drilling fluid
or air through the borehole. Petroleum bits typically use a liquid
drilling fluid which is pumped downwardly through the drill pipe
and out of the bit. As the drilling fluid flows up out of the
borehole, the chips and cuttings are carried along in a slurry.
Mining bits typically do not employ a liquid drilling fluid;
rather, air is used to remove chips and cuttings.
[0005] The earth-disintegrating action of the rolling cone cutters
is enhanced by a plurality of cutter elements. Cutter elements are
generally inserts formed of a very hard material which are
press-fit into undersized apertures or sockets in the cone surface.
Due to their toughness and high wear resistance, inserts formed of
tungsten carbide dispersed in a cobalt binder have been used
successfully in rock-drilling and earth-cutting applications.
[0006] Breakage or wear of the tungsten carbide inserts limits the
lifetime of a drill bit. The tungsten carbide inserts of a rock bit
are subjected to high wear loads from contact with a borehole wall,
as well as high stresses due to bending and impacting loads from
contact with the borehole bottom. Also, the high wear load can
cause thermal fatigue in the tungsten carbide inserts which can
initiate surface cracks on the inserts. These cracks are further
propagated by a mechanical fatigue mechanism caused by the cyclical
bending stresses and/or impact loads applied to the inserts. This
may result in chipping, breakage, and/or failure of inserts.
[0007] Inserts that cut the comer of a borehole bottom are subject
to the greatest amount of thermal fatigue. Thermal fatigue is
caused by heat generation on the insert from a heavy frictional
loading component produced as the insert engages the borehole wall
and slides into the bottom-most crushing position. When the insert
retracts from the borehole wall and the bottom of the borehole, it
is quickly cooled by the circulating drilling fluid. This
repetitive heating and cooling cycle can initiate cracking on the
outer surface of the insert. These cracks are then propagated
through the body of the insert when the crest of the insert
contacts the borehole bottom, as high stresses are developed. The
time required to progress from heat checking to chipping, and
eventually, to breaking inserts depends upon formation type,
rotation speed, and applied weight.
[0008] Thermal fatigue is more severe in mining bits because more
weight is applied to the bit and the formation usually is harder,
although the drilling speed is lower and air is used to remove
cuttings and chips. In the case of petroleum bits, thermal fatigue
also is of serious concern because the drilling speed is faster and
liquid drilling fluids typically are used.
[0009] Cemented tungsten carbide generally refers to tungsten
carbide ("WC") particles dispersed in a binder metal matrix, such
as iron, nickel, or cobalt. Tungsten carbide in a cobalt matrix is
the most common form of cemented tungsten carbide, which is further
classified by grades based on the grain size of WC and the cobalt
content.
[0010] Tungsten carbide grades are primarily made in consideration
of two factors that influence the lifetime of a tungsten carbide
insert: wear resistance and toughness. As a result, existing
inserts are generally formed of cemented tungsten carbide particles
(with grain sizes in the range of about 3 .mu.m to 6 .mu.m) and
cobalt (the cobalt content in the range of about 9% to 16% by
weight. However, thermal fatigue and heat checking in tungsten
carbide inserts are issues that have not been adequately resolved.
Consequently, inserts made of these tungsten carbide grades
frequently fail due to heat checking and thermal fatigue when high
rotational speeds and high weights are applied.
[0011] For the foregoing reasons, there exists a need for a new
cemented tungsten carbide grade with the desired toughness, wear
resistance, and improved thermal fatigue and shock resistance so
that better inserts may be manufactured from the new grade, and
better drilling bits may be made using these inserts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 shows thermal conductivity data for existing cemented
tungsten carbide grades and TFR-improved grades as a function of
cobalt content as fitted by a non-linear curve.
[0013] FIG. 2 shows thermal conductivity data for existing cemented
tungsten carbide grades and TFR-improved grades as a function of
cobalt content as fitted by a straight line.
[0014] FIG. 3 is a perspective view of an earth-boring bit made in
accordance with an embodiment of the invention.
[0015] FIG. 4 is a cross-sectional view of a rolling cone in
accordance with an embodiment of the invention.
[0016] FIG. 5 shows thermal conductivity data for existing cemented
tungsten carbide grades and TFR-improved grades obtained through
the test described in Example 1.
[0017] FIG. 6 shows fracture toughness data for existing cemented
tungsten carbide grades and TFR-improved grades obtained through
the test described in Example 2.
[0018] FIG. 7 shows thermal conductivity data for existing cemented
tungsten carbide grades and TFR-improved grades obtained through
the test described in Example 3.
[0019] FIG. 8 shows fracture toughness plotted against wear number
for existing cemented tungsten carbide grades and TFR-improved
grades.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Embodiments of the invention meet the need for an improved
thermal fatigue and shock-resistant material by providing a
composition including tungsten carbide in a cobalt binder matrix.
The composition has a thermal conductivity exceeding a
predetermined value. Such a composition not only has good thermal
fatigue and shock resistance, but also meets the desired toughness
and wear resistance. Therefore, the composition is suitable for
forming inserts and other cutting elements.
[0021] For a wear-resistant material, the associated thermal
fatigue and shock resistance depends on various material
properties, such as thermal properties and mechanical properties.
It is believed that the following formula describes the dependency
of thermal fatigue and shock resistance on various properties of
the material: 1 TFR ( 1 - r ) K ( a ) K1c ( E ) ( 1 )
[0022] where TFR is thermal fatigue and shock resistance, r is
Poisson's ratio, K is thermal conductivity, a is coefficient of
thermal expansion, K1c is fracture toughness, and E is elastic
modulus. It is noted that fracture toughness (K1c) may be replaced
by transverse rupture strength in the formula and a similar
correlation will result.
[0023] For cemented tungsten carbide, Poisson's ratio is generally
in the range of about 0.20 to 0.26. Although the actual value
varies with different carbide compositions, Poisson's ratio is not
a significant factor in influencing thermal fatigue and shock
resistance of cemented tungsten carbide. On the other hand, the
ratio of 2 K a
[0024] represents a composite thermal index which does affect
thermal fatigue and shock resistance. Furthermore, the ratio of 3
K1c E
[0025] represents a composite mechanical index which also
influences thermal fatigue and shock resistance. Therefore, it is
desirable to optimize the product of the composite thermal index
and the composite mechanical index to obtain optimal thermal
fatigue and shock resistance.
[0026] Because tungsten carbide in a cobalt matrix is
representative of wear-resistant material, embodiments of the
invention are explained with reference to a WC/Co system. However,
it should be understood that embodiments of the invention are not
limited to a WC/Co system.
[0027] It also should be noted that existing carbide grades are
formulated to achieve desired toughness and wear resistance. For a
WC/Co system, it typically is observed that the wear resistance
increases as the grain size of the tungsten carbide particles or
the cobalt content decreases. On the other hand, the fracture
toughness increases with larger grain size tungsten carbide and
greater content of cobalt. Thus, fracture toughness and wear
resistance (i.e., hardness) tend to be inversely related, i.e., as
the grain size or the cobalt content is decreased to improve the
wear resistance of a specimen, the fracture toughness of the
specimen will decrease, and vice versa.
[0028] Due to this inverse relationship between fracture toughness
and wear resistance (i.e., hardness), the grain size of the
tungsten carbide particles and the cobalt content have been often
adjusted to obtain the desired wear resistance and toughness. For
example, a higher cobalt content and larger WC grains are used when
a higher toughness is required, whereas a lower cobalt content and
smaller WC grains are used when a better wear resistance is
desired.
[0029] It should be noted that a higher composite mechanical index
is obtained by using larger WC grains and a higher cobalt content.
However, an increase in the composite mechanical index may result
in a decrease in wear resistance. Therefore, a balance between
toughness and composite mechanical index is desired. Existing
cemented tungsten carbide grades maintain this balance by using
relatively small WC grain size and relatively high cobalt content.
But, due to small WC grain size and high cobalt content, such
grades generally have a low composite thermal index. Consequently,
the thermal fatigue and shock resistance of such grades is
relatively poor.
[0030] Efforts to improve the thermal composite index leads to
different formulations of cemented tungsten carbide, such as large
tungsten carbide grains with a low cobalt content. It is believed
that the thermal conductivity of cemented tungsten carbide
generally is inversely proportional to the cobalt content, i.e., as
the cobalt content decreases, the thermal conductivity of cemented
tungsten carbide increases. On the other hand, the coefficient of
thermal expansion generally is directly proportional to the cobalt
content. As a result, as the cobalt content decreases, the
composite thermal index increases significantly because of the
increase in the thermal conductivity and the decrease in the
coefficient of thermal expansion.
[0031] This increase in the composite thermal index is further
enhanced by increasing the grain size of tungsten carbide. It is
believed that the thermal conductivity of cemented tungsten carbide
increases as the grain size of tungsten carbide increases.
Consequently, using larger or coarser tungsten carbide grains
effects an increase in the composite thermal index and the
composite mechanical index, which, in turn, enhances the thermal
fatigue and shock resistance of cemented tungsten carbide.
[0032] With the above considerations, it is believed that cemented
tungsten carbide grades using relatively coarse tungsten carbide
grains and a relatively low cobalt content are desirable to improve
the thermal fatigue and shock resistance. Coarse or large tungsten
carbide grains generally refer to those having nominal particle
sizes exceeding 4 .mu.m, and a low cobalt content generally refers
to weight percentages lower than 14%. It should be understood,
however, that these ranges are preferred embodiments and other
ranges are acceptable so long as the thermal conductivity exceeds a
predetermined value as described herein.
[0033] Although embodiments of the invention are described with
reference to improving the composite thermal index, it should be
understood that improvements in the composite thermal index should
not be obtained at the expense of a satisfactory composite
mechanical index.
[0034] As discussed above, the product of the composite thermal
index and the composite mechanical index is representative of the
thermal fatigue and shock resistance of a cemented tungsten
carbide. A person of ordinary skill in the art will recognize that
an optimal thermal fatigue and shock resistance may be obtained by
maximizing the product of the composite thermal index and the
composite mechanical index. One method of optimizing the thermal
fatigue and shock resistance is to study the dependency of fracture
toughness, elastic modulus, thermal conductivity, and coefficient
of thermal expansion on various factors, such as grain size, cobalt
content, and WC purity. Such studies will reveal desirable ranges
for WC grain size, cobalt content, and WC purity.
[0035] It should be noted that the above formulations are not
likely to result in a decrease in the composite mechanical index.
Although toughness generally is decreased as a result of using a
lower cobalt content, this decrease in toughness is offset by an
increase in toughness due to use of large WC grains. Therefore,
carbide formulations in accordance with embodiment of the invention
effect an increase in the composite thermal index without
decreasing the composite mechanical index. Consequently, the
thermal fatigue and shock resistance of the carbide formulations is
improved.
[0036] For existing grades of cemented tungsten carbide, the
coefficient of thermal expansion is generally in the range of
4.times.10.sup.-6 to 7.times.10.sup.-6/.degree. C. Furthermore, the
thermal conductivity of existing grades of cemented tungsten
carbide generally falls below a value as defined by the following
equation:
K.sub.min=0.00102X.sup.2 -0.03076X+0.5464 (2)
[0037] K.sub.min is the minimal thermal conductivity in the unit of
cal/cm.multidot.s.multidot..degree. K, and X is cobalt content by
weight. Embodiments of the invention utilize cemented tungsten
carbide with a thermal conductivity in excess of approximately
K.sub.min determined by Equation 2.
[0038] It should be noted that Equation 2 is derived from existing
thermal conductivity data for various grades used in the art. FIG.
1 is a graph showing thermal conductivity as a function of cobalt
content. The solid squares represent thermal conductivity of
existing cemented tungsten carbide grades. A quadratic curve
divides the graph into two regions: 10 and 15. Region 15 represents
thermal conductivity which has been achieved by existing carbide
grades, whereas region 10 represents thermal conductivity of the
carbide grades used in embodiments of the invention. It should be
understood that any data points which fall within region 10 are
within the scope of embodiments of the invention.
[0039] It should also be noted that region 10 alternatively may be
defined by a straight line which is illustrated in FIG. 2. The
linear curve may be expressed by the following equation:
K.sub.min=0.38-0.00426X (3)
[0040] FIG. 2 is a graph showing thermal conductivity having a
linear relationship with cobalt content. In constructing this
figure, the same data in FIG. 1 is used, however a linear-curve
fitting method was used. Although it is not clear which equation
represents the true relationship between thermal conductivity and
cobalt content, a skilled person in the art will recognize that
routine experiments may be conducted to make the determination. It
is expected that one of them represents the relationship between
thermal conductivity and cobalt content without large deviations.
For the purpose of illustrating embodiments of the invention,
Equation 2 is used with the understanding that Equation 3 also may
be used.
[0041] While thermal conductivity is specified with reference to
its value at the ambient condition, i.e., room temperature and
pressure, it should be understood that thermal conductivity depends
on various factors, including temperature and pressure. Therefore,
the thermal conductivity of cemented tungsten carbide inserts under
operating conditions may differ from the values disclosed herein
because they are subjected to a higher temperature and/or pressure.
Such variations are immaterial because embodiments of the invention
are described with reference to the thermal conductivity values at
room temperature and pressure.
[0042] It should be understood that the improved thermal fatigue
and shock resistance obtained in embodiments of the invention
alternatively may be represented by the composite thermal index,
which is the quotient of the thermal conductivity over the
coefficient of thermal expansion.
[0043] Another factor which influences the thermal conductivity of
cemented tungsten carbide is the purity of the carbide. It is
believed that as the carbide purity increases, the thermal
conductivity will increase. In a stoichiometric WC crystal, the
carbon content is at 6.13% by weight of WC. Either excess tungsten
or excess carbon (also referred to as "free carbon") may be present
in the carbide. Furthermore, iron, titanium, tantalum, niobium,
molybdenum, silicon oxide, and other materials also may be present.
These materials are collectively referred to as "impurities." These
impurities may adversely affect the thermal conductivity of the
cemented tungsten carbide.
[0044] In some embodiments, conventionally carburized tungsten
carbide is used. Conventionally carburized tungsten carbide is a
product of the solid state diffusion of tungsten metal and carbon
at a high temperature in a protective atmosphere. It is preferred
to use conventionally carburized tungsten carbide with an impurity
level of less than 0.1% by weight.
[0045] In other embodiments, tungsten carbide grains designated as
WC MAS 2000 and 3000-5000 (available from H. C. Starck) are used.
It is noted that similar products may be obtained from other
manufacturers. These tungsten carbide grains contain a minimum of
99.8% WC and the total carbon content is at 6.13.+-.0.05% with free
carbon in the range of 0.04.+-.0.02%. The total impurity level,
including oxygen impurities, is less than about 0.16%.
[0046] Another reason that the MAS 2000 and 3000-5000 grades are
preferred is that the particles are larger. Tungsten carbide in
these grades is in the form of polycrystalline aggregates. The size
of the aggregates is in the range of about 20-50 .mu.m. After
milling or powder processing, most of these aggregates break down
to single-crystal tungsten carbide particles in the range of about
7-9 .mu.m. These large single-crystal tungsten carbide grains are
suitable for use in embodiments of the invention.
[0047] It is recognized that thermal fatigue and shock resistance
is not the only factor that determines the lifetime of a cutting
element. Wear resistance, i.e., hardness, is another factor. In
some embodiments, after the ranges of acceptable WC grain sizes,
cobalt content, and carbide purity have been determined, the
desirable wear resistance is selected. Because Rockwell A hardness
correlates well with wear resistance, desirable wear resistance may
be determined on the basis of Rockwell A hardness data. It is known
that the hardness of cemented tungsten carbide depends on the
cobalt content and the tungsten carbide grain size. A preferred
hardness for embodiments of the invention exceeds a value
designated as "H.sub.min" according to the following equation:
H.sub.min=91.1-0.63X (4)
[0048] H.sub.min is minimal Rockwell A scale hardness, and X is
cobalt content by weight.
[0049] In some embodiments, rock bits will be manufactured using
rolling cones with inserts formed of the above formulations. A
typical rock bit is illustrated in FIG. 3. Referring to FIG. 3, an
earth-boring bit 10 made in accordance with one embodiment of the
invention includes a bit body 20, having a threaded section 14 on
its upper end for securing the bit to a drill string (not shown).
Bit 10 has three rolling cones 16 rotatably mounted on bearing
shafts (hidden) that depend from the bit body 20. Bit body 20 is
composed of three sections or legs 22 (two of the legs are visible
in FIG. 3) that are welded together to form bit body 20. Bit 10
further includes a plurality of nozzles 25 that are provided for
directing drilling fluid toward the bottom of a borehole and around
cones 16. Bit 10 further includes lubricant reservoirs 24 that
supply lubricant to the bearings of each of the cutters. Cones 16
further include a frustoconical surface that is adapted to retain
the inserts that are used to scrape or ream the sidewalls of a
borehole as cones 16 rotate. FIG. 4 illustrates a cross-section of
one of the cutter cones. The frustoconical surface 17 will be
referred to herein as the "heel" surface of the cone 16, although
the same surface may be sometimes referred to by others in the art
as the "gage" surface of the cone.
[0050] Each cone 16 includes a plurality of wear-resistant inserts
15, 18, and 30, which may be formed of a carbide formulation in
accordance with embodiments of the invention. These inserts have
generally cylindrical base portions that are secured by
interference fit into mating sockets drilled into the lands of the
cone, and cutting portions that are connected to the base portions
and that extend beyond the surface of the cone. The cutting portion
of the inserts includes a cutting surface that extends from cone
surfaces 24 and 27 for cutting formation material. As to the
construction of the cutter cones, reference is made to only one
cone for illustration, with the understanding that all three cones
usually are configured similarly (although not necessarily
identically). Cone 16 includes a plurality of heel row inserts 30
that are secured in a circumferential row in the frustoconical heel
surface 17. Cone 16 further includes a circumferential row of gage
inserts 15 secured to cone 16 in locations along or near the
circumferential shoulder 29. Cutter 16 further includes a plurality
of inner row inserts 18 secured to cone surfaces 24 and 27 and
arranged and spaced apart in respective rows. Although the
geometric shape of the inserts is not critical, it is preferred
that they have a semi-round top, a conical top, or a chiseled
top.
[0051] It should be understood that mining rock bits can be
constructed as described above. In typical mining bits, there is no
need for grease reservoirs 24, but the remaining configuration is
equally applicable. Furthermore, it is foreseeable that a mining
rock bit with grease reservoirs may be developed. Embodiments of
the invention also are suitable for this type of mining bits.
[0052] The following examples illustrate embodiments of the
invention and are not restrictive of the invention as otherwise
described herein. For the sake of brevity, carbide formulations
according to embodiments of the invention are referred to
hereinafter as "TFR-improved grades."
EXAMPLE 1
[0053] This example shows that a TFR-improved grade has a thermal
conductivity higher than K.sub.min. Thermal conductivity may be
measured by various methods conventional in the art. In this
example, thermal conductivity is obtained by the flash method in
accordance with the American Standard Testing Manual ("ASTM")
standard E 1461-92 for measuring thermal diffusivity of solids.
Thermal conductivity is defined as the time rate of steady heat
flow through unit thickness of an infinite slab of a homogeneous
material in a direction perpendicular to the surface, induced by
unit temperature difference. Thermal diffusivity of a solid
material is equal to the thermal conductivity divided by the
product of the density and specific heat. The specific heat of a
WC/Co system can be measured by differential scanning calorimetry
based on ASTM-E 1269-94 and is generally in the range of about 0.05
cal/g.multidot..degree. K for carbide grades used in rock bit
applications.
[0054] In the flash method, thermal diffusivity is measured
directly, and thermal conductivity is obtained by multiplying
thermal diffusivity by the density and specific heat capacity. To
measure thermal diffusivity, a small, thin disc specimen mounted
horizontally or vertically is subjected to a high-density short
duration thermal pulse. The energy of the pulse is absorbed on the
front surface of the specimen and the resulting rear surface
temperature rise is measured. The ambient temperature of the
specimen is controlled by a furnace or cryostat. Thermal
diffusivity values are calculated from the specimen thickness and
the time required for the rear surface temperature rise to reach
certain percentages of its maximum value. This method has been
described in detail in a number of publications and review
articles. See, e.g., F. Righini, et al., "Pulse Method of Thermal
Diffusivity Measurements, A Review," High Temperature-High
Pressures, vol. 5, pp. 481-501 (1973).
[0055] A series of specimens was prepared according to the standard
test procedure. The specimens included the following TFR-improved
grades: 7 .mu.m WC/8% Co ("708"), 7 .mu.m WC/10% Co ("710"), 7
.mu.m WC/12% Co ("712"), 8 .mu.m WC/8% Co ("808"), 8 .mu.m WC/10%
Co ("810"), and 8 .mu.m WC/12% Co ("812"). Thermal diffusivity of
these specimens was measured by the flash method, and thermal
conductivity was calculated accordingly. The thermal conductivity
data shows that the TFR-improved grades of cemented tungsten
carbide have a thermal conductivity greater than K.sub.min as
determined by Equation 1. FIG. 5 shows thermal conductivity data
for standard grades and TFR-improved grades having various
percentages of cobalt by weight. In the plot, squares are used to
represent the standard grade while circles are used to represent
the TFR-improved grades, or coarse grain grades. It can be seen
that the coarse grain grades have thermal conductivities higher
than those of the standard grades. Also, all the coarse grain
grades have thermal conductivities higher than K.sub.min.
EXAMPLE 2
[0056] This example shows that TFR-improved grades with a lower
cobalt content have improved toughness compared to conventional
grade carbides at a similar hardness. Hardness is determined by the
Rockwell A scale. To evaluate the toughness of a carbide, the ASTM
B771 test was used. It has been found that the ASTM B771 test,
which measures the fracture toughness (K1c) of cemented tungsten
carbide material, correlates well with the insert breakage
resistance in the field.
[0057] This test method involves application of an opening load to
the mouth of a short rod or short bar specimen which contains a
chevron-shaped slot. Load versus displacement across the slot at
the specimen mouth is recorded autographically. As the load is
increased, a crack initiates at the point of the chevon-shaped slot
and slowly advances longitudinally, tending to split the specimen
in half. The load goes through a smooth maximum when the width of
the crack front is about one-third of the specimen diameter (short
rod) or breadth (short bar). Thereafter, the load decreases with
further crack growth. Two unloading-reloading cycles are performed
during the test to measure the effects of any residual microscopic
stresses in the specimen. The fracture toughness is calculated from
the maximum load in the test and a residual stress parameter which
is evaluated from the unloading-reloading cycles on the test
record.
[0058] Two groups of specimens were prepared according to the
standard test method. One group consisted of specimens of the
following conventional grades: 4 .mu.m WC/11% Co ("411"), 5 .mu.m
WC/10% Co ("510"), 5 .mu.m WC/12% Co ("512"), 6 .mu.m WC/14% Co
("614"), and 6 .mu.m WC/16% Co ("616"). The other group consisted
of specimens of the following TFR-improved grades: 708, 710, 712,
808, 810, and 812. FIG. 6 shows the resultant fracture toughness
data plotted against hardness. It can be seen that the fracture
toughness of the coarse grain grades are similar to, or greater
than, those of the standard grades.
EXAMPLE 3
[0059] This example provides wear resistance data for the
TFR-improved grades which are compared with the wear resistance
data of conventional grades as shown in FIG. 7. Wear resistance can
be determined by several ASTM standard test methods. It has been
found that the ASTM B611 correlates well with field performance in
terms of relative insert wear life time.
[0060] The test was conducted in an abrasion wear test machine
which has a vessel suitable for holding an abrasive slurry and a
wheel made of annealed steel which rotates in the center of the
vessel at about 100 RPM. The direction of rotation is from the
slurry to the specimen. Four curved vanes are affixed to either
side of the wheel to agitate and mix the slurry and to propel it
toward a specimen. The testing procedure is described below.
[0061] A test specimen with at least a {fraction (3/16)} inch
thickness and a surface area large enough so that the wear would be
confined within its edges was prepared. The specimen was weighed on
a balance and its density determined. Then, the specimen was
secured within a specimen holder which is inserted into the
abrasion wear test machine and a load is applied to the specimen
that is bearing against the wheel. An aluminum oxide grit of 30
mesh was poured into the vessel and water was added to the aluminum
oxide grit. Just as the water began to seep into the abrasive grit,
the rotation of the wheel was started and continued for 1,000
revolutions. The rotation of the wheel was stopped after 1,000
revolutions and the sample was removed from the sample holder,
rinsed free of grit, and dried. Next, the specimen was weighed
again, and the wear number (W) was calculated according to the
following formula: 4 W = D L ( 5 )
[0062] where D is specimen density and L is weight loss.
[0063] Two groups of specimens were prepared: one group consisted
of specimens of the TFR-improved grades: 708, 710, 712, 808, 810,
and 812; the other group consisted of specimens of the following
conventional grades: 411, 510, 512, 614, and 616. FIG. 7 shows the
wear number plotted against hardness. As in the other plots,
squares are used to represent the standard grade and circles are
used to represent TFR-improved grades or coarse grain grades. It
can be seen that the wear numbers of the TFR-improved grades are
similar to those of the standard grades. It is important to
recognize that wear resistance was not sacrificed with the increase
in fracture toughness. FIG. 8 is a plot of fracture toughness
versus wear resistance. As both wear number and fracture toughness
relate to hardness, plotting these values against one another is
useful in showing the TFR-improved grades have higher overall
performance characteristics.
[0064] As described above, TFR-improved grades of cemented tungsten
carbide may have many advantages, including improved thermal
fatigue and shock resistance while maintaining the required
toughness and wear resistance. Tungsten carbide inserts formed of
these TFR-improved grades will experience reduced thermal fatigue
and thermal shock, thereby increasing the lifetime of rock bits
which incorporate such inserts.
[0065] While the invention has been disclosed with respect to a
limited number of embodiments, those skilled in the art will
appreciate numerous modifications and variations therefrom. For
example, wear-resistant materials suitable for use in embodiments
of the invention may be selected from compounds of carbide and
metals selected from Groups IVB, VB, VIB, and VIIB of the Periodic
Table of the Elements. Examples of such carbides include tantalum
carbide and chromium carbide. Binder matrix materials suitable for
use in embodiments of the invention include the transition metals
of Groups VI, VII, and VIII of the Periodic Table of the Elements.
For example, iron and nickel are good binder matrix materials.
Although embodiments of the invention are illustrated with respect
to tungsten carbide inserts in a rock bit, the TFR-improved grades
also may be used to form any cutting elements. It should be
understood that a rock bit using three rolling cones is a preferred
embodiment. Embodiments of the invention may be practiced with any
suitable number of rolling cones. It is intended that the appended
claims cover all such modifications and variations as fall within
the true spirit and scope of the invention.
* * * * *