An earth-boring device such as a drill bit is disclosed. The earth boring device includes a rotary main cutting structure, an insert on the main cutting structure. The insert is formed of a composition having tungsten carbide and cobalt. The cobalt makes up less than approximately 9% by weight of the composition. The composition has a Rockwell A hardness greater than approximately an H.sub.min as determined by the following formula: H.sub.min =91.1-0.63X, where X represents a cobalt content by weight of the composition.

FIELD OF INVENTION
 The invention relates to improved earth-boring bits and more particularly
 to rock bits utilizing tungsten carbide inserts.
 BACKGROUND
 Earth-boring drill bits are commonly used in drilling oil and gas wells or
 mineral mines. Typically, an earth-boring drill bit is mounted on the
 lower end of a drill string. As the drill string is rotated at the
 surface, the drill bit is rotated down in the borehole as well. With the
 weight of the drill string bearing down on the drill bit, the rotating
 drill bit engages an earthen formation and proceeds to form a borehole
 along a predetermined path toward a target zone.
 A rock bit, typically used in drilling oil and gas wells, generally
 includes one or more rotatable cones that perform their cutting function
 due to the rolling and sliding movement of the cones acting against the
 formation. 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-fitted into
 undersized apertures or sockets in the cone surface. Due to their
 toughness and high wear resistance, inserts formed of tungsten carbide in
 a cobalt binder are commonly used in rock-drilling and earth-cutting
 applications.
 Breakage or wear of tungsten carbide inserts limits the lifetime of a drill
 bit. In a rock bit, inserts are subjected to high wear loads from contact
 with a borehole wall. Additionally, the inserts are exposed to high stress
 due to bending and impacting loads resulting from contact with a borehole
 bottom. The high wear load can also cause thermal fatigue which initiates
 surface cracks on the carbide inserts. These cracks are further propagated
 by a mechanical fatigue mechanism that is caused by the cyclical bending
 stress and/or impact loads applied to the inserts. The cracks may result
 in chipping, breakage, and failure of inserts.
 Inserts that cut the corner of a borehole bottom generally are subject to
 the greatest amount of thermal fatigue. Thermal fatigue is caused by heat
 generation on the gage side of the insert. The heat results from a heavy
 frictional loading component that is produced as the insert engages the
 borehole wall and slides to the bottom-most crushing position. When the
 insert retracts from the bottom, it is quickly cooled by the surrounding
 circulating drilling fluid. This repetitive heating and cooling cycle can
 initiate cracking on the outer surface of the insert. These cracks then
 propagate through the body of the insert when the crest of the insert
 contacts the borehole bottom. The time required to progress from heat
 checking to chipping and eventually to a broken insert depends upon the
 formation type, rotation speed, and applied weight. Despite lower drilling
 speeds and air cooling, the problem of thermal fatigue is more severe in
 mining bits because greater weight is applied to the bit and the
 formations usually are harder. In petroleum bits, thermal fatigue also is
 a serious concern because of the faster bit rotation speed and cooling
 with drilling mud.
 Cemented tungsten carbide generally refers to tungsten carbide ("WC")
 particles dispersed in a binder metal matrix (i.e., iron, nickel or
 cobalt). Tungsten carbide in a cobalt matrix is the most common form of
 cemented tungsten carbide. This type of tungsten carbide is further
 classified by grades based on the grain size of WC and the cobalt content.
 Existing tungsten carbide grades for inserts have been adjusted for
 desired wear resistance and toughness only. These carbide inserts
 frequently fail when high rotational speed and high weight are applied due
 to heat checking and thermal fatigue.
 Because thermal fatigue plays a critical role in limiting the lifetime of a
 tungsten carbide insert and because existing carbide grades are not
 formulated to minimize thermal fatigue in inserts, there exists an
 unfulfilled need for inserts formed of an improved tungsten carbide
 composition which will minimize thermal fatigue while maintaining desired
 toughness and wear resistance.
 SUMMARY OF INVENTION
 In some aspects the invention relates to an earth-boring cone, comprising,
 a rotating surface, a plurality of inserts that extend from the rotating
 surface, wherein the inserts are formed of a composition comprising
 tungsten carbide and cobalt, and wherein the composition of the inserts
 has a minimum Rockwell A hardness as determined by the formula: H.sub.min
 =91.1-0.63X, wherein H.sub.min is the minimum Rockwell A hardness, and X
 is the percentage cobalt content by weight.
 In an alternative embodiment, the invention relates to an earth-boring
 cone, comprising, a rotating surface, a plurality of inserts that extend
 from the rotating surface, and means for increasing thermal fatigue
 resistance of the inserts without decreasing fracture toughness or wear
 resistance of the inserts.
 In an alternative embodiment, the invention relates to a method of boring
 an earth formation comprising, providing a rock bit, providing a plurality
 of cones that are rotatably attached to the rock bit, wherein each cone
 comprises, a rotating surface, a plurality of inserts that extend from the
 rotating surface, wherein the inserts are formed of a composition
 comprising tungsten carbide and cobalt, and wherein the composition of the
 inserts has a minimum Rockwell A hardness as determined by the formula:
 H.sub.min =91.1-0.63X, wherein H.sub.min is the minimum Rockwell A
 hardness, and X is the percentage cobalt content by weight, placing the
 cones in contact the earth formation, and rotating the rock bit.

DETAILED DESCRIPTION
 Exemplary embodiments of the invention will be described with reference to
 the accompanying drawings. Like items in the drawings are shown with the
 same reference numbers.
 Embodiments of the invention provide an improved tungsten carbide
 composition that includes large WC grains with a lower cobalt content.
 Such an improved tungsten carbide composition minimizes thermal fatigue in
 tungsten carbide inserts and still maintains desired toughness and wear
 resistance. Therefore, the improved composition in accordance with
 embodiments of the invention is suitable for manufacturing inserts used on
 the main cutting structure of a rock bit.
 A typical rock bit is illustrated in FIG. 1. An earth-boring bit 10
 generally includes a bit body 20, having a threaded section 14 on its
 upper end for securing the bit to a drill string (not shown). The bit 10
 has three cones 16 rotatably mounted on bearing shafts (hidden) that
 depend from the bit body 20. The bit body 20 is composed of three legs 22
 (two legs are shown in FIG. 1) that are welded together to form bit body
 20. The bit 10 further includes a plurality of nozzles 12 that are
 provided for directing drilling fluid toward the bottom of the borehole
 and around cones 16. The bit 10 further includes lubricant reservoirs 24
 that supply lubricant to the bearings of each of the cutters. It should be
 understood that mining rock bits can be similarly constructed as described
 above. This configuration is applicable to mining bits, but typically
 there is no need for grease reservoirs 24. However, it is foreseeable that
 mining bits with grease reservoirs will be developed. A person skilled in
 the art will recognize that embodiments of the invention are suitable for
 these bits.
 FIG. 2 illustrates a cross-section of a cone 16. The cone 16 generally
 includes a frustoconical surface 17 and a main cutting structure 32. The
 frustoconical surface 17, often referred to as the "heel row surface, " is
 adapted to retain heel row inserts 30 that scrape or ream the sidewall of
 a borehole as the cone 16 rotates about the borehole bottom. The heel row
 inserts 30 primarily function to maintain a constant diameter of the
 sidewall of a borehole. The main cutting structure is defined to include a
 gage row 27 and inner rows 26. On the gage row 27, a plurality of gage
 inserts 15 are secured to cone 16 in locations along or near the
 circumferential shoulder 29. The gage row inserts primarily function to
 cut the corner of a borehole. This requires the gage row inserts to cut
 both the sidewall and the bottom of the borehole. On the inner rows 26,
 the inner row inserts 18 are sized and configured to cut the bottom of the
 borehole.
 In general, the cutting action operating on the borehole bottom typically
 is a crushing or gouging action. In contrast, the cutting action operating
 on the sidewall is a scraping or reaming action. Ideally, a crushing or
 gouging action on the borehole bottom requires a tough insert which is
 able to withstand high impact and compressive loading. The scraping or
 reaming action on the sidewall calls for a very hard, wear-resistant
 insert. Therefore, a hard and wear-resistant material is desirable for
 heel row inserts, while a tough material is desirable for inserts on the
 main cutting structure.
 For a WC/Co system, it is typically observed that the wear resistance
 increases as the grain size of tungsten carbide or the cobalt content
 decreases. However, the fracture toughness decreases as the grain size of
 tungsten carbide or the cobalt content decreases. Thus, fracture toughness
 and wear resistance (i.e., hardness) tend to be inversely related: as the
 grain size or the cobalt content is decreased to improve the wear
 resistance of a specimen, its fracture toughness will decrease and vice
 versa. Due to this inverse relationship, it is generally accepted that one
 grade of cemented tungsten carbide cannot optimally perform both cutting
 actions because it cannot be as hard as desired for scraping or reaming
 the sidewall and be as tough as desired for crushing or gouging the
 bottom. Typically, different tungsten carbide grades have been used for
 heel row inserts and for inserts on the main cutting structure.
 To obtain carbide grades with different toughness and wear resistance, the
 grain size of tungsten carbide and the cobalt content often are adjusted
 to obtain desired wear resistance and toughness. Generally, a particular
 WC grain size is selected to obtain a desired wear resistance. Then the
 cobalt content is used to adjust the toughness to a desired value. Due to
 the high wear resistance requirement for heel row inserts, existing
 carbide grades suitable for heel row inserts are typically limited to WC
 grains in the range of 2-4 .mu.m and a cobalt content in the range of
 6-11%. For example, carbide grades of 2 .mu.m WC/8% Co, 3 .mu.m WC/11% Co
 and 4 .mu.m WC/6% Co are commonly used for heel row inserts. The
 relatively small WC grains used in these grades render them highly wear
 resistant, albeit not very tough.
 For inserts on the main cutting structure, tougher carbide grades are
 generally required. Existing grades suitable for such inserts further
 depend on the extension-to-diameter ratio ("extension ratio") of the
 inserts. FIGS. 3a-3c illustrate the definition of extension ratio for an
 insert 18A, made in accordance with the invention, mounted differently in
 a cone: the insert is mounted flush to the cone 34 in FIG. 3a; the insert
 is mounted recessed in the cone 34 in FIG. 3b; and the insert is mounted
 protruding from the cone 34 in FIG. 3c. In all cases, the extension 38 is
 measured from bending point 40 to the tip of the insert. An extension
 ratio is the ratio of extension 38 over diameter 36. In general, a higher
 extension ratio requires tougher carbide. Furthermore, a tough carbide is
 preferred due to the crushing and gouging action of the inserts on the
 main cutting structure. As a result, existing carbide grades used for
 inserts on the main cutting structure typically are different from those
 used for heel row inserts.
 Existing carbide grades for inserts on the main cutting structure typically
 are limited to the following ranges: for extension ratios of 50% or more,
 cobalt contents are 10-16% and grain sizes are 4-6 .mu.m; for extension
 ratios less than 50%, cobalt contents are 9-14% and grain sizes are 3-5
 .mu.m. For example, carbide grades of 3 .mu.m WC/11% Co, 4 .mu.m WC/9% Co,
 4 .mu.m WC/11%, 4 .mu.m WC/14% Co and 5 .mu.m WC/10% Co are commonly used
 for inserts on the main cutting structure with an extension ratio of less
 than 50%. Carbide grades of 4 .mu.m WC/11% Co, 5 .mu.m WC/10% Co, and 6
 .mu.m WC/15% are commonly used for inserts on the main cutting structure
 with an extension ratio of 50% or more. Although some grades may be used
 for all extension ratios, other grades may be used only for extension
 ratios of less than 50%.
 While existing tungsten carbide grades are formulated to achieve desired
 toughness and wear resistance, they are not made to minimize thermal
 fatigue in tungsten carbide inserts. Efforts to minimize the thermal
 fatigue in tungsten carbide inserts have led to different formulations
 such as carbide grades with larger WC grains and a lower cobalt content.
 The magnitude of thermal fatigue generally depends on a number of physical
 properties such as thermal fatigue and resistance to thermal shock.
 Thermal fatigue stress may be expressed in the following equation:
EQU .sigma..sub.f =.alpha..multidot.E.multidot..DELTA.T
 where .sigma..sub.f is thermal fatigue stress, .alpha. is coefficient of
 thermal expansion, E is Young's modulus or elastic modulus, and .DELTA. T
 is temperature differential. The thermal shock resistance of a material
 may be expressed in the following equation:
 ##EQU1##
 where TFR is thermal fatigue and shock resistance, r is Poisson's ratio, K
 is thermal conductivity; and klc is fracture toughness.
 Thermal fatigue cracks in tungsten carbide/cobalt are believed to be caused
 by dissimilar thermal properties of tungsten carbide and cobalt. For
 example, the coefficient of thermal expansion for cobalt is about twice
 the coefficient of thermal expansion for tungsten carbide. Specifically,
 the coefficients of thermal expansion for cobalt and tungsten carbide are
 13.0-14.0.times.10.sup.-6 /.degree. C. and 5.0-7.0.times.10-6/.degree. C.,
 respectively. Additionally, the thermal conductivity of cobalt is
 approximately half of that of tungsten carbide. Specifically, cobalt has a
 thermal conductivity of about 0.70 W/cm.sec..degree. K., whereas tungsten
 carbide has a thermal conductivity of about 1.3-1.5 W/cm.sec..degree. K.
 Because of the large differences in thermal conductivity and coefficient
 of thermal expansion between tungsten carbide and cobalt, a stress is
 induced when the composite material is heated and cooled rapidly. Repeated
 expansion and contraction of the composite material leads to cyclical
 stress that eventually can form cracks at the weakest point in the
 composite material. These cracks normally form on the surface of the
 insert, where temperature fluctuation and matrix distortion are the
 highest. Damage begins at the surface as a network of cracks develops
 along the carbide particles. Once these cracks are started, crack growth
 accelerates rapidly and the insert begins to chip and break.
 A carbide grade that uses a reduced amount of cobalt may suffer less
 thermal damage. Lower cobalt volumes lead to lower distortion at the
 cobalt/carbide interface and therefore reduce thermally-induced stress.
 Further, decreasing the cobalt content tends to minimize cobalt depletion
 or cobalt extrusion, which can be a cause of cobalt erosion during
 operation. Cobalt erosion also contributes to insert failure. In addition
 to reducing thermal fatigue stress and cobalt erosion, a lower cobalt
 content also results in increased thermal conductivity of cemented
 tungsten carbide. Thermal conductivity of cemented tungsten carbide
 generally is inversely proportional to the cobalt content. Specifically,
 as the cobalt content decreases, the thermal conductivity of cemented
 tungsten carbide increases. Additionally, the coefficient of thermal
 expansion generally is directly proportional to the cobalt content. As
 such, when the cobalt content decreases, the thermal fatigue and shock
 resistance increases significantly because of the increase in the thermal
 conductivity and the decrease in the coefficient of thermal expansion.
 This increase in the thermal fatigue and shock resistance is further
 enhanced by increasing, the grain size of tungsten carbide. The thermal
 conductivity of cemented tungsten carbide increases as the grain size of
 tungsten carbide increases. As a result, using larger or coarser tungsten
 carbide grains results in an increase in the thermal fatigue and shock
 resistance of cemented tungsten carbide. Another attendant advantage of
 using tungsten carbide with larger grains is that it increases the
 toughness of the cemented tungsten carbide. This increase in toughness, by
 using larger WC rains, offsets the decrease in toughness when the cobalt
 content is reduced. This is important in that the carbide formulations in
 accordance with embodiments of the invention improve the thermal fatigue
 resistance of cemented tungsten carbide without decreasing its toughness.
 A person skilled in the art will recognize that the numerical ranges for
 rain sizes of tungsten carbide are either a nominal number for particle
 size or an average particle size. In a typical cemented tungsten carbide
 formulation, the tungsten carbide particles have a size distribution.
 Therefore, the numerical range for tungsten carbide grain size is only a
 convenient way to refer to the relative size of tungsten carbide particles
 in a metal matrix. They are not precisely accurate numbers. However, it is
 known that Rockwell A hardness correlates to the cobalt content and the
 tungsten carbide grain size. In fact, a carbide composition or formulation
 may be defined with a fair degree of precision by cobalt weight percentage
 and Rockwell A scale hardness. Because both cobalt content and Rockwell A
 scale hardness can be easily and accurately ascertained, they are the
 preferred parameters to define embodiments of the invention. Table 1 shows
 preferred embodiments with respective cobalt content and Rockwell A scale
 hardness.
 TABLE 1
 Hardness Range (HRa)
 % Co by Weight Most Preferred Preferred
 4% 89.0-93.0 88.0-93.0
 5% 88.0-92.5 87.0-92.5
 6% 87.0-92.0 86.0-92.0
 7% 87.0-91.5 86.5-91.5
 8% 85.0-90.7 85.0-90.7
 9% 85.0-90.5 85.0-90.5
 In some embodiments, it is preferred that a carbide formulation is made to
 have a hardness greater than a minimum hardness value (H.sub.min) as
 determined according to the following formula:
EQU H.sub.min =91.1-0.63X
 where H.sub.min is a minimal Rockwell A hardness and X is cobalt content by
 weight. With a given cobalt content, a certain grain size is selected to
 formulate a tungsten carbide grade to render its hardness greater than the
 H.sub.min for that cobalt content.
 In embodiments of the invention, it is preferred that the cobalt content is
 equal to or less than 9% by weight for extension ratios of less than 50%.
 For extension ratios of 50% or more, it is preferred that the cobalt
 content be equal to or less than 10% by weight.
 In some embodiments, rock bits are constructed from cones with inserts
 formed of the above carbide formulations. Rock bits in accordance with
 embodiments of the invention may be of the type illustrated in FIGS. 1 and
 2, except that the inserts on the main cutting structure are formed of the
 above carbide formulations. 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.
 The following examples illustrate embodiments of the invention and are not
 restrictive of the invention as otherwise described herein. For the sake
 of brevity, a carbide formulation according to embodiments of the
 invention is referred hereinafter as a "thermally-improved grade."
 EXAMPLE 1
 This example indicates that thermally-improved grade carbides with a lower
 cobalt content have similar impact strength to conventional grade carbides
 with a higher cobalt content. To evaluate the toughness of a carbide, the
 ASTM B771 test was used. It has been found that the American Standard
 Testing Manual ("ASTM") B771 test, which measures the fracture toughness
 (klc) of cemented tungsten carbide material, correlates well with the
 insert breakage resistance in the field.
 Briefly, 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 chevron-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.
 Two groups of specimens were prepared according to the standard test
 method. One group consisted of carbides of conventional grade. The carbide
 compositions of conventional grade were as follows: 5 .mu.m WC/10% cobalt;
 4.5 .mu.m WC/11% cobalt; 4 .mu.m WC/11% cobalt; 4 .mu.m WC/9% cobalt; and
 3 .mu.m WC/11% cobalt. The other group consisted of tungsten carbides of
 thermally-improved grade. The compositions of the thermally-improved grade
 were as follows: 6 .mu.m WC/8% cobalt; 6 .mu.m WC/6% cobalt; 5 .mu.m WC/8%
 cobalt; 4 .mu.m WC/9% cobalt; and 4 .mu.m WC/6% cobalt. Three specimens
 consisting of 6 .mu.m WC/8% cobalt were made, as well as two specimens
 consisting of 6 .mu.m WC/6% cobalt. Table 2 shows impact strength for each
 tested specimen.
 TABLE 2
 THERMALLY-IMPROVED
 CONVENTIONAL GRADE GRADE
 Grain Size Cobalt Impact Grain Size Cobalt Impact
 (.mu.m) Volume (%) Strength (.mu.m) Volume (%) Strength
 5 10 13.5 6 8 13.0
 4.5 11 13.5 6 8 13.0
 4 11 12.4 5 8 12.0
 6 6 12.1
 4 9 10.0 6 6 12.1
 3 11 9.5 4 6 9.3
 3 11 9.5 4 9 10.0
 Because impact strength correlates with toughness of a carbide insert in
 the field, the toughness of the thermally-improved grade carbides may be
 predicted based on these data. Table 2 shows that the impact strength of a
 thermally-improved grade using 6 .mu.m grain size WC and 8% cobalt is
 similar to that of a conventional grade using 5 .mu.m WC and 10% cobalt.
 Furthermore, the impact strength of a thermally-improved grade using 4
 .mu.m WC grains and 6% cobalt is similar to that of a conventional grade
 using 3 .mu.m WC grains and 11% cobalt. Moreover, the thermally-improved
 grades of 6 .mu.m WC/6% cobalt and 5 .mu.m WC/8% cobalt have impact
 strength similar to a conventional grade using 4 .mu.m tungsten carbide
 grains with 11% cobalt.
 EXAMPLE 2
 This example shows that carbides of a thermally-improved grade have better
 wear resistance than ones of a conventional grade with equivalent
 toughness. 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.
 The test was conducted in an abrasion wear test machine which had a vessel
 suitable for holding an abrasive slurry and a wheel made of annealed steel
 which rotated in the center of the vessel at about 100 RPM. The direction
 of rotation was from the slurry to the specimen. Four curved vanes were
 affixed to either side of the wheel to agitate and mix the slurry and to
 propel it towards a specimen. The testing procedure is briefly described
 as follows: a test specimen with at least a 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
 was determined; the specimen was placed in and fastened to a specimen
 holder which was inserted into the abrasion wear test machine; a load was
 applied to the specimen that was bearing against the wheel; aluminum oxide
 grit of 30 mesh was poured into the vessel and water was added to the
 aluminum oxide grit; just as the water had seeped into the abrasive grit,
 the rotation of the wheel was started and it continued for 1,000
 revolutions; the rotation of the wheel was stopped after 1,000
 revolutions; the sample was then removed from the sample holder, rinsed
 free of grit and dried; the specimen was weighed again, and the wear
 number (W) was calculated according to the following formula:
EQU W=D/L
 where D is specimen density and L is weight loss.
 Two groups of specimens were prepared: one group consisted of
 thermally-improved grades; the other group consisted of carbides of a
 conventional grade. The compositions of thermally-improved grades were as
 follows: 5 .mu.m WC/8% cobalt; 6 .mu.m WC/8% cobalt; 6 .mu.m WC/6% cobalt;
 and 4 .mu.m WC/6% cobalt. Compositions of conventional grades were as
 follows: 5 .mu.m WC/10% cobalt; 4 .mu.m WC/11% cobalt; and 3 .mu.m WC/11%
 cobalt. Data from the tests according to the ASTM B611 procedure for both
 groups are summarized in Table 3.
 TABLE 3
 THERMALLY IMPROVED GRADE
 WITH IMPROVED EQUIVALENT
 IMT STRENGTH
 CONVENTIONAL GRADE TO A CONVENTIONAL GRADE
 Grain Grain
 Size Cobalt B611 Wear Size Cobalt B611 Wear
 (.mu.m) Volume (%) Resistance (.mu.m) Volume (%) Resistance
 6 8 4.5
 5 10 3.7 6 8 4.5
 4 11 4.0 5 8 4.0
 6 6 4.9
 3 11 6.1 4 6 10.0
 The data in Table 3 indicate that a thermally-improved grade has equivalent
 or better wear resistance than a conventional grade having equivalent
 toughness. Specifically, according to Table 2, a thermally-improved grade
 of 6 .mu.m WC/8% cobalt has similar toughness to a conventional grade with
 5 .mu.m WC/10% cobalt. According to Table 3, the thermally-improved grade
 using 6 .mu.m WC/8% cobalt has better wear resistance than its equivalent
 (i.e., a conventional grade using 5 .mu.m WC/10% cobalt). Surprisingly,
 the thermally-improved grade using 4 .mu.m WC/6% cobalt has far better
 wear resistance than the conventional grade using 3 .mu.m WC/11% cobalt,
 although they have comparable toughness. Similarly, the thermally-improved
 grade of 6 .mu.m WC/6% cobalt is more wear resistant than the conventional
 grade of 4 .mu.m WC/11% cobalt. These data clearly support that reducing
 cobalt contents in tungsten carbide and simultaneously increasing tungsten
 carbide grain sizes result in better wear resistance while still
 maintaining the desired toughness. As explained above, such compositions
 also minimize thermal fatigue in tungsten carbide inserts made from these
 compositions. Therefore, it is possible to manufacture a
 thermally-improved tungsten carbide grade which has equivalent or better
 wear resistance without sacrificing the required toughness.
 EXAMPLE 3
 This example indicates that carbides of a thermally-improved grade have
 higher hardness than ones of a conventional grade with similar toughness.
 Hardness is determined by the Rockwell A scale. It is known that hardness
 correlates with wear resistance.
 Table 4 summarizes the testing results. Samples of conventional grades and
 thermally-improved grades were tested according to the standard procedure.
 It is noted that carbide with 5 .mu.m WC/8% cobalt has hardness similar to
 a conventional grade with 4 .mu.m WC/11% cobalt. These two kinds of
 carbide have similar impact strength. This is also true for a
 thermally-improved grade with 6 .mu.m WC/6% cobalt. On the other hand, a
 thermally-improved grade of 4 .mu.m WC/6% cobalt has a higher hardness
 than its equivalent conventional grade (i.e., 3 .mu.m WC/11% cobalt).
 Similarly, a thermally-improved grade using 6 .mu.m WC/8% cobalt is harder
 than a conventional grade with 5 .mu.m WC/10% cobalt, although they have
 similar impact strength.
 These data further support that reducing cobalt contents in cemented
 tungsten carbide and simultaneously increasing tungsten carbide grain size
 result in higher hardness while maintaining the desired toughness.
 Therefore, it is possible to manufacture a thermally-improved tungsten
 carbide grade that has better wear resistance without sacrificing the
 required toughness.
 TABLE 4
 THERMALLY
 CONVENTIONAL GRADE IMPROVED GRADE
 Grain Grain
 Size Cobalt Rockwell A Size Cobalt Rockwell A
 (.mu.m) Volume (%) Hardness (.mu.m) Volume (%) Hardness
 4.0 11 88.4-89.2 5 8 88.3-89.1
 6 6 88.6-89.4
 3.0 11 89.0-89.9 4 6 90.4-91.2
 5.0 10 87.7-88.5 6 8 88.2-89.0
 4.0 10 88.6-89.6 5 8 88.3-89.1
 6 6 88.3-89.4
 To compare the performance of a thermally-improved grade and a conventional
 grade, field tests were conducted with respect to rock bits using inserts
 formed of the following thermally-improved grades: 4 .mu.m WC/6% cobalt
 (the "406 grade"); 4 .mu.m WC/9% cobalt (the "409 grade"); 6 .mu.m WC/6%
 cobalt (the "606 grade"); and 6 .mu.m WC/8% cobalt (the "608 grade").
 These thermally improved grades are compared with the following
 conventional grades: 3 .mu.m WC/11% cobalt (the "311 grade"); and 5 .mu.m
 WC/10% Co (the "510 grade"). A bit size of 77/8 inches was used for the
 406 grade, whereas a bit size of 121/4 inches was used for the 409, 606
 and 608 grades.
 EXAMPLE 4
 This example shows that the 406 grade resulted in about a 60% increase in
 total rock bit life with no loss in drilling efficiency. A 77/8" diameter
 three-cone rotary rock bit was constructed using the 311 conventional
 grade for drill medium hardness formations. The rock formation being
 drilled consisted of compacted sandstone with large grain nodules. This
 rock bit achieved an average life of 40 hours and produced 5200 feet of
 drilling distance. The bit exhibited a dull condition with severe wear on
 all gage inserts. Consequently, the drill bit was discarded.
 In contrast, a series of five test bits using the 406 thermally-improved
 grade in the gage inserts were run at the same location. The bits achieved
 a median life of about 63 hours and a drilling distance of about 8200
 feet. This was approximately a 60% increase in total rock bit life without
 a decrease in drilling efficiency.
 EXAMPLE 5
 This example shows that the 608 grade achieved about a 10% reduction in
 volume loss over a conventional grade in a split cone test. In a split
 cone test, each rolling cone was fitted with a conventional grade in half
 of the gage inserts and a thermally-improved grade in the other half. In
 this example, the conventional grade was the 510 grade, and the
 thermally-improved grade was the 608 grade. A small indicator insert was
 placed on each rolling cone where the carbide grades were alternated.
 A rock bit using these split cones was tested in a mine in Tucson, Ariz.,
 which contained an abrasive ore with quartzite and pyrite deposits. In
 this type of formation, a medium formation drill bit achieves a median
 life of about 55 hours and a drilling distance of 3500 feet. Primary
 carbide wear failures at this mine are mainly attributed to material loss
 on the gage row inserts. This rock bit achieved an average life of 40
 hours and produced 5200 feet of drilling distance. After the bit was run
 to the median life of a standard assembly, the bit was examined for gage
 row insert wear. The 608 grade exhibited a visible reduction in volume
 loss of about 10% over the 510 grade. Furthermore, the 608 grade showed
 very little chipping and very few heat-checking cracks. There were
 indications that the 510 grade would have continued to wear and eventually
 break gage inserts due to heat-checking cracks if the tests had continued.
 EXAMPLE 6
 This example demonstrates that the 606 grade resulted in visibly
 significant decrease in volume loss of about 20% compared to the 510
 grade. A split cone was prepared using a 606 grade and a 510 grade in the
 gage row inserts. Rock bits incorporating such split cones were tested in
 a copper mine which included an abrasive ore with quartzite and pyrite
 deposits. This formation is softer than the formation tested in Example 5.
 In this formation, the median bit life is about 80 hours and the median
 drilling distance is about 13,500 feet. The test bit was run for the
 median hours and examined for volume loss on gage inserts. It was observed
 that the 606 grade resulted in a 20% reduction in volume loss as compared
 to the 510 grade.
 EXAMPLE 7
 A split cone was also prepared using a 409 grade and a 510 grade and tested
 in the same method as in Example 5. It was observed that the 409 grade
 achieved a visible reduction in volume loss of about 20%. Further, there
 were fewer large heat-checking cracks, and the overall insert condition
 was improved.
 As demonstrated above, thermally-improved carbide formulations using larger
 WC grains and a lower cobalt content may have many advantages, including
 improved wear resistance while maintaining the required toughness.
 Tungsten carbide inserts formed of such formulations experience reduced
 thermal fatigue, thereby increasing the lifetime of rock bits which
 incorporate such inserts.
 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, carbide 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 the invention include the transition metals of group
 VIII of the Periodic Table of the elements. For example, iron and nickel
 are also good binder matrix materials. Although embodiments of the
 invention are illustrated with respect to tungsten carbide inserts in a
 rock bit, the improved carbide formulations may also be used to form
 cutting elements in raise bore and shaft drill cutters. It should be
 understood that a rock bit or an earth-boring bit using three cutter cones
 is a preferred embodiment. The invention may be practiced with any number
 of cutter 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.
 While the invention has been disclosed with reference to specific examples
 of embodiments, numerous variations and modifications are possible.
 Therefore, it is intended that the invention not be limited by the
 description in the specification, but rather the claims that follow.