Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention i5 directed to improvements in
the construction of roc~ bits. More particularlyl the present
invention is directed to cutter cones of rock bits having
metallurgically bonded cutting inserts.
Rock bits used for drilling in subterranean fo~nations
when prospecting for oil, gas or minerals, have a main body
which is connected to a drill string, and a plurality, typically
three, cutter cones rotatably mounted on journals. The journals
extend at an angle from the main body of the rocX bit~
As the main body of the rock bit is rotated either
from the surface through the drill string, or by a downhole
motor, the cutter cones rotate on their resyective journals.
During their rotation, teeth provided in the cones come into
contact with the subterranean formation, and provide the drill-
ing action.
As is known, the subterranean environment is often
very harsh. Highly abrasive drilling mud is continuously
circulated from the surface to remove debris of the drilling,
and for other purposes. Furthermore, the suhterranean for-
mations are composed of rock with a wide range of compressive
strength and abrasiveness.
Generally speaking, the prior art has provided two
types of cutter cones to cope with the above-noted conditions
and to perform the above-noted drilling operations. The first
typc of drilling cone is known as "milled-tooth" cone becausc
the cone has relatively sharp cutting teeth obtained by appro-
priate rnilling of the cone hody. Milled tooth cones, generally
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33~
have a short life and are used ~or drilling in low compressive
strength (soft) subterran~an formations.
A second type of cutter cone, used for drilling in
higher compressive strength (harder) formations, has a plurality
o very hard cermet cu~ting inserts which are typically com-
prised of tungsten carbide and are mounted in the cone to
yro~ect outwardly therefrom. Such a rock bit having cutter
cones containing tungsten carbide cutter inserts is shown, for
exarnple, in United States Patent No. 4,358,384 wherein the
general mechanical structure of the roc~ bit is also descxibed.
The cutter inserts, which typically have a cylindrical
base, are usually mounted through an interference fit into
matching openings in the cutter cone. This method, however, of
mounting the cutter inserts to the cone is not entirely satis-
factory because the inserts are often dislodged from the cone by
excessive force, repetitive loadings or shocks which unavoidably
occur during drilling.
Another problem encountered in the manufacture of rock
bits, relates to the number of machining and other steps
required to fabricate the cutter cone. Conventional cutter
cones are fabricated in several machining operations, which are,
generally speaking, labor intensive and expensive.
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Furthermore, the internal portion of the cutter cone
includes a friction bearing wherethrough the cone is mounted to
the respective journal. It also includes bearing races for
balls to retain the cone on the journal. These internal bearing
surfaces of the cone must be sufficiently hard to avoid undue
wear and to support the loads encountered in drilling. To
accomplish this, it has been customary in the prior art to
selectively carburise certain pre-machined internal surfaces of
the cone.
None of the prior art processes are entirely
satisfactory from the standpoint of providing rock bit cutter
cones in sufficiently simple (and therefore inexpensive~
procedures with sufficient ability to retain the cutter inserts
under severe load conditions.
SUMMARY OF THE INVENTION
The cutter cone according to the invention has a tough
shock resistant core, and hard, cuttin~ inserts fi~ted in
cavities provided in the core. A hard cladding is disposed on
the outer surface of the cone having been metallurgically bonded
thereto, preferably in a suitable mold by a powder metallurgy
process.
Preferably, metallurgical bonding of the cladding
occurs through hot isostatic pressing. The cu~ting inserts are
also metallurgically bonded to the core and to the cladding as a
result of the formation of the cladding through hot isostatic
pressing or like powder mctallurgy processes.
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The interior of the cone incorporates conventionally
machined bearing surfaces and races for attachment of the cutter
cone to a respective journal of the rock bit~ As a preferred
alternative, however, the bearing surfaces and bearing races are
forrned in the interior of the cone from a metal powder or cermet
in the same or similar powder metallurgical bonding process
wherein the exterior cladding is bonded and hardened. As still
another alternative, the bearing surfaces are formed in a
separate piece which is subsequently affixed into a bearing
cavity provided in the core.
In order to prevent degradation of the cutting inserts
into undesirable "eta" phase, by diffusion of carbon from the
insert into the underlying core during the powder metallurgical
bonding process, and to accomodate the mismatch in thermal
expansion coefficients between the cuttin~ insert and the
ferrous core body, a thin coating of a suitable material is
deposited on the inserts prior to placement of the inserts int-o
corlesponding cavities in the core. Examples of such material
are copper, copper alloys, silver, silver alloys, cobalt, cobalt
alloys, tantalum, tantalum alloys, gold, gold alloys, pa.lladium,
palladium alloys, platinum platinum alloys, and nickel or
nickc;l alloys.
~ nother alternative to prevent degradation of the
cutting inserts .is to provide an alternate source of carbon such
as a graphite laycr, in the vicinity of the cuttinc3 inserts.
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According to the present invention there is provided a
cutter member of a rock bit comprising:
a core including a plurality of cavities on its exterior
surface;
a plurality of hard cutter inserts in the cavities in the
core;
a powder metallurgy cladding metallurgically bonded on
the exterior surface oE the core, and being metallurgically bonded
ko the cutter inserts for retaininy the cutter inserts in the
core; and
means disposed on the cutter inserts for substantially
preventing diffusion of carbon from the cutter inserts into the
core and the cladding during heating of the cladding for
metallurgically bonding the same to the core.
Also according to the present invention there is provided
a process for making a cutter member of rock bit having a
plurality of tungsten-carbide cutter inserts, the process being
characterized by the steps of:
depositing a thin layer of a material selected from a
group consisting of graphite, copper, copper alloys, silver,
silver alloys, cobalt, cobalt alloys, tantalum, tantalum alloys,
go:Ld, gold alloys, palladiurn, palladium alioys, platinum, platinum
alloys, and nickel or nickel alloys on a plurality o~ cutter
lnserts;
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placing the plurality of cutter inserts into cavities
formed in the outer surface of a solid core of t~le cutter member,
depositing a powder composition on the outer s-lrface of the core
so as to partially embed the cutter inserts;
presslng the powder in a mold to substantially conform to
the to the desired final exterior configuration of the cutter
melrlber; and
heating the powder to metallurgically bond said powder to
the melnber and thereby provide an exterior cladding of the cutter
member Eor retaining the cutter inserts in the cavities.
Further to the present invention there is provided a
process Eor securing at least one cemented carbide body to a steel
body comprising the steps of:
depositing a thin layer oE a material selected from a
group consisting of graphite, copper, copper alloys, silver,
silver alloys, cobalt, cobalt alloys, tantalum, tantalum alloys,
gold, gold alloys, palladium, palladium alloys, platinum, platinum
alloys, nickel and nickel alloys on such a carbide body;
placing such the carbide body into a cavity formed in the
outer surface of a solid body of steel, the cavity being
di.rnensioned to accept the cemented carbide body without
sllbstantial interference;
applying a powder composition on the outer surEace oE the
steel body so as to partially ernbed the cemented carbide body;
pressing the powder in a mold to substantiall.y conEorm to
a desired Einal exterior conEiguration; and
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heating the powder to metallurgically bond said powder to
the steel body and thereby provide an exterior cladding of the
steel body for retaining the carbide body in the cavity.
Further to the present invention there is provided a
cutter member oE a rock bit, comprising:
a core, including an interior opening, wherethrough the
cutter Inernber rnay be mounted to a pin connected to a drill string,
sa:id core also including, on its exterior surEace, a plurality of
cavities;
a plurality of hard cutter inserts, the cavities and the
cutter inserts having substantially matching dimensions so that
the cutter inserts are accommodated in the cavities without
substantial interference;
a cladding disposed on the exterior surface of the core,
the cladding having been deposited by a powder metallurgy
technique including a step wherein compacted powder o~ the
cladding is heated to metallurgically bond said powder to the
core, the cladding being substantially harder than the core, said
cladding partially embedding the cutter inserts and
rneta:Lluryically bonding sai.d inserts to thC core and to the
c.ladd.ing, and
means disposed on the cutter inserts for substantially
preventing diffusion Oe carbon fcom the cutter inserts into the
core and the cladding during the step wherein compacted powder of
the c.ladcliny is heated to metallurgically bond the same to the
core.
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Further to the present invention there is provided a
cutter cone of a rock drilling bit used for drilling in
subterranean forrnations and adaptecl for mounting to a journal leg
of the rock drilling bit~ the cone comprising:
A tough, shock-resistant steel core having an interior
opening wherethrough the cone is rotatably mounted to the journal,
and a plurality of cavities disposed on its exterior surface;
a plurality of hard cutter inserts comprising
tungsten-carbide and being dimensioned Eor mounting into the
exterior cavities of the core without substantial interference;
a cladding comprising material selected from a group
consisting of tool steel and cermets, said cladding substantially
coverng the exterior surface of the core, partially embedding the
cutter inserts and being metalurgically bonded thereto, having a
hardness of at least 50 Rockwell C hardness units and having been
deposited on the core by a powder metallurgy process, including a
step of placing a suitable powder on the exterior surface of the
core to which the inserts are mounted, and heating the powder to
metallllrgically bond the powder to the core, the cladding having
substantially 100 percent density, and
a coating disposed on the cutter inserts comprising a
material. which substantially prevents difEusion of carbon from the
cutter inserts into the core during the powder metallurgy process.
Further to the present invention there is pcovided a
cutter cone rotatably mountable on a journal oE a rock bit oE the
type having a plurality of journals disposed anc3ll1arly relative to
the rotational axis of the rock bit, the cone comprising:
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a tough~ shock-resistant, solid steel core, the core
having ar, interior opening wherethrough the cone is mounted on its
respective journal, the core also having means disposed on its
surface for accepting, through a slip fit, a plurality of cutter
inserts;
a plural.ity of tungsten-carbide cutter inserts, each of
the cutter inserts being mounted into the means disposed on the
exter:ior surface of the core;
an exterior cladding disposed on the core partially
embeddirly the cutter inserts, having a hardness of at least 50
Rockwell C units, said cladding having been deposited on the core
by a powder metallurgy process including a step wherein a suitable
metal powder is heated under high isostatic pressure to
metallurgically bond said powder to the core and to
metallurgically bond the cutter inserts to the core and cladding,
and
a thin layer of a diffusion preventing metal disposed
between each cutter insert and the core, said layer comprislng
means for preventing diffusion of carbon from the tungsten-carbide
insert into the core during the step of heating under high
isostatic pressure.
Further to the present invention there is provided a
process for makin~ a cutter member of a rock bit of the type
rnounted throu~h a pln to a drill string, the cutter member having
a plurality o~ tungsten-carbide cutter inserts, the process
compr;.sincJ the steps o~:
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depositing a thin layer of a material selected from a
group consisting of graphite, copper, copper alloys, silver,
silver alloys, cobalt, cobalt alloys, tantalum, tantalum alloys,
gold, gold alloys, palladium, palladium alloys, platinum, platinum
alloys, nickel, and nickel allo~s on the cutter inserts;
after said step of depositing, placing a plurality of the
cutter inserts into cavities formed in the outer surface of the
sol:id core of the cutter member, said cavities being dimensioned
to accept the cutter inserts without substantial interference;
depositing a suitable powder composition on the outer
surface of the core so as to partially embed the cutter inserts,
and
heating and pressing the powder in a suitable mold to
metallurgically bond said powder and said cutter inserts to the
member and thereby to provide an exterior cladding of the cutter
member, said cladding having a hardness of at least 50 Rockwell C
units, substantially conforming to the desired final exterior
configuration of the cutter member, and being comprised of a
Material selected from a group consisting of metals and cermets.
Further to the present invention there is provided a
cutter cone to be mounted on a journal of a rock bit comprising:
a solid core including an interior opening wherethrough
the cutter cone may be rotatably mounted to a journal o the rock
bit, said core also inclucling, on its exterior surface, a
p:Lurality o cavities;
a plurality o~ hard cutter inserts in the cavit:ies in the
core, and
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a powder metallurgy cladding metallurgically bonded on
the exterior surface of the core, and comprising means for
rnetallurgically bonding the cutter inserts to the core and to the
cladding and for Letaining the cutter inserts in the core.
Further to the present invention there is provided a
process for making a cutter cone for a rock bit of the type having
at least one journal on which the cutter cone is rotatably
rrlounted, the cutter cone having a plurality of cutter i.nserts, the
process cornprising the steps of:
placing a plurality of cutter inserts into cavities
Eormed in the outer surface of a solid core of the cutter cone;
depositing a powder composition on the outer surface of
the solid core so as to partially embed the cutter inserts,
pressing the powder in a mold to substantially conform to
the desired final exterior configuration of the cutter cone, and
heating the powder to bond said powder to the cone, an
exterior cladding of the cutter cone being formed in said steps of
heating and pressing, and said cladding serving as means for
retaining and metallurgically bonding the cutter inserts in the
cavities.
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The features of present invention can be best under-
stood, together with further objeets and advantages, from the
following deseription taken together with the appended drawings
wherein like numerals indicate like parts.
BRIEF DESCRIPTION OF THE DRAWINGS
Fiqure 1 is a perspective view of a rock bit ineor-
por~ting the cutter cone of the present invention.
Figure 2 is a eross seetional view of a journal leg of
~ rock bit with the cutter cone of the present invention mounted
thereon;
Figure 3 is a schematie eross sectional view of an
intermediate in the fabrication of the cutter cone of the
present invention, the intermediate having a solid core;
Figure 4 is a schematie cross-seetional view of an
intermediate in the process of fabricating another embodiment of
the eutter cone of the present invention;
Figure 5 is a schematie eross-sectional view of a
tungsten carbide cobalt (cermet3 insert coated with a layer of
nickel, which is encorporated in the cutter cone of the present
invention, and
Figure 6 is schematie representation of a Scanning
Electron Microscope (SE~) micrograph of the boundary layers
between the tungsten carbide cobalt insert and a nick~l coating
on the cne hand, and the nickel coating and underlying mild
steel core, cn the other hand.
3~
D~SCRIPTION O~ TH~ PREF~RRED E~iBODI~NTS.
Referring now to the drawing figures, the perspective
view of Figure 1 shows a rock bit 8 wherein a cutter cone of the
present invention is mounted. The cross-sectional view of
Figure 2, shows mountinq of a first embodiment of the cutter
cone 10 of the present invention to a journal leg or journal 12
of the rock bit 8.
It should be noted at the outset, that the mechanical
configurations of the rock bit 8, the journal 12 and of the
cutter cone 10 are conventional in many respects, and therefore
need to be disclosed here only to the extent they differ from
well known features of conventional rock bits. For a descrip-
tion of the conventional features of a rock bit, the specifica-
tion of United States Patent No. 4,358,384 is incorporated
herein by reference.
For the purpose of explaining the several features of
the cut-ter cone, it is deemed to sufficient to note that,
in conventional rock bit cons:ruction internal friction bearing
surfaces 14 and ball races 1;i are lubricated by an internal
supply of a lubricant ~not shown). The bearing surfaces 14 and
ball races 16 are sealed from extraneous material, such as
drilling mud and drilling debris, by a suitable seal, such as an
elastic O-ring seal 20. The conventional internal bearings are
usually of the "hard-on-soft" type; e~g. a hard metal bearing
surface of the journal 12 engages a bronze bearing surface ~4 of
the cutter cone 10~
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Furthermore, in conventional cutter cone construction,
a plurality of tungsten carbide cobalt (cerme~) cutter inserts
26 are interference fitted into corresponding circular holes
which are drilled individually in the cutter cone 10. This
procedure is not only labor intensive, but provides a cutter
cone which may have, under severe drilling conditions, less than
ad~q~late retention of the cutter inserts 26.
~ eferring now principally to Figure 3, a solid core 28
o~ the cutter cone 10 is shown in a first embodiment. The core
2~ comprises touyh, shock resistant steel, such as mild steel,
~or example A.I.S.I. 9315 steel, or A.I.S.I. 4815 steel. In
alternative embodiments, the core 28 itself, may be made by
powder metallurgy techniques.
A plurality of cavities 30 are provided in the outer
surface 32 of the core 28 to receive, preferably by a sliding
fit, a plurality of cutter inserts 26. The cavities 30 may be
configured as circular apertures, shown on Figure 3, but may
also comprise circumferential grooves (not shown) on the
exterior surface 32 of the core 28. Furthermore, the ~utter
inserts 26 may be of other than cylindrical configuration. They
may be tapered, as is shown on Figure 5, or may have an annulus
~not shown) comprising a protrusion~ Alternatively, the inserts
m.l~ be tApered and oval in cross-section. What is important in
khis regard is that the cutter inserts 26 are positioned into
the cavities 30 without force fitting, or without the need for
pre~cision fitting each individual insert 26 into a precisely
matching hole, thereby eliminating significant labor and cost.
The cutter inserts 26 are typically made of hard cermet
m~teriaL. Ir. accordance with ~sual practice in the art, the
~Z3~
cutter inserts comprise tungsten-carbide cobalt cermet.
~lowever, other cermets which have the required hardness and
mechanical properties, may be used. Xuch alternative cermets
are tungsten-carbide in iron, iron-nickle, and tungsten-carbide
in iron-nickle cobalt. In fact, tungsten-carbide-iron bas~d
rnetal cermets often match better the thermal expansion
coe~icient of the underlying steel core 28, than
t~ngsten-carbide-cobalt cermets.
Subsequent to positioning the cutter inserts 26 into
the cavities 30, a powdered metal or cermet composition i5
applied to the exterior surface 32 of the core 28, to eventually
become a hard exterior cladding of the eutter cone 10.
The metal or cermet composition is schematically shown
on Figure 3 as a layer or cladding bearing the reference num~ral
3~. As is explained below, one function of the cladding is to
retain the insert 26 in the core 28.
The metal or cermet composition comprising the
cladding, should satify the following requirements. It should
be capable of being hardened and metallurgically bonded to the
underlying core 2~ to provide a substaintially one hundred
percent dense cladding of a hardness of at least 50 Rockwell C
units. Many tool steel, and cermet compositions satify these
recluirements. For example, commercially available, well known,
A.~.S.I. D2, M2, M~2, ancl S2 tool and high strength steels are
suitable for the cladclincJ. An excellent cladding for the
pre~ent invention is the tool steel composition which comprises
2.~5 weig}lt percent carbon, 0.5 percent macJnese, 0.9 percent
silicon, 5.25 percent chromium, 9.0 percent vanadium, 1.3
percent molybdenum, 0.07 percent sulphur, with the remainder of
~ _
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the composition being iron. This composition is well known in
the metallurgical arts under the CPM lOV design~ti~n of the
Crucible Metals Division of Colt Industries. Still another
excellent cladding material is a proprietary alloy of the
above-noted Crucible Metals Division, known under the
development number 516,892.
Instead of powdered steel compositions, such powdered
cerm~ts as tungsten carbide cobalt (WC-Co),
titanium-carbide-nickel-molybdenum, (TiC-Ni-Mo) or
titanium-carbide-iron alloys (Ferro-TiC alloys) may also be used
~or the cladding 34.
The application of the powdered material of the
cladding 3~ and metallurgical bonding to the underlying core 28
and its subsequent hardening are performed in accordance with
well known powder metallurgy processes and conventional heat
treatment practices. Although these well known processes need
not be disclosed here in detail, it is noted that the powder
metallurgy processes suitable for use include the use of a mold
(not shown) which determines the exterior configuration of the
cutter cone 10.
Futhermore, the powder metallurgy proces~ involves
application of high pressure to compact the powder, and a step
oE heating the powdered cladding in the mold (not shown) at a
hiCJh temperature, but below the melting temperature of the
powder, to transform the powder into dense metal, or cermet, and
to metallurgically bond the same to the underlying core 28.
Thus, the cladding 34 incorporated in the cutter cone lO may be
obtained by cold pressing or cold isostatic pressing the
powdered layer 3~ on the core 2~, followed by a step of
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sintering.
A preferred process for obtaining the hard cladding 34
for th~ cutter cone 10 is, however, hot isostatic pressing
(HIPping). Details of this process, including the preparatory
steps to the actual hot pressing of the cutter cone 10, are
described in United States Patent Nos. 3,700,435 and 3,804,575,
the specifications of which are hereby expressly incorporated by
ref~rence. When the Crucible CPM-lOV powdered steel composition
i~ u.~ed for the cutter cone 10, the hot isostatic pressing step
i~ preEerably performed between approximately 1900 to 2000F,
~or approximately 4 to 8 hours, at approximately 15,000 to
30,000 PSI.
After the hot isostatic pressing step, certain further
heat treatment steps, well known in the art, such as quenchiny
and tempering, are performed on the cutter cone 10. The
conditions for quenching and tempering are perferably those
recommended by the suppliers of the powdered steel composition
which is used for the cladding 34.
Referring still principally to Figures 2 and 3, the
cutter cone 10 obtained in the above described manner has an
exterior configuration which corresponds to the final, desired
configuration of the cutter cone 10 useable in a rock bit. In
other words, little, if any, machining is required on the
~xt~r.ior of the cutter cone 10. Thickness of the cladding i5
not critical the cladding may, for example, be 1/8 inch (3.2 mm)
th:ick.
~ further, very s.ignificant advantage is that the
autter :inserts 26 are affixed to the core 28 and to the cladding
3~ by metallurgical bonds. ~xperience has shown that a tungsten
-- 11 --
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carbide cob~lt insert (of the size normally used for rock bits,
having 0.5" diameter and 0.310" "grip") affixed to the cutter
cone 10 as described herein requires on the average a pulling
~orce in excess of 21,000 lbs. to dislodge the insert from the
cone lOo In contrast, conventional, interference fitted inserts
are dislodged from the cone 10 by a force o~ approximately 7,000
to 10,000 lbs.
1'he cladding 34 of the cone 10 is substantially one
hurldred percent (99.995%) dense, and has a surface hardness of
at least 50 Rockwell C Units.
rrhe interior of the solid intermediate cutter cone 10
shown on Figure 3 may be machined independently of the hot
isostatic pressing process, to provide the cutter cone interior
shown on Figure 1. Alternatively, the core 28 itself may be
formed by powder metallurgy in steps separate from the
above-described steps. Furthermore, conventional, bearing
surfaces, for example, aluminum-bronze, or hard metal bearings,
for example, cobalt based hard facing alloys may be applied in~o
the interior of the cone 10 in accordance with state of the art.
~ s still another alternative, the bearing surfaces may
be formed separately from the fabrication of the core 28. In
this case, a separate bearing insert piece (not shown) is fitted
into the hollow core.
Referring now to Figure 4, a second embodiment of the
cutter cone 36 is shown. This embodiment has interior bearing
sur~aces 38 and races 40 obtained by a powder metallurgy
process, preferably a process including a hot isostatic pressing
.step. Thus, in order to obtain the cutter cone~ 36 shown on
E'igure 4, a forged mild steel core is provided by a machined
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interior cavity, or opening 42, and a plurality of e~terior
cavities or aperatures 30. The exterior aperatures 30 receive
cutter inserts 26 in a sliding fit, as it was described in
connection with the first embodiment. The exterior cladding 34
is applied to the core 10 in the manner described in connection
with the first embodiment.
However, simultaneously with, or subsequent to the
powder metallurgy process wherein the cladding 34 (not
separately shown in E'igure 4) is bonded, a powdered metal or
cermet composition is also bonded in the interior ~avity 42
through a powder metallurgy process, to provide the bearing
races 40 and bearing surface 38. In this case, the interior
surfaces of the cutter cone 36 emerge fro~l the hot isostatic
pressing process in a "near-net" shape, and therefore do not
require extensive finish machining.
There is a significant advantage of obtaining very hard
bearing surfaces 38 and races 40, such as tungsten-carbide
cobalt, in the cutter cone 36. Mamely, when such bearing
surfaces and races have "hard" counterparts on the rock bit
journal 12, then external lubrication and cooling may be
aEfected by circulating drilling mud, rather than by an internal
supply of a lubricant. This, of course, eliminates the need for
a sealing device such as an 0 ring seal 20 (shown on Figure 2)
ancl ~liminates problems associated with degradation or wear of
the seal 20. Rock bits having no seal, but rather bearings open
to the ambient environment, are known in the art as "open
bearing" bits.
Referring now to Figure 5, still another feature of the
-improved cutter cone 10 is disclosed. In accordance with this
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feature, the tungsten carbide cobalt cutter inserts 26 have a
thin coating or layer 44 of a material which prevents diFfusion
of carbon from the tungsten carbide into the underlying steel
core 28 during the high temperature hot isostatic pressing or
sintering process. As is known, such diffusion has a
significant driving force because the carbon content of the
steel core 28 typically is low. Loss of carbon from the
tungsten carbide results in formation of "eta" phase of the
tUnCJsten carbide, which has significantly less desirable
mechanical properties than the original tungsten carbide insert.
It was discovered, however, that the above-noted
difEusion, undesirable "eta phase" formation, and degradation of
mechanical properties of the tungsten carbide inserts 26 may be
prevented by providing a layer of copper, copper alloys, silver,
silver alloys, cobalt, cobalt alloys, tantalu~l tantalum alloys,
gold , gold alloys, palladium, palladium alloys, platinum,
platinum alloys, and nickel or nickel alloys on the cutter
inserts 26 before the inserts 26 are incorporated into the core
28.
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Alternatively, A layer of graphite (not shown) also
prevents degradation because it provid~s an alternate source of
carbon. A layer of graphite is readily placed on or near the
insert 26 for example, by applying a suspension of graphite in a
volatile solvent, such as ethanol, on the insert 26. The
graphite prevents or reduces diffusion of carbon from the
tunysten carbide because it eliminates the driving force of the
diffusion.
The other metals noted above, prevent or reduce dif-
fusion of carbon by virtue of the limited solubility of carbon
in these metals at the temperatures and pressures which occur
during the hot isostatic pressing process.
The metal coatings may be applied to the cutter
inserts 26 by several methods, such as electroplating, electro-
less plating, chemical vapor deposition, plasma deposition and
hot dipping. The metal layer or coating 44 on the cutter
inserts is preferably approximately 25 to 100 microns (0.001 to
0.004") thic~.
The metal layer 44 deposited on the cutter insert
preferably should not melt during the hot isostatic pressing or
sintering process. It certainly must not boil during said
processes. Nickel or nickel alloys are most preferred materials
for the coatiny or layer 44 used in the present invention.
~ 15
The metal coating 44 on the inserts 26 not only
prevents the undesirable neta" phase formation in the insert 26,
but also provides a transition layer of intermediate thermal
expansion coefficient between the tungsten carbide inserts 26
and the surrounding ferrous metal cladding 34 and core 28~ In
the absence of such a transition layer the boundary cracks
readily. Nevertheless, as it was noted above, test results in
the absence of such a metal coating still show significant
improvement over non-metallurgically bonded i~serts with regards
to the force required to dislodge the inserts 26. Figure 6
schematically illustrates a Scanning Elec~ron Microscope (SEM)
micrograph of the boundary layers bétween the tungsten carbide
cutter insert 26 and a nickel layer 44 on the one hand, and the
nickel layer 44 and the underlying core 28, on the other hand.
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