Note: Descriptions are shown in the official language in which they were submitted.
~237~
ROCK BITS HAVING METALLURGICALLY BO~DED
_ _ _
~UTTER INSERTS
BACKGROUND OF THE INVENTION
1. Field of the Invention
_ _
The present invention is directed to improvements
in the construction of rock bits. More particularly,
the present invention is directed to cutter cones of
roller cone rock bits and drag bits having metallurgi- ~
cally bonded cutter inserts.
2. Descri tion of the Prior Art
P
Roller cone rock bits used for drilling in subter-
ranean formations when prospecting for oil, gas orminerals have a main body which is connected to a drill
string and a plurality, typically three, of cutter cones
rotatably mounted on journals. The journals extend at
an angle from the main body of the rock bit.
As the main body of the rock bit is rotated either
from the surface through the drill string, or by a down- ¦
hole motor, the cutter cones rotate on their respective
journals. During their rotation, teeth provided in the
cones come into contact with the subterranean formation
and provide the drilling action.
Drag bits (or shear bits), on the other hand, are
typically one piece, having no rotating parts. The cut-
ting structure may include, for example, diamond chips
imbedded in a matrix on the cutting face of the bit,
~237~.2~
synthetic polycrystalline cutters mounted to the face
of the bit body, or synthetic polycrystalline discs
mounted to tungsten carbide shanks, the shanks being
subsequently interference fitted within complementary
holes formed in the face of the drag bit body.
As is known, the subterranean environment is often
very harsh. Highly abrasive drilling mud is continu-
ously circulated from the surface to remove debris of
the drilling, and for other puxposes. Furthermore,
the subterranean formations are composed of rock with
a wide range of compressive strength and abrasiveness.
One type of cutter cone, used for drilling in
higher compressive strength (harder) formations, has a
plurality of very hard cermet cutting inserts which are
typically comprised of tungsten carbide and are mounted
in the cone to project outwardly therefrom. Such a rock
bit having cutter cones containing tungsten carbide cut-
ter inserts is shown, for example, in ~nited States Patent
No. 4,358,384 wherein the general mechanical structure of
2~ the rock bit is also described.
The cutter inserts, which typically have a cylindrical
base, are usually mounted through an interference fit into
matching openings in the cutter cone or in the face at the
cutting end of a drag bit. This method, however, of mounting
,~
~371~:
the cutter inserts to the cone and within holes formed
in the drag bit face is not entirely satisfactory be-
cause the inserts are often dislodged from the cone
or the drag bit face by fluid particle erosion of body
material, 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 bit.
~onventional ~it are fabricated in several machin-
ing opera~ions which are, generally speaking, laborintensive and expensive.
None of the prior art processes are entirely
satisfactory from the standpoint of providing rock bit
cutter cones and drag bit bodies with sufficient abil-
ity to retain the cutting structure (including insert
type cutters) under severe load conditions.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide
a drag bit body wherein hard cutting inserts are affixed to the
face of the drag bit by a metallurgical bond.
It is another object of the present invention to provide
a drag bit face with cutting structures in the matrix of the
face of the bit wherein inadvertent degradation of cutter
inserts is avoided during fabrication of the drag bit.
~23'7~2
It is still another object of the present invention to
provide a drag bit body which has a tough resilient core and a
hard outer cladding obtained by a powder metallurgy process.
It is yet another object of the present invention to
provide a drag bit which the tunysten carbide cutter supports,
metallurgically bonded to the drag bit face are joined to
polycrystalline diamond blanks in a separate processing operation
- the purpose of the diamond blanks being to provide a highly wear
resistant rock cutting surface.
These and other objects and advantages are attained by a
drag bit body which has a tough shock resistant core, and hard,
cutting inserts fitted in cavities or on projections provided in
the core or matrix face of the bit. A hard cladding is disposed
on the outer surface of the drag bit face, having been
metallurgically bonded thereto in a suitable mold by a powder
metallurgy process.
In accordance with the present invention there is
provided a drag type rock bit comprising:
a drag bit core body forming an interior chamber therein,
said core forming a first cutter end and a second pin end, said
interior chamber being op~n to said pin end, said core further
including on its-exterior surface at said first cutter end, a
plurality of cavities;
a plurality of hard cutter inserts, the exterior cavities
and the cutter inserts having substantially matching dimensions so
that said cutter inserts are accommodated in the cavities without
substantial interference; and
-4
~237~22
a cladding disposed on at least the exterior sur~ace of
the core, the cladding having been deposited by a powder
metallurge technique including a step wherein compacted powder of
the cladding is heated to metallurgically bond said powder to the
core, the cladding being harder than the core, said cladding
partially embedding the cutter inserts and metallurgically bonding
said inserts to the core and to the cladding.
Also in accordance with the present invention there is
provided a process for making a drag bit type of rock bit having a
steel body and a plurality of diamond cutting tips extending from
the body at a cutting end thereof, the process comprising the
steps of:
depositing a powder composition on an outer surface of
the drag bit body;
first, heating and pressing the powder in a mold to
metallurgically bond said powder to the drag bit body and thereby
to provide an exterior cladding of the body, said cladding
substantially conforming to the desired :Einal exterior
configuration of the drag bit, and being comprised of a material
selected from a group consisting of metals and cermets having a
hardness greater than the steel body;
second, heating and pressing in a separate cycle, diamond
cutting tips onto said drag bit at a sufficiently low temperature
to avoid damage to the diamond cutting tips.
Further to the present invention there is provided a drag
bit type of a rock drilling bit used Eor drilling in subterranean
formations, the bit comprising:
,~>,
-4a-~
.
a core bit body comprising tough shock-resistant mild
steel having a first cutting end and a second pin end, said core
further comprising an interior chamber formed therein, said second
pin end being open to said chamber, and a plurality of cavities
disposed on its exterior first cutting end surface
a cladding comprising material selected from a group
consisting of tool steel and cermets;
a plurality of hard cutter inserts being dimensioned for
mounting into the exterior cavities of the first cutting end of
said core without substantial interference, the cladding
substantially covering the exterior first cutting end surface of
the core, partially embedding the cutter inserts and being
metallurgically bonded thereto, the cladding having a hardness of
at least 50 Rockwell C hardness units and having been deposited on
the core by a powc1er metallurgy process including a step of
placing a suitable powder on the exterior surface of the core to
~hich the inserts are mounted, and heating the powder to
metallurgically bond the powder to the core, the cladding having
substantially lO0 percent density, the cutter inserts comprising
tungsten-carbide, and further comprising a coating disposed on the
inserts, said coating comprising a material which substantially
prevents diffusion of carbon from the cutter insert into the core
during the powder metallurgy process.
Further to the present invention there is provided a drag
bit type of rock bit comprising:
a tough, shock-resistant, solid steel core body, the core
body having a first cutter end and a second pin end, said core
~,~
-4b--
~;~3~ 2
defining an interior chamber opened to said second pin end of said
core body, the core also having means disposed on its first cutter
end surface for accepting, through a slip fit, a plurality of
cutting inserts;
a plurality of tungsten-carbide cutter insert studs, said
insert studs having a diamond cutting element metallurgically
bonded to an end of said stud, each of the diamond inserts being
mounted into the means disposed on the exterior first cutter end
surface of the core;
an exterior cladding disposed on the core partially
embedded the diamond 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;
a means for protecting the diamond cutting elements
bonded to said tungsten-carbide stud during said cladding process,
and
a thin layer of a diffusion-preventing metal disposed
~etween each diamond cutter insert stud and the core, said layer
comprising means for preventing diffusion or carbon from the
tungsten-carbide insert stud into the core during the step of
heating under high isostatic pressure.
Further to the present invention there is provided a
process for making a drag bit type of rock bit, said drag bit
having a plurality of tungsten-carbide diamond-tipped cutter
insert studs, the process comprising the steps of:
-4c--~
~IL2~7~
depositing a thin layer of a material selected from a
group consisting of graphite, copper, copper alloys, silver,
silver alloys, cobalt, cobalt alloys, tantalum, tantalLIm alloys,
gold, gold alloys, palladium, palladium alloys, platinum, platinum
alloys, nickel and nickel alloys on the diamond tipped cutter
insert studs;
placing a plurality of the diamond tipped cutter insert
studs into cavities formed into an outer surface of a first cutter
end of a solid core of a drag bit body, said cavities being
dimensioned to accept the diamond tipped cutter insert studs with
a slip fit, the diamond tipped cutter insert studs having the thin
layer of the material selected from the group consisting of
graphite, copper, copper alloys, silver, silver alloys, cobalt,
cobalt alloys, tantalum, tantalum alloys, gold~ gold alloys,
palladium, pallad:ium alloys, platinum, platinum alloys, nickel and
nickel alloys;
depositing a suitable powder composition on the outer
surface of the drag bit body;
first, heating and pressing the powder in a suitable mold
to metallurgically bond said powder to the drag bit body and
thereby to provide an exterior cladding o the body, said cladding
having a hardness of at least 50 Rockwell C units, substantially
conforming to the desired final exterior configuration of the drag
bit, and being comprised of a material selected from a group
5 consisting of metals and cermets, and
second, a step comprising means for heating and pressing
the powder in said mold sufficiently to bond said diamond insert
-4d-~
st~lds to said outer surface of said drag bit body into a two-step
process, without destroying the diamond cutting elements
metallurgically bonded to said tungsten-carbide studs.
Further to the present invention there is provided a
process for making a drag bit type of rock bit, said drag bit
having a plurality of diamond tipped tungsten-carbide studded
inserts in a cutter end of said drag bit, the process comprising
the steps of:
depositing a thin layer of a metallic material on the
tungsten-carbide studs minus their diamond cutting tips;
placing a plurality of said coated tungsten-carbide studs
into an outer surface of a first cutter end of a solid core of a
drag bit body, said cavities being dimensioned to accept the
coated tungsten-carbide studs with a slip fit;
depositing a suitable powder composition on the outer
surface of the drag bit body;
heating said powder composition between 1900F. and
2300F. in a suitable mold for 4 to 10 hours;
pressing said powder composition during said heating
20 cycle between 15,000 and 30,000 pounds per square inch to
consolidate said powder composition on said drag bit body
providing an exterior cladding thereon, said cladding having a
hardness of at least 50 Rockwell C units, substantially conforming
to the desired final exterior configuration of the drag bit, and
being comprised of a material selected from a group consisting of
metals and cermets; and pressing and heating, in a separate cycle,
diamond cutting tips to said coated tungsten-carbide studs, a
A -4e-~
~:~37~22
nickel shim is first placed between each of said diamond cutting
tips and said tungsten-carbide studs, said heating cycle having
temperatures between 1~00E~. (650~C) and 1385F (750C) for 0.5 to
4 hours, said pressiny cycle taking place simultaneously with said
heating cycle, said pressing cycle having pressures between 15,000
and 30,000 pounds per square inch to bond said diamond tips to
said studs.
Further to the present invention there is provided a
process for making a drag bit type of rock bit, said drag bit
having a plurality of projection extending from a body of said
drag bit at a cutting end of said drag bit, the process comprising
the steps of:
depositing a suitabie powder composition on the outer
surface of the drag bit body;
heating said powder composition between 1900F. and
2300F. in a suitable mold for 4 to 10 hours;
pressing said powder composition duriny said heating
cycle between 15,000 and 30,000 pounds per square inch to
consolidate said powder composition on said drag bit bod~
providing an exterior cladding thereon, said cladding having a
hardness of at least 50 Rockwell C units, su~stantially conforming
to the desired final exterior configuration of the drag bit, and
being comprised of a material selected from a group consisting of
metals and cermets, and pressing and heating, in a separate cycle,
diamond cutting tips to said projections extending from the
cutting end of said drag bit, a nickel shim is first placed
between each of said projections, said heating cycle having
-4f-~
~l237~2
temperatures between 1200F. (650C.~ and 1385F. (750C.) for 0.5
to 4 hours, said pressing cycle taking place simultaneously with
said heating cycle, said pressing cycle having pressures between
15,000 and 30,000 pounds per square inch to bond said diamond tips
to said studs.
Preferably, metallurgical bonding of the cladding occurs
through hot isostatic pressing (~IP or HIPPING). The cutting
inserts and/or drag bit studs 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 metalluryy
processes.
In order to prevent degradation of the cemented car-
-49-
~3'7~Z2
bide studs for drag bits into undesirable "eta" phase, by
diffusion of carbon from the insert into the underlying
core duriny the powder metallurgical bonding process, and
to accommodate the mismatch in thermal expansion coeffi-
cients between the cutting insert and the ferrous core
body, a thin coating of a suitable material is depositedon the inserts prior to placement of the inserts into
corresponding cavities in the core. Examples of such
material are copper, copper alloys, silver, silver alloys,
cobalt, cobalt alloys, tantalum, tantalum alloys, gold,
gold alloys, palladium, palladium alloys, platinum,
platinum alloys, and nickel or nickel alloys.
Another alternative to prevent degradation of the
cutting inserts is to provide an alternative source of
carbon, such as a graphite layer, in the vicinity of the
cutting inserts.
With regard to drag or shear bits, the preferably
mild steel core of the bit body has machined therein a
chamber to admit hydraulic fluid ("mud") that is di-
rected through one or more nozzles strategically placedin the cutting face of the drag bit body. The interior
walls of the chamber may be clad with metal powder
or cermet in a manner similar to the powder metallurgi-
cal bonding process of the interior bearing surfaces of
the rock bit cones. An alternative to simply cladding
~237~
the walls of the nozzles in the drag bit body is to
form the nozzles such that the cladding initially fills
the nozzle bore which is later machined to the proper
diameter. In this alternative, it is preferable that
the hardness of the cladding prior to machining be rea-
sonably soft, preferably less than 40 Rockwell C.
With regard to drag matrix or shear bits, the fabri-
cation cycle is preferably a combination of stud forma-
tion and/or bonding in association with the attachment
of polycrystalline diamond (PCD) pieces to the studs or
projections in the drag or matrix bit face in a second,
separate lower temperature/pressure HlPping cycle. The
purpose of this second lower temperature/pressure cycle
is both to prevent degradation of the PCD, while permit-
ting the preferred HIP bond to be established between the
PCD and the stud or supporting projection in the bit face.
The features of the present invention c~n be best
understood, together with further objects and advantages,
from the following description ta~en together with the
appended drawings wherein like numerals indicate like
parts.
--6--
.
~237~2
sRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional view of a typical
drag bit body;
Figure 2 is a view of a synthetic polycrystalline
disc mounted to a protrusion formed in the powder metal-
lurgically formed face of the drag bit;
Figure 3is an alternative embodiment wherein a poly-
crystalline disc is bonded to a tungsten carbide stud,
the stud being interference fitted or metallurgically
bonded within a complementary recess in the face of the
drag bit; and
Figure 4 is a chart illustrating the preferred fab~
rication cycle to fabricate the drag bit. The first cy-
cle is used to form and/or bond the cladding and/or the
studs to the drag bit face. The second cycle is used for
bonding the polvcrystalline diamond pieces to the studs
and/or projection in the bit face.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In conventional drag bit construction, a plurality
of tungsten-carbide-cobalt (cermet) cutter inserts or
diamond tipped insert studs are interference fitted into
corresponding circular holes which are drilled individually
in the cutting face of a drag bit. This procedure is not
only labour intensive, but provides a drag bit which has,
under severe drilling conditions, less than adequate
retention of the cutter inserts.
~237~Z%
Referring now specifically to Figure 1, the drag
bit core, generally designated as 128, consis-ts of a machined
steel forging or body 112. The body is preferably fabri-
cated from A.I.S.I. 9315 or A.I.S.I. 4815 steel. However,
the body could be forged from a 4000 series mild steel, such as
as 4120, 4310, 4320 and 4340. These materials would be
interchangeable with 9315 steel. Regardless of the material
from which the core is made, the pin end 114 (the end that
threadably engages a drillstring) must be protected from the
cladding process 134 to facilitate the pin threading
operation (not shown).
A nozzle bore 120 may be provided in the head or
face end 116 of body 112. The internal surface of the
cylinder bore 120 may or may not be clad with the
cladding material i34, depending upon the type of hy-
draulic nozzle to be secured within the bore.
A preferable alternative to cladding the nozzle
bore 120 is to form the drag bit body such that the
intended nozzle is completely filled with cladding
material after consolidation in such a manner that
after consolidation the cladding is sufficiently soft
(preferably less than 40 Rockwell C) such that the bore
could be readily machined.
The cladding thickness may be varied on the exterior
surface 115 of the core body 112 as well as the interior
surface 113 that forms internal chamber 118.
The metal or cermet composition comprising the cladding
should satisfy the following requirements. It should
7~L22
be capable of being hardened and metallurgically bonded
to the underlyin~ core 128 to provide a substantially
one hundred percent dense claddinq of a hardness of at
least 50 Rockwell C, Many tool steel and cermet
compositions satisfy these requirements. For example,
commercially available, well-known A.I.S.I. D2, M2, M42
and S2 tool and high-strength steels are suitable for
the cladding. An excellent cladding for the present in-
vention is the tool steel composition which consists
essentially of 2.45 weight percent carbon, 0.5 percent
manganese, 0.9 percent silicon, 5.25 percent chromium,
, . ~ , .. . . .
9.0 percent vanadium, 1.3 percent molybdenum, 0.07 per-
cent sulfur, with the remainder of the composition be-
ing iron. This composition is well-known in the metal-
lurgical arts under the CPM-lOV designation 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 Nun~er 516,892.
Instead of powdered steel compositions, such pow-
dered cermets as tungsten-carbide-cobalt (~C-Co),
titanium-carbide-nickel-molybdenum (TiC-Ni-Mo), or
titanium-carbide-iron alloys (Ferro-TiC alloys) may
also be used for the cladding 134.
~5
~23~
The application of the powdered material of the clad-
ding 134 and metallurgical bonding to the underlying
core 128 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 pro-
cesses suitable for use in the present invention include
the use of a ceramic molding process (not shown) which
determines the exterior configuration of the drag
bit 1~.
Furthermore, the powder metallurgy process involves
application of high pressure to compact the powder and
heating the powdered cladding in the ceramic mold ~not
shown) at a high temperature-but below the melting tem-
perature of the powder - to transform the powder into
dense metal, or cermet, and to metallurgically bond the
same to the underlying core 128. Thus the cladding
134 incorporated in the drag
bit 100 of the present invention may be obtained by
cold pressing or cold isostatic pressing the powdered
layer 134 on the core 1~, followed ~y a step of
sintering.
A preferred process for obtaining the hard cladding
134 for the draq bit 100 of the
present invention is, however, hot isostatic pressing
(HIPping). Details of this process, including the
preparatory steps to the actual hot isostatic pressing
, _ --10--
~3~ 2
of the drag bit 100, are described in United States
Patents Nos. 3,-/00,435 and 3,804,575. When the Crucible
CPM-lOV powered steel composition is used for the drag
bit 100 of the present invention, the hot isostatic
pressing step is preferably performed between approximately
1040C to 1200C, for approximately 4 to 10 hours, at
approximately 10,500 to 30,000 g/mm2.
An ideal temperature for the pressing cycle is 1175
-- 15~ Centigrade, at a pressure of 10,500 ~ 350 g/mm2
for 8 hours.
With reference to Figures 2 and 3, the protrusions
125 and 138 are formed in the powder metallurgy mold
to provide a means to mount, for example, polycrystal-
line diamond discs, generally designated as 140 (Fig-
ure 2). These discs, as well as the diamond tipped in-
sert studs referred to earlier, are fabricated on a
tungsten carbide substrate, the diamond layer being
composed of a polycrystalline material. The synthetic
polycrystalline diamond layer (PCD) is manufactured by
the Specialty Material Department of General Electric
Company of Worthington, Ohio. The foregoing drill cut-
ter blank is known by the trademark name of STRATAPAX
drill blank.
The diamond capped tungsten carbide stud, generally
designated as 150, is provided with a complementary non-
interference sized hole 145 in protrusion 138 (Fiqure 3) so
that the insert 150 may be metallurgically bonded to the
cladding 134 on face 116 of core body 112.
~37~2~
In accordance with the presentinvention, a plurality
of cavities 145 may be provided in the outer surface of
the core 112 to receive, preferably by a slip fit, a
plurality of cutter inserts 150. The cavities 145 may be
configured as circular apertures, shown on Figure 9, but
may also comprise circumferential grooves (not shown) on
the exterior surface of the core 112. Furthermore, the
cutter inserts 150 may be of other than cylindrical con-
figuration. They may be tapered, or may have an annulus
comprising a protrusion. Alternatively, the inserts may
be tapered and oval in cross section. What is important
in this regard is that the cutter inserts 150 are positioned
into the cavities 145 without force fitting, or without
the need for fitting each individual insert into a precisely
matching hole, thereby eliminating significant labor and cost.
The cutter inserts 150 are typically made of hard
cermet material. In accordance with usual practice in
the art, the cutter inserts comprise tungsten-carbide-
cobalt cermet. ~owever, other cermets which have the
required hardness and mechanical properties may be used.
Such alternative cermets are tungsten carbide in iron,iron-nickel, and tungsten carbide in iron-nickel-cobalt
matrices. In fact, tungsten-carbide-iron based metal
cermets often match better the thermal expansion coeffi-
cient of the underlying steel core than the tungsten-
carbide-cobalt cermets.
-12-
~2~ 2
Subsequent to positioning the cutter inserts in-to the
cavities, a powdered metal or cermet composition is applied
to the exterior surface of the core to eventually become
a hard exterior cladding of the drag bit.
Since polycrystalline diamond discs are preferred
as a cutting structure for drag or shear bits, two sep-
arate hot isostatic pressing cycles may be required as
is illustrated in Figure 4. The first high-temperature/
high-pressure cycle consolidates the cladding 134 to
the core body 112 and bonds, for example, the tungsten
carbide studs 142 (Figure 3) within the cladding mate-
rial. When Crucible CPM-lOV powdered steel composition
is used during the first HIPping cycle for the drag bit
100 of the present invention, the hot isostatic press-
ing step is preferably performed between approximately
1040C to 1200C, for approximately 4 to 10 hours, àt
approximately 10,500 to 21,000 g/mm2.
After the hot isostatic pressing step, certain fur-
ther heat treatment steps well-known in the art, such
as quenching and tempering, may be performed on the
drag bit 100. The conditions for
quenching and tempering are preferably those recommended
by the suppliers of the powdered steel composition which
is used for the cladding 134.
Alternatively J for drag bits, once the cladding is
consolidated, a sufficiently hard (greater than 50 Rock-
well C) and abrasion resistant surface layer may be ob-
tained by rapid cooling the bit, thereby requiring no
-13-
further heat treatment. Such a cooling cycle is typi-
cally available in hot isostatic pressing units equipped
with a convective cooling device. A cold inert gas flow
may also adequately cool the bit.
The second cycle (less temperature and pressure)
serves to metallurgically bond the PCD (polycrystalline
diamond) disc 140 to the cladding material ~130, Fig-
ure 2) or the disc 150 to the tungsten carbide stud 142
(130, Figure 3). In Figures 2 and 3, a nickel shim 131
may be used to bond the PCD discs 140/150 to the pro-
trusion 126 or to the tungsten carbide stud body 142,
~Where the nickel shim is used as a diamond
bonding agent, the temperature should be between
650 and 750~C, at a pressure between
10,500 to 21,000 g/mm~ for 0.5 to 4 hours. The
Dreferred conditions of this bondinq vrocess are
650C at 10,500 g/mm2 for about 2 hours.
Where the PCD discs 140/150 are silver brazed to
the protrusion 126 or to the stud body 142, a tem~era-
ture of about 350C, at pressures ranging from 10,500
to 21,000 g/mm2 will accomplish the task. It
should be emphasized that the process as outlined above
will work equally well for both ~he steel projections
126 and the tungsten carbide studs 142.
The drag bit 100, obtained in the above-described
manner, has an exterior configuration which corresponds
to the final, desired configuration. In other words,
-14-
~37~L22
little, if any machining is required on -the exterior of the
drag bit 100 obtained in accordance with the presen-t invention.
Uniform thickness of the cladding is illustra-ted in Fig. 1,
however, it could well be an advantage to clad the head 116 of
drag bit body 112 heavier or thicker than the cladding on the
rest of the body for extended performance. The cladding on
the head 116 of the drag bit could; for example, be 5mm thick
while the rest of the drag bit body 112 (with the exception
of the threaded pin end 114) could be 3mm thick. The walls
113, forming chamber 118, could be uniformly clad to the
thickness of the drag bit body 112 or the cladding 134 on walls
113 may be thinner than the exterior cladding since the
interior of the bit is subjected to less abrasive action than
the exterior surfaces of drag bit 100.
A further, very significant advantage is that the cutter
inserts 150 and diamond disc 140 are affixed to the core 128
and to the cladding 134 by metallurgical bonds. Experience
has shown that, for example, a tungsten-carbide-cobalt insert
having an 0.5 inch (12O7mm) diameter and a 0.310 inch (7.87mm)
"grip", affixed to a steel body in accordance with the present
invention re~uires on the average a pulling force in excess
of 9500 kg to dislodge the insert from the steel body. In
contrast, conventional interference fitted inserts are dislodged
from a steel body by a force of approximately 3,200 to 4,500 kg.
Similarly, for drag bits, the metallurgical bond-
ing of the studs and/or projections into the bit face
is a substantial advantage over present art. Typi-
cally, drag bit studs/cutters interference fitted into
holes in the bit face are lost in service through
-15-
~Z3~7~22
erosion of the bit face being especially aggressive at
the base of the cutters such that a substantial portion
of the grip length of the stud/cutter can be eroded away.
The loss of these studs/cutters in service not only de-
creases the rate of drilling but introduces highly un-
desirable and difficult debris into the well which, if
not removed, will damage and/or destroy every bit put
into the well afterward. Therefore, the metallurgical
bonding of the studs into the bit face will significantly
reduce the frequency of stud/cutter loss, thereby increas-
ing the overall life of the drag bit as well as decreasing
the likelihood of an expensive fishing operation, neces-
sary to remove debris from the hole.
The cladding 134 of the cone 10 and the drag bit
100, obtained in accordance with the present invention,
is substantially one hundred percent (99.995%) dense and
has a surface hardness of at least 50 Rockwell C.
The interior of the drag bit body is internally
clad through the powder metallurgy process;
preferably a process that includes the hot isostatic
pressing step. The forged mild steel drag bit core
body 112 is provided with a machined chamber 118 and a
nozzle bore 120. A counterbore 122 may also be machined
in the body 112 to accommodate a threaded nozzle body
(not shown). Obviously, the cladding 134 resists the
abrasive effect of pressurized hydraulic drilling mud
during a drilling operation. A "wash-out" of the in-
ternal nozzle cavity has been a problem with both roll-
-16-
~23~2:~
ing cone and drag type rock bits, hence internally cladsurfaces would inhibit this type of catastrophic damage
to the cutting tools.
In accordance with still another feature of the
improved drag bit lO0 of the present invention, the tungsten-
carbide-cobalt cutter inserts has a thin coating or layer
143 of a material which preven-ts diffusion of carbon from
the tungsten carbide into the underlying steel core 128
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 128
typically is low. Loss of carbon from the tungsten
carbide results in formation of "eta" phase of the tung-
sten carbide, which has significantly less desirablemechanical properties than the original tungsten car-
bide insert.
It was discovered, in accordance with the present
invention, however, that the above-noted diffusion, un-
desirable "eta" phase formation and degradation of me-
chanical properties of the tungsten carbide inserts
150 may be prevented by providing a layer of copper,
copper alloys, silver, silver alloys, cobalt, cobalt
alloys, tantalum, tantalum alloys, gold, gold alloys,
palladium, palladium alloys, platinum, platinum alloys,
and nic.~el or nickel alloys on the cutter inserts
~237~2Z
lS0 before the inserts 150 are incorporated into
the core 128.
Alternatively, a layer of graphite (not shown) also
prevents degradation because it provides an alternate
source of carbon. A layer of graphite is readily
placed on or near the insert 150 by, for example,
applying a suspension of graphite in a volatile solvent,
such as ethanol, on the insert 150. The graphite
prevents or reduces diffusion of carbon from the tung-
sten carbide because it eliminates the driving forceof 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 in-
serts lS0 by several methods, such as electroplating,
electroless plating, chemical vapor deposition, plasma
depositionl and hot dipping. The metal layer or coat-
ing 143 on the cutter inserts is pre~erably approxi-
mately 25 to 100 microns thick.
The metal layer -143, deposited on the cutter in-
sert preferably, should not melt during the hot iso-
static pressing or sintering process. It certainly
2~ must not boil during said processes. Nickel or nickel
alloys are most preferred materials for the coating or
layer 143 used in the present invention.
o -18-
~237 31L;~:~
The metal coating 143 on the inserts 150 not
only prevents the undesirable "eta" phase formation in
the inserts 150, but also provides a transition layer
of intermediate thermal expansion coefficient between
the tungsten carbide inserts 150 and the surrounding
ferrous metal cladding 134 and core 128. In the
absence of such a transition layer, the boundary may
crack. Nevertheless, as it was noted above, test re-
sults in the absence of such a metal coating still show
significant improvement over nonmetallurgically bonded
inserts with regards to the force required to dislodge
the inserts~
A "cemented carbide" is defined as a solid and coherent
mass made by pressing and sintering a mixture of powders
of one or more of the metallic carbides and a much smaller
amount of a metal, such as cobalt, to serve as a binder.
--19--
