Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to rotary drill bits for
use in drilling or coring deep holes in subsurface
formations. In particular, the invention is applicable to
5 rotary drill bits of the klnd comprising a bit body having
an external surface on which are mounted a plurality of
cutting elements for cutting or abrading the formation,
and an inner passage for supplying drilling fluid to one
or more nozzles at the -external surface of the bit. The
nozzles are so located at the surface of the bit body that
drilling fluid emerging from the nozzles flows past the
cutting elements, during drilling, so as to cool and/or
clean them.
Although not essential to the present invention,
the cutting elements may be in the form of so-called
"preform" cutting elements in the shape of a tablet, often
circular, having a superhard cutting face formed of
polycrystalline diamond or other superhard material.
Conventionally there are two main methods of
manufacturing such drill bits. In one common type of
drill bit the bit body is machined from steel and the
surface of the bit body is formed with sockets which
receive pegs or studs on which the cutting elements are
mounted. In the second common type of bit the-bit body is
formed by a powder metallurgy process in which a hollow
mould is first formed, for example from graphite, in the
configuration of the bit body. The mould is packed with
powdered material such as tungsten carbide, usually around
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a s~eel blank, and the powdered material is then
infiltrated with a metal alloy in a furnace so as to form
a hard matrix. If the cu~ting elements are of a kind
which are not thermally stable at the infiltration
-temperature, formers are mounted on the interior surf-ace
of the mould so as to define in the finished bit body
sockets or other locations where cutting elements may be
subsequently mounted.
Steel bodied bits are generally simpler and
cheaper to manufacture than matrix bodied bits. However,
they are more susceptible to erosion during drilling and,
for this reason, they are sometimes provided with a hard
surface coating, for example of tungsten carbide, which
adds to the complexity and cost of production.
On the other hand, although matrix ~odied bits
are more resistant to erosion, their manufacture is
complex and costly due largely to the high material cost
and to the additional processes involved. Matrix bodied
bits also may be made with the matrix at the surface more
erosion resistant than that inside the bit body.
The present invention sets out to provide a new
method of manufacturing a rotary drill bit using cast
iron, in which at least some of the disadvantages of the
known steel-bodied and matrix-bodied bits may be overcome.
British Paten~ Specification No. 1,574,884
discloses the use of cast iron in the manufacture of
cutting bodies suitable for working or cutting material,
such as cutting tips or inserts in rock drilling or chip
~ 3 ~ ~ r,~
forming machines. In such products it is necessary for
surface portions of the product to be sufficiently hard to
be resistant to erosion and damage during use of the
product. Specification No. 1,574,884 describes a method
- 5 of providing a hard surface which requires the encasting
of a hard metal, such as a sintered hard carbide, e.g.
tungsten carbide, in the cast iron. Such methods have
not, however, proved satisfactory in practice due to the
high cost of the process and the difficulties of ensuring
that the carbide is securely bonded within the cast iron.
~.S. Patent Specification No. 4,499,795
describes the use of cast iron in the manufacture of a
rotary drill bit. In this case the inner surface of a
mould is packed or coated over selected portions with
paiticles of sintered tungsten carbide or similar sintered
refractory hard metal and then cast iron is melted and
poured into the mould. In this case, also, therefore, the
hard surface of the finished bit is provided by tungsten
carbide.
The present invention provides methods of
manufacturing a rotary drill bit using cast iron where the
cast iron itself forms the necessary hardened surface
portions of the drill bit, the steps of the method being
such as to effect the necessary hardening of the cast iron
during the casting process.
According to the invention there is provided a
method of manufacturing a rotary drill bit comprising
a m~th~ of m~nu~ctur~n~ ~ ro~ry ~rlll ~1
having a bi~ body, ~omp~lgl~g ~he ~tep~ o~:
(a) ~ormln~ a hollow mould ln he ~o~lr~
con~iguration of ~t lea~t ~ ~or~ion o~ th~ blt bo~y5
(b) addlng to ~ ~ir~t body of molt~n ~t
ron an lron carblde-~orming Add1tlve;
~ c) pourin~ the ~lrst bo~y o~ molt~n ~t
ir~n into the lowermo~t p~rt o~ ~he ~ouldl
(d~ coolln~ ~he mould 60 that said ~rst ~ody
of cast ~ron ~olld1fl~s and, upon solidificAtlon, ~orm#
maB~ive lnclu~ion~ of cement~te;
~ e) pourin~ a ~econd body of molt~n o~st
lron, without ~i~nificant amounts of 6uch lron carb~e-
formlng additlve, into the mould on top of th~
~olidif1 ed flrst ~ody oP cast iron; and
~ ) aoolin~ the mould ~o th~t said secon~
~ody of cast iron ~olidifle~.
Said hardening treatment ~ay comprise adding to
at least part of the molten cast iron, before it is poured
into the mould, an ixon carbide-forming additive, whereby
said part of the cast iron, upon solidification, contains
massive inclusions of cementite. Said additive may
comprise boron or tel1uri~m. Approximately 1/2% of
additive may be added t~ the molten cast iron.
If reguired the molten cast iron may be poured
int~ the mould in two successive pourings, said additive
being added only to the cast iron in ~he first pouring ~o
as to harden only the portion of the bit b~dy which is
lowermost in the mould.
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In an altern~tive methDd according ~o the
invention said hardening treatment comprises the ~tep of
chill hardening a portion of the surface of ~he bi~ body
by accelerated cooling of said surface portion during the
solidification part of ~he casting process, said
accelerated cooling being at a rate to produce massivP
~nclusions uf cementite in the ~olidified surf~ce p~rtion.
Said chill hardening may be effe~t~d by lDcating
a metal heat sink in the mould adjacent said ~urface
portion of the bit b~dy which is to be hardened.
There may be mounted on the interiDr of the
mould, prior to pouring the cast iron into the mould,
formers which project into the mould cavity so as to form
sockets in the solidified cast iron bit body, said formers
being in close thermal contact with said metal heat sink,
whereby ~he interior of each socket i5 chill hardened.
In a further method according to the invention
said hardening treatment comprises the step, before
pouring the cast iron int~ the mould, of coating at least
part of the interior surface of the mould with a material
which rea ts with ~he cast iron, during solidification
thereof, to form massive inclusions of cementite in at
least one surface portion of the bit body. Said coating
material may be tellurium.
In a still further method according to the
inventi~n said hardening treatment comprises controlling
the rate of cooling of the bit b~dy, after solidification
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of the cast iron and during ~che ~ubsesluent cooling portion
of the casting process, in such manner as to ef f ect the
formation of martensite in at least one surace porkion of
the bi t ~ody .
~he invention includes within its ~cope
a rc~t~Lry ~rlll bl'c comprl~ L b~t bo~y
havin~ an external Ecur~ce on which ~Lre mour,Lted ~L
plurality of cu~cting elemant~ for eu~ct~ng or abrtL~lln~
the fonnatl~n belng drilled, a number o~ nozzle~ ~t ~clle
external sl;r~ace o~ the bit, and an ln~Ler pa6s~L~e ~or
6upplylng ~rillin~ f lui~l to the nozzle~;, at lea~t ~ ma~n
port ~ on of the ~lt body belng f ormed ~rom C~Lst ~ro~ ~n~
at lea~t a p~Lr~ of the ~urface of the ceL~t iron
comprlsing a 6eparat~1y c~Lst outer layer of aaet lron
c~ontaining n a6sive lnclusion~ of cementito Pormed by the
addition of a c~r~de-formlng additive ~o ~he molt~n
cast ~ron, ~uch masfii~re lnclusion~ of cem~n~ite l~lng
absent from said malrL portion of the bit bo~y.
In a drill bit manufactured according to the
me~hods of the invention, it is preferably those parts of
the bit body which are particularly subject to erosi~n
durin~ drilling, for examples the areas around the nozzles
and cutting elements, which are hardened to increase their
resistance to such erosion. At the same time, provided a
sui~able form of cast iro~ is used, the rest of the bit
bGdy may be accurately machined or otherwise worked after
it has been cast. Thus the present invention may combine
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the simplicity o~ m~nufqc~ure of ~eel bodied ~its wi~h
the er~sion resistance of matrix bodied bit~ and, indeed,
the erosion resi~tance of B drill bit according to the
inventi~n will be ~uperivr to that of a ~teel b~died ~it
and may also be ~uperior ~o tha~ of ~ matrix b~died bit.
Embodiments of the present invention will now be
described by way of example only with reference to the
accompanying drawings in which:
Figure 1 is a ~ide elevation ~f a typical drill
bit of ~he kind to which ~he invention is applicable,
Figure 2 is an end elevation ~f ~he drill bit
shown in Figure 1,
Figure 3 is a vertical section ~hrough a mould
showing the manufacture of a drill bit by one method
according to the invention, and
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Figure 4 is a vertical section through a mould
showing the manufacture of a drill bit by ano'cher method
according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referrirlg to Figures 1 and 2, the body 10 of the
drill bit is formed of cast iron by the method to be
described, and has a threaded shank 11 at one end for
connection to the drill string.
The operative end face 12 of the bit body is
formed with a number of blades 13 radiating from the
central area of the bit, and the blades carry cutting
structures 14 spaced apart along the length thereof.
The bit has a gauge section including kickers
16 which contact the wall of the borehole to stabilise the
bit in the borehole. A central passage (not shown~ in the
bit body and shank delivers drilling fluid through nozzles
17 in the end face 12 in known manner to clean and/or cool
the cutting elements.
In the particular arrangement shown each cutting
structure 14 comprises a preform cutting element mounted
on a carrier in the form of a stud which is located in a
socket in the bit body. Normally, each preform cutting
element is circular and comprises a thin facing layer of
polycrystalline diamond bonded to a backing layer of
tungsten carbide. ~owever, it will be appreciated that
this is only one example of the many possible variations
of the type of bit to which the invention is applicable,
including bits where each preform cutting element
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comprises a unitary layer of thermally stable
polycrystalline diamond material.
Figure 3 illustrates a method of manufacturing a
bit body of the kind shown in Figures 1 and 2.
Referring to Figure 3, a two-part mould 19 is-
formed from suitable material, such as sand. The two-part
mould comprises an upper part 20 and a lower part 21 which
between them define a mould cavity 22. The mould cavity
has an internal configuration corresponding generally to
the required surface shape of the bit body or a portion
thereof. A passage 23 is formed in the upper mould part
20 and leads to the mould cavity 22 from a pouring chamber
24 at the upper surface of the mould part 20.
To produce the cutting face confiyuration of the
drill bit shown in Figures 1 and 2, the mould may be
formed with elongate recesses corresponding to the blades
13 of the drill bit. Spaced apart along each blade
forming recess will be a plurality of sockets each of
which receives a cylindrical former (not shown), the
object of the formers being to define in the bit body
sockets to receive the studs on which the cutting elements
are mounted. Accordingly, the formers will be of the same
cross-sectional shape as the ~studs, for example circular
or rectangular.
There is also provided in the mould 19, at each
desired location for a nozzle 17, a socket (not shown)
which receives one end of an elongate former (not shown)
which extends into the mould space so as to form in the
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bit body a socket in which a nozzle may be subsequently
inserted.
In one method according to the invention cast
iron is melted and poured into the mould via ~he chamber
-5 24 and passage 23. While ~he cast iron is molten and
before it is is poured into the mould, there is added to
the molten cast iron an additive having the property of
causing iron carbide (cementite) to be formed in the cast
iron during solidification. Suitable additives are boron
and tellurium and approximately 1/2% of additive may be
added to the molten cast iron.
After the mould cavity 22 has been filled with
the molten cast iron with the additive, the mould is
allowed to cool and by the time solidification of the cast
iron h~s occurred, the additive will have caused the
inclusion of massive bodies of cementite in the cast iron,
thus substantially hardening the cast iron.
As previously mentioned, it may be desirable for
only the lower surface portion of the bit body to be
hardened, the upper portion of the bit body being softer
so as to be machinable. To achieve this, the cast iron is
poured into the mould in two portions. The first portion
contains the boron or tellurium additive and i5 sufficient
to fill only the bottom of the mould up to a certain
level, as indicated for example by the line 25 in Figure
3. The remainder of the mould cavity is then filled with
cast iron without the additive. As a resu:Lt, only the
lower surface portion of the body will be hardened by the
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inclusion of massive cementite.
In another method in accordance with the
invention the internal surface of the mould cavity is
coated, before introduction of the molten cast iron into
the mould, with a material which reacts with the cast
iron, during solidification, to effect surface hardening
thereof by the production of massive inclusions of
cementite. A suitable material is a wash of tellurium
over the interior surface of the mould or a part thereof.
Thus, where it is only required that the lower portion of
the bit body be hardened, the tellurium wash is coated
over only the lower part of the mould cavity, that is to
say up to the level of the line 25 in Figure 3.
In a further method according to the invention,
the hardening of the cast iron is effected by controlling
the rate of cooling of the bit body after solidification
has occurred. Thus, after the cast iron in the mould has
solidified suitable controlling of the rate of cooling of
the mould will result in the production of martensite
which has the effect of hardening the cast iron.
A still further method in accordance with the
invention is illustrated with respect to Figure 4.
In this case the lower part 21 of the mould is
provided with a heat sink in the form of a chill plate 26.
The size and initial temperature of the chill plate 26,
which may comprise a body of graphite, machined steel or
cast iron, is selected so as to accelerate the rate of
cooling of the cast iron in the lower part of the mould
y~
cavity and thus effect hardening of the cast iron in that
locality by ~he process as known "chill hardening". As is
well known, in chill hardening the production of massive
inclusions of cementite in the cast iron occurs.
Preferably the chilling at the surface of the
bit body effected by the chill plate 26 is at a rate
sufficient to cause the formation of cementite ~iron
carbide) so that at the surface itself, the material is
almost entirely composed of cementite, the proportion of
cementite decreasing with distance inwardly from the
chilled surface.
As previously mentioned, normally cutting
elements, or carriers for cutting elements such as studs
or pegs, are secured within sockets in the surface of the
lS finished bit body by brazing. Cast iron is normally
difficult to braze due to the presence of surface graphite
and, accordingly, it is advantageous to chill harden the
surface of the body within sockets being formed in the bit
body during its formation to receive cutting elements or
their carriers, since the hardening reduces the amount of
graphite present at the surface of each socket, thus
facilitating brazing. Thus the aforementioned formers may
be of metal and in close thermal contact with the chill
plate 26 so as to chill harden the cast iron defining the
sockets.
Although any form of cast iron may be employed
in the methods according to the invention, it preferably
comprises a spheroidal graphite cast iron in which, in
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known manner, graphite is precipitated in nodular forrn by
the addition of magnesiurn or some other suitable material.
Al~ernatively the cast iron may be of the type known as
"grey" cast iron, or may be compacted graphite cast iron
~ontaining titaniurn or some other material.
