Friction bearing rock bit and segment, and method for making them

A rock bit segment has a cutter rotatably mounted on a journal. Loads radially of the axis of the journal are largely taken by a friction bearing pad of hard material. Ball bearings track in circular races on the journal and cutter and retain the cutter on the journal. A seal at the junction of the journal and the balance of the segment keep lubricant from escaping from the interface between the cutter and the journal and formation material from entering this interface. To provide hardened wear surfaces for the seal on the journal and segment in the ball race on the journal, on radial surfaces of the journal adjacent the pad, and at the nose of the journal, the entire rock bit segment is subjected to a carbon enrichment of the surface after the segment has been machined. Thereafter, the segment is quenched to effect a hardened case, and then the segment is tempered. Carbon in the case varies from 0.5% to 0.9% at the surface to base metal concentrations at depth, and the case is 0.010 to 0.015 inches thick.

BACKGROUND OF THE INVENTION 
The present invention relates to rotary rock bits, and, more in particular, 
to an improved rotary rock bit of the friction bearing type and a method 
for manufacturing the improved rock bit. 
Rotary rock bits are used in earth boring, for example, for access to 
formations containing petroleum values. Typically, each rock bit has one 
or more legs or segments, with each segment having a journal rotatably 
carrying a cone cutter. The axis of the journal lies at an acute angle to 
both the vertical and the horizontal. Each cone cutter is compressively 
loaded between the bottom of the journal by the weight of the drill string 
and the radial surface at the bottom of the hole being drilled. 
These compressive loads can be very high because of the weight of the drill 
string on the cutters, for the drill string can be thousands of feet in 
length. A friction type journal bearing can withstand these high loads 
better than anti-friction ball or roller bearings, so long as the friction 
bearing is continuously lubricated. A sealed lubricant system in 
communication with the friction bearing provides the lubrication, as 
described in U.S. Pat. No. 3,917,028. The lubrication system includes a 
variable volume reservoir that expands and contracts in response to 
pressure differentials between the lubricant and the drilling environment. 
The system then can adjust for changes in pressure due to drilling mud 
head and expansion and volatilization of the lubricant at the high 
temperatures of deep holes. 
To improve wear of the journal bearing surface on the journal, it has been 
the practice to provide a hard bearing metal wear pad on the journal on 
that portion which sustains heavy loading. It has also been the practice 
to offset the axis of curvature of the wear pad from the axis of the 
journal to avoid squeezing a seal too much. 
Ball bearings tracking in a race at the journal and on the cone cutter 
secure the cutter to the journal. To avoid Brinneling the race of the 
journal, the race and adjoining lands have been hardened by carburization 
and subsequent heat treatment. A wear resistant surface adjacent the 
O-ring seal between the journal and the leg has also been provided by 
carburizing and heat treatment. 
The journal has a thrust face lying normal to the axis of the journal 
between a nose of the journal and the ball race to take the thrust of the 
cone cutter. Thrust may also be taken at the end of the nose of the 
journal which lies close to the axis of the bit. These areas, being 
subject to wear, have also been carburized and heat treated, or hardfaced. 
In the past, the selective carburization and heat treatment has been 
arduous, requiring much handwork. The machined areas of the faying 
surfaces, where the segment abuts neighboring segments, and journal as 
well as other areas were stopped off. Stop off was either by ceramic based 
paint or by electroplated masking material. Non stopped off areas were 
carburized. Carburization was at an elevated temperature, for a long 
period of time, and in a carburizing atmosphere. For example, a 
carburizing atmosphere of 0.6% to 0.8% carbon potential for 15 hours and 
at about 1700.degree. F. was used in carburizing a segment to obtain 0.040 
to 0.060 inches depth of carburization. The part was then slowly cooled so 
as to avoid hardening the carburized surfaces. When the ceramic based 
stop-off was used, this material was then removed by hand sandblasting in 
the selected areas of where the grease reservoir and nozzle were to be 
formed. The part was then finish machined. Finish machining includes 
development of the grease reservoir and the nozzle. After finish 
machining, the part was heat treated by heating, quenching and tempering. 
Thereafter, all remaining ceramic stop-off was removed and the parts, wear 
pad and other journal wear surfaces were finished ground. 
Quite obviously, the selective carburization process used in the past 
consumes both time and money. 
In the selective carburization technique it was thought that some areas 
required deep carburization to withstand imposed loads, for example, the 
roller races. Deep carburization in other areas would be bad, however, 
because those areas are not thick enough to provide a meaningful soft and 
tough core. Examples of such thin areas include the ball flanges and 
shirttail. The faying surfaces and areas adjacent thereto must be 
maintained at a low carbon level because of the adverse effect excessive 
carbon has on the integrity of the welds at the faying surfaces. For 
example, cracks could develop creating leak paths. For those reasons 
carburization in rock bits was thought to require the selective area 
technique described. 
SUMMARY OF THE INVENTION 
The present invention provides an improved friction type rock bit segment, 
rock bit and method for their construction in which the surface of the 
entire friction bearing rock bit segment is carbon enriched and heat 
treated simultaneously to develop the requisite hardness in the zones of a 
journal and adjacent the journal. 
It has been discovered that the heavy, carburized case on the journal areas 
is not necessary in friction bearing rock bits. A thin carbon enriched 
skin appropriately hardened in wear sensitive areas suffices. The thin 
skin is thin enough so that areas requiring a tough core will have a tough 
core. As a consequence, the entire leg or segment can be provided with a 
carbon enriched skin. Therefore, the carbon enrichment step and the 
hardening step can, in a sense, be combined by doing all the machining 
beforehand and then quenching the part from a temperature achieved for the 
carbon enrichment. The method of the present invention considerably 
reduces the task of preparing a rock bit segment by eliminating many 
procedures and much of the handwork done before. 
In a particular form, the present invention contemplates completely 
fabricating a leg or segment of a rock bit, except for carbon enrichment 
of a thin layer of skin and heat treatment and the mounting of the cutter 
cones, bearings, and lubrication system. The segment is completely 
machined. Thus the faying surfaces, lubrication passages, threads and ball 
races are finish machined. After the machining, the entire segment surface 
is carbon enriched to develop a case of comparatively high carbon 
percentage content and then quenched to develop the desired hardness in 
the case and core. After quenching, the segment may be tempered. 
Preferably, carbon enrichment takes place in a carburizing atmosphere 
formed of dissociated methane and at a temperature of about 1500.degree. 
F. to 1600.degree. F. for about three hours. A case thickness of 0.005 to 
0.020 inches thick with a carbon concentration of 0.4% to 0.9% at the 
surface grading down to base metal concentrations is satisfactory. 
Preferably case thickness is from about 0.010 to 0.015 inches and carbon 
concentration at the surface is from about 0.5% to about 0.7%. The 
carburizing atmosphere has a carbon potential of at least about 0.9% to 
1.5% carbon. The process can be carried out under different operating 
conditions to achieve the same result. For example, higher temperatures 
and shorter times. Raising the carbon potential also shortens the time. 
Also the desired results may be obtained by methods using solid 
carburizing materials. Atmospheres containing both carbon and nitrogen may 
be used to produce a carbo-nitrided case. 
The present invention contrasts with the prior art in the invention's 
simplicity. In the prior art a segment was preliminarily machined. Then 
stop-off was applied to control the areas to be carburized. The segment 
was then carburized in the journal area. After carburization, the segment 
was cooled slowly, selectively hand cleaned, finish machined, and then 
heat treated followed by final overall cleaning and grinding. The 
invention is markedly simpler. 
Thses and other features, aspects and advantages of the present invention 
will become more apparent from the following description, appended claims 
and drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIGS. 1 and 2, a single steel leg or segment 10 of a rock 
bit assembly 11 forms one element of the assembly. Typically, three such 
segments array around a vertical axis 12 and are welded together along 
faying surfaces to form a unitary drill bit. Such a weld is shown at 13 in 
FIG. 1. 
A segment or leg includes an upper pin segment 14 having threads 15 for 
attachment to a drill string. An outer surface 16 of major radius of the 
rock bit is generally cylindrical in shape with a slight back draft. Each 
segment has a 120.degree. angle between faying surfaces. A depending leg 
18 provides a relief at 20 for rotation of a cone cutter 22. A shirttail 
23 at the base of leg 18 protects the cone cutter. A journal 24 extends 
inwardly and downwardly from leg 18 and receives cone cutter 22 for 
rotation of the latter. The axis of the journal, indicated at 26, extends 
downwardly and inwardly towards the vertical axis of revolution of the 
rock bit assembly. Leg 18 adjacent the foot of journal 24 is machined at 
28 to define an annular wear surface extending radially of the journal. 
The external surface of the journal is machined at 30 and surface 30 joins 
surface 28 through a radius 32. A seal, such as an O-ring 34, on journal 
24 and abutting against surfaces 28, 32, and 30 provides a seal from the 
drilling environment into the interface between cutter 22 and the journal. 
Rotary cutter 22 has a generally conical external surface and a cylindrical 
bore 36 in receipt of journal 24. The cutter can have tungsten carbide 
insert teeth 38, or the teeth can be integral, both being conventional. 
The anterior end of journal 24 has a reduced diameter nose or spindle 40 in 
receipt of a friction bushing 42. The friction bushing mounts in the cone 
cutter. A thrust button 44 at the base of bore 36 of cutter 22 bears on a 
friction surface 46 of spindle 40 at the extreme anterior end of journal 
24. A thrust face 47 of the journal at the base of the spindle extends 
radially of the axis of the journal to a diameter larger than the spindle. 
This thrust face takes the considerable thrust load exerted upon the 
journal by cone cutter 22. Spindle end 46 can take some of this thrust. 
Cutter 22 mounts on journal 24 through a plurality of ball bearings 48 that 
track in an annular race 50 of the journal and an annular race 52 of the 
cutter. These races are radially aligned. These races are defined as 
surfaces of revolution around axis 26 generated by arcs of circles. A 
passage 54 opening into race 50 allows balls 48 to be inserted into the 
race after cutter 22 is positioned on the journal. A retainer plug 56 
placed in passage 54 after balls 48 are in place in races 50 and 52 keeps 
the balls in position. The retainer plug can be welded in place as at 58. 
A longitudinal slot 60 in plug 56 provides for passage of lubricant to 
balls 48 and their races. The function of balls 48 is primarily to hold 
the cutter to the journal and not to take out thrust loads. 
As is conventional, lubrication is provided from a pressure compensated 
reservoir 62. A boot 64 within the reservoir has one side exposed to 
drilling environment pressures by a passage 65 through a retaining plug 
66. A cup 68 within a cavity 70 has an external flange 72 on a seat 73 of 
the leg. An annular ring 76 of boot 64 clamps between flange 74 and a 
flange 78 of plug 66. Grease fills the space exteriorly of boot 64 and 
within cavity 70. This grease communicates with the bearing surfaces of 
the segment through a passage 80 and a passage 82, both formed by 
drillings in the leg. Passage 80 opens into passage 82 and passage 82 in 
turn opens into slot 60. A radial drilling 90 into a groove 92 provides 
lubricant for the interface between the cone cutter and the journal. 
Groove 92 extends only part of the way around the journal. A hard metal 
bearing wear pad 94 extends around the bottom portion of the journal and 
bears the considerable radial compressive load produced by the weight of 
the drill string about the bit. To take load off the journal and avoid 
excessive compression of the O-ring, the wear pad extends out slightly 
from the surface of the journal. This extension is not shown. 
As previously mentioned, O-ring 34 seals the bearing surfaces between the 
rotary cutter and the journal. The O-ring resides in an annular groove 98 
of the cutter. 
Pressure changes between grease inside the reservoir and drilling fluid 
outside the rock bit occur as the bit operates in changing environments of 
temperature, load, and external pressure. Changes occur in the effective 
volume of the grease that could cause loss of grease from the system or 
intrusion of foreign material into the bearings past O-ring 34, except for 
the pressure reservoir. Changes in grease volume produce changes in the 
space occupied by rubber boot 64 and the lubricant pressure therefore 
matches the environment outside the lubricant galleries. 
What has been described up to here has been typical of friction bearing 
rock bit segments. The invention contemplates a modification of the 
segment in the manner of providing wear surfaces. Previously, the practice 
was to carburize only the wear surfaces on the journal and the annular 
wear ring at the foot of the journal against which the O-ring bears. 
Stop-off applied to the rest of the segment prevented it from carburizing. 
After carburization of the journal a slow cool and clean-up to get rid of 
undesired stop-off followed. The rest of the segment was then finish 
machined. Finally, the segment was heat treated to develop the hard case 
desired at the wear surfaces. This heat treatment required the heating of 
the part again, its quenching, and tempering. 
By the present invention, this technique has been substantially simplified. 
No stop-off is applied and no clean-up required. Prior to heat treatment, 
the segment is finish machined. This includes drilling the grease 
reservoir and galleries, the tapping of threads for a jet nozzle 100 (see 
FIG. 1), all journal machining, and faying surface machining. The segment 
is then placed in a carbon enriching environment at an elevated 
temperature to get the required carbon content and depth of penetration in 
the wear surfaces of the journal and adjacent wear ring, and then the leg 
is rapidly cooled to develop the hardness desired. Tempering may follow. 
Heat treatment, then, accompanies carbon enrichment. 
In one practical embodiment, the carbon enrichment environment was 
endothermically cracked methane enriched with methane corresponding to a 
carbon potential of 0.9% to 1%. Carburizing grade steel such as 4815 or 
8720 is suitable. Carbon enrichment took place in the endothermically 
cracked methane enriched by methane at a temperature of 1550.degree. F. to 
1600.degree. F. for three hours. The part was quenched in oil and 
tempered. The case was about 0.015 inches thick with carbon content of the 
steel at the surface of 0.6% to 0.9% grading down to base metal carbon 
content. In another practical example, case thickness of 0.010 to 0.015 
inches with a carbon content of 0.5% to 0.7% at the surface grading down 
to base metal proved satisfactory. Carbon enrichment took place at 
1550.degree. F. for about three hours in a carbon enriching atmosphere of 
endothermically cracked methane with a carbon potential above 0.5% carbon. 
The segment was quenched in oil and tempered to develop the hardness 
desired in the carbon enriched surface and the core. 
It is thought that higher enrichment temperatures can be used, for example, 
the 1700.degree. F. used in the old carburizing technique. With higher 
temperatures, the enrichening time must be reduced. Also, the carbon 
potential can be varied. The carbon content at the surface can be between 
about 0.4% to about 0.9%. Below 0.4% there is insufficient carbon for the 
hardness required. The preferred range is from about 0.5% to about 0.7% 
carbon at the surface. 
The thickness of the carbon enriched case is important. It should be within 
the range of from about 0.005 to about 0.020 inches. A case thickness of 
between about 0.010 to 0.015 inches is preferred. If the case thickness is 
too great, impact sensitive areas such as the threads of nozzle jet 100, 
shirttail 19, and the lands of ball race 50, will not have an ample core 
of soft and tough material. On the other hand, too thin a case results in 
wear surfaces of inadequate life. 
If desired, the case can be carbo-nitrided by the techniques described here 
and with the addition of nitrogen to the atmosphere that gives carbon 
enrichment. The nitrogen atmosphere can be supplied by ammonia of up to 
10% by volume. A carbo-nitrided case will be a little harder and a little 
more wear resistant than a carbon enriched case. 
FIG. 3 illustrates carbon gradient versus depth from the surface. Curve "A" 
represents the prior art standard. Curve "B" represents the invention. As 
the curve "B"shows, the carbon content in the steel falls off below about 
0.005 inches depth. This means a loss of hardness. With a case thickness 
less than 0.005, it can be seen that the effective skin thickness to 
resist wear becomes very thin. The carbon enriched skin thickness does not 
give accurate Rockwell "C" hardness because it is too thin. Hardness on 
the 15N scale converted by standard references to Rockwell "C" should be 
between about 55 to about 62 R.sub.c. The carbon and depth tolerances are 
shown for both curves. The curve limits extend outside tolerance overlap 
so that the curves can be picked up. 
It has been found with this technique of carbon enriching that the segment 
can be welded satisfactorily in the assembly of a complete rock bit. While 
the hardened surfaces in the journal have proven satisfactory for wear 
resistance, it is still necessary to provide a wear pad to take the 
considerable weight load of the drill string on the journal. The wear pad 
can be applied by a hard facing of such material as a nickel or cobalt 
based material. Other material suitable for rock bit journal bearings can 
also be used such as aluminum bronze. The wear pad may be finished by 
grinding to develop a slight offsetting, as described. 
The present invention has been described with reference to a preferred 
embodiment. The spirit and scope of the appended claims should not, 
however, necessarily be limited to the foregoing description.