Earth-Boring Bit Having an Improved Hard-Faced Tooth Structure

1. Field of the Invention:

The present invention relates generally to the tooth structure of earth-boring bits of the rolling cutter variety. More particularly, the present invention relates to improving the wear-resistance of mill- or steel-tooth earth-boring bits.

2. Background Information:

The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made that success possible. Only soft formations could be commercially penetrated with the earlier drag bit, but the original rolling-cone rock bit invented by Howard R. Hughes, U.S. Pat. No. 939,759, drilled the hard caprock at the Spindletop Field, near Beaumont, Texas, with relative ease.

That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill for miles. Many individual improvements have contributed to the impressive overall improvement in the performance of rock bits.

The early rolling-cone earth-boring bits had teeth formed integrally with the cutters. These bits, commonly known as "steel-tooth" or "mill-tooth" bits, are still in common usage for penetrating relatively soft formations. The strength and fracture-toughness of the steel teeth permits relatively long teeth with long crests, which provide the aggressive gouging and scraping action that is advantageous for the rapid penetration of relatively soft formations.

However, it is rare that a formation interval will consist entirely of soft material with low compressive strength. Often, there are streaks of hard or abrasive materials that a steel-tooth bit must be able to penetrate economically, without damage to the bit.

Although steel teeth possess good strength, their abrasion resistance generally is not adequate to permit rapid penetration of hard or abrasive streaks without damage to the bit. Consequently, it is conventional in the art to provide a layer of wear-resistant material or hard-facing over at least a portion of the teeth of a steel tooth bit. These wear-resistant materials or hard-facings are conventional, and typically consist of particles of tungsten carbide or other hard metal dispersed in a steel or cobalt binder matrix. Such hard-facing materials are applied by melting the binder of the hard-facing material and applying the material over the surfaces of the tooth. The proper application of hard-facing material to steel tooth bits requires considerable skill on the part of the welder.

Internal records at Hughes Christensen Company indicate that the practice of hard-facing steel teeth was initiated in approximately 1929. With the introduction of the tungsten carbide insert (TCI) bit by Hughes Tool Company in the 1950's (see U.S. Pat. No. 2,687,875, Aug. 31, 1954, to Morlan, et al.), the focus of the drilling industry turned to the use of TCI bits. More recently, however, attention again has focused on the improvement and development of earth-boring bits of the mill- or steel-tooth variety because of advances in bearing and seal technology.

It is difficult to apply a relatively thick layer of hard-facing material over the crest or ends of teeth within tolerance. A tooth with a crest hard-faced to a thickness beyond the tolerance can cause the tooth to interfere with or "strike" an opposing cone. This condition requires expensive and time-consuming grinding of the hard-faced crest to reduce the thickness and eliminate interference. At least as early as 1989, one corner of the steel teeth in one row was beveled to permit application of hard-facing over the crest and ends of the teeth without causing the aforementioned interference between teeth.

U.S. Pat. No. 5,152,194, Oct. 6, 1992, to Keshavan, et al. discloses a method of hard-facing a steel-tooth earth-boring bit, wherein a substantially uniform thickness of hard-facing is provided over the tooth. Each corner of each tooth is rounded to achieve uniform hard-face thickness. That disclosure does not address the difficulty of applying a thick layer of hard-facing material over a tooth of a steel-tooth earth-boring bit without incurring the problem of tooth strike, which requires costly and time-consuming grinding operations to bring the hard-faced tooth within the clearances and tolerances necessary to avoid strike.

A need exists, therefore, for an earth-boring bit having hard-faced steel tooth structure that permits and facilitates application of hard-facing material in substantial thicknesses over the tooth, while avoiding over-application of hard-facing material.

It is a general object of the present invention to provide an improved earth-boring bit having an improved hard-faced tooth structure.

This and other objects of the present invention are achieved by providing an earth-boring bit having a bit body and at least one cutter rotatably secured to the bit body. The cutter has a plurality of teeth formed integrally thereon and arranged in circumferential rows. Each of the teeth includes a pair of ends, a pair of flanks and a crest substantially transversely connecting the ends and flanks. The crest of the at least one of the plurality of teeth has a concave depression formed at each intersection of the ends and the crest. A wear-resistant material is applied over at least the crest and a portion of at least the ends and flanks of the at least one of the plurality of teeth, wherein the thickness of the wear-resistant material is substantially greater over the concave depressions than elsewhere on the tooth.

The concave depressions facilitate application of the wear-resistant material over the tooth. According to a preferred embodiment of the present invention, the earth-boring bit has three cutters, each of the cutters having a plurality of teeth formed integrally thereon. Each of the crests of the plurality of teeth is provided with the concave depressions.

Other objects, features, and advantages of the present invention will become apparent to those having skill in the art with reference to the drawings and detailed description, which follow.

FIG. 1 is a perspective view of an earth-boring bit of the type contemplated by the present invention.

FIG. 2 is a schematic representation of a steel tooth, which depicts the various surfaces of such a tooth.

FIG. 3 is a fragmentary section view of a prior-art hard-faced steel tooth.

FIG. 4 is a fragmentary section view of the hard-faced steel tooth structure according to the present invention.

Referring now to FIG. 1, an earth-boring bit 11 according to the present invention is depicted. Earth-boring bit 11 includes a bit body 13 having threads 15 at its upper extent for connecting bit 11 into a drillstring (not shown). Each leg of bit 11 is provided with a lubricant compensator 17, a preferred embodiment of which is disclosed in U.S. Pat. No. 4,276,946, Jul. 7, 1981, to Millsapps. At least one nozzle 19 is provided in bit body 13 for spraying cooling and lubricating drilling fluid from within the drillstring to the bottom of the borehole.

At least one cutter, in this case three (one of which is obscured from view in the perspective of FIG. 1), 21, 23 is rotatably secured to each leg of bit body 13. A plurality of teeth 25 are arranged in generally circumferential rows on cutters 21, 23. Teeth 25 are integrally formed from the material of cutters 21, 23, which is usually steel.

FIGS. 2 and 3 illustrate a prior-art hard-faced steel or milled tooth 25. FIG. 2 schematically represents tooth 25 to illustrate its various surfaces. FIG. 3 is a fragmentary section view of a prior-art tooth similar to that schematically represented in FIG. 2. Tooth 25 has a number of surfaces, including a pair of flanks 27, 29, a pair of ends 31, 33, and a crest 35 substantially transversely connecting flanks 27, 29 and ends 31, 33. A layer 41 of wear-resistant material, commonly known as hard-facing is provided over flanks 27, 29, ends 31, 33 and crest 35. Hard-facing layer 41 is provided to increase the hardness and wear resistance of tooth 25. Hard-facing materials are conventional in the art and generally consist of particles of tungsten carbide or other hard metal dispersed in a binder matrix of cobalt, steel, or an alloy thereof. Hard-facing materials generally are applied by melting the binder and applying the hard-facing over tooth 25 using a gas torch. Considerable welding skill is required to obtain a relatively even layer 41 that covers all of the tooth surfaces that are desired to be hard-faced.

One limitation on the thickness t of hard-facing layer 41 is the clearance or tolerance necessary to avoid striking or interference between teeth 25 and opposing cutters 21, 23. Thus, a welder must strike a careful balance between applying a hard-facing layer 41 that is sufficiently thick to effectively increase the wear resistance of tooth 25, while insuring that tooth 25 remains within tolerance and does not strike an opposing cutter 21, 23. If thickness t of hard-facing layer 41 is so great that it causes striking or interference, costly grinding operations are necessary to bring tooth 25 back into tolerance necessary to avoid striking. If thickness t is insufficient, the resulting performance of bit 11 may be less than expected. A conventional thickness t of hard-facing layer 41 is approximately 0.062 or 1/16 inch.

FIG. 4 illustrates, in fragmentary section view, a hard-faced tooth 125 according to the present invention. Like tooth 25 illustrated in FIG. 2 and 3, tooth 125 according to the present invention includes a pair of ends 131, 133, a pair of flanks (not shown in FIG. 4) and a crest 135 substantially transversely connecting the flanks and ends 131, 133. At each intersection of ends 131, 133 and crest 135, tooth 125 is provided with a concave depression 138, 139. Preferably, each concave depression takes the form of an inverted radius or fillet.

A layer of hard-facing 141 is provided over tooth 125, preferably covering at least crest 135, concave portions 137, 139, and a portion of ends 131, 133 and the flanks. Concave depressions 137, 139 provide the ability to obtain a greater thickness T of hard-facing layer 141 over concave depressions 137, 139 at the corners of crest 135, which are highly susceptible to abrasive wear. Concave depressions also provide a guide to the welder, who simply fills concave depressions with hard-facing material and then applies a standard thickness t of hard-facing over crest 135 and remainder of tooth 125. Thus, the necessity of extremely careful buildup of a layer of hard-facing of increased thickness is obviated, and welding skill requirements are relaxed.

Preferably, concave depressions 137, 139 have a radius equal to or greater than the thickness t of a conventional hard-facing layer (41 in FIG. 3). Thus, when hard-facing operations are complete, the thickness T of hard-facing layer 141 over depression 135 is substantially twice that of conventional hard-facing over the remainder of tooth 125. A minimum radius of 0.5t is necessary to insure that thickness T of hard-facing layer 141 over concave depressions 137, 139 is at least 50% greater than standard thickness t. According to the preferred embodiment of the invention, all of teeth of bit 11 that conventionally are hard-faced are provided with the concave depressions 137, 139 at the intersections of ends 131, 133 and crest 135.

With reference to FIGS. 1-4, the operation of earth-boring bit 11 according to the present invention will be described. Bit 11 is connected by threads 15 into a drillstring (not shown). Drillstring and bit 11 then are rotated, wherein cutters 21, 23 roll and slide over the bottom of the borehole. As cutters 21, 23 roll and slide over the bottom of the borehole, teeth 25 gouge and scrape formation material, resulting in penetration of the formation. Drilling fluid from within drillstring exits nozzle 19, cooling and lubricating cutters 21, 23, and lifting fragments of formation material away from the bottom of the borehole.

Improved hard-faced teeth 125 remain sharp because of their improved wear-resistance.

A principal advantage of the present invention is the provision of an earth-boring bit having improved wear resistance. The improved tooth structure disclosed herein permits the economical manufacture of a more wear-resistant earth-boring bit that is adapted to be manufactured by minimally skilled welders without the need for costly finish-grinding of teeth after hard-facing operations.

While the invention has been shown in only one of its preferred embodiments, it is thus not limited. It will be apparent to those having skill in the art that the present invention is subject to variation and modification without departure from the scope thereof.