Abstract:
An earth-boring bit has a bit body with at least one earth disintegrating cutter mounted on it. The cutter is generally conically shaped and rotatably secured to the body. The cutter has a plurality of teeth formed on it. The teeth have underlying stubs of steel which are integrally formed with and protrude from the cutter. The stubs have flanks which incline toward each other and terminate in a top. A carburized layer is formed on the flanks and the top to a selected depth. The stub has a width across its top from one flank to the other that is less than twice the depth of the carburized layer. A layer of hardfacing is coated on the tops and flanks of the stub, forming an apex for the tooth.

Description:
TECHNICAL FIELD 
     This invention relates to improvement to earthboring tools, especially to steel tooth bits that use hardfacing containing carbide particles to enhance wear resistance. 
     BACKGROUND ART 
     The earliest rolling cutter earth boring bits had teeth machined integrally from steel, conically shaped, earth disintegrating cutters. These bits, commonly known as “steel-tooth” or “mill-tooth” bits, are typically used for penetrating relatively soft geological formations of the earth. The strength and fracture-toughness of steel teeth permits the effective use of relatively long teeth, which enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths. 
     However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel tooth bit should penetrate economically without damage to the bit. Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks. 
     Consequently, it has been common in the art since at least the early 1930s to provide a layer of wear resistant metallurgical material called “hardfacing” over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast or macrocrystalline tungsten carbide dispersed in a steel, cobalt or nickel alloy binder or matrix. Such hardfacing materials are applied by heating with a torch a tube of the particles which welds to the surface to be hardfaced a homogeneous dispersion of hard particles in the matrix. After hardfacing, the cone is preferably heat treated, which typically includes carburizing and quenching from a high temperature to harden the cone. The particles are much harder than the matrix but more brittle. After hardening, the matrix has a hardness preferably in the range from 53 to 68 Rockwell C (RC). The mixture of hard particles with a softer but tougher steel matrix is a synergistic combination that produces a good hardfacing. 
     There have been a variety of different hardfacing materials and patterns, including special tooth configurations, to improve wear resistance or provide self sharpening. Generally, the hardfacing applied to the teeth of new bits is in a preapplication ratio range of 50 to 80 percent carbide particles, typically about 70 percent, in a metal matrix of iron, nickel, cobalt or their alloys. The thickness of the hardfacing deposit on new bits is usually about {fraction (1/16)} to ⅛ inch over the flanks, end portions and top of the crest of the tooth. Portions of the hardfacing may be somewhat thicker. The thicker portions are generally at the corners where the flanks intersect the crest. These thicker portions may be up to double that of other areas. 
     Worn bits have been retipped by adding a type of hardfacing to the teeth after they have been worn. Often a substantial part of the original hardfacing would be worn off along with a portion of the underlying steel teeth. The retipping hardfacing materials typically used are about 35-50% by weight of carbide particles with a fairly soft copper, bronze, brass or iron matrix. The soft matrix allows the retipper to shape the new tooth being formed. Depending on the extent of wear, the hardfacing may be quite thick, even greater than {fraction (3/16)} inch on top of the top of the underlying steel tooth. Retippers normally do not heat treat the retipped bit. Because of the softer matrix and the lack of heat treating the hardness of the matrix after application on a retipped tooth would normally be considerably less than a new bit tooth. While satisfactory for very soft drilling, such as water well drilling, the retipped hardfacing is not as wear resistant as the original equipment hardfacings described above, which contain a higher percentage of carbide particles and a harder matrix metal. 
     While hardfacing provides good wear resistance for a steel tooth bit, teeth are still susceptible to breakage. Breakage is generally thought to occur due to portions of the teeth being too brittle. Brittleness, particularly in smaller diameter drill bits, is at least partially caused by the underlying carburized layer. The standard manufacturing procedure is to carburize the steel cone after it is hardfaced to harden the surface for resisting erosion. The carburizing is performed in a furnace, using either a gas or a pack process. This process adds carbon throughout the hardfacing, and also increases the carbon content in a carburized layer near the surface of the steel, the layer having a depth of about 0.030 to 0.140 inch depending upon bit size and application. The carburizing process creates a carburized layer even below the hardfacing. 
     If the tooth crest is fairly sharp as in smaller cones, the carburized layer becomes deeper at the crest of the tooth because the carburized layers on the two flanks and sharp crest tend to merge. This makes the crest brittle. Even though subsequently carburized, this brittle area can be subject to premature tooth failure. 
     DISCLOSURE OF INVENTION 
     In this invention, the underlying steel tooth or stub is formed with a shorter length than conventional. The flanks of the tooth stub will be sufficiently far from each other at the crest or top of the tooth stub to prevent the carburized layers on the flanks and crest from merging. Therefore there is no increase in carburized layer depth at the crest, unlike the prior art teeth with sharp crests. The distance from one flank to the other, measured perpendicular to the axis of the tooth at the crest, is greater than twice the depth of the carburized layers on the flanks. 
     A layer of hardfacing is applied to the top and flanks of the tooth stub, forming an apex for the tooth. The layer of hardfacing is much thicker than normally used, preferably equal to or greater than {fraction (3/16)} inch on the crest. The hardfacing layer has an axial depth that is preferably at least 15 percent the axial length of the tooth stub. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of an earth-boring bit of the steel tooth type constructed in accordance of this invention. 
     FIG. 2 is a sectional view of a tooth of an earth-boring bit as in FIG. 1, but showing a prior art design. 
     FIG. 3 is a sectional view, taken along the line  3 — 3  of FIG. 4, of a tooth constructed in accordance of this invention. 
     FIG. 4 is a sectional view of the tooth of FIG. 3, taken along the line  4 — 4  of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an earth-boring bit  11 , modified in accordance with 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 drill string (not shown). Each leg of bit  11  is provided with a lubricant compensator  17 . At least one nozzle  19  is provided in bit body  13  for directing pressurized drilling fluid from within the drill string and bit against the bottom of the borehole. 
     The cutters  21 ,  23 , generally three (one of which is obscured from view in FIG.  1 ), are rotatably secured to respective legs of bit body  13 . A plurality of inner row teeth  25  are arranged in generally circumferential rows on cutters  21 ,  23 , being integrally formed on the cutters, usually by machining. Heel row teeth  29  are located at the outer edges of each cutter  21 ,  23  adjacent gage surface  30 . 
     FIG. 2 illustrates a tooth  27  which in the prior art would be in a heel row in place of heel row teeth  29  (FIG. 1) in the cutter  21  of FIG.  1 . Prior art tooth  27  is formed with a milling cutter which forms a root  31 , inclined flanks  33 ,  35  and an elongated crest  37 . One of the flanks  33 ,  35  is a leading flank and the other a trailing flank, considering the direction of rotation of cutter  21 . 
     Tooth  27  has an axis  39  which is substantially perpendicular to the cutter axis  40  of rotation (FIG.  4 ). A carburized layer  41  is formed in the underlying steel of tooth  27  in a conventional process. Carburized layer  41  is generally in the depth range from about 0.030 to 0.140 inch depending upon bit size and application. The depth of carburizing layer  41  is not uniform because of the sharpness of crest  37 . Because of the short distance from one flank  33  to the other flank  35  at crest  37 , a deeper area  41   a  of carburizing layer  41  will result at crest  37 . Carburized portion  41   a  becomes deeper because of the merging of the carburized layers  41  underlying flanks  33 ,  35 . The distance from flank  33  to flank  35 , measured perpendicular to axis  39  at crest  37 , is less than twice the average depth of carburized layer  41  on flanks  33 ,  35 . 
     A layer of hardfacing  43  is applied over tooth  27 . It may be of various types, typically containing tungsten carbide granules in an alloy steel matrix. The thickness of hardfacing  43  on flanks  33 ,  35  and on top of crest  37  is about {fraction (1/16)} to ⅛ inch. Heat treating, which includes carburizing, is usually performed after hardfacing. In another type of prior art tooth, shown in U.S. Pat. No. 5,351,771, curved recesses are located at the junctions of the flanks with the crest. If tooth  27  had those recesses, the thickness of hardfacing  43  would be about double in the recesses than on the top of crest  37  and on flanks  33 ,  35 . In another type of prior art tooth, a slot is located on the leading flank as in U.S. Pat. No. 5,445,231. If tooth  27  had such a slot, the thickness of hardfacing  43  on the flank over the slot would be about double that of the rest of tooth  27 . 
     FIG. 3 shows a heel row tooth  29  constructed in accordance with this invention. Tooth  29  has a steel stub  47  which is integrally formed with cutter  21  in a conventional manner by milling. Stub  47  is shorter than the steel portion of tooth  27  of the prior art. Stub  47  extends upward from roots  49 , has flanks  51 ,  52  that incline toward each other, and outer and inner ends  53 ,  55 . Roots  49  are the valleys between teeth  29 , as shown in FIG.  1 . During rotation about cutter axis  40  (FIG.  4 ), one flank  51 ,  52  leads while the other trails. Flanks  51 ,  52  join outer and inner ends  53 , 55 , terminating in a top or crest  57 . Top  57  is shown to be flat and perpendicular to tooth axis  58 , but could be of other configurations. 
     Stub  47  has a carburizing layer  59  that is uniform and of a depth of about 0.080 to 0.120 inch. Carburized layer  59  is formed conventionally after hardfacing. Carburized layer  59  does not have an increased depth layer at the top  57 . The distance between flanks  51 ,  52 , measured perpendicular to tooth axis  58  at the junction with top  57 , is substantially greater than twice the depth of carburized layer  59 . The carburized layers  59  on flanks  51 ,  52  do not merge with each other at top  57 . 
     A hardfacing layer  61  is applied to tooth stub  47  in a conventional manner. Hardfacing  61  may be of a variety of types, but preferably includes tungsten carbide granules or particles in an alloy steel matrix. The matrix binder may contain iron, nickel, cobalt and their alloys and has a hardness after application on tooth stub  47  and heat treating in the range from about 53 to 68 RC. The tungsten carbide particles are in a pre-application ratio in a hardfacing tube of about 50 to 80 percent by weight, preferably about 70 percent. Because of its extra thickness on top  57 , hardfacing  61  will be applied in multiple passes, but without allowing the earlier passes to cool substantially. After hardfacing  61  is applied, cutter  21  is heat treated in a conventional manner. The heat treating process creates the carburized layer  59  and also enhances the hardfacing  61 . 
     Hardfacing  61  is shaped generally to form an extension or apex of stub  47  to resemble the configuration of prior art tooth  27 . The apex of hardfacing  61  includes flanks  63 ,  65  which extend generally in the same direction from flanks  51 ,  52 , respectively, terminating in a crest  67 . The apex of hardfacing  61  also has outer and inner end portions  69 ,  71  which extend in the same direction from tooth stub outer and inner end portions  53 ,  55 , respectively. Hardfacing  61  also may have a thinner portion, typically about 0.047 to 0.125 inch, that will cover a portion of tooth stub flanks  51 ,  52  and outer and inner ends  53 ,  55 . 
     Flanks  63 ,  65  of hardfacing  61  converge to a fairly sharp crest  67 . The overall length of tooth  29  from root  49  to crest  67 , measured along tooth axis  58 , is conventional. However, the thickness  75  of hardfacing  61  measured from top  57  of stub  47  to crest  67  is much greater than previously utilized with this type of hardfacing, being at least {fraction (3/16)} inch. Thickness  75  will normally be twice or more the thickness of hardfacing  61  covering tooth stub flanks  51 ,  52  and outer and inner ends  53 ,  55 . In the embodiment shown, tooth stub  47  has a shorter axial length  73 , measured along axis  58  from root  49  to top  57 , than axial thickness  75  of hardfacing  67 . However, tooth stub length  73  could be longer than hardfacing thickness  75 . Tooth stub length  73  should not be so long so as to decrease the distance between tooth stub flanks  51 ,  52  to a point where their carburized layers  59  merge and become extra deep. For a very large diameter bit having long teeth, the minimum axial thickness  75  of {fraction (3/16)} inch of hardfacing  61  will be not less than 15 percent the axial length  73  of tooth stub  58 . For smaller diameter bits, 8¼ inch or less, the minimum axial thickness  75  of {fraction (3/16)} inch divided by axial length  73  will normally be higher, at least 35 percent. 
     The invention has significant advantages. Utilizing an extra-thick hardfacing layer reduces the width of the underlying steel crest from flank-to-flank. This blunter underlying or tooth stub top avoids extra-deep carburizing layers at the top of the tooth stub. A shorter tooth stub and a thicker hardfacing layer on top can reduce brittleness and the possibility of breakage without reducing overall tooth length. 
     While the invention has been shown in one of its forms, it should be susceptible to various changes without departing from the scope of the invention. For example, although shown only on a heel row tooth, the hardfacing in accordance with this invention could also be applied to inner row teeth and various tooth geometries.