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BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
       [0001]    The present disclosure generally relates to a roller cone drill bit with improved erosion resistance. 
       Description of the Related Art 
       [0002]    U.S. Pat. No. 4,303,137 discloses a method for manufacturing tungsten carbide insert-type cutter cones for a rock bit for drilling oil wells and the like. A cone blank is formed from medium to high carbon steel by forging and machining. The cone blank has a generally conical external surface, a generally cylindrical internal bearing cavity, and a circumferentially extending ball bearing race in the bearing cavity. The cone blank is heat treated by quenching and tempering to a desired core hardness. Insert holes are drilled in the external surface of the heat treated cone blank for insertion of tungsten carbide inserts. The surface of the ball race is selectively hardened by heating and quenching for forming a surface layer having a higher hardness than the core hardness. Selective hardening of the ball race is obtained by applying energy to the surface of the ball race by induction heating, an electron beam or a laser beam to austenitize a surface layer which is rapidly cooled for hardening. 
         [0003]    U.S. Pat. No. 4,708,752 discloses that a medium to high carbon steel body of a roller cone for a drilling bit is machined to final dimensions, and is thereafter rendered absorbent to laser light by application of black paint or black etch. Holes for hard tungsten carbide or like inserts are drilled in the light absorbent steel body. The entire steel body, including the holes, is subjected to a laser treatment which, however, is effective to raise to above austenitizing temperature only the dark light absorbent surfaces. Walls of the insert holes, being shiny, reflect the laser light and are not effected by it. Rapid self-quenching of the laser heated surfaces results in a hard martensitic layer in the external surface, with a surface hardness of 57 to 60 Rockwell C units. The seal gland, heel, and spindle bore areas of the roller cones are hardened similarly by exposure to laser light. In an alternative process, the hard tungsten carbide or like inserts are press fitted into the holes before the laser treatment. The subsequent laser treatment does not affect the inserts adversely, because the inserts, too, have shiny light reflective surfaces, and therefore do not absorb the laser light. 
         [0004]    U.S. Pat. No. 5,975,223 discloses a rock drill bit for rotary crushing machining of rock. The rock drill bit includes legs, each of which carry a journal provided with bearing surfaces cooperating via bearing elements with bearing races in a rotatable roller provided with inserts or chisels. Each leg is made from a parent material with substantially homogeneous hardness and each leg comprises a leg tail provided to resist entrance of drill cuttings into the bearings. The bearing surfaces have higher wear resistance than the parent material and the leg tail is at least partly of the same material condition as the bearing surfaces. The invention further relates to a method of manufacturing a rock drill bit. 
         [0005]    U.S. Pat. No. 7,210,377 discloses a method of forming a drill bit structure, the method including fixing spacers to the drill bit structure. The spacers are arranged at preselected locations on an outer surface of the drill bit structure. A hardfacing material is then applied to the drill bit structure, and the spacers are removed. Holes are machined in the drill bit structure at the preselected locations, and drilling inserts are positioned in each hole. A method of forming a drill bit structure, the method including applying a hardfacing material to selected surfaces of the drill bit structure. The hardfacing material includes a perforated carbide infiltrated material and a perforated powder infiltrated material. The perforations in the powder infiltrated material correspond to the perforations in the carbide infiltrated material. Holes are machined in the drill bit structure at the locations of the perforations, and drilling inserts are positioned in each hole. 
         [0006]    U.S. Pat. No. 8,307,920 discloses an earth boring drill bit that includes a cutting cone with a cutting disk. Compacts are inserted within the disk having a chisel shaped end set flush with the cutting disk periphery. The compact crests and cutting disk periphery form a generally seamless cutting surface. The cutting cone can further include cutting teeth thereon also having flush mounted compacts. The compacts can be made from a material such as cemented carbide, hardfacing, tungsten, tungsten alloys, tungsten carbide and the cutter made from steel. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    The present disclosure generally relates to a roller cone drill bit with improved erosion resistance. In one embodiment, a method of manufacturing a roller cone for a drill bit includes: selectively carburizing a land of the roller cone between a plurality of spots on the land for protection against erosion; after carburization, forming sockets in the roller cone at the spots; and mounting cermet inserts in the sockets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
           [0009]      FIGS. 1A-1D  illustrate spot facing of a first roller cone, according to one embodiment of the present disclosure. 
           [0010]      FIG. 2A  illustrates the first roller cone having flats produced by the spot facing.  FIG. 2B  illustrates applying inhibitor to one of the flats.  FIG. 2C  illustrates the first roller cone having the inhibitor applied to the flats. 
           [0011]      FIG. 3A  illustrates carburizing of the first roller cone.  FIGS. 3B and 3C  illustrate forming a socket at one of the flats. 
           [0012]      FIG. 4A  illustrates a second roller cone having the sockets formed therein at the flats.  FIG. 4B  illustrates pressing a cutter insert into one of the sockets. 
           [0013]      FIG. 5  illustrates the first roller cone having the cutter inserts pressed into the sockets. 
           [0014]      FIG. 6  illustrates a drill bit having the roller cones. 
           [0015]      FIG. 7  illustrates carburization of a first roller cone, according to another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIGS. 1A-1D  illustrate spot facing of a first roller cone  1   a , according to one embodiment of the present disclosure. The first roller cone  1   a  may start from a forged steel body of a conical shape. A handling socket  2  may be formed into a rear of the steel body for mounting thereof onto a spindle  3  of a machine tool, such as a lathe. The lathe may be manually operated or CNC. One or more lands, such as a heel land  4   a , a gage land  4   b , an inner land  4   c , and a nose land  4   d  may be formed in an outer surface of the body, such as by turning, using the lathe. 
         [0017]    Once the lands  4   a - d  have been formed, the spindle  3  may be locked and a spot facer  5  may be spun  6  and plunged  7   p  into an outer surface of one of the lands, such as the gage land  4   b , until a flat  8  is formed. The spot facer  5  may be a counterbore bit (shown) or an end mill (not shown) and, if an end mill, may be articulated to form the flat  8  after plunging  7   p . The spot facer  5  may be plunged  7   p  manually or be plunged and articulated by the CNC machine. Once the flat  8  has been formed, the spot facer  5  may be raised  7   r  and the spindle  3  may be unlocked and rotated  9  by a predetermined increment. Once rotated by the increment, the spindle  3  may be relocked and the spot facer  5  used to form a second flat. The spot facing process may be repeated until a set of flats has been formed around the selected land  4   b.    
         [0018]    Although implied that the spot facer  5  is in a vertical orientation, the spot facer may be in a horizontal orientation instead. 
         [0019]      FIG. 2A  illustrates the first roller cone  1   a  having flats  8  produced by the spot facing. Once the selected land  4   b  has been spot faced, the spot facing process may be repeated for the other lands  4   a,c,d . Each flat  8  may be circular and may correspond to a location where an insert may later be mounted. Once the flats  8  have been formed, the handling socket  2  may be enlarged, such as by machining, to form a bearing shaft receptacle (not shown) for mounting the first roller cone  1   a  to a leg (not shown) of a drill bit. 
         [0020]      FIG. 2B  illustrates applying inhibitor  10  to one of the flats  8 . Once the bearing shaft receptacle has been formed, the inhibitor  10  may be applied to one of the flats  8  for protection of a region of the first roller cone  1   a  adjacent to the flat from carburization. The inhibitor  10  may be metallic paint or paste and may be applied using a brush  11 . The inhibitor paint or paste may be formulated with a base metal having little or no affinity for carbon, such as copper. The inhibitor  10  may be applied manually or the application may be automated, such as by a robot. 
         [0021]    Advantageously, for manual application of the inhibitor  10 , the flats  8  provide a clear visual guide. 
         [0022]    Alternatively, the inhibitor  10  may be applied using a sprayer, such as an airbrush (shown) or a spray gun (not shown). Alternatively, the spot facing may be omitted and the inhibitor  10  may be manually applied by covering the first roller cone  1   a  cone with a template (not shown) having holes formed therein to guide application of the inhibitor onto spots (corresponding to the flats  8  but without altering the outer surface of the first roller cone  1   a ) of the lands  4   a - d  thereof. Alternatively, the spot facing may be omitted and the inhibitor  10  may be applied by a robot (not shown) programmed to apply inhibitor onto spots (corresponding to the flats  8  but without altering the outer surface of the first roller cone  1   a ) of the lands  4   a - d  thereof. 
         [0023]      FIG. 2C  illustrates the first roller cone  1   a  having the inhibitor  10  applied to the flats  8 . The application of the inhibitor  10  may then be repeated for the rest of the flats  8  of the roller cone  1   a.    
         [0024]      FIG. 3A  illustrates carburizing of the first roller cone  1   a . Once the inhibitor  10  has been applied to all of the flats  8  and has been allowed to cure, the first roller cone  1   a  may be loaded into a carburization furnace  12 . The carburization furnace  12  may include a housing  13 , a heating element  14 , a controller, such as programmable logic controller (PLC)  15 , a temperature sensor  16 , a carbon potential sensor  17 , a reactor  18 , and a power supply (not shown). 
         [0025]    The reactor  18  may be mounted to a sidewall of the housing  13  and extend through an opening therein. The reactor  18  may include a plurality of concentric tubes, such as a feed tube  18   f , a combustion tube  18   c , a heater tube  18   h , and a generator tube  18   g . The reactor  18  may further include a manifold  18   m  having a first inlet in fluid communication with the feed tube  18   f  for supplying fuel  19  thereto. The reactor  18  may further include a diffuser  18   d  connected to a distal end of the feed tube  18   f  and an igniter (not shown) disposed within the combustion tube  18   c  adjacent to the diffuser. The manifold  18   m  may further have a second inlet in fluid communication with the combustion tube  18   c  for supplying an oxidizer, such as air  20 , thereto. The air  20  may flow down an annulus formed between the feed tube  18   f  and the combustion tube  18   c  to the diffuser  18   d  for mixing with the fuel  19 . An end of the combustion tube  18   c  may be closed for diverting exhaust  21  resulting from combustion of the fuel  19  and air  20  up an annulus formed between the combustion tube  18   c  and the heater tube  18   h  to an outlet of the manifold  18   m . The manifold  18   m  may also have a baffle isolating the outlet from the second inlet. 
         [0026]    The manifold  18   m  may further have a third inlet in fluid communication with the generator tube  18   g  for supplying a mixture  22  of air and enriching gas thereto. The enriching gas may be a hydrocarbon or carbon-oxide. The mixture  22  may flow down an annulus formed between the generator tube  18   g  and the heater tube  18   h  for being heated by the counter-flowing exhaust  21  and/or radiation from the combustion. The reactor  18  may further include catalyst  18   y  packed along a portion of the annulus formed between the generator tube  18   g  and the heater tube  18   h  to promote cracking of the mixture  22 . The catalyst  18   y  may be metallic, such as electrolytic nickel. The cracked mixture (not shown) may be discharged from the generator tube into a chamber formed in the housing  13  to establish a carburizing atmosphere therein. 
         [0027]    Before loading of the first roller cone  1   a , the furnace  12  may be preheated to a carburizing temperature and the reactor  18  operated to establish the carburizing atmosphere. The first roller cone  1   a  may be loaded and allowed to sit in the furnace for a predetermined period of time  23  sufficient for carburization of an uninhibited portion thereof. The inhibitor  10  may protect the flats  8  from carburization thereof. 
         [0028]    Once carburization of the first roller cone  1   a  is complete, the first roller cone may be unloaded from the carburization furnace  12  and quenched. The first roller cone  1   a  may then be loaded into a heat treatment furnace (not shown) and tempered therein. 
         [0029]    Alternatively, the reactor  18  may be omitted and the first roller cone  1   a  may instead be packed with carburizing material. Alternatively, the reactor  18  may be a separate unit from the furnace  12  and the cracked mixture may be piped thereto. 
         [0030]      FIGS. 3B and 3C  illustrate forming a socket  24  at one of the flats  8 . Once the first roller cone  1   a  has been quenched and tempered, a drill bit  25   d  may be spun  6  and plunged  7   p  into the flat  8  to form a pilot hole  26  in the first roller cone  1   a . Once the pilot hole  26  has been formed, a reamer  25   r  may be spun  6  and plunged  7   p  into the pilot hole  26  to form the socket  24  in the first roller cone  1   a . Once the socket  24  has been formed into the flat  8 , the drilling and reaming process may be repeated for the rest of the flats  8  to form the sockets  24  in the first roller cone  1   a . Each flat  8  may be slightly oversized relative to the respective socket  24  to prevent leakage of carburization into the respective socket  24 . A diameter of each flat  8  may be greater than a diameter of the respective socket  24 , such as ten to sixty percent greater, to prevent the leakage. The oversizing may be limited such that the remainders of the lands  4   a - d  are still adequately protected from abrasion and/or erosion by the carburization. 
         [0031]      FIG. 4A  illustrates a second roller  1   b  cone having the sockets  24  formed therein at the flats  8 . As the spot facing, applying inhibitor  10 , carburizing, and socket forming processes are being performed on the first roller cone  1   a , the processes may be contemporaneously or immediately thereafter performed on second  1   b  and third  1   c  ( FIG. 6 ) roller cones. 
         [0032]      FIG. 4B  illustrates pressing a cutter insert  27  into one of the sockets  24 . Each cutter insert  27  may be made from a cermet, such as a cemented carbide. The cemented carbide may be cobalt-tungsten carbide. Each cutter insert  27  may have a cylindrical base and a nose extending from the base. Each nose may be dome-shaped, conical, or chisel-shaped, such as each cutter insert  27  of the gage lands  4   b  having a chisel-shaped nose and each cutter insert of the inner  4   c  and nose  4   d  lands having a conical nose. Each base may have a diameter slightly greater than a diameter of the respective socket  24  to form an interference fit therebetween. To mount each cutter insert  27  into the respective socket  24 , a sleeve  28  may be fit onto the nose of the respective cutter insert. The base of the respective cutter insert  27  may be aligned with the respective socket and a mallet  29  may be swung  30  to tap the sleeve  28 , thereby pressing  31  the cutter insert  27  into the respective socket  24 . 
         [0033]    Alternatively, the base of each cutter insert  27  may have a diameter slightly less than a diameter of the respective socket  24  and the cutter insert may be mounted in the respective socket, such as by brazing, instead of interference fit. 
         [0034]      FIG. 5  illustrates the first roller cone  1   a  having the cutter inserts  27  pressed into the sockets  24 . Once the cutter insert  27  has been pressed into the respective socket  24 , the pressing may be repeated for each of the remaining sockets  24  of the lands  4   b - d . For the heel land  4   a , stabilizer inserts  32  may be pressed into the respective sockets  24  thereof. Each stabilizer insert  32  may be made from a cermet, such as a cemented carbide. The cemented carbide may be cobalt-tungsten carbide. Each stabilizer insert  32  may be cylindrical and may have a diameter slightly greater than a diameter of the respective socket  24  to form an interference fit therebetween. Each stabilizer insert  32  may be pressed into the respective socket in a similar fashion to that of the cutter inserts  27 , discussed above. 
         [0035]    Alternatively, the base of each stabilizer insert  32  may have a diameter slightly less than a diameter of the respective socket  24  and the stabilizer insert may be mounted in the respective socket, such as by brazing, instead of interference fit. 
         [0036]      FIG. 6  illustrates a drill bit  33  having the roller cones  1   a - c . The drill bit  33  may include a body (not shown) and the roller cones  1   a - c . The nose lands  4   d  and the inner lands  4   c  of the roller cones  1   a - c  may be offset relative each other to form a continuous cutting profile. The third roller cone  1   c  may further have an outer land  4   e  formed adjacent to the gage land  4   b  thereof and in an outer surface thereof. The flats  8 , sockets  24 , and cutter inserts  27  of the outer land  4   e  may be oversized relative thereto, thereby overlapping with the gage land  4   b  thereof. 
         [0037]    The body may have an upper shank and a lower leg for each roller cone  1   a - c . The body may be made from a metal or alloy, such as steel. Each leg may be attached to the shank, such as by welding. The legs may be equally spaced around the body, such as three at one hundred twenty degrees. The shank may have a coupling, such as a threaded pin, formed at an upper end thereof for connection to another member of a bottomhole assembly of a drill string for drilling a wellbore. A bore may be formed in the shank and may extend from an upper end thereof to a plenum formed therein adjacent to a lower end thereof. 
         [0038]    Each leg may have an upper shoulder, a mid shirttail, a lower bearing shaft, and a ported boss. The shoulder, shirttail, ported boss, and bearing shaft of each leg may be interconnected, such as by being integrally formed and/or welded together. Each ported boss may be in fluid communication with the plenum via a respective port formed in the shank and may have a nozzle fastened therein for discharging drilling fluid onto the respective roller cone  1   a - c . Each bearing shaft may extend from the respective shirttail in a radially inclined direction. Each bearing shaft may have a journal for supporting rotation of the respective roller cone  1   a - c  therefrom. Each leg may have a lubricant reservoir formed therein and a lubricant passage extending from the reservoir to the respective journal bearing formed between the bearing shaft and the respective roller cone  1   a - c . The lubricant may be retained within the each leg by a seal, such as an o-ring, positioned in a seal gland between the respective cone  1   a - c  and the bearing shaft. Each leg may also have a fill port in fluid communication with the lubricant reservoir and closed by a pressure compensator. 
         [0039]    Each roller cone  1   a - c  may be mounted to the respective leg by a plurality of balls (not shown) received in a race formed by aligned grooves in each roller cone and the respective bearing shaft. The balls may be fed to each race by a ball passage formed in each leg and retained therein by a respective ball plug. Each ball plug may be attached to the respective leg, such as by welding. Upper and lower edges of each shirttail may be protected from erosion and/or abrasion by respective hardfacing with a ceramic or cermet material. An outer surface of each shirttail may also be protected from erosion and/or abrasion by stabilizer inserts secured into sockets thereof, such as by interference fit or brazing. 
         [0040]    Alternatively, each cutter of the inner land  4   c  and/or nose land  4   d  of any one or all of the roller cones  1   a - c  may be a milled tooth hardfaced by a ceramic or cermet material instead of the cutter insert  27 . Alternatively, the lubricant system may be omitted from the drill bit  33  and the drill bit may be used in a mining operation instead of a wellbore drilling operation. 
         [0041]      FIG. 7  illustrates carburization of a first roller cone  36 , according to another embodiment of the present disclosure. Instead of using the inhibitor  10  to selectively carburize the first roller cone  1   a , a promoter  37  may be applied to the first roller cone  36  at all surfaces thereof except for the flats  8 . The first roller cone  36  may be otherwise identical to the first roller cone  1   a . The promoter  37  may be a carbon-rich paint or paste and may be applied using the brush  11  or any alternatives therefor discussed above. 
         [0042]    Once the promoter  37  has been applied and has been allowed to cure, the first roller cone  36  may be loaded into a furnace  34 . The furnace  34  may include a housing  35 , the heating element  14 , the PLC  15 , the temperature sensor  16 , and the power supply (not shown). Before loading of the first roller cone  36 , the furnace  34  may be preheated to a carburizing temperature. The first roller cone  36  may be loaded and allowed to sit in the furnace for the predetermined period of time  23  sufficient for carburization of the promoted portion thereof. 
         [0043]    Once carburization of the first roller cone  36  is complete, the first roller cone may be unloaded from the furnace  34  and quenched. The first roller cone  36  may then be loaded into a heat treatment furnace (not shown) and tempered therein. Once heat treatment has been completed, the sockets may be formed into the first roller cone  36  at the flats  8  and the inserts may be pressed into the sockets as discussed above for the first roller cone  1   a . The other two roller cones may be processed in a similar fashion and the three cones mounted to legs of a drill bit as discussed above. 
         [0044]    Alternatively, the spot facing may be omitted and the promoter  37  may be manually applied by covering the first roller cone  1   a  cone with a template (not shown) to guide application of the promoter between spots (corresponding to the flats  8  but without altering the outer surface of the first roller cone  36 ) of the lands thereof and to the rest of the surfaces thereof. Alternatively, the spot facing may be omitted and the promoter  37  may be applied by a robot (not shown) programmed to apply inhibitor between spots (corresponding to the flats  8  but without altering the outer surface of the first roller cone  36 ) of the lands thereof and to the rest of the surfaces thereof. 
         [0045]    While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the invention is determined by the claims that follow.

Summary:
A method of manufacturing a roller cone for a drill bit includes: selectively carburizing a land of the roller cone between a plurality of spots on the land for protection against erosion; after carburization, forming sockets in the roller cone at the spots; and mounting cermet inserts in the sockets.