This invention relates in general to hardfacing on earth-boring bits and, in particular, to a hardfacing containing a mixture of ultrahard sintered tungsten carbide pellets with other types of tungsten carbide pellets.
Hardfacing has been used for many years on earth-boring bits to reduce the abrasive and/or erosive wear. The hardfacing typically comprises hard metal particles dispersed within a metal matrix. The hard metal particles are often formed of tungsten carbide. Sintered tungsten carbide, also called cemented carbide, comprises tungsten carbide grains within a binder powder, such as cobalt. The tungsten carbide grains utilized in sintered tungsten carbide pellets are generally less than ten microns in diameter. During this sintering process, which employs heat and pressure, the cobalt will enter a liquid stage while the tungsten carbide grains remain in the solid stage. As a result of this process, the cobalt cements the tungsten carbide grains to create sintered tungsten carbide. The ductile cobalt metal offsets the characteristic brittleness of the tungsten carbide particles, resulting in a pellet that has enhanced toughness and durability. Sintered tungsten carbide pellets can be formed into generally spherical shapes or irregular shapes. Also, sintered tungsten carbide in a crushed form is available.
Cast tungsten carbide particles are formed in a casting process, and, thus, are harder than sintered tungsten carbide and do not have a binder of a soft metal such as cobalt. Cast tungsten carbide particles may be spherical, irregular or crushed. Spherical cast carbide pellets are typically smaller in diameter than standard spherical sintered tungsten carbide pellets. Cast tungsten carbide particles are thus harder than sintered tungsten carbide particles but more brittle.
Prior art hardfacing for earth-boring bits contains a variety of sizes and volume fractions of standard spherical sintered tungsten carbide pellets, crushed sintered tungsten carbide particles, spherical cast tungsten carbide pellets, crushed cast tungsten carbide particles, as well as other types of cast tungsten carbide, such as monocrystalline or macrocrystalline particles. The matrix that contains and binds the hardfacing pellets and particles is often iron, but it also may contain nickel and/or other alloys.
The hardfacing composition described herein includes particles referred to herein for convenience as “ultrahard” particles. The ultrahard particles are sintered and consist of tungsten carbide grains, cobalt and vanadium. The ultrahard particles are dispersed within a matrix metal of iron, nickel or alloys thereof. In one embodiment, the ultrahard particles comprise 4 to 8% cobalt, 0.25% to 2% vanadium, with the remainder being tungsten carbide.
The composition may also contain conventional sintered tungsten carbide particles, typically of a larger size than the ultrahard particles. The ultrahard particles have a greater hardness than the sintered tungsten carbide particles. The composition may also include cast tungsten carbide particles. The ultrahard particles have a lesser hardness than cast tungsten carbide particles but greater toughness. The ultrahard particles may be in a spherical form or a crushed form.
Bit 11 contains hardfacing in various places to prevent wear on the steel components. In this embodiment, bit leg outer surface hardfacing 27 covers the entire outer surface of each bit leg 17 except for ball plug 23 and fixture dimple 25. Hardfacing 27 extends from the lower end, or shirttail, of each bit leg 17 to the recess containing pressure compensator cap 21. A leading edge hardfacing 29 extends over the leading edge of each bit leg 17. A trailing edge hardfacing 31 extends over the trailing edge of each bit leg 17. Leading edge hardfacing 29 and trailing edge hardfacing 31 join outer surface hardfacing 27.
A robotic process may serve as the method of applying hardfacing layers 27, 29 and 31. In a plasma transferred arc (PTA) process, hardfacing powder flows down a nozzle to an arc. The arc moves relative to the bit leg 17 during the application. Other methods are available, such as using an oxyacetylene torch and a rod. Some earth-boring bits 11 may have outer surface hardfacing 27 applied only on the lower edge or shirttail. Some bits may have only leading edge hardfacing 29 and not trailing edge hardfacing 31. The compositions of outer surface hardfacing 27, leading edge hardfacing 29 and trailing edge hardfacing 31 may be the same or may differ.
Cones 19 also contain layers of hardfacing, particularly if it is a milled tooth type. In a milled tooth bit, cones 19 have rows of machined or milled teeth 33 that are formed integrally with the body of each cone 19. Teeth 33 contain layers of teeth hardfacing 35. Teeth hardfacing 35 covers the leading and trailing flanks and the inner and outer sides of each tooth 33. Each cone 19 has a gage surface that may contain a layer of gage hardfacing 37 for engaging the side wall of the bore hole. Teeth hardfacing 35 and gage surface hardfacing 37 are typically applied by heating with an oxyacetylene torch a metal tube filled with hard metal particles. The hardfacing layers 35, 37 on cones 19 often have different compositions than hardfacing layers 27, 29 and 31 on bit leg 17.
Sintered tungsten carbide, also called cemented carbide, comprises tungsten carbide grains within a binder powder, such as cobalt. The tungsten carbide grains utilized in standard spherical sintered tungsten carbide pellets 39 are generally less than ten microns in diameter. During this sintering process, which employs heat and pressure, the cobalt will enter a liquid stage while the tungsten carbide grains remain in the solid stage. As a result of this process, the cobalt cements the tungsten carbide grains to create sintered tungsten carbide. The ductile cobalt metal offsets the characteristic brittleness of the tungsten carbide particles, resulting in a pellet that has enhanced toughness and durability. Sintered tungsten carbide pellets can be formed into generally spherical shapes or irregular shapes. Also, sintered tungsten carbide in a crushed form is available. The hardness of standard spherical sintered tungsten carbide pellets 39 ranges from about 1368 KHN (Knoop hardness), which is approximately 89.5 HRA (hardness Rockwell A), to about 1587 KHN (approximately 91.7 HRA).
Spherical cast tungsten carbide pellets 41 are formed in a casting process, and thus, are harder than sintered tungsten carbide and do not have a binder of a soft metal such as cobalt. Cast tungsten carbide particles may be spherical, irregular or crushed. Spherical cast tungsten carbide pellets 41 are typically smaller in diameter than standard spherical sintered tungsten carbide pellets 39. Hardness levels for spherical cast tungsten carbide pellets 41 range from about 1992 KHN (approximately 95.7 HRA) to about 2223 KHN (approximately 97.9 HRA). Typical sizes for spherical cast tungsten carbide pellets 41 in bit hardfacing are in the range from 44-250 microns. Spherical cast tungsten carbide pellets 41 are thus harder than standard spherical sintered tungsten carbide pellets 39 but more brittle. Standard spherical sintered tungsten carbide pellets 39 are tougher than spherical cast tungsten carbide pellets 41. Prior art hardfacing for earth-boring bits contains a variety of sizes and volume fractions of standard spherical sintered tungsten carbide pellets, crushed sintered tungsten carbide particles, spherical cast tungsten carbide pellets, crushed cast tungsten carbide particles, as well as other types of cast tungsten carbide, such as monocrystalline or macrocrystalline particles. The matrix that contains and binds the hardfacing pellets and particles is often iron, but it also may contain nickel or other alloys.
Referring to
In the example of
In
In
In
In
The examples of
The various compositions described result in an extremely wear and/or erosion resistant material. The ultrahard particles provide more hardness than conventional sintered tungsten carbide particles. Although not as hard as cast tungsten carbide particles, ultrahard particles provide more toughness. Ultrahard particles may be used as a replacement for or in addition to cast tungsten carbide particles.
While several examples have been shown, it should be apparent to those skilled in the art that various changes may be made to these compositions.
This application is a continuation of U.S. patent application Ser. No. 12/893,953, filed Sep. 29, 2010, pending, which claims the benefit of U.S. Provisional Patent Application Ser. No. 61/246,711, filed Sep. 29, 2009, the disclosure of each of which is hereby incorporated herein in its entirety by this reference.
Number | Date | Country | |
---|---|---|---|
61246711 | Sep 2009 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 12893953 | Sep 2010 | US |
Child | 13953351 | US |