Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
1999-05-11
2001-06-19
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S239000, C075S240000, C051S307000, C175S374000
Reexamination Certificate
active
06248149
ABSTRACT:
TECHNICAL FIELD
This invention relates to the composition of hardfacing materials applied to surfaces subjected to abrasive wear to increase their wear resistance. More particularly, this invention relates to hardfacing compositions applied to one or more surfaces of earth-boring bits of the roller-cutter variety.
BACKGROUND OF INVENTION
It is a long-standing practice in the design and manufacture of earth-boring bits to apply wear-resistant hardfacing materials to the surfaces of such bits that are subjected to abrasive wear during drilling operations. In earth-boring bits of the rolling cutter variety, these surfaces include the teeth of bits of the milled or steel tooth variety, the gage surfaces of the rolling cutters and the shirttails of the bit legs comprising the bit body.
In the past, these hardfacing compositions generally comprise carbides of the elements of Groups IVB, VB and VIB in a matrix metal of iron, cobalt or nickel and alloys and mixtures thereof. The hardfacing is applied by melting the matrix and a portion of the surface to which the hardfacing is applied with an oxyacetylene or atomic hydrogen torch. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal lends the hardfacing fracture toughness. A hardfacing composition must strike an adequate balance between wear resistance (hardness) and fracture toughness. A hardfacing composition that is extremely hard and wear-resistant may lack fracture toughness, causing the hardfacing to crack and flake prematurely. Conversely, a hardfacing with adequate fracture toughness, but inadequate hardness and wear resistance, is eroded prematurely and fails to serve its purpose.
Many factors affect the suitability of a hardfacing composition for a particular application. These factors include the chemical composition and physical structure of the carbides employed in the composition, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
One early advance in hardfacing compositions for use in earth-boring bits is disclosed in commonly assigned U.S. Pat. No. 3,800,891, Apr. 2, 1974 to White et al. This patent discloses a hardfacing composition comprising sintered tungsten carbide in an alloy steel matrix. Sintered tungsten carbide comprises grains or particles of tungsten carbide sintered with and held together by a binder of non-carbide material, such as cobalt. The sintered tungsten carbide possesses greater fracture toughness than the more conventional cast tungsten carbide, such that the resulting hardfacing composition possess good fracture toughness without sacrificing hardness and wear resistance.
U.S. Pat. No. 4,836,307, Jun. 6, 1989 to Keshavan et al., discloses a hardfacing composition employing particles of cemented or sintered tungsten carbide and relatively small particles of single crystal monotungsten carbide, sometimes referred to as “macrocrystalline” tungsten carbide, in a mild steel matrix. This composition purports to possess the advantages of sintered tungsten carbide, as disclosed in U.S. Pat. No. 3,800,891, with the advantages of single crystal monotungsten carbide, which is harder than the cemented or sintered tungsten carbide, yet is less brittle than the alternative cast carbide.
U.S. Pat. No. 5,089,182, Feb. 18, 1992, to Findeisen, et al. discloses a method of manufacturing cast carbide pellets that are generally spherical in shape and have improved mechanical and metallurgical properties over prior-art carbide pellets. These cast pellets are not truly spherical, but are sufficiently symmetrical that residual stresses in the pellets are minimized. Also, the generally spherical shape of these pellets eliminates corners, sharp edges and angular projections, which are present in conventional crushed particles, that increase residual stresses in the particles and tend to melt as the hardfacing composition is applied to the surface.
U.S. Pat. No. 5,663,512, Sep. 2, 1997 to Schader et al. discloses a hardfacing composition which includes a quantity of spherical sintered tungsten carbide granules and a quantity of cast spherical cast tungsten carbide granules in a eutectic form of WC/W
2
C.
During application, some melting of the sintered spherical carbide granules occurs, which precipitates into the metal matrix and coats the spherical WC/W
2
C granules. This composition provides a good balance between hardness and fractures toughness.
DISCLOSURE OF INVENTION
In this invention, the hardfacing composition comprises a quantity of macrocrystalline tungsten carbide particles and a quantity of cast carbide pellets, which are generally spherical granules. The cast carbide pellets are a eutectic of WC/W
2
C. Each of the tungsten carbide particles comprises a single crystal of monotungsten carbide WC in an irregular shape. During the application, the crystal monotungsten carbide particles melt slightly, smoothing irregular edges. The melted material partially dissolves in the matrix during cooling and the subsequent heat treating process. The melted material precipitates out into significantly smaller monotungsten carbide particles and coats on the cast eutectic carbide pellets. The metal matrix tends to adhere to this layer or coating better than to cast carbide pellets having no coating. This reduces loss of carbides during use.
The size of the macrocrystalline tungsten carbide particles may be relatively the same as the cast carbide pellets or the sizes may differ. Also, the quantity of macrocrystalline tungsten carbide particles in a preapplication ratio may be approximately the same as the cast carbide pellets by weight or they may differ. The metal matrix is conventional and from the group consisting of nickel, iron, cobalt and alloys and mixtures thereof. The relatively small irregular shaped macrocrystalline particles combined with small spherical particles allows a high density of carbide to metal matrix in the composition.
REFERENCES:
patent: 3800891 (1974-04-01), White et al.
patent: 4414029 (1983-11-01), Newman et al.
patent: 4944774 (1990-07-01), Keshavan et al.
patent: 5492186 (1996-02-01), Overstreet et al.
patent: 5619000 (1997-04-01), Ederyd et al.
patent: 5663512 (1997-09-01), Schader et al.
patent: 5715899 (1998-02-01), Liang et al.
patent: 5791422 (1998-08-01), Liang et al.
patent: 5944127 (1999-08-01), Liang et al.
patent: 5988302 (1999-11-01), Sreshta et al.
“A Handbook of Hard Metals”; W. Dawihl; 1955.
Massey Alan J.
Overstreet James L.
Baker Hughes Incorporated
Bracewell & Patterson L.L.P.
Bradley James E.
Mai Ngoclan
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