Alloys or metallic compositions – Cobalt base – Chromium containing
Reexamination Certificate
2000-07-27
2002-11-12
Sheehan, John (Department: 1742)
Alloys or metallic compositions
Cobalt base
Chromium containing
C420S440000, C148S425000
Reexamination Certificate
active
06479014
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to alloys for tips to be welded or brazed onto wood cutting saws.
Wood cutting saw blades deteriorate at the cutting tips at a high rate, especially in modernized saw mills equipped with high speed saws. When saw tips become dull, cutting efficiency is greatly reduced. Most saw mills have their own shop to sharpen or “re-tip” the saw blades.
Commonly used saw tip materials comprise tungsten carbide composites, which are fine tungsten carbide particles dispersed in a cobalt matrix. These composites have excellent abrasion resistance as is required for saw tip applications, but tend to deteriorate in the presence of corrosive wood juice. Another disadvantage of tungsten carbide tips is that they are often brazed onto saws using cadmium-containing brazing alloys, and cadmium is considered to be hazardous. Furthermore, the strength of the brazing material is often inadequate, such that the tips break off at the bond, especially when cutting frozen wood in winter months.
Cobalt-chromium-tungsten alloys, namely, certain alloys distributed under the trade designation STELLITE®, have also been used in saw tip applications. The constituents in these cobalt-based alloys which have primary effect on hardness are as follows:
Composition (wt %)
Alloy
C
Cr
W
Hardness (HRC)
STELLITE 12
1.5
29
8.5
45
STELLITE 1
2.5
29
13
52
STELLITE 19
1.8
31
11
50
These alloys contain complex carbides rich in chromium. Although there is some partition of tungsten in the carbides, much of the tungsten remains in solid solution. This is due to the large size of the tungsten atoms, which results in sluggish diffusion through the matrix. Tips manufactured from these alloys have enhanced corrosion resistance as compared to tungsten carbide composites. Furthermore, these alloys can be welded onto saws by electric resistance welding or gas-tungsten-arc welding, which welding techniques produce a stronger metallurgical bond than brazing. These alloys, however, are typically not selected for applications such as non-corrosive or mildly corrosive wood cutting, because their abrasion resistance is generally inferior to conventional tungsten carbide composites.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a saw tip which is attachable to saw blades by welding, and which has abrasion resistance comparable to conventional tungsten carbide tips; to provide a saw tip which has enhanced corrosion resistance as well as abrasion resistance comparable to abrasion resistance of tungsten carbide tips; to provide a saw tip having enhanced corrosion and abrasion resistance as compared to cobalt-chromium-tungsten saw tips; and to provide a saw tip having enhanced corrosion resistance in reducing-type corrosive wood cutting environments.
Briefly, therefore, the invention is directed to a saw blade Co-based saw tip for attachment to a saw blade for cutting wood, the saw tip comprising Mo in an amount from about 4 to about 20% by weight, Cr, and C.
The invention is also directed to a saw tip for attachment to a saw blade for cutting wood, the tip comprising, by approximate weight %:
C 0.8-3.5
Cr 25-35
Mo 4-20
Mn, Si, Ni, B plus Fe 0-10
Co Balance.
The invention is also directed to a saw tip for attachment to a saw blade for cutting wood, the tip consisting essentially of, by approximate weight %:
C 0.8-3.5
Cr 25-35
Mo 4-20
Mn, Si, Ni, B plus Fe 0-10
Co Balance.
Another embodiment of the invention is directed to a saw tip for attachment to a saw blade for cutting wood, the tip comprising 4-10% by weight Mo and 0.8-2.0% by weight C.
A further embodiment is directed to a saw tip for attachment to a saw blade for cutting wood, the tip comprising 10-20% by weight Mo and 1.5-3.0% by weight C.
The invention is also directed to a saw tip for attachment to a saw blade for cutting wood, the tip comprising, by approximate weight %:
C 0.8-3.5
Cr 25-35
Mo 4-20
W 1-4
Mn, Si, Ni, B plus Fe 0-10
Co Balance.
Still further embodiments of the invention are directed to wrought or powder alloys for forming saw tips comprising each of the foregoing compositions, and rods from which tips are segmented upon attachment to saw blades.
Other objects and features of this invention will be in part apparent and in part point out hereinafter.
DETAILED DESCRIPTION OF THE INVENTION
The saw tip alloy of the invention comprises about the following, by weight percent:
C 0.8-3.5
Cr 25-35
Mo 4-20
Co Balance
An alternative embodiment comprises about the following, by weight percent:
C 0.8-3.5
Cr 25-35
Mo 4-20
W 1-4
Co Balance
Of the foregoing embodiments, there are especially preferred alternative high Mo/high C and low Mo/low C embodiments. The high Mo/high C embodiment comprises about the following, by weight percent:
C 0.8-3.5
Cr 25-35
Mo 10-20
W 1-4 (or 0-4)
Co Balance;
more preferably,
C 1.5-3.5
Cr 25-35
Mo 10-20
W 1-4 (or 0-4)
Co Balance.
The low Mo/low C embodiment comprises about the following, by weight percent:
C 0.8-3.5
Cr 25-35
Mo 4-10
W 1-4 (or 0-4)
Co Balance;
more preferably
C 0.8-2.0
Cr 25-35
Mo 4-10
W 1-4 (or 0-4)
Co Balance
Further preferred embodiments contain elemental percentages in each of the above ranges, except that there are no intentional W additions, and the incidental occurrence of W is avoided to the extent possible.
According to this invention, Mo is employed rather than some or all of the W in Co—Cr—W alloys for saw tips in order to enhance abrasion resistance. Molybdenum atoms are much smaller than W atoms, and with an atomic weight roughly half the atomic weight of W, there are roughly twice as many Mo atoms for a given weight percentage. Molybdenum has a greater affinity for C than does W, and due to its smaller size diffuses much more quickly, thereby favoring the formation of carbides which impart abrasion resistance. Furthermore, Mo imparts greater corrosion resistance than does W in acidic environments of a reducing nature, which are often encountered in wood cutting applications.
The Mo content is in the range of about 4% to about 20% by weight. While the corrosion resistance imparted by Mo is believed to be imparted by Mo in solid solution, the wear resistance is imparted primarily by the formation of Mo carbides. The high Mo/high C embodiment is especially preferred for cutting softer woods such as pine, and in some instances is less preferred for harder woods because it is more brittle than the low Mo/low C embodiment.
Other elements such as Mn, Si, Ni, B and Fe may be present as incidental impurities, or as intentional additions to improve melting characteristics. In particular, up to about 10 wt %, preferably up to about 8 wt %, of these elements cumulatively are included in the alloy.
The alloys are prepared as powder metallurgy preforms, i.e., pre-shaped teeth or rods for cutting or automatic tipping, as cast rods for welding onto saw blades, and as solid or tubular wires for welding onto saw blades.
One further advantage of the current invention as compared to the prior tungsten carbide saw teeth is that the current alloy permits the use of a hook angle of up to as much as about 35 degrees, which corresponds to a reduction in cutting forces.
REFERENCES:
patent: 5242758 (1993-09-01), Hitchcock et al.
patent: 62037355 (1987-02-01), None
McKee Bradley
Raghu Damadoran
Wu James B. C.
Deloro Stellite Company Inc.
Oltmans Andrew L.
Senniger Powers Leavitt & Roedel
Sheehan John
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