Note: Descriptions are shown in the official language in which they were submitted.
1090523
The present invention relates to tungsten carbide
products.
Cobalt-bonded tungsten carbide materials are well
known for their resistance to abrasive wear; they are
widely used because they combine this property with good
strength and impact resistance. In 1968, it was shown
by the inventors herein that iron-nickel bonded tungsten
carbide has superior transverse rupture strength to the
cobalt-bonded materials, provided certain conditions were
observed and precautions were taken to prevent formation
of a deleterious "eta" phase by control of carbon (see
U.S. Patent No. 3,384,465). Conditions necessary to
successfully produce high strength iron-nickel bonded WC
material were: the sintered WC grain size must be below
5 microns, and the nickel content of the binding metal
should be between S and 40% by weight. However, these
conditions and precautions are necessary but not sufficient
to yield materials of optimal abrasive wear resistance.
In accordance with the present invention, there is
provided a method of making iron-bonded tungsten carbide
powder compacts comprising: (a) forming a powder mixture
blend consisting of tungsten carbide powder, 0.1 to 1.5 wt%
vanadium carbide, and a binder powder constituting about 3
to 30 wt% of the mixture, the binder powder consisting
essentially of 7.0 to 15 wt% nickel and ~he remainder iron;
(b) adding additional carbon in an excess amount of 0.2
to 1.0 wt% over and above the carbon required to satisfy
stoichiometric WC; (c) homogenizing the blended powders;
(d) cold compacting the homogenized powder with sufficient
pressure to form a coherent compact; and (e) liquid phase
sintering the compacts at 1300 to 1500C for sufficient
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time to achieve a uniformly bonded microstructure.
The presence of the small amount of vanadium carbide
powder and the adjustment of the carbon content of the
powder mixture results in a hard sintered compact which
is useful for cutting tools and other applications where
a high resistance to abrasive wear is required while
not sacrificing transverse rupture strength and impact
resistance.
The present invention also provides a hard
sintered compact consisting essentially of tungsten car-
bide and 0.1 to 1.5 wt% vanadium carbide particles bonded
by an alloy consisting essentially of 7 to 15 wt% nickel,
and the remainder iron, carbon in the compact being
adjusted to assure the substantial absence of an eta
phase and a deleterious amount of graphite, substantially
all of the grains of the ~intered tungsten carbide having
a grain size of not over S microns.
The invention is described further, by way of
illustration, with reference to the accompanying drawings,
in which:
Figure 1 is a graphical illustration plotting the
variation of abrasion resistance factor with the percent
binder; several plots are illustrated, one of which repre-
sents data generated employing the inventive teaching
herein;
Figure 2 is a sectional view of an apparatus
employed to determine abrasive resistance factor;
Figures 3 and 4 are micro-photographs of tungsten
carbide material produced respectively with and without
the addition of VC in accordance with the present invention;
and
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Figure 5 is a graphical illustration of wear resis-
tance factor as varied with the nickel content of the
binder.
Abrasion resistance of cemented carbides is determined
by a commonly used test approved by the cemented carbide
Producers Association, procedure P-112. The test employs a
suitable vessel 10, such as that shown in ~igure 2, which
holds a wet abrasive in the form of an aluminum oxide
slurry 11; an abrading wheel 12 is disposed partly immersed
in the slurry. The wheel has mixing vanes 13 on each side
to lift and swirl the slurry against the specimen 14. The
steel wheel normally rotates in the center of the vessel at
about a 100 r.p.m.; the direction of the rotation is as
shown. A specimen holder 15 causes the specimen 14 to
bear against the periphery of the wheel. The holder 15 is
L-shaped and pivots about the apex 16. The specimen holder
must be mounted so that there is no more than 0.002 inch
side play occurring at the line of contact between the
specimen and the wheel. The specimen is so placed that it
is tangent to the wheel at about the centerline of the
wheel. A 25 lb. weight 17 is attached to the end of lever
arm 16a of holder 15. With a lever advantage of 2 to 1,
a load of 50 lbs. is thereby applied at the specimen at the
line of contact 18.
The procedure for the test is essentially as follows:
(al A sample is weight to the nearest 0.0001 gram. (b)
The density is determined, (c) The specimen is placed on
the holder, and inserted into the wear test machine. (d)
The 25 lb. weight is released causing a load to be applied
to the specimen causing it to bear against the wheel. (e)
The bottom drain of the vessel is closed and 30 grains of
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aluminum oxide (A1203), is poured into the vessel to within
1 inch of the center of the wheel. Water is added to the
aluminum oxide in a ratio of 1 cc per 4 grams of grit.
When water has seeped into the abrasive grit, rotation
of the wheel is
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started and run for 1300 revolutions (determined by means of
a counter). The slurry is stirred to insure uniformity.
(g) The weight of the specimen is then weighed. The abrasion
resistahce factor is computed by the formula:
Abrasion resistance (cm3? = ~eight loss in gms. x 105
factor (REV) 1300 REV x Density
of Specimen
As shown in Figure 5, wear resistance appears to be
optimal for nickel contents in the range from 7 to 15% by
weight of the binder. Plot 2 represents abrasion resistance
for an as-sintered composition according to the invention
herein; plot 20 is for the same composition which has been
additionally subjected to a treatment at -196C. The Rockwell
"A" hardness follows a similar trend (not shown), the hardest
compositions falling within the same nickel range. Treatment
at -19~C produces additional improvement in both properties,
due to conversion of retained austenite in the binder to the
harder martensite.
We have now found that iron-nickel bonded tungsten
carbide materials will be superior to cobalt bonded materials
in abrasion resistance and superior to iron-nickel bonded
tungsten carbide materials by the employment of controlled
amounts of vanadium carbide. The increase in abrasion resis-
tance is best illustrated by reference to Figure 1 wherein a
comparison is made between materials produced according to
prior methods and materials produced according to the inventive
method herein. Plot 1 represents a variation in the abrasion
resistance with percent binder for a cobalt-bonded tungsten
carbide material. Plot 2 is for tungsten carbide material
employing an iron-nickel binder, the nickel representing 20%
of the binder. The lower factor values for each binder
i
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percentage of plot 2 represent material subjected to a
cold treatment at -196~C. Plot 3 represents an iron-
nickel bonded tungsten carbide material employing 10~
nickel in the binder. Again the lowest factor values for
each binder percentage of plot 3 represent material
subjected to a cold treatment of -196C. Plot 4 represents
a tungsten carbide material in accordance with the present
invention wherein vanadium carbide has been added in an
amount of up to 1~ by weight. Plot 3 particularly shows
the effect on abrasion resistance factor keeping the nickel
content of the binder at about 10%, and varying the binder
content; plot 2 exhibits less desirable abrasion resistance
factor which is directly related to the high nickel
content. Plot 3 is superior to plot 1 containing a cobalt
bonded material, irrespective of the binder content.
By deploying the same control limits as that for the
material of plot 3, but additionally adding vanadium
carbide, a synergistic effect in abrasion resistance factor
was observed (see plot 4). Notably, the small vanadium
carbide addition reduced the abrasion resistance factor
to a value approximately 1/2 of that with no addition.
Of particular significance, insofar as the practical
application of the material is concerned, is the fact
that the improvement in abrasion resistance factor does
not come at the expense of a loss in strength. Transverse
rupture strength is approximately equal to that of the
cobalt bonded composition of an equivalent binder content.
Several samples were prepared and tested to evaluate
the present invention; the samples were tested for hardness
and abrasion resistance. In each case the RA hardness
value was at least 92 and the samples had a transverse
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1090523
rupture strength of at least 200,000 psi. The resulting
data for six samples is shown in Table I below:
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1090523
Table I
Composition Treatment Abrasion Resistance Hardness
Factor (10-5 cm3/Rev) (RA)
94-3/4% WC + 1/4% VC
+ 5% (Fe 10 Ni)-196C (twice) 0.52 94.0
97% WC + 3% Co ~Jone 2.09 92.6
89-1/2% WC + 1/2% VC
+ 10~ (Fe 10 Ni)-196C (twice) 1.31 93.75
92% WC + 4% TaC
+ 4% Co None 3.4~ 92.0
81-1/2% ~C + 1% VC
+ 17-1/2% (Fe 10 Ni) -196C (twice) 6.81 92.65
87% WC + 13% Co None - 15.8 89.2
The following procedure was used to produce the above
samples. Tungsten carbide powder, less than 3 microns in
average particle size and containing 6.1 weight percent com-
bined carbon, was added to a stainless steel mill loaded with
tungsten carbide-based balls, together with required amounts
of hydrogen reduced electrolytic iron powder, carbonyl or
electrolytic nickel powder, and spectroscopically pure
graphite powder. Graphite powder or excess carbon was added
or present in an amount of at least .2-1.0 wt. ~ over and
above the amount required to satisfy stoichiometric WC; this
completely inhibits eta phase (Fe3w3C). Sufficient benzene
was added to cover the charge, which was then ball milled for
four days. 2% paraffin was dissolved in the benzene and
uni~ormly distri~uted throughout the slurry; the benzene was
then completely evaporated. The dry powder was screened
through a 20 mesh sieve and then pressed into segments at
a pressure of 20,000 psi. The paraffin was removed by
dewaxing at 750F (400C) under dry hydrogen or vacuum.
Specimens were sintered by heating under vacuum for 1 hour
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at (2550-2600F) or about 1400C, while resting on graphite
trays on which 100 mesh crystallites of tungsten carbide
had been sprinkled.
Another typical composition which may be used is
74 wt% WC, 25 wt% Fe-Ni and 1.25 wt% VC.
Figures 3 and 4 demonstrate that the presence of
the required amounts of VC do not significantly act as a
grain refiner for the microstructure of the sintered
compact. Figure 3 shows a sample under the electron
microscope containing 95% WC, 5~ binder consisting of 10%
Ni and the remainder iron; no VC was employed. Figure 4
shows a sample having 94.75% WC, 5% binder containing 10%
Nickel and the remainder iron; .25~ VC is employed. There
is little difference between the WC grain size in each
figure. The magnification for each figure is 5012X.
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