U.S. patent number 3,660,050 [Application Number 04/835,817] was granted by the patent office on 1972-05-02 for heterogeneous cobalt-bonded tungsten carbide.
This patent grant is currently assigned to E. I. du Pont de Nemours and Company. Invention is credited to Ralph K. Iler, Eugene G. Rigby.
United States Patent |
3,660,050 |
Iler , et al. |
May 2, 1972 |
HETEROGENEOUS COBALT-BONDED TUNGSTEN CARBIDE
Abstract
Strong, hard, impact-resistant bodies comprising tungsten
carbide bonded with from 3 to 25% by weight of heterogeneous
cobalt-tungsten solid-solution alloy, useful as cutting tools, are
prepared by heating an intimately mixed cobalt/tungsten carbide
powder to a temperature above 1000.degree. C and consolidating the
powder to a density of at least 98% of its theoretical density,
having either. 1. mixed a carbon-rich and a carbon-deficient powder
together prior to consolidation to produce a non-homogeneous
binder; 2. added free carbon to a carbon-deficient powder to
produce local areas where tungsten is removed from the binder alloy
as tungsten carbide; or 3. allowed a portion of the carbon in the
tungsten carbide to oxidize during consolidation to produce areas
in the binder phase which are then carbon deficient and high in
tungsten.
Inventors: |
Iler; Ralph K. (Wilmington,
DE), Rigby; Eugene G. (Wilmington, DE) |
Assignee: |
E. I. du Pont de Nemours and
Company (Wilmington, DE)
|
Family
ID: |
25270545 |
Appl.
No.: |
04/835,817 |
Filed: |
June 23, 1969 |
Current U.S.
Class: |
75/240; 75/950;
148/905; 419/18 |
Current CPC
Class: |
C22C
29/08 (20130101); C22C 29/067 (20130101); Y10S
148/905 (20130101); Y10S 75/95 (20130101) |
Current International
Class: |
C22C
29/06 (20060101); C22C 29/08 (20060101); C22c
029/00 () |
Field of
Search: |
;29/182.8 ;75/204 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Quarforth; Carl D.
Assistant Examiner: Tate; R. L.
Claims
We Claim:
1. A dense body consisting essentially of tungsten carbide bonded
with from 3 to 25% by weight of a heterogeneous cobalt-tungsten
alloy, said alloy consisting essentially of cobalt and an average
of from 5 to 25% by weight of tungsten, and said alloy comprising
regions containing less than 8% by weight of tungsten interspersed
with regions containing more than 8% by weight of tungsten, the
difference between the percentages of tungsten in the varying
regions being at least 1, the body having a density in excess of
98% of its theoretical density.
2. A dense body of claim 1 bonded with from 8 to 12% by weight of
heterogeneous cobalt-tungsten alloy.
3. A dense body of claim 1 in which the tungsten carbide is present
as grains having an average diameter of less than 2 microns.
4. A dense body of claim 1 in which the tungsten carbide is present
as grains having an average diameter of less than 1 micron.
5. A dense body of claim 2 in which the tungsten carbide is present
as grains having an average diameter of less than 2 microns.
6. A dense body of claim 2 in which the tungsten carbide is present
as grains having an average diameter of less than 1 micron, and the
body has a density in excess of 99% of its theoretical density.
7. A tool for shaping metal having a cutting edge of a composition
of claim 1.
8. A tool for shaping metal having a cutting edge of a composition
of claim 6.
Description
BACKGROUND OF THE INVENTION
This invention relates to hard metal compositions of tungsten
carbide bonded with a heterogeneous cobalt-tungsten alloy, to
methods for preparing them, and to the use of the final products in
cutting or shaping very hard materials.
The products of this invention will ordinarily be referred to
herein as cobalt-bonded tungsten carbide, a term commonly employed
to describe a well-known class of compositions, but it will be
understood that the cobalt binder phase contains appreciable
amounts of tungsten and is thus in reality a cobalt-tungsten
alloy.
As shown in copending application Ser. No. 660,986, filed Aug. 16,
1967, now U.S. Pat. 3,451,791 substantially non-porous compositions
of tungsten carbide bonded with a cobalt alloy having a novel
combination of strength and hardness are obtained when the cobalt
phase contains over about 8 percent by weight of tungsten in solid
solution, the grain size of the tungsten carbide is less than a
micron and the composition is homogeneous in composition and
structure.
We have discovered a further class of useful carbide structures in
which the composition of the cobalt alloy binder is not homogeneous
but varies from region to region on a microscopic scale throughout
the composition. In this new class of carbide structures, regions
bonded with cobalt solid-solution alloys rich in tungsten, with
high strength and hardness but greater brittleness, are
interspersed on a microscopic scale with regions bonded with a
weaker but more ductile and tougher cobalt phase containing less
tungsten than in the aforesaid regions. In this way regions of high
strength, modulus and hardness are interdispersed with regions of
high ductility and toughness.
The effect of tungsten in solid solution in the cobalt binder, on
the acid resistance of the cobalt phase is well known in the art,
which teaches that for at least some tungsten to be free to
dissolve in the cobalt phase, the atomic ratio of
carbon-to-tungsten in the system must be less than 1.0. However, a
carbon-deficiency is generally considered to be undesirable because
during sintering it promotes at least a partial reaction of the
tungsten-containing cobalt binder phase with tungsten carbide to
form the brittle eta phase, Co.sub.3 W.sub.3 C, with attendant loss
of desirable strength properties, especially resistance to
impact.
Thus, it has been demonstrated by Kubota, Ishida and Hara in the
Indian Institute of Metals Transactions Vol. 9, 132-138, September,
1964, that in carbon-deficient, cobalt-bonded, tungsten carbide
compositions, the higher the concentration of tungsten in solid
solution in the cobalt the greater the resistance of the metal
phase to attack by concentrated hydrochloric acid.
The relationship between acid resistance and the amount of tungsten
in the cobalt metal phase calculated from the data of the above
authors for compositions containing 5% and 25% of cobalt is shown
in their FIG. 1. Such behavior is characteristic of a body in which
the metal binder phase is homogeneous, the amount of tungsten in
the cobalt phase being relatively uniform throughout the body.
As shown by these authors, the yield strength of cobalt-tungsten
alloys increases with tungsten content. However, Adkins, Williams
and Jaffee show these alloys also become more brittle in "Cobalt"
(1960) page 8, and 16-29.
Kubota and associates show that in metal-bonded tungsten carbide
compositions, when the tungsten carbide average particle size is
less than 3 microns, those bodies which are carbon deficient are
inferior in strength. The carbon deficiency, of course, produces a
tungsten-rich cobalt binder phase. Such fine-grained bodies are
reported to be weaker than bodies of similar fine-grain size which
are not carbon-deficient and thus have less tungsten in the cobalt
phase.
Regardless of grain size of the tungsten carbide, bodies with more
tungsten dissolved in the cobalt are known to have more acid
resistance.
We have now found it possible to prepare cobalt-bonded tungsten
carbide bodies in which the cobalt binder contains on the average
more than 8% by weight of tungsten dissolved in the cobalt, yet in
which the cobalt phase is low in acid resistance. The variations in
the concentration of tungsten in solid solution in the cobalt
binder result in lower average resistance of the binder phase to
removal by hydro-chloric acid, than when the same amount of
tungsten is uniformly distributed in the cobalt phase. The products
of this invention are thus characterized by having a relatively low
resistance to attack by acid in spite of having a substantial
amount of tungsten dissolved in the cobalt phase.
It is also well-known in the art, that in cobalt-bonded tungsten
carbide compositions, carbon deficiency leads to formation of eta
phase, Co.sub.3 W.sub.3 C, during consolidation at high
temperature. Formation of eta phase leaves less ductile cobalt
binder phase and thus causes brittleness in the resulting bodies.
The localized carbon deficiency in the compositions of this
invention surprisingly does not result in formation of brittle
cobalt-bonded compositions. On the contrary, the compositions of
this invention are surprisingly tough, frequently possessing
transverse rupture strengths in excess of commercially available
cobalt-bonded tungsten carbide compositions which are free of eta
phase.
The dense, cobalt-bonded compositions of this invention are
prepared by hot-pressing heterogeneous powder mixtures of
cobalt/tungsten carbides. Variations necessary for heterogeneity
are induced in the powders by any one of the following
techniques:
Blending dissimilar powders. Variations can be achieved by
selecting and blending dissimilar tungsten carbide powders, each
respectively having atomic ratios of carbon to tungsten of greater
and less than one. Even after ballmilling the tungsten carbide with
cobalt and consolidating to dense bodies, local variations in the
amount of tungsten dissolves in cobalt occur on a microscopic
scale, as a result of admixing the different powders.
Blending carbon with powder. Variations can also be achieved by
admixing and dispersing small amounts of finely divided carbon in a
cobalt/tungsten carbide mixture which has a carbon:tungsten atomic
ratio less than one. When the composition is heated during
consolidation, the carbon particles dissolve and carburize the
region around each particle, raising the local carbon:tungsten
ratio to one or higher.
Oxidizing the powder. Variations can also be obtained by slightly
oxidizing cobalt/tungsten carbide powders having a carbon:tungsten
ratio of 1.0 or slightly higher. Thus, finely milled powder, when
dried, will absorb from 0.1 to 1.0 percent by weight of oxygen when
exposed to air. Oxidation of powder is generally non-uniform, the
outer surfaces of granules being oxidized first (in a mass,
generally the upper surface of the powder is oxidized more than the
interior).
SUMMARY OF THE INVENTION
In summary, this invention is directed to dense bodies consisting
essentially of tungsten carbide bonded with from 3 to 25% by weight
of a heterogeneous cobalt-tungsten alloy, said alloy consisting
essentially of cobalt and an average of from 5 to 25% by weight of
tungsten, said alloy comprising regions containing less than 8% by
weight of tungsten interspersed with regions containing more than
8% by weight of tungsten. This invention is further directed to the
use of the dense bodies as cutting tools, and to a method for
preparing these compositions comprising milling finely divided
carbon with cobalt and tungsten carbide in sufficient amounts to
produce a mixture containing from 0.01 to 0.5% free carbon and
having a carbon:tungsten ratio about 1; heating the milled mixture
in an inert atmosphere at an elevated temperature; densifying the
composition to at least 98% of its theoretical density by
hot-pressing; and then rapidly cooling the dense composition.
The dense bodies of this invention demonstrate an unusual
combination of extremely good strength and hardness while
sacrificing little in such properties as ductility and toughness.
As a result, the bodies are useful in a variety of applications as
cutting tools and bits with particular advantage in uses usually
confined to high speed steel cutting tools.
The overall or average atomic ratio of carbon:tungsten can range
from 0.85 to 1.02, depending on the cobalt content. However, the
dense bodies of this invention have a surprising combination of
strength and toughness in view of prior art teachings, and are
exceptionally effective for use as tips or bits in metal cutting
and metal-removal operations.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts X-ray diffractometer tracings of twelve various
cobalt-bonded tungsten carbide compositions, indicating the degree
of heterogeneity of tungsten in the cobalt binder, along with two
reference curves for sodium chloride.
FIG. 2 depicts two X-ray diffraction curves for sodium chloride
superimposed at the extreme ends of a diffraction curve for
cobalt.
FIG. 3 is a plot of the profiles of 14 cobalt-bonded tungsten
carbide compositions analyzed according to procedure B described
hereinafter.
FIG. 4 is a plot of the profiles of FIG. 3 after adjusting the
relative positions of the sample patterns by the estimated error in
determining the sodium chloride peak position.
DESCRIPTION OF THE INVENTION
As stated above, this invention is directed to dense bodies of
tungsten carbide bonded with a cobalt-tungsten alloy. The bodies
have a density of at least 98% of their theoretical density and
comprise a polycrystalline, three-dimensional network structure of
tungsten carbide, the pores of which are interpenetrating and
filled with from 3 to 25% by weight of a three-dimensional,
continuous, cobalt-tungsten alloy binder phase containing an
average of from about 5 to 25% and preferably 8 to 12% by weight of
tungsten in solid solution in the cobalt. The cobalt-tungsten
binder phase is characterized by being heterogeneous and
substantially less resistant to dissolution in concentrated
hydrochloric acid at room temperature than the cobalt phase in
bodies of similar constitution in which the tungsten is
homogeneously and uniformly distributed throughout the cobalt
phase. The dense bodies of this invention are prepared by several
means including by suitably admixing separately prepared tungsten
carbide-cobalt powders and consolidating the powders to density to
produce bodies consisting of interspersed regions smaller than
about 100 microns in cross-section, having substantially different
concentrations of tungsten in cobalt, there being present cobalt
regions containing more than 8% by weight of tungsten as well as
cobalt regions containing less than 8% by weight of tungsten as
indicated by the cobalt lattice constants as determined by X-ray
diffraction, the difference between the varying regions ordinarily
exceeding 1 to 2% and preferably being at least 2 to 3% or
more.
1. DENSE COMPOSITIONS OF THIS INVENTION
a. Structure
The bodies of this invention consist of two interpenetrating
continuous phases, the major one is tungsten carbide and the minor
one is cobalt-tungsten alloy. The latter is also referred to as a
binder phase because it has generally been thought that it
surrounded and bound together the grains of tungsten carbide. Since
it greatly contributes to the strength of the composition, it must,
in fact, bind the structure together. We have found additional
proof of this by accurately measuring the length of a thin bar of a
tungsten carbide body of this invention, containing 10 percent by
weight of cobalt-tungsten alloy, and then removing the tungsten
carbide phase without disturbing the metal phase, which is porous
but coherent, and measuring the length, of this metallic skeleton.
We found that it is about two percent shorter than the original
length, showing that in the original composition, the metal phase
was subjected to a two percent elongation. This shows that the
cobalt in bodies of this invention is under considerable tension
and strain, and that it thus keeps the tungsten carbide phase under
compression and truly acts as a "binder."
The amount of cobalt metal present as binder in the bonded
compositions of this invention, ranges from about 3 to 25% by
weight, preferably from 5 to 12% by weight. Bodies containing an
amount of cobalt within the 3 to 25% range have a very desirable
combination of strength, hardness and toughness, and those bodies
containing from 5 to 12% by weight are particularly suitable,
because of their toughness, for replacing high speed steel
tools.
The tungsten carbide phase, also referred to as the tungsten
carbide skeleton, contributes markedly to the outstanding
properties of the dense bodies of this invention. The tungsten
carbide skeleton is polycrystalline; that is, it consists of many
small crystals separated by grain boundaries. Some of these
boundaries are scarcely visible when a polished section is etched
with acid, which removes cobalt, but can be revealed by etching
with a suitable reagent for dissolving tungsten carbide by methods
known to those skilled in the art. By these means the individual
grains making up the carbide skeleton can be distinguished through
an optical microscope and surface replicas can be made and examined
by the electron microscope.
A significant characteristic of the tungsten carbide in the
preferred bodies of this invention is the presence of a substantial
proportion of the structure as a fine grain structure. The carbide
grains, as measured in metallographic polysections described
hereinafter, consist of a substantial proportion having an average
grain diameter of less than a micron, but the remainder may be
larger than 1 micron.
The tungsten carbide skeleton contributes substantially to the
overall strength and hardness of the dense compositions of this
invention.
Another structural characteristic of the compositions of this
invention is the heterogeneous nature of the cobalt-tungsten alloy
binder. Thus, where compositions of the prior art are generally
characterized as containing a binder phase which is essentially
homogeneous throughout the composition the products of this
invention are characterized by a variety of cobalt-tungsten ratios
throughout the binder phase.
b. Tungsten in the cobalt
In prior art compositions the cobalt phase contains an amount of
tungsten that is related to the atomic ratio of carbon:tungsten in
the body. The tungsten which is not combined with carbon as
tungsten monocarbide, WC, could be present in one of the possible
states which have been described in the prior art in
carbon-tungsten-cobalt ternary systems, namely: ditungsten carbide
W.sub.2 C; various cobalt tungsten carbide phases such as kappa or
eta (Co.sub.3 W.sub.3 C), this latter also being known in some
countries as "delta"; metallic tungsten; the intermetallic compound
Co.sub.3 W or in solid solution in the face-centered cubic form of
cobalt which is the main constituent of the binder phase.
Regardless of the heterogeneous distribution of tungsten in the
cobalt phase, in bodies of this invention, it is preferred to have
most of the tungsten which is present in the bodies and which is
not present as tungsten monocarbide, in solid solution in cobalt.
By suitably relating the atomic ratio of carbon: tungsten to the
cobalt content, maintaining the tungsten carbide with a very fine
grain size, permitting at least some of the tungsten to dissolve in
the cobalt phase before hot pressing and then pressing and cooling
rapidly, we have found that it is possible to maintain a large
portion of the tungsten in solution in the cobalt and to minimize
formation of eta and other solid phases. By controlling the
conditions during preparation of these compositions, we have found
it possible to vary the amount of tungsten in the cobalt from
region to region throughout the composition. These regions can be
ascertained and characterized by a variety of techniques. The
regions in which the tungsten concentration in the cobalt is less
than 8% by weight and the binder phase is readily attacked by acid,
may be as large as 100 microns, or they may be very small, such as
less than a micron in cross-sectional diameter. Where they are
large, as when powders in granular form, high and low in carbon are
mechanically mixed and hot pressed, the regions can be detected
easily by metallographic procedures. The low carbon regions often
contain some eta phase which is easily distinguished. In such
cases, the attack of the binder by acid occurs irregularly and can
be observed in microscopic cross-section. On the other hand, when
the low-tungsten-in-cobalt regions are only a micron or so in size
they can be detected by X-ray diffraction, as will be further
described.
c. Carbon:tungsten ratio
We have found that if the tungsten content of the cobalt binder
phase exceeds about a third of the metal binder phase by weight, it
becomes very difficult to prevent the conversion of substantial
amounts of the cobalt binder to the more brittle eta phase. For
this reason, the atomic ratio of carbon: tungsten ordinarily ranges
from 0.85 up to 1.02 and should be greater than about
[1.0-0.0062(P-1)] and less than about [1.0 - 0.00166 (P-15)] where
P is percent by weight of cobalt in the composition. A preferred
lower limit is about [1.0-0.004(P-1)], or a minimum C:W ratio of
about 0.90.
On the other hand, the carbon deficiency must be sufficient to
provide a measurable amount of tungsten in the cobalt phase, and
the deficiency must be greater as the amount of cobalt in the
composition is increased. Thus for example, when the cobalt
concentration is less than 12% by weight, only a minute carbon
deficiency, scarcely, scarcely measurable by analytical means, such
as a carbon:tungsten atomic ratio of 0.99, will provide sufficient
tungsten to reach a concentration of an average of 8% by weight in
the cobalt phase. On the other hand, a carbon:tungsten ratio of
0.94 will provide up to an average of 24% tungsten in the cobalt.
It is desirable that the free carbon content be as low as possible,
preferably less than 0.15 percent.
d. Eta phase
With a deficiency of carbon, a part of the tungsten carbide-cobalt
bond may consist of eta phase, Co.sub.3 W.sub.3 C, although this is
generally undesirable. Up to 10 or 20 percent by weight of eta
phase may be present in the binder phase isolated after removal of
tungsten carbide, but less than 5% is preferred, since as much of
the tungsten as possible should remain in solid solution in the
cobalt and as little as possible consumed to form eta phase. The
presence of tungsten dissolved in the cobalt binder phase is at
least partly responsible for the unusual combination of properties
of the products of this invention.
e. Anisodimensional tungsten carbide
One of the preferred products of this invention is a dense body
comprising anisodimensional tungsten carbide platelets bonded with
from 3 to 25% by weight of heterogeneous cobalt-tungsten alloy.
The term isodimensional means having the same dimensions, while
anisodimensional means not having the same dimensions. A particle
that is isodimensional is therefore one having approximately equal
length, breadth and width. The term isodiametric is employed in the
same sense, an isodiametric particle being one having equal
diameters when measured in different directions. A sphere is
perfectly isodiametric; a grain of sand or of sugar is
approximately isodiametric and can also be described as being
isodimensional. The size and shape of ultimate particles and their
arrangement in aggregates is more fully described by Dr. A. Von
Buzagh, in "Colloid Systems," published by the Technical Press,
Ltd. (London, 1937).
Finely divided tungsten carbide of the prior art has ordinarily
been obtained by pulverizing coarser crystals. The finely divided
particles so obtained are, broadly speaking, isodimensional. When
milled tungsten carbide is bonded with metal by the processes of
the prior art to form hard, cemented carbide bodies, there occurs a
recrystallization and grain growth of the tungsten carbide. By
metallographic methods, the size and shape of the resulting carbide
grains can be observed. A review of published micrographs of the
grain structure of commercial cemented carbides, as well as
examination of a range of cobalt-bonded tungsten carbide products
of commerce, indicates that the tungsten carbide grains are
isodimensional. While in some instances the polished cross sections
of individual grains indicate a length or maximum dimension two or
even three times that of the minimum dimension, this is the
exception rather than the rule. In micrographs, grains give the
impression of being anisodimensional when a substantial proportion
of the grains show a maximum dimension at least three times that of
the minimum dimension.
For purposes of this invention, anisodimensional particles are
therefore those having a maximum dimension at least three times
that of their minimum dimension. Some of the products of the
present invention consist largely of anisodimensional tungsten
carbide crystals of which the maximum dimension is at least three
and preferably at least four times that of the minimum dimension.
In such products the tungsten monocarbide grains, which appear to
be crystals, are typically present as triangular platelets, the
thickness of which is no more than one-fourth and usually no more
than one-sixth the length of the side of the platelet. Preferred
anisodimensional tungsten carbide particles are from 0.05 to 1
micron in thickness and from 0.2 to 4 microns in length or breadth.
The commonest particles are triangular platelets, although
polygonal platelets are also observed.
To obtain a body containing tungsten carbide platelets, the
physical state of the starting powder is important. The tungsten
carbide in the powder admixed with cobalt before being heated must
have a crystal size of less than 0.1 microns, and preferably has a
crystal size of less than 0.05 microns as calculated from X-ray
line broadening or specific surface area.
f. Impurities
Foreign materials such as organic dirt, mineral dust or fragments
of enamel or glass such as from equipment should be scrupulously
avoided in preparing the bodies of this invention. Organic matter
can result in holes or inclusions of carbon in the final body and
mineral materials such as silicates leave inclusions of glass which
is very harmful because the inclusions cause localized internal
stresses upon cooling, thereby contributing to brittleness. Other
mineral dust as well as glass or enamel fragments are similarly
deleterious. Localized carburization of powder during manufacture
results in regions high in carbon in the final structure. If a
portion of powder, such as the outer layer of powder inclosed in a
graphite mold, is carburized before it is pressed, there may be
found homogeneous regions near the surface of pressed pieces that
contain an excess of carbon and show excessive grain growth. Such
homogeneously carbon-rich regions due to gross contamination with
carbon are to be avoided and they do not correspond to the type of
heterogeneity of regions found throughout the structures of the
present invention.
g. Properties of the compositions
1. Acid resistance
As shown by Kubota, Isheda and Hara in the reference mentioned
above, in cobalt-bonded tungsten carbide bodies of the prior art in
which the distribution of tungsten in cobalt is apparently
homogeneous, a small decrease in the atomic ratio of
carbon:tungsten to a ratio less than 1.0 remarkably increases
resistance of the metal phase to dissolution in hydrochloric acid.
This is believed due to the increased amount of tungsten in solid
solution in the cobalt phase.
In the products of the present invention, while some regions of
cobalt are rich in tungsten, others are low in tungsten and are
therefore not acid resistant. The acid penetrates the structures
via these non-acid resistant regions, with the result that the
overall acid resistance is low, in spite of a relatively high
average tungsten content in the cobalt.
The amount of tungsten in solid solution in the cobalt can be
determined as described by Kubota, et al. A preferred method for
measuring the amount of tungsten in the cobalt is described
hereinafter in the methods of characterization. The dense bodies of
the prior art disclosed in copending application Ser. No. 660,986,
which contain an average of more than 8% tungsten in the cobalt,
are characterized as having a resistance to etching, R, of greater
than 50 hours, where resistance is expressed in terms of number of
hours required at room temperature for concentrated hydrochloric
acid to remove 0.25 milligrams of metal per square centimeter of
surface area per percent of metal present in the original
sample.
The dense bodies of the present invention, which contain an average
of from 5 to 25% tungsten in the cobalt phase are low in acid
resistance, characteristically having an acid resistance of less
than 50 and generally less than 30 hours. Bodies in which the acid
resistance is over 50 hours are termed "acid resistant." Bodies of
the present invention are not "acid-resistant."
2. Strength
The unusual strength of the dense bodies of this invention is
described in greater detail in the subsequent sections. Much of the
strength of the dense bodies of this invention is of course
attributable to the skeletal strength of the tungsten carbide,
however, the cobalt phase also quite evidently contributes
substantially to the overall strength.
Characteristically, a body of this invention containing about 12
percent cobalt has a transverse rupture strength of about 500,000
psi and possesses a carbide skeleton with a strength of over 60,000
psi. Most commercial tungsten carbide bodies of the same cobalt
content characteristically will have a transverse rupture strength
of only 380,000 psi and a skeleton with a strength of about 46,000
psi.
Removal of the tungsten carbide from the dense bodies of this
invention by anodic etching leaves a coherent but porous and weak
metal structure. Conversely, removal of the metal from the dense
bodies of this invention leaves a porous body of tungsten carbide
which has a much lower transverse rupture strength than before
removal of the metal. It is the combination of the interpenetrating
metal and carbide phase which provide the high strength.
3. Hardness
The hardness of the dense bodies of this invention, measured at
ordinary and high temperatures is higher than that of many of the
prior art tungsten carbide bodies of equivalent cobalt content.
This is one of the most important characteristics of the bodies of
this invention. High hardness at high temperatures is of special
value in cutting tools. A representative dense body of this
invention containing 10 to 12 percent cobalt will measure 87 on the
Rockwell A scale at 800.degree.C., while most commercially
available tungsten carbide bodies prepared by methods of the prior
art containing 12% cobalt have a hardness of only about 75 Rockwell
A, and many commercially available carbides containing as little as
6% cobalt have a hardness of only 83 Rockwell A.
The unusual hardness of bodies of this invention is largely
dependent upon the structure of the tungsten carbide skeleton which
bears most of the load in the composite body. The hardness
increases with finer grain size of tungsten carbide in the carbide
skeleton. In the compositions of this invention the hardness is not
appreciably reduced by the fact that the tungsten content in the
cobalt phase is not uniform, so long as over half of the metal
binder contains more than 8% tungsten. This is true because
hardness is mainly determined by the grain size and coherent nature
of the tungsten carbide phase.
4. Density
The relation between the apparent density of bodies of this
invention and their theoretical density as calculated from the
volumes and individual densities of the components, permits an
estimate of the internal porosity. The bodies of this invention
have an apparent density of over 98% of the theoretical density,
and preferably at least 99% of the theoretical density. Expressed
in another way, the volume of a given weight of a preferred body of
this invention is generally equal to the sum of the volumes of the
components calculated from the weight of each component divided by
its density.
2. PREPARATION OF COMPOSITIONS OF THIS INVENTION
a. Preparation of the Powder Mixtures
1. Starting materials
Starting materials for use in this invention are tungsten carbide
and cobalt which are substantially pure, that is, containing no
more extraneous matter than is found in the tungsten carbide and
cobalt powders conventionally employed in making cobalt-bonded
tungsten carbide cutting tools. Small amounts of iron, up to 0.5%,
may be present from erosion of process equipment; but other than
iron, the total impurities amount to less than 0.5% by weight, and
preferably are present only in spectroscopically detectable
amounts.
Suitable colloidally subdivided tungsten carbide powder is
described in copending application Ser. No. 772,810 filed Nov. 1,
1968, now abandoned. This tungsten carbide is in the form of
crystallites of colloidal size well under half a micron in
diameter, typically 30 or 40 millimicrons in diameter, the
crystallites being linked together in porous aggregates, prepared
by forming and precipitating tungsten carbide from a reaction
medium of molten salt.
Cobalt suitable for use in this invention includes any source of
cobalt metal which can be used to prepare an interdispersion of
cobalt with tungsten carbide powder; for example, finely divided
powder such as "Cobalt F," sold by the Welded Carbide Tool Co. The
metal is preferably more than 99.5% pure cobalt, and should be free
from impurities that would be harmful to the properties of cemented
tungsten carbide.
2. Blending Components
The cobalt and tungsten carbide powders suitable to be used in this
invention must be intimately mixed. Extensive milling of the
tungsten carbide with the metal is ordinarily employed to achieve
an intimate mixture.
It is preferred to use a mill and grinding material from which a
negligible amount of metal is removed, and it is usually preferred
to use ballmills or similar rotating or vibrating mills. Suitable
materials of construction for such mills are steel, stainless
steel, or mills lined with cobalt-bonded tungsten carbide. The
grinding medium, which is more susceptible to wear than the mill
itself, should be of a hard, wear-resistant material such as a
metal-bonded tungsten carbide. Cobalt-bonded tungsten carbide
containing about 6% cobalt is a preferred grinding medium. The
grinding medium can be in various forms as balls or short
cylindrical rods about one-eighth to one-quarter inch in diameter,
which have been previously conditioned by running in a mill in a
liquid medium for several weeks until the rate of wear is less than
0.01% loss in weight per day. Mill loadings and rotational speeds
should be optimized as will be apparent to those skilled in the
art.
In order to avoid caking of the solids on the side of the mill, a
sufficient amount of an inert liquid medium is ordinarily used to
give a thin slurry of the tungsten carbide powder charged to the
mill. One of the liquid media which are suitable for this purpose
is acetone.
Ballmilling tungsten carbide in the presence of cobalt reduces the
particle size of the tungsten carbide and distributes the cobalt
uniformly among the fine particles of carbide. It is often
advantageous to have at least 25% of the carbide smaller than a
micron, and most preferably the average particle size is less than
a micron. When it is necessary to reduce the particle size of the
tungsten carbide it is preferred to mill the tungsten carbide
separately prior to interspersing the carbide with cobalt. It is
advantageous to start with the preferred colloidal tungsten carbide
disclosed in copending application Ser. No. 772,810 referred to
above, since it is not necessary to mill the tungsten carbide
before it is milled with cobalt.
Milling of cobalt/tungsten carbide mixtures is continued until the
cobalt is homogeneously interspersed with the finely divided
tungsten carbide. Homogeneous interspersion is evidenced by the
fact that it is essentially impossible to separate the cobalt from
the tungsten carbide by physical means such as sedimentation or a
magnetic field.
The mill is ordinarily fitted with suitable attachments to enable
it to be discharged by pressurizing it with an inert gas. The
grinding material can be retained in the mill by means of a
suitable screen over the exit port. The liquid medium is separated
from the milled powder such as by distillation and the powder is
then dried under vacuum. Alternatively the solvent can be distilled
off directly from the mill. The dry powder is then crushed and
screened, while maintaining an oxygen-free atmosphere such as with
nitrogen or argon, or by maintaining a vacuum.
3. Adjusting the Carbon:Tungsten Ratio
Various means are known in the art for adjusting the ratio of
carbon:tungsten in cobalt/tungsten carbide compositions. Thus, the
ratio can be adjusted by simply adding suitable amounts of finely
divided tungsten, ditungsten carbide, or carbon to the mill. For
the purposes of this invention, it is necessary to produce a carbon
deficiency in the powder compositions which will result in carbon
deficient regions in the dense bodies. The term "carbon deficient"
will be understood to mean "containing less than one atom of carbon
per atom of tungsten after consolidation at 1,300.degree. to
1,500.degree.C." Similarly when referring to the "atomic ratio of
carbon:tungsten" of a powder, it will be understood that this means
the atomic ratio after consolidation at high temperature. In other
words, although the powder has some carbon:tungsten ratio, it is
not this ratio that is significant as it changes during heating.
Thus, it is the ratio after heating which is significant.
Carbon deficiency can be produced in tungsten carbide or mixtures
of tungsten carbide and cobalt binders by
a. synthesizing tungsten carbide of colloidal particle size such
that the surface of the particles consists mainly of tungsten atoms
which are not accompanied by corresponding carbon atoms.
b. making a composition of tungsten monocarbide intermingled with
ditungsten carbide or finely divided tungsten metal or phases such
as Co.sub.3 W.sub.3 C or eta phase, in which there is less than one
carbon atom per tungsten atom.
c. oxidizing part of the tungsten or intermingled cobalt to an
oxidized from which during subsequent heating with the remaining
tungsten monocarbide reacts to form carbon oxides which escape
leaving carbon deficient regions in the final product corresponding
to the oxidized regions.
If only a small carbon deficiency, such as an atomic ratio of
carbon:tungsten of 0.97 or 0.99 is to be created, small amounts of
other metals such as tantalum or titanium can be used in place of
tungsten. However, in determining the carbon:tungsten ratio in
final compositions, the presence of such added metals or their
carbides must be taken into account. Of titanium and tantalum, it
is preferred to use tantalum because its carbide acts as a grain
growth inhibitor, and enhances hardness at high temperature.
4. Heterogeneity in the powder
Means for deliberately producing heterogeneity or local variations
in the carbon:tungsten ratio have not been described in the prior
art. Such variations can be produced by any of the following
methods:
Blending dissimilar powders
A powder mixture of tungsten carbide and cobalt which is carbon
deficient can be blended with a tungsten carbide or cobalt/tungsten
carbide powder mixture which contains a theoretical amount or a
slight excess of carbon over that required to form tungsten
monocarbide, and then the blend is consolidated at high
temperature. For example, the carbon deficient powder can be a
mixture of tungsten carbide and cobalt which has been milled to
develop a specific surface area in excess of three square
meters/gram which is permitted to absorb oxygen; this can be
blended with a powder which is not carbon deficient, such as a
milled powder of the tungsten carbide and cobalt of the prior art
commonly used for producing cemented tungsten carbide bodies with
an atomic ratio of carbon to tungsten of from 1.0 to 1.03, as
commonly employed in carbide cutting tools. The carbon deficient
powders can also be prepared by ballmilling a composition of cobalt
and tungsten carbide along with finely divided tungsten powder to
provide the carbon deficiency; this powder can be blended as
described with a powder which is not carbon deficient. Powders
having atomic C:W ratios as low as 0.80 and as high as 1.1 can be
used for blending.
Blending carbon with powder
Another method for preparing a heterogeneous powder comprises
milling finely divided carbon with tungsten carbide and cobalt,
preferably in sufficient amounts to produce a carbon : tungsten
atomic ratio of about 1.0. Generally more than 0.01 and less than
0.5 percent by weight of carbon is added based on the weight of
tungsten carbide. In the consolidated body the region around each
carbon particle is carburized as the carbon dissolves, producing
local regions which are not carbon-deficient, the remainder of the
body being carbon-deficient and having a higher tungsten
concentration in the cobalt. The tungsten carbide should have a C:W
ratio of at least 0.8 prior to addition of the carbon, and should
have a ratio between 0.85 and 1.02 after addition of the
carbon.
Many commercially available carbon blacks have a particle size in
the millimicron range and any of these is a suitable source of
carbon. Ordinarily it is preferred that the carbon be in a form
which, after the milling step, will have a particle size of less
than 5 microns and most preferably less than 1 micron.
Oxidizing the powder
Still another method for producing heterogeneity in the
consolidated bodies comprises partially oxidizing a pelletized
powder prepared by ballmilling cobalt and tungsten carbide
containing a slight excess of carbon, subsequently pelletized by
tumbling, for example, so that the outer surface of the pellets
becomes more highly oxidized than the interior. Thus a mixture of
tungsten carbide and cobalt powders each from 1 to 10 microns in
ultimate particle diameter can be ballmilled in an acetone medium
for several days and then the mixture can be removed from the mill
and the powder dried without exposure to the air. A small amount of
non-volatile organic matter from the acetone remains on the powder.
The powder can then be screened through a mechanically shaken
screen of 60 meshes per inch under nitrogen, producing spherical
pellets 10 to 100 microns in diameter. The pelleted powder will
ordinarily be stored under nitrogen containing a low concentration
of oxygen which is absorbed to an amount of from about 0.1 up to
about 1% by weight. The powders can alternatively be brought slowly
into the air providing the exposure is gradual enough to avoid
local heating and excessive oxidation. Such an oxidized powder
gives a consolidated body containing an average atomic ratio of
carbon to tungsten of greater than 1.0, yet the body contains
cobalt having more than 8% by weight of tungsten in solid solution.
If oxidation is excessive, as much as 20% by weight of tungsten on
the average is found in the cobalt phase, and the acid resistance
may approach 50 hours. It is believed that the surface regions of
the spherical pellets becomes more highly oxidized than the
interior, and that when the powder is compressed and the
composition is heated there results a three-dimensional continuum
of carbon deficient composition derived from the surface regions of
the pellets in which the cobalt binder phase is rich in dissolved
tungsten, while those portions of the body derived from the
interior regions of the pellets remain as regions less deficient in
carbon, containing little or no tungsten and having low acid
resistance. It is believed that these interior-derived regions of
the composition reduce brittleness because of the ductility of the
pure cobalt binder.
Identification of the heterogeneous regions is sometimes difficult.
However, by metallographic procedures, X-ray diffraction analysis,
electrical resistivity measurements and Curie temperature
measurements, regions high in carbon and cobalt regions low in
tungsten can be identified in the presence of regions low in
carbon, and cobalt regions high in tungsten.
Heterogeneity preferably occurs only on a microscopic scale, but
may occur in regions as large as a tenth of a millimeter. Thus, 50
micron-sized granules of cobalt/tungsten carbide powder which have
been heated in hydrogen at 900.degree. C. and have a
carbon:tungsten ratio of 0.95 can be blended with granules of a
similar powder which have been heated in hydrogen containing enough
methane to deposit a small amount of free carbon and have a
carbon:tungsten ratio of 1.03. Polished cross-sections of
consolidated bodies made from such mixed powders show localized
regions high and low in carbon, about 50 microns in size,
corresponding to the size of granules of the respective
powders.
Preferred powders are those which produce bodies in which the
heterogeneous regions are so fine and intermixed that they cannot
be identified under the microscope but are still known to be
present from either X-ray diffraction patterns of the cobalt phase
or from the fact that the acid resistance is lower than that of a
similar body having the same degree of porosity in which there is
the same overall concentration of tungsten in cobalt, but the
tungsten is homogeneously distributed. Homogeneous distribution of
tungsten in cobalt is attained when steps are taken to eliminate
the causes of heterogeneity as described above.
5. Reducing the powder
When the dried milled mixture of tungsten carbide and cobalt
contains over about 0.1 percent by weight of free carbon or more
than about 0.5 percent by weight of oxygen, it is preferred to
remove these impurities by treatment at a minimum elevated
temperature in a very slightly carburizing atmosphere. Under these
conditions extreme local variations in carbon to tungsten ratio are
corrected, but the desirable variations within the limits of the
present invention are not affected.
Oxygen as well as excessive free carbon can be removed during this
purification, and at the same time the combined carbon content can
be adjusted, all by heating the powder in a stream of hydrogen
containing a carefully regulated concentration of methane. The
powder can be charged to shallow trays made from a high temperature
alloy, such as Inconel, and the trays loaded directly from the
inert atmosphere environment to a tube furnace also made from
Inconel or some similar high temperature alloy.
The powder in a stream of the reducing gas is brought to a
temperature ranging from 750.degree. to 1,000.degree. C., depending
on the metal content of the powder, in from 3 to 5 hours, taking
half an hour to raise the temperature the last hundred degrees. For
a cobalt content of about 1%, 1,000.degree. C. is used, and for
powders containing 12% cobalt, the temperature is
800.degree.-900.degree. C.
The reducing gas should consist of a stream of hydrogen containing
methane and about 10 percent of inert carrier gas such as argon.
Thus, at 1000.degree. C. the stream should contain 1 mole percent
of methane in hydrogen; at 900.degree. C., 2 mole percent of
methane; and at 800.degree. C., 4 mole percent of methane in the
hydrogen. The reduction/carburization at the maximum temperature is
carried on for a period of 0.5 to 3 hours, and after cooling to
room temperature under argon the powder is discharged to an inert
atmosphere environment where it is screened through a seventy mesh
screen. If desired this powder can be stored for extended periods
in sealed containers or it can be used directly in the next step of
this process.
Care must be employed to assure that in the reduction/carburization
step an excess of methane is avoided so that an undesirable amount
of free carbon is not introduced into the powder. It is to be noted
that although the reaction conditions are such that free tungsten
metal would ordinarily be converted to tungsten carbide,
nevertheless very finely divided tungsten carbide used in this
invention remains slightly deficient in carbon and is not
carburized completely to a stoichiometric ratio for tungsten
carbide.
For compositions in which the desired atomic ratio of
carbon:tungsten is less than about 0.97, and where oxygen is to be
removed by the foregoing reduction step, methane or other
carburizing environments should be avoided and only hydrogen used.
Generally speaking, with compositions of higher cobalt content,
lower atomic ratios of carbon:tungsten can be employed. However,
the minimum average atomic ratio of carbon:tungsten, R.sub.Min, is
found to be
R.sub.Min = 1.0 - 0.0062(P-1),
where P is percent by weight of cobalt.
An optimum ratio will be between this minimum and 1.02. Thus, for a
composition containing 10% by weight of cobalt, for example, the
minimum ratio is about 0.94. For a body containing 25% cobalt, the
minimum ratio is about 0.85. A ratio above 0.90 is preferred. A
maximum ratio, R.sub.max for most purposes is R.sub.max =
1-0.00166(P-15). For a composition containing 3% cobalt the maximum
ratio is about 1.02.
b. Consolidation of the powder
The consolidated bodies of this invention are prepared from
interspersed cobalt/tungsten carbide powders. Generally speaking,
consolidation is carried out in the manner described in copending
application Ser. No. 660,986, filed Aug. 16, 1967, i.e. by heating
and compressing the powders.
It is important that when the powder composition is being heated
for the first time it should not be subjected to excessive pressure
or mechanical constraint, especially when in a graphite or carbon
container. Pressure can be applied providing it is not sufficient
to keep the sintering billet in intimate contact with the graphite
walls of the mold. With some powders, a pressure of up to 1000
p.s.i. can be applied during the heating step, since even under
such pressure the billet shrinks away from the mold and is not
seriously carburized. The harm that is caused by excessive
compression may be due either to shearing forces which disturb the
internal structure of the composition at the beginning of
recrystallization and sintering, or it may be due to chemical
effects from contact with material such as graphite which is
ordinarily used to apply the pressure. Thus it has been observed
that application of pressure to the composition while in an alumina
mold is less harmful to the resultant bodies, even using pressures
higher than 1000 p.s.i. The harm also may be due to trapping of
gases in pores that are collapsed by the pressure. In the absence
of pressure such pores would not normally become closed at this
stage of sintering.
If the powder is first heated without application of pressure to a
prescribed temperature it can thereafter be consolidated to density
and molded by hot pressing in a carbon mold without absorbing
undesirable amounts of carbon. We have found that after the
tungsten has dissolved in the cobalt phase during the heat
treatment it is much less readily carburized.
Heat treatment is carried out in an inert atmosphere or in a
vacuum. An inert atmosphere is one that does not react with the
powder, such as argon or hydrogen. Heat treatment is carried out at
a temperature T.sub.s which is above 1000.degree. C., but generally
below the final consolidating temperature, T.sub.m, and the
treatment lasts for about t.sub.s to 20 t.sub.s minutes, where:
log.sub.10 t.sub.2 = 13250/(T.sub.s + 273) - 8.2 minutes
and
where P = percent by weight of metal in the composition.
Thus the composition is heated to temperature T.sub.s and held for
a minimum of t.sub.s minutes. The maximum time of heating is not
critical at temperatures below which no appreciable grain growth of
tungsten carbide occurs, namely below about 1200.degree.C. However,
above 1,200.degree.C., the time should not exceed about 20 t.sub.s.
For example, at 1000.degree.C., it is necessary to heat for at
least 21/2 hours and preferably several times this long; at
1100.degree.C. the composition is heated for at least 13 minutes;
at 1200.degree.C. the hold time is a minimum of about 5 minutes and
not over 2 hours; at 1400.degree.C. the hold time is less than 10
minutes, and at 1500.degree.C. it is less than 4 minutes.
It should be noted that the temperatures and times required vary to
some extent with the size of samples, dimensions of equipment,
heating rates attainable and the like. For example, it is possible
to carry out the heating step either on loose powder or
preconsolidated billet while the sample is being heated to the
temperature at which it is to be finally consolidated. Such heating
should be carried out rapidly in the range above 1,200.degree.C.,
providing the sample is heated relatively uniformly throughout its
volume. An integrated combination of temperatures and times
equivalent to the fixed times and temperatures described, is in
keeping with the spirit of the invention, and will be apparent to
those skilled in the art.
A preferred method of fabrication is by hot pressing the powders in
the manner described below. Various types of hot pressing equipment
are known in the art. Depending on press design and desired
operating characteristics, heating can be by resistance heating,
induction heating, or plasma torch heating. Short heating times of
a few seconds duration are attainable by resistance sintering under
pressure.
Temperature can be measured very near the sample itself by means of
a radiation pyrometer and can be cross-checked for accuracy with an
optical pyrometer. Such instruments should be calibrated against
primary standards and against thermocouples positioned in the
sample itself so that actual sample temperatures can be determined
from their readings. Automatic control of heat-up rate and desired
temperature can be achieved by appropriate coupling mechanisms
between a radiant pyrometer and the power source.
The mold can be of a variety of shapes but is usually cylindrical,
with a wall thickness of up to an inch or more. It is particularly
advantageous to use a cylinder with a cross-section which is
circular on the outside and square in the inside in pressing bodies
to be used as cutting-tip inserts, thereby fabricating them as near
as possible to their final desired dimensions.
As an example, for a 1 inch in diameter finished pressed round
disc, the shell is cylindrical, 1 inch in inside diameter, 11/2
inches in outside diameter, 4 inches in length. Thin graphite discs
one-fourth inch in thickness and 1 inch in diameter are loaded in
the cylinder on top and bottom of the material to be pressed. The
surface of the graphite discs in contact with the sample can have a
conical depression one-eighth inch in diameter at the center to
form a tip on the sample and keep it positioned in the center of
the mold when it shrinks away from the sides due to sintering.
Graphite pistons 1 inch in diameter and 2 inches long are loaded in
both ends of the cylinder in contact with the one-fourth inch discs
and protruding from the cylinder.
Graphite parts used in the press tend to oxidize at the pressing
temperatures used, and it is therefore necessary to maintain a
non-oxidizing atmosphere or vacuum within the press. In addition to
prolonging the life of the graphite parts, the use of a vacuum or
an inert atmosphere makes it possible to remove the mold containing
the hot pressed body from the heart of the induction heated furnace
and cool the sample much more quickly than if it were left to cool
in the hot zone of the furnace after shutting off the power. The
press can be arranged to permit the mold to be removed from the hot
furnace, and when this is done the mold cools very rapidly by
radiation. Thus the mold described above, removed from the furnace
at 1400.degree.C., cools to dull red heat, about 800.degree.C., in
about 3 minutes.
Powders which are pyrophoric or absorb oxygen upon exposure to air,
should be loaded into the mold in a non-oxidizing atmosphere, for
example in a glove box filled with inert gas. The appropriate discs
and pistons can then be inserted and the loaded mold can be handled
with the contained powder essentially loosely packed or, for
example, with no more pressure than can be applied to the pistons
with the fingers. However, it is often convenient to apply about
200 to 400 p.s.i. pressure with a small press, to give a more
compacted sample for greatest ease in handling.
In a preferred aspect of this invention, a cobalt/colloidal
tungsten carbide powder mixture is pressed at about 200 p.s.i. as
it is loaded into the mold, it is then brought to the maximum
temperature with no pressure on the pistons, and held for 2 to 5
minutes at maximum temperature before applying any pressure. During
the period at maximum temperature with no pressure applied, the
body shrinks due to sintering. At the end of the period, the body
attains 80-90% of theoretical density and its diameter is about 60%
of the mold diameter. The pressure is then applied, reaching
maximum in 15 to 30 seconds, and the presintered body is reformed
into conformity with the mold. Maximum pressure and temperature are
applied until complete densification is attained, as indicated when
movement of the rams ceases. This ordinarily does not require more
than 5 minutes, and usually only one minute, after which the sample
is immediately removed from the hot zone and permitted to cool
rapidly by radiation to below 800.degree.C. in about 5 minutes or
less.
The conditions which give rise to the preferred dense cobalt-bonded
bodies are quite important and should be precisely established for
the particular composition and the type of structure desired.
Unduly long presintering times before application of pressure can
be harmful due to excessive crystallite growth and the development
of too extensive and rigid a cross-linked carbide structure. Too
early an application of pressure can also be harmful as pointed out
above. Holding the sample for too long a time at maximum
temperature should also be avoided, not only because of a tendency
towards carburization but also because secondary crystallite growth
tends to cause a coarsening of the structure and eventually the
development of porosity. Cooling too slowly can also be detrimental
if the sample remains at high temperature long enough for
undesirable crystallite growth and structural changes to occur.
These structural changes can include changes in the composition of
the cobalt binder phase. Thus with a low carbon content and the
corresponding large amount of tungsten initially in the cobalt
phase, precipitation of eta phase occurs at elevated temperatures.
This can be minimized by brevity of hot pressing and rapidity of
cooling of the pressed product. Generally speaking, it is
undesirable to have more than about 20% by weight of eta phase in
the binder, and it is preferred to have less than 5% eta phase in
the binder.
While it is preferred that the products of this invention be made
by heating and sintering lightly compacted finely divided
cobalt/tungsten carbide powders, followed immediately by
application of pressure, it is sometimes desirable to carry out the
sintering step as a separate operation.
Thus, in order to achieve maximum productivity from a hot press,
the initial sintering step can be carried out in a separate furnace
in an inert atmosphere. This can be accomplished in several ways.
For example, the starting powder can be loaded or lightly compacted
into molds to be later used for hot pressing, and then heated
rapidly in an inert atmosphere to a temperature within from
50.degree. to 200.degree. of the final hot pressing temperature to
be employed. The mold and its partially sintered contents, while
still hot, can be passed directly into a hot pressing
operation.
The maximum temperature at which the bodies should be pressed is
largely dependent on the cobalt content, although the proper
temperature is to some extent dependent on the size of the molded
piece, the heating rate, and the available pressure as well. The
compositions of this invention are conveniently subjected to a
temperature of T.sub.m for a period of t.sub.m to 20 t.sub.m
minutes, where
and
log.sub.10 t.sub.m = 13250/(T.sub.m + 273) - 8.2 minutes
where P is the percent by weight of metal in the composition.
Thus, for compositions containing 6% cobalt T.sub.m is about
1450.degree.C., and for compositions containing 12% cobalt, T.sub.m
is about 1400.degree.C.
It is preferred to bring the sample to the desired temperature as
rapidly as possible. For example, a sample 1 inch in diameter can
be heated to 1400.degree. C. in 4 to 5 minutes, or to
1850.degree.C. in 6 to 7 minutes, by introducing the mold into a
preheated graphite block, the limiting factor being the rate of
heat transfer from the graphite equipment via the mold to the
sample. Rapidity of heating is especially important in compositions
which have an atomic ratio of carbon:tungsten close to 1.0.
Pressure can be applied to the cobalt/tungsten carbide composition
in a hot press through the action of remotely controlled hydraulic
pneumatic rams. Applying pressure simultaneously through two rams
to the top and bottom gives more uniform pressure distribution
within the sample than does applying pressure through only one ram.
An indicator can be attached to each ram to show the amount of ram
movement, thereby allowing control of sample position within the
heat field and indicating the amount of sample compaction. The end
section of the rams, which are exposed to the high temperature zone
should be made from graphite.
A variation of 100.degree. from the mean specified temperature
provides to some extent for the variables mentioned above. Thus, in
order to attain temperature equilibrium in the interior without
overheating the exterior, larger bodies require a lower
temperature, which also permits a longer heating time. Higher
temperatures and shorter times can be employed when high molding
pressures can be used and smaller molded bodies are being made.
The most important factor in determining consolidation conditions
is the physical nature of the heat-treated composition of the
invention. When the composition is a heat-treated powder, for
example, it can be loaded into graphite molds and heat and pressure
simultaneously applied until the material reaches the recommended
temperature range, T.sub.m at which the pressure is maintained for
the specified time. The required pressure may be as low as 100 to
200 pounds per square inch for compositions such as those
containing 15 to 25 percent by weight of cobalt and which are soft
at the pressing temperature. Several thousands of pounds per square
inch is required for bodies containing one to three percent cobalt,
although pressures of not more than 4000 pounds per square inch are
usually used where operations are in graphite equipment.
For compositions containing from 5 to 12 percent cobalt the
required pressure can also vary according to the physical nature of
the composition. Thus if a sintered powder composition is used,
which has been heat-treated at a temperature T.sub.s close to the
maximum allowable temperature T.sub.m, a high pressure such as 4000
p.s.i. is preferably applied over a prolonged period, often
continuously, while the mass is heated from 1000.degree.C. to
temperature T.sub.m.
On the other hand, if degassed powder is preconsolidated to
relatively high density such as about 50 percent of theoretical
density, so that voids or pores larger than about ten microns are
eliminated, and this compact is then heat-treated at temperature
T.sub.s, it shrinks spontaneously to a coherent body. If T.sub.s is
then raised to T.sub.m, sintering continues and a relatively dense
body is obtained which can then be molded by brief application of
pressure at temperature T.sub.m.
Compositions of the invention require application of pressure at
the defined maximum temperature, T.sub.m, to eliminate voids. In
such instances the consolidation is carried out until the body of
the invention reaches a density of greater than 9 percent and
preferably greater than 99 percent of theoretical, corresponding to
a porosity of less than one percent by volume. However, for many
uses even this degree of porosity may be too high. The porosity of
the bodies of this invention is characterized by preparing polished
cross-sections of the bodies for examination under a metallurgical
microscope. Pores observed in this way are classified according to
a standard method recommended by the American Society for Testing
Materials (ASTM) and described on pp. 116 to 120 in the book
entitled "Cemented Carbides," published by the MacMillan Company of
New York (1960). Thus, bodies of this invention are preferably
pressed until a porosity rating of A-1 is obtained especially where
the material is to be subjected to heavy impact or compression.
This corresponds to a density of essentially 100% of theoretical or
a volume porosity of about 0.1%. However, porosities as great as
A-3 or A-4 are suitable for many uses, since such bodies
nevertheless have very high transverse bending strength. Even a
porosity rating of A-5 which corresponds to a density of about 98
percent and a porosity around 2 percent, is acceptable for the
compositions of this invention.
Pressures of from 500 to 6000 psi can be used in graphite
equipment, but generally speaking not over 4000 psi can be applied
without danger of breaking the equipment, unless the graphite mold
and plungers are reinforced with a refractory metal such as
tungsten or molybdenum.
Instead of loading a powder into a mold, preconsolidated compacts
in the form of billets can be prepared and heat-treated and then
loaded in a mold for hot pressing. Such heat-treated, sintered
billets can also be shaped by rolling or forging in an inert
atmosphere.
After final consolidation to a dense billet the compositions of
this invention can be further shaped by bending, swaging, or
forging at about temperature T.sub.m. Similarly, pieces can be
welded together by bringing two clean surfaces together under
pressure.
3. CHARACTERIZATION OF DENSE COMPOSITIONS
a. Chemical analysis
The chemical composition of the bodies of this invention can be
determined by conventional chemical analysis for the elementary
constituents. Samples can be pulverized as in a Plattner steel
mortar and screened before sampling for analysis. The more
convenient methods of analysis for tungsten, cobalt, total carbon,
free carbon, oxygen, and density are described in copending
application Ser. No. 660,986 referred to above.
b. Examination with optical microscope
To examine homogeneity of the overall structure and detect gross
inclusions or localized coarse grain structure polished surfaces
can be examined quite satisfactorily at magnification up to 2000X
with the light microscope. In order to examine individual tungsten
carbide grains and their structural arrangement in consolidated
bodies, it is advantageous to fracture a sample and examine the
fractured surface or to etch the polished surface with chemical
agents which due to the different rates of chemical attack dissolve
a thin layer from the exposed grains, enhancing the contrast
between the tungsten carbide and metal phases and making grain
boundaries more readily visible. Techniques commonly used for
preparing fractured and etched surfaces and analysis of those
surfaces are fully described in copending application Ser. No.
660,986 referred to above.
c. Examination with electron microscope
Because of the unusually fine-grained structure, especially in
preferred bodies of the invention in which over half of the grains
of tungsten carbide are less than 0.75 microns in diameter, it is
necessary to use the electron microscope to measure the grain size.
To measure the grain size of tungsten carbide both the boundaries
between tungsten carbide grains and the tungsten carbide-metal
phase boundaries must be outlined. Furthermore, the metal phase
must be distinguished from tungsten carbide so that the former can
be avoided when counting the grain size of tungsten carbide. A
multi-step chemical etch accomplishes this objective. The procedure
described in copending application Ser. No. 660,986 referred to
above is employed in characterizing the products of this
invention.
d. Transverse rupture strength
Many suitable procedures have been described in the literature for
the measurement of transverse rupture strength. We prefer to use
the method described in application Ser. No. 660,986 referred to
above.
e. Magnetic characteristics
The Aminco-Brenner "Magne-Gage," basically a torsion balance made
by the American Instrument Company, Silver Springs, Maryland, is a
device which permits quantitative determination of the relative
force required to pull a magnet away from a specimen containing
magnetic material.
Use of the "Magne-Gage" and preparation of samples for analysis are
fully described in application Ser. No. 660,986 referred to
above.
f. Acid resistance
The method of measuring the acid resistance of metal-bonded
tungsten carbide bodies is also described in application Ser. No.
660,986 referred to above
As pointed out there, the samples to be tested are cut into small
bars 0.006 .times. 0.006 .times. 0.55 inches. The sample bars are
then cleaned and measured to the nearest 0.001 inch, weighed to the
nearest tenth of a milligram and suspended individually from a
glass rod so that the bars hand about 1 inch below the rod.
The surfaces of the bars are then cleaned again by suspending the
bars in boiling trichloroethylene and then washing them with water
and acetone. The bars and their support wires are then weighed to
the nearest tenth of a milligram and the bars are then immersed in
25.degree.C. hydrochloric acid containing 35% by weight of hydrogen
chloride. 50 milliliters of acid are used for each bar, and the
acid is agitated throughout the test. The samples are removed
periodically and are measured and weighed.
Acid etch resistance R is expressed in terms of the number of hours
required for the acid to remove 0.25 milligrams per square
centimeter of surface area per percent of metal originally present
in the sample.
In measuring acid resistance it is important that the surfaces of
the test samples be clean and smooth, free from roughness or
scratches. It is also important that the samples be free of cracks
and porous defects which lead to low values for R. The pores
provide avenues of attack of the cobalt phase by the acid, so that
by the above method the acid resistance may appear to be
irregularly low. In such instances, the pores can be filled with a
resin or wax by impregnating the specimens, for example in hot
beeswax, the excess wax is then wiped from the surface with a cloth
soaked in acetone and the outer surface of the specimen further
cleaned with an aqueous detergent in an ultrasonic cleaning device
until the surface is water-wettable, indicating that wax has been
removed from the exterior. By this procedure, the fine pores remain
blocked and a true value of acid resistance can then be
obtained.
g. Tungsten content of the cobalt
A preferred method for measuring the tungsten content of the cobalt
is to 1) polish a section of sample; 2) remove tungsten carbide by
anodic etching for an hour in a solution containing 10 percent by
weight of potassium hydroxide and ten percent of potassium
ferricyanide; 3) rinse; 4) remove the residual metal binder layer
by dissolving it in a ten percent solution of hydrochloric acid;
and 5) then again etch to remove tungsten carbide, thus leaving a
film of metal binder a few thousandths of an inch in thickness. The
sample is then examined by X-ray diffraction and the lattice
constant of the cobalt determined. The percentage of tungsten in
the cobalt is calculated, based on the information given in
"Handbook of Lattice Spacings and Structure of Metals," Vol. 1,
page 528, Pergamon Press, 1958, by W. B. Pearson. When no tungsten
is present, the lattice constant of cubic cobalt is 3.545
angstroms, and when the initial binder contains 21% by weight of
tungsten and 79% by weight of cobalt in solid solution, the lattice
constant is 3.570.
We have found that the metal binder phase can be isolated by
electrolytically etching a body of the invention, using it as an
anode, in the potassium hydroxide-potassium ferricyanide solution
for 24 hours at a current density of 3 amperes per square inch,
then rinsing in water and removing the layer of cobalt alloy, which
is from 0.005 to 0.010 inches thick, and drying at 60.degree.C.
under nitrogen. The tungsten content determined by X-ray
diffraction from powder patterns, corresponds within the limit of
error to the ratio of weights of tungsten to tungsten plus cobalt,
determined by chemical analysis, providing no substantial quantity
of Co.sub.3 W or carbide phases are present. In this recovered
metal phase, tungsten carbide and cobalt-tungsten carbide phases
such as eta, Co.sub.3 W.sub.3 C are determined by heating the
sample in 35% hydrochloric acid at 80.degree.C. for 1 hour, then
filtering and weighing the washed and dried insoluble residue which
will contain the carbides which are insoluble. If the intermetallic
compound Co.sub.3 W is present, it will dissolve in the acid, but
it is seldom present in the unannealed bodies of this
invention.
When eta phase is present in a body of this invention which is
rapidly cooled, it is in a form rich in tungsten and corresponds to
the conventional formula Co.sub.3 W.sub.3 C which is reported to
have a face-centered cubic lattice constant of 11.08 angstroms.
However, when bodies of the present invention are slowly cooled
from 1400.degree. or 1300.degree.C. at 5.degree.C. per minute, the
eta phase apparently absorbs cobalt or loses tungsten, so that the
ratio of cobalt to tungsten changes from 3:3 to 3:2, and the
lattice constant changes continuously from 11.09 to 10.75
angstroms. The lattice spacing of the eta phase, when present,
serves to indicate whether a body has been rapidly or slowly
cooled.
h. Density
The method of measuring apparent density should be selected
according to the type of specimen available. Most conveniently the
actual density of any given composition is measured on a convenient
size sample by weighing the sample first in air and then immersed
in water previously boiled to remove dissolved air. The density is
then calculated from the equation:
d = (W.sub.1 .times. S)/(W.sub.1 - W.sub.2)
where
d = actual density in grams/cubic centimeter;
W.sub.1 = weight in grams in the air;
W.sub.2 = weight in grams in the water; and
S = specific gravity of water at the temperature of
measurement.
The theoretical density of a composition is determined by the
equation:
t = 1563s/cs + 15.63 (100-c)
where
t = theoretical density in grams/cubic centimeter;
c = weight percentage of tungsten carbide; and
s = specific gravity of the tungsten-cobalt alloy binder phase.
Percentage of theoretical density is then calculated by the
expression
percentage of theoretical density = d/t .times. 100.
A method for measuring actual density of irregularly shaped
specimens employs mercury displacement, as described by
Maczymillian Burke, Roczniki chem., 31, 293-295 (1957), "Pykometer
for Determining the Bulk Density of Porous Materials," and further
referred to in J. Am. Chem. Soc., 45, (7), p. 352-353 (1962), by
the same author.
i. Heterogeneity of tungsten in the cobalt
Variations in the concentration of tungsten in solid solution in
the cobalt phase can be observed by careful examination of the
X-ray diffraction lines of the cubic cobalt phase of the recovered
metal binder. When tungsten is uniformly distributed as in products
of the prior art, the lattice spacing of the cobalt is uniform as
evidenced by sharp single peaks in the diffraction lines of the
cobalt, whereas in products of the present invention different
regions of cobalt contain different amounts of tungsten in solid
solution so that the diffraction lines, recorded as intensity
versus diffraction angle show broadening, or shoulders due to two
or more unresolved peaks, or even two or more separate peaks,
depending on the degree of resolution obtained and the irregularity
of distribution of tungsten in the cobalt phase.
Attainment of resolution in X-ray diffraction is discussed in some
detail by Emmett F. Kaelble "Handbook of X-ray," McGraw Hill Book
Co., (1967), pages 9-14 to 9-30.
Measurement of exact line position includes the technique of
incorporating into the sample of cobalt-tungsten binder a uniform
amount of sodium chloride to serve as an internal standard. (Refer
to H. P. Klug and L. E. Alexander, "X-ray Diffraction Procedures,"
John Wiley & Sons, Inc., N.Y. (3rd printing 1962) pages 452-3.)
By this means the measured angle two-theta for a cobalt line from
cubic cobalt is corrected by the difference between the measured
angle for a neighboring sodium chloride line and its known standard
value. From the corrected value of two-theta for the cobalt line,
the unit cell dimension of the cobalt is calculated as on p. 343 of
the foregoing reference.
The tungsten carbide-cobalt composition of this invention is cut or
ground to produce a specimen with a smooth surface having an area
of several square centimeters. The exposed surface must be
representative of the interior of the specimen, outer layers which
might be oxidized or carburized by previous treatments being ground
away to a depth of at least 0.06 inches.
The method consists in anodically etching the smooth, cleaned
surface for 24 hours in alkaline ferricyanide solution to remove
the tungsten carbide phase to a depth of up to 1/32 inch, scraping
off and recovering the residual, porous cobalt phase, and examining
it by X-ray diffraction.
Variations in the "d" spacing for the strongest cobalt line
indicate variations in the amount of tungsten in solid solution in
the cobalt lattice; and the ratio of the intensity of the strongest
eta line to that of the strongest cobalt line, serves as an
empirical indication of the relative amount of eta phase present in
the cobalt. This is called the "eta ratio."
It appears that the eta phase is precipitated within the cobalt
metal phase, since it is not removed by the anodic attack which
removes tungsten carbide, in spite of the fact that, by itself, eta
phase is quite soluble in this reagent. Similarly, tungsten within
the cobalt lattice is not attacked by the anodic etch.
While not ordinarily encountered, destruction of eta phase may
occur during anodic attack if most of the cobalt has been converted
to eta phase, so that little of the cobalt is left to surround and
protect the eta phase. Also, is some unusual specimens of
cobalt-bonded tungsten carbide, other than that of Ser. No. 660,986
referred to above, when much eta phase is present the cobalt may be
very finely divided, so that the recovered powder is oxidized in
air and partially destroyed. These factors have been taken into
account in devising the procedure described below.
The surface is cleaned by immersing in boiling dimethylformamide
followed by rinsing in acetone and drying. Alternatively, the
sample may be held over a gas flame until it is just red hot and
permitted to cool slowly, after which it is scrubbed with steel
wool in water and dried.
Electrical contact is made with the sample by wrapping a fine
platinum wire around the specimen or using platinum clips. The
lead-in wire for electrical contact should be covered with rubber
insulation. The sample is hung in a 100 cubic centimeter plastic or
glass beaker and connected to the positive source of direct
current, thus making the specimen an anode.
A cathode of platinum sheet 1 inch .times. 3/4 inch welded to a
platinum lead wire is also hung inside the wall of the beaker using
the wire as a hook and connected to the negative source of direct
current.
Up to four cell-assemblies of this type can be connected in series
to a 12 volt DC source. The specimens are connected toward the
positive terminal of a 12 volt storage battery, or other source
providing a current of up to 1 ampere.
The electrolyte is made by dissolving 100 grams of potassium
ferricyanide and 100 grams of potassium hydroxide, adding these to
about 30 ml. of distilled water and stirring until the mixture
becomes hot, and then diluting to about 1 liter. Sufficient
solution should be added to each beaker to cover the sample and
most of the cathode. Usually about 75 milliliters of solution is
required. The beakers may be covered with a sheet of plastic if
desired, to reduce electrolyte spray during electrolysis which is
continued for 24 hours with a current to each specimen of 0.7
amperes.
At the end of the electrolysis period, the specimen is removed and
rinsed in water to remove alkali without losing any cobalt.
The cobalt within 1/32 inch of the edges of the cut surface, which
have been in contact with graphite, is then trimmed away and
discarded. Some specimens yield cobalt films which are cracked and
only very lightly adherent. In such cases, the cobalt film can be
removed from the center of the cut surface leaving the cobalt
around the edges.
The cobalt is collected by scraping it off under water in a pan,
excess water is decanted and the cobalt is washed with distilled
water into a 3 inch diameter porcelain mortar, along with about 10
milliliters of water. Excess water should be decanted from the
mortar. The cobalt is then ground by about 10 strokes of the
porcelain pestle, to break up the flakes. Excessive grinding must
be avoided, since it may affect the cobalt structure. The powder is
then washed with distilled water into a thin plastic bag held so
that the cobalt will collect in one corner. The cobalt in
suspension is drawn toward the corner by holding the latter next to
a small magnet. The cobalt is then held in the corner with the
magnet while the water is decanted off and replaced with 10
milliliters of n-propyl alcohol. The powder is then suspended and
again drawn to the corner and the alcohol discarded.
The corner containing the cobalt is closed off by twisting it
several times, tying it loosely, and cutting off the rest of the
bag.
Depending on the amount of cobalt in the original composition and
the size of the specimen, one or more preparations of this type
from the same piece of material may be required to obtain from 50
to 250 milligrams of cobalt powder for examination. In repeating
the preparation the etched surface is well scraped, or preferably
sand-blasted before being again anodically etched.
Two somewhat different procedures have also been employed in
diffraction analysis of the cobalt phase. They are referred to
hereinafter as procedure A and procedure B.
In procedure A, described more fully below, a 75 milligram sample
is used with a parafocusing device to obtain a diffractometer
tracing from which the line position, shape, and intensity is
employed to estimate the percent tungsten in the cobalt, and the
variation in the amount of tungsten in different portions of the
sample, i.e., the heterogeneity of distribution of tungsten.
In procedure B, also described in detail below, a sample of greater
weight is used with a flat sample holder, automatic stop scanning
every 0.04.degree. and accumulating the same number of counts at
each point. The time at each point is recorded on punch tape fed to
a computer from which a profile of intensity versus angle is
constructed. The punch tape is converted to punch cards fed to a
computer programmed to calculate intensity profile at each point,
then interpolate to a finer mesh of points spaced apart by
one-tenth the separation of alpha-1 and alpha-2 at that point,
using a Lagrangian polynomial fit covering a range of eight
consecutive points, then applying the Keating correction for the
alpha doublet whereby the alpha-2 contribution is subtracted from
the total intensity at each point using a series approximation,
thus leaving the equivalent alpha-1 profile. From this the amount
and distribution of tungsten in the cobalt is more accurately
determined than by procedure A. Details of these procedures are as
follows:
PROCEDURE A
Apparatus
North American Philips Diffractometer
Power Supply -- Type No. 12045
X-ray Target Source -- Cobalt with iron filter to give cobalt alpha
radiation
Wide Range Goniometer/type No. 42202
Electronic Circuit Panel -- Type No. 12049
Advance Metal Research Corp. Autofocusing Attachment, Model No.
3-201
SAMPLE PREPARATION
Thirty-five one hundreths of a gram of sodium chloride is ground in
an agate mortar along with a 0.075 gram sample of the powder to be
tested, and the mixture is screened through a 325 mesh screen. The
salt is present to provide representative peaks of known spacings
at 1.99 and 1.628 angstroms. The screened powder is placed on a
fiberglass sample holder along with 0.2 milliliters of amyl acetate
and 1 drop of 25% collodian solution. These are mixed to disperse
the powder and spread it to a film on the holder over an area 3
inches long and 1 inch wide at the center and one-half inch at the
ends.
INSTRUMENT CONDITION
The X-ray apparatus is fitted with a cobalt target and iron filter
at 25 kilovolts and 20 milliamps. Scanning speeds of 1/2.degree.
and 1/8.degree. per minute are used. The chart speed is 30 minutes
per hour. The diversion slit is -4.degree.; the receiving slit
0.010 inch wide. Scintillation counter detector, 950 volts, 6 volt
base line, gain zero, scale factor 8, multiplier 0.8 and time
constant 4.
CALCULATION PROCEDURE
The scans are examined, and the d spacings corrected if necessary
from the spacings of the known sodium chloride lines, corresponding
to 53.3.degree. 2.theta.corresponding to 1.994 d A.
Cobalt Lattice and Cobalt-Eta Co.sub.3 W.sub.3 C Ratio
The region 48.degree. 2.theta.to 52.degree. 2.theta. is scanned at
1/2.degree. per minute. The counts per second for cobalt in the
region corresponding to a d value of around 2.06 A. are
recorded.
Cobalt Peak or Multiple Cobalt Peaks
The region 51.degree. 2.theta. to 54.degree. 2.theta. is scanned at
1/8.degree. per minute. The location of the cobalt peak or multiple
cobalt peaks and the sodium chloride internal standard are read in
degrees 2.theta.. The alpha-1 and alpha-2 peaks are averaged for
sodium chloride and cobalt. A correction for the sodium chloride
2.theta. degree location is appropriately made to the cobalt
2.theta. degree location. The corrected d spacing is then
calculated for the lattice constant or constants of cobalt.
The amount of tungsten alloyed with the cobalt phase is calculated
from the following linear relationship.
Tungsten Cobalt Lattice Constant in Cobalt 2.theta. (Angstroms) (%
by wt.
__________________________________________________________________________
51.82 3.544 0 51.74 3.550 5 51.68 3.554 8 51.60 3.560 13 51.44
3.570 21 51.28 3.580 29
__________________________________________________________________________
Products of this invention show the cobalt line with the 2.06
spread out or with pronounced shoulders or even with multiple
peaks. This indicates that there are multiple cobalt-tungsten
alloys present with different concentrations of tungsten in solid
solution. It has been observed that when at least some of the
cobalt contains less than 8% tungsten the product has a low
resistance to removal of cobalt with hydrochloric acid even though
the average concentration of tungsten is well above 8% and even
through the amount of alloy which contains less than 8% tungsten is
a minor one.
If the tungsten is evenly distributed in the cobalt, all cobalt
crystals have the same lattice constant and the upper portion of
the line corresponding to a d-spacing of 2.06 angstroms is
symmetrical about its peak or maximum height, as shown in Curve A
of FIG. 1, and its breadth at half height is about the same as that
of the nearby sodium chloride line as shown in Curves B and C of
FIG. 1 which are symmetrical about a center line.
On the other hand when the 2.06 angstrom line of cobalt clearly
consists of several peaks as in curves D and E of FIG. 1 it is
evident that there are different regions in the cobalt which
contain two or more different levels of tungsten and thus have
different lattice constants. In such instances, heterogeneity is
obvious.
The small dimensions and even distribution of these heterogeneous
regions is shown by cutting successive thin slices of the
composition and showing that in each the cobalt shows the same
heterogeneity in regard to tungsten content.
When the cobalt peak shows a shoulder as in portion d of the curve
F of FIG. 1, as seen by comparison with the right side a.sub.1 of
curve F, the component peaks can be identified by inspection only
after considerable experience, but may be more easily located by a
simple graphical procedure: a dotted line a.sub.2 is drawn to be
symmetrical about the midline between the .alpha.-1 and .alpha.-2
peaks at 51.47.degree. 2.theta. corresponding to a lattice constant
of 3.568 angstroms. The X-ray radiation is not quite monochromatic
and consists of two slightly different wavelengths which causes
double peaks known as alpha-1 (.alpha.-1) and alpha-2 (.alpha.-2)
for each d-spacing. The symmetrical curve a.sub.1 a.sub.2 would be
characteristic of cobalt containing a single homogenous
concentration of 19 percent tungsten. However, the curve d which is
above a.sub.2, indicates that some portions of the cobalt also
contain lower concentrations of tungsten. The difference in
intensity at each angle, between curve d and a.sub.2 is plotted as
a difference, curve bb.sub.1. A curve b.sub.2 is now drawn
symmetrical to the side b.sub.1 about the line at angle
51.7.degree.. Then a new curve c is drawn plotted as a difference
between the lower portion of curve d and curve b.sub.2. This new
curve c is centered at 51.9.degree..
Thus, the curve da.sub.1 can be empirically resolved into three
component peaks centered at 51.47, 51.7 and 51.9 degrees. These
peaks correspond to regions of cobalt containing 19%, 8% and 0%
tungsten in the cobalt lattice, and the relative height of the
peaks indicates the relative amounts of the different regions
present.
This method was employed on three different samples of cobalt
recovered from the same specimen of composition of this invention
and the following observations were reported indicating the
reproducibility of the method:
Relative Sample % Tungsten Present Peak Height
__________________________________________________________________________
1st 19 100 8 20 0 9 2nd 19 100 5.5 27 0 15 3rd 19 100 4 24 0 12
__________________________________________________________________________
Curve G of FIG. 1 is the X-ray diffraction intensity curve of the
strong line of a different cobalt sample obtained from the same
composition used for curve F, and graphically analyzed the same
way, giving essentially the same results. Curve H of FIG. 1 is a
similar graphical analysis of the cobalt peak of another product of
this invention.
In curves I and K of FIG. 1, the position of component peaks can be
seen by inspection. Where the range of heterogeneity is still
greater as in J and L, while it is still possible to observe
directly that numerous peaks are present, it becomes more difficult
to identify them, and in M and N it is scarcely possible to say
what components may be present.
The cobalt in compositions similar to those of this invention
except that the distribution of tungsten is homogeneous, is
ordinarily not sufficiently fine-grained to cause line broadening.
Thus, the shape of the peak is similar to that of the sodium
chloride line used for comparison. As the binder phase in the
products of this invention is fine-grained the cobalt line in
products of this invention is broadened, and the broadening is due
to the presence of several component peaks with different
d-spacings corresponding to different tungsten levels in the
cobalt, i.e., heterogeneity.
We have devised means for analyzing these curves into component
peaks, simultaneously taking into account the alpha-1 and alpha-2
components.
PROCEDURE B
The cobalt powder isolated by removal of the tungsten carbide as
described above is mixed with an equal volume of sodium chloride,
and prepared as an X-ray diffraction powder sample. The X-ray data
is gathered by an automatic step-scanning diffractometer, using
chromium radiation, and the intensity is recorded on a punched
paper tape. The tape is then converted to punched cards which are
processed using a digital computer. The entire section of the X-ray
pattern is first corrected for the K.alpha. doublet broadening
using the method described by Keating (Rev. Sci. Inst. 30, 752
(1959)). The corrected pattern then gives the instrumental
broadening in the form of the (220) sodium chloride peak, as well
as the observed (111) cobalt alloy peak. The limits of the sodium
chloride peak are chosen as the points where the intensity dropped
to the level of the background intensity. The limits of the cobalt
alloy peak are chosen so that when the sodium chloride pattern is
superimposed at the extreme ends of the cobalt peak, the maximum of
the sodium chloride peak falls in the low intensity region of the
cobalt peak. This is illustrated in FIG. 2 wherein 1 and 3
represent the superimposed sodium chloride peaks and 2 represents
the cobalt peak.
The sample broadening curve is then computed for all data points
between those defined by the two positions of the sodium chloride
maximum described above.
The computer program first subtracts the background from both
peaks. It then computes the sample broadening using a method
similar to that described by Patterson (Proc. Phys. Soc. A63, 477
(1950)). The program uses three different schemes to minimize the
residuals of the diffraction peak. In phase one, it selects the
largest available residual, and reduces it by 10%. If the
corresponding subtraction or addition of the instrumental peak
reduces the sum of the squares of the residuals, the adjustment is
accepted as part of the solution; if not, the step is reversed. In
this and all other phases, the condition is imposed that the sample
broadening curve may not be negative at any point. The points on
either side of the maximum residual are then reduced by 10% if such
reduction decreases the sum of squares of residuals. When no
further improvement can be made by reducing the largest residual or
its two neighbors, the program enters phase two, where each of the
residuals is first increased, then decreased, in turn. If an
adjustment produces a reduction in the overall sum of squares, it
is made a part of the solution. If an improvement of the fit of the
solution is achieved during this process, the program returns to
phase one and begins again, if not, it goes on to phase three. This
involves the simultaneous increase and decrease of adjacent pairs
of residuals within the available range. If improvement is made in
the solution, the program returns to phase one and restarts; if no
further improvement can be made, it prints out the results.
In summary, the (111) cobalt alloy peak and the (220) sodium
chloride peak are scanned, using chromium radiation. The
diffraction pattern is recorded and processed by Keating's
technique to remove the K.alpha..sub.2 portion of the diffraction
pattern. The sodium chloride peak is then used as an instrument
broadening profile, and is employed to determine the nature of the
sample broadening present in the cobalt alloy peak, using
Patterson's method. The relaxation takes place in three stages:
first, the maximum residual and its adjacent neighbors are reduced;
when that is no longer effective, each of the residuals is reduced
in turn; finally, adjacent pairs of residuals are increased and
decreased simultaneously. This process then yields the profile
which corresponds to the X-ray broadening of the cobalt-tungsten
alloy, free of the effects of instrumental broadening.
To correct for instrumental errors, all calculated profiles are
shifted to the angular position which would place the sodium
chloride reference peak at its proper diffraction angle. The
resulting peak positions for the cobalt phase recovered from 14
cobalt-bonded tungsten carbide compositions are shown in FIG. 3,
together with a plot of percent by weight of tungsten vs. peak
position, and a histogram showing the number of peaks whose
estimated ranges fall at a given point. The error range shown is
.+-. 0.0125.degree. 2.theta. for each peak and zero for the sodium
chloride peak (not shown). These data are then replotted, adjusting
the relative positions of the sample patterns within a range of
.+-. 0.015.degree. 2.theta., the estimated error in the
determination of the sodium chloride peak position. The relative
positions of peaks for a given sample are not changed, but are
shifted as a unit. These data are plotted in FIG. 4, with the error
estimate increased to .+-. 0.0175.degree. 2.theta., together with
the plot of tungsten concentration and the frequency histogram. As
may be seen, a strong correlation in the peak spacing is now
evident from the histogram.
These data are itemized in Table I, which lists the peaks for each
sample, their indicated compositions, and the closest weight % of
tungsten grouping in the histogram. These histogram groupings are
summarized in Table II, stated in weight % of tungsten, and
converted to atomic % of tungsten, together with possible atomic
ratios. The occurrence of integral atomic ratios suggests the
possibility of ordered structures at the various compositions. Such
order would seem to be required to explain the segregation of
composition in these alloys.
Of the 14 different compositions analyzed by the preceding
procedure, and identified by the sample numbers 136C to 192C, it
will be noted from FIG. 4 that 6 out of the 14 did not contain
tungsten-cobalt regions giving peak positions greater than about
67.8.degree. 2.theta., and which therefore contained no regions
containing less than 8% by weight of tungsten in the cobalt. These
six compositions are therefore not examples of compositions of this
invention. One of these six, numbered 158C, corresponds to the
composition represented by Curve A of FIG. 1. The remaining eight
compositions are all heterogeneous, containing regions having less
than 8% tungsten in the cobalt. Of these eight, number 184A
corresponds to the composition represented by Curve D of FIG. 1,
which is the composition of Example 4; 136C corresponds to Curve E
of FIG. 1 and to Example 192C 184C corresponds to Example 5; 192C
corresponds to Example 7; 192B corresponds to Example 8; and 192A
corresponds to Example 9.
TABLE I
Closest Sample No. of Peaks wt% W Group
__________________________________________________________________________
136C 6 25.7.+-..5 26.0 21.2.+-..5 21.5 18.0.+-..6 17.2 12.0.+-..7
11.4 5.6.+-..8 5.0 2.1.+-..9 1.5 154E 6 23.4.+-..5 23.9 21.6.+-..5
21.5 17.5.+-..6 17.2 14.5.+-..6 14.3 12.0.+-..7 11.4 5.4.+-..8 5.0
158A 1 25.9.+-..5 26.0 158B 4 21.5.+-..5 21.5 17.2.+-..6 17.2
13.8.+-..6 14.3 7.5.+-..7 7.5 158C 2 23.9.+-..5 23.9 21.0.+-..5
21.5 172B 2 23.6.+-..5 23.9 22.0.+-..5 21.5 184A 8 26.2.+-..5 26.0
24.2.+-..5 23.9 22.0.+-..5 21.5 17.2.+-..6 17.2 11.3.+-..7 11.4
8.5.+-..7 7.5 5.2.+-..8 5.0 0.5.+-..9 1.5 184B 3 26.4.+-..5 26.0
24.3.+-..5 23.9 21.8.+-..5 21.5 184C 4 16.4.+-..6 17.2 14.0.+-..6
14.3 10.1.+-..7 11.4 7.0.+-..7 7.5 184D 2 26.2.+-..5 26.0
24.4.+-..5 23.9 184E 2 25.4.+-..5 26.0 24.3.+-..5 23.9 192A 6
18.2.+-..6 17.2 15.0.+-..6 14.3 10.8.+-..7 11.4 7.3.+-..7 7.5
3.8.+-..8 5.0 2.8.+-..8 1.5 192B 5 14.8.+-..6 14.3 11.6.+-..7 11.4
8.0.+-..7 7.5 5.0.+-..8 5.0 1.3.+-..9 1.5 192C 4 11.5.+-..7 11.4
7.0.+-..7 7.5 4.5.+-..8 5.0 1.4.+-..9 1.5
__________________________________________________________________________
---------------------------------------------------------------------------
TABLE II
Possible wt % Group At % W Atomic Ratio
__________________________________________________________________________
26.0.+-..3 10.12.+-..15 1/10 (10.0%) 23.9.+-..3 9.15.+-..13 1/11
(9.09%) 21.5.+-..35 8.11.+-..16 1/12 (8.33%) 17.2.+-..4 6.24.+-..17
1/16 (6.25%) 14.3.+-..4 5.08.+-..16 1/20 (5.00%) 11.4.+-..4
3.96.+-..15 1/25 (4.00%) 7.5.+-..4 2.53.+-..14 1/40 (2.50%)
5.0.+-..4 1.66.+-..14 1/60 (1.67%) 1.5.+-..5 0.485.+-..16 1/180
(0.556%)
__________________________________________________________________________
4. Utility
Some of the bodies of this invention are extremely dense, impact
resistant, wear resistant, extremely hard, and are very strong.
They are therefore suitable for use in the numerous ways in which
such refractory materials are conventionally used. Some of the
other uses to which the bodies of this invention can be put include
cutting tools, drilling bits, as binders or matrices for other hard
abrasives, and many other specific uses apparent to those skilled
in the art.
Bodies of this invention are used in tools in which unusual
strength is required in combination with high hardness. They are
particularly advantageous in tools in which conventional
cobalt-bonded tungsten carbide tools fail by flaking, chipping, or
cracking, such as in tools for form cutting, cut-off, milling,
broaching and grooving. Thus they find extensive use where, because
of the inadequacies of cobalt-bonded tungsten carbide of the prior
art, high speed steel tools are still employed.
Because of the unusual fine grain size, compositions of this
invention are useful in tools where extremely small cross-sections
are encountered, as for example in rotary tools smaller than an
eighth of an inch in diameter such as end mills, drills and
routers; knives having a cutting edge with an included angle less
than about 30.degree.; and steel-cutting tools which cut with high
rake angles such as broaches, thread chasers, shaving or planing
tools, rotary drills, end mills, and teeth for rotary saws. While
the compositions of this invention containing more than about 12%
cobalt are not stronger than compositions of this invention
containing from 5 to 12% cobalt, nevertheless, the impact strength
and toughness is higher. These are generally useful where tool
steels are normally employed, and have the advantage of higher
hardness than tool steels. For highest impact strength,
compositions containing from 12 to 25% cobalt are employed, as in
some dies and punches. However where a balance of impact strength
and wear resistance is required, compositions containing 5 to 12%
cobalt also find uses in dies and punches employed in operations
involving high volumes and long production runs.
The products of this invention are further illustrated in the
following examples wherein parts and percentages are by weight
unless otherwise noted.
EXAMPLE 1
This is an example of a composition of this invention in which a
heterogeneous distribution of tungsten in the cobalt binder phase
is produced by hot pressing a powder of very finely divided
tungsten carbide and cobalt containing a very small amount of
uniformly distributed, finely divided free carbon. The tungsten
carbide employed is made as described in copending application,
Ser. No. 772,810 filed Nov. 1, 1968.
By analysis this powder contains 93.4% tungsten, 5.95% total
carbon, 0.14% free carbon and 0.46% oxygen. Thus there is 5.81%
carbon bound in the tungsten carbide and the atomic ratio of
chemically combined carbon to tungsten is 0.95.
The product gives the X-ray diffraction pattern of tungsten carbide
and from the broadening of the X-ray lines, the average crystallite
size is calculated to be 35 millimicrons. The specific surface area
is 6.6 square meters/gram. Electron microscopic examination of the
powder shows it to consist of porous aggregates of colloidal
crystallites in the size range 20 to 50 millimicrons. The
aggregates are mainly in the size range of from 1 to 10 microns,
although some aggregates as large as 50 microns can be
observed.
This material will hereafter be referred to as aggregated colloidal
tungsten carbide powder.
Incorporation of the cobalt bonding phase is accomplished by
milling the cobalt in powder form with aggregated colloidal
tungsten carbide powder prepared as described above. To an 8 inch
diameter one gallon steel mill the following are charged: (a)
14,000 parts of "Carboloy" grade 883 cobalt bonded tungsten carbide
cylinders, one-quarter of an inch in diameter, and one quarter inch
long, the rods being previously conditioned by tumbling for 2
weeks; (b) 1500 parts of the aggregated colloidal tungsten carbide
powder prepared above; (c) 205 parts of a fine cobalt powder,
having a specific surface area of 0.7 square meters per gram and a
grain size of about 1 micron. This charge occupies about half the
volume of the mill. Milling under acetone is continued for 7 days
by rotating the mill at 45 revolutions per minute, after which time
the mill lid is replaced by a discharge cover and the contents are
transferred to a container maintaining an atmosphere of nitrogen
throughout the system while this is being done. Three portions of
acetone of 395 parts each are used to wash out the mill. The solids
in the drying flask are allowed to settle and the bulk of the
acetone is siphoned off. The flask is then evacuated and when the
bulk of the acetone is evaporated, the temperature of the flask is
brought to 125.degree.C., maintaining a vacuum of less than a tenth
of millimeter of mercury. After about 4 hours, the flask is cooled,
filled with pure argon and transferred to an argon glove box. In
this inert environment the solids are removed from the drying flask
and screened through a 70 mesh sieve.
The screened powder is charged to shallow trays which are then
loaded directly from the argon filled box to a five inch diameter
Inconel tube furnace, where the powder is brought to 900.degree. C.
at a uniform rate in about 3 hours. The gas passing through the
furnace consists of hydrogen, at a flow-rate of four liters per
minute, with methane introduced at a flow-rate of 40 milliliters
per minute. This treatment removes oxygen impurities, adjusts the
carbon content and makes the powder less susceptible to reaction
with air. The powder is held in this gas stream at 900.degree. C.
for 2 hours, then is cooled and passed through a 40 mesh per inch
screen in an argon filled box. Samples are taken under argon for
analysis.
The resulting heat-treated tungsten carbide powder is characterized
by analysis as follows: tungsten 82.3%, total carbon 5.21%; free
carbon 0.01%; cobalt 12.1%; oxygen 0.27%. The carbon content found
by analysis corresponds to an atomic weight of combined carbon of
0.965 per atomic weight of tungsten. The free carbon is uniformly
distributed throughout the powder as particles generally less than
a micron in size.
Forty-five parts of the powder described above is charged in an
oxygen-free environment to a cylindrical carbon mold and
close-fitting carbon pistons are inserted in each end. The mold
containing the powder is pressed at 200 psi and is then transferred
to a vacuum hot press. After evacuation the sample, under no
pressure, is brought to 1420.degree. C. by induction heating in 7
minutes and held at this temperature with no application of
pressure for 5 minutes. During the heating the sample sinters and
shrinks away from contact with the carbon surface, thus avoiding
carburization.
Hydraulic pressure is then applied to both pistons and the pressure
on the sample in the mold is brought to 4000 psi in a period of
half a minute. The sample is subjected to a pressure of 4000 psi at
1420.degree. C. for 1 minute at which time no further movement of
the pistons is observed. The mold containing the sample is then
ejected from the hot zone and allowed to cool to 800.degree. C. in
2 minutes in the evacuated chamber of the press. After cooling to
less than 100.degree.C., the mold is removed from the vacuum
chamber and a dense sample in the form of a cylindrical disc or
billet, 1 inch in diameter and a quarter of an inch thick, is
recovered.
The disc is cut into two segments, using a one hundred and eighty
grit diamond saw, and one of the segments is further cut into bars
0.070 .times. 0.070 inches in cross-section for measurement of
strength and hardness. The modulus of rupture of the hot pressed
composition as measured by applying a load at the center of a span
of 0.5 inches, is 566,000 psi, the Rockwell A hardness is 91.8. The
density of the hot pressed body is measured as 14.60 grams per
cubic centimeter which corresponds to a composition containing 9.5%
of cobalt. The body contains no free carbon, indicating that the
carbon particles have dissolved and reacted during hot pressing.
The reduction in cobalt content as compared with the powder is due
to the extrusion of some metal during fabrication.
The cobalt phase contains tungsten heterogeneously distributed, the
tungsten content being about 17% and 7% in different regions as
determined by procedure A, described above. Two different specimens
of cobalt recovered from different portions of the interior of the
billet are scanned by X-ray diffraction repeatedly at one-eighth of
a degree per minute in the region of the strongest cobalt line.
That cobalt line shows a main peak and a shoulder, and gives
lattice constants in angstroms, as follows:
First sample: from peaks: 3.5650, 3.5646; from shoulders 3.5520,
3.5515. Second sample: from peaks 3.5646, 3.5643, 3.5646; from
shoulders 3.5527, 3.5527, 3.5558, 3.5552. Average for peaks --
3.565; average for shoulders -- 3.553. These values correspond to
about 17 and 7% of tungsten in solid solution in the cobalt. The
fact that both of these values are obtained on different samples
from different parts of the billet tends to prove that the regions
high and low in cobalt, respectively, are uniformly interspersed in
the same way throughout the billet. Furthermore, the regions low in
tungsten are apparently derived from the free carbon particles
originally present, which dissolve and combine with the tungsten in
the surrounding cobalt, forming tungsten carbide and leaving cobalt
regions low in tungsten.
EXAMPLE 2
This is an example of a product of this invention made by blending
a tungsten carbide which is deficient in carbon, with graphite
powder and cobalt so that in regions surrounding the graphite,
which mostly dissolves during hot pressing, the cobalt binder is
low in dissolved tungsten while else where the carbon deficient
tungsten carbide furnishes tungsten in solid solution in the cobalt
binder.
Very finely divided tungsten carbide having a specific surface area
of 7.1 square meters per gram is employed. It contains 5.77% total
carbon and 0.06 free carbon, and thus the atomic ratio of combined
carbon to tungsten is 0.93. It contains 1.41% oxygen. Three
thousand six hundred parts of this tungsten carbide are mixed with
15 parts of powdered graphite which has passed a screen of 200
meshes per inch, and 500 parts of fine cobalt powder, and milled as
in Example 1 for 5 days. The milled powder is dried, screened and
reduced as in Example 1. The resulting powder contains 5.34% of
total carbon, 0.02% of free carbon and 0.26% oxygen. Thus the
atomic ratio of carbon to tungsten is 0.99, with the free carbon
evenly distributed through the powder as particles of graphite of
around a micron in size. This powder is hot pressed in a graphite
mold as a billet 1.8 inches wide, 3.12 inches long and 0.6 inches
in thickness.
The powder is compacted by pushing in the pistons at room
temperature with a pressure of 400 pounds/square inch. The compact
is then heated in the mold with no pressure applied for a period of
16 minutes, at which time the sample has reached 1390.degree. C. A
pressure of 4,000 pounds/square inch is then applied for 1 minute
and the mold is then removed from the heated zone.
The resulting dense cobalt-bonded tungsten carbide composition is a
very fine-grained, hard, strong material suitable for use as a
cutting edge on tools used for high-speed cutting of ferrous
alloys, especially in tools such as drills, reamers and form tools
where conventional metal-bonded carbides are not strong enough and
where tool steels are employed only at low cutting speeds. This use
is made possible by the fact that the transverse rupture strength
of this product is 540,000 pounds per square inch, which is almost
twice that of conventional carbide tooling, and approaches that of
tool steels, while the hardness is over 91, Rockwell A scale.
Examination of the microstructure shows that there are carbon
particles at the polished surface 10 to 20 microns apart, on the
average. These are from about 2 to less than 1 micron in size.
These carbon particles are thus within a matrix of cobalt-bonded
tungsten carbide in which there is an overall carbon deficiency.
However, X-ray diffraction measurements, by procedure B described
above, on the recovered cobalt phase show that some of the cobalt
contains less than 8% tungsten in solid solution, whereas the
remaining cobalt contains more than 8% tungsten. A low-tungsten
region surrounds each grain of carbon. This is evidenced by the
fact that near the carbon grains the tungsten carbide grains are
larger than elsewhere, indicating there is no carbon deficiency in
those regions and thus little tungsten in the cobalt in those
regions.
EXAMPLE 3
This example describes the preparation of a dense body of tungsten
carbide bonded with 12% cobalt possessing unusual high strength and
hardness, having an extremely fine grain size and low porosity, and
having a heterogeneous distribution of tungsten in the cobalt, made
by preparing a very finely divided intimate mixture of cobalt and
tungsten carbide powders, cold pressing and sintering under
conditions to be described.
To a steel mill having a capacity of about one gallon and a
diameter of 8 inches, are charged 14,000 parts of grinding
cylinders one-fourth inch long and one-fourth inch in diameter of
tungsten carbide bonded with 6% cobalt. The cylinders have been
previously conditioned by tumbling in acetone in the mill for 2
weeks in order to wear off all sharp corners. This
"pre-conditioning" is continued until the rate of wear under
milling conditions is less than about 10 parts in 5 days when used
to mill compositions of this invention.
Into the mill is also charged 1800 parts of fine commercial
tungsten carbide powder, 2 parts of powdered graphite passed
through a screen of 200 meshes per inch and 1450 parts of acetone.
The fine tungsten carbide powder has a specific surface area as
determined by nitrogen adsorption of 0.66 square meters per gram.
By X-ray line broadening the average crystallite size is 370
millimicrons. Examination of the powder with an electron microscope
reveals dense aggregates in the size of 2 to 10 microns, the
aggregates being comprised of tungsten carbide grains in the size
range from 0.5 to 2 microns, with an average of around 1/2 or 1
micron. Chemical analysis of this powder is 93.2% tungsten, 5.90%
total carbon, and 0.31% of oxygen.
The charge occupies about half of the volume of the mill. Milling
is carried out by rotating the mill at 45 revolutions per minute,
the lid being tightly sealed to prevent loss of contents. Milling
is continued for 48 hours. The mill is then permitted to cool and
is opened. Two hundred and fifty parts of cobalt powder are added.
The cobalt powder has a specific surface area of 0.7 square meters
per gram and an average grain size of about 1 micron. The mill is
closed and milling continued for 72 hours, at a rate of 45 rpm. The
mill is then permitted to cool and the lid is replaced by a
discharge cover and fitted with inlet and outlet connections so
that the contents are transferred to a container maintained in an
atmosphere of nitrogen throughout the operation. Three portions of
acetone of 395 parts each are used to wash out the mill. The solids
in the receiver flask are allowed to settle and the bulk of the
acetone is siphoned off. The flask is then evacuated and warmed
from the exterior to distill off the acetone and the temperature of
the flask is brought to 125.degree.C. after the distillation is
completed. The contents are maintained at that temperature under a
vacuum of less than a tenth of a milliliter of mercury for about 4
hours. The flask is then cooled and filled with pure nitrogen and
transferred to a nitrogen filled glove box. In this inert
environment the solids are removed from the flask and screened
through a sieve having 70 meshes per inch to give essentially
spherical pellets.
Analysis of the powder, which is maintained continuously under
nitrogen is 5.15% total carbon, 0.09% free carbon, 0.46% oxygen,
12.76% cobalt and the remainder being tungsten. The specific
surface area by nitrogen adsorption is 2.8 square meters per gram
and the crystallite size of the tungsten carbide by X-ray
diffraction is 80 millimicrons. The density of this powder when
tapped in a container to maximum settling is 35 percent of the
theoretical density.
This powder has an atomic ratio of combined carbon to tungsten of
0.97 and the free carbon is uniformly distributed throughout the
powder as particles less than a micron in size.
Fifty-five parts of the powder described above is charged in an
oxygen-free environment to a 1 inch diameter cylindrical graphite
mold and close-fitting graphite pistons are inserted in each end.
The mold containing the powder is pressed at 200 psi and is then
transferred to a vacuum hot press. After evacuation the sample,
under no pressure, is brought to 1400.degree.C. by induction
heating in 7 minutes and held at this temperature for 5 minutes.
During the heating the sample sinters and shrinks away from contact
with the mold surface, thus avoiding carburization.
Hydraulic pressure is then applied to both pistons and the pressure
on the sample in the mold is brought to 4000 psi in a period of
half a minute. The sample is subjected to a pressure of 4000 psi at
1400.degree.C. for 1 minute, by which time no further movement of
the pistons is observed. The mold containing the sample is then
ejected from the hot zone and allowed to cool to 800.degree.C. in 2
minutes in the evacuated chamber of the press. After allowing to
cool to less than 100.degree.C. the mold is removed from the vacuum
chamber and a dense sample in the form of a cylindrical disc or
billet, 1 inch in diameter and a quarter of an inch thick is
recovered.
The resulting hot pressed billet has a transverse rupture strength
of 545,000 psi and a hardness of 91.4 Rockwell A. Examination of
the microstructure shows extremely low porosity, with an ASTM
rating of Al. The cobalt distribution is extremely uniform, the
tungsten carbide grains are substantially all smaller than 1
microns, are generally equiaxed, no eta phase is observed, and the
mean grain diameter is 0.5 micron. The carbon content is 5.30 and
the atomic ratio of carbon to tungsten is 0.96.
The cobalt recovered after removing the tungsten carbide contains
24 percent of tungsten, as determined by X-ray diffraction
according to procedure A. The distribution of tungsten in the
cobalt phase ranges from 7% to 29% with most at about 24% as
measured by procedure B.
EXAMPLE 4
This is an example of a product of this invention prepared by the
methods described in Example 1, except that the reduced powder
which is hot pressed contains even more free carbon and the overall
level of tungsten in the cobalt is less.
The tungsten carbide is similar to that employed in Example 1
except that it has an atomic ratio of combined carbon to tungsten
of 1.0 and contains 0.26% by weight of free carbon. After milling
with cobalt and reduction, it contains 0.07% free carbon and the
atomic ratio of combined carbon to tungsten is 0.96; the cobalt
content is 12.2 percent by weight. After being hot pressed, the
dense product has a transverse rupture strength of 547,000 psi, a
hardness of Rockwell A = 92.4, and contains 9.03 percent of cobalt
and an atomic ratio of carbon to tungsten of 0.958; no free carbon
is found. The acid resistance is low -- about 15 hours. The
distribution of tungsten dissolved in the cobalt phase is
heterogeneous, regions being identified by procedure B as
containing 26%, 23.9%, 21.5%, 17.2%, 11.4%, 7.5%, 5.0% and 1.5% of
tungsten. No eta phase is present in spite of the relatively low
average carbon content. By procedure A, regions averaging 22% and
6% tungsten in the cobalt phase are evident, but procedure B shows
the value for the regions that gives these averages. Curve D of
FIG. 1 reflects the analysis by procedure A, and sample 184A of
FIG. 4 and Table 1 reflect the analysis by procedure B.
The billet is cut into cutting tool inserts for turning a high
temperature alloy. Less chipping of the cutting edge is encountered
than with standard carbide compositions of the prior art.
EXAMPLE 5
This is an example of the invention in which heterogeneous
distribution of tungsten in the cobalt phase is effected by
blending two lots of reduced tungsten carbide-cobalt powder, one
containing more and the other less carbon than required to furnish
consolidated bodies of superior strength. The reduced powders are
prepared as in Example 1, the first powder is made from tungsten
carbide containing 5.23 percent of total carbon, 0.06 percent free
carbon and 1.18 percent oxygen, thus having an atomic ratio of
bound carbon to tungsten of 0.85; the second powder is made from
tungsten carbide containing 6.70% total carbon, 0.79% free carbon
and 0.51% oxygen. Each powder contains 12.2% of cobalt. During the
screening of the powders through a screen of 70 meshes per inch,
the horizontal screen and attached receiving pan are vibrated in a
direction parallel to the plane of the screen. The resulting
screened powders are obtained in the form of spheres about 50 to
150 microns in size formed by aggregation of the much finer powder
components. During the reduction step at 900.degree. C., these
spheres are slightly sintered and increase in strength so they can
be tumbled in a mixer without breaking apart.
The first powder after reduction contains 4.54% total carbon, no
free carbon and has an atomic ratio of carbon to tungsten of 0.85.
When this powder is separately hot pressed as in Example 1, the
resulting billet contains 10.96% cobalt and has an atomic ratio of
carbon to tungsten of 0.83, a Rockwell A hardness of 91.9 and a
transverse bending strength of only 404,000 psi.
The second powder after reduction contains 5.53 percent total
carbon, 0.14% free carbon, and an atomic ratio of carbon to
tungsten of 1.03. When separately hot pressed as in Example 1, it
gives a billet containing 8.2 percent cobalt, 5.73 present total
carbon, and an atomic ratio of total carbon to tungsten of 1.02, a
small amount of free carbon being present. The hardness is 92.0 on
the Rockwell A scale and the transverse rupture strength is 375,000
psi.
To prepare a composition of this invention, 25 parts by weight of
the first reduced powder and 75% of the second reduced powder are
thoroughly blended by tumbling. This mixture is hot pressed as in
Example 1 and gives a billet containing 9.2% cobalt, 5.49% total
carbon, an atomic ratio of total carbon to tungsten of 0.99, a
hardness of 91.6 on the Rockwell A scale, and a transverse bending
strength of 540,000 psi. The microstructure shows regions in which
the grain size of tungsten carbide is less than 1 micron,
interspersed with regions containing some coarse tungsten carbide
two or three microns by 8 microns in cross-section. The latter
coarseness is indicative of regions in which the atomic ratio of
carbon to tungsten is about 1.0.
The acid resistance for this product is 18 hours. By analysis of
the strongest X-ray diffraction line of cobalt, using procedure B,
regions of cobalt are found to be present, containing 17.2%, 14.3%,
11.4%, and 7.5% tungsten, respectively. This heterogeneity is
reflected in sample 184C of FIG. 4 and Table 1 which correspond to
the composition of this example. By procedure A an average value of
14% tungsten in the cobalt is observed. A photomicrograph of a
polished cross-section of the composition, etched lightly to reveal
the tungsten carbide grains shows the presence of carbon particles
in the structure. However, there are regions from 10 to 50 microns
in diameter, comprising about a quarter of the area of a typical
cross-section, that are free from carbon and in which the tungsten
carbide grains are smaller than 2 microns. These are the portions
of the structure which are derived from the carbon-deficient
powder. At high magnification the carbon particles appear as
irregular clusters, 1 or 2 microns in size, and in the regions of
the cross-section where they are present, they are 10 to 30 microns
apart. There are also pores in the areas containing free carbon,
these being distinguished as separate rounded solid black areas, in
these regions a substantial portion of the tungsten carbide grains
are from 2 to 10 microns in size.
The composition is found to be not only very strong, but also very
resistant to chipping under impact, being equal in this respect to
many tungsten carbide bodies of the prior art which contain more
cobalt, and thus have a hardness of less than R.sub.A = 90. The
composition is fabricated into an insert for a cut-off tool and
used on a screw machine for cutting off stainless steel parts
without chipping under conditions where most carbide tools of the
prior art chip and break.
EXAMPLE 6
This is an example of the preparation of a product of this
invention by starting with two different tungsten carbide powders,
one containing more carbon than the other. The tungsten carbide
powders are prepared in the same manner as that of Example 1, by
incorporating different amounts of carbon in the synthesis. Both
powders consist of porous aggregates from 1 to 10 microns in size,
of colloidal crystallites of tungsten carbide about 40 millimicrons
in average diameter.
The first tungsten carbide powder is low in carbon, the total
carbon content being 6.07%. The powder contains 0.09% free carbon
and 0.36% oxygen. The second tungsten carbide powder contains 6.19%
total carbon, 0.12% free carbon, and 0.43% oxygen.
Equal parts of each of these two powders are ball-milled with
sufficient cobalt powder as in Example 1 to provide a mixture
containing 12.4% of cobalt. The resulting milled and dried powder
is reduced also as in Example 1. The reduced powder contains 5.28%
total carbon, less than 0.01% free carbon, 0.23% oxygen and has an
atomic ratio of carbon to tungsten of 0.985.
A billet 1 inch in diameter and a quarter of an inch in thickness
is pressed by the procedure described in Example 1, resulting in a
very strong composition containing 8.61% of cobalt, 5.45% of
carbon, and an atomic ratio of carbon to tungsten of 0.97.
The hardness of this composition is 92.0 on the Rockwell A scale,
and the transverse rupture strength is 593,000 psi. The
distribution of tungsten in the cobalt phase determined by
procedure A (see Curve E of FIG. 1) shows the presence of regions
containing approximately 20, 10 and 3% of tungsten. The more
sensitive procedure B shows that the major region containing an
average of 20% tungsten is an average for regions containing 26.0%,
21.5% and 17.2%; the 10% region was 11.4%, and the 3% region is an
average of 5.0% and 1.5% regions. The procedure B analysis is
reflected in sample 136C of FIG. 4 and Table I.
The microstructure shows most of the tungsten carbide grains are
under one micron in size and the average grain diameter is less
than 1 micron. Regions about 10 microns in area and about 20 to 50
microns apart are present and contain coarser tungsten carbide
grains up to 5 microns in size. No eta phase is present and no
carbon particles evident in micrographs of polished sections.
The composition is converted into twist drills 0.060 inches in
diameter and used for drilling electronic circuit boards without
breaking.
EXAMPLE 7
This is an example of a composition of this invention in which the
overall atomic ratio of carbon to tungsten is 1.0, but some regions
contain free carbon while others are carbon-deficient. In the
carbon-deficient regions there is 11.4% of tungsten dissolved in
the cobalt phase, while in the carbon-rich regions less than 8%
tungsten is found in the cobalt. A very finely divided tungsten
carbide similar to that employed in Example 1 having a specific
surface area of 9.2 square meters/gram, an atomic ratio of combined
carbon to tungsten of 0.98 and containing 0.39% free carbon, is
milled with sufficient cobalt powder to give a composition
containing 12.3% of cobalt. This powder is then reduced as in
Example 1 by heating at 900.degree. C. in hydrogen containing
methane. The heat-treated powder contains a total of 5.4% of carbon
and the overall atomic ratio of total carbon to tungsten is 1.0.
However, 0.08% of free carbon is present, so that the atomic ratio
of combined carbon to tungsten is 0.99.
This composition is hot pressed as in Example 1 and gives a product
containing 8.9 percent by weight of cobalt, having a transverse
bending strength of 521,000 pounds per square inch and a hardness
of Rockwell A = 91.9. The atomic ratio of total carbon to tungsten
is 1.0. The cobalt phase contains an average of 7% of tungsten but,
as determined by procedure B, different regions of cobalt contain
11.4%, 7.5%, 5.0%, and 1.5%. respectively of tungsten. The analysis
by procedure B is reflected in sample 192C of FIG. 4 and Table 1.
No eta phase, Co.sub.3 W.sub.3 C, is present. The acid resistance
of the composition is 12 hours. Microscopic examination of a
polished etched section shows particles of free carbon less than 2
microns in size, from 30 to 80 microns apart. In the regions near
the free carbon particles, there are tungsten carbide grains having
cross-sections as large as 3 by 10 microns. In regions between the
carbon grains and more distant from them, the tungsten carbide
grains are, for the most part, smaller than about 1 micron, and
have an average grain size of less than 2 microns. This composition
is not as strong as others of this invention in which there are
cobalt regions containing more tungsten, but nevertheless, its
performance as a form tool on a screw machine cutting mild steel is
superior to most compositions of the prior art which have the same
overall chemical composition but which do not have the unique
distribution of tungsten dissolved in cobalt. The form tool is also
very resistant to chipping in interrupted cuts.
EXAMPLE 8
This is an example of a composition similar to that of Example 7,
except that the powder mixture contains 11% of cobalt before being
hot pressed. It is prepared from finely divided tungsten carbide
similar to that employed in Example 1, containing 6.36% of total
carbon and 0.33% free carbon, and has a specific surface area of
8.8 square meters per gram. The heat-treated, reduced
cobalt-tungsten carbide powder composition also contains 0.09% of
free carbon uniformly distributed throughout the mass as particles
smaller than five microns. The atomic ratio of combined carbon to
tungsten is 0.985. After being hot pressed as in Example 1, the
resulting billet contains 8.9% cobalt and an overall ratio of total
carbon to tungsten of 0.99; the transverse rupture strength is
520,000 psi, the hardness 92.0 Rockwell A, and impact tests show
that the material is very resistant to chipping. The cobalt phase
is isolated and found to contain 8% of dissolved tungsten, which by
procedure B is shown to exist in regions containing 14.3%, 11.4%,
7.5%, 5.0%, and 1.5% tungsten. Analysis by procedure B is reflected
in sample 192B of FIG. 4 and Table I. The acid resistance is very
low, 7 hours. A polished cross-section of a specimen shows the
presence of regions high in carbon, about 3 microns across, these
regions being on the average about 20 microns apart, the high
carbon regions being characterized by the presence of free carbon
and grains of tungsten carbide larger than 3 microns in size, while
the intermediate regions are characterized by showing no free
carbon and having a grain size smaller than 2 microns. This
composition is used in fabricating a punch employed in a punch and
die assembly for punching razor blades from steel ribbon. It is as
chip-resistant as the standard compositions of the prior art
containing from 15 to 25% of cobalt, but nevertheless is much
harder and more wear resistant, giving two to five times the wear
life of such prior art compositions.
EXAMPLE 9
This is a composition similar to that of Example 7, except that in
the reducing step only two-thirds as much methane is employed in
the gas stream and the reduced powder contains only 0.02% free
carbon and an overall ratio of combined carbon to tungsten of 0.99.
After being hot pressed, the composition contains 8.9% cobalt, the
overall atomic ratio of total carbon to tungsten is 0.988, and
there is no apparent free carbon in the composition. The
transverse, rupture strength is 572,000 psi and the Rockwell A
hardness is 91.9.
The overall concentration of tungsten in the cobalt phase is 9%,
and by procedure B the tungsten is shown to be present in different
regions containing 17.2%, 14.3%, 11.4%, 7.5%, 5.0%, and 1.5%,
tungsten respectively. The procedure B analysis corresponds to
sample 192A of FIG. 4 and Table I. The acid resistance is about 15
hours. The material is very chip resistant and is employed in a
punch similar to that described in Example 8.
EXAMPLE 10
This is an example similar to that of Example 3, except that more
cobalt is added so that the composition contains 25% of cobalt.
After cold pressing, degassing, sintering at 1300.degree.C. for 5
minutes, and then cooling to below 800.degree.C. in 5 minutes, a
dense, pore-free billet is obtained having an overall atomic ratio
of total carbon to tungsten of 0.99. The billet contains particles
of free carbon about 1 micron in diameter, uniformly distributed
over a polished cross-section, on the average about 30 microns
apart. The cobalt binder contains an average of 15% of tungsten and
by Procedure B it is shown that the tungsten is present in
different regions at varied concentrations in the cobalt ranging
from 26.0 to 1.5%. The transverse rupture strength is 625,000 psi,
the hardness is 88 on the Rockwell A scale. The average grain size
of the tungsten carbide is less than one micron. Although grains
near the carbon particles range up to 5 microns, in intermediate
regions they are smaller than 1 micron. The composition is very
resistant to chipping and breakage by impact, and is employed as a
knife having an included angle of 20.degree. at the cutting edge,
in shears employed for cutting steel sheet. The composition is much
harder and more wear resistant than most compositions of the prior
art having the same cobalt content and also it is much stronger in
bending.
EXAMPLE 11
This is an example of a composition of the invention containing 6%
of cobalt made by a procedure like that of Example 3. The body is
sintered at 1450.degree.C. for 10 minutes and is then cooled within
20 minutes to less than 800.degree.C.
The milled and dried powder is kept from exposure to the
atmosphere, while being cold pressed into 3/4 inch square blanks,
1/4 inch thick, on an automatic powder press maintained in a
nitrogen atmosphere. The powder contains 0.04% free carbon
uniformly distributed as particles from 0.5 to 2 microns in size
throughout the mass and the atomic ratio of combined carbon to
tungsten is 0.99. The sintered body has a carbon to tungsten atomic
ratio of 0.98, a transverse rupture strength of 480,000 psi, and a
hardness of 92.5 Rockwell A. A polished cross-section shows a
microstructure containing uniformly distributed particles of free
carbon about 1 micron in size, their distance apart ranging from 20
to 50 microns. The average grain size of the tungsten carbide is
less than 2 microns. The cobalt phase contains an overall average
of 18% of tungsten in solid solution which is present in different
regions at concentrations which range from 23.9% to 5.0% of
tungsten, respectively. The billets are converted to positive rake
cutting inserts and are used for turning a nickel-based high
temperature alloy.
EXAMPLE 12
The powder of Example 11 is hot-pressed in a manner similar to that
described in Example 1 except that the maximum temperature is
1450.degree.C. The cobalt content of the hot-pressed billet is
5.2%, the transverse rupture strength is 510,000 pounds/square
inch, and the hardness is 93.0 on the Rockwell A scale.
The atomic ratio of total carbon to tungsten is 0.98 and free
carbon is present as finely divided particles as in the product of
Example 11. The cobalt contains an average of 15% of tungsten,
present in varied regions having concentrations ranging from 5.0%
to 21.5%.
This product is useful as a cutting tool insert similar to that of
the product of Example 11.
EXAMPLE 13
This is an example of this invention in which two heat-treated and
reduced powders, similar to those described in Example 5, are
mixed. The powders differ not only in the ratio of carbon to
tungsten, but also in cobalt content. Thus, the first powder is
prepared from colloidal tungsten carbide similar to that of Example
1, except that it is more deficient in carbon, containing an
overall atomic ratio of total carbon to tungsten of 0.93. This is
admixed with cobalt to produce a composition containing 6% of
cobalt, and is then ballmilled, screened on a vibratory machine to
aggregate the powder in the form of spheres about 60 microns in
average diameter, and reduced as in Example 1 at 900.degree. C. The
resulting powder contains no free carbon and the atomic ratio of
total carbon to tungsten is 0.95.
A second powder is prepared identical with the second powder of
Example 5, containing 12% of cobalt, an overall atomic ratio of
total carbon to tungsten of 1.03, and containing 0.14% of free
carbon in the form of particles smaller than 3 microns uniformly
distributed through the mass. It is reduced at 900.degree.C., as in
Example 1. The reduced powder consists of small spherical
aggregates of the same size as those of the first powder.
Equal parts of these two powders are thoroughly blended in a
mechanical tumbler. The mixture is loaded into a graphite mold
without compaction, and heated to 1400.degree.C. over a period of
about 20 minutes in a vacuum. At this point, 2000 pounds per square
inch pressure is applied to a graphite piston compressing the
powder into the mold for one minute. The mold and contents are then
removed from the heated zone of the furnace and permitted to cool
under vacuum, the temperature of the mold dropping to less than
800.degree.C. in 15 minutes. The hot-pressed composition has a
transverse rupture strength of 510,000 pounds/square inch and a
Rockwell A hardness of 92.7, thus combining very high hardness with
very high strength. The resistance to chipping is much greater for
this product than that of standard compositions of the prior art
containing 9% cobalt, as shown by using milling cutter inserts of
this material for face milling rough cast iron engine blocks. The
cobalt binder on the average contains 20% of tungsten, which in
different regions ranges from 5% to 25%. The atomic ratio of carbon
to tungsten is 0.98. The microstructure examined in a polished
cross-section indicates that the body consists of interpenetrating
networks of regions low in cobalt and high in tungsten carbide,
containing no free carbon, and regions high in cobalt and lower in
tungsten carbide containing particles of carbon about one micron in
size and about 10 to 30 microns apart. The overall cobalt content
of the hot pressed body is 8.1%.
* * * * *