Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
Since 1940, wear-resi~stant parts for wear-prone tools and
equipmen~ have been made of cemented carbide alloys consisting
of a finely dispersed hard-carbide phase based on metals chosen
from Groups IVB, VB and VIB of ~he Periodic table, cemented by
cobalt or nickel or bo~h. Produced by compacting finely-milled
powders followed by liquid-phase sintering to achieve consolida-
tion~ cemented carbide alloys possess microstructures character-
ized by hard carbide grains generally in the 1 to lS micron
range.
The use of iron or steel as binder materials has proven
difficult because the finely-divided state and high specific
surface of the dispersed hard phases promote the forma~ion of
comparatively brittle blnary interstitial alloys of tungsten
and iron with carbon, thus reducing the free binder volume
fraction and embrittling the sintered body, more or less,
depending on the precision maintained in the formulation and
sintering parameters and on the free carbon additions m~ade to
satisfy the affinity between iron and carbonO
Unlike cobalt and nickel, iron forms a stable carbide,
Fe3C, and has a greater tendency to form brittle binary carbides
than cobalt or nickel binder materials. Carbon ~ransfer from
the hard carbide phase or phases to iron is promoted by the
presence of the liquid or plastic state of an iron or steel
binder during liquid-state sintering, carried ou~ a-~ tempera-
tures near to, at, or above the melting point of the b-inder.
More recently, useEul wear parts have been made by casting
a liquid steel or cast iron melt into a prepared bed of
comparatively coarse particulate, e.g., 1/8 inch ~o 3/16 inch
sintered, cemented carbide.
The present lnvention may be distinguished from the molten-
steel casting method of Charles S. Eaum, United States Patent
Nos. 4~024,902 and 4,140,170 and the molten-cast iron method of
Sven Karl Gustav Ekemar in United States Patent No. 43119,459
by two main factors: (1) a powder compact of steel or iron and
graphite con-taining di.spersed particulates of sintered, cemented
carbide, or a number of pieces of dimension~d sintered cemented
carbide, or primary, unmilled macrocrystallîne carbide crystals
is sintered at a temperature below the melting temperature of
steel or cast iron, and (2) in place of the use of matrix-alloy
melt;ng temperatures to achieve alloy densification, high
compaction ~mit pressures, both before and after sintering~ are
used, thereby avoiding degradation of the dispersed hard phase
particle surfaces by decomposition, melting or carbon diffusion
reactions.
Foundry methods, also, lack ~he well-known economic
advantages inherent in powder metallurgy methods~ notably, when
a multiplicity of wear parts either small or of thin section are
to be made. Also, because of the necessarily relatively high
processing temperatures and liquidity~ excessive amounts of
unwanted binary carbides may form despite the use of compara-
tively coarse~ low-surface area carbide particles.
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Since both the conventional powder metallurgy method of
pressing and sintering finely-milled steel-cementPd carbide
powders and me-~hods involving casting liquîd steel or liquîd
cast iron -into particulate cemented carbide prearranged in
molds result in problems hereînbefore descrîbed, ît is the
prîmary objec~ive of this înven~ion to develop a method by which
a steel-cemented hard carbîde allo'y can be fabricated essentially
free of bînary -intersti~îal alloys of iron and tungsten wîth
carbon and în which the dispersed hard carbide phase îs free of
boundary area decomposîtion, melting or thermal cracking and is
f mly bo~md în a steel matrix essentially free of macroporosity.
It is also an object of this înventîon to produce a
compositîon of matter having dispersed hard carbîde material
firmly and adherently bonded in a metallic matrix by powder
metallurgical techniques of compaction and high temperature and
high pressure dîffusion bondîng~
It is a fwrther object of this învention to manufacture
tools having hard carbîde wear or cutting înserts embedded in
and bonded to a consolidated steel powder matrix or composîtion
of matter according to this invention.
It is a further object of this învention to manufac~ure
parts being substantially nonmachinable and of sufficient împact
resistance to make them suîtable for use as security pla~es and
padlock components.
BRIEF SUMMARY OF I~IE INVENTION
_ _ _ . _
The method o~ the present invention învolves blendîng and
mixing sintered, cemented tungsten carbîde particles or prîmary
unmilled macrocrystall-ine (i.e., greater than 400 mesh) tungsten
carbide crystals with a matrix oE iron and graphite powders or
steel powder 7 cold isostatical.ly pressing the composite in a
preform mold to a desired shape, then solid-state sintering at
a comparatively low tempera~ure~ specificallyl at a temperature
below the melting temperature of the s~eel, preferably, betweell
1900 degrees and 2250 degrees Fahrenheitg then hot isostat.ically
pressing (HIP) the sintered body at a temperature well below the
melting point of steel to achieve final densification. A
diffusion bond is formed between the hard carbide particles and
the surrounding steel powder, which holds the wear-resistant
hard carbide particles in place~
A critical factor of ~he present invent~on is high-pressure
densification, both cold and hot, to avoid pxocess temperatures
which produce liquidity of the steel binder phase and, thus,
promote the aforementioned undesirable reactions be-tween the
steel binder material and hard dispersed phaseO The technique
is reinforced in thls respect by the use of a hard dispersed
particle or particles of low specific surface. The method also
provides a significant advance in production capability for the
manufacture of steel-carbide wear parts of comparatively small
size or of th-in section or intricate design, as compared with
methods as disclosed in United States patents hereinbefore
enumerated in which molten steel or molten cast iron are poured
into a mold preloaded with particles of cemented carbide.
Further, both chemical control of and compositional
flexibility of the matrix alloy are superior to molten-metal
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casting methods. The avoidance oE high processing temperatures
required to melt and pour steel or cast iron provides better
economy of molds, which may be reused, and matrix me-tals, which
are not subject to pouring loss and recycle cost. The method of
the present invention is well suited for the formation of parts
that must withstand highly abrasive wear Eorces as well as
impact forces. The process is ideally suited -to form wear
resistant parts and cutting tools for equipment for agriculture~
road and highway construction and maintenance~ crushing,
comm-inuting, excavation, and processing. Since the wear
resistance of the products produced by this process is so high,
so as to make them practically nonmachinable, they are also
ideall~ suited for use as security plates in safes. This wear
resistance in combination with the impact resistance of these
compositions makes them also suitable for use in padlocks.
BRIEF DESCRIPTION OE THE DRAWINGS
The exact nature of the present invention will become
more clearly apparent upon reference to the following detailed
specification taken in connection with the accompanying drawings
in which:
Figure 1 is a photomicrograph at 1500 magnification
showing a cemented carbide particle having a cobalt binder
embedded in and bonded to a consolidated steel powder ma~rix.
Figure 2 is a cross sec-tionalized perspective vîew of a
wear plate having cemented carbide inserts embedded in and
bonded to a consolidated steel powder matrixO
Figure 3 is a cross sectional view oE part of a cutting
~ool having cemented carbide button embedded in and bonded to
a consolidated steel powder matrix.
DETAILED ~ESCRIPTION OF THE INVENT~ON
Prealloyed steel matrix powder, or a mixture of iron
powder and graphite powder, comprising 2~ weight per cent (w/o)
to 70 w/o of the final mixture is blended and mixed with 30 w/o
to 80 w/o of hard car'bide particles of W, Ti, Ta, Nb~ or Zr,
V, Hf, Mog B, Si Cr or a mixture of these, either as sintered
cemented carbide particles or as primary, uncemented, unsintered,
unmilled carbide crystalsO About 3 per cent of nap'htha or
o~her liquid hydrocarbon is added to the powder blend during
mixing to prevent segregation of higher density car'bide particles
during mi~ing and mold filling, specifically when the dispersed
hard phase is composed of hard carbide particles coarser than
about 250 microns.
For dispersed hard phase particles finer than about 250
microns, paraffin wax or a solid lubricant such as zinc
stearate may be used, because the possibility of component
particle segrega~ion during mixing is then diminished.
Next, the matrix powder containing the dispersed hard
carbide phase is packed in a preform mold made of polyurethane
or other elastomeric plastic. Steel powders of different
chemical compositions (such as carbon, alloy or stainless steel
powders) or elemental powders such as iron, copper or nickel,
may also be packed in the same mold with the main composite
steel powder-carbide blend, in any desired location~ adjacent
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to and in contact with the body containing the hard carbide
dispersed phase, or surrounding such body, or enveloping a
dimensioned, sintered cemented carbide insert. The packed
mold with a suitable fitted cover is -then sealed and placed in
a rubber bag or balloon which is then evacuatedg sealed and
isos~atically pressed~ preferably at about 35,000 psi, but
not less than 10,000 psi.
The compac-ted powder preform ls then removed from the
mold and heated in vacuum or in a proteckive or reducing gas
atmosphere, e.g., hydrogen~ to a temperature below the melting
temperature of the steel matrix9 preferably between 1900 degrees
and 2100 degrees Fahrenheit, for between 20 and 90 minutes.
~ n alternative preforming method consists of packing the
composite mixture containing preferably liquid hydrocarbon,
e.g., naphtha, preferably 7 w/o and methyl cellulose, preferably
0.5 w/o, as pressing lubricant and green-state binder,
respectively1 in a steel preform mold. The green preform is
then air dried at room temperature, in the mold, then removed
from the mold and placed in a rubber bag which is then evacuated
and sealed, r~ady for cold isostatic compaction as hereinbefore
described.
Compacts thus solid-state sintered retain some porosity;
shrinkage duri.ng sintering does not exceed 1 per cent. It has
been found, however, that densification achieved by high-
pressure isostatic compacti.ng followed by sintering as herein
described is sufficient to eliminate any interconnected pore
network and that the sintered bodiesg therefore~ qualify for
~ 3~
efEective final densification by known hot isostatic pressing
(HIP) methods.
Hot isostatic pressing for -Lhe purposes of this invention
is applied in an inert atmosphere~ preferably at 1600 degrees
to 2300 degrees Fahrenheit or at any temperature below the
melting temperature of the steel ~or from 20 to 90 minutes at
a minimum pressure of 10,000 psi but, preferably at a pressure
of about 15,000 psi for 60 minutes. Equally important, an
alloy layer is formed at the interfaces of cemented carbide
par~icles and steel matrix. This interfacial alloy bond, which
first forms during sintering and is later enhanced during hot
isostatic pressing, consists of a thin border area betwe~ for
example, a 0.75 per cent carbon steel ma~rix and dispersed
cobalt-cemented carbide particles in a 1/8 inch to 3/16 inch
size range. The bond is typically less than 40 microns thick,
and no greater than 50 microns thick. The interfacial bonding
alloy under these conditions is composed of, principally,
cobalt and iron. Bond formation becomes iimporta~t especially
when the hard dispersed phase is of comparatively coarse
particles, because these are apt to pull out if not securely
anchored in the matrix alloy.
Cemented tungsten carbide par~icle sizes comprising the
dispersed hard phase are selected from within the general size
range of 2.5 mesh to 100 mesh, in the U. S. Sieve Series,
preferred size ranges being -12 ~20 mesh, -5 ~12 mesh, and
-4 +6 mesh. Speciific selected mesh ranges may be prepared by
known methods of crushing and sizing sintered, cemen~ed carbide
tool components, and which alloys are more commonly of a
cobalt or nickel-cemented t~mgsten carbide (WC) base, sometimes
containing al50 TiC~ TaC or NbC or combinations of these hard
carbides.
An additional useful aspect in the process of the present
invention is to apply a coati.ng oE an alloy or metal, preferably
Corson bronæe or nickel 3 on the surfaces o:E a dimens-loned
~intered cemented t~mgsten carbide insert of selected shape and
size, or a number of such inserts, which are then embedded in
a steel or iron-graphite matrix powder at selected loca-~ions
within a preform mold before the filled mold is isostatically
compacted. The corson bronze coating usecl may be either of the
two nominal compositions shown in Table I.
TABLE I
CORSON BRONZE COMPOSITIONS
A D
2.5 w/o Ni 10 w/o Mn
0.6 w/o Si ~ w/o Co
Oo25 w/o Mn 86 w/o Cu
Balance Cu
Following cold isostatic compaction and d~ring subsequent
sintering and hot isostatic pressing of the carbide~s~eel
compact~ the coating on the cemented carbide body au~ogenously
forms a diffusion bond, to increase the bonding strength wi~h
which dimensioned cemented carbide bodies are held in the
matrix. By this method, a cemented carbide body, or a n~mber
of them~ of specific shape and size may replace a dispersed
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hard carbide phase of particulate nature, and thereby form a
wear-resistant body or a tool for cutting or drilling metal
or rock.
It is recognized that the comparatively low processing
temperatures employed in the process of ~his invention may~ in
cases in which steel matrix powder compositions are used which
do not bond well to particles of a dispersed hard carbide phase,
result in inadequate bond strength ak ~he matrix~carbide par-
ticle interface. In such ~Lses ~ for example when alloy steel
powders are used which are known to be lesx sinterable at the
comparatively low solid-state sintering tempera~ures described
in the process of this invention9 it has been found beneficial
to precoat the hard carbide particles with nickel or copperS
for example, by known processes such as electroless metal
coating or by vacuum vapor-phase coating. Nickel coatings thus
applied to the hard carbide dispersed fraction, prior to blend
ing, have been found to improve carbide particle bonding strength.
Such precoating of the hard carbide particles would also be
beneficial when stainless steel powders are being used.
A further a~d useful part of the foregoing method îs the
incorporation of a dispersed hard carbide phase in a steel or
iron-graphite powder compact consisting of unmilled macro-
crystalline carbi.de crystals in size range ~ractions between
60 mesh and 400 mesh, -in the U. S. Sieve Series, and in
preferred mesh rallges, e.g., m-inus 60 plus 100 mesh, minus 80
plus 200 mesh, or minus 150 mesh plus 325 mesh, instead of and
in place of particles of cemented carbide. The method of the
present -invention for formulating and form:ing macrostructured
cemented carbide compositions is exactly as heretofore described.
The relatively low processing temperature practiced
reswlts in a macrostructure essent-lally free of brittle double
carbides of iron and t~mgsten (eta phase) and gross porosity.
The tendency for liquid-phase sintered, microstructured;
cemented tungsten carbide alloys ernploying a steel binderg for
example, in place of the usual cobalt binder, -to develop
brittle eta-type phases is well known. It is believed that the
avoidance of liquid phase sintering and consequen-~ly the
avoidance of carbon-transfer that such practice encourages, as
well as the uniquely low specific surface of the unmilled
macrocrystalline carbide particles comprising the dispersed
hard phase are essential for the success~ul forma~ion of the
two phase, steel-carbide macrostructures produced by this
method. It should be understood that liquid phase sintering
as referred to herein means sintering at a temperature at which
the steel binder is at least partially liquidO The prohibition
of liquid phase eintering in this invention, therefore, does
not apply to any lower melting point metals or alloys (e.g.,
copper or corson bronze) which may be added as a powder or
coating to promote bonding or densification, and may in~entionally
become liquid d~tring sintering or hot isostatic pressing.
The use of unmilled macrocrystalline hard carbide crystals
as a dispersed hard phase is a preferred embodiment of the
method of this invention, as an efficient means of maintaining
a hard phase possessing low specific surface. It is recognized,
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however~ that essentially binderless, hard aggregates of finer
or milled hard carbides may be so used.
An important aspect of the aforementioned macrostructured
bodies is the avoidance of ball milling or other co~inution
of ~he matrix-carbide powder mixtures~ or of either of these
two materials separa~ely, prior to cold isostatic compaction,
sinkering and HIPo The former practice, widely considered
essential to so~md commercial cemented carbide structures,
leads to enhanced reaction between carbi~es and iron-base matrix
powders with resultant formation of brittle double carbides.
Avoidance of powder milling also reduces cos~.
The method of the invention may employ any of the macro-
crystalline carbides 7 or combinations or solid solutions of
them, specifically WC, TiC, TaC or NbC, all exhibiting the low
specific surface and angular, blocky shapes typifying these
coarsely-crystalline mono and binary carbides. It is known
that primary macrocrystalline carbide materials may be finely
milled, together with cobalt or nickel, to form cemented-carblde
microstructures by liquid-phase sintering in the temperature
range 2400 degrees to 2800 degrees Fahrenheit~ in which the
resultant dispersed hard carbide phases are typically between
one micron and about ten microns. The method of the invention,
in contrast, results in dispersed, single macrocrystalline
carbide grains in size ranges selected from within the much
coarser extremes of 2S0 microns to about 40 microns.
This invent:ion is further explained by the following
egamples:
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EXAMPLF. N0
Wear resistant cutting tips were fabrlcated for rotary
sugar cane shredding machines. A uniformly blended mixture
composed of approximately 55 w/o 1/8 inch to 3/16 inch cobalt
cemented tungsten carbide granules, approximately 44.67 w/o
minus 100 mesh atomized iron powder and 0O33 w/o of minus 325
mesh graphite powder was prepared. During blending 5 w/o of
naphtha was added to minimi~e segregation of the higher~density
cemented carbide particles. The damp mixture was manually
compacted in~o an elastomeric polyurethane mold cavity of the
desired tool shape, d-imensioned to allow for cold isostatic
powder compaction plus one per cent sintering shrinkage.
Following cold isostatlc compaction a~ 35,000 psi, the compacted
preform was removed from the mold and vacuum sintered at 2000
degrees Fahrenheit for 60 minutes, following which ~he sintered
body was isosta~ically pressed at 2250 degrees Fahrenheit for
60 minutes at lS,000 psi under heliumO
Metallographic examination disclosed a matrix structure
composed of mostly pearlite and a little ferrite typlcal of
conventional slow-cooled 0.75 per cent carbon steel of low
porosity. The cemented carbide-matrix interfaces were occupied
by bands of a width of about 5 microns of an alloy believed to
be composed of iron and cobalt, principally. The cemented
carbide dispersed particles appeared unimpaired by thermal
cracking and no evidence of dissolution, melting or decomposi-
tion of the dispersed carbide phase exis~ed at or near the
interfacial boundaries, such boundaries being sharp except for
36
the aforementioned iron-cobalt alloy diffusion zone. No
potentially harmEul concentrations of eta phase were observed.
Test bodies were manually bent over a mandrel by ha~,mering at
room ~ perature and found to have a high resistance to impact
loading and to be essentially free of brit-tle fracture.
Figure 1 is a photomicrograph of a typical area in a
composite produced according to Example 1, except that sinter~
ing was done at 2100 degrees FahrenheitO A cobalt cemented
tungsten carbide granule 40 is shown metallurgically bonded to
a plain carbon steel having a mos~ly pearli-tic structure 50 by
a diffusion zone 45 containing iron and cobalt. The diffusion
~one 45 is approximately 3 microns thick.
EXAMPIE N0. 2
A wear-resistant, two inch square by 3/8 inch thick plate
was fabricated consisting of 60 w/o of unmilled minus 60 plus
100 mesh ma~rocrystallin~ WC cemented by 40 w/o of 0.75 per cent
C steel containing 2 w/o Cu. A uniformly dry blended migture
of minus 60 plus 100 mesh macrocrystalline WC crystals, minus
325 mesh graphite powder, minus 100 mesh iron powder, and minus
325 mesh copper powder were dry blended, unmilled, to a uniform
mixture~ then dampened by blending with liquid naphtha and
methyl cellulose equal, respectively, to 7 per cent and 0.5 w/o
of the dry mixture, and then packed into a steel preform mold
to a firm, green, plate shape of dimensions equal to approxi-
mately 102 per cent of the desired final dimension.
Following a-ir drying in the mold at room temperature, the
compact was removed from the mold~ placed in a rubber bag and
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further processed by cold isostatic compaction, sintering and
HIP as described in Example No. 1. Metallographic examination
revealed a macros-tructure of macrocrystalline WC evenly
dispersed -throughout a steel matrix. A 5 micron thick bond
layer of unknown composition was observed at WC-steel inter-
faces. These interfaces were free of brittle b-inary carbide
phases and cracks~
EX~MP E _ 0 3
A composite 1 1/2 inch cubic wear-r2sistant body of steel
enclosing a dimensioned plate of sinteredg cemented 5 w/o
cobalt~tungsten carbide was fabricated, purposefully embedd-ing
the dimensioned plate of sintered, cemented carbide in the
green powder prior to iso-compaction so that its outer surface
was flush with the outer surface of the steel cube. A dry
unmilled blend comprised of 97.25 w/o minus 100 mesh atomized
iron powder, 2 w/o minus 325 Cu powder and 0.75 w/o graphite
was made, then blended with naphtha and methyl cellulose equal
to, respectively, 5 w/o and 0.3 w/o of the dry blend. This was
then packed into an elastomeric mold following which a one inch
square by 1/4 inch thick plate of sint.ered cemented carbide
was pressed down into the iron powder mix so that the outer
surfaces were congruent.
The mold9 after sealing, was placed in a rubber bag,
evacuated, sealed and at this point was isostatically compacted,
removed from the mold, sintered and hot isostatically compacted
as in Example No. 1~ Metallographic examination revealed that
the prepositioned sintered carbide plate was bonded by a
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5 micron interfacial bond phase to the s~eel matrix surround-
ing it on th:ree sides and that the entire structure appeared
so~md and free of cracks.
Figure 2 presents a description of a wear plate 20 manu-
factured in the manner described in this example, except that
three rather one cemented carbide .inserts 30 are embedded in
the plate 20 such that a surface 45 of each insert 30 is sub-
stantiially flush with the working end 40 of the tool 20. It
will be noted that the interfacial bond 35 is substantially
uniform and continuous and forms a -tough and adherent bond
between ~he cemented carblde and the consolidated carbon steel
and copper matrix 25.
In certain wear applications, depending on the corrosion
nature of the environment ln whiich the wear plate will be used,
stainless steel or alloy steel powders may be advantageously
su~stittlted ~r the iron, carbon and copper powders utilized
in this example.
Figure 3 provides a cross sectional view of another
embodiment of a tool according to the present invention. This
~0 tool 1 can be manufactured substantially as described in
Example 3, except that the cemented carbide insert 5 is allowed
to have its working end 2 extend outward and beyond the steel
body 10 of tool 1. As shown in this figure, the insert 5 bonded
to the steel body 10 by a diffusion zone 15 which was formed by
the interdiffusion of cobalt from the insert 5 and iron from the
steel body 10 duriing high temperature and h1gh pressure sinter-
ing operations.
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3~ii
Modifications may be made with:in the scope of the
appended claims.
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