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BACKGROUND OF THE INVENTION
The invention is directed to the fabrication of steel wire of various types
by a method which is capable of producing a quality product at high rates
of productivity. The method, which is a complete departure from
conventional practice, has particular significance when used in the
manufacture of filamentary steel wire of very fine diameter, for example,
in the range of from about 0.5 to 20 mils.
Although the conventional practice for manufacturing steel wire is capable
of producing a high quality product, the need for casting, repeated
mechanical reductions, intervening heat treatments and other required
operations render the resulting wire product relatively expensive. This
becomes apparent when one considers the steps involved merely to obtain
the intermediate steel wire rod product. That is, molten steel is cast
into ingots which are subsequently rolled into blooms from which billets
are formed. Finally, the billet is hot rolled to produce the steel wire
rod. The wire rod must then undergo a series of elaborate and costly
metal-drawing and heat treating operations to obtain a steel wire product
of the desired cross-section and mechanical properties.
It is, of course, obvious that the smaller the diameter of the wire
produced by the afore-mentioned processing procedures, the greater will be
cost of production. Yet, there has been an increasing demand for wire
having diameters in the range of 10 mils or less, and in some applications
less than 5 mils is desired. This demand has come about largely as a
result of the growing use of filamentary steel wire as a reinforcing
element in composite materials. For example, fine diameter steel is now
widely used to reinforce the rubber carcass of pneumatic tires. The steel
tire cord employed for this purpose is generally made from high carbon
steel, i.e., from 0.6 to 0.8 percent by weight of carbon. Because of the
relatively low product yield, wire drawing of such high carbon material to
attain fine diameters becomes excessively expensive. Moreover, because of
the loss of ductility resulting from the need to pull the wire through
numerous drawing dies, frequent intermediate heat treating steps are
needed to restore the ductility required for further drawing.
There is, therefore, a desire and a need for an alternative to the
conventional practice for producing steel wire wherein a product of
substantially equivalent properties can be produced at considerably less
cost.
A number of previous attempts have been made to meet this need, but for one
reason or another the methods proposed have not proved to be entirely
successful in practice. Perhaps, most noteworthy of the prior proposals is
a method wherein certain techniques of the ceramic arts are utilized. Such
method is described in U.S. Pat. No. 3,671,228 and involves a procedure
wherein a powdered agglomerate of iron oxide is mixed with a binder and
the mixture is placed in a die chamber where it is compacted and extruded
with a hydraulic press to form filaments. The filaments are then subjected
to a reducing atmosphere at a temperature below the sintering temperature
to effect reduction of the metal oxide to the metallic state followed by a
sintering of the reduced compact to form wire.
Although this prior method constitutes a significant advance in the art,
the brittleness of the precursor filaments makes further handling in the
conversion operations difficult. Moreover, the high pressures required to
form the precursor add to processing costs.
It is, therefore, an object of this invention to provide an entirely new
approach to the production of filamentary wire.
It is further object of this invention to provide a method for producing
filamentary steel wire which is substantially less costly than the
conventional practice.
It is a still further object of this invention to provide a method for
producing steel wire products which have outstanding mechanical
properties.
SUMMARY OF THE INVENTION
In carrying out the process, a precursor filament consisting of an acrylic
polymer with particles of iron oxide entrained therein is first formed.
This is accomplished by employing wet-spinning techniques such as are
commonly used in the textile arts for the production of acrylic fibers.
That is, a spinning dope is made up consisting of a uniform dispersion of
iron oxide particles in an acrylic polymer solution with the ratio by
weight of iron oxide to acrylic polymer being in the range of about 3:1 to
7:1. The iron oxide containing acrylic polymer spin dope is then spun
through a spinnerette and directly into a coagulation bath to form the
precursor filament. The filamentary precursor is converted to steel wire
by exposing the filament to a reducing atmosphere (e.g., a gaseous mixture
of hydrogen and carbon monoxide) for a period ranging between about 3 to 8
minutes at a temperature in the range of from about 900.degree. C. to
1150.degree. C. Under these conditions, the iron oxide particles are
reduced to the metal state, and the polymer in the precursor is pyrolized
to carbon and by-product gases. The carbon is absorbed by the metallic
iron, and the individual metal particles are caused to sinter to form
continuous steel wire.
Optionally, the precursor filament may be drawn or stretched following the
formation thereof to improve its tenacity for further handling. Also, the
toughness of the precursor may be improved by a shrinking operation which
can be conducted immediately subsequent to the drawing procedure. In
addition, the tensile properties of the ultimate steel wire product can be
enhanced by a conventional heating and quenching treatment to produce a
tempered martensitic structure. It is also within the purview of the
invention to combine other reducible metal compounds in particulate form
together with the iron oxide particles when making up the spin dope in
order to produce steel alloy wire.
DESCRIPTION OF THE INVENTION
In the context of this invention, the term "acrylic polymer" refers to a
fiber-forming polymer and includes polyacrylonitrile and copolymers and
terpolymers of acrylonitrile. That is, those copolymers and terpolymers
are included which are obtained by polymerizing acrylonitrile with
monomers such as vinyl acetate, methyl acrylate, vinyl pyridine and others
which are known by those skilled in the art to be polymerizable with
acrylonitrile to give satisfactory fibers and filaments.
As used herein, the term "iron oxide" is intended to include both hematite
(Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4) or mixtures thereof.
Also, in the context of this invention the term "filamentary steel wire"
has reference to an elongated structure which may be either circular or
rectangular in cross-section. When rectangular, the structure has a ribbon
configuration with the aspect ratio of thickness to width being generally
in the range of about 1:20. The elongated structures of circular
cross-section generally have diameters in the range of from about 0.5 to
20 mils and may be solid or hollow. In the latter case, a thin wall tubing
is provided.
As indicated, the iron oxides suitable for the purposes of this invention
consist of either hematite (Fe.sub.2 O.sub.3) or magnetite (Fe.sub.3
O.sub.4) or mixtures of the two. The iron oxide needs to be in particulate
form and in order to achieve the density desired in the ultimate wire
product the metal particles should possess a good distribution in particle
size. However, the average diameter of the particles should not exceed
about 5 microns, with an average diameter of about 1 micron or less being
usually preferred.
An excellent source of hematite is the by-product obtained in the
regeneration of hydrochloric acid pickling solutions which are used in the
iron and steel industry to remove mill scale and other forms of iron oxide
from iron and steel products. The procedure includes a reaction chamber
which converts the ferrous chloride to ferric oxide and regenerates
hydrogen chloride gas. The regenerated hydrogen chloride gas is absorbed
in water and the hydrochloric acid obtained is recycled to the pickling
bath. The hematite recovered as by-product is in the form of small
particles caused by the turbulence of the hot gases in the reaction
chamber.
Another source of suitable iron oxides is the high grade iron ore
concentrates (more than 95 percent by weight of iron oxide) which are
available in various parts of the world. An example is the MAC Maimberget
A concentrate ore from Sweden which contains over 98 percent by weight of
iron oxide (i.e., 96.23 percent magnetite and 2.24 percent hematite).
For convenience, the invention will now be described in terms of its
utilization in the production of steel wire, although as previously noted,
the method is also applicable in the production of steel alloy wire.
In making up the spin dope from which the precursor filaments are produced,
the iron oxide particles are incorporated into a typical acrylic polymer
spinning solution in the form of a uniform dispersion. The solvent may be
selected from those commonly used in the wet-spinning of acrylic polymers
(e.g. dimethylacetamide, dimethylformamide and dimethylsulfoxide) with the
ratio by weight of solvent to polymer being in the range of from 3.5:1 to
6:1, and preferably 3.8:1 to 4.5:1, respectively. The iron oxide particles
are added in an amount such that the ratio by weight of metal oxide to
acrylic polymer is in the range of about 3:1 to 7:1, respectively.
Although not required, it is sometimes advantageous to add small amounts
of a wetting agent to the dope (e.g., less than 1.0 percent by weight of
sorbitan monopalmitate). Following make-up, the dope components are mixed
by well known methods to solubilize the polymer and to obtain a uniform
dispersion of the metal oxide.
Filamentary structures are formed from the afore-described spin dope by
continuously extruding the dope through a desired number of shaped
orifices in a spinnerette and directly into a coagulation bath. The
pressures required to give satisfactory extrusion rates are nominal and
generally do not exceed 50 psig, with the normal range being from about 10
to 50 psig. The orifice design will, of course, determine the
configuration of the filament. Aside from the standard filament of
circular cross-section produced by a round orifice, a rectangular slit
will produce a filament having a ribbon configuration.
Also, many orifice designs are known in the art for producing a hollow or
tubular filamentary structure such as, for example, a segmented arc
configuration, plug-in-orifice and others such as disclosed in U.S. Pat.
No. 3,405,424.
As is typical in the wet-spinning of acrylic fibers, the coagulation bath
contains both a precipitant and a solvent for the acrylic polymer. The
precipitant or coagulant is generally either water or ethylene glycol. And
although a wide variety of solvents are applicable, solvents such as
dimethylacetamide, dimethylformamide and dimethylsulfoxide are generally
of preference both in conventional acrylic fiber spinning and in the
practice of this invention. For convenience, it is usually desirable to
employ the same solvent as was used in preparing the spinning dope.
For the purposes of this invention a binary mixture of water and
dimethylacetamide or ethylene glycol and dimethylacetamide is usually
preferred. When employing the former the solvent is generally present in
the range of from about 30 to 70 percent by volume, with from 50 to 60
percent being preferred. When employing ethylene glycol as the coagulant
in lieu of water, the dimethylacetamide solvent generally constitutes from
about 15 to 85 percent by volume of the mixture, with from about 40 to 60
percent being preferred.
With water/dimethylacetamide systems the bath temperatures are those
conventionally employed and can range between 28.degree. C. and 70.degree.
C., with from about 35.degree. C. to 60.degree. C. being preferred. In the
case of ethylene glycol/dimethylacetamide mixtures, the bath temperature
may range between 0.degree. C. to 95.degree. C., with 10.degree. C. to
30.degree. C. being usually preferred. An especially preferred coagulation
system is one comprised of a mixture of ethylene glycol and a
dimethylacetamide solvent, with the solvent constituting from about 40 to
60 percent by volume of the mixture. In operation, the coagulating bath
containing these components is preferably maintained at a temperature in
the range of from about 10.degree. C. to 30.degree. C.
In some instances, it may be desirable to add an acrylic plasticizer (e.g.,
N, N-dimethyl lauramide) to the coagulation bath. When used, this optional
ingredient is generally present in an amount not exceeding 0.1 percent by
weight of the coagulation composition.
Although acceptable precursor filaments are produced following the
afore-mentioned filament-forming operations, improvements can be imparted
by an additional stretching or attenuation step. That is, the coagulation
step may be followed by a polymer orientation step in which the filaments
are stretched from about 1 to 3 times their initial length in a
conventional hot water or boiling water stretch bath. This orientation and
attenuation procedure, which greatly improves filament strength and
productivity, is generally referred to as a "hot cascade" stretch.
Stretching is accomplished by correlating the linear entry rate of the
filaments into the stretch bath with the rate of withdrawal. When the
latter is at a higher rate, stretching of the filament will, of course,
occur.
Although again optional, further advantages can be realized by following
the stretch operation with a shrinking step. This is also accomplished by
continuously passing the filaments through a hot or boiling water bath.
However, in contrast to the stretching procedure, the filaments are
withdrawn from the bath at a speed sufficiently slower than the feed speed
to allow relaxation and shrinkage to occur. The extent of shrinkage is
usually much less than the stretch originally imparted. In general, the
ratio of the length of the filaments before and after shrinking is in the
range of from about 1:0.9 to 1:0.7, respectively. The purpose of this
processing step is to improve the toughness of the precursor filaments and
to minimize the extent of shrinking which occurs when converting the
precursor to steel wire.
Conversion of the precursor filaments to steel wire is effected by exposing
the filaments to a reducing atmosphere at a temperature in the range of
from about 900.degree. C. to 1150.degree. C. over a time span of from
about 3 to 8 minutes. Under these conditions, the iron oxide particles are
reduced to iron, the polymer in the precursor is converted to carbon and
by-product gases with the carbon being absorbed by the iron, and the
individual metal particles sinter to form continuous steel wire.
It has been found that good results are achieved when the reducing
atmosphere is comprised of a gaseous mixture consisting of about 80 to 98
percent by volume of hydrogen, from 2 to 15 percent by volume of carbon
monoxide and from 0 to 10 percent by volume of a carburizing gas. In
addition to contributing to the reduction of iron oxide to iron, the
carbon monoxide serves to control the absorption of carbon into the iron.
An especially efficient reduction is realized when the hydrogen component
of the reducing atmosphere contains a mixture of both atomic and molecular
hydrogen. That is, atomic hydrogen will diffuse more readily into the
interstices of the metal oxides than will molecular hydrogen because of
its smaller size and weight. This faster diffusion rate will, of course,
facilitate reduction. In addition, the presence of atomic hydrogen
increases the inherent reduction power of the system. A carburizing medium
may be included in the reducing gas mixture, if desired, to provide an
additional source of carbon to further enhance the tensile strength of the
ultimate steel wire product. When used, the carburizing gas may be
selected from the hydrocarbon gases commonly used in the steel industry as
a carburizing medium to supply a quantity of carbon for absorption and
diffusion into steel. Included among such gases are methane, ethane,
propane and butane, with methane and propane being especially preferred.
In a preferred mode for carrying out the precursor conversion step of the
process, the precursor filaments are continuously processed through an
elongated furnace which has been heated to an appropriate temperature. The
reducing gases are caused to flow within the furnace in a reverse
direction to the direction of movement of the wire being formed. In this
manner the wire never "sees" an oxidizing environment until the process is
complete and the wire exits the furnace to a take-up device.
In addition to effecting a reduction, the reducing gases cool the moving
wire at the point of contact therewith to produce a pearlite structure of
relatively fine grain structure. The tensile properties of the resulting
steel wire product may be improved by conversion to a tempered martensite
structure. This can be accomplished by well-known methods which involve
heating to a relatively high temperature, quenching and then reheating to
a lower temperature. For example, the steel wire may be heated
continuously in a furnace to the austenitic temperature and then quenched
in oil or water. This is followed by a post-tempering in oil.
Attention is now directed to the attached drawing which illustrates the
types of apparatus which may be employed in carrying out the method of
this invention.
FIG. 1 is a side elevational view partly in section showing an apparatus
arrangement of the type which can be used to form the precursor filaments.
FIG. 2 is a schematic side elevational view partly in section illustrating
a furnace arrangement suitable for use in converting the precursor
filaments to steel wire.
Referring now to FIG 1, a spin dope consisting of an acrylic polymer
solution with iron oxide particles uniformly dispersed therein is pumped
from supply tank 10 by pumping means 12 through filter 14 and thence to
spinnerette assembly 16. The dope is extruded through the filament shaping
orifices of the spinnerette and passes directly into coagulation bath 18
where the filaments are formed. From the coagulation bath the filaments
are withdrawn over guide means 22 by positively driven filament advancing
rolls 24 and 26. When on these rolls, the filaments are water washed to
complete the coagulation and to remove residual solvent. The water is
supplied from a spray or shower head 28, with the wash water being
collected in a container or tray 30. It will be recognized that the
washing operation can be conducted in more than one stage of the process
and by the employment of other known washing means. After leaving rollers
24 and 26, the filaments are directed into a "hot cascade" bath 32 which
contains hot or boiling water. The filaments are withdrawn therefrom by
means of driven rollers 34 and 36, which are operated at a peripheral
speed greater than that of rolls 24 and 26 so that the filaments are
caused to stretch during passage through hot water bath 32. After leaving
rollers 34 and 36, the filaments are directed into a second hot or boiling
water bath 38. They are withdrawn from bath 38 by means of rolls 40 and 42
which are driven at a peripheral speed less than that of rolls 34 and 36
so that the filaments are permitted to relax and thereby shrink during
passage through the bath. In order to keep the filaments moist and thereby
faciliate processing, water is dripped on rolls 34 and 36 through pipes 44
and 46. Likewise water is dripped onto rolls 40 and 42 through pipes 41
and 43. From rolls 40 and 42 the filaments are passed over guides 48 and
50 and onto take-up device 52.
Referring now to FIG. 2 which illustrates a type apparatus which may be
used to convert the precursor filaments to steel wire. An elongated
heating chamber 54 is shown having its mid-section encased in an insulated
housing member 56 in which resistance heating elements (not shown) are
embedded. A gas inlet tube 58 for introducing reducing gases is inserted
into one end of the elongated heating chamber 54 and flare tubes 60 and 62
for gas burn-off are provided at each of the opposing ends of the chamber.
An endless steel belt 64, is provided for carrying the filaments being
processed through heating chamber 54 in a direction opposite to the flow
of gas entering the system from gas inlet tube 58. Upon exiting the
heating chamber the steel wire obtained passes through the nip of spring
loaded tension rolls 66 and 68 and onto a take-up device 86.
To further supplement the description of this invention, the following
illustrative Examples are presented.
EXAMPLE 1
This example illustrates a run in which the iron oxide particles employed
consisted of hematite (Fe.sub.2 O.sub.3).
A solvent mix, consisting of 850 cc of dimethylacetamide, 0.5 cc of
ethylene glycol, and 1.2 cc of sorbitan monopalmitate, was intimately
mixed with 1000 grams of hematite in a rod mill for 10 hours. The
resulting slurry was then transferred to a large Waring blender where it
was chilled to a temperature of 5.degree. C. after which a copolymer
consisting of 93 percent by weight of acrylonitrile and 7 percent by
weight of vinyl acetate was added. The solvent was chilled to reduce its
solvency so that the polymer could be dispersed mechanically with only
small amounts going into solution. The Waring blender was then brought to
high speed and further blending of the oxide and complete solution of the
polymer took place. The blender was turned off when a final temperature of
42.5.degree. C. was obtained as sensed by a thermocouple in the mixture.
The heat for the temperature rise resulted from the degradation of
mechanical energy supplied by the blending device. During the mixing
period, a vacuum of 22 inches of mercury was pulled on the contents of the
blender to reduce the amount of air entrapment in the precursor mix.
The contents of the blender were transferred to the dope pot of a
wet-spinning line where the precursor mix was subjected to a vacuum of 22
inches of mercury for 1/2 hour and then pressurized to 35 psi. for 1/4 of
an hour. This step was undertaken to again reduce entrained air that could
cause voids in the precursor filaments. A positive displacement pump was
used to deliver 14.6 cc per minute of the precursor dope through a filter
stack having a final stainless steel screen of 120 mesh and then through a
cup spinnerette which had five holes each of 20 mils in diameter. Upon
emerging from the spinnerette, the dope threadlines entered a coagulation
bath which was at a temperature of 24.degree. C. The coagulation system
employed consisted of a mixture of 50.2 percent by volume of ethylene
glycol and 49.8 percent by volume of dimethylacetamide. An acrylic
plasticizer (N,N-dimethyl lauramide) was also present in an amount of 0.1
percent by weight based on the weight of the coagulating mixture. The
threadline was taken up at the first godet (thread advancing rolls) at 20
feet per minute and washed with the bath solution to continue the gentle
coagulation process. The second godet received the threadline at the rate
of 20 feet per minute. Here the threadline was washed with water to
complete the coagulation. Then the precursor threadline was stretched in
boiling water, to orient the fibers. This step occurred between the second
godet and the third godet which moved at a rate of 50 feet per minute.
Relaxation of the threadline occurred in boiling water between the third
and fourth godet which rotated at the rate of 40 rpm. On leaving the
fourth godet the precursor threadline was taken up on a Leesona winder.
The take-up bobbin from the spinning line was placed in the feed position
of a furnace-conversion system, and a threadline was fed at a rate of
approximately 17 inches per minute into the furnace on a belt moving at a
rate of 7.5 inches per minute. The precursor filaments remained in the
furnace for 3.2 minutes at a temperature of 1070.degree. C. The difference
in the rates of movement between the belt and the precursor feed takes
into account the shrinkage of the threadline which occurs during the
conversion operation. To coordinate the feed rate of the threadline with
the belt movement, the threadline position before entering the furnace was
sensed by a photoelectric relay. A mixture of reducing gases was fed into
the furnace near its exiting end at a rate of 15 liters per minute. The
composition of this gas mixture consisted of 92.0 percent by volume of
hydrogen, 4.6 percent by volume of methane, and 3.4 percent by volume of
carbon monoxide. The steel wire product obtained was of an essentially
ferritic-pearlitic structure with a carbon content of 0.70 percent .+-.
0.10 percent. Instron measurements gave a tensile strength of 122,000 psi.
at a 3.4 percent elongation.
To convert into a tempered martensite, the wire was heated continuously in
a furnace to 830.degree. C. and then quenched in water to give a
martensitic structure. Post-tempering to 250.degree. C. in oil for 5
minutes gave a tempered-martensitic structure having a tensile strength of
215,000 psi.
EXAMPLE 2
This example describes a run wherein the metal oxide employed consisted of
a mixture of hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4).
Five hundred grams of hematite, 500 grams of magnetite, and 250 grams of a
copolymer consisting of 93 percent acrylonitrile and 7 percent vinyl
acetate were intimately mixed in a rod mill for 10 hours. A solvent mix
consisting of 850 cc of dimethylacetamide and 0.5 cc of ethylene glycol
was chilled to 10.degree. C. and placed into a large Waring blender. The
mixture of oxides and polymer was then transferred to the blender and
stirred-in by hand to give a reasonably uniform mixture. The solvent was
chilled to 10.degree. C. to reduce its solvency and allow the polymer to
be dispersed mechanically with only small amounts going into solution. The
Waring blender was then brought to high speed and further blending of the
oxide and complete solution of the polymer took place. The blender was
turned off when a final temperature of 42.5.degree. C. was attained as
sensed by a thermocouple in the mixture. The heat for the temperature rise
resulted from the degradation of mechanical energy supplied to effect
mixing. During the mixing period, a vacuum of 22 inches of mercury was
pulled on the contents of the blender to reduce the amount of air
entrapment into the precursor mix.
The contents of the blender were transferred to the dope pot of a filament
spinning line. Here the precursor mix was subjected to a vacuum of 22
inches of mercury for one-half hour and then pressurized to 35 psi. for
1/4 of an hour. This step was undertaken to again reduce entrained air
that might cause voids in the precursor fiber. A positive displacement
pump was used to deliver 14.6 cc per minute of the precursor dope. The
dope was first passed through a filter stack having a final stainless
steel screen of 120 mesh and then entered a cup spinnerette which had five
holes each of 20 mils in diameter. Upon emerging from the spinnerette, the
dope threadlines entered a coagulation bath which was at a temperature of
24.degree. C. The coagulation system employed consisted of a mixture of
50.2 percent by volume of ethylene glycol and 49.8 percent by volume of
dimethylacetamide. An acrylic plasticizer (N,N-dimethyl lauramide) was
also present in an amount of 0.1 percent by weight based on the weight of
the coagulating mixture. The threadline was taken up at the first godet at
20 feet per minute and washed with the bath solution to continue the
gentle coagulation process. The second godet received the threadline at
the rate of 20 feet per minute. Here the threadline was washed with water
to complete the coagulation. The precursor filaments were then stretched
in boiling water. This step occurred between the second godet and the
third godet which rotated at a rate of 50 feet per minute. Relaxation of
the threadline occurred in a boiling water bath between the third and
fourth godet which rotated at the rate of 40 feet per minute. On leaving
the fourth godet the precursor threadline was taken up on a Leesona
winder.
The bobbin from the spinning line was placed in the feed position of a
furnace conversion system. A threadline from the bobbin was fed at a rate
of approximately 13 inches per minute into the furnace on a belt moving at
a rate of 5.0 inches per minute. The precursor filaments remained in the
furnace for 4.8 minutes, with the furnace being at a temperature of
1100.degree. C. The difference in the rate of movement between the belt
and the precursor feed accounts for the shrinkage of the threadline during
the conversion operation. To coordinate the feed rate with the belt
movement, the position of the threadline before entering the furnace is
sensed by a photoelectric relay. The reducing gases were fed into the
furnace near the exit end at a rate of 15.6 liters per minute. The
composition consisted of 88.2 percent by volume of hydrogen, 6.7 percent
by volume of methane, and 5.1 percent by volume of carbon monoxide. The
steel wire product obtained was of an essentially pearliticferritic
structure with a carbon content of 0.70 percent .+-. 0.10 percent.
Instron measurements gave a tensile strength of 142,000 psi. at a 3.9
percent elongation.
To convert into a tempered martensite, the wire was heated continuously in
a furnace to 830.degree. C. and then quenched in oil at 100.degree. C. to
give a martensitic structure. Post-tempering to 280.degree. C. in oil for
5 minutes gave a tempered-martensitic structure having a tensile of
265,000 psi and an elongation of 1.6 percent.
As shown by the above examples, the method of this invention is capable of
producing steel wire with outstanding tensile properties. That is, steel
wire of an essentially ferritic-pearlitic structure and a carbon content
in the range of from 0.6 to 0.8 by weight can be produced with tensile
properties exceeding 140,000 psi, and when converted to tempered
martensite, tensile properties substantially in excess of 260,000 psi are
attainable (see Example 2). In addition, proportionally high densities are
realized. That is, products exhibiting a density of between 97.6 percent
and 98.6 percent of that which is theoretically possible have been
produced routinely.
Although the invention has been described with particular reference to
steel wire, the method may also be employed to produce high density, steel
alloy wire. This is readily accomplished by merely combining one or more
other metal oxides with iron oxide when making up the spin dope used to
form the precursor filament. Such spin dope will then contain a mixture of
metal oxide particles dispersed in an acrylic polymer solution, with the
particles having an average diameter of less than about 5 microns and the
weight ratio of combined metal oxide to acrylic polymer being in the range
of from 3:1 to 7:1. Any metal oxide may be used in combination with iron
oxide so long as the range of conditions by which it may be reduced and
sintered overlap with those of iron oxide. Among others, nickel oxide and
cobalt oxide are exemplary of compounds which may be suitably combined
with iron oxide to produce alloyed steel wire. The proportions of the
various metal oxides can be widely varied according to the properties
desired in the ultimate product.
Although the invention has been described with respect to details of the
preferred embodiments, many modifications and variations which clearly
fall within the scope of the invention as defined by the following claims
will become apparent to those skilled in the art.
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