Note: Descriptions are shown in the official language in which they were submitted.
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~ he pre~ent invention relates to a novel process
for preparing titanium carbide base powder~ for the production
of cemented carbide alloys.
Conventionally, cemented carbide P-loy machine~,
tools and abra~ion-resistant parts are produced by press-
molding a powder composition such as WC-Co base, WC-TiC-
TaC base or like composition and sintering the resulting
molded mass by powder metallurgy techniques.
However, since tungsten is heavier than other
metals, the product prepared therefrom i9 difficult to handle
and install. ~urther because of the limited availability
of the tungsten ore, cemented carbide alloys produced there-
from are very expensive.
Titanium appears to be free of these problems and
serviceable as a substitute for tungsten used as a base metal
of cemented carbide alloys. For use in place of tungsten
as a material of cemented carbide~, titanium must be con-
verted to titanium carbide. ~or this purpose, three methods
are known: a first method in which TiO2 i9 reduced directly
with carbon, a ~econd method in which TiH2 is carbonized
with addition of carbon, and a thlrd method which is the
so called "menstruum process." With the first method, the
higher the temperature at which TiO2 is treated in a high
degree of vacuum, the higher will be the content of combined
carbon in the resulting TiC, but titanium oxide is produced
during the reducing process in the form of a eutectic mixture,
i.e. TiC-TiO, which is very difficult to reduce. Similarly,
the second method involves the formation of a TiC-Ti eutectic
mixture which is very difficult to carbonize. According to
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the third ~ethod, Ti i~ reduced in a bath of low-melting
reducing metal such as aluminum. In the course of the
reduction, Ti reacts with carbon mixed therewith or with
the carbon contained in the reaction vessel itself, forming
TiC crystals. The titanium carbide obtained by this method
has a combined carbon content most approximate to the
theoretical value. However, whichever of the foregoing
method~ is resorted to, the combined carbon content of TiC
industrially available at present is limited to 19.2 to
19.6g,which is lower than the theoretical value of 20~05U~
Thus it i9 very difficult to produce highly pure
TiC having a combined carbon content approximately equal to
the theoreticPl value. In other words, titanium carbide
heretofore available contains a relatively large amount of
free carbon. As a result, TiC-TiO,.TiC-Ti or like eutectic
.
mi~ture reacts with the nickel or cobalt binder used,
forming a double carbide phase, seriously reducing the
binding effect of the binder metal. ~oreover, TiC particle~
having a highly reactive surface tend to undergo ùxidation
on the surface, failing to combine with the binder metal with
` high bond strength. Accordingly although sintered alloys
-- ~ prepared from TiC particles can be harder than WC base alloys,
they have lower flexural strength and lower toughness than
WG base alloys. Thè TiC particles produced by the foregoing
methods fail to give alloys having the properties required
of cemented carbide alloy machine~, tools and abrasion-
re~istant parts.
Various other attempts have also been made for the
deYelopment of TiC b~se cemented carbide alloys. The TiC-
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Mo2C-Ni base alloy developed by Ford Company of U.S. in 1964
is ~nown to have high hardness of 90.5 in terms of Rockwell
A hardness as a TiC base cemented carblde alloy, but the
flexural strength of the alloy is as low as 60 to 120 kg/mm2,
thus failing to fulfill the requirement for use as a cemented
carbide alloy. Presumably, the inferior properties are
attributable to the fact that the TiO and Ti contained in -~
the TiC component impair the properties of Ni used as a
binder and also to the presence of an oxide which is formed
on the surface of TiC particles during sintering and handling
and which prevents effective bonding between the TiC particles
with the Ni binder. Because of these problems, hardly any
progress has been made ever since in the development of and
research on TiC-Ni base or TiC-Co base cemented carbide
alloys.
The present invention provides a novel process
for preparing titanium carbide base powders for the production
of cemented carbide alloys which process has been
accomplished based on extensive research I have carried out
in order to o~ercome *he drawbacks of the conventional
processes described.
One advantage of this invention is the provision of
titanium carbide base powders for the production of cemented
~ carbide alloys which are lightweight, easy to handle and
inexpensive to manufacture, without using tungsten which is
heavy and expensive because of the limited resource.
Another advantage of this invention is the provision
of titanium carbide base powders for the production of cemented
carbide-alloys whereln the oxide covering the surface of
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TiC particles has been removed to the greatest possible
extent and which contain TiC having a high content of com-
bined carbon approximate to the theoretical value, the
titanium carbide particles thus being rendered very compatible
wlth a binder metal and bondable with greatly improved
strength so as to give cemented carbide alloys fulfilling
the desired requirements in respect of hardness, flexural
strength and toughness.
Another advantage of this invention is the provision
of titanium carbide base powders which can be manufactured ..
with ease because an intèrmediate product thereof obtained ~ -
by sintering the starting material in the course of production
is readily crushable.
According to the present invention, there is provided
a process for prepraring a titanium carbide base powder for the
production of cemented carbide alloys comprising the steps of
molding a mixture of TiC powder starting material and an amount
of binder metal powder required for sintering.the TiC material,
- .the TiC material having a combined carbon content lowerthan the
theoretical value; sintering the molded mixture by heating
the Dixture at a high temperature of 1550 to 2500C under
conditions inert to TiC; maintaining the sintered product at the
: high.temperature to fully remove the resulting oxide from the
surface of the TiC particles and to thereby render the TiC -:
particles highly compatible with the binder metal, causing the
binder metal to fuse onto the surface of the TiC particles while
permitting the binder metal to form pores by its partial
pressure within the resulting binder metal phase of the
sintered product; and crushing the sintered product after
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cooling the product to thereby obtain a powder composed of
~iC particle~ having the highest po~sible content of com-
bined carbon approximate to the theoretical value, provided
with binder metal fused to the surface of the particles
~ith high strength and exhibiting high amenability to sintering.
The TiC powder used as the starting material i8
one commercially available and having a lower combined
carbon content than the theoretical value. With this in-
vention, the combined carbon content of the powder can be as low
as about 18.00~. Useful TiC powders include a mixture of
Ti powder and an amount of carbon required for preparing
TiC.
Examples of useful binder metals are those generally
used; e.g. Ni and Co which are used alone or admixture. The
ratio in amount of the binder metal or metals to the TiC
powder starting material is 50-10:50-90 by weight. Usually
the binder metal is thoroughly mixed with the TiC powder in
a ball mill for a prolonged period of time. Mo2C, WC and
TaC may further be added to the mixture as a sintering re-
20~ actlon accelerating agent. The resulting mixture is usuallypress-molded with use of a mold, but use of the mold is not
critical; for example the mixture may be manually molded.
The molded mixture is then sintered by being heated at a
high temperature of 1550 to 25~0 C under conaitions inert
to ~iC, namely in a vacuum or in an argon gas, nitrogen gas
br like inert gas atmo~phere. Experiments have revealed
that the most preferable sintering temperature is about 2200
C. With this invention, the heat treatment is not discontinued
immediately after the molded mixture ~s been sintered, but
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the sintered product i8 continuously maintained at a temper-
ature within the foregoing range for a period of time which
is variable with the heating temperature. Generally the
sintered product i9 maintained at the above-mentioned temper-
ature for a period of time required to fully deoxidize thesurface of the TiC particles, causing the binder metal to
firmly fuse onto the surface, and to increase the combined
carbon content of the TiC to the highest possible value
close to the theoretical valuei while permitting part of the
binder metal phase precipitated out in the sintered product
to vaporize and form pores in the metal phase due to the
partial pressure of the metal.
According to the present invention the oxide on
the surface of the TiC particles is~ removable to the greatest
possible extent and the combined carbon content of the ~iC
is increasable nearly to the theoretical value by the high-
- - temperature heat treatment, whereby the TiC particles are
rendered highly compatible with the binder metal, enabling
the binder metal to fuse to the surface of the TiC particles
with high strength. Consequently the titanium carbide base
- -` ~ powder obtained by the pre~ent proces~ i8 much more amenable
to sintering than the powders of the same type heretofore
-~ - available, with the resulting advantage that ~he cemented
carbide alloys prepared from the powder by re-sintering
have the hardness required of cemented carbide alloys as
well as satisfactory flexural strength and toughness and
are therefore well-suited for the manufacture of cemented
carbide alloy machines, tools and various wear-resistant
parts. -The titanium carbide base powder of this invention
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has another advantage that because expensive and heavy tungsten
is not used as a material, the cemented carbide alloy products
and parts prepared therefrom are lightweight, easy to handle
and inexpensive. Furthermore according to thi~s invention,
the sintered product obtained is readily crushable to the
desired powder due to the presence of the pores which are
fonmed by vaporizing part of the binder metal phase of the
sintered body through the high temperature treatment.
E~amples of this invention are given below.
:
Example 1
To 75~ by weight of TiC powder starting material
comprising commercial TiC powder (containing 19.7~ by weight
- of combined carbon and 0.1~ by weight of free carbon) and
0.55% by weight of carbon added to the TiC powder was added
~5 25~ by weight of carbonyl nickel as Ni, the ingredients
were~mixed together in a ball mill for 48 hours, and the
mixture was pressed mla-~old. The molded mixture was then
heated for sintering at 1500 C in a vacuum furnace to
obtain a test piece, which was found to have the following
20 ~ properties.
lexural strength: 90 k ~mm2.
Hardness, Rockwell A: 90.2.
Free carbon in the
sintered product: Corre~ponding to ASTM C4.
Pores in the
~sintered product: Corresponding to ASTM B4.
Thus the test piece had poor flexural strength. The
test piece was then heated at about 2400 C for 1 to 1.5
hours in an argon gas atmosphere which is inert to TiC,
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causing part of the nickel phase between the TiC particles
to vaporize and produce pores. The resulting product was
readily crushable due to the presence of the pores. The
TiC base powder obtained had a combined carbon content
approximate to the theoretical value and was composed of
TiC particles subsbantially free of oxide on their surface
and having Ni fused to the surface with high bond strength.
To 93.7~ by weight of the TiC-Ni powder ( 24~o by
weight of Ni and 1.6 ~ in particle size) were added 6.~o
by weight of Ni and 0.3~ by weight of C, the ingredients
were mixed together in a ball mill for 24 hours, and the
mixture was pressed in a mold with use of paraffin wax as
a binder-lubricant. The molded product was then dewaxed by
- - being heated to 1200 C and thereafter heated at 1400 C
;~ ~ 15 in a vacuum furnace for sintering. ~he sintered product
was found to have the following properties.
FlexuraI strength: 161 kg/mm2.
Hardness, Rockwell A: 88.80.
Free carbon in the
20~ sintered product: Corresponding to ASTM C2.
- Pores in the
sintered product: Corresponding to ASTM A1.
Thus the test piece had greatly improved flexural
strength.
~25 When the above procedures were repeated in the
ame manner as above except that 25~ by weight of Ni was
replaced by 25~ by weight of Co, or by equal amounts of Ni
and Co, subQtantially the same results as above were obtained.
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Example 2
To 71~ by weight of TiC powder starting material
comprising commercial TiC powder (containing 19,6~/o by weight
of combined carbon and 0.30~ by weight of free carbon) and
0.45~ by weight of carbon added to the TiC powder were
- added 25~ by weight of carbonyl nickel as Ni and 4.0~0 by
weight (theoretical amount) of Mo2C as a sintering reaction
accelerating agent, and the ingredients were mixed together
and molded in the same manner as in Example 1. The molded
mixture was then heated for sintering at 1520 C in a
vacuum furnace to obtain a test piece, which was found to
have the following properties.
Flexural strength: 112 kg/mm2.
Hardness, ~ockwell ~: 89~6~
Thus the test piece had poor flexural strength.
The test piece was then heated at a progressively increa~ing
temperature of from about-1500 to 1700 C and finally heated
to about 2200 C. The heating was continued for 1 to 1.5
houræ ln a hydrogen gas atmosphere which 1s inert to TiC,
; 20~ causlng part of the nickel phase between the TiC particles
to vaporizè and produce pores, whereby a skeletal product
wa~ obtained. The product was readily crushable due to the
presence of the pores. The ~ic base powder obtained had a
combined carbon content approximate to the theoretical ~alue
and was composed of TiC particles substantially free of
oxide on their ~urface and having Ni completely fused to
the 3urface.
To 94.75k by weight of the TiC-Mo2C-Ni powder
were added 5U~o by weight of Ni and 0. 25~o by weight of C, the
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ingredients were mixed together in a ball mill for 24 hours,
and the mixture was pressed in a mold with use of paraffin
wax as a binder-lubricant. The colded product was then
dewaxed by being heated to 1150 C and thereafter heated at
- 5 1470 C in a vacuum furnace for sintering. The sintered
product was found to have the following properties.
Flexural strength: 180 kg/mm2.
Hardness, Rockwell A: 88.20.
Free carbon in the
sintered product: Corresponding to ASTM C2.
Pores in the
sintered product: Corresponding to ASTM A1.
Thus the test piece had outstanding flexural strength.
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