Metallurgical process

The present invention discloses a method for densifying previously sintered parts constructed of powdered metals, ceramics or the like to nearly 100% theoretical density. The method of the present invention comprises heating the parts containing binder and hard phase above their liquid phase temperature and then applying a pressure in a predetermined range to the parts for a predetermined period of time and simultaneously maintaining the parts at or above their liquid phase temperature. This pressure range is set so that the pressure is below the pressure necessary to overcome the capillary force acting on the binder to keep the binder from entering the voids but above the pressure necessary to physically move or collapse the microstructure inwardly, thus filling the voids with a homogenous mixture of binder and hard phase. The method of the present invention achieves complete closure of even large voids and the elimination of substantially all porosity within the part.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
The present invention relates to a method for densifying previously 
sintered parts of powdered metals, ceramics, hard metals and the like 
which are sintered in the presence of a liquid phase. 
II. Description of the Prior Art 
In the liquid phase of sintering powdered metals, ceramics, hard metals and 
the like, the powdered material which may comprise a powdered hard phase 
and powdered metal binder, is first intermixed with a fugitive binder 
which holds the part in the desired shape after cold pressing. Usually 
this fugitive binder consists of paraffin, polyethelene glycol, or a metal 
containing hydrocarbon or other organic plasticizer. The cold pressed part 
is conventionally known as a preform. 
The preforms are then subjected to a presintering step in which the 
preforms are slowly heated, thus vaporizing the fugitive binder and the 
vaporized binder is removed from the part by a wash gas, vacuum pumping or 
other means. Following the presintering step, the parts retain their shape 
due to some solid state sintering of the powdered binder. 
The parts are then subjected to a sintering operation in which the parts 
are raised to a temperature where some liquid phase appears and some 
solutioning of the hard phase occurs. This appearance of the liquid phase 
relatively rapidly densifies the part due to capillary action and 
solutioning effects. Following the sintering of the parts, the parts are 
sufficiently dense and hard for many applications. 
In the WC-Co system, these sintering parts comprise hard phase paticles 
such as tungsten carbide, held together by a binder such as cobalt. 
Following the sintering process, the parts may contain voids surrounded by 
a mix of hard particles and binder in which the hard phase particles are 
spaced from each other by a distance less than the width of the void size. 
For applications requiring still further densification, greater strength of 
the sintered part or better internal integrity, the parts may be further 
treated. Several types of treatments to accomplish this reduction in 
porosity are known in the prior art. 
All of the previously described methods, although effective in reducing 
porosity and/or voids, suffer from problems in effectively producing a 
homogenous structure free from stress risers or internal stresses which 
can be detrimental to the performance of the material. 
WC-Co alloys are ternary in nature due to the three constituents involved; 
mainly tungsten, carbon and cobalt. Because of this when the solidus or 
eutectic temperature is reached, all of the binder does not melt but 
rather this melting occurs over a range of temperatures. Also, some amount 
of solutioning of the WC occurs as the temperature is increased which may 
decrease the amount of solid remaining and increase the amount of liquid. 
The amount of liquid present at any given temperature in the three phase 
region where melting occurs, is very dependent on the exact composition of 
the WC-Co alloy under consideration. 
FIG. 17 shows a vertical quasi-binary section of the WC-Co ternary phase 
diagram parallel to the C-W binary at 16% Co. This figure is from a paper 
by J. Gurland--Journal of Metals 6,285,1327 (1957). Similar phase diagrams 
will apply to other hard metals. 
For WC-Co alloys to perform properly, they must avoid free carbon or carbon 
deficient phases in the microstructure. FIG. 17 shows that the useful area 
for WC-Co alloy of 16% cobalt is the WC+.gamma. field which ranges from 
6.00-6.13 wt. % carbon. (.gamma. represents solid binder). A typical 16% 
WC-Co alloy would contain carbon in this range. When sintering such an 
alloy, some liquid phase should be present and this can be obtained in the 
WC+.gamma.+Liq field or in the WC+Liq field, i.e. all binder (.gamma.) has 
dissolved in the latter case. This would preferably be in the WC+Liq field 
as this would give the largest possible amount of liquid. The temperature 
at which the WC+liquid field is entered however, ranges from 1360.degree. 
C. to 1450.degree. C. depending on the exact carbon content of the alloy. 
At temperatures below this down to approximately 1300.degree. C., smaller 
and smaller amounts of liquid are available. For WC-Co alloys of other Co 
percentages, the temperature shown for the various fields will go up or 
down, depending on composition. In particular lower Co contents raise 
these temperatures and higher cobalt contents lower these temperatures. 
Also lower cobalt contents narrow the WC+.gamma. field and higher Co 
contents widen the WC+.gamma. field. 
The amount of liquid phase and the amount of WC solutioning have a great 
effect on all of the methods used to reduce porosity and voids in WC-Co 
alloys. This effect is due to the changes in capillary and other forces 
upon the physical situation of the material at the time that the porosity 
reduction process is applied. 
One such process if commonly referred to as hot isostatic pressing (HIP). 
In this process, the parts are placed in a pressurizable furnace and 
pressurized to approximately 5000 psi cold and then elevated to the 
solidus temperature and above. At this temperature (above approximately 
1250.degree.-1350.degree. C.) the pressure is increased to above 10,000 
psi due to the thermal expansion of the gas used to initially pressurize 
the system at room temperature. The primary advantage of HIP processing is 
to virtually eliminate all porosity within the part as well as greatly 
minimize or eliminate larger randomly spaces slits, holes or fractures 
which may be present in the part provided that such holes, slits, or 
fractures are not open to the surface. During the HIP process, as the 
parts are heated above the solidus, the binder, e.g. cobalt, begins to 
become molten and the spaces between the hard particles begin to form 
capillary passageways which are open to the voids in the part. In the 
absence of pressure applied to the part, the capillary force created by 
these passageways would prevent any molten binder contained within the 
part from entering the voids in the part. These capillary forces pulling 
the molden binder away from voids in the part can range from 20 to 1500 
psi depending on the size of the flaws present and the size of the 
capillaries between the grains. These factors depend on the specific WC-Co 
alloy under consideration and the specific temperature for which the 
calculations are done. 
During the HIP process, extremely high pressures (10,000-15,000 psi) are 
applied to the parts at a temperature below the sodius and subsequently 
the temperature is raised to above the solidus. When the solidus 
temperature is reaches, some liquid phase is formed. However, the 
structure will still be rather rigid since all of the binder will not have 
melted (WC+.gamma.+Liq field of FIG. 17). The high pressure applied to the 
parts at this time can then overcome the capillary forces and push the 
binder into any voids which might be present. This is well known in the 
art as "binder laking". This "binder laking" is one indication that some 
carbide users look for to be sure that a material was indeed HIP 
processed. 
An example of such "binder laking" is shown in prior art FIG. 15 in which 
16% WC-Co alloy part was subjected to the HIP process (1500.times. 
magnification). FIG. 15 (500.times. magnification) also shows a Hughes 
Tool Co. M.P.D. grade 168 after HIP processing. Large cobalt lakes are 
evident throughout the parts in both FIGS. 15 and 16. Although laking is 
preferable to porosity, it is much less desirable than a more homogeneous 
microstructure. Any discontinuity in a material including a "binder lake" 
will act as a stress riser when the material is subjected to stress and 
thus may shorten its useful life in a given application. 
A further disadvantage of HIP processing is that due to the high 
temperatures and extremely high pressures used during the HIP processing, 
the previously known HIP equipment is extremely massive in construction 
and expensive to acquire. Also, HIP processing is a secondary process and 
requires that the parts be dewaxed and sintered in other equipment prior 
to being placed into the HIP equipment for that process to be effected. 
Other methods wherein the sintering and hot isostatic processing may be 
carried out in one piece of equipment are disclosed in various prior art 
publications, some of which are discussed below. 
Dr. Wolfgang Schedler, Reutte in Austrian Pat. No. 314,212 discusses a 
process for sintering alloys with a liquid phase in which, after reaching 
the eutectic temperature of the binder phase and a stage in the sintering 
shrinkage resulting in external sealing of still existing pores, the 
powder compacts are exposed to the isostatic pressure of an inert gas. He 
further states that "At about 50 vol % binder metal, a pressure of several 
bars is sufficient to achieve the effect of the invention," (hole closure) 
"while the final pressure should be about 200 bar or more below a binder 
metal of 10%. 
In his example #1, a 25% cobalt material is heated to 1300 Degrees C. under 
vacuum and then subjected to 15 bar of argon pressure. The material is 
then further heated to 1320 Degrees C. Material prepared in this way 
performed 11/2 times better in a stamping application than did material 
prepared in a similar manner but without pressurization. 
Similar results were achieved in example #2 with a 9% cobalt WC alloy 
processed at 1400 Degress C. and 100 bar of argon pressure. 
Example #3: A 5% cobalt material using 1420 Degree C. and 150 bar of 
pressure. 
Example #7: 10% cobalt WC material using 1390 Degress C. at 180 bar of 
argon pressure. 
Example #9: An 8% cobalt WC material at 1350 Degrees C. and 130 bar of 
argon pressure. 
In all of the above cases, especially #1 and #9, the material was 
pressurized at a temperature where the material would have minimal liquid 
phase present (FIG. 17) and to a pressure that would have overcome 
capillary forces and yet the bulk material would have considerable 
resistance to the macro movement of the microstructure due to the high 
proportions of the solid phases. This would be indicated by the statement 
that these relatively large, 100 to 200 bar (greater than capillary 
forces), pressures are needed to effect hole closure. Thus the voids and 
or porosity may, to some degree, be filled by the eutectic liquid and give 
relatively the same performance improvements as HIP processed material 
would indicate. Material processed in such a manner would also be subject 
to the same disadvantages as HIP processed material, mainly cobalt laking, 
and the fact that equipment capable of high temperatures and up to 3000 
psi (recommended for materials less than 10% Co) are expensive to acquire 
and utilize a great deal of the gas used to pressurize which must be 
continually cleaned in order not to effect the chemical balance of the 
alloys being processed. 
A. Hara and N. Yoshida in a Japanese Pat. No. Sho 46-9528 discuss a method 
similar to the Schedler patent wherein powders for the manufacture of 
cemented carbide are molded and sintered, and pressure is then applied to 
the sintered product using high pressure gas after the shrinkage is almost 
complete. 
In the example, they discuss a 7 wt% Co WC alloy which is heated to 1300 
Degrees C. for 2 hours under vacuum and then pressurized to 1000 
Kg/cm.sup.2 (12000 psi). When pressurization is complete, the temperature 
is raised to 1400 Degrees C. and held for one hour to complete the 
processing. By treating such a material in this manner, pressurization 
again is carried out at a temperature where there may be minimal liquid 
phase available. Since however the pressure is very high, the small amount 
of eutectic liquid which may be present (if any) may be forced into any 
voids present by overcoming capillary action and thus effecting void 
closure. Again, this process may produce "cobalt lakes" which are 
preferable to voids but not as desireable as a more homogeneous structure. 
Also in this case, due to the very high pressures and the high 
temperatures used, the equipment necessary to accomplish this process is 
massive and expensive to acquire. 
A somewhat different approach to densify carbide by pressurizing after 
sintering was taken by Johan Romp as described in U.S. Pat. No. 2,263,520. 
In this patent he describes a method of sintering carbide in a gaseous 
mixture of 85% N.sub.2 and 15% H.sub.2 or in 100% hydrogen for 
approximately 1 hour and then increasing the pressure by using a mixture 
of 85% N.sub.2 and 15% H.sub.2 to a pressure of 50 atm. 
In his example I, he uses a 6% Co-WC alloy which he sintered at atmospheric 
pressure in an atmosphere of 85% N.sub.2 and 15% H.sub.2 at atmospheric 
pressure and 1450 Degrees C for 1 hour. At this point, the density was 
14.5 gm/cu. Upon continued heating at 1450 Degrees C. and pressurizing to 
50 atomospheres with the 85% N.sub.2 --15% H.sub.2 mixture and 
subsequently cooled at the increased pressure, the density was increased 
to 14.85 gm/cu. 
In example II he used a mixture of 79% WC, 15% TiC, and 6% Co sintered for 
1 hour at atmospheric pressure in hydrogen at 1500 Degrees C. At this 
time, the density was 11.1 gm/cu. On further heating at 1500 Degrees C. in 
a gas mixture of 85% N.sub.2 and 15% H.sub.2 pressurized to 50 stmospheres 
and subsequently cooled under this high pressure the specific weight is 
increased to 11.45 gm/cu. 
Using this procedure, as can be seen from the rather low densities observed 
after initial sintering, there is a great deal of porosity in the parts at 
the end of initial sintering. These pores, although sealed from the 
surface due to the liquid phase sintering at the high temperatures used, 
would be filled with the gaseous mixture used during sintering. This would 
especially be true for the mixture of 85% N2 and 15% H2. Thus, even though 
the porosity is substantially reduced by pressurization, the gaseous 
mixture trapped in the pores will remain since the pressure inside the 
void will remain nearly equal to the pressure outside the part as the 
outside pressure is increased and the voids begin to shink. This will 
necessarily result in an increase in density. However, the number of voids 
in the part will remain the same although they will be smaller. This is 
pointed out in the body of the patent in that he states "the increased 
pressure is preferably maintained even during cooling to avoid any risk 
that when the gaseous pressure is decreased while the body is still hot 
and slightly plastic, the effect of the invention may be influenced 
detrimentally". This indicates that if the pressure (50 atm) was removed 
before the material was solid and able to withstand internal stresses, the 
gas pressure in the remaining pores would cause the pores to again expand 
and decrease the density. Thus, by using this procedure, the size of the 
pores are reduced thereby increasing the density but the number of pores 
may not be substantially reduced; therefore, porosity is not totally 
eliminated. This residual porosity is detrimental to the performance of 
the material although not as detrimental as the original porosity level 
(and pore size) might be. 
A further disadvantage of this process is that the current furnaces used 
for sintering carbide contain graphite in the hot zone and typically the 
parts themselves are placed on graphite trays for sintering. If such a 
furnace contained H.sub.2 above about 1200 Degrees C., an equilibrium 
mixture of H.sub.2 and CH.sub.4 (formed by the reaction 2H.sub.2 +C 
solid=CH.sub.4) would be formed and this mixture would by definition have 
a carburization potential of 1 and thus add carbon to the WC-Co alloy such 
that it would exhibit free carbon in the microstructure which would be 
detrimental to its performance. 
In order to sinter carbide in a hydrogen containing atmosphere in the 
presence of graphite, the parts are typically buried in A1203 sand which 
forms a local balanced atmosphere around the parts and protects them from 
carburization. Using this procedure, however, in a pressurizable furnace 
would greatly reduce the capacity of the equipment due to the volume of 
A1203 and sand used. 
A further disadvantage is that equipment capable of high temperatures and a 
pressure of 50 atom (750 psi) is massive in construction and expensive to 
acquire. 
SUMMARY OF THE PRESENT INVENTION 
The present invention provides a method for densifying and removing 
porosity in previously sintered carbide or other liquid phase sintered 
part which overcomes all of the disadvantages of the various proposed 
processes discussed above. 
In brief, the method of the present invention comprises having previously 
sintered parts within a heatable pressurizable chamber. These parts may be 
either vacuum or hydrogen sintered and, similarly, may be cooled or not 
cooled following the sintering step. The parts are then heated to a 
temperature such that they are completely or nearly completely within the 
WC+liquid phase of the ternary phase diagram. In any case, such that 
considerble liquid is present and an equilibrium or nearly equilibrium 
amount of solutioning of the hard phase has occured, the sintered preform 
is in a relatively weak and mobile condition. The specific temperature 
will depend upon, of course, the specific material under question. 
Typically in the range of 1350 to 1600 Degrees C. With the parts in the 
appropriate regions of the phase diagram (WC+liq) the pressure vessel is 
pressurized with a gas, such as argon to a range with is below the 
pressure necessary to overcome the capillary force acting on the binder to 
prevent it from entering the voids but sufficient to physically move or 
collapse the structure inwardly. This pressure will vary from material to 
material but typically is in the range of 50 to 500 psi. The parts are 
maintained within the pressure vessel at the appropriate temperature for a 
relatively short time, typically 30 to 60 minutes. The parts may be heated 
first and then pressurized or pressurized first and then heated or 
simultaneously pressurized and heated under the following restrictions. 
If the material has been previously hydrogen sintered, the parts should be 
heated under a vacuum to a temperature such that they are in or nearly in 
the WC+Liq region of their phase diagram before pressurization. This will 
help any hydrogen which may be trapped in the pores to dissolve in the 
liquid metal binder and escape before pressurization in order to trap any 
gas in such pores. 
If the final pressure to be used is over that necessary to overcome the 
capillary forces in the material the parts should be heated in a vacuum to 
the appropriate temperature before pressurization. In such a case as the 
pressure increases, the proper closure will occur when a pressure is 
reached that will collapse the structure and yet not move significant 
amounts of binder. Thus further pressurization will not cause structure 
degradation such as "binder laking" since the voids are already filled 
with a homogeneous structure when the pressure might reach a level where 
it may be able to move the binder against the capillary forces. 
Once processing is complete, the pressure may be relieved or left on during 
cooling without degrading the effect of the invention. 
The preferred method of the invention is to dewax the parts (vacuum or 
hydrogen or wash gas), sinter the parts under vacuum (0.001 to 2 torr) and 
then effect pore closure by pressurization at the appropriate temperature 
for that material, as consecutive steps in a single piece of equipment as 
a single process. 
Consequently, in the method of the present invention, the temperature and 
pressure used in such that the capillary force imposed on the molten 
binder (and dissolved hard phase) prevents the binder from entering the 
voids. The physical state of the material (yield strength) at the 
appropriate temperature for that material is such that the pressure 
applied externally to the part is sufficient to physically move or 
collapse the structure inwardly, thus filling the voids with a relatively 
homogeneous mixture of hard phase and binder and virtually eliminating 
"binder laking". 
In practice, the method of the present invention substantially eliminates 
all porosity within the parts as well as closing larger randomly spaced 
holes in a manner superior to the previously known methods. The result of 
this is increased transverse rupture strength as compared to HIP and 
conventional sintering. This method is also more practical and less 
expensive to perform since the preferable pressure range is 50 to 500 psi 
and more preferably 50 to 300 psi and the equipment needed to perform such 
an operation is much less expensive to purchase and less expensive to 
operate then the equipment neded for the previously known processes. Also, 
since there may be an advantage to pressurizing slowly i.e. allow time for 
viscous flow of the structure into the voids thus equipment for fast 
pressurization is not necessary.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
The method of the present invention is designed to further densify parts 
which are liquid phase sintered such as cemented carbides, powdered 
metals, ceramics or the like. 
In brief, in the method of the present invention in the case of WC-Co 
alloys, the sintered parts are contained in a heatable presurizable 
chamber. The parts are then heated to a temperature such that they reside 
in or nearly in the two phase (WC+liq) portion of the ternary diagram 
(FIG. 17). Most importantly, they are heated to a temperature such that 
all or nearly all of the binder plus dissolved hard phase (WC or other 
carbides) is in a liquid state and that an equilibrium or nearly 
equilibrium amount of the hard phase is dissolved in this liquid. (In the 
case of WC-Co alloys the Co may dissolve in excess of 50% of its own 
weight of WC). In this state the structure is in a state of minimum 
resistance to gross structural movement (low viscosity). 
It should be noted that this temperature should be below that which might 
cause undesirable discontinuous grain growth. The parts should also have 
been sintered in a manner such that only gases which can be easily 
dissolve in the liquid phase are present in the voids, (such as H.sub.2). 
Any such trapped gasses would preclude total closure of the voids or 
porosity. This may be accomplished by vacuum sintering or hydrogen 
sintering in a conventional manner. The heated parts are then pressurized 
with a gas such as argon to a pressure which is less than, and thus 
insufficient to overcome, the capillary force acting on the liquid binder 
so that the liquid binder is not forced into the voids in the part. This 
pressure, however, is to be greater than that necessary to physically move 
or flow the bulk material and thus sufficient to physically move or 
collapse the structure inwardly thus forcing a relatively homogeneous 
mixture of hard phase and binder into the voids. Although the precise 
pressure varies between different hard phase and binder compositions, the 
preferred range is between 50 to 500 psi. The parts are maintained at the 
appropriate temperature and pressure for a relatively short time, 
typically 15 to 60 minutes. The chamber is then cooled and depressurized 
or depressurized and cooled and the parts removed. 
The capillary force of the liquid binder is calculated as follows: 
Assume: 
1. Surface tension (.gamma.) of Co-WC solution at 1400 degrees C. in vacuum 
is 100.phi. dynes/Cm.sup.2. 
2. Volume of binder phase (liquid) in WC+liq field of phase diagram is 
double that of the binder phase in the WC+.gamma. field. 
3. Cubic grains. 
.DELTA.P, i.e. force necessary to move Co-WC solution out of capillaries 
into voids, is calculated from the following equation: 
##EQU1## 
Where r.sub.1 =radius of space between grains of WC 
R.sub.2 =void radius 
However, r.sub.2 is much greater than r.sub.1 so that the above equation 
reduces to: 
##EQU2## 
TABLE A 
______________________________________ 
Wt % Co r.sub.1 Grain Size P 
______________________________________ 
6% .083 u 2.5 micron 240 bar 
6% .333 " 10 " 60 bar 
10% .133 " 2.5 " 144 bar 
10% .555 " 10 " 36 bar 
16% .222 " 2.5 " 90 bar 
16% .95 " 10 " 22 bar 
25% .30 " 2.5 " 65 bar 
25% 1.25 " 11 " 16 bar 
______________________________________ 
Tests have shown that the previously sintered parts may be heated first 
(vacuum) and then pressurized, pressurized first and then heated or heated 
and pressurized simultaneously while remaining within the scope of the 
invention and without affecting the parts. It has also been found that the 
rate at which the material is pressurized and whether or not the material 
has been cooled or not after sintering has no effect on the parts. In the 
case of parts that have previously been sintered in hydrogen, there may be 
some advantage in first heating the parts in vacuum to allow any hydrogen 
trapped in the voids or porosity to dissolve in the binder and diffuse to 
the part surface and be removed prior to pressurizing. 
In the preferred embodiment of the invention, green (as pressed) preforms 
are placed in an appropriate furnace such as described in U.S. Pat. No. 
4,398,702 and the following procedure is followed. 
1. The temperature slowly raised to a temperature above the vaporization 
point of the fugitive binder and below a temperature at which the 
hydrocarbons in the fugitive binder might crack (400 degrees C.) the 
hydrocarbon vapors are removed by vacuum pumping. Alternately a wash gas, 
such as hydrogen, argon or nitrogen, is used to convey the hydrocarbon 
vapors to a surface in the equipment where they may be condensed and 
appropriately removed. 
2. Once the fugitive binder is completely removed the temperature is raised 
under vacuum (0.01-2.0 mm Hg) to the appropriate sintering temperature for 
the material in question (typically this would be temperature used for 
porosity and hole closure during pressurization). This temperature would 
be held for the time period necessary to accomplish the liquid phase 
sintering of the material and to solution an equilibrium or nearly 
equilibrium amount of the hard phase in the binder. This would be 
typically 15 to 30 minutes. 
3. The sintered parts would then be pressurized to the appropriate pressure 
with an inert gas, such as argon. This pressure would be typically in the 
range of 50 to 500 psi. The pressure and temperature would be held for 
approximately 15 to 30 minutes. Since these pressures (50-500 psi) are 
less than the pressure necessary to overcome capillary forces (see Table A 
above), binder laking is avoided. However, the pressurization must be 
sufficient to physically collapse to structure of the part. 
When processing the parts in this manner the maximum pressure used need not 
be determined precisely since the proper void closure will occur (collapse 
of the structure into the voids) as the pressure is increased and thus if 
the pressure necessary to overcome capillary action is exceeded, the 
binder would not be able to fill the voids or porosity and form binder 
lakes because they would already be filed with a nearly homogeneous 
structure. If the vessel is pressurized first and then the parts heated 
the pressure obtained at the eutectic temperature must be below that 
necessary to overcome capillary action or the binder will fill the voids 
and binder lakes will result. 
If pressurization is to be carried out after heating and or sintering and 
the temperature is not such that the parts are substantially in, i.e. in 
or nearly in, the two phase WC+liq phase field (see reference character 10 
in FIG. 1) i.e. (just over the eutectic temperature and in the 
WC+.gamma.+liq field of FIG. 17). Then even though there may be enough 
liquid phase for sintering, the parts may have a substantial yield 
strength and viscosity. In this case, the voids may not close at very low 
pressures due to structure collapse but rather in order to close the voids 
the pressure must be reached which will overcome capillary action such 
that closure will occur by binder movement thus producing binder laking. 
Test results have established that the present invention effectively 
eliminates all porosity within the part as well as closing large holes or 
flaws by filling such holes with a relatively homogeneous hard phase and 
binder mixture. Since binder laking is eliminated, parts produced by the 
present method are superior to those produced by the prior art. 
Also the present invention may be carried out in equipment which need be 
capable of only approximately 300 psi as opposed to prior art which needs 
equipment capable of 3000 to 10,000 psi and above therefore the necessary 
equipment is much less costly to acquire and operate. 
The following examples indicate how the method of the present invention may 
be used to close a large flaw as well as decrease the porosity in the 
part. 
EXAMPLE 1 
Conventional vacuum sintering to show a large flaw. 
1. Material--(90% WC--10% Co) medium gram size material. RA 88.6. Carbon 
content of WC 6.12%. 
2. Place 15 gm of powder in 1" diameter mold. 
3. Place parafin shaving--1/2" long, approximately 0.02" diameter--on 
powder to produce medium size flaw. 
4. Add 15 gm of powder. 
5. Place parafin shaving--1/2" long approximately 0.05" diameter--on powder 
to produce large flaw. 
6. Add another 15 gm of powder. 
7. Press powder mechanically at 30,000 psi. 
8. Vacuum dewax bar at 420 Degrees C. 
9. Sinter cycle--temperature 1415 Degrees C., pressure 100 microns Hg, time 
90 minutes, then cool. 
The resulting cemented carbide sample from Example 1 has two flaws; one of 
which is shown in FIG. 1 at 784 magnification. 
EXAMPLE 2 
The parts produced by the steps described in Example 1 were then subjected 
to the following steps: 
1. Maintained at 1415 Degrees C. following sintering. 
2. Pressurized with argon gas to a pressure of 250 psi--17 bar. 
3. Time 30 minutes. 
FIGS. 2 and 3 illustrate the complete closure of a large flaw at 754 and 
1500.times. magnification, respectively, and with an absence of cobalt 
laking. 
EXAMPLE 3 
The parts produced by the steps described in Example 1 were then subjected 
to the following steps: 
1. Parts maintained at 1415 degrees C. following sintering. 
2. Pressurize with argon to 90 psi--6 bar. 
3. Time 30 minutes. 
FIGS. 4 and 5 illustrate complete closure of the large flaw at 775.times. 
and 1500.times. magnification, respectively, with an absence of cobalt 
laking. 
EXAMPLE 4 
The parts from the lot of Example 1 were then subjected to the following 
steps: 
1. Pressurized with argon to 160 psi--10 ar at room temperature. 
2. Heated to 1415 Degrees C. whereupon the pressure rises to 250 psi--17 
bar. 
3. Maintained at temperature and pressure for 30 minutes. 
FIGS. 7 and 8 show complete closure of the large flaw at 1500.times. and 
75.times. magnification, respectively, with the absence of cobalt laking. 
EXAMPLE 5 
The parts from the lot of Example 1 were treated the same as Example 2 
except the parts were cooled to room temperature following sintering and 
reheated to 1415 Degrees C. 
FIGS. 11 and 12 show complete closure of the large flaw at 75.times. and 
1500.times. magnification, respectively. 
EXAMPLE 6 
The parts were processed in a manner identical to Example 1 except that a 
16% cobalt WC alloy powder was used. 
Material: 16% Co--WC alloy 
Hardness: 86.5 RA 
Carbon Content of WC--6.12% 
The following steps were performed: 
1. Heat parts to 1415 Degrees C. 
2. Pressurize to 50 psi--3 bar. 
3. Hold for 30 minutes. 
FIGS. 13 and 14 illustrate complete closure of the flaws at 75.times. and 
1500.times. magnification, respectively. 
All of the above examples as can be seen from FIG. 17 were performed at a 
temperature and carbon content such that the material at the time of 
pressurization was in the two phase (WC+Liq) region of the phase diagram 
(reference character 10). 
Test results have also shown that with 10% cobalt material complete closure 
of the flaws is not possible at 50 psi--3 bar and 1415 Degrees C. At that 
temperature, a pressure of 50 psi--3 bar is below the pressure necessary 
to overcome the capillary force imposed on the molten binder, but is also 
insufficient to physically move the material to collapse the voids to 
obtain void closure. Also tests have shown that at 300 psi--20 bar and 
1400 Degrees C. that it is not possible to close voids in a 6% Co., 89% 
WC, 4.5% TiC, 0.5% TaC material whereas 300 psi at 1450 Degrees C. is 
fully effective in closing voids in such material. Thus at 1400 Degrees C. 
a higher pressure would be necessary to close voids in this material and 
at such a higher pressure the binder may be moved into the voids and cause 
binder laking. When the material was further heated to increase the amount 
of liquid phase and hard phase solutioning the voids were easily closed at 
300 psi, a pressure below that necessary to overcome capillary action. 
This resulted in complete void closure in this material with no binder 
laking. 
The following table illustrates test results showing that parts processed 
in accordance with the present invention achieve not only greater 
resistance to transrupture than sintered parts but also greater resistance 
to transrupture than parts that have undergone hot isostatic pressing. 
This greater resistance to transrupture is thought to result from the more 
homogenous microstructure 
__________________________________________________________________________ 
ANALYTICAL TESTS PERFORMED ON 
KNOWN GRADES OF MATERIAL FOR COMISON VERIFICATION 
TEST PIECES 
VACUUM SINTERED TEST PIECES SINTERED AT 
TEST PIECES THEN HOT ISOSTATICALLY 
ULTRA TEMP UNDER POSITIVE 
VACUUM SINTERED 
PRESSED (10,000 PSI) 
PRESSURE METHOD 
__________________________________________________________________________ 
GRADE 9% COBALT 
BALANCED WC 7 PIECES 1 TEST CUBE 
7 PIECES 1 TEST CUBE 
7 PIECES 1 TEST 
__________________________________________________________________________ 
CUBE 
Density 14.59 14.58 14.62 
Hardness 91.2 RA 91.2 RA 91.2 RA 
Trans-Rupture 
315,098 psi 345,517 psi 391,669 psi 
Microscopic A.sub.1 Porosity 
A.sub.O Porosity A.sub.O Porosity 
Material Structure 
Normal Normal Normal 
Comments: Standard Material 
Standard Material 
Standard Material 
__________________________________________________________________________ 
GRADE 15% Cobalt 
BALANCE WC 6 PIECES 1 TEST CUBE 
6 PIECES 1 TEST CUBE 
6 PIECES 1 TEST 
__________________________________________________________________________ 
CUBE 
Density 13.72 14.00 13.99 
Hardness 88.6 RA 88.9 RA 89.0 RA 
Trans-Rupture 
54,433 psi 82,299 psi 126,063 psi 
Microscopic Macro porosity -- -- 
Material Structure 
Normal Normal Normal 
Comments Known intentional voids 
Voids all closed but had 
Voids all closed and had 
approx. .020" cobalt laking minimal cobalt 
__________________________________________________________________________ 
laking 
obtained by the present invention (i.e. minimal binder laking) than parts 
that have been HIP processed. 
The densification and microstructural development of sintered parts 
obtainable by the present method is comparable or superior to the 
corresponding densification and microstructural development obtainable 
from the previously known art. The present invention, however, is 
advantageous over the prior art in that the present method employs 
comparitively much lower pressures than those needed in prior art, namely, 
one tenth of that used in the Rutter patent and less than one one 
hundredth of that used in HIP processing. Also there is no negative effect 
by pressurizing slowly. As such, the machinery and equipment necessary to 
practice the method of the present invention is much less massive and, 
therefore, much less expensive in construction than the corresponding 
machinery and equipment necessary for the process of prior art. Also the 
method precludes the possibility of trapping gases in the pores as would 
be the case in the Romp patent. 
A still further advantage of the present invention is that the voids are 
filled with material having a homogeneous microstructure, thus, minimizing 
and even eliminating "binder laking". 
Although many types of metallurgical furnaces can be used to practice the 
method of the present invention, preferably, the metallurgical furnace 
described in U.S. Pat. No. 4,398,702 entitled "Metallurgical Furnace", 
issued on Aug. 16, 1983 is used to practice the method of the present 
invention. 
Although the preferred pressure range of the present invention is between 
50 and 500 psi, for micro grain materials presures up to 1500 psi are 
necessary in order to overcome grain interlocking and still achieve the 
desired gross structural movement. 
Having described my invention, however, many modifications thereto will 
become apparent to those skilled in the art to which it pertains without 
deviation from the spirit of the invention as defined by the scope of the 
appended claims.