Process for producing refractory powder

In a process for producing a refractory metal carbide grade powder, a metal carbide powder is heated above melting point of a wax prior to forming a blended mixture of metal carbide and wax which is subsequently milled with a metal binder in liquid milling medium incompatible with the wax and the mixture is dried to form a powder.

CROSS REFERENCE TO RELATED APPLICATIONS 
Copending U.S. patent application Ser. No. 304,759 relates to a process 
wherein an aqueous mixture containing a dispersed phase of wax is attritor 
milled with metal carbide and metal binder particles to coat the particles 
with wax and produce a slurry which is spray dried to produce a powder. 
The present invention relates to refractory powders, and particularly to 
refractory metal carbide grade powders of the type containing particles of 
refractory metal carbide, a binder metal and an organic binder. 
BACKGROUND OF THE INVENTION 
Grade powders are pressed and sintered to form cemented carbide articles 
such as drill tools, cutting bits and wear parts. Grade powders comprise a 
mixture of fine powders of metal carbides and a binder metal. A wax binder 
may be included in the grade powder to promote the flowability of the 
grade powder into die cavities, to aid in pressing by a lubricating 
action, and to impart sufficient green strength to permit handling after 
pressing. 
Prior to sintering, the wax binder is typically removed during the step 
known as "dewaxing" by heating in a furnace at a temperature of from about 
ambient to 500.degree. C. in a protective atmosphere or vacuum. 
As set forth in U.S. Pat. No. 4,070,184 to Scheithauer et al. a process is 
described wherein water is used as a milling fluid and a water soluble, 
long chain polyvinyl alcohol is added to the milled slurry of metal 
carbide and binder metal after milling but prior to spray drying. 
According to this process, the wax-like binder is water soluble or 
compatible with water to ensure adequate distribution of the wax in the 
grade powder. 
Paraffin type waxes which are insoluble in water are typically incorporated 
into grade powders by use of an organic solvent which dissolves the 
paraffin wax so that good dispersion of the wax in the powder is assured. 
The organic fluid is removed by several drying methods to give a wax 
containing grade powder. Due to the flammability of organic solvents, if 
spray drying is used, a closed cycle spray drying system is required which 
utilizes an oxygen free atmosphere such as nitrogen. Generally, this 
system is characterized by a high initial cost as compared to a water 
system which can be an open system. 
In the water system, water is the milling liquid and a water soluble binder 
is utilized. The water system is clearly desirable from a safety 
standpoint as compared to the organic system. However, the water system is 
undesirably limited to the types of waxes that can be used since the waxes 
must be compatible with the system to adequately disperse the wax in the 
powder for achieving desired powder properties. 
It is desirable to develop systems for waxing grade powders which permit 
more flexibility in the utilization of waxes. 
SUMMARY OF INVENTION 
In accordance with the present invention, there is provided a process for 
producing a refractory powder containing an organic binder comprising 
heating particles of a refractory material to a temperature above the 
melting point and below the decomposition point of said organic binder to 
form a heated powder of refractory material, mixing said organic binder 
with said heated powder to form a blended mixture, milling said blended 
mixture with a liquid milling medium to produce a slurry, said organic 
binder being substantially insoluble in said liquid milling medium, and 
removing substantially all of said milling medium by evaporating below 
said decomposition temperature and forming a refractory powder comprising 
refractory particles having an adherent coating of organic binder. 
In accordance with a more particular aspect of the present invention, there 
is provided a process for producing a powder mixture of metal carbide 
particles, metal binder particles and wax suitable for making a cemented 
metal carbide comprising heating a mixture of refractory metal carbide 
particles to a temperature above the melting point of said wax to form a 
heated powder, mixing said wax with said heated powder to form a blended 
mixture, milling said blended mixture including metal binder particles 
with a liquid milling medium to produce a slurry, said wax being 
substantially insoluble in said liquid milling medium, and drying said 
slurry to remove said milling medium and form a powder mixture comprising 
metal carbide particles and metal binder particles and said wax. 
DETAILED DESCRIPTION 
Grade powders which may be prepared by the process of the present invention 
are intimate mixtures of refractory metal carbide powders plus a metallic 
cementing phase or matrix called a metal binder. Generally the grade 
powders include an organic binder which also serves as a pressing 
lubricant. A typical grade powder is a mixture of tungsten carbide, 
cobalt, and paraffin wax. The carbide powder may consist of other carbides 
or mixtures thereof and are generally the refractory carbides which 
include carbides of the metals from the Groups IV, V and VI of the 
Periodic Table that have a melting point above about 1895.degree. C. 
Cobalt is the most common matrix for tungsten carbide. Nickel, iron, and 
molybdenum, either singularly or in combination, particularly in 
combination with cobalt are typically used when refractory metal carbides 
other than tungsten carbide are used. For example, the matrix phase for 
titanium carbide is typically either nickel or a nickel-molybdenum alloy. 
As used herein, the matrix metal is selected from the iron group of metals 
and alloys of the iron group of metals. 
The amount of binder metal or matrix metal may be from about 2 to about 90 
percent by weight of the total weight of refractory carbide and matrix 
metal. From about 5 to about 20 percent is more typical. 
The average particle size of the refractory metal carbide is generally from 
slightly less than one micron to about 25 microns. The most common 
tungsten carbide generally is between 1 to 2 microns. Grain inhibitors and 
other additives may typically be employed in the grade powder mix. 
Materials commonly used are molybdenum carbide, vanadium carbide and 
chromium carbide. The refractory metal particles may be conveniently 
blended with the metal binder particles to form a blended power mixture. 
It is also contemplated that additions of binder metal or metal carbide 
particles may be performed during subsequent mixing operations. 
Grade powders may be processed by any suitable method known in the art to 
produce a hard body that is particularly desirable for use as the working 
surfaces of tools. Typically sintered metal carbide bodies are prepared by 
pressing the grade power in hard steel or carbide lined steel molds at 
pressures usually ranging from 5 to 30 tons per square inch depending on 
the size and shape of the compact. Sintering is then performed usually at 
temperatures ranging from 1350.degree. to 1500.degree. C. for times of 
from about 30 to 60 minutes. Sintering is generally performed in a 
protected or non-oxidizing atmosphere. 
One type of waxes utilized in the process of the present invention is 
insoluble in water. Typical water-insoluble waxes are low-melting mixtures 
or compounds of high molecular weight which are solid at room temperature 
and generally similar in composition to fats and oils. The waxes are 
thermoplastic and possess the properties of water repellency, smooth 
texture, and nontoxicity. The major types of waxes include animal wax such 
as bees wax, spermaceti, lanolin, and shellac wax. The vegetable waxes 
include carnauba, candelilla, and others. Mineral waxes include earth 
waxes such as ceresin and petroleum waxes such as paraffin. Various 
synthetic waxes include ethylenic polymers, polyol ether-esters, 
chlorinated naphthalenes and various hydrocarbon waxes. Of the 
water-insoluble waxes petroleum waxes such as paraffin waxes are 
preferred. The water-insoluble waxes are typically soluble in organic 
solvents such as acetone. 
Another type of waxes utilized in the process of the present invention is 
insoluble in organic solvents. Typical organic solvent-insoluble waxes 
include polyglycol, polyethylene glycol, hydroxyethylcellulose, tapioca 
starch, and carboxymethylcellulose. Some of the organic-insoluble waxes 
may be soluble in water. 
In accordance with the present invention, metal carbide particles which may 
include metal binder particles are heated to a temperature above the 
melting point of the wax and intimately or thoroughly mixed to form a 
substantially uniform blend of heated carbide powder and wax. Preferably 
the temperature is at least about 20 degrees centigrade and more 
preferably about 30 degrees centigrade above the melting point of the wax. 
The temperature should be below the decomposition temperatures of the wax. 
Generally longer mixing times tend to give a better dispersion of the wax 
in the powder. The metal carbide particles and wax should be sufficiently 
mixed so that the particles are at least partially coated with or imbedded 
in the wax. If metal binder particles are present with metal carbide 
particles, the mixing should be carried out in a non-oxidizing 
environment, i.e. inert atmosphere or vacuum to prevent the oxidation of 
the metal binder. The amount of wax typically used should be sufficient to 
impart green strength to a pressed compact. Increased amounts of wax up to 
a certain level tend to increase the green strength of a compact. The 
amount of wax varies according to the type of desired grade powder. The 
amount of wax employed is typically from about 0.5 to about 5 percent by 
weight based on the total weight of the final grade powder. 
Next the uniformly blended mixture of wax and metal carbide is preferably 
cooled. The cooling promotes adherence of wax to particles. Tumbling 
during cooling aids in obtaining smaller clumps of mix. Cooling also 
reduces tendency of metal binder oxidation. 
Milling the resulting blended metal carbide containing wax and metal binder 
powder is an important feature of the present invention. If metal binder 
is not present during the heating and mixing of wax and metal carbide 
particles, metal binder particles are added either prior to or during 
milling to produce a milled mixture of metal carbide, metal binder and 
wax. The milling of relatively coarse fraction of refractory material is 
often desirable to reduce the powder to a particle size suitable for 
sintering. When the refractory material is a metal carbide, compactability 
of alloy powders is improved by milling with a metal binder. The milling 
mixture is milled with a liquid milling medium to produce a slurry. The 
liquid milling medium is selected so that the wax is insoluble therein. 
The hereinbefore waxes insoluble in water may be used when water is a 
milling fluid. When other waxes hereinbefore mentioned are used, an 
incompatible or insoluble fluid is utilized. 
Preferably the milling is attritor milling. Attritor milling aids in 
rapidly dispersing the metal binder throughout the powder. The time 
required to properly disperse the metal binder particles or reduce the 
particle size during attritor milling is dependant on the particular 
attritor mill used, the type of powders used, the speed of the mill and 
various other factors. Generally it has been found that times as short as 
one hour are sufficient to properly disperse the particles and obtain a 
desirable size reduction. 
After the appropriate milling time, the slurry is discharged from the mill. 
This may require additional milling fluid to thin the slurry and rinse the 
mill. During discharge, the slurry may be passed through a 100 mesh screen 
to permit the removal of any contamination than may have been introduced 
from the milling balls. Milling fluid may be decanted from the screened 
slurry to obtain the desired solids concentration for spray drying. 
Generally, this ranges from 70-90% by weight. It is desirable to avoid 
using excess milling fluid during milling so that the drying step may be 
carried out without prior decanting or filtering. 
Spray drying may be carried out using commercially available spray drying 
equipment. The inlet and outlet air temperatures should be maintained 
below about 370.degree. C. and 190.degree. C., respectively, to prevent 
substantial oxidation or decarburization of the slurry constituents. The 
spray drying is carried out under conditions to produce an agglomerated 
powder mixture consisting essentially of aglomerated particles of metal 
carbide, metal binder and wax. Typically the size range of the 
agglomerated particles is from about 20 to about 150 microns. During spray 
drying, the slurry is generally heated to about 50.degree. C. and 
agitated. A suitable spray dryer is a Protco-Schwartz spray dryer with 
two-fluid-top nozzle atomization. When water is the milling fluid, typical 
drying parameters may be an air pressure of 20 psi, drying temperature of 
200.degree.-230.degree. C. and an outlet temperature of 
100.degree.-130.degree. C. When organic solvents are used as the milling 
fluid, the spray drying is preferably performed in absent air. Spray 
drying temperatures are dependent on the volatility of the solvent. The 
spray dried agglomerates may be classified by screening to obtain a 
desired fraction. 
Although spray drying is the preferred method of drying, it is contemplated 
that other drying methods which produce an agglomerate or a 
non-agglomerate may be utilized. The milling fluid may be conveniently 
evaporatively removed at a temperature below the decomposition temperature 
of the organic binder. When it is desirable to produce a non-agglomerated 
powder, the milled refractory powder and binder may be tumbled during 
evaporative drying to prevent formation of agglomerates. The drying should 
be carried out to such an extent that substantially all the milling fluid 
is removed from the powder. The powder consists essentially of refractory 
particles, organic binder and intentional additives.

To more fully illustrate the invention, the following example is presented. 
All parts, proportions, and percentages are by weight unless otherwise 
indicated. 
EXAMPLE 1 
About 169 kilograms of tungsten carbide powder is placed in a Ross 
double-planetary mixer equipped with a heating/cooling jacket. The 
tungsten carbide powder is pre-heated to a temperature of 165.degree. C. 
About 3.6 kilograms of synthetic amide wax is added. The wax which is 
insoluble in water is obtained from Kindt Collins Co. and has a melting 
point of about 135.degree. C. The mixing is continued for about 5 minutes 
after addition of the wax while the temperature is maintained at about 
160.degree. C. The wax-metal carbide mixture is mixed additionally while 
the mixer is changed to a cooling mode and the powder mixture is cooled to 
room temperature. About 25 liters of deionized water is placed in an 
attritor mill and the mill is rotated at slow speed. The waxed metal 
carbide powder and 10.8 kilograms of cobalt metal powder are slowly added 
and dispersed in the milling solution. The mill is then rotated at high 
speed, about 100 r.p.m. for about 1/2 to 3 hours. The slurry is removed 
from the mill and transferred to a holding tank where it is agitated prior 
to spraying. The spray drying results in the formation of agglomerated 
particles of metal carbide, metal binder and wax which are suitable for 
pressing into compacts and sintering. 
EXAMPLE 2 
About 282 grams of tungsten carbide powder is placed in a heater mixer and 
heated to 90.degree. C. The temperature is maintained while about 6 grams 
of Carbowax 8000, a polyethylene glycol which is water soluble with a 
melting point about 65.degree. C., is added. The mixing is continued for 
another 5 minutes while the 90.degree. C. temperature is maintained. The 
mixer is changed to a cooling mode to cool the tungsten carbide-wax 
mixture to room temperature. Mixing is continued during cooling to prevent 
the formation of large agglomerates. About 180 milliliters of heptane is 
placed in an attritor mill. The carbowax is insoluble in the heptane. 
While the mill is rapidly rotated, the tungsten carbide-wax mixture and 
about 18 grams of cobalt powder are slowly added to the mill. The powders 
are permitted to disperse in the milling medium for a short period of 
time. The mill is then rotated at high speed, about 200 r.p.m. for about 2 
hours. The slurry is removed and placed in a pan which is dried over a 
steam table while spatulating. The powder produced in a non-agglomerated 
powder of tungsten carbide, metal binder particles and wax.