US3674443A - Titanium carbide-nickel compositions - Google Patents

Abstract

DENSE, HOMOGENEOUS COMPOSITIONS HAVING (A) A DENSITY IN EXCESS OF 95% OF THEORETICAL AND (B) AN AVERAGE GRAIN SIZE LESS THAN 10 MICRONS AND CONSISTING OF:

(1) 80-97 PERCENT BY VOLUME OF TITANIUM CARBIDE AND (2) 3-20 PERCENT BY VOLUME OF NICKEL, THE NICKEL BEING PRESENT MAINLY AS A BINDER PHASE UNIFORMLY DISTRIBUTED AT THE GRAIN BOUNDARIES, ARE USEFUL AS CUTTING EDGES FOR CUTTING TOOLS.

Description

nited S tes Patented July 4, 1972 3,674,443 TITANIUM CARBIDE-NICKEL COMPOSITIONS Alma U. Daniels, Wilmington, Del., assiguor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed May 28, 1969, Ser. No. 828,699 Int. Cl. B22f 3/ 14 US. Cl. 29182.8 4 Claims ABSTRACT OF THE DISCLOSURE Dense, homogeneous compositions having (a) a density in excess of 95% of theoretical and (b) an average grain size less than 10 microns and consisting of:

( 1) 80-97 percent by volume of titanium carbide and (2) 3-2 percent by volume of nickel, the nickel being present mainly as a binder phase uniformly distributed at the grain boundaries, are useful as cutting edges for cutting tools.

BACKGROUND In the past, many attempts have been made to make strong dense compositions which have small average grain sizes by sintering titanium carbide and nickel compositions. Such titanium carbide-nickel bodies have very large average grain sizes and the nickel has not been uniformly distributed, possibly due to inadequate wetting of the titanium carbide by the nickel, and furthermore the bodies have been lacking in strength. To overcome this problem, it has been the practice to use molybdenum along with the nickel. See Swinehart, Cutting Tool Materials Selection A.S.T.M.E., Dearborn, page 89 (1963). The use of molybdenum along with the nickel gives improved wetting of the carbide phase and enables a strong sintered composition to be made.

It has now been discovered that strong, dense titanium carbide-nickel bodies can be prepared in the absence of molybdenum when the process as taught in the instant invention is employed.

SUMMARY OF THE INVENTION It has been found that dense titanium carbide-nickel bodies with average grain sizes smaller than microns can be prepared when particular process conditions are used.

Thus, the invention relates to the following compositions of matter: homogeneous compositions having (a) a density in excess of 95% of theoretical and (b) an average grain size less than 10 microns consisting of:

(1) 80-97 percent by volume of titanium carbide and (2) 3-20 percent by volume of nickel, the nickel being present mainly as a binder phase uniformly distributed at the grain boundaries, substantially all of said binder phase having a thickness of less than 0.5 micron.

These compositions demonstrate exceptional advantages over sintered titanium carbide-nickel compositions and as a result of their exceptional properties the compositions of this invention are useful as cutting tools in machining materials such as cast iron and alloy steel.

The above compositions can be made by:

(I) Intimately milling together a mixture of fine titanium carbide and nickel powder in a hydrocarbon oil and separating, drying and screening the powder in a nonoxidizing atmosphere;

(II) Charging in a non-oxidizing atmosphere the titanium carbide-nickel powder to a graphite mold provided with close fitting graphite pistons and in either sequnce (a) applying to the power by means of said pistons a pressure of 0-500 p.s.i. and (b) introducing the mold into a vacuum furnace preheated to a temperature in the range of from 1000 C. to 1475 C.;

(III) Heating said mixture under vacuum to a selected temperature in the range of from 1350" C. to 1475' C. in a period of time ranging from about 2 minutes to 20 minutes while maintaining a pressure of from 0-500 p.s.i.;

(IV) Maintaining the final temperature and pressure of Step III for a period of time ranging from zero minutes to 5 minutes;

(V) Maintaining the temperature of Step III and applying to the sample by means of the graphite pistons a pressure in the range of from 500 p.s.i. to 4000 p.s.i. for a period of time ranging from 2 minutes to 6 minutes; and

(VI) Thereafter immediately releasing the pressure on the pistons, removing the die from the furnace and rapidly cooling the resulting titanium carbide-nickel composition.

DETAILED DESCRIPTION Starting materials Titanium carbide is used in the compositions of this invention in amounts ranging from -97 percent by volume. At least 80 volume percent titanium carbide is required to provide adequate hardness. Preferred amounts of carbide range from -95 percent by volume to achieve an optimum balance of strength and hardness.

Titanium carbide can be obtained commercially or can be synthesized by methods well known to the art. The titanium carbide should be finely divided, that is it should have a particle size of less than 5 microns and preferably less than 2 microns. If the starting material is appreciably larger than 5 microns in particle size it can be pre-ground to reduce its size to that which is acceptable. Of course the milling of the components of this invention, which is carried out to obtain a high degree of homogeneity, will result in some comminution of the carbide and the other starting component, nickel.

Nickel is used in the compositions of this invention in amounts ranging from 3-20 percent by volume. At least 3 volume percent of nickel is required to provide adequate strength. The preferred amounts of nickel are 5-5 percent by volume since an optimum balance of strength and hardness are achieved in this range.

The starting materials used in the compositions of this invention should be pure. In particular it is desired to exclude impurities such as oxygen which tends to have deleterious effects on the dense compositions. On the other hand minor amounts of many impurities can be tolerated with no appreciable loss of properties. Thus the metal can contain small amounts of other metals such as titanium, zirconium, tantalum or niobium as minor impurities, although low melting metals like lead should be excluded. Small amounts of carbides other than titanium carbide, such as several percent of tungsten carbide, which is sometimes picked up in grinding, can be present. Even oxygen can be tolerated in small amounts such as occurs when titanium carbide has been exposed to air resulting in a few percent of titanium oxy-carbide. However, after the powder components have been milled together and are in a highly reactive state, oxidation, particularly of the metals, occurs easily and should be avoided.

PROCESS OF MANUFACTURE The manner in which the compositions of this invention are prepared is important, because the characteristics of the compositions are achieved in part at least as a result of the manner in which they are prepared. Thus the use of fine-grained starting materials and thorough milling of the mixed components are directly related to the fine grain size and uniform homogeneity of the compositions. Other precautions which should be observed in preparing the compositions which have important effects on the products are:

1) The prevention of excessive contamination from grinding media and moisture or oxygen in the air;

(2) Hot-pressing or presintering under conditions which permit the escape of volatile materials prior to densification;

(3) Avoiding undue absorption of carbon from pressing molds by limiting their contact under absorption-promoting conditions;

(4) Avoiding excessive component recrystallization and resultant segregation by avoiding prolonged subjection to very high temperatures.

(1) Milling and powder recovery Milling of the components, to homogeneously intermix them and obtain very fine grain sizes is carried out according to the practices common in the art. If roller milling is employed, optimum milling conditions will ordinarily involve a mill half-filled with a grinding medium such as cobalt bonded tungsten carbide or alumina balls or rods, a liquid medium such as a hydrocarbon oil, an inert atmosphere, grinding periods of from a few days to several weeks, and powder recovery also in an inert atmosphere. The recovered powder is ordinarily dried at temperatures of around ISO-300 C. under vacuum, followed by screening and storage when desirable in an inert atmosphere. Other methods of milling, such as vibratory milling can also be used in preparing the powders.

(2) Consolidation The compositions of this invention are ordinarily consolidated to dense, pore-free bodies by sintering under pressure. Consolidation is ordinarily carried out by hot pressing the mixed powders in a graphite mold under vacuum.

The hot pressing process consists of loading milled and dried powder into a graphite mold or die provided with close fitting graphite pistons and subjecting it to a pressure of -500 p.s.i., applied to the pistons by means of hydraulically activated rams, inserting the mold into the heated zone (1000 C.-1475 C.) of a vacuum hot press, thus allowing volatile impurities to escape before the composition is densified. Full pressure (500-4000 p.s.i.) is then applied to the sample at or near maximum temperature (1350l475 C.) employed for a period of time ranging from 2 minutes to 6 minutes.

The pressure is then released and the mold containing the densified body is immediately ejected from the heated zone of the hot press and cooled rapidly in the course of a few minutes to dull red heat while still under vacuum.

Maximum or goal temperatures for hot pressing range between 1350 C. and 1475 C. depending on the amount of nickel which is present. The temperatures will ordinarily be about 1400 C. Full pressures used during hot pressing ordinarily range between 500 and 4000 p.s.i., with low pressures being used in combination with lower temperatures for compositions with a high nickel content. Conversely, higher pressures and temperatures are employed for compositions low in nickel.

As would be expected, at higher temperatures and pressures, some of the nickel may tend to squeeze out of the compositions when pressure is applied. This tendency can be compensated for by starting with a little more nickel than is desired in the final body when operating at a high temperature and pressure. By this procedure, some of the nickel will be squeezed out during pressing, leaving the body with the desired metal content. Generally speaking, appreciable squeeze-out of metal is to be avoided not only because it changes the composition but also because the metal causes sticking to and damaging of the molds.

It is important that during hot pressing, the compositions not be heated to a goal temperature for a period of time which is much in excess of that required to eliminate porosity and achieve density. Such higher temperatures or longer times can result in excessive grain growth, coarsening of the structure, the development of secondary porosity due to recrystallization, or in the formation of undesirable phases.

The products of this invention are ordinarily subjected to pressure at maximum temperature for 2-6 minutes after which the product is immediately removed from the hot zone. The resultant bodies are fine grained, homogeneous, essentially pore-free, and are characterized by high hardness and excellent transverse rupture strength.

CHARACTERISTICS OF THE COMPOSITIONS In addition to characterizing the compositions of this invention on the basis of the components discussed above, the compositions can also be characterized on the basis of their structural characteristics, i.e., fine grain size and homogeneity.

The dense bodies of this invention are characterized as having a fine average grain size smaller than 10 microns and preferably smaller than 5 microns in average grain diameter. Moreover, the grain size is uniform throughout the compositions and there is essentially no porosity in the dense compositions of this invention. The fine grain size and low porosity of dense compositions of this invention contribute greatly to its hardness and thus result in bodies which are exceptionally abrasion-resistant. For example, cutting tools made from dense bodies of this invention resist abrasion when coming in contact with the hard carbide inclusions that are found in cast iron.

Distribution of the titanium carbide and nickel in dense bodies of this invention is uniform and homogeneous, and, generally speaking, any area 100 microns square which is examined microscopically at l000 magnification will appear the same as any other area 100 microns square within conventional statistical distribution limits. The nickel is present mainly as a binder phase uniformly distributed at the grain boundaries, substantially all of said binder having a thickness of less than 0.5 micron, in contrast to compositions prepared by conventional sintering where the nickel is largely present as heterogeneously distributed inclusions in a coarse-grained weak body. The combination of fine grain size and homogeneity of distribution of the components in dense bodies of this invention results in bodies which are resistant to thermal shock both as regards shattering and as regards surface heat-cracking.

The products may contain small amounts of iron, cobalt, tungsten carbide, alumina, and other impurities, which are generally picked up during the milling process. When metallic impurities are present, they will generally constitute less than about 1% by volume of the total composition. When alumina inserts are used during milling, for example in a vibratory mill, there may be as much as 5% by volume of alumina present.

The products of this invention will have actual densities in excess of of theoretical, preferably in excess of 98% of theoretical. Theoretical density for any given composition is determined by the following equation:

p =4.95V 1c+8.90V 1 wherein:

=theoretical density in grams per cubic centimeter;

V =desired volume in cubic centimeters of titanium carbide per cubic centimeter of product;

V =desired volume in cubic centimeters of nickel per cubic centimeter of product.

A comparison of actual density to theoretical density is of course an indication of the degree of porosity.

EXAMPLES The invention will be better understood in reference to the following examples wherein parts are by weight unless otherwise noted.

Example 1 This is an example of a composition containing 93 volume percent titanium carbide and 7 volume percent nickel.

The titanium carbide used is a fine powder with a specific surface area of 3 m. /g., as determined by nitrogen adsorption. An electron micrographs shows that the titanium carbide grains are approximately 2 microns in diameter, and are clustered in the form of loose aggregates. The carbon content is 19.0% and the oxygen analysis indicates a titanium dioxide content of about 2.5%.

The nickel used is a fine powder containing 0.15% carbon, 0.07% oxygen, and less than 300 parts per million iron. The specific surface area of the nickel powder is 0.48 m. g. and its X-ray diffraction pattern shows only nickel which has a crystallite size of 150 millimicrons as calculated from line broadening. Under the electron microscope, the powder appears as polycrystalline grains, 1 to microns in diameter.

The powders are milled by loading 8300 grams of cylindrical 6% cobalt-bonded tungsten carbide inserts, A" long and A in diameter in a 1.8 liter steel rolling mill that is completely lined with 6% cobalt-bonded tungsten carbide. The inserts have been previously worn in so that contamination of powder batches with tungsten carbidecobalt will be kept to a few percent. The mill is charged with a mixture of 500 ml. of Soltrol 130 (a saturated parafiinic hydrocarbon with a boiling point of approximately 130 C.), 230 grams of the titanium carbide and 31.2 grams of the nickel as above described.

The mill is then sealed and rotated at 90 r.p.m. for 5 days. The mill is then opened and the contents emptied while keeping the miling inserts inside. The mill is then rinsed out with Soltrol 130 several times until essentially all of the milled solids are removed.

The milled powder and liquid are then transferred to a vacuum evaporator and the excess hydrocarbon is decanted off after the suspended material has settled. The wet residual cake is then dried under vacuum with the application of heat until the temperature within the evaporator is between 200 and 300 C., and the pressure is less than about 0.1 ml. of mercury. Thereafter, the powder is handled entirely in the absence of air.

The dry powder is passed through a 70 mesh screen in a nitrogen atmosphere, and then stored under nitrogen and sealed in plastic containers.

A consolidated billet is prepared from this powder by hot pressing the powder in a cylindrical gra hite mold having a cylindrical cavity 1 in diameter and which is equipped with opposingclose-fitting graphite pistons. One piston is held in place in one end of the mold cavity, while 22 grams of the powder is dropped into the cavity under nitrogen and evenly distributed by rotating the mold and tapping it lightly on the side. The upper piston is then put in place under hand pressure. The assembled mold and contents are then placed in the vacuum chamber of a vacuum hot press. The mold is held in a vertical position and the pistons extending above and below are engaged between opposite graphite grams of the press under pressure of about 100 p.s.i. Within a period of a minute, the mold is raised into the hot zone of the furnace at 1000 C., and at once the temperature of the furnace is increased while the pressure is maintained at 100 p.s.i. during the heat-up period. The temperature is raised from 1000 C. to 1400 C. in about 4 minutes, and the temperature of the mold is then held at 1400 C. for another 2 minutes to ensure uniform heating of the sample. A pressure of 4000 p.s.i. is applied to the billet through the pistons for 4 minutes. The pressure is then released and the mold and contents are immediately moved out of the furnace into a cool zone and cooled to dull red heat in about 5 minutes.

After further cooling, the mold and contents are removed from the vacuum furnace and the billet is removed from the mold and sand-blasted to remove any adhering carbon.

The hot pressed billet is found to be essentially nonporous, having no visible porosity under 1000 magnification. An electron micrograph at 20,000 magnification on a sample etched with a mixture of hydrofluoric, nitric and sulfuric acids shows a homogeneous composition having an average grain size of about one micron, with a nickel-rich binder phase uniformly distributed at the grain boundaries and having a thickness of 0.05 to 0.3 micron. Few of the grains exceed 2 or 3 microns in size. The density is 99% of the theoretical density.

X-ray diffraction analysis reveals primarily a sharp, strong face-centered cubic type pattern with atomic spacing of 4.323 angstroms indicating titanium carbide, and also diffraction lines showing the presence of free nickel.

Chemical analysis shows, in addition to titanium, carbon and nickel, the presence of about 2.7% of tungsten and 0.15 of cobalt. The tungsten and cobalt are presumably picked up from attrition of the milling inserts and mill lining. The billet, which is 1" in diameter, and about 0.30" in thickness, is cut so that a piece slightly larger than /2 square is removed from the center. Strips 0.070" in thickness are cut from the material remaining to each side of the center piece, and are further cut into 0.070" x 0.070" square bars, for testing transverse rupture strength. Other portions of the billet are used for indentation hardness tests and for other product characterizations. The transverse rupture strength as measured by breaking the 0.070" x 0.070" test bars on a span is about 211,000 p.s.i. The hardness of the billet is 93.0 on the Rockwell A scale.

The square center piece is finished as a cutting tip to /2" x /2" x 7 and the corners are finished with a radius, a style known in the industry as SNG-432.

The tip is used to turn a cylinder of Class 30 grey cast iron at a speed of 1250 surface feet per minute, a depth of 0.050" and a feed of 0.005" per revolution. After ten minutes the Wear land on the flank of the cutting edge is only 0.006" deep, which is only half the flank wear measured on a commercial titanium carbide based cutting tip after ten minutes turning under the same conditions. When used to mill 4340 steel having a Brinnell hardness number of 340 at 1000 surface feet per minute, a width of cut of 2", a depth of 0.050" and a feed of 0.0057" per revolution, for a single tooth 6" diameter milling head, two corners of the insert run an average distance of 24" before failing by chipping. Two corners of a commercial titanium carbide based cutting tip tested under the same condition give an average cutting distance of only 16" 'before failing due to chipping. Thus the composition of the invention shows a considerable improvement over conventional titanium carbide based compositions, both with respect to Wear and chip resistance in machining operations on two important classes of metal.

EXAMPLE 2 The procedure of Example 1 is repeated, except that the proportions of the components are changed to give a powder containing 86 volume percent of titanium carbide and 14 volume percent of nickel.

A dense body made by hot pressing this powder as described in Example 1 is found to have a transverse bend strength of 207,000 p.s.i. and a Rockwell A hardness of 92.2. Examination of an electron micrograph at a magnification of 20,000 on a sample etched as in Example 1 shows a homogeneous composition having an average grain size of about 0.8 micron and a nickel-rich binder phase uniformly distributed at the grain boundaries and having a thickness of 0.1 to 0.5 micron. The density is 98.5% of the theoretical density.

An SNG-432 cutting tip made from the hot pressed body is found to yield excellent results in the semi-finishing turning of Class 30 gray cast iron and in the finish milling of Class 30 gray cast iron.

Example 3 The procedure of Example 1 is used, changing the proportions of the components to give a powder containing 97 volume percent of titanium carbide and 3 volume percent of nickel. The powder is hot pressed as described in Example 1, modifying the conditions as follows. The sample in the mold is pressed at 500 p.s.i., before being introduced into the hot zone of the vacuum hot press at 1200 C. It is brought to 1475 C. maintaining the pressure on the sample at 500 p.s.i. during the heat-up period of about three minutes. After holding at 1475 C. for five minutes to ensure uniform heating of the sample, the pressure is increased from 500 p.s.i. to 40-00 p.s.i. and this pressure and temperature of 1475 C. are maintained for a further six minutes. The pressure is then released and the mold and contents are immediately moved out of the furnace into a cool zone and cooled to dull red heat in about five minutes.

The resulting dense body is found to have a transverse rupture strength of 155,000 p.s.i. and a Rockwell A hardness of 93.5. Electron micrograph examination of a sample etched as in Example 1 shows a homogeneous composition having an average grain size of about three microns and a nickel-rich binder phase uniformly distributed at the grain boundaries and having an average thickness of 0.02 to 0.2 micron. The density is 97% of the theoretical denesity.

A seal vane in a centrifugal pump for pickling acid in a steel plant is made from the above composition, and after being in service for six months, inspection of the part shows very little wear or corrosive attack and the part is returned to service.

UTILITY The pressed products show excellent performance as cutting tools and have advantages over conventional TiC-based tools in certain applications such as turning cast iron and milling alloy steel. The products can also be used as wear parts and refractory and corrosion resistant parts in high temperature equipment.

I claim:

1. A homogeneous composition consisting of to 97% by volume of titanium carbide and 3 to 20% by volume of nickel and having (a) an average grain size of less than 10 microns, (b) a binder phase consisting essentially of nickel uniformly distributed at the grain boundaries, substantially all of said binder phase having a thickness of less than 0.5 micron, and (c) a density in excess of 95% of the theoretical density.

2. A composition of claim 1 having a density in excess of 98% of the theoretical density.

3. A composition of claim 2 consisting of to by volume of titanium carbide and 5 to 15% by volume of nickel.

4. A composition of claim 1 wherein the average grain size is less than 5 microns.

References Cited UNITED STATES PATENTS 2,752,666 7/1956 Goetzel et al. 29182.8 2,714,245 8/1955 Goetzel 29182.8

FOREIGN PATENTS 961,181 6/1964 Great Britain. 406,633 7/ 1932 Great Britain.

OTHER REFERENCES Swarzkopf et al.: Refractory Hand Metals, 1953, Mac- Millan Company, pp. 376-7.

Jones: Fundamental Principles of Powder Metallurgy, 1960, Edward Arnold Ltd., pp. 4867.

CARL D. QUARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner U.S. Cl. X.R.