Tool Alloy Compositions And Methods Of Fabrication

Abstract

A composition of material comprising a titanium molybdenum carbide and an iron group metal binder is disclosed which is particularly useful as a metal cutting tool. The carbide phase of the composition is substantially richer in carbon than the prior art. For any given metal exchange ration, the carbide phase contains more carbon than the composition line TiC-Mo.sub.2 C for the same ratio. Also, the carbide phase can contain higher amounts of molybdenum than was used in the prior art.

Description

The present invention relates to improved cemented carbide alloys and more particularly to improved monocarbide alloys of titanium and molybdenum which are substantially richer in carbon than the prior art alloys of those metals and which, with a proper binder from the iron metal group, exhibits superior wear resistant characteristics.

Early efforts to improve the wear-resistance of sintered cemented carbides, such as WC-Co, and WC-TiC-TaC-Co, for steel cutting resulted in tool materials based on the system TiC-Mo.sub.2 C-Ni. Such a system was proposed as early as 1931 in Austrian Patent No. 160,172. More recent developments of such systems are reported by R. Kieffer and F. Benesovsky in Hartmetalle (Vienna, Springer, 1965). Certain improvements of the tool materials based on TiC-Mo.sub.2 C-Ni through addition of molybdenum to the nickel binder to enhance wettability of the titanium carbide and thus increase strength of the sintered carbide is disclosed in U.S. Pat. No. 2,967,349. This finding of improved performance of the carbide alloys at carbon concentrations less than corresponding to the composition line TiC-Mo.sub.2 C was essentially subsstantiated by mechanical property and hardness measurements in a study by R. Kieffer and D. Fister reported at Plansee berichte fur Pulvermetallurgie, Bd. 18 (1970), pp 246-253.

Wear resistance of these improved tools, which can be termed TiC-Mo.sub.2 C-Mo-Ni tools, in cutting soft and medium hard steels is generally much higher than the tungsten carbide-based sintered carbides, but tool reliability is usually low because of their brittleness and their tendency to notch at the scale line. As a result, the full potential of their inherent high wear and cratering resistance is rarely realized in practice. An additional shortcoming of the tool materials is their inability to machine hard steels, such as steels having a Rockwell hardness (R.sub.c) of 50 or higher, at competitive metal removal rates. Current usage is thus limited to machining of soft to medium hard low alloy steels under light cutting conditions, and machining of cast irons and steels.

Efforts to adapt the TiC-Mo.sub.2 C-Mo-Ni tool materials for roughing steels by decreasing their brittleness through higher binder contents met only with limited success. When this is done, their wear resistance appears substantially impaired and the high binder tools show a marked tendency to plastically deform at increased thermal loads.

It is accordingly an object of the present invention to provide an improved composition of material which exhibits improved wear resistant characteristics.

It is another object of the present invention to provide an improved monocarbide alloy of titanium and molybdenum which is substantially richer in carbon than such prior art alloys and which, generally, also contains more molybdenum than is proposed by the prior art.

It is another object of this invention to provide such alloys which are stable in the presence of iron group metal binder alloys, in particular nickel, and which afford cemented carbide tool materials superior in their overall characteristics to the TiC-Mo.sub.2 C-Mo-Ni tool alloys.

It is a further object of this invention to provide such alloys which can be cemented using cobalt and iron, without causing embrittlement of the alloys by formation of brittle intermetallic phases of or of .eta.-carbides, such as observed in carbon alloys with compositions falling in the area bounded by TiC-Mo.sub.2 C-Mo and TiC-Mo.sub.2 C-Mo-Ti.

Briefly stated, in accordance with the present invention, cemented carbide alloys are provided which are based on the monocarbide solid solution (Ti, Mo)C and which have carbon contents higher than the composition line TiC-Mo.sub.2 C. This monocarbide solid solution is cemented with an iron group metal binder which forms between 5 and 25 percent by weight of the total composition. The carbon content higher than the composition line TiC-Mo.sub.2 C is greater for higher exchange ratios of molybdenum for titanium. For example, it has been discovered in the present invention that from 6 to 60 percent of the titanium in the monocarbide can be exchanged for molybdenum. For the composition (Ti.sub..94 Mo.sub..06) C.sub.z the value of z can range from 0.972 to 0.985. This is contrasted with a value of z of 0.969 on the composition line TiC-Mo.sub.2 C for this metal exchange ratio. For the composition (Ti.sub..40 Mo.sub..60) C.sub.z, in which 60 percent of the titanium has been exchanged for molybdenum, the value of z can range from 0.71 to 0.90. This is contrasted with a value of z of 0.70 on the composition line TiC-Mo.sub.2 C for this metal exchange ratio.

In the preferred range of the present invention, from 12 to 50 percent of the titanium in the monocarbide has been exchanged for molybdenum. For the composition (Ti.sub..88 Mo.sub..12) C.sub.z, the value of z can range from 0.95 to 0.97. For the composition (Ti.sub..50 Mo.sub..50) C.sub.z, the value of z can range from 0.78 to 0.88. In the preferred range of the invention, the monocarbide solid solution within the ranges just described is cemented with between 8 and 12 percent by weight of an iron group metal binder, preferably nickel or cobalt or a combination of these metals.

For a complete understanding of the present invention, together with an appreciation of its other objects and advantages, please see the following detailed description of the invention and the attached drawings, in which:

FIG. 1 is a graphical representation of the monocarbide solid solution phase of the present invention and also shows the composition of the carbide alloys of the prior art tool developments in this area which were discussed above;

FIGS. 2 and 3 wear curves comparing the wear of tools according to the present invention and according to the prior art when subjected to identical test conditions; and

FIGS. 4 through 6 show the wear rate of tools in accordance with the present invention as a function of the exchange ratio of molybdenum for titanium (or the mole percent of molybdenum in the total metal content) for different test conditions.

The compositions of the carbide component used in the fabrication of the carbide-binder metal composites of the invention can be expressed either in atomic percent of the constituent elements, for example as Ti.sub.u Mo.sub.v C.sub.w (u + v + w = 100), where u, v, and w are, respectively, the atomic percent of titanium, molybdenum and carbon present in the alloy; or as relative mole fractions of metal and interstitial elements in the form (Ti.sub.x Mo.sub.y)C.sub.z, (x + y = 1), whereby x and y are, respectively, the relative mole fractions (metal exchange) of titanium and molybdenum, and z measures the number of gramatoms carbon per gramatom metal.

It is noted that 100.sup.. y defines mole percent molybdenum exchange in (Ti.sub.x Mo.sub.y)C.sub.z, and 100.sup.. x defines mole percent titanium exchange. The two sets of composition variables are readily interconverted by the relations

u = 100.sup.. x/1+z v = 100.sup.. y/1+z w = 100.sup.. z/1+z x = u/u+v y = v/u+v z = w/u+v

The latter method of defining the overall composition of the carbide component, the designation (Ti.sub.x Mo.sub.y)C.sub.z, is particularly useful in describing the concentration spaces of interstitial alloys and is used throughout the remainder of this specification.

FIG. 1 is a graphical representation of the gross composition of the monocarbide solid solution (Ti.sub.x Mo.sub.y)C.sub.z and illustrates this phase of the composition of the present invention as well as the composition of the carbide alloys of the prior art tool developments in this area. The format of FIG. 1 was selected to show these compositions rather than the conventional triangular graph for showing ternary systems because in such a graph the ranges discussed below would be poorly shown due to excess compressions.

The ordinate in FIG. 1 is the subscript z in the composition (Ti.sub.x Mo.sub.y)C.sub.z, while the abscissa is the subscript y. Of course, the abscissa also defines x, since x + y = 1. The ordinate is also shown as atomic percent carbon of the gross composition, with this atomic percent being equal to z/1 + z. The ordinate is linear for z, and is thus somewhat non-linear for atomic percent carbon. The abscissa is also shown as the percent exchange ratio of molybdenum for titanium, or the mole percent of molybdenum in the total metal content. This mole percent is equal to 100.sup.. y.

The line 10 in FIG. 1 represents the composition line Ti-C-Mo.sub.2 C, or the composition in the chosen notation of a mixture of TiC and Mo.sub.2 C for varying ratios of the two compounds. The area bounded by ABCD represents the composition range investigated by R. Kieffer and D. Fister in the above referenced report. The area bounded by AB'C'D' represents the gross composition of most current commercial tools in this area. The composition at the point D', (Ti.sub..87 Mo.sub..13)C.sub..936 approximately corresponds to the prior art comparison tool TiC-Mo.sub.2 C-Ni discussed below, and the point midway between B' and C', or (Ti.sub..09 Mo.sub..91)C.sub..91, when combined with a binder of 10 percent nickel, represents the optimum composition for the tools described in the above mentioned U.S. Pat. No. 2,967,349. This is the approximate composition of the prior art comparison tool TiC-Mo.sub.2 C-Mo-Ni discussed below.

The gross carbide composition of the tool alloys in accordance with the present invention generally fall within the composition area bounded by EFGH, but preferably within the more confined area E'F'G'H'. Carbide alloys located outside the area E'F'G'H', but inside the area EFGH, provide alloys of lesser quality when employed as cutting tools, but have other useful applications. In the chosen notation, the composition point, E' corresponds to (Ti.sub..87 Mo.sub..13)C.sub..97, point F' to (Ti.sub..87 Mo.sub..13)C.sub..95, point G' to (Ti.sub..50 Mo.sub..50)C.sub..78, and point H' to (Ti.sub..50 Mo.sub..50)C.sub..88. Point E corresponds to (Ti.sub..94 Mo.sub..06)C.sub..985, point F to (Ti.sub..94 Mo.sub..06)C.sub..972, point G to (Ti.sub..40 Mo.sub..60)C.sub..71, and point H to (Ti.sub..40 Mo.sub..60)C.sub..90.

It is seen from FIG. 1 that the carbide phase of the composition of the present invention is characterized by being substantially richer in carbon than the prior art composition. For any given metal exchange ratio, the composition contains more carbon than the composition line TiC-Mo.sub.2 C for the same ratio. The prior art had assumed that this composition line represented the maximum possible carbon content for any given exchange ratio. Also, in accordance with the present invention, higher amounts of molybdenum can be used than was possible with the prior art compositions. As is discussed in more detail below, this enables tools to cut under severe machining conditions as well as the light machining conditions to which the prior art TiC-Mo.sub.2 C-Mo-Ni tools were limited.

The carbide-metal composites of the invention may be fabricated by several different powder metallurgy techniques. A typical fabrication procedure is as follows: A mixture of carbide and binder alloy in the desired proportions are ball-milled in stainless steel jars for 3 to 4 days, using tungsten carbide-cobalt alloy balls and naphta or benzene as milling fluid. Depending on the power density, 3 to 5 weight percent pressing lubricant, usually paraffine, is added in solution with a suitable solvent such as benzene. The solvent for the paraffine is then evaporated, and the dry powder mixture compacted into the desired shapes at pressures ranging between 6 and 10 tons per square inch. The pressing lubricant is then removed by heating at temperatures between 200.degree. and 700.degree.C under vacuum and the compacts, stacked on suitable support materials such as graphite, are sintered for 1 to 11/2 hrs. at temperatures between 1350.degree. and 1450.degree.C under vacuum. For the evaluation of the alloys of the invention are machine tools, the sintered parts are ground on diamond wheels to the desired tool geometry.

A typical fabrication procedure for an alloy containing titanium and molybdenum in the molar ratios 6:4 and having a carbon content of 47.9 atomic percent, is described below. The composition of this alloy in the previously described notation is (Ti.sub..60 Mo.sub..40) C.sub..87.

A carefully blended powder mixture consisting of 46 weight percent TiC, 52.17 weight percent Mo.sub.2 C and 1.83 weight percent carbon was hot pressed in graphite dies at approximately 2000.degree.C to a density corresponding to about 75 percent of the theoretical. The compacts were then placed in a graphite container, homogenized for 3 hours at 2000.degree.C under a vacuum of 3 .times. 10.sup.-.sup.5 torr. and then crushed and ball milled to yield a grain size of less than 47 micrometers. The master alloy powder was then analyzed and homogeneity was ascertained by X-ray diffraction.

Compositions formed in the manner just described can be termed prehomogenized solid solutions. Another method for making the compositions of the present invention is to first form such a prehomogenized solid solution which is a molybdenum rich monocarbide solid solution, such as (Ti.sub..30 Mo.sub..70)C.sub..85 and then reactively sintering it with suitable quantities of titanium monocarbide and binder material to bring the gross composition of the material to the desired levels.

Aside from the routine fabrication variables, choice of composition of the carbide ingredient for a given gross composition of the composite strongly influences microstructure and phase distribution and, as a result, the properties of the sintered compacts. Thus, for example, very fine-grained composites are obtained by the reactive sintering method just discussed. The fine-grained structures in such composites are attributable to dissolution of the carbide components in the binder at sintering temperatures and reprecipitation of the more stable, equilibrium carbide solid solution from the liquid binder alloys. Concurrent with these dissolution/precipitation reactions are reactions involving a preferential transport of molybdenum-rich monocarbide solid solution to the titanium carbide grains to form an alloyed surface layer which, owing to its higher molybdenum content, is better wetted by the binder alloy and thus affords a stronger bond than is possible between TiC and binder metal alone. In general, the bulk of the carbide grains in such composites, if properly fabricated, will have a grain size equal or less than that in the as-milled condition. Wear-resistance, but especially top catering resistance, in machine-tool applications appears markedly improved by the presence of unreacted TiC in the core of a fraction of the carbide grains. Tendency towards plastic deformation under high cutting loads is somewhat higher than in composites prepared from preformed solid solutions and binder alloy with the same gross composition. Consequently, a lower binder content is used in reactively sintered composites for machine tool applications.

In general, studies of the performance of the alloys of this invention as machine tools revealed that fabrication of the composites by reactive sintering is advantageous for alloys in which the gross metal exchange in the carbide is less than 40 mole percent molybdenum. Performance of reactively sintered composites with more than 40 mole percent molybdenum in exchange in cutting soft to medium hard steels (R.sub.c -45) is slightly better than of tools prepared from prehomogenized solid solutions, while the performance is about equal in cutting hard steels.

Those skilled in the art can devise other methods of making the compositions of the invention. For example, carbide deficient gross compositions could be formed, mixed with binder and sintered in a carburizing atmosphere.

The following tables and graphs show the performance of a large number of tools having different composotions within the range of the invention and also give comparison data for a number of prior art tools which were subjected to the same test conditions. Four different test conditions were used. These are designated Test Condition A, Test Condition B, Test Condition C and Test Condition D. The test bars consisted of 4340 steel in four different hardness ranges, R.sub.c 22 to 29, R.sub.c 33 to 38, R.sub.c 46 to 50, and R.sub.c 50 to 55. Unless otherwise noted, test conditions referred to in the tables were:

TEST CONDITION A (wear test)

4340 steel, R.sub.c 22 to 29; cutting speed, 500 surface feet per minute; feed rate, 0.0151 inch per revolution; depth of cut, 0.060 inch; no coolant. SNG 433 inserts.

TEST CONDITION B (roughing test)

4340 steel, R.sub.c 22 to 29; cutting speed, 500 surface feet per minute; feed rate, 0.0203 inch per revolution; depth of cut, 0.125 inch; no coolant. SNG 433 inserts.

TEST CONDITION C

4340 steel, R.sub.c 33 to 38; cutting speed, 500 surface feet per minute; feed rate, 0.0102 inch per revolution; depth of cut; 0.060 inch; no coolant. SNG 433 inserts.

TEST CONDITION D (finishing hardened steel)

4340 steel, R.sub.c 46 to 55; cutting speed, 250 surface feet per minute; feed rate, 0.0051 inch per revolution; depth of cut, 0.050 inch; no coolant. SNG 432 inserts.

The wearland was measured at suitable time intervals with the aid of a tool microscope. Plastic deformation of the cutting edge and crater depth were measured on a metallograph. The flank data presented in the graphs and tables refer to the uniform wear zone of the tools.

To obtain a comparative performance evaluation of the composites of the invention, a cross section of representative tools from different manufacturers were also tested under identical conditions and the best performing tools selected as comparison standards. Tools from the C-5 and C-6 class carbides manufactured by different companies proved fairly equivalent and are thus not specifically identified in the tables. Large differences in the cutting performance of the C-7 and C-8 class of carbides were noted, however, in machining fully hardened 4340 steel. The grade K7H manufactured by the Kennametal Company, 1000 Lloyd Avenue, Latrobe, Pennsylvania was selected as the main comparison tool for this particular application because this is a tool which is widely used commercially for this type of machining.

The following four examples, which are representative of the compositions of the present invention, describe in detail four specific compositions and the manner in which they were fabricated. FIGS. 2 and 3 and the following Tables 1 through 4 show the performance of these four examples and of the leading prior art tools when subjected to the above described test conditions.

EXAMPLE 1

A powder blend consisting of 91.50 weight percent of an alloy (Ti.sub..60 Mo.sub..40)C.sub..87 and 8.50 weight percent nickel was prepared as a prehomogenized solid solution in the manner described above and the compacts sintered for 1 hour and 10 minutes at 1385.degree.C under vacuum. Average linear shrinkage during sintering was 16.4 percent. Average grain size of the carbide phase was approximately 4 micrometers and the hardness was 93.0 on the Rockwell A scale.

EXAMPLE 2

A powder blend consisting of 48.4 weight percent of a prehomogenized solid solution having the composition (Ti.sub..30 Mo.sub..70)C.sub..80, 42.1 weight percent TiC [resulting in a gross carbide composition of (Ti.sub..70 Mo.sub..30)C.sub..914 ], and 9.50 weight percent nickel were reactively sintered in the manner described before and sintered for 1 hour and 30 minutes at 1380.degree.C under vacuum. Linear shrinkage during sintering was 16.1 percent and the grain size of the carbide phase approximately 4 micrometers. The microstructure of the carbide grains consisted of a core of unconverted titanium carbide surrounded by an outer shell of compositionally graded titanium-molybdenum monocarbide solid solutions, with the outermost layer approximately corresponding to a carbide containing approximately (Ti.sub..45 Mo.sub..55)C.sub.z. A hardness of 92.9 on the Rockwell A scale was measured for the sintered composite.

EXAMPLE 3

A powder blend consisting of 33.6 weight percent of a prehomogenized solid solution having the composition (Ti.sub..30 Mo.sub..70)C.sub..80, 55.1 weight percent TiC [resulting in a gross carbide composition of (Ti.sub..80 Mo.sub..20)C.sub..944 ], 5.65 percent nickel and 5.65 percent cobalt, was reactively sintered in the manner described before the compacts sintered for 1 hour and 30 minutes at 1395.degree.C under vacuum. The carbide phase in the sintered compact had an average grain size of 3 micrometers and the measured hardness of the composite RA = 93.1.

EXAMPLE 4

A powder blend of 56.7 weight percent (Ti.sub..85 Mo.sub..15)C.sub..951, 31.3 weight percent (Ti.sub..70 Mo.sub..30)C.sub..91 [resulting in a gross carbide composition of (Ti.sub..80 Mo.sub..20)C.sub..94 ], and 12 percent cobalt was processed in the manner described before and resulting compacts sintered for 1 hour and 15 minutes at 1395.degree.C under vacuum. Average carbide grain size in the sintered part was approximately 3 microns and the hardness R.sub.A = 92.8.

Table 1 __________________________________________________________________________ Wear Pattern of the Tools Described in Examples 1 through 4 in Comparison to Commercial Sintered Carbides. Test Condition A Total Cutting Time WEAR PATTERN* Crater Tool Minutes A B C D Depth Remarks __________________________________________________________________________ Example 1 40.00 <.001" .006" .010" .005" .0031" -- Example 2 60.20 <.001" .005" .007" .003" .003" -- Example 3 31.79 <.001" .007" .006" .002" .002" -- Example 4 79.05 <.001" .008" .012" .002" .0034" -- K7H 49.89 .002" .010" .021" .006" .0085" -- TiC-Mo.sub.2 C-Mo-Ni 46.03 <.001" .007" .012" .038" .0023" Failed .times..020" by chipping at scale line __________________________________________________________________________ (*) Wear Pattern Nomenclature: A....Notch due to crater breakout B....Corner wear C....Flank wear D....Notch at scale line

Table 2 __________________________________________________________________________ Wear Pattern of the Tools Described in Examples 1 through 4 in Comparison to Commercial Sintered Carbides. Test Condition B Total Cutting Time Wear Pattern* Crater Edge Re- Tool Minutes A B C D Depth Deform marks __________________________________________________________________________ Example 1 8.18 <.001" .004" .004" .006" .0015" <.0005" not failed Example 2 10.05 <.001 .006+" .005" .004" .0015" <.0005 not failed Example 3 5.06 <.001" .007" .006" .003" .001" .002" -- K7H 9.82 .020" .008" .006" .014" .0052" <.0005" chipped at scale line TiC-Mo.sub.2 C-Mo-Ni 4.68 <.001" .010+" .008" .022" .0017" .0028" chipped at scale line C-5 1.70 .006" .020" .016" <.002" .0067" .0045" -- __________________________________________________________________________ (*) Wear pattern nomenclature same as Table 1.

Table 3 __________________________________________________________________________ Wear Pattern of the Tools Described in Examples 1 and 2 in Comparison to Commercial Sintered Carbides Test Condition C Total Cutting Time Wear Pattern* Crater Tool Minutes A B C D Depth Remarks __________________________________________________________________________ Example 1 45.58 <.001" .006" .008" .002" .002" -- Example 2 60.66 <.001" .005" .006" <.002" .0021" -- K7H 34.87 .001" .009" .012" .004" .005" -- TiC-Mo.sub.2 C-Mo-Ni 53.24 .004" .005" .007" .007" .0015" -- __________________________________________________________________________ (*) Wear Pattern Nomenclature same as Table 1.

Table 4 __________________________________________________________________________ Wear Pattern of the Tools Described in Examples 1 and 2 in Comparison to Commercial Sintered Carbides - Test Condition D Total Hardness Cutting Time Wear Pattern* Crater Re- Tool of Steel Minutes A B C D Depth marks __________________________________________________________________________ Exam- R.sub.c 51 to 54 107.42 <.001" .006-" .008" <.001" .00075" not ple 1 failed Exam- R.sub.c 55 to 57 9.82 <.001" .004-" .004" <.001" -- not ple 1 failed Exam- R.sub.c 47 to 49 157.77 <.001" .005" .006" <.001" .0007" not ple 2 failed K7H R.sub.c 49 to 52 88.47 .021" .007" .010+" .001" -- chipped at (A) K7H R.sub.c 55 to 57 9.96 <.001" .005+" .024" .001" -- edge break- down __________________________________________________________________________ (*) Wear Pattern Nomenclature same as Table 1

FIGS. 2 and 3 are graphical representations of some of the tests which provided the data for Tables 1 and 4 respectively. FIG. 2 shows the averaged corner and flank wear as a function of cutting time for tools formed from the above Examples 1 and 2 and the prior art tools TiC-Mo.sub.2 C-Mo-Ni and K7H when subjected to the Test Condition A. Curve 12 shows the wear of the K7H tool, curve 14 the wear of the TiC-Mo.sub.2 C-Mo-Ni tool, curve 16 the wear of the Example 1 tool and curve 18 the wear of the Example 2 tool.

FIG. 3 shows the averaged corner and flank wear as a function of cutting time for tools formed from the above Examples 1 and 2 and the prior art tool K7H when subjected to the Test Condition D. No curve for the prior art tool TiC-Mo.sub.2 C-Mo-Ni is shown since this tool failed almost instantly when subjected to this test condition. Curve 20 shows the wear of the K7H tool when machining steel having a hardness of R.sub.c = 49 to 52. Curve 22 shows the wear of the Example 1 tool when machining steel having a hardness of R.sub.c = 51 to 54. Curve 24 shows the wear of the Example 2 tool when machining steel having a hardness of R.sub.c = 47 to 49.

It is seen from the curves of FIGS. 2 and 3 that the same compositions according to the present invention have superior wear characteristics over both the best prior art tool for the moderate Test Condition A, the tool TiC-Mo.sub.2 C-Mo-Ni, and the best prior art tool for the more severe Test Condition D on hardened steel, the tool K7H.

The following Table 5 shows the wear rate of a large number of tools formed from specific compositions in accordance with the present invention and also of several prior art tools when subjected to the Test Condition A.

Table 5 __________________________________________________________________________ Gross Observed Wear Rates Composition of Mils per Minute Carbide Phase Binder Flank Crater Remarks __________________________________________________________________________ (Ti.sub..5 Mo.sub..5)C.sub..85 12% Co .21 .13 (Ti.sub..6 Mo.sub..4)C.sub..82 3.6%Ni,7%Co .40 .09 Brittle (Ti.sub..7 Mo.sub..3)C.sub..90 6%Fe,6%Ni .09 .07 Slight plastic deform. (Ti.sub..65 Mo.sub..35)C.sub..82 9% Co -- -- Brittle (Ti.sub..60 Mo.sub..40)C.sub..84 9% Ni -- .10 -- (Ti.sub..60 Mo.sub..40)C.sub..82 9.5%Ni .17 .075 -- (Ti.sub..50 Mo.sub..50)C.sub..85 8.8%Co .125 .071 -- (Ti.sub..80 Mo.sub..20)C.sub..93 12% Co .068 .04 -- (Ti.sub..80 Mo.sub..20)C.sub..92 5.65%Ni,5.65%Co .085 .063 Slight edge deform. (Ti.sub..70 Mo.sub..30)C.sub..90 5%Ni, 5%Co .10 .089 -- (Ti.sub..86 Mo.sub..14)C.sub..94 11%Ni .066 .04 -- (Ti.sub..87 Mo.sub..13)C.sub..94 12.5%Ni .11 .067 -- (Ti.sub..80 Mo.sub..20)C.sub..92 11%Ni .063 .04 -- (Ti.sub..75 Mo.sub..25)C.sub..89 12%Ni .08 .067 Slight edge deform. (Ti.sub..75 Mo.sub..25)C.sub..92 10.5%Ni .065 .05 -- (Ti.sub..70 Mo.sub..30)C.sub..90 11.5%Ni .08 .063 Slight edge deform (Ti.sub..70 Mo.sub..30)C.sub..90 9.8%Ni .065 .05 -- (Ti.sub..68 Mo.sub..32)C.sub..89 10.5%Ni .06 .053 -- (Ti.sub..60 Mo.sub..40)C.sub..87 10.3%Ni .12 .10 Slight edge deform (Ti.sub..60 Mo.sub..40)C.sub..84 10.5%Ni .16 .08 Slight Edge deform (Ti.sub..60 Mo.sub..40)C.sub..87 8.5%Ni .08 .073 -- (Ti.sub..54 Mo.sub..46)C.sub..84 8.5%Ni .12 .09 -- (Ti.sub..40 Mo.sub..60)C.sub..81 8%Ni .25 .27 -- (Ti.sub..50 Mo.sub..50)C.sub..81 7%Ni .128 .084 -- (Ti.sub..50 Mo.sub..50)C.sub..82 12%Ni .16 .097 -- (Ti.sub..54 Mo.sub..46)C.sub..894 8.5%Ni .123 .085 trace excess carbon. Prior Art Tools __________________________________________________________________________ C-5, C-5A 1.3-1.7 .40-.80 typical tool life 7 to 10 minutes C-6, C-7 .28-.40 .12-.18 -- TiC-Mo.sub.2 C-Ni .15-.25 .03-.06 Notching at scale line TiC-Mo.sub.2 C-Mo-Ni .09-.21 .05 Severe notching at scale line. Cast Carbide (Ti-W-C base) .30-.35 .09 -- TiN coated C-5 (for life of coating) .12-.18 .03-.10 Typical tool life 35 minutes TiC coated C-6 (for life of coating) .14-.16 .08-.12 -- __________________________________________________________________________

The following Table 6 shows the wear rate of representative prior art commercial carbide tools when subjected to the Test Condition B:

Table 6 __________________________________________________________________________ Carbide Range of Observed Wear Rates Class Mils per Minute Flank Crater REMARKS __________________________________________________________________________ C-5, C-5A 9. to 12 4 to 5 3 to 5 mils thermal deformation at 1.5 minutes C-6, C-7 .55 to .85 .5 to .7 1 to 3 mils thermal deformation after 5 to 10 minutes. TiC-Mo.sub.2 C-Mo-Ni .7 to .9 .35 to .45 Frequent tool break- age, 2 to 3 mils thermal deformation after 4 to 6 minutes __________________________________________________________________________

The corresponding wear rates for tools formed from compositions according to the present invention are shown in FIG. 5, described below.

The following Table 7 shows the wear rates of representative prior art commercial carbide tools when subjected to Test Condition D:

Table 7 __________________________________________________________________________ Carbide Hardness of Range of Observed Class Test Steel Flank Wear Rates, Mils per Minute Remarks __________________________________________________________________________ C-2 R.sub.c 51 to 55 Rapid Failure Poor finish C-5 R.sub.c 51 to 55 Rapid Failure Poor finish C-6,C-7 R.sub.c 52 to 55 1.5 to 3.0 Frequent fail- ure by corner delamination on steels with R.sub.c >54 C-6,C-7 R.sub.c 46 to 48 .08 to .15 -- K7H R.sub.c 49 to 52 .08 Good finish TiC-Mo.sub.2 C-Ni R.sub.c 51 to 54 -- Failure on contact TiC-Mo-Ni R.sub.c 49 to 52 -- Failure by corner delamin- ation after .1 to 5 minutes TiC-Mo-Ni R.sub.c 46 to 49 -- Failure by corner delamin- ation usually at start of second pass __________________________________________________________________________

The corresponding wear rates for tools formed from compositions according to the present invention are shown in FIG. 6, described below.

FIGS. 4, 5 and 6 show the wear rate of tools formed from compositions in accordance with the present invention as a function of the exchange ratio of molybdenum for titanium, or the mole per cent molybdenum in the total metal content of the carbide phase, for different test conditions. In FIG. 4 the curve 26 shows the averaged corner and flank wear rates and the curve 28 shows the top cratering rate for Test Condition A. In FIG. 5 the curve 30 shows the averaged corner and flank wear rates and the curve 32 shows the top cratering rate for Test Condition B. FIG. 6 shows averaged corner and flank wear rates for Test Condition D. The curve 34 shows these rates for machining hardened steel having an R.sub.c = 46 to 49. The curve 36 shows these rates for machining hardened steel having an R.sub.c = 50 to 55. These curves, when viewed together, show that, within the composition range of the present invention, for progressively more severe machining test, it is desirable to have higher exchange ratios of molybdenum for titanium.

It is noted that the curves of FIGS. 4 through 6 are shown independent of the carbon content, or the subscript z in (Ti.sub.x Mo.sub.y)C.sub.z. This is because it has been found that, within the range of the present invention as shown in FIG. 1, the performance of a tool is a function of the metal exchange ratio, but for a given metal exchange ratio varies very little with changes in carbon. This is contrasted with the prior art compositions in which the carbon content was critical, and in which slight changes in the carbon content could change a satisfactory material into a very brittle material. Thus, it is much easier to formulate the compositions of the present invention.

The composition of the present invention is formed from the above described carbides bonded with a binder from the iron metal group, such as nickel, cobalt and iron. The binder can form from 5 to 25 percent by weight of the composition. If too little binder is used, the composition will be too brittle. If too much binder is used, the composition will be too soft and will deform. Preferably, the binder forms from 8 to 12 percent by weight of the composition.

The selection of the proper binder is somewhat dependent upon the mole per cent molybdenum in the carbide phase of the composition. For compositions in which the carbide phase contains less than 40 mole per cent molybdenum, cutting performance of cobalt or nickel bonded tools is considered equivalent. Carbides with more than 55 mole per cent molybdenum show embrittlement when using a cobalt binder. Iron binders are useful only for carbides containing less than 35 mole per cent molybdenum. Carbides with more than 60 mole percent molybdenum are unstable in the presence of cobalt and iron and decompose under formation of titanium-richer monocarbide solutions, .eta.-carbides, and free carbon. .eta.-carbide formation is especially pronounced in substantially carbon-deficient carbide solutions.

The properties of the carbide-metal composites can be extensively modified by alloying. The following summary of the effects of the principal alloying ingredients are based on performance studies of the composites as tool materials in turning 4340 steel. However, low level alloying with other elements may also be accomplished without departing from the spirit of the invention.

1. Substituting vanadium for up to 20 percent of the amount of titanium present proves essentially inert with respect to cutting performance. Carbide grain growth during sintering, however, appears enhanced, necessitating lower sintering temperatures and more careful temperature control in the fabrication of vanadium-alloyed composites.

2. Niobium, tantalum, and hafnium added in exchange for up to 30 mole per cent of the titanium in molybdenum-rich alloys, or alloys containing more than 40 mole per cent molybdenum, decrease tool deformability and thus permits the use of higher binder contents than is possible with the unalloyed composite. Alloying of high titanium alloys with these elements does not result in beneficial effects.

3. Addition of zirconium in exchange for titanium causes a marked decrease in the performance of the composites as machine tools.

4. Tungsten can be alloyed in substantial quantities (up to 50 mole per cent) in exchange for molybdenum in the composites of the invention without impairment of performance. Small additions of tungsten (less than 5 mole per cent in exchange for molybdenum) are beneficial in retarding grain growth during sintering. Titanium carbide-rich alloys containing more than 70 mole per cent TiC are especially insensitive towards higher concentrations of tungsten, while molybdenum-rich alloys tend to reject free graphite from the solid solution if the tungsten exchange for molybdenum exceeds 20 mole per cent. Formation of free graphite can be avoided by lowering the carbon content of the alloys between 0.5 and 1.5 atomic per cent. The use of cobalt and iron-base binders is not recommended for substantially carbon-deficient, tungsten-containing carbides, as they invariably cause embrittlement of the composite by .eta.-carbide formation.

5. Low level (less than 10 mole percent) additions of chromium in exchange for molybdenum (or for molybdenum and tungsten if tungsten is also used) appeared to be inert, while larger additions resulted in a pronounced decrease in the performance of the composites as machine tools.

6. Addition of nitrogen in partial replacement (up to 25 mole percent) for carbon slightly improves wear-resistance of the composites under light cutting conditions.

Alloying with other transition metal carbides was accomplished by using preformed solid solutions of these carbides with TiC, such as (T,W)C.sub. z and (Ti,Hf)C, or with MoC.sub.z, such as, for example, (Ta,Mo)C.sub.z and (V,Mo)C.sub.z. Preparation of these carbide solid solutions was analogous to the procedures described for the titanium-molybdenum carbide alloy.

The following Table 8 shows the wear rates for a number of tools formed from compositions incorporating some of the alloy substitutions just discussed when these tools were subjected to Test Condition A:

Table 8 __________________________________________________________________________ Gross Observed Wear Rates Composition of Mils per Minute Carbide Phase Binder Flank crater Remarks __________________________________________________________________________ (Ti.sub..4 Ta.sub..1 Mo.sub..5)C.sub..85 11%Ni .28 .19 Brittle (Ti.sub..4 Ta.sub..1 Mo.sub..5)C.sub..85 13%Ni .19 .16 -- (Ti.sub..4 V.sub..1 Mo.sub..5)C.sub..84 12%Ni .18 .13 -- (Ti.sub..72 Nb.sub..08 Mo.sub..20)C.sub..94 13%Ni .12 .07 -- (Ti.sub..72 V.sub..08 Mo.sub..20)C.sub..93 13%Ni .12 .07 -- (Ti.sub..6 Mo.sub..3 W.sub..1)C.sub..93 9%Ni -- -- Excess carbon (Ti.sub..6 Mo.sub..36 W.sub..04)C.sub..87 9%Ni .08 .07 -- (Ti.sub..8 Mo.sub..15 W.sub..05)C.sub..94 11.3%Ni .06 .04 -- (Ti.sub..5 Mo.sub..3 W.sub..1)C.sub..82 9.3%Co -- -- Brittle (Ti.sub..5 Hf.sub..1 Mo.sub..4)C.sub..86 9%Ni .09 .08 -- (Ti.sub..4 Zr.sub..1 Mo.sub..5)C.sub..86 11%Ni .60 -- Edge chipping (Ti.sub..7 Mo.sub..3)(C.sub..76 N.sub..14) 10%Ni .06 .04 -- __________________________________________________________________________

The composites of the invention also may be modified and adapted for special applications by surface coatings of wear-resistant alloys based on carbides and nitrides of the refractory transition metals.

The data shown in the above discussed tables and graphs is representative of many other alloys within the range of the invention which were prepared and tested. It becomes evident from a comparison of the performance data that the carbide-metal composites of this invention afford a substantial improvement of the existing carbides in terms of tool reliability, wear performance, and versatility of application. It is noted from the results presented that the molybdenum-richer compositions are particularly suited for rough cutting soft and medium hard steels and for machining hardened steels, while the titanium-richer composition (greater than 70 mole percent titanium) are better suited for light cutting and finishing of soft and medium hard steels. The wear data also indicate that no performance advantage in cutting steels is being gained by increasing the titanium exchange in the composites of the invention substantially above 80 mole percent (approximately 69 weight percent TiC): tool reliability decreases rapidly, especially in somewhat heavier cuts, and the failure mechanisms become similar to those of the prior art TiC-Mo.sub.2 C-Mo-Ni tool materials.

While the principal application of the new composite materials of the invention is envisioned to be in the area of machine tools, their high wear-resistance will make them also suitable for applications where currently tungsten carbide-based cemented are used, such as wear-resistant linings, gage-blocks, bearings, etc., and for milling and drilling in the mining industry.

While the invention is thus disclosed and many specific embodiments described in detail, it is not intended that the invention be limited to those shown embodiments. Instead, many modifications will occur to those skilled in the art which fall within the spirit and scope of the invention. It is intended that the invention be limited only by the appended claims.

Claims

What is claimed is:

1. A composition of material comprising sintered carbide-binder metal in which said carbide has titanium and molybdenum as its base metal and has a gross composition falling within the area EFGH of FIG. 1, and in which said binder is selected from the iron group metals and comprises between 5 and 25 weight percent of the composition.

2. A composition of material according to claim 1 in which said carbide has a gross composition falling within the area E'F'G'H' of FIG. 1.

3. A composition of material according to claim 2 in which said binder is selected from the group consisting of nickel and cobalt and comprises between 8 and 12 weight percent of the composition.

4. A composition of material according to claim 1 in which up to 30 mole percent of the total amount of titanium in the composition is replaced by a material chosen from the group consisting of hafnium, niobium and tantalum.

5. A composition of material according to claim 1 in which up to 50 mole percent of the total amount of molybdenum in the composition is replaced by tungsten.

6. A composition of material according to claim 5 in which up to 10 mole percent of the combined amounts of molybdenum and tungsten in the composition is replaced by chromium.

7. A composition of material according to claim 1 in which up to 25 mole percent of the total amount of carbon in the composition is replaced by nitrogen.

8. A composition of material according to claim 1 in which up to 20 mole percent of the total amount of titanium in the composition is replaced by vanadium.

9. The method of forming a composition of material comprising a sintered carbide-binder metal in which said carbide has titanium and molybdenum as its base metal and has a predetermined gross composition falling with the area EFGH of FIG. 1, comprising the steps of:

forming a homogeneous solid solution powder alloy of (Ti,Mo)C which contains a higher mole percent molybdenum than said predetermined gross composition,

adding a sufficient amount of titanium monocarbide to said homogeneous solid solution powder alloy so that said powder alloy and said titanium monocarbide together have said predetermined gross composition,

adding binder material to said mixture,

mechanically blending said mixture until it has a uniform consistency,

compacting said mixture into a desired shape, and

sintering the composites so formed at an elevated temperature.

10. The method of claim 9 which further comprises the step of grinding the sintered composite into predetermined shape to form a metal cutting tool.