US2942971A - Process of making cemented carbide products - Google Patents

Description

June 28, 1960 w. w. WELLBORN 2,942,971

PROCESS OF MAKING csmzuwsn CARBIDE PRODUCTS Filed Feb. 3, 1955 2 Sheets-Sheet. 1

INVENTOR M/A' m surf/w, 7% W+W ATTORNEY June 28, 1960 w. w. WELLBORN PROCESS OF MAKING CEMENTED CARBIDE PRODUCTS Filed Feb. 3, 1955 INVENTOR. WLL/AMl K/VELLBORN fimjlyemihflv 2 Sheets-Sheet 2 ZORGOQSOU W M R In the drawings: Figure 1 is a micrograph showing structure under 1500 magnification of a composition, alloy or product made from an unsaturated solid solution of TiC and WC and 'sintered as a mixed crystal with cobalt and tungsten carbide;

Figure 2 is a micrograph showing structure under 1500 magnification of an exemplary composition, alloy or product of my invention and produced in accordance with my invention and thus, essentially employing a saturated and as an optimum, a super-saturated solid solution of titanium and tungsten carbides as a mixed crystal and as comminuted, admixed and sintered with tungstencarbide and cobalt; p

Figure 3 is a micrograph showing structure under 1500 "magnification'ot another exemplary composition, alloy or product of my invention and produced in accordance with my invention; it will be noted that Figure 2 relates to alloy A of Figure 4 and that Figure 3 relates to alloy B of the same figure;

' Figure 4 is a triangular co-ordinate plot or graph by total volume percentages of tungsten and titanium carbides with cobalt that are employed in accordance with an are-a (shown with cross lines) within which a cutting index of greater than a 300 (V-60) rating on a general purpose test is attained. The shaded (cross hatched) area defined by dotted line portion 4 within overlapping portions'of curves 1 and 4 represents an area of volume content of my invention, wherein the three properties represented by curves or lines 1, 3 and 4 are obtained, as a minimum, and in combination' Points A' and B indicate representative alloys of my invention which are hereinafter discussed in some detail. The area defined by the solid lines of the parallelogram 5 lies fully within the area of 4' and thus, is aneasily defined area within which the proper ties of my invention are attained. By way of example, the parallelogram 5 represents an area containing about 56.5% to 62.0% WC, about 26.5% to 36.0% TiC and about 7.5% to 11.5% C0 by volume.

It should be noted that the two compositions illustrated in Figures 1 and 2 were produced by employing the same crushed sizes of ingredients in both cases and involving the same total volumes of WC, T iC and Co.

In determining the composition of my solid metal carbide alloy, I recognized that a cobalt binder normally clings closely when fused to tungsten carbide, but has less affinity for titanium carbide. It has thus been'customary to use nickel as a binder for a titanium carbide composition, product or tool.

Another factor which had to be considered is that the strength of a solid metal carbide ordinarily depends on its type of boundary, since the latter is considered stronger than the individual crystals. Thus, it is ordinarily easier to break a coarse grain structure because the proportioning of boundary to crystal is less. However, tungsten carbide, itself, has a peculiar characteristic in that the boundary is first broken before the crystal structure is pulled apart.

Taking the above and other factors into consideration,

. I have been able to produce a composition employing both titanium and tungsten carbides and of such a type that the boundaries are strengthened and depend essentially on the use of cobalt.- The composition or product is such that the overall content'of tungsten carbide is 3 always greater on a volume basis than that of the titanium carbide, the titanium carbide is only employed as a fully tungsten-carbide-saturated mixed crystal with the binder metal, and additional tungsten carbide is employed with the mixed crystal and the binder metal. When this holds true, among other things, the titanium carbide is unable to have any adverse efiect on the clinging action of the binding metal and, in fact, I have determined that cobalt is the best suited binder metal for my composition. In addition, there appears to be a double strengthening type of action in the composition such that resistance to break is increased, both in the boundaries as well as in the crystal structure and to such extent that the ratio I or proportioning between the two approaches unity. That is, the solid solution or crystal has, substantiallyv the same strength or resistance to break as the grain boundaries. In addition, grain growth or granulation of the crystal structure is limited during the processing of the composition and is prevented once the composition has been produced and shaped in solid form for and used as a cutting element.

After having determined that desired results could be obtained employing titanium and tungsten carbides with cobaltas a binder, I discovered that it is essential in producing the new and improved product to preliminarily prepare and to then employ a mixed crystal of the two carbides, essentially in which the titanium carbide is at least fully saturated by dissolved tungsten carbide. There is a limitation that the titanium carbide of the preliminarily formed mixed crystal or solid solution is in amount by volume that, as a maximum, is not over 57% of such crystal. Also, it is essential that the mixed crystal be used as the major ingredient of the carbide composition or product, in the sense that its volume content predominates over that of the WC and Co (the other additions of the second or final part of the procedure).

It is necessary to provide a mixed crystal that is at least a fully saturated solution of tungsten carbide in I titanium carbide, as produced by comminuting or powdering, admixing, pressing and shaping, and sintering the elements or ingredients involved and at a temperature and for a period suflicient to assure the attainment of a tion, grain growth, and to control fully saturated solid solution. This was determined to be necessary to limit the sintering time of the second operathe temperature required for such operation, and and produce a better resultant grain structure and a desired metallographic pattern.

If the mixed crystal used is not fully saturated, it adversely affects the later added ingredients and the required structure cannot be obtained. From a processing standpoint, the heating time usually required in the second sintering operation is about 60 to 90 nnnutes and at a temperature of about 2725 F., as compared to about 30 to 45 minutes (half the time) and at a temperature of 2700 F. (25 lower) when a saturated crystal is used in accordance with my invention. A relatively short sintering time and low sintering temperature is required to obtain a full densifying of the composition compact of 7 my invention.

' such as WC, TiC, and Co,

In this particular art where there are three components,

' being held substantially constant in volume with respect not true employing my invention, in

' see the cross hatched area defined by the curve line 4' of Figure 4 of the drawings.

'Those skilled in the art would expect that transverse rupture strength would drop as the titanium carbide content is increased. But, employing my invention, strength is actually greater in the area defined by curve 4' than in a straight tungsten carbide, cobalt grade composltion and, contrary to common belief.

In producing the composition or product of my invenand with the cobalt content of 60 minutes.

blended with comminuted tungsten carbide and'cobalt.

The mixed powder is then compacted (shaped) and sintered. More particularly, by way of example:

(a) Take about 55% by volume of TiC and about 45% by volume of WC, with a permissible variation of up to a maximum of 57% by volume of TiC. Intimately admix the ingredients in powdered form, heat to a temperature of about 3800 F. (or to a higher temperature which is the proper solution temperature where less volume TiC is used and more WC is to be dissolved) in a non-contaminating ambient atmosphere and hold for about one hour; thereafter cool to provide the mixed crystal as a saturated or super-saturated solid solution of WC in TiC. As an alternate, Ti ,'carbon andvtu'ngsten in powdered form are used and the solution forming operation is conducted as before, but preferably in .a carbon tube furnace. It appears that a higher combined carbon content is made possible by this method and that purity is better than by using TiC and WC directly; (b) (hush and grind the mixed crystal toforma powder; 7 i m4 (c) Mix, blend and grind the mixed crystal or about and about 8.3% by volume of cobalt:

.(d) Compact the mixed powder at a suitable pressure p of about 10m 15 tons per square inch;

(a) Sinter ,at about 2700 F. within 'a non-contamior for an optimum of 30 minutes employing a hydrogen atmosphere. Vacuum sintering may also be used.

Ihave found that a practical approach that workswell is to produce with; a permissible variation of ":2% of the TiC, the WC and the mixed crystal of steps (a) and (c). 5

79% by volume of TiC and 21% by volume of 'WC'and heat to 3800" F. for one hour, followed by cooling, as in step (c), The mixed crystal would be crushed and ground as before, see (b). The crystal in an amount of about 42.3% by volume wouldthen be mixed, 'blended and ground with about 49.4% by volume of tungsten carbide and about 8.3% by volume of cobalt. Compact as in (d Then sinter in hydrogen or a'vacuurn .at about 2725 F. for 60 to 90, minutes, with an optimum From the above, it will be apparent that the temperatures and period of application of temperatures required for the last step employing the 'old' method is much greater than in my process. The basic structureand nature of the composition produced, see the micrograph of Figure l, is entirely at variance with that of a com-v position produced in accorda'ncewith or having the controlled'content of my inventive disclosure, see the microraph of Figure 2. 5

My procedure is conducted in such a manner as t provide an area of composition wherein a superior grade of carbide product suitable for general purpose steel cutting and milling is obtained. The compositions are better grades than those outside the area by reason of their contentalone, but as produced following the procedur'e-of my invention, in which anessentially saturated 'or' super saturated mixed crystal is employed, the results ,ar'e farsuperior, I have also determined thata proper "employment of a cobalt binder is important in obtaining the results of my invention. Itappears from my work that tantalum carbide may, be substituted for-titanium carbide in the mixed crystal up to about by volume to also obtain a'compositiori or product having improved properties over the prior art.

' To further exemplify from the standpoint of the product produced, I have taken two representative alloys or Compositions A and B (see the micrographs of Figures 2 and 3) that are located within the limitations of the area 4' of Figure 4. In this connection, Table I shows the specific composition and properties of Alloy A.

TABLE I Alloy A Content for Resultant first part of Content for second overallconprocedure, part of procedure, tent, percent percerit by percent by vol. by vol.

T10, Mixed crystal, 60.3 Tiqaas w W0, 31.4 W0, 58.4

'- In producing Alloy B, the same mixed crystal content 60.3% by volume with about"-3 l.4%by 'volumeof WC nating ambientfatmosphere for about 30 to'45 minutes,

as shown in column one of Table I' is employed. However, .thevolume content for the second step and the overall resultant content of the ingredients in the prod- Although both Alloys A and B have the superior prop- I erties of my new composition, it will be noted that Alloy B has less titanium carbide than; A and is thus tougher and softer thanA. It also has a slightly less cutting ability than A However, both are tougher and harder than any heretofore 'known carbide. Table III shows the comparative properties of these two compositions.

The cutting index represents surface feet per minute for a tool life of sixty minutes before-regrind, based on a general'purpose-test, employing adepth of cut of 0.125

of an inch, with a feed of 0.020 of an inch on an AISI steel test log having a B.H.N. hardness of 285. An article entitled Carbide Tool Evaluation, by H. O.

Warnock, inthe February 1954 issue of the Tooling and Productionmagazine, discusses such a type of test.

, TABLE 111 Alloy A 7 Alloy B Strength (transverse 210,000 240,000.

rupture p s Hardness 92% Rockwell A 91% Rockwell A. Outtinglndex 3 for V-60 300 for V-.-60.

Alloys produced in accordance with my invention have a hard dense structure andwhen employed as a cutting tool'or element, show an increase in operating life to microns.

about 1,000 hours (from about ZOOhours).

sities of the three principal ingredients employed in;mak-

ing the composition. In this connection, -TiC-has a gramsper cubic centimeter density of 4.25, WC of 15.7 and Co of 8.9. In arrivingat my;invention, weight, ,per-

"7 centages give no definite idea of the amount of material elfecting an action and this is the reason why I employ a volume and to assure a volume balance inthe composition.

Figures 1 and 2 are micrographs of two alloys having the same ultimate content or composition, namely, about 58.4% WC, 33.3% TiC, and 8.3% of Co, allby volume. The composition of Figure l isbased on the use of an unsaturated mixed crystal of 42.3% by volume as employed with tungsten carbide of 49.4% by volume and cobalt of 8.3% by volume in the second part of the procedure (the final sintering operation).

On the other hand, in the second part employed in producing the composition of Figure 2, a fully saturated mixed crystal constitutes about 60.3% by volume, the tungsten carbide 31.4% by volume, and the cobalt 8.3% by volume.

The mixed crystal employed in the second part in producing the composition of Figure 1 is not saturated. Thus, during sintering, there is a tendency for the titanium carbide to continue towards saturation and this tendency is strong enough to cause excessive grain growth which is further enhanced by the longer sintering time and higher temperature that is required.

The composition of Figure 2is characterized by its volume balanced relationship-finer grain structure, increased hardness and surprisingly, by its increased strength and toughness, all in combination. The micrograph shows a matrix ,of TiC-WC solid solution (essentially fully saturated) and uniformly dispersed finer than usual WC grains. In this connection, a slight excess of WC is employed to assure full saturation of the mixed crystal, as produced in accordance with the first part of my procedure. Normally in sintering, as occurs in connection with the second part of the procedure used in providing the composition of Figure 1, there is an absorption of WC into the structure while in the composition of Figure 2, there is an equilibrium of structure as to the mixed crystal, itself, and between the mixed cryastlal and the additional tungsten carbide and the co a t.

A uniform, finer and homogeneous structure with more interrupted or jagged break lines defining a more tortuous path is also shown in the structure of Figure 2. There is no evidence of unreacted titanium carbide, as such, and the cobalt content is primarily in grain boundaries, is spread more thinly and is not in the solution of the crystal. There'is a larger solution area (the snow area of Figure 2) of matrix and with more widely dispersed tungsten carbide grains and essentially, of a finer structure. It ,is a precipitation-interrupted structure and is basically different and improved in its characteristics from prior art compositions and from one employing an unsaturated mixed crystal or one having an unsaturated final composition (see Figure l) and even when, as in the case of Figure 1, the same total ingredient content exists in both compositions;

The product of my invention represents a new concept in carbide metallurgy, since it gives increased hardness with increased strength to thus provide greater Wear resistance combined with increased shock resistance, as in milling applications; When used asa cutting tool, it provides an increased amount of metal removed" in mass production operations to materially reduce unit costs through its uniform performance. In tests against ten competitive grades, it has proved superior in all cases, showing from 20 to 25% less Wear at 5 cuts and 23% less wear at 20 cuts. Six of the tend grades were worn beyond repair before the test was completed.

Such tests were conducted under severe breakdown conditions on a inch milling machine, using a 6 inch R.H. cutter with one tooth of 1 inch square having a ,5 of an inch corner radius. The material used for the work piece was a S.A.E. 1020, normalized, forged steel bar that was climbmilledat 1375 s.f.p'.m. with 4% titanium carbide solid solution.

' powdering,

inch feed per minute, resulting in about .0053 of an inch chip load. The depth of the cut was .050 of an inch and the over-hang of the work was 3 inches.

Summarized briefly, I have produced a sintered or hard metal cemented carbide composition that has a precipitation-interrupted micro-structure consisting of fine-grain precipitated tungsten carbide grains or particles, some retained coarser medium-grain tungsten carbide grains or particles and all, as uniformly and widely dispersed in a matrix of a saturated tungsten carbide- The composition is bound together between its grainsor crystals with a 'thin layer of an evenly distributed cobalt binder; it has a volume-balanced composition and essentially, of the ingredients making up its crystal structure.

'My invention is made possible by the production of a volume-balanced composition whose total content falls within the area enclosed by the line 4 of Figure 4 of the drawings. Itutilizes a saturated or supersaturated mixed crystal in which the titanium'carbide is in an amount of not over 57% by volume of such crystal; it has a new and improved combination of strength, hardness and cutting index properties; and, it has a new and improved microstructure and strength relationship between its carbide crystals and its cobalt binder.

Whatl claim is:' i

1. A process, of making a cemented carbide alloy hard metal product characterized by its superior combination of transverse rupture strength, hardness and cutting index and by its improved microstructure which comprises, mixing, and compacting about 45% tungsten carbide by volume and about 55% titanium carbide by volume -2% of each ingredient, sintering the compaced ingredients at an elevated temperature and for a period sufficient to supersaturate the titanium carbide by the 'tungsten'carbide and while maintaining the defined proportioning of carbides until the titanium carbide is supersaturated by the tungsten carbide, and cooling to form a solid solution crystal and in order to maintain the titanium carbide supersaturated; finely comminuting the supersaturated solid solution crystal and also tungsten carbide and cobalt auxiliary ingredients, admixing them in a volume proportioned relationship in which the supersaturated solid solution crystal content is employed in volume predominance over the individual content of the auxiliary ingredients, the total tungsten carbide content of the admixture is employed in volume predominance over each of the other individual ingredients thereof, and the total volume content of the individual ingredients thereof is maintained within a range of about 54.5 to 63.7% tungsten carbide, about 25.5 to 36.0% titanium carbide, and about 6.3 to 12.0% cobalt; and compacting and sintering the admixture into a hard metal product.

2. A process as defined in'claim 1 wherein tantalum carbide 'is substituted for titanium carbide in the mixed crystal up to 50% by volume of the titanium carbide, and the total volume content of the tantalum and titanium carbides is maintained within the range of about 25.5 to

36.0% for the titanium carbide of claim 1.

3. A process as defined in claim 1 wherein the supersaturated solid solution crystal of a volume of about 44.3 to 62.3% is admixed with the auxiliary ingredients in a volume of about 29.4 to 44.9% of tungsten carbide and about 8.3 to 10.8% of cobalt, and always with the solid solution crystal in volume predominance over the auxiliary ingredients.

4. A process in making a cemented carbide alloy hard metal product characterized by its superior combination of transverse rupture strength, hardness and cutting index and by its improved microstructure which comprises, powdering, mixing and compacting about 45 tungsten carbide by volume and about 55% titanium carbide by volume :2% of each ingredient, sintering the compacted ingredients in a non-contaminating atmosphere at a solution temperature of a minimum of about 3800" F. while holding for about an hour and maintaining the defined proportioning of carbides until the titanium carbide is supersaturated by the tungsten carbide, cooling to form a solid solution crystal in which the titanium carbide is supersaturated; finely comminuting the supersaturated solid solution crystal and also tungsten carbide and cobalt auxiliary ingredients, admixing them in a volume proportioned relationship in which the supersaturated solid solution crystal content is employed in volume predominance over the individual content in the auxiliary ingredi- 10 cuts, the total tungsten carbide. content is employed in volume predominance over each of the other individual ingredients thereof, and the total volume content of the individual ingredients thereof is maintained within a range of 54.5 to 63.7% tungsten carbide, about 25.5 to 36.0% titanium carbide and about 6.3 to 12.0% cobalt; and

compacting the admixture and sintering it at about 2700 F. for about thirty to forty-five minutes in a non-containdnating atmosphere into a hard metal product.

References Cited in the file of this patent UNITED STATES PATENTS Schroter et a1. Sept. 24, 1935 Lucas Oct. 18, 1938 Dawihl et al. Aug. 26, 1941 Schwarzkopf Dec. 2, 1941 Kurtz Aug. 19, 1952 Wulff Aug. 10, 1954 Goetzel Aug. 2, 1955 Lucas et a1. Jan. 24, 1956 FOREIGN PATENTS Great Britain Nov. 16, 1931 Great Britain May 11, 1933 Great Britain J an. 30, 1939 OTHER REFERENCES

Claims (1)

1. A PROCESS OF MAKING A CEMENTED CARBIDE ALLOY HARD METAL PRODUCT CHARACTERIZED BY ITS SUPERIOR COMBINATION OF TRANSVERSE RUPTURE STRENGTH, HARDNESS AND CUTTING INDEX AND BY ITS IMPROVED MICROSTRUCTURE WHICH COMPRISES, POWDERING, MIXING, AND COMPACTING ABOUT 45% TUNGSTEN CARBIDE BY VOLUME AND ABOUT 55% TITANIUM CARBIDE BY VOLUME $2% OF EACH INGREDIENT, SINTERING THE COMPACED INGREDIENTS AT AN ELEVATED TEMPERATURE AND FOR A PERIOD SUFFICIENT TO SUPERSATURATE THE TITANIUM CARBIDE BY THE TUNGSTEN CARBIDE AND WHILE MAINTAINING THE DEFINED PROPORTIONING OF CARBIDES UNTIL THE TITANIUM CARBIDE IS SUPERSATURATED BY THE TUNGSTEN CARBIDE, AND COOLING TO FORM A SOLID SOLUTION CRYSTAL AND IN ORDER TO MAINTAIN THE TITANIUM CARBIDE SUPERSATURATED, FINELY COMMINUTING THE SUPERSATURATED SOLID SOLUTION CRYSTAL AND ALSO TUNGSTEN CARBIDE AND COBALT AUXILIARY INGREDIENTS, ADMIXING THEM IN A VOLUME PROPORTIONED RELATIONSHIP IN WHICH THE SUPERSATURATED SOLID SOLUTION CRYSTAL CONTENT IS EMPLOYED IN VOLUME PREDOMINANCE OVER THE INDIVIDUAL CONTENT OF THE AUXILIARY INGREDIENTS, THE TOTAL TUNGSTEN CARBIDE CONTENT OF THE ADMIXTURE IS EMPLOYED IN VOLUME PREDOMINANCE OVER EACH OF THE OTHER INDIVIDUAL INGREDIENTS THEREOF, AND THE TOTAL VOLUME CONTENT OF THE INDIVIDUAL INGREDIENTS THEREOF IS MAINTAINED WITHIN A RANGE OF ABOUT 54.5 TO 63.7% TUNGSTEN CARBIDE, ABOUT 25.5 TO 36.0% TITANIUM CARBIDE, AND ABOUT 6.3 TO 12.0% COBALT, AND COMPACTING AND SINTERING THE ADMIXTURE INTO A HARD METAL PRODUCT.

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