5“ 91H!" U“ s ‘ $1: "WU”. Bat. .‘ “I K. 0 “Wu.“ 1...! 3., ..~.. . \ , ‘4.- x-.& “.0.“ L1“ .m‘u . ‘3 ...u ‘3. .. T. «v n... r. 56 3." r31. . .2 p.75 4 I. x .2 a, 3... a. .. S a? w... Am h... Wm m. 3 mm...— "qu fiat; m2:— “Wu ‘4” an”; , A ..u .. a . e: w; Am a: 3,» E} a. “a ”N 3 ..,:. .3». s. U 4 .. ".5. c 9.. . C a; 9. h aw. .u an an. .3 .5 an an 5.... a. .. Q . E r .1; r ,. 11-15515 THE EFFECT OF POWDER SIZE AND SIIT'I'ERII‘JG TEEH’ERATURE ON THE PHYSICAL PROPERTIES OF SOME TUNGSTEN CARBIDE - COBALT ALLOYS By Alfred D. Stevens A THESIS Submitted to the School of Graduate Studies of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of METALLURGICAL ENGINEER Department of Metallurgical Engineering 1955 THESIS ‘ti‘ v r 1-“ _ / )‘Q’b’w ABSTRACT THE EEFEET OF POI'JDER SIZE AND SDTTERD‘IG TE-IPTTRATUBE ON THE PHYSIJAL PROPERTIES OF SOME TUNGSTEN CARBIDE - COBALT ALLOYS BY Alfred B. Stevens The history and methods of manufacture of tungsten carbide - cobalt alloys are reviewed. Then a series of investigations into the properties of several alloys of this type is described. Initially, an experiment in.milling alloys of 3%, 5%, 8%L and 11% cobalt is discussed. Fbllowing heating these alloys to various temperatures, their physical prOperties ( hardness, transverse rupture strength, density, porosity, and etched structure ) were investigated. Secondly, two 5% cobalt, 9h% tungsten carbide, lS tantalum carbide alloys were milled to different average particle sizes, heated to various temperatures, and their physical prOperties were studied. Finally, three 11% cobalt, 88% tungsten carbide, 1% tan- talum.carbide alloys were milled to different average particle sizes, heated to various temperatures, and their physical properties were studied. A.discussion of the variation of properties is included. (3 $8881 INTRODUCTION Tungsten carbide-cobalt alloys, commonly referred to as "tungsten carbide", or simply "carbide", have gained wide industrial usage since their first commercial application by Schroter in the early 1920's. Originally only economically use- ful for cast iron and non-ferrous machining applications, the usefulness of these alloys has been extended into the steel and special alloy machining fields, and for wear and shock-resisting applications. This was accomplished through addition of other metallic carbides to the fundamental tungsten carbide-cobalt alloy base, and through variation in this alloy proper. Engle (l)1 and MbKenna (2) each present information on the addition of carbides of titanium, tantalum, and columbium to the basic tungsten carbide-cobalt allqys for use in steel working. Tantalum carbide was the first to be so added, and has been followed by the others. Both economics and effectiveness are considered in choosing the additional carbide used. Through the thirty years of carbide tool utilization, the carbide manufacturing industry has grown in a "trade secret" manner. Each of the approximately dozen American companies, and< the foreign ones, too, have closely guarded their manufacturing know-how. l - The numbers in parentheses refer to the list of references appended to this paper. To qualify as good tool material, these tungsten carbide- cobalt alloys must be carefully controlled throughout the manu- facturing process. The process is a powder metallurgical one involving minute particles and requiring extremely rigid methods to secure reproducible high quality. Basically, the properties of carbides are dependent upon the raw materials used and the processing cycle of mamlfacture. The former can be controlled through purchase specification, and the latter may be varied to fashion desired properties in the carbide. Knowledge of the effect of processing variation on the properties of the final sintered piece is a prime pre- requisite to production of satisfactory carbides. A general outline of manufacturing of tungsten carbide-cobalt alloys in use today follows . MANUFACTUR DIG PROCESS A mixture in monatomic ratio of finely divided tungsten powder (less than 10 micron average) and powdered carbon is heated in the absence of air to approximately ZBOOOF. to get chemical combination, forming the monocarbide WC. Cobalt oxide powder is reduced under hydrogen to metallic cobalt. Tungsten carbide and cobalt are then mixed in the desired proportion, placed in a ball mill, and milled to the desired particle size over a period of days. The milling medium varies among manufacturers, water, acetone, and naphtha being common. Following discharge from the mill, the mixture is dried and then formed under pressure to the desired shape. Dimension- al allowance is made for shrinkage during heating. The compact is then sintered. THEORETICAL ASPECTS A considerable amount of research has been reported on the theoretical aspects of tungsten carbide-cobalt alloys. Two phases have been studied: the tungsten-cobalt-carbon.ternary equilibrium system itself, and the role of cobalt in the pyro- merging reaction at elevated temperatures. . The ternary equilibrium.system has been investigated by Takeda (3) and Rautala and Norton (h). General agreement exists between the two works. The function of the cobalt in the pyromerging or sintering reaction, as tungsten carbide-cobalt alloys are heated to temp- eratures approximating 27000F. and cooled to room temperature, is discussed by wyman and Kelley (S), Dawihl and Hinnuber (6), Sandford and Trent (7), Kieffer (8), and Gurland and Norton (9). Unanimous agreement has not yet been reached on whether the'WC ' grains actually form a bridge bond or whether the cdbalt pro- vides a thin interposing layer of near molecular dimensions between WC grains, creating a true cemented bond. PRACTICAL ASPEJTS Some significant practical aspects of tungsten carbide- cobalt alloys are their measurable physical properties. The performance in metalworking operations of these alloys are a function of their properties. Stevens and Redmond (10), and the tentative standards on carbides of the ASTM (11) present informa- tion on testing of tungsten carbide-cobalt alloys. Common measured properties are: indentation hardness, trans- verse rupture strength, density, porosity, and structure. The significance of these test properties is discussed by Stevens (12), Engle (l), and McKenna (2). The measurement of the above listed properties is described following. Hardness is commonly measured on the Rockwell A scale (60 kg load). Hardness testing is dependent on variability of specimen surface, accuracy of the geometry of the diamond penetrator point, and precision of the testing instrument. Standard reference blocks are used, and special oenetrators are calibrated using the blocks. The hardness range of commercial carbides is limited, being 8b to 9h on the Rockwell A scale. In order to properly evaluate the materials, the indicated hardness is determined to tenths of a division. variation greater than 0.3 total in a specimen is undesir- able and indicative of nonauniform.microstructure. Commercial repro- ducibility in a given.charge is 0.3 point total range. Transverse rupture strength values are determined by the method cited for brittle materials in the ASTM Standards. This involves suspending the test beam across two parallel 1/8" rounds, 9/16" apart, and center loading the beam to fracture. All testing reported herein was conducted with Specimens 0.200" x 0.375" in cross-section. The testing was conducted with the 0.200" dimension.being parallel to the direction of loading and the 0.375" dimension at right angles to the direction of loading. The transverse rupture value is dependent upon the quality of the prepared surface as well as the inherent material strength. Small surface or internal irregularities result in extreme reduc- tion of transverse rupture strength. Commercial variation of trans- verse rupture strength in material giving satisfactory performance is in the order of 15%, with manufacturing quality standards being set for a minimum value as a qualifying requirement. Density is normally determined by the liquid displacement method. water is the commonly used liquid, with a wetting agent added. Density tolerance in commercial materials is 0.2 unit. For a given alloy, the density figure determined on a specimen is a rough indication of the amount of porosity contained. The porosity of any specimen is determined by grinding and metallographically polishing a section of it. Diamond lapping tech- niques of considerable precision are utilized. These are summarized in the General Motors Standards (13). Examination of the polished section at 200x has been accepted as a tentative standard by the ASTM (11), and the presentation of the section is compared to the reference photomicrographs contained in these tentative standards. The microstructure of carbide alloys are studied at lSOOx following etching of the polished surface with alkali ferricyanide. The etching solution attacks the tungsten carbide (WC) grains vpreferentially'to give them a gray color. The cobalt is unaffected, remaining white. Additional carbides will be attacked at a different rate from that of tungsten carbide (WC). SCOPE OF THIS INVESTIGATION This investigation is in three parts. First is a study of the effect of variation of cobalt content upon milling action. Secdnd is a study of two 5% cobalt, 9h%'WC, 1% TaC alloys having different average particle sizes after milling. Third is a study of three 11% cobalt, 88% NC, 1% TaC alloys having different average particle sizes after milling. EFFECT OF COBALT CONTENT UPON 'MILLING Tungsten carbide-cobalt alloys having 3%, 5%, 8%, and 11% cobalt content (remainder tungsten carbide) were investigated. Charges numbered 1, 2, 3, and h were prepared with these respective analyses, using a single lot of tungsten carbide and of cobalt in order to assure a common universe for all charges. The milling time, total charge weight, mill size, and total ball weight were identical for all four charges. Table I presents the resultant particle size after milling for 72 hours. TABLE I Charge Co - % Particle Size in microns cruinaeJ boovuo HyHH \1 WM \ogooxo It is evident that increasing the cobalt content of the alloy retards the efficiency of the milling action. This retardation is approximately proportional to the increase of cobalt content in the range presented. PROPERTIES OF THE 3%, 5%, :3, AND 115:: COBALT ALLOYS Test tip specimens were formed from each alloy, and then heated in a vacuum furnace (less than 200 microns absolute pressure) for a period of 30 minutes at temperature. Several maximum temperatures were investigated with the expectation of finding an.optimum for each analysis. Following their return.to room temperature, the test tips were ground to the 0.200" x 0.375" cross-section with a 220 grit diamond wheel. The transverse rupture strength was determined, the Rockwell A hardness and the density were secured, and the specimens were exam- ined for porosity and etched structure. Photomicrographs of both porosity and etched structure of specimens of each charge after heating to each of four maximum temperatures are included as Figures 1, 2, 3, and h, appended. Table II presents the results of the physical tests. These values are averages for two or more test tips, and averaging excludes greater than 20% variation in transverse rupture strength. Where the hardness variation of specimens is greater than 0.3 point, it is so indicated by V. TABLE II Charge Specimen Heating Temp.°F. TR% RA*% Density 1 A 2h00 — 30 min. 112 83 v 13.0 1 B 2500 " " 155 89.0 13.7 1 c 2700 " n 186 91.2 1A.6 1 D 2800 (60 min.) 223 93.0 15.0 2 A 2h00 - 30 min. 229 89.0 13.9 2 B 2500 " " 211 91.0 1A.6 2 C 2700 n n 260 92.0 1A.9 2 D 2800 (60 min. 260 92.0 1h.9 3 A 2hOO - 30 min. 273 90.3 1h.h 3 B 2500 n n 328 91.0 1A.6 3 C 2700 " " 328 91.0 lh.7 3 D 800 (60 min.) 3h? 91.0 1A.7 h A 2&00 - 30 min. 3hl 89.6n 1h.3 h B 2500 n " 35h 90.3 1h.3 h C 2700 " n 372 90.0 1h.3 A D 2800 (60 min.) 328 90.0 1A.3 * Transverse rupture strength in thousand psi ** Hardness - Rockwell A scale The significant facts indicated by Table II are several. The transverse rupture strength is less sensitive to the maximum temp- erature of heating as the percentage of cobalt increases from 3% to 11%. The hardness likewise is less sensitive to the heating temperature as the percentage of c0halt increases from 3% to 11%. The density values also follow a like pattern. Study of the microstructures of the various specimens provides significant reason for the differences in properties of the alloys after the various heating cycles. Both the inherent porosity (at 200x) and the etched structure (at 1500x) are included in the exam- ination. The 3% cobalt analysis (charge 1) has reduced porosity with higher pyromerging temperature (see Figure 1). This is in conforme ance with changes in the transverse rupture strength, the hardness, and the density. Examination of the etched structures of specimens l—A and l-B was hampered by their extreme porosity, but evidence of any significant tungsten carbide grain growth was absent over the temperature range studied. Only specimen l-D approaches commercial quality. The 5% cobalt analysis (charge 2) also has reduced porosity with higher sintering temperature (see Figure 2). This is in con- formance with changes in the transverse rupture strength, the hard- ness, and the density. The range of porosity is less extensive than in the 3% cobalt alloy, and the minimun porosity (at the 2800°F. temperature) is slightly less than the minimum porosity of the 3% cobalt alloy. Examination of the etched structure discloses no sig- nificant tungsten carbide grain growth over the temperature range studied. Specimen 2-D is the only one approaching commercial quality. The 8% cdbalt analysis (charge 3) follows the trend of lower porosity with higher sintering temperature (see Figure 3). A rel- atively sound structure is reached at 2500°F., and porosity is well within commercial range after heating to 2700°F.. This progression conforms to values of transverse rupture strength, hardness, and density previously determined. Examination of the etched structure again discloses no significant tungsten carbide grain growth over the temperature range studied. Both specimens 3—C and 3-D are of commercial quality. The 11% cobalt analysis (charge h) has significantly less porosity after the 2h00°F. pyromerging temperature than the other analyses (see Figure h), and the higher sintering temperatures reduce the porosity to an insignifiéant amount. This early densi- fication is reflected in the stabilized values of transverse rup- ture strength, hardness, and density in Table II. Close examina- ‘tion of the etched structure reveals a slight amount of tungsten carbide grain growth at 2800°F.. EFFECT OF PARTICLE SIZE UPON PROPERTIES A second investigation undertaken was a study of the effect of the particle size of milled powder upon the properties of file. resultant compact after sintering. Two analyses were studied: 5% Co, 911%I'E, 1% Tao, and 1155 Co, 8853 WC, 1% TaC. PROPERTIES OF Two 5% Co, 91153910, 1:: TaC ALLOYS The analysis 5% cobalt, 9h% WC, 1% TaC was milled to two conditions of amerage particle size. Charge 5 averaged 1.h0 mi- crons, and charge 6 averaged 0.95 microns. Standard test tips were fabricated from each charge, and these test tips were heated to various temperatures in the pyromerging range. The physical data obtained from these test tips is presented in Table III. 10 TABLE III Specimen Heating Temp.°F. TR* RA%* Density 5-A 2AOO - 30 min. _ 223 89.0 13.9 5-B 2500 - " " 211 91.0 1h.6 5-0 2700 - " n 261 92.0 1A.9 5-D 2800 - 60 min. 273 92.0 lh.9 6-A 2hOO - 30 min. 1A9 85 v 13.3 6-B 2500 - n n 186 89.9 1h.2 6-0 2700 -. n n 229 91.5 1h.8 6-D 2800 - 60 min. 266 91.3 1h.9 * Transverse rupture strength in thousand psi -x-x- Hardness - Rockwell A scale The comparison of properties indicates that charge 5 pro- duces the more uniform, higher quality'materigl. maximum.trans- verse rupture strength and maximum.hardness are greater for charge 5 than for charge 6. For any sintering temperature, the properties of charge 5 are superior to those of’charge 6. The most significant property advantage possessed by charge 5 with respect to charge 6 is the temperature latitude for secur- ing maximum values of transverse rupture strength, hardness, and density. This latitude is at least 100°F. for charge 5. Examina- tion of microstructures of the various specimens disclosed no significant differences. The property of temperature latitude is extremely desirable in commercial operations, as it permits greater reproducibility of alloy properties from charge to charge and from heat to heat. PROPERTIES OF THREE 11% Co, 88% WC, 1% TaC ALLOYS The analysis 11% cobalt, 88% tungsten carbide, 1% tantalum carbide was studied in a similar manner to the 5% cobalt charges 5 and 6 previously discussed. Three charges ( 7, 8, and 9 ) were milled to varying particle size ranges. The average sizes were: Charge Average size (microns) 7 2.00 8 1.85 9 1.30 Standard test tips were then fabricated from each charge and heated to various pyromerging temperatures. These test tips were then examined in usual manner. The physical data obtained is presented in Table IV. TABLE IV Specimen Heating Temp.°F. TR* RA%* Density 7-A 2hOO - 30 min. 285 89.0 lh.l 7"B 2500 "' II n 273 9000 11103 7-0 - 2700 - " " 322 90.0 lh.3 7-D 2800 - 60 min. 273 90.5 1h.3 8-A 2h00 - 30 min. 30h 90.0 1h.3 8-B 2500 - n n 298 90.0 1A.2 8-0 2700 - " " 3h? 89.9 lh.3 8-D 28OO - 60 min. 3h? 90.0 1h.3 9-A 2h00 - 30 min. hlS 89.5 1h.3 9-B 2500 - " " 385 89.8 1h.3 9-0 2700 - n n 372 89.5 1h.3 9-D 2800 - 60 min. 390 89.5 1h.3 iv Transverse rupture strength in thousand psi ** Hardness - Rockwell A scale Significant is the fact that maximum.density is reached in all specimens with the exception of 7-A. The early completion of the shrinkage reaction is principally a function of the increased cobalt content of charges 7, 8, and 9 as compared to charges 5 and 6. The cobalt content by volume in charges 5 and 6 is 7.6%, and in charges 7, 8, and 9 is 17.8 %. Iardness values are relatively uniform for the three charges. Specimen 7-A.did not reach full densification, accounting for the lower hardness reading of 89.0 RA. Specimen 7-D possesses higher hardness than any other specimen, but coupled is a lower transverse rupture value than attained in specimen 7-0. Significant is the decrease in hardness of charge 9 as compared to charges 7 and 8. The transverse rupture strength increases with decreased aver- age powder particle size. The values secured in charge 7 would.be sub-par for commercial material of this analysis. The values secur- ed in charge 8 are satisfactory for commercia1.materials. The rup- ture values of charge 9 are in the premium range. All three charges 7, 8, and 9 exhibit wide temperature lat- itude for production of maximum.physical properties. Charge 9 is outstanding in this respect. The transverse rupture values are all of a high order for this material. Observation of the etched structures (see Figure 5) indicates that grain growth occurs more markedly in charge 9 as compared to charge 7. Specimens 9-B, 9-C, and 9-D present a gradual increase 111the average apparent grain size. Also evident is a diminution 13 of the amount of grains of size less than 2 microns. Charge 7 does not exhibit these type reactions to any marked degree. The lower hardness exhibited by charge 9 is a function of this grain growth. CONCLUSION The properties of tungsten carbide - cObalt alloys are pri- marily dependent on the alloy composition itself. Higher cobalt content provides greater latitude in the pyromerging cycle. The average particle size following milling is roughly pro- portional to the cobalt content in the 3% to 11% cObalt range studied. The minimum.temperature necessary to develop commercial properties increased with decreasing cobalt content in the range studied. For a given alloy, maximum physical properties apparently result from an optimum.average particle size and an optimum.max- imum.sintering temperature. Too small average particle size in the powder results in excessive grain growth in pyromerging, which gives an indicated hardness lower than maximum. Too large average particle size results in lower than.maximum strength. Fbr an 11% cobalt alloy, the optimum.average particle size appears to lie between 1.85 microns and 1.30 microns. Hardness in WC - Co alloys of low cobalt content (up to 12% is a function of WC grain size as well as the cobalt content. Fbr a single alloy, variations in hardness result from variation in WC grain size of the milled material. This is due to variation of mass reaction rate of solution of WC in cobalt during the pyro- merging cycle. The cobalt-rich solid solution phase is formed first, and is followed by formation of file liquid phase which apOroaches eutectic composition. If a significant proportion.of’the milled EC grains are under approximately 0.5 micron effective diameter, the proportion of WC going into solution in the cobalt—rich phase and melt is extensive at the elevated temperature involved. Upon cooling the mass, the dissolved WC must be rejected from solution due to its limited solubility (less than 1%) in cobalt at room temperature. Two possibilities exist: to nucleate new grains or to pre- cipitate out on existing grains. The latter requires less energy, and thus the predominant characteristic is increased apparent grain size in the product. The mass hardness (as measured on the Rockwell A scale) is lower than maximum. If the milled alloy has a minimum.of fine WC (less than 0.5 micron), less than the critical amount for the alloy, the cobalt- rich solutions are relatively deficient in WC at the pyromerging temperature. Upon cooling, little WC is rejected, and, in precip- itating, provides incomplete bonding in the mess. Porosity may also be present. 15 When the milling cycle is such to produce the optimum average grain size with proner proportion of minus 0.5 micron material, the amount of WC dissolved in cobalt and then precip- itated upon cooling is such to produce maximum WC grain'bonding to surrounding grains without significant apparent grain growth. Hardness of the mass is at a maximum.for the alloy. The solubility reaction rate of WC in cObalt at elevated temperatures is of critical importance in the pyromerging reaction, and offers opportunity for further profitable research. 16 2hOOOF l-B 25OO°F l-C 27OOOF v '." O ’ O . ' g -t‘ . . e. . .5 ' l-D ’ .. s H . ZOOOOF . . -‘ 0 .' " 200x - Porosity’ 1500x - Etched Structure Figure 1 3% CO - 97% WC Alloy 200x - Porosity 1500x - Etched Structure Figure 2 5?: To - 9558 we Alloy - 0 0. v , n... C C " . . s o 3-OB . 2500 F .b . O 3 - C 27OOOF ' K 3-D ZBOOOF 200? - Porosity 1500x - Etched Structure Figure 3 % CO - 92% NC Alloy .—___, 200x - Porosity t - B 25000F h - c 27OOOF h - D 28OOOF 1500x - Etched Structure Figure h 11% CO - 89% we Alloy Charge 7 Figure 5 - Etched Structure (1509x) 9. 10. 11. REFERENCES E.W. Eagle, Cemented Carbides, Powder Metallurgy, edited by J. IWHf, ASI‘I’ Ch. 39, pH36o P.M. McKenna, Tool Materials Powder Metallurgy, edited by J. Wulff, ASH, , c , ph5h. S. Takeda, A Metallographic Stud of the ActiOn of the Cementin MateriaI Tor Came-555d :sten C‘a-rbi'd e §1ence Hanoi-ts, Tom ImperiEl University, HoWrsary Volume, 1936, 193, 138611. P. Rautala and J .T. Norton, Tungsten-Cobalt-Carbon System, Trans. AIME, October, 1952, JournaI of EétaE, pIOIIS. L.L. Wyman and F.C. Kelley, Cemented Tungsten Carbide, A Study of the Cementihg Material, Trans. All/IE 1931, 93, p208. W. Dawihl and J. Hinnuber, Uber Den Aufbau der Hartmetal- lergierungen, Kolloid Ztsch. 1915, p233. E.J. Sandford and E.M. Trent, The Physical Metallurgy of Sintered Carbides, Br. Iron and Steel Institute Symposium on Powder Metallurgy, Spec. Report 38, 19117, p811. R. Kieffer, Theoretical Aspects of Sinterin of Carbides Physics of Powder Metallurgy, McGraw-Hill, ew York, N.Y., 19 l. J. Gurland and J.T. Norton, Role cf the Binder Phase in Cemented sten Carbide-Cobalt Alloys, Trans. AIME, Oct. 1952 Journal 0 Metals, 791051. A.D. Stevens and J.C. Redmond, Methods of Testi "Cemented Carbide Compositions) ASTM Special: TecHn-icaI FEE. IIIO, I952, 553. Transactions of the ASTM Subcommittee B-9, Sec. 3—c, Tentative Standards on Porosity, Density, and Hardness. A.D. Stevens, How QualitT can Make or Break Carbide ToolsJ Tooling and ProductiOn, e ruary, 191111. I Metallggraphic Ebcamination of Tungsten Carbides, GM 1111519, Genera Motors jSEandards, p775. I . .l' "1"! '7! T. ”'Crfir'lllfiufljly[Witlilsfii'llyiflW“ ll"