Handbook Corrosion (1992) WW Part 10 ppsx

Fig. 1 Microstructures of WC-Co (a, c, and e) and WC-TaC-TiC- Co (b, d, and f) cemented carbides. In a, c, and e, the white areas are cobalt binder phase. In b, d, and f, the darker, more rounded grains are the W x Ta y Ti z C cubic solid-solution phase. (a) and (b) Fine g rain structures. (c) and (d) Medium grain structures. (e) and (f) Coarse grain structures. All 1500×. Source: Ref 1 and 2 The first key to the successful development of cemented carbides was that these refractory metal compounds, particularly WC, are best produced as powders. In fact, the only logical way to produce tungsten is the hydrogen reduction of WO 3 or ammonium paratungstate powder into tungsten metal powder. The carburization of tungsten to WC also results in a fine powder. The second key was the discovery of the eutectic system WC-Co (Fig. 2). Liquid-phase sintering is possible well below the melting point of the WC and even below the melting point of cobalt. Fig. 2 Quasi-binary phase diagram for the WC-Co system Cemented WC is produced by mixing from 3 wt% or less up to as much as 30 wt% of cobalt metal powder with a balance of WC powder. The mixed powders are ball milled, generally in volatile solvents, for times ranging from a few hours to as long as 7 days. Alternatively, the powders are milled in an attritor for 1 to 10 h. A suitable transient binder is added to the powder, which is then pelletized and pressed to form the shape. Finally, the part is sintered at temperatures between 1300 and 1600 °C (2370 and 2910 °F), most often in vacuum. Because a liquid phase is formed during sintering, virtually 100% density is achieved. More information on the production of cemented carbides is available in the articles "Cermets and Cemented Carbides" and "Production Sintering Practices" in Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook. Effect of Composition on Properties The two most common variables in cemented carbides are the cobalt or binder content and the grain size. As shown in Fig. 3, increased grain size decreases hardness, and increased cobalt content also decreases hardness (Ref 6). Increased contents of cobalt or other binders, however, are necessary to increase strength. As shown in Fig. 4, strength increases with increased cobalt content; although a maximum appears to occur at about 15 to 18% Co, this is true only for transverse rupture strength (Ref 6). Very high impact strength requires very high cobalt contents (up to 25 or 30 wt%) and coarse-grain carbide. In corrosion applications, however, the binder content ranges from virtually nil (there are some so- called "binderless" compositions that actually contain 1 to 2% binder) up to about 10%, with exceptions running up to 15% binder. Fig. 3 Effect of cobalt content and grain size on the hardness of WC-Co cemented carbides Fig. 4 Effect of cobalt content and grain size on the transverse rupture strength of WC-Co cemented carbides Cemented carbides are not selected for corrosion applications per se. They are extremely important in corrosion conditions in which high hardness, wear resistance, or abrasion resistance is required. When this is the case and the selection of a cemented carbide is logical, the corrosion-resistant properties are examined. For ordinary corrosion resistance, many metals and ceramics are better choices, but when wear resistance is also a requirement, the cemented carbide is needed. Binder Composition and Content. The corrosion resistance of cemented carbides is based on the two very different components. The cobalt binder has very poor corrosion and oxidation resistance, and the WC has excellent corrosion resistance and good oxidation resistance. Alternate binders, such as nickel, have better corrosion resistance than cobalt and are used in spite of their lower hardness and strength. Nickel is a superior binder for cemented TiC and therefore is used in all cemented TiC materials regardless of the need for corrosion resistance. In some applications, cemented TiC shows repair corrosion resistance, and in other applications, cemented WC is better. The addition of nickel to the usual cobalt binder used for WC, or the substitution of it entirely for cobalt, always improves corrosion resistance. There is, however, a sacrifice in strength, hardness, and wear resistance. A chromium addition also enhances corrosion resistance. The most important variable in the corrosion of cemented carbides is the binder content. Because the binder corrodes more than the carbide, the smaller the amount of binder the better. On the other hand, decreasing the binder decreases the strength. Carbides. Additions of TaC and TiC to the WC-Co materials are common for the compositions used for machining steel. These additives give the carbide crater resistance. Cratering on the top of a metal-cutting insert is the result of a physicochemical reaction. The addition of TaC and/or TiC will slow this reaction; indeed, it has been found that TaC also enhances the outright chemical corrosion resistance of these materials. Other additives, such as chromium carbide (Cr 2 C 3 ), molybdenum carbide (Mo 2 C), niobium carbide (NbC), and vanadium carbide (VC), are often added in small quantities as grain growth inhibitors. Little has been published about their effect on corrosion, but chromium has been shown to be a beneficial binder additive to WC-Ni binder compositions (Ref 7). Vanadium carbide and Mo 2 C will probably have a weakening effect on the strength of a WC-base hardmetal. For TiC-base hardmetals, Mo 2 C is invariably added to the composition, but there are no known studies of the effect of molybdenum on corrosion resistance. The molybdenum has always been added to enhance the liquid-phase sintering of the TiC-base compositions. In general, these compositions have been made for their hardness and strength characteristics, with corrosion resistance being a secondary consideration. Most rescent TiC-base compositions have titanium nitride (TiN) added, and this has been shown to improve the corrosion resistance (Ref 8). Perhaps it is not surprising that compositions developed primarily for machining should show improved corrosion resistance. In machining, there is heat with resultant oxidation and often corrosionlike mechanisms. Thus, some of the improved machining compositions also show better corrosion behavior. On the other hand, optimum corrosion resistance is obtained by tailoring the composition and amount of the binder phase. This can result in lower-strength materials with limited usefulness in machining applications. Because carbon is the basis of cemented carbides, its variation within a given composition is very important to properties and corrosion resistance. Figure 5 shows the range of carbon content allowable in the simple WC-Co compositions as cobalt content is varied (Ref 9, 10, 11). Corrosion-resistant compositions have three problems: • The lower the cobalt or binder content, the better the resistance to cor rosion, but this limits the safe zone, in which neither carbon porosity nor phase (hard, brittle M 6 C or M 12 C intermetallics) exist • The lower the carbon content, the better the corrosion resistance, but falling into the - phase zone results in embrittlement of the material • The addition of alternate binders, such as nickel, decreases the safe zone In making corrosion-resistant cemented carbides, manufacturers must be aware of these problems and limitations. Information on the metallography and microstructures of these materials is available in the article "Cemented Carbides" in Metallography and Microstructure, Volume 9 of ASM Handbook, formerly 9th Edition Metals Handbook. Fig. 5 Effect of cobalt content and carbon content on the phases present in WC-Co cemented carbides Applications of Cemented Carbides The major applications of cemented carbides actually involve environments that are inherently corrosive. For example, the major use of cemented carbides is for metal-cutting (machining) applications. In these applications, extreme heat is generated whether or not coolants are used, and in those cases in which coolants are used, the corrosive attack of the coolant is a factor in the performance of the cutting tool. In general, however, very little heed is paid to this factor; cemented carbides are more often chosen for their wear resistance in such applications as mining and oil well drilling. In actuality, there is a corrosive environment to be contended with in mining (Ref 12) and oil well drilling; the natural waters and other fluids involved are often very corrosive. Other well-known examples in which cemented carbide is performing in a corrosive environment include balls for ball point pens and dental drills. In both of these examples, the corrosion resistance of the most frequently used WC-6Co composition was serendipitous. The material was selected for its wear resistance. It just happens to have good corrosion resistance in the saline and ink solutions. The dulling of cemented carbide saw tips used for sawing green or unseasoned wood is a corrosive as well as a wear phenomenon (see the section "Saw Tips and Corrosion" in this article). Examples of the use of cemented carbide in true corrosion applications include the following: • Ball point pen balls • Dental drills and burrs • Surgical and orthodontic tweezers, pliers, and clamps • Valve seats • Valve balls and valve stems • Valve and shaft seals (seal rings) • Spray nozzles • Pulverizing hammers • Compressor plungers • Bearings • Cage mills • Ball mill linings and balls • Internal parts in industrial meters The article "Cermets and Cemented Carbides" in Powder Metal Technologies and Applications, Volume 7 of the ASM Handbook contains more information on applications for cemented carbides. Selection of Cemented Carbides for Corrosion Applications The selection of cemented carbides is a very difficult problem for the user. There has been a lack of standardization on the part of the producers, and this lack has not been answered by any national or international standards organization. Some attempts have been made to standardize with regard to metal-cutting applications. There is International Organization for Standardization (ISO) standard 513 for metal-cutting applications for carbide (Ref 13). It is widely used in Europe and most other parts of the industrial world, but it is not recognized in the United States (Ref 11). In addition, there is no ISO standard for cemented carbides used for wear, mining, or corrosion applications, and if any exist in other industrialized countries, the producers choose to ignore them, or they may be so broad that a given producer can have three or more grades falling into one category (Ref 14, 15). The producers also tend to disregard attempts at standardization in the hope of having a unique product. Even in the established WC-6Co grades, the producers offer several different varieties based on different grain size or different minor element additions. For example, the company that developed the WC-6Co composition about 70 years ago offers five different grades of this composition, and two of them have identical published properties. They are not alone. In some cases, three grades are shown with the same composition and properties. A good example is the nine differently designated 6% Co compositions of one company. Five of the nine are indeed different because of small TaC additions or grain size, but one of the compositions has three designations, and two of them have two designations. More often, the reason for the multiple designations of the same composition is that one designation is for cutting tools, another for wear parts or dies, and another for mining. Another problem area is the selection of composition by the manufacturer. For example, if one producer establishes a WC-25Co composition, another producer will make and market a grade with 24% Co, and another a product with 26% Co. Despite these problems, Table 2 lists the properties of various representative grades for corrosion applications; Table 3 lists approximate compositions and proprietary designations for a number of corrosion-resistant grades. Table 2 Some physical properties of corrosion-resistant cemented carbide grades Properties of a carbon steel, a tool steel, and a cast cobalt alloy are included for comparison. Nominal composition, wt% Transverse rupture strength Thermal conductivity Special attributes Proprietary designation WC Co TaC TiC Ni Cr Mo 2 C Hardness, HRA Density, g/cm 3 MPa ksi Abrasion resistance factor (a) Coefficient of thermal expansion, m/m · K W/m · K cal/cm·s·°C Abrasion-resistant, wear, and structural grades GU-2 (b) 96.5 3 0.5 . . . . . . . . . . . . 93.3 15.30 1655 240 1.8 4.9 125.5 0.30 PWX (b) 94.0 5.5 0.5 . . . . . . . . . . . . 92.5 15.05 2137 310 2.1 5.2 108.8 0.26 A (b) 94.0 6.0 . . . . . . . . . . . . . . . 91.8 15.00 2206 320 3.4 5.5 104.6 0.25 B (b) 91.0 9.0 . . . . . . . . . . . . . . . 90.8 14.70 2758 400 6.8 5.5 96.2 0.23 Maximum abrasion resistance BB (b) 87.0 13.0 . . . . . . . . . . . . . . . 89.5 14.28 3103 450 17 6.2 87.9 0.21 Toughness GU-1 (b) 81.5 18.0 0.5 . . . . . . . . . . . . 88.4 13.84 3448 500 32 6.8 83.7 0.20 474 (b) 79.0 12 9 . . . . . . . . . . . . 89.6 14.29 2241 325 16.5 5.8 87.9 0.21 Gall resistance GG (b) 60.0 12 28 . . . . . . . . . . . . 89.0 14.09 2069 300 18 7.1 83.7 0.20 Titan 80 (b) . . . . . . . . . 74 12.5 . . . 13.5 93.0 5.63 1379 200 22 7.8 16.7 0.04 Oxidation resistance Titan 60 (b) . . . . . . . . . 70.5 17.5 1.0 11.0 91.7 5.71 1724 250 28 8.4 16.7 0.04 Titan 50 (b) . . . . . . . . . 66.5 22.5 1.0 10.0 K602 (c) 88.2 1.8 10.0 . . . . . . . . . . . . 94.3 15.6 759 110 . . . 4.9 . . . . . . K701 (c) 85.8 10.1 . . . . . . . . . 4.1 . . . 92.0 14.0 1138 165 . . . 6.5 62.8 0.15 (d) K703 (c) 93.3 5.8 . . . . . . . . . 0.9 . . . 91.5 14.7 1931 280 . . . 4.5 . . . . . . K714 (c) 88.4 6.1 4.5 1.0 . . . . . . . . . 92.5 13.1 1827 265 1.8 (d) 4.0 . . . . . . K801 (c) 93.7 . . . 0.3 . . . 6.0 . . . . . . 90.0 14.8 2103 305 17 (d) 5.6 96.2 0.23 (d) Special corrosion resistance K803 (c) 89.0 . . . . . . 1.0 10.0 . . . . . . 91.0 14.4 2000 290 . . . 5.6 . . . . . . Grades for heading and forming dies HD-15 (b) 85.0 15 . . . . . . . . . . . . . . . 87.4 14.10 3172 460 30 6.5 83.7 0.20 HD-20 (b) 80.0 20 . . . . . . . . . . . . . . . 85.3 13.60 3103 450 45 6.8 83.7 0.20 Impact resistance HD-25 (b) 75.0 25 . . . . . . . . . . . . . . . 83.5 13.15 2965 430 65 7.5 83.7 0.20 HD-20T (b) 75.0 20 5 . . . . . . . . . . . . 85.3 13.55 2896 420 46 7.1 83.7 0.20 Gall resistance HD-25T (b) 70.0 25 5 . . . . . . . . . . . . 83.5 13.15 2827 410 67 7.8 83.7 0.20 Mining grades 575 (b) 94.0 6 . . . . . . . . . . . . . . . 90.8 15.00 2413 350 8.1 4.9 104.6 0.25 569 (b) 90.0 10 . . . . . . . . . . . . . . . 88.6 14.51 2930 425 13 5.8 104.6 0.25 783 (b) 89.0 11 . . . . . . . . . . . . . . . 88.1 14.41 3103 450 19 5.8 104.6 0.25 Strength and impact resistance 502 (b) 88.0 12 . . . . . . . . . . . . . . . 87.6 14.31 2965 430 21 6.2 104.6 0.25 Noncarbide metals To 1379 200 Carbon steel . . . . . . . . . . . . . . . . . . . . . . . . To 79 7.8 (tensile strength) >140 14.8 50.2 0.12 T1 tool steel . . . . . . . . . . . . . . . . . . . . . . . . To 87 8.7 3448 500 70 12.6 . . . . . . Cast Co-Cr-W alloy . . . . . . . . . . . . . . . . . . . . . . . . To 83 8.6 2069 300 110 13-16 . . . . . . Source: Ref 16, 17 (a) Determined in accordance with ASTM B 657 (Ref 2). The lower the number, the better the resistance to abrasion. (b) Adamas designation. (c) Kennametal designation. (d) Values estimated from available data. [...]... to corrosion resistance only (c) Coupled to brass Table 8 Corrosion of WC-Co cemented carbides in mineral acids Corrosion rates for AISI type 304 stainless steel are shown for comparison Cobalt content, wt % Weight loss mg/mm2 37% HCl 5% HCl, 10% H2SO4 5% H2SO4 10% HNO3 5% HNO3 Room temperature 100 °C (212 °F) 100 °C (212 °F) Room temperature 100 °C (212 °F) Room temperature 100 °C (212 °F) 10 h 100 ... K68 GT05 K1 H10T GTi05 CS10 HI HA 94.0 6.0 K6 GT10 K2 H16T GTi10 HML G10E H6 91.0 9.0 K9 GT15 MK30 H30T GTi15 H10F G3 H8 87.0 13.0 GT3H H40T GTi20 R4 G5 H81 81.5 18.0 0.5 GTi40 74.0 12.5 13.5 K165 FO5T NX33 CN02 70.5 17.5 1.0 11.0 F10T NX55 T12A 66.5 22.5 1.0 10. 0 TTF T12B 88.2 1.8 10. 0 K602 85.8 10. 1 4.1... K801 WC6Ni 89.0 1.0 10. 0 K803 TCR30 85.0 15.0 SP212 BT40 G3 B50T CT60 G6 MPD160 80.0 20.0 G4 H60T GTi40 CT75 G7 75.0 25.0 GT55 G5 H70T CT85 G8 75.0 20.0 5.0 K91 ND20 70.0 25.0 5.0 K90 ND25 94.0 6.0 K3404 BT10 K3 B10T CT30 HAN6 90.0 10. 0 K3070 BT25 MK35 B30T CT45 G3 MPD10 89.0 11.0 K3047 MK40... for a particular application Table 4 Selected mechanical properties of corrosion- resistant cemented carbide grades Poisson's ratio Charpy V-notch impact resistance(a) Tensile strength Compressive strength Modulus of elasticity J Proprietary designation in.·lb MPa ksi MPa ksi GPa 106 psi GU-2(b) 0.21 1.24 11 103 4 150 6068 880 662 96 PWX(b) 0.21 1.36 12 1241 180 5929 860 652 94.5 A(b) 0.23 1.47 13 1 310. .. designations of corrosion- resistant cemented carbide grades oprietary designations Adamas Carbide Anderson Strathclyde Carbidie Carmet Danit General Carbide General Electric Carboloy GTE Valenite GU2 CA CD20 CA8 K04 GC003 999 VC3 PWX CF CD24 CA306 K10 GC005 895 VC32 A CG CD30 CA4 K20 GC106 883 B CD35F CA12 K30 GC009 VC152 BB CD40 CA10 GC313 258 VC11 GU1 CD650 474 VC047 Titan 8 CA100 ... and 14 for corrosion resistance in other media Source: Ref 7 and 28 Fig 8 Corrosion resistance of cemented carbides in 37.8% HNO3 at room temperature See Fig 7 for key to identification and compositions See also Table 9 and Fig 9, 10, 11, 12, 13, and 14 Source: Ref 7 and 28 Fig 9 Corrosion resistance of cemented carbides in 9.8% H2SO4 at room temperature See also Table 9 and Fig 7, 8, and 10, 11, 12,... cemented carbide show very acceptable corrosion resistance in these warm acids These results are to be expected, because the cobalt and nickel binders are completely soluble in these acids Table 10 Weight losses of cemented carbides immersed in various acids at 50 °C (120 °F) for 72 h Composition Weight loss, mg/cm2/d HCl, % H2SO4, % HNO3, % 5 10 37 10 50 98 5 10 50 WC-6Co 2.29 2.43 0.79 8.72 2.82... hand, the corrosion- resistant grades do offer significant benefits in corrosion resistance in many media (Table 7) These grades include the WC + Ni binder, the WC + Co-Cr binder, and the so-called binderless WC, which generally contains about 10% TaC and between 1 and 2% Co In addition, there are other special grades, such as the 0.1 to 1.0% Pt addition patented as an improvement toward ink corrosion. .. also noted by composition, such as WC-10Co-4Cr Grades are also listed by a grade number that can be used when referring to Fig 7, 8, 9, 10, 11, 12, 13, and 14 Table 9 Properties of corrosion- resistant cemented carbide grades See Fig 7, 8, 9, 10, 11, 12, 13, and 14 for the corrosion resistance of 12 of these grades in various media Proprietary designation Grade number(a) Composition symbol(a) Composition,... 0.3TaC 1850 92.9 15.3 1400 203 H10T(b) 2 WC-6Co 94.5 5.5 (c) 1730 92.4 15.0 1900 276 H30T(b) 3 WC-9Co 90.4 9 0.2TiC, 0.4TaC 1450 90.7 14.6 2000 290 H40T(b) 88 12 (c) 1340 89.7 14.3 2600 377 K701(d) 4 WC-10Co-4Cr 85.8 10. 1 4.1 1645 92.0 14.0 1140 165 WC6Ni(b) 5 WC-6Ni 94 6 1400 90.2 15.0 1500 218 WC9Ni(b) 6 WC-9Ni 91 9 1150 87.6 14.6 1800 261 TCR10(b) 7 WC-6NiCr 94 5.7 0.3 1520 . GT05 K1 H10T GTi05 CS10 HI HA 94.0 6.0 . . . . . . . . . . . . . . . K6 GT10 K2 H16T GTi10 HML G10E H6 91.0 9.0 . . . . . . . . . . . . . . . K9 GT15 MK30 H30T GTi15 H10F G3 H8. . . . . . . 66.5 22.5 1.0 10. 0 K602 (c) 88.2 1.8 10. 0 . . . . . . . . . . . . 94.3 15.6 759 110 . . . 4.9 . . . . . . K701 (c) 85.8 10. 1 . . . . . . . . . 4.1 . . . . . . . . . . . . K3404 BT10 K3 B10T . . . CT30 . . . HAN6 90.0 10. 0 . . . . . . . . . . . . . . . K3070 BT25 MK35 B30T . . . CT45 G3 MPD10 89.0 11.0 . . . . . . . .