INTRODUCTION TO HARDMETALS
1.02 Fundamentals and General Applications of Hardmetals
1.02.2 Metallurgy of Hardmetals
thin-film hard coatings like TiN, TiC, TiCN, Al2O3, TiAlN, etc. became available. This has led to large increases of productivity especially in machining applications (Prakash & Roebuck, 2013).
1.02.1.1 Market Size of WC-Co Hardmetals
Currently about 50–55 GT of WC/cobalt is manufactured globally (2012). About 14 GT is manufactured in western Europe and almost 20 GT in China. The production market share for Europe is about 28%, China 39%, NAFTA 13%, Japan 10% and 10% for the rest of the world. The global market sales volume (2011) for hard- metal goods is in excess ofV22 billion. In terms of global turnover and global consumption, the following distribution is estimated (Table 1).
The range of cutting-tool material classes with an optimal combination of hardness and toughness is shown schematically inFigure 5and a comparison of typical properties of some hard materials inTable 2 (Schedler, 1988).
in the iron–cobalt–nickel system with properties equivalent to, or superior to conventional cobalt-bonded materials (Prakash & Gries, 2009).
The calculated ternary phase diagram of the WC–Co system at 1350 C is shown inFigures 6 and 7. It is apparent that the two-phase region of WC–Co is quite narrow and bordered with three phase regions that contain carbon-deficient complex mixed carbides of type M6C or free carbon at carbon-rich com- positions. Third phases are to be avoided to prevent undue loss of strength.Figure 6 highlights the narrow carbon range for a WC-10% Co as a function of temperature. The stoichiometric carbon content corre- sponds to 5.52 mass% C. The two-phase regions are limited to 5.38 to 5.54 mass% carbon. If additional carbides such as TiC, TaC, NbC, Cr3C2, etc. are added, the cobalt-rich liquid becomes stable at lower temperatures.
Even within the two-phase region of WC–Co, the amount of dissolved tungsten in the binder phase after sintering depends primarily on the carbon content. Higher tungsten contents have been measured in the binder phase at low carbon contents and vice versa. At 1250C, it is predicted that the content of W and C in the solid binder at equilibrium with WC varies between approximately (at%) 9Wþ1C and 3Wþ3C when the total C is increased in the material (Andren, 2001). The amount of dissolved tungsten has a major influence on the strength and ductility of the binder phase at ambient and elevated temperatures. It is well accepted that the solubility of tungsten in the binder is higher in the case of fine-grained WC hardmetals compared with coarse-grained ones. It is also now accepted that WC–Co hardmetals consist of two Figure 5 Schematic wear resistance–toughness relationship for cutting tool material classes.
Table 2 Properties of important wear-resistant hard materials (Schedler, 1988)
Property
High-speed steel
WC–Co cemented carbides
TiC-based cermets
Ceramics (oxides, nitrides)
Polycrystalline cubic BN
Polycrystalline diamond
Density, g/cm3 8.0–9.0 9.0–15.0 5.0–9.0 3.2–5.0 3.45 3.5
Vickers hardness, HV 750–800 800–1900 1500–2200 1500–2500 4000 8000
Transverse rupture strength, N/mm2 3000–4000 1000–3000 900–1800 400–600 500–800 600–1100 Hot transverse rupture strength (1000C),
N/mm2 500 900–1500 600–1500 400–600 500–700 600–1000
Compressive strength, N/mm2 3500–4000 4000–7000 3000–5000 3000–4000 6000 7600
Modulus of elasticity, kN/mm2 210 400–680 300–450 300–400 680 850
Thermal expansion coefficient, 106/K1 12 5.0–7.0 6.0–8.0 2.5–7.5 3.5 1.2
Fundamentals and General Applications of Hardmetals 33
interconnected skeletons of the hard carbide phase and the ductile cobalt phase. The grain boundaries be- tween the carbide grains can have up to a 0.5 monolayer of cobalt between them.
Variations in the carbon content of the sintered alloys influence the physical, mechanical, electrical and thermal properties as well as the shrinkage (warpage) after sintering (Table 3).
The manufacturing performance and product reliability of hardmetal components has profited by combining experimental approaches as well as simulation and modelling approaches. Currently phase diagrams and diffusion kinetics can be calculated based on available thermodynamic data (Frisk et al., 2001).
As it has been discussed in literature, the carbide grain size has a strong influence on material properties (Table 4). The“run of mill”hardmetals have an average grain size of 1–2mm. The tendency to decrease the grain size of the WC and thus improve on properties has been ongoing work in the past decades.Figures 8and9are the examples that show the influence of grain size on properties.
Figure 7 Calculated vertical section of the ternary phase diagram Co–W–C at 10 mass% Co (Petersson, 2004).
Figure 6 Calculated ternary phase diagram of the WC–Co system at 1350 C (Petersson, 2004).
The main advantage of a submicron grain size is the high hardness as well as toughness combined with a high compressive strength, high edge strength and superior abrasive wear resistance. Currently the proportion of submicron and smaller grained WC usage is in excess of 30% of all WC used. This is especially true for applications using round tools in metal, nonferrous, wood composites and man-made industrial materials.
During manufacture of hardmetal parts, WC grain growth occurs during sintering, which is nominally in the range of 1400–1500C. Grain growth inhibitors such as tantalum, chromium or vanadium as carbides are commonly used either individually or in varying combinations to design microstructure and properties of submicrometer hardmetals. The large body of research published by Chinese researchers (Liu, 2009) clearly points out that rare-earth additions are also beneficial for WC grain growth retardation. Even though WC–Co materials are a prime example of liquid-phase sintered composites, a major part of densification proceeds during heating up in the solid state. The sintering behavior is dependent on material composition such as carbide grain size, cubic carbide additions, binder volume fraction, carbon content and previous milling history of the powder raw materials. Details about sintering mechanisms will be dealt with in Chapter 1.08. A typical sintering cycle showing general sintering mechanisms is shown inFigure 10.
The advent of electrically resistance heated sintering furnaces of batch type that permit the use of vacuum or protective gases as sintering atmospheres until almost full density of the sintered compacts combined with a gas overpressure consolidation step (between 1 and 10 MPa) have led to a quantum leap in the quality and reli- ability of hardmetal products. The overpressure treatment (mainly using Argon gas), also termed Sinterhip, practically eliminates bulk residual porosity and leads to more uniform product properties on an industrial Table 3 Example of typical WC–Co hardmetal grades and properties (Element Six)
HM Code Cobalt (%) Grain size Density (g cm30.10) Coercivity (Oe)
Hardness
TRS (MPa) HV1050 HRA0.3
T6 6 Coarse 14.95 130–160 1450 90.6 2800
B20 8 14.7 90–110 1250 88.7 2800
B25 10 14.5 75–110 1230 88.5 2900
B30 11 14.4 75–100 1150 87.7 2900
B40 15 14 65–90 1050 86.5 2800
B15N 6.5 Extra coarse 14.9 55–75 1100 87.2 2200
B20N 8.6 14.65 45–60 1050 86.5 2300
B25SN 9.5 14.55 50–64 1050 86.5 2300
G10 6 Medium 14.95 190–220 1600 91.9 2800
G15 8 14.7 160–200 1480 90.9 3000
G20 11 14.4 130–170 1320 89.4 3300
G30 15 14 110–150 1200 88.2 3200
G40 20 13.5 90–120 1050 86.5 3100
G55 26 13 70–100 870 84.4 2900
K04 4 Submicron 15.1 350–450 1850 93.4 2000
K05 5 15 310–360 1800 93
K06 6 14.9 270–350 1750 92.7
K07 7 14.7 260–320 1700 92.5
K010 10 14.4 21–290 1620 92
K015 15 13.9 190–250 1400 90.2 3700
Table 4 Classification WC grain size
Grain Size (mm) Designation
<0.2 Nano
0.2–0.5 Ultrafine
0.5–0.8 Submicron
0.8–1.3 Fine
1.3–2.5 Medium
2.5–6.0 Coarse
>6.0 Extra coarse
Fundamentals and General Applications of Hardmetals 35
scale. This has been especially of advantage for applications involving chipless forming tools such as Sendzimer rolls, wire rolls, wire-drawing dies and stamping and punching tools.
Since sintering is mainly a diffusion-controlled process that is time dependent, the incentive to control WC grain growth and also improve production productivity by using rapid sintering techniques such as microwave sintering, spark plasma sintering, electric discharge sintering, etc. has been innovative. These processes though not as mature as conventional sintering processes are making production headway in a number of niche applications.
Figure 8 Influence of WC grain size on hardness of WC–Co hardmetals (Richter & Ruthendorf, 1999).
25
20
15
10
5
0
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40
Grain size (μm) Fracture toughness (MPa m0.5)
Hardmetals of different binder content
WC–Co WC ceramics
Figure 9 Influence of WC grain size on fracture toughness of WC–Co (Richter & Ruthendorf, 1999).
1.02.2.1 Hardmetals for Metal Cutting
To improve the steel machining performance of WC–Co hardmetals, cubic transition metal carbides (TiC, TaC, and NbC) were added to the basic WC–Co composition even as early as in the 1930s. WC does not dissolve any of these transition metal carbides, but forms a solid solution with these carbides. This leads to a microstructure with an enhanced creep resistance, higher oxidation resistance and higher adhesive wear resistance against long-chipping materials. In practice, this increased the productivity of machining operations since the tool was capable of resisting higher temperatures without suffering catastrophic crater wear. The ISO classification of cutting tools that is summarized in Tables 5 and6shows an example of some hardmetal compositions prevalent in metal cutting (Böhlerit). In applications that call for higher toughness, higher binder contents (mainly Cobalt) were chosen. Further productivity improvements in metal cutting have been achieved since the 1970s by coating the hardmetal substrates with a thin wear-resistantfilm of TiN, TiC, TiCN, TiAlN, Al2O3, etc. by CVD or PVD. Nitride and carbonitride layers possess high hardness and high oxidation resistance, whereas the chemically inert Al2O3 layer serves as an oxidation-resistant heat barrier to avoid excessive heating and subsequent softening of the substrate.
1600 1400 1200 1000 800 600 400 200
00 100 200 300 400 500 600
Time (min)
Temperature (°C)
Eutectic temperature
Solid state sintering
Liquid phase sintering
Oxide reduction De-waxing
Figure 10 Schematic WC–Co sintering process. Cycle time 12 to>24 h (Andren, 2001).
Table 5 ISO 513 classification of cutting tools according to use
Symbol Workpiece material
Color code
Designation in increasing order of wear resistance and decreasing order of toughness in each category (in increments of 5)
P Ferrous metals with long chips Blue P01, P10–P 40
M Ferrous metals with long or short chips, nonferrous
metals Yellow M10–M40
K Ferrous metals with short chips, nonferrous metals,
non metallic materials Red K01, K10–K40
Table 6 Typical compositions and properties of metal cutting grades Type P (Böhlerit)
Grade
ISO appl.
code WC
(wt%)
TiCþTac (wt%)
Co (wt%)
WC grain size (mm)
Density (g/cm3)
Hardness HV30
Comp.
strength (N/mm2)
TRS (N/mm2)
Fracture toughness, K1c
(MPa m0.5)
Modulus of elasticity (kN/mm2)
ISO 513 ISO 3369 ISO 3878 ISO 4506 ISO 3327 ISO 3312
SB10 P05–P15 57.5 33 9.5 2.5 10.3 1575 5300 2000 9.3 530
SB20 P15–P25 69 22 9 2.5 11.20 1550 5200 2000 9.3 540
LW225 P20–P40 72.7 17.3 10 1.25 12.55 1525 5100 2300 9.8 550
SB29 P20–P40 74.1 15.9 10 1.25 12.35 1525 5100 2200 9.8 550
SB30 P25–P30 69 21 10 2.5 11.60 1500 5100 2200 10.0 550
SB40 P35–P45 77 12 11 5.3 13.15 1400 5000 2400 12.0 560
Fundamentals and General Applications of Hardmetals 37
1.02.2.2 Cermets
Another important family of wear-resistant cutting tool alloys, also developed in the early days of hardmetals and which is very popular (more than 30% usage) in Japan, is based on TiC, but alloyed with a number of other elements like Ta, Nb, V, Cr, and Mo with the binder alloy being nickel with cobalt and Al.
The name cermet has been coined from the syllables “Cer” from ceramics and “met” from metals and connotes a wide range of materials consisting of hard ceramic phases bonded by metal with properties superior to that attained by any one single component.
Modern cermet alloys exhibit multicomponent microstructures based on titanium carbonitride and varying amounts of Mo, W, Ta, Nb, V, and perhaps other elements. The binder is an NiCo alloy and contains considerable amounts (20–40 mass%) of Ti, Mo, W, V and other metals like Al in solution, depending on composition and sintering conditions. Earlier TiC-based cermets were created to conserve the strategic important metal tungsten in WC–Co. However, the thermal conductivity, thermal shock resistance, toughness, bending strength and fatigue resistance of these TiC-based materials were inferior to WC–Co. The breakthrough in cermet properties to be on a par with WC–Co was achieved only after the introduction of TiN to the basic Ti, MoC–NiMo alloys (Figure 11).
The microstructures of cermets, whose hard particles often have a “core–rim” structure, are much more complex than those of relatively straightforward WC–Co hardmetals (Figure 4). The core is essentially undis- solved TiCN and/or TiN. The rim is enriched in heavier elements W, Ta/Nb and Mo and has the same cubic crystal structure as the core. With increasing nitrogen content in cermets, the grain size of the hard particles and the thickness of the rim generally decrease. The performance of modern cermets to machine steels at high speed is as good as or even better than that of coated hardmetals.
Nitrogen also plays an important role as far as coated hardmetals are concerned. The microstructure in the surface region of coated substrates has been modified by introducing nitrogen to develop a cobalt-enriched, cubic-carbide-free layer (CFL) (Schwarzkopf, Exner, & Fischmeister, 1988). The lifetime of coated hardmetals using CFL substrates can be significantly extended due to increased toughness beneath coating layers. In the mid 1990s, functionally graded material (FGM) cermet was developed by sintering of TiCNþWCþCo/Ni under nitrogen atmosphere (Tsuda, Ikegaya, Isobe, Kitagawa, & Nomura, 1996). The FGM cermet consists of a high wear-resistant cubic-carbide-enriched surface layer, a tough core containing WC, cubic carbides/car- bonitrides and CoNi binder and an intermediate zone with graded compositions. The FGM cermet displays better wear resistance compared to standard cermet and coated hardmetals (Chen, Lengauer, & Dreyer, 2000;
Glühmann et al., 2013).
The main binder used in cermets has been alloys of nickel and cobalt. Interest in perhaps using steel binders has been growing.Figure 12shows an example of a calculated phase diagram (Alvaredo, P., Mari, D., & Gordo, E. (2013)).
Figure 11 Example for microstructural features of a WC–Co (left) compared to a WC–TiC–TaC–Co HM (right) (SEM micrograph (EPMA)).