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Characterization of WC–(W,V)C–Co made from pre-alloyed (W,V)C
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 229–233
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Characterization of WC–(W,V)C–Co made from pre-alloyed (W,V)C Nobom Gretta Hashe a,*, Susanne M. Norgren b, Hans-Olof Andrén c, Johannes H. Neethling a a b c
Department of Physics, P.O. Box 77000, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa Sandvik Tooling, R&D Materials and Processes, SE-126 80 Stockholm, Sweden Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 6 June 2008 Accepted 29 September 2008
Keywords: Microstructure Cubic carbides Electron microscopy
a b s t r a c t Large (W,V)C cubic carbides in WC–VC–Co cemented carbides are undesirable as their presence is one of the causes for poor properties in the material. Earlier attempts to reduce the (W,V)C cubic carbide grain sizes in the WC–VC–Co cemented carbide have been published before. The present investigation strives to reduce the cubic carbide grain size by using a pre-alloyed (W,V)C powder in the place of VC, to reduce the driving force for the formation of (W,V)C during sintering. This should in turn reduce the possibility of forming large (W,V)C grains. WC–VC–Co was prepared using WC, (W,V)C, and Co powders. The compositions were 8.1 wt% V and 12 wt% Co, with the balance comprising W and C. XRD diffraction patterns confirmed that the bulk of WC–(W,V)C–Co contains WC grains, cubic (W,V)C grains, and a Co-rich binder phase. SEM–EDS measurements yielded an average composition of (W0.31V0.69)Cx. TEM–EDS of the cubic carbide was in good agreement with the SEM–EDS measurement, yielding the average composition of (W0.30V0.70)Cx. The average grain size of the cubic carbide of WC–(W,V)C–Co material after sintering had grown to only the same size as that of the starting powder, 1.4 lm. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Basic cemented carbides are materials consisting of hard carbide grains embedded in a ductile binder phase, usually rich in cobalt. The hard phase is usually WC, but often other carbides or carbonitrides with the sodium chloride structure are present. A common application of cemented carbides is metal cutting. Small amounts of vanadium carbide (about 0.5 wt%) are usually added in the WC–Co industry to produce a fine gained WC–Co material. The hardness of WC–Co improves with fine WC grain sizes. Additions of more than about 2 wt% V (above the solubility limit of V for the liquid binder) create a problem in that during liquid state sintering V dissolves in the liquid binder and then reprecipitates as very large grains of (W,V)C resulting in very brittle material [1]. Luyckx found in 1994 that the WC–VC–Co material was more brittle than the WC–Co material [2]. She suggested that the toughness of WC–VC–Co material can be improved by decreasing the size of the (W,V)C grains. In the previous reports attempts have been made to reduce the grain size of (W,V)C cubic carbides by replacing half of the of the V atoms with Ti atoms producing WC–VC–TiC–Co. Ti was added because its solubility in the liquid binder phase is lower than that of V [3]. Earlier attempts were also made to reduce the cubic carbide grain sizes by sintering the materials in the presence of nitrogen [4,5], since it decreases the solubility of V during sintering. * Corresponding author. Tel.: +27 41 504 4299; fax: +27 41 504 1508. E-mail address: [email protected] (N.G. Hashe). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.020
This investigation attempts to reduce the cubic carbide grain size by using a pre-alloyed (W,V)C instead of VC. The pre-alloyed (W,V)C was aimed to have the equilibrium composition at 1410 °C. The use of the pre-alloyed (W,V)C reduces the driving force for the formation of (W,V)C during sintering, in turn reducing the probability of forming large (W,V)C grains. The results of this study are compared with the results of the materials analysed previously [3–5]. The comparison is based on the composition and size of cubic carbide or cubic carbonitride grains and the mechanical properties. An overview of all five materials studied (designated SA1–SA5) is given in Table 1. 2. Experimental procedure WC–VC–Co was prepared by Sandvik Tooling in Sweden, using WC, (W,V)C, and Co powders. The material was designated SA5 and had a composition of 8.1 wt% V and 12 wt% Co with the balance comprising W and C. The carbon content was carefully monitored in order to reduce the possibility of forming undesired phases like graphite and the g phase. The compositions of all five materials were specifically selected to attain the same volume fraction of 16% of the binder phase and same vol% of cubic carbides or carbonitrides. The grain sizes of the starting powders are tabulated in Table 2. The powder mixtures were ball milled for 80 h, pressed, and sintered at 1410 °C for 1 h in vacuum. The sintered WC–WVC–Co material was analysed using scanning (SEM) and transmission electron microscopy (TEM), energy dispersive X-ray spectrometry
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Table 1 Materials used in the present and previous studies.
1800
Sample
1600
WC–VC–TiC–Co WC–VC–Co WC–VC–TiC–Co WC–VC–Co WC–(W,V)C–Co
Vacuum Vacuum Nitrogen Nitrogen Vacuum
Powder mixture WC
VC
TiC
Co
X X X X X
X X X X
X
X X X X X
X
(W,V)C
X
INTENSITY (arb. units)
SA1 SA2 SA3 SA4 SA5
Sintering atmosphere
(W,V)C starting powder (W,V)C
1400 (W,V)C
1200 1000 800
(W,V)C
600 400 200 0
0
20
40
60
80
100
120
2-THETA
Table 2 Grain sizes of the starting powders. Powder
Raw materials supplier
Grain size (in lm) FSSSa
VC WC TiC Co (W0.30,V0.70)C
H.C. Starck H.C Starck (DS150) H.C. Starck OMG Treibacher
1.2–1.8 1.45–1.55 1.2–1.8 1.3–1.6 1.4
a
(W,V)C
Fig. 2. An XRD pattern confirming that the (W,V)C starting powder consists of a single phase.
Fischer sub-sieve size.
(EDS), and X-ray diffractometry (XRD) with copper Ka radiation (k = 1.54 nm). The quantitative energy dispersive X-ray spectrometry analyses were carried out on the SEM and TEM using EDAX DX4 systems and the EDAX standardless quantification software. The accuracy of the EDS values is estimated to be ±2 at%. 3. Results and discussion 3.1. Microstructure of SA5 A typical secondary electron SEM image of the (W,V)C starting powder is shown in Fig. 1. It was observed that the grain size of the powder particles is rather uniform and that the powder grains are small in size (1.4 lm). SEM–EDS measurements of the grains yielded an average composition of (W0.30,V0.70)Cx. No evidence of a composition gradient was found in the grains. The (W,V)C powder was found to be single phase, as evidenced by the absence of additional reflections in the XRD diffraction pattern (Fig. 2). Only reflections that could be attributed to the cubic carbide phase were observed in the XRD diffraction pattern. During milling the (W,V)C powder grain size decreases, but it increases again during sintering. Fig. 3 is a typical secondary elec-
Fig. 3. A typical secondary electron SEM image of WC–(W,V)C–Co material. Bright contrast is WC, grey is (W,V)C, and dark is binder phase.
tron image of the as-sintered WC–(W,V)C–Co material. The nature of the grains shown in Fig. 3 was determined by SEM–EDS and found to be (W,V)C (greyish regions), WC (light regions), and Corich phase (dark regions). No evidence of a core–rim structure was found in the cubic carbide grains in SA5 as was found in the same material (SA2) with VC as a starting powder. The SEM–EDS measurements yielded an average composition of (W0.31,V0.69)Cx which is the same composition as the starting powder, showing that the pre-alloyed composition is the same as the equilibrium composition at 1410 °C. The cubic carbide grains were more evenly distributed in the present material compared to SA2.
WC-(W,V)C-Co
Intensity (arb. units)
140
WC
WC
120 100 80
WC
(W,V)C WC
WC
60
(W,V)C
Co
40 20 0
0
10
20
30
40
50
60
70
80
2 THETA Fig. 1. A typical secondary electron SEM image of the (W,V)C starting powder.
Fig. 4. A typical XRD pattern of the bulk of the WC–(W,V)C–Co material.
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Fig. 4 is an XRD diffraction pattern which confirms that the bulk of SA5 contains WC grains, cubic (W,V)C grains and a Co-rich binder phase. A bright-field TEM micrograph of the microstructure of the WC– (W,V)C–Co (SA5) material is shown in Fig. 5. No evidence of core– rim formation in the cubic carbide was found, which would have formed if the composition of the pre-alloyed (W,V)C was not the equilibrium composition at the sintering temperature. No interfacial dislocations were observed in the cubic carbide grains as was the case between the core and the rim in SA2. In contrast to the two phases observed in SA4, the cubic carbide in SA5 was a single phase. SA4 consisted of two types of cubic carbonitrides, one with a composition (W0.17,V0.83)(C,N) and the other with the composition of (W0.24,V0.76)(C,N). TEM–EDS of the cubic carbide of SA5 yielded the average composition of (W0.30,V0.70)Cx. The Co-rich binder phase is indicated by the arrows in Fig. 5. TEM–EDS of the binder yielded an average composition of 95 at% Co, 3 at% W, and 2 at% V. The composition of the binder phase of SA5 determined by TEM–EDS is approximately the same as that of SA2 (93.76 ± 0.21 at% Co, 4.24 ± 0.18 at% W, 1.94 ± 0.12 at% V, and 0.06 ± 0.02 at% C) determined by an atom probe field ion microscope. Small additions of VC in WC–Co materials are used in the cemented carbides since VC inhibits WC grain growth which leads to an increase in hardness. Not only the WC grain size, but also the morphology of the WC/Co interfaces is affected by the addition of VC. The interfaces of WC/Co have a tendency to form facets with multi-steps of (0 0 0 1) and (1 0 1 0) habits when small amounts of V are added [5]. Yamamoto et al. explained the mechanism of grain growth inhibition as follows: during sintering, VC dissolves in the binder and then the segregation of (W,V)C to the WC/Co interfaces takes place. At a temperature of 1200 °C, a (W,V)C layer surrounds the WC grains. This layer is faceted with fine multi-steps. After the formation of the segregation layer, the dissolution–reprecipitation process of WC grains may then be limited [6]. In this study, steps on the WC grains at the WC/Co interface have also been observed (Fig. 6). The presence of these steps may imply that V added as (W,V)C has the same effect on the WC grain growth as inhibitor amounts of VC. Qualitative comparison of the WC grain size in SA5 with the WC grain size in other materials in this study shows that the grain sizes are approximately the same. This may lead to the conclusion that it does not matter whether the starting powder is VC or (W,V)C as long as V is present in the mate-
231
Fig. 6. A bright-field TEM micrograph showing a multi-stepped WC grains.
Fig. 7. A bright-field TEM micrograph showing steps in the WC grain along the WC/ (W,V)C interface.
rial to inhibit WC grain growth. The steps in the WC grains previously observed along the WC/Co interface [6] can also be seen in the WC/(W,V)C interface in Fig. 7. 3.2. Grain size and mechanical properties of SA5
Fig. 5. A typical bright-field TEM micrograph showing the microstructure of SA5.
The average grain sizes of the cubic carbides were measured using the intercept method as described elsewhere [7]. The total analysed area was about 6500 lm [2] per sample and the number of lines used for the calculation was 170 (and 169 for SA3). For each material ten micrographs were analysed. The average grain size of the cubic carbide of SA5 after sintering had grown to the same size as that of the starting powder. Since the driving force for nucleation and growth of (W,V)C is eliminated, only a coarsening process occurs during sintering by dissolution of small grains and reprecipitation onto large (W,V)C grains. The coarsening process is probably slower than the growth process, giving a smaller cubic carbide size. Compared to the SA1, SA2, SA3, and SA4 grain size distributions, using (W,V)C as a starting powder in SA5 inhibited the
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Grain size distribution
accumulated grain distribution (%)
120
100
80 SA4 SA3 SA1 SA2 SA5
60
40
20
0 0
1
2
3
4
5
6
7
8
9
Cubic carbide (cc) grain size in microns Fig. 8. Grain size distributions of SA1–SA5.
development of very coarse grains. The width of the distribution (d90–d10)/d50 (where d10 is the grain size at 10% of the accumulated grain distribution, etc.) for SA5 is 0.86 which is narrower compared to the distribution widths for all the other materials (Fig. 8). The material was further characterized by mechanical and magnetic measurements, and the results are given in Table 3. Porosity was also measured for the materials, in the present and earlier studies. Porosity measurements of the material are important in quality control, since the properties of the material depend on the density which in turn depends on the composition and porosity [8]. According to the industrial standard (ISO4505) the porosity is defined by three types. Type A covers pore diameters <10 lm. Type B covers pore diameters between 10 and 25 lm, and type C covers porosity developed by the presence of graphite. The degree of porosity is given by four numbers ranging in value from 02 to 08, which provides a measure of pore volume as a percentage of total volume of the sample [8]. Neither porosity nor graphite was detected in SA5. All studied materials had acceptable porosity values. However, graphite had formed in SA3 during sintering which should reduce the hardness. The relative magnetic saturation of SA5, for materials free of graphite or g phase, was inside the window for a graphite or g phase material. The relative magnetic saturation for both SA3 and SA4 materials was close to 1. However, it was only SA3 that graphite was detected using the light optical microscope. The coercivity of SA5 was measured and can be compared with the values of other materials (Table 3). For WC–Co materials of given Co content the coercivity is used as an indirect measure of the
WC grain size [9]. The coercivity of the present material is lower than that of the other materials (SA1–SA4) of the same Co volume and sintering conditions. It is well known that coercivity and hardness increases with decreasing WC grain size. The hardness of this material is higher than that of the other material (SA1–SA4) whereas the coercivity value for SA5 is lower than that of other materials in Table 3. This finding is probably due to the fact that the Co-rich binder is not uniformly distributed through the material, which could be responsible for the lower coercivity in this material. From the results of this investigation on WC–(W,V)C– Co materials no clear correlation between coercivity and WC grain size was found. From Table 3 it can be seen that SA5 has the highest hardness of all the materials, which is most likely due to the small cubic carbide grain size. Starting from SA2, with largest cubic carbide grain size, we expect the hardness to increase as the cubic grain size decreases, since the WC grains appears to be approximately the same in all five materials. The only exception is SA3 which has an unexpected low hardness, most likely due to the presence of graphite. It should be noted that the hardness of all the materials fall in the same range as that of the cemented carbides used in various machining applications (1100–2000 H V) [8]. The toughness of SA5 was measured using the Palmquist technique. The toughness of this material is low compared to the other materials of the same Co content and sintering conditions (Fig. 9). It is assumed that the reason for the low toughness is due to the small cubic carbide grains. The reason for this assumption is that the cubic carbides of this study are smaller than those of the other
Table 3 Mechanical, magnetic properties, and cubic carbide grain sizes of SA1, SA2, SA3, SA4, and SA5. Sample
SA1
SA2
SA3
SA4
SA5
HCa(kA/m) wt% Co Sb Density (g/cm3) Porosity(ISO4505) HV30 (Vickers) Average grain size of cubic carbide/cubic carbonitrides (diameter in lm)
16.46 13.2 0.87 12.48 A02B00C00 1549 1.7 ± 0.8
18.01 12.0 0.86 12.53 A00B00C00 1519 2.0 ± 1.2
17.02 13.2 0.99 12.34 A06B00C06 1505 1.7 ± 0.6
15.75 12.0 1.0 12.48 A02B02C00 1576 2.0 ± 0.6
16.53 12.0 0.95 12.35 A00B00C00 1606 1.4 ± 0.4
a b
Coercivity. magnetic saturation relative to pure Co (by weight).
N.G. Hashe et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 229–233
may imply that (W,V)C has the same effect as VC the WC grain growth inhibitor. These steps were also observed in the WC/ (W,V)C interface. The cubic carbide grain size in SA5 was the same as the starting powder, 1.4 lm. This means that the use of a pre-alloyed powder of a composition close to equilibrium at the sintering temperature efficiently prevented the formation of large (W,V)C grains.
1640
SA2 1620
SA4
Hardness (Hv30)
1600 1580
SA3
1560
SA1
1540
SA5
233
Acknowledgements
1520
The contributions made by the late Prof. S. Luyckx in introducing the NMMU group to the hard metals field is greatly acknowledged. Dr. Bo Jansson at Seco Tools is thanked for many fruitful discussions.
1500 1480 1460
9.0
10.0
11.0
12.0
13.0
14.0
1/2
Toughness (MPa m ) Fig. 9. Graph of hardness vs. toughness of SA1-SA2,
materials. Another reason could be that the Co is not well distributed in the material affecting the mean free path of the binder phase. 4. Conclusions A WC–Co based cemented carbide containing large additions of (W,V)C was sintered in vacuum to produce the WC–(W,V)C–Co material (SA5). The material in this study was compared to similar materials, with additions of VC instead of (W,V)C, sintered in either vacuum (SA2) or nitrogen atmosphere (SA4). Unlike SA2, the cubic carbides in SA5 did not exhibit any core–rim structure. The cubic carbides in SA5 were of a single phase unlike the SA4 material which was sintered in nitrogen. Steps on the WC grains at the WC/Co interfaces were observed. The presence of these steps
References [1] Osborne C, Cornish L, Luyckx S. On the preparation of fine V8C7–WC and V4C3– WC powders. Int J Refract Met Hard Mater 1997;15:163–8. [2] Luyckx S. Vanadium carbide in hardmetals. XVth CMMI Congress Johannesburg. SAIMM 1994;2:11–3. [3] Hashe NG, Neethling JH, Andrén H-O, Norgren S, Berndt PR. A comparison of the microstructure of WC–VC–TiC–Co and WC–VC–Co cemented carbides. Int J Refract Met Hard Mater 2007;25:207–13. [4] Hashe NG, Norgren S, Andrén H-O, Neethling JH, Berndt PR. The influence of sintering in nitrogen gas on the microstructure of a WC–VC–TiC–Co cemented carbide. Int J Refract Met Hard Mater 2008;26:404–10. [5] Hashe NG, Neethling JH, Andrén H-O, Berndt PR. Reduction of carbide grain growth in WC–VC–Co by sintering in nitrogen atmosphere. Int J Refract Met Hard Mater 2009;20–5. [6] Yamamoto T, Ikuhara Y, Sakuma T. High resolution transmission electron microscopy study in VC-doped WC–Co compound. Sci Technol Adv Mater 2000:97–104. [7] Exner HE, Hougardy HP. Quantitative image analysis of microstructures. Germany: Verlag, Informationsgesellschaft; 1988. [8] Santhanam AT, Tierney P, Hunt JL. Metal’s handbook, vol. 2. 10th ed., ASM International; 1990. pp. 950–77. [9] Rolande U, Weinl G, Zwinkels M. Effect of Ta on structure and mechanical properties of (Ti, Ta, W) (C, N)–Co cermets. Int J Refract Met Hard Mater 2000;19:325–8.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Characterization of WC–(W,V)C–Co made from pre-alloyed (W,V)C Nobom Gretta Hashe a,*, Susanne M. Norgren b, Hans-Olof Andrén c, Johannes H. Neethling a a b c
Department of Physics, P.O. Box 77000, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa Sandvik Tooling, R&D Materials and Processes, SE-126 80 Stockholm, Sweden Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 6 June 2008 Accepted 29 September 2008
Keywords: Microstructure Cubic carbides Electron microscopy
a b s t r a c t Large (W,V)C cubic carbides in WC–VC–Co cemented carbides are undesirable as their presence is one of the causes for poor properties in the material. Earlier attempts to reduce the (W,V)C cubic carbide grain sizes in the WC–VC–Co cemented carbide have been published before. The present investigation strives to reduce the cubic carbide grain size by using a pre-alloyed (W,V)C powder in the place of VC, to reduce the driving force for the formation of (W,V)C during sintering. This should in turn reduce the possibility of forming large (W,V)C grains. WC–VC–Co was prepared using WC, (W,V)C, and Co powders. The compositions were 8.1 wt% V and 12 wt% Co, with the balance comprising W and C. XRD diffraction patterns confirmed that the bulk of WC–(W,V)C–Co contains WC grains, cubic (W,V)C grains, and a Co-rich binder phase. SEM–EDS measurements yielded an average composition of (W0.31V0.69)Cx. TEM–EDS of the cubic carbide was in good agreement with the SEM–EDS measurement, yielding the average composition of (W0.30V0.70)Cx. The average grain size of the cubic carbide of WC–(W,V)C–Co material after sintering had grown to only the same size as that of the starting powder, 1.4 lm. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Basic cemented carbides are materials consisting of hard carbide grains embedded in a ductile binder phase, usually rich in cobalt. The hard phase is usually WC, but often other carbides or carbonitrides with the sodium chloride structure are present. A common application of cemented carbides is metal cutting. Small amounts of vanadium carbide (about 0.5 wt%) are usually added in the WC–Co industry to produce a fine gained WC–Co material. The hardness of WC–Co improves with fine WC grain sizes. Additions of more than about 2 wt% V (above the solubility limit of V for the liquid binder) create a problem in that during liquid state sintering V dissolves in the liquid binder and then reprecipitates as very large grains of (W,V)C resulting in very brittle material [1]. Luyckx found in 1994 that the WC–VC–Co material was more brittle than the WC–Co material [2]. She suggested that the toughness of WC–VC–Co material can be improved by decreasing the size of the (W,V)C grains. In the previous reports attempts have been made to reduce the grain size of (W,V)C cubic carbides by replacing half of the of the V atoms with Ti atoms producing WC–VC–TiC–Co. Ti was added because its solubility in the liquid binder phase is lower than that of V [3]. Earlier attempts were also made to reduce the cubic carbide grain sizes by sintering the materials in the presence of nitrogen [4,5], since it decreases the solubility of V during sintering. * Corresponding author. Tel.: +27 41 504 4299; fax: +27 41 504 1508. E-mail address: [email protected] (N.G. Hashe). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.09.020
This investigation attempts to reduce the cubic carbide grain size by using a pre-alloyed (W,V)C instead of VC. The pre-alloyed (W,V)C was aimed to have the equilibrium composition at 1410 °C. The use of the pre-alloyed (W,V)C reduces the driving force for the formation of (W,V)C during sintering, in turn reducing the probability of forming large (W,V)C grains. The results of this study are compared with the results of the materials analysed previously [3–5]. The comparison is based on the composition and size of cubic carbide or cubic carbonitride grains and the mechanical properties. An overview of all five materials studied (designated SA1–SA5) is given in Table 1. 2. Experimental procedure WC–VC–Co was prepared by Sandvik Tooling in Sweden, using WC, (W,V)C, and Co powders. The material was designated SA5 and had a composition of 8.1 wt% V and 12 wt% Co with the balance comprising W and C. The carbon content was carefully monitored in order to reduce the possibility of forming undesired phases like graphite and the g phase. The compositions of all five materials were specifically selected to attain the same volume fraction of 16% of the binder phase and same vol% of cubic carbides or carbonitrides. The grain sizes of the starting powders are tabulated in Table 2. The powder mixtures were ball milled for 80 h, pressed, and sintered at 1410 °C for 1 h in vacuum. The sintered WC–WVC–Co material was analysed using scanning (SEM) and transmission electron microscopy (TEM), energy dispersive X-ray spectrometry
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Table 1 Materials used in the present and previous studies.
1800
Sample
1600
WC–VC–TiC–Co WC–VC–Co WC–VC–TiC–Co WC–VC–Co WC–(W,V)C–Co
Vacuum Vacuum Nitrogen Nitrogen Vacuum
Powder mixture WC
VC
TiC
Co
X X X X X
X X X X
X
X X X X X
X
(W,V)C
X
INTENSITY (arb. units)
SA1 SA2 SA3 SA4 SA5
Sintering atmosphere
(W,V)C starting powder (W,V)C
1400 (W,V)C
1200 1000 800
(W,V)C
600 400 200 0
0
20
40
60
80
100
120
2-THETA
Table 2 Grain sizes of the starting powders. Powder
Raw materials supplier
Grain size (in lm) FSSSa
VC WC TiC Co (W0.30,V0.70)C
H.C. Starck H.C Starck (DS150) H.C. Starck OMG Treibacher
1.2–1.8 1.45–1.55 1.2–1.8 1.3–1.6 1.4
a
(W,V)C
Fig. 2. An XRD pattern confirming that the (W,V)C starting powder consists of a single phase.
Fischer sub-sieve size.
(EDS), and X-ray diffractometry (XRD) with copper Ka radiation (k = 1.54 nm). The quantitative energy dispersive X-ray spectrometry analyses were carried out on the SEM and TEM using EDAX DX4 systems and the EDAX standardless quantification software. The accuracy of the EDS values is estimated to be ±2 at%. 3. Results and discussion 3.1. Microstructure of SA5 A typical secondary electron SEM image of the (W,V)C starting powder is shown in Fig. 1. It was observed that the grain size of the powder particles is rather uniform and that the powder grains are small in size (1.4 lm). SEM–EDS measurements of the grains yielded an average composition of (W0.30,V0.70)Cx. No evidence of a composition gradient was found in the grains. The (W,V)C powder was found to be single phase, as evidenced by the absence of additional reflections in the XRD diffraction pattern (Fig. 2). Only reflections that could be attributed to the cubic carbide phase were observed in the XRD diffraction pattern. During milling the (W,V)C powder grain size decreases, but it increases again during sintering. Fig. 3 is a typical secondary elec-
Fig. 3. A typical secondary electron SEM image of WC–(W,V)C–Co material. Bright contrast is WC, grey is (W,V)C, and dark is binder phase.
tron image of the as-sintered WC–(W,V)C–Co material. The nature of the grains shown in Fig. 3 was determined by SEM–EDS and found to be (W,V)C (greyish regions), WC (light regions), and Corich phase (dark regions). No evidence of a core–rim structure was found in the cubic carbide grains in SA5 as was found in the same material (SA2) with VC as a starting powder. The SEM–EDS measurements yielded an average composition of (W0.31,V0.69)Cx which is the same composition as the starting powder, showing that the pre-alloyed composition is the same as the equilibrium composition at 1410 °C. The cubic carbide grains were more evenly distributed in the present material compared to SA2.
WC-(W,V)C-Co
Intensity (arb. units)
140
WC
WC
120 100 80
WC
(W,V)C WC
WC
60
(W,V)C
Co
40 20 0
0
10
20
30
40
50
60
70
80
2 THETA Fig. 1. A typical secondary electron SEM image of the (W,V)C starting powder.
Fig. 4. A typical XRD pattern of the bulk of the WC–(W,V)C–Co material.
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Fig. 4 is an XRD diffraction pattern which confirms that the bulk of SA5 contains WC grains, cubic (W,V)C grains and a Co-rich binder phase. A bright-field TEM micrograph of the microstructure of the WC– (W,V)C–Co (SA5) material is shown in Fig. 5. No evidence of core– rim formation in the cubic carbide was found, which would have formed if the composition of the pre-alloyed (W,V)C was not the equilibrium composition at the sintering temperature. No interfacial dislocations were observed in the cubic carbide grains as was the case between the core and the rim in SA2. In contrast to the two phases observed in SA4, the cubic carbide in SA5 was a single phase. SA4 consisted of two types of cubic carbonitrides, one with a composition (W0.17,V0.83)(C,N) and the other with the composition of (W0.24,V0.76)(C,N). TEM–EDS of the cubic carbide of SA5 yielded the average composition of (W0.30,V0.70)Cx. The Co-rich binder phase is indicated by the arrows in Fig. 5. TEM–EDS of the binder yielded an average composition of 95 at% Co, 3 at% W, and 2 at% V. The composition of the binder phase of SA5 determined by TEM–EDS is approximately the same as that of SA2 (93.76 ± 0.21 at% Co, 4.24 ± 0.18 at% W, 1.94 ± 0.12 at% V, and 0.06 ± 0.02 at% C) determined by an atom probe field ion microscope. Small additions of VC in WC–Co materials are used in the cemented carbides since VC inhibits WC grain growth which leads to an increase in hardness. Not only the WC grain size, but also the morphology of the WC/Co interfaces is affected by the addition of VC. The interfaces of WC/Co have a tendency to form facets with multi-steps of (0 0 0 1) and (1 0 1 0) habits when small amounts of V are added [5]. Yamamoto et al. explained the mechanism of grain growth inhibition as follows: during sintering, VC dissolves in the binder and then the segregation of (W,V)C to the WC/Co interfaces takes place. At a temperature of 1200 °C, a (W,V)C layer surrounds the WC grains. This layer is faceted with fine multi-steps. After the formation of the segregation layer, the dissolution–reprecipitation process of WC grains may then be limited [6]. In this study, steps on the WC grains at the WC/Co interface have also been observed (Fig. 6). The presence of these steps may imply that V added as (W,V)C has the same effect on the WC grain growth as inhibitor amounts of VC. Qualitative comparison of the WC grain size in SA5 with the WC grain size in other materials in this study shows that the grain sizes are approximately the same. This may lead to the conclusion that it does not matter whether the starting powder is VC or (W,V)C as long as V is present in the mate-
231
Fig. 6. A bright-field TEM micrograph showing a multi-stepped WC grains.
Fig. 7. A bright-field TEM micrograph showing steps in the WC grain along the WC/ (W,V)C interface.
rial to inhibit WC grain growth. The steps in the WC grains previously observed along the WC/Co interface [6] can also be seen in the WC/(W,V)C interface in Fig. 7. 3.2. Grain size and mechanical properties of SA5
Fig. 5. A typical bright-field TEM micrograph showing the microstructure of SA5.
The average grain sizes of the cubic carbides were measured using the intercept method as described elsewhere [7]. The total analysed area was about 6500 lm [2] per sample and the number of lines used for the calculation was 170 (and 169 for SA3). For each material ten micrographs were analysed. The average grain size of the cubic carbide of SA5 after sintering had grown to the same size as that of the starting powder. Since the driving force for nucleation and growth of (W,V)C is eliminated, only a coarsening process occurs during sintering by dissolution of small grains and reprecipitation onto large (W,V)C grains. The coarsening process is probably slower than the growth process, giving a smaller cubic carbide size. Compared to the SA1, SA2, SA3, and SA4 grain size distributions, using (W,V)C as a starting powder in SA5 inhibited the
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Grain size distribution
accumulated grain distribution (%)
120
100
80 SA4 SA3 SA1 SA2 SA5
60
40
20
0 0
1
2
3
4
5
6
7
8
9
Cubic carbide (cc) grain size in microns Fig. 8. Grain size distributions of SA1–SA5.
development of very coarse grains. The width of the distribution (d90–d10)/d50 (where d10 is the grain size at 10% of the accumulated grain distribution, etc.) for SA5 is 0.86 which is narrower compared to the distribution widths for all the other materials (Fig. 8). The material was further characterized by mechanical and magnetic measurements, and the results are given in Table 3. Porosity was also measured for the materials, in the present and earlier studies. Porosity measurements of the material are important in quality control, since the properties of the material depend on the density which in turn depends on the composition and porosity [8]. According to the industrial standard (ISO4505) the porosity is defined by three types. Type A covers pore diameters <10 lm. Type B covers pore diameters between 10 and 25 lm, and type C covers porosity developed by the presence of graphite. The degree of porosity is given by four numbers ranging in value from 02 to 08, which provides a measure of pore volume as a percentage of total volume of the sample [8]. Neither porosity nor graphite was detected in SA5. All studied materials had acceptable porosity values. However, graphite had formed in SA3 during sintering which should reduce the hardness. The relative magnetic saturation of SA5, for materials free of graphite or g phase, was inside the window for a graphite or g phase material. The relative magnetic saturation for both SA3 and SA4 materials was close to 1. However, it was only SA3 that graphite was detected using the light optical microscope. The coercivity of SA5 was measured and can be compared with the values of other materials (Table 3). For WC–Co materials of given Co content the coercivity is used as an indirect measure of the
WC grain size [9]. The coercivity of the present material is lower than that of the other materials (SA1–SA4) of the same Co volume and sintering conditions. It is well known that coercivity and hardness increases with decreasing WC grain size. The hardness of this material is higher than that of the other material (SA1–SA4) whereas the coercivity value for SA5 is lower than that of other materials in Table 3. This finding is probably due to the fact that the Co-rich binder is not uniformly distributed through the material, which could be responsible for the lower coercivity in this material. From the results of this investigation on WC–(W,V)C– Co materials no clear correlation between coercivity and WC grain size was found. From Table 3 it can be seen that SA5 has the highest hardness of all the materials, which is most likely due to the small cubic carbide grain size. Starting from SA2, with largest cubic carbide grain size, we expect the hardness to increase as the cubic grain size decreases, since the WC grains appears to be approximately the same in all five materials. The only exception is SA3 which has an unexpected low hardness, most likely due to the presence of graphite. It should be noted that the hardness of all the materials fall in the same range as that of the cemented carbides used in various machining applications (1100–2000 H V) [8]. The toughness of SA5 was measured using the Palmquist technique. The toughness of this material is low compared to the other materials of the same Co content and sintering conditions (Fig. 9). It is assumed that the reason for the low toughness is due to the small cubic carbide grains. The reason for this assumption is that the cubic carbides of this study are smaller than those of the other
Table 3 Mechanical, magnetic properties, and cubic carbide grain sizes of SA1, SA2, SA3, SA4, and SA5. Sample
SA1
SA2
SA3
SA4
SA5
HCa(kA/m) wt% Co Sb Density (g/cm3) Porosity(ISO4505) HV30 (Vickers) Average grain size of cubic carbide/cubic carbonitrides (diameter in lm)
16.46 13.2 0.87 12.48 A02B00C00 1549 1.7 ± 0.8
18.01 12.0 0.86 12.53 A00B00C00 1519 2.0 ± 1.2
17.02 13.2 0.99 12.34 A06B00C06 1505 1.7 ± 0.6
15.75 12.0 1.0 12.48 A02B02C00 1576 2.0 ± 0.6
16.53 12.0 0.95 12.35 A00B00C00 1606 1.4 ± 0.4
a b
Coercivity. magnetic saturation relative to pure Co (by weight).
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may imply that (W,V)C has the same effect as VC the WC grain growth inhibitor. These steps were also observed in the WC/ (W,V)C interface. The cubic carbide grain size in SA5 was the same as the starting powder, 1.4 lm. This means that the use of a pre-alloyed powder of a composition close to equilibrium at the sintering temperature efficiently prevented the formation of large (W,V)C grains.
1640
SA2 1620
SA4
Hardness (Hv30)
1600 1580
SA3
1560
SA1
1540
SA5
233
Acknowledgements
1520
The contributions made by the late Prof. S. Luyckx in introducing the NMMU group to the hard metals field is greatly acknowledged. Dr. Bo Jansson at Seco Tools is thanked for many fruitful discussions.
1500 1480 1460
9.0
10.0
11.0
12.0
13.0
14.0
1/2
Toughness (MPa m ) Fig. 9. Graph of hardness vs. toughness of SA1-SA2,
materials. Another reason could be that the Co is not well distributed in the material affecting the mean free path of the binder phase. 4. Conclusions A WC–Co based cemented carbide containing large additions of (W,V)C was sintered in vacuum to produce the WC–(W,V)C–Co material (SA5). The material in this study was compared to similar materials, with additions of VC instead of (W,V)C, sintered in either vacuum (SA2) or nitrogen atmosphere (SA4). Unlike SA2, the cubic carbides in SA5 did not exhibit any core–rim structure. The cubic carbides in SA5 were of a single phase unlike the SA4 material which was sintered in nitrogen. Steps on the WC grains at the WC/Co interfaces were observed. The presence of these steps
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