Adding rhenium to the binder in cemented carbide final
1. Materials Science and Engineering
KTH Royal Institute of Technology
Stockholm, SWEDEN
Adding Rhenium to the Binder in Cemented Carbide
A project by
Eyvind Engblom, Jenny Linden, Joakim Larsen, Kristoffer
Pettersson and Patricia Lind
Stockholm,
May
2013
Analysis
and
Design
of
Materials
2. 1
Abstract
The aim of the project was to investigate an alternative binder to cobalt (Co) in cemented
carbides, for use in for example cutting tools. The problem of the currently used binders is
that they soften at the working temperature (800°C). Alternative binder phases to cemented
carbides, besides Co, include nickel (Ni) and iron (Fe). Common supplements to the binder
phase are super-alloys and noble metals. This study is focused on the effects of using rhenium
(Re) in addition to Co in the binder.
In order to evaluate the effects of adding Re, two samples were investigated; one containing
WC-Co-Re and one reference tool containing WC-Co. The samples were evaluated using
Vickers hardness test, SEM/EDS and light optical microscopy.
The Rhenium sample showed an increase in hardness of 150 MPa. The SEM/EDS analysis
showed that Rhenium was dissolved together with cobalt in the binder.
4. 3
1. Introduction
Cemented carbide is a composite material consisting of W carbide (WC) and a binder phase,
most commonly used is Co. The material is characterized by hardness, corrosion- and wear
resistance and toughness. Cemented carbides are used in many types of applications, due to
the fact that different material compositions change the properties of the product. Applications
include, for instance, drilling equipment, mining equipment and cutting tools for machining.
The normal working temperature for machining operations is 800°C or above. However,
during these operations above 800°C, the Co binder softens making the cutting tool blunt. In
order to improve the material properties, the composition of the binder phase could be altered,
either through changing the main component of the binder phase or adding a supplement. The
main component of the binder phase is usually Co, Fe, Ni or a combination of those. Common
supplements to the binder phase are carbides like TiC, TaC, NbC and Mo2C, or elements like
Cr, Fe, Cu or Al.
A study by Lisovskii [1] indicates that a Re-Co-binder phase could be preferable to Co alone.
The study also shows that Re as a supplement to Co increases the hexagonal structure in the
binder phase resulting in increased temperature stability and hardness. Norgan et al.,
investigated the effect of Re and super-alloy addition to the binder phase in machining tools.
The study showed results such as improved tool life, machining speeds and tool wear [2]. The
study also concludes that “no clear effect was evident from varying the super alloy-to-cobalt
ratio in the alternative binders” which is an indication that Re caused the improvements
mentioned above [2].
The aim of the project is to study an alternative binder phase that maintains its properties
during machining operations above 800°C. The effects of Re addition to a Co binder phase, as
a potential deformation hardening effect, was investigated.
5. 4
2. Background
2.1 Manufacturing of Cemented Carbide
2.1.1 Milling
Milling is the first step in the manufacturing process of cemented carbides through liquid-
phase sintering. Milling reduces the grain size and mixes WC powder with binder powder.
The grain size of WC usually varies between 1-5 µm [7]. In the milling process, a milling
liquid and milling bodies are added. Milling liquid must be chosen in consideration to which
pressing aid will be used. Milling bodies are added in order to reduce the grain size and
receive a homogenous powder mixture.
2.1.2 Drying/agglomeration
After milling, the powder mixture is agglomerated and dried through spray drying. During the
drying process of the powder, the milling liquid is evaporated and the powder agglomerate is
formed from the particles and pressing aid. The agglomerates are spherical with a diameter of
100-200 µm [7]. This gives the powder the capability of floating, which is important for the
pressing process.
2.1.3 Pressing
The agglomerate is then compromised through pressing into the dimensions of the product,
but with a greater volume than the final product. There are two different pressing methods,
uniaxial, which is the most common, and double sided pressing [7]. The pressing aid is used
not only to create good pressing properties for the powder, but also to reduce the wear of the
pressing tools.
2.1.4 Sintering
The next step in the manufacturing process of cemented carbides is liquid-phase sintering.
During this process, the porosity decreases, resulting in an increase in density and strength of
the product [7]. The driving force for the microstructure evolution during the sintering process
is an increase in grain size. The sintering temperature varies between 1350-1520°C and is the
process in which cavity shrinks and will be filled with the binder [7]. During the liquid-phase
sintering the binder phase will melt and dissolve W and C. Pure Co, most commonly used as a
binder phase, has a melting point of 1495°C, however, the solubility of both W and C
decreases the melting point and the lowest existing melting point of Co is at the ternary
eutectic between graphite, FCC-Co and WC at 1275°C [7]. See figure 1.
6. 5
2.2 Binder Phases
Co is commonly used as a binder phase for cemented carbide due to its properties of high
melting point at 1495°C, the capability to form a liquid phase with WC at 1275°C, and its
high temperature strength [3]. Usually the Co content in cemented carbide varies between 3-
30%, which affects the material properties [4]. When Co wets WC, W and carbon stabilizes in
its cubic form, resulting in Co being stronger as a binder than in its pure form [3]. Co exists in
two different atom structures depending on temperature. At high temperature it has a cubic
structure, FCC, Co Beta. At lower temperature Co takes a hexagonal form, HCP, called Co
epsilon.
Ni and Fe can be used as potential binders [5], [6]. Fe is cheaper than Co, and has the
possibility of martensitic hardening [5]. Having a slightly higher melting temperature than Co,
Fe provides more difficulties during manufacturing through liquid-phase sintering. Another
drawback is that Fe has a possibility to form Fe3C during processing [5], [7]. Ni, on the other
hand, does not form a carbide phase. Ni has a lower melting temperature than Co, making it
possible to sinter at lower temperatures. A combination of Ni and Fe increases the toughness
as well as avoiding the unwanted Fe3C phase[5]. A binder consisting of 75% Fe and 25% Ni
shows maximum strength values [5]. However, a binder containing Co remains as the harder
and tougher alternative of binder phases in cemented carbides [5].
Figure 1: Phase diagram of WC-Co-C, with 10 wt% Co
7. 6
2.3 Rhenium
Re has a high melting point at 3186°C and maintains a high degree of hardness at high
temperature. It has a HCP structure and a density of 21.02 g/cm3
. It has a young’s modulus of
463 MPa and a Vickers hardness of 2450 MPa. It exists as 10-4
ppm in the earth crust [8].
Re is a noble metal and does not form carbides in WC-Co alloys. This is very important since
less noble metals form carbides, which changes the role of the binder.
2.4 Adding Rhenium to Cobalt in the Binder Phase
In the WC-Co-system, the presence of Re increases formation of HCP-structured Co [2]. This
is believed to increase the hardness on the microscopic scale, which is presumed to give an
overall increase in strength and brittleness for the binder phase.
Re also reduces the stacking-fault energy in Co by a factor of three [2]. Lower stacking-fault
energy means wider stacking faults. Stacking faults is an interruption of the regular structure
in a material. For example, in a FCC-metal the sequence of atoms is ABCABC- etc. In case of
a stacking fault, the sequence may become ABABCA- etc, as can be seen in figure 2.
Stacking faults decrease cross slip, which is mobility of dislocations outside the slip planes. In
other words, large stacking faults which is large dislocations stops dislocation movement and
the material will be deformation hardened [9].
2.5 Sustainability
W and WC have low solubility in water. W is considered to be a lithophilic (soil binding)
element. Areas most exposed to emissions of particles are within the proximity of industries,
mines, production plants, etc. Studies show that very high concentration of W can be harmful
to living organisms [10]. Most health risks are however occupational. The toxic effects of W
and WC are still being investigated [11].
Co is widely spread naturally in air, water and ground. It is not considered a risk to health
except if consumed in high doses [12].
Figure 2: Illustration of stacking faults in a FCC structure. To the right regular FCC
structure, and the left showing stacking faults in the FCC structure [24].
8. 7
Although Co is widely spread in nature it only occurs in approximately 10 ppm of the earth
crust. W is a rare metal, estimated to occur in about 1.5 ppm. Re is even rarer with an average
concentration of 0.001 ppm [13].
WC can be recycled by gathering of used material which is then processed in various ways.
An estimation of the average global recycling of used material is about 30 % [14]. W is
considered a conflict mineral because of unethical practices associated with mining in The
Democratic Republic of Congo [15].
9. 8
3. Characterization Techniques
3.1 X-ray Diffraction (XRD)
X-ray diffraction (XRD) is an analysis technique used to investigate and quantify the crystal
structures in materials [16]. The x-rays are reflected by planes of atoms, which make up the
crystal structure, and scattered into specific directions. Specific angles of incidence will
generate an equal angle of reflection from the plane [17]. This is linked to the concept of
constructive interference, which means that the x-rays will have the same phase after the
reflection, even though they travel over a different distance (see figure 2) [18]. These angles
can be calculated using ‘Bragg’s law’, 𝑛𝜆 = 2𝑑 ∗ sin (𝜃), where d=distance between planes,
θ=the incidence angle, λ = wavelength of the x-ray and n is an integer. Bragg’s law describes
the relation between the wavelength of the x-ray and the distance between the planes in the
crystal [17], [18].
In practice, a detector measures the intensity of the x-rays (with known wavelength) that are
reflected and the angle. This information can be used to determine both the type of the crystal
structures and quantity in the material [17].
In figure 3, two beams of equal wavelength and phase are scattered by two different atom
planes. Constructive interference occurs when the distance 2𝑑 ∗ 𝑠𝑖𝑛(𝜃) is equal to an integer
times the wavelength [18].
3.2 Scanning Electron Microscopy (SEM)/ Electron-dispersive X-ray Spectroscopy (EDS)
With microscopy techniques it is possible to get information on the microstructure and
composition of the material.
Scanning Electron Microscopy (SEM) is a technique used to create an image of a specimen by
scanning it with a beam of electrons. The best SEM-microscopes are able to enlarge objects
up to 100 000 times [19]. Compare this to a light optical microscope, which can magnify up to
1000 times. SEM has a greater depth of field than light optical microscope.
There are different ways in which SEM-microscopes can be used to gather information from
the specimen. Two different detectors can be used in a SEM (as seen in figure # 4): a
secondary electron (SE) detector and a backscatter electron (BSE) detector. SE reveals the
Figure 3: Illustrates diffraction of two beams of equal wavelength when constructive
interference occurs. [18].
10. 9
morphology and topography of the sample through inelastic interactions between the beam
electrons and the electrons in the atoms. An inelastic reaction means that there is energy loss.
SE are produced when a beam electron hit the sample and excite an electron in the sample
[20]. The excited electron can then escape from the sample if it has enough energy. If the
detector picks up more SE, the image gets brighter. This method can collect information at a
depth of up to 10 Å [20]. BSE are produced by elastic reactions between the beam electrons
and the nucleus, which means that there is no energy loss. Scattered angles range up to 180°,
but the average angle is around 5° [21]. This reveals knowledge of the composition of the
sample.
Another detector that is used in SEM is the EDS (energy-dispersive X-ray spectroscopy)
detector. With this technique the chemical composition of the material can be detected. When
the electrons hit the surface of the specimen, the impact can excite an electron from one of the
inner shells, allowing another electron in an outer shell to jump inwards. The energy
difference from this reaction is specific for each element. This information can therefore be
used to classify the existing elements in the specimen. [22].
Figure 4: illustration of a set up in a SEM microscope [19]
11. 10
4. Experimental work
In order to determine the properties due to Re additions to the binder in cemented carbides,
two samples were investigated. One sample contains Re and Co in the binder phase (WC-Co-
Re) and the other only Co (WC-Co), as a reference. Both samples had a binder phase content
of 10 wt%. The sample with Re addition contains 22 wt% Re in the binder phase, which is 2.2
wt% of the entire WC-Co-Re sample.
From the binary phase diagram of Re and Co, a suitable sintering temperature was determined
as 1520°C in order for the binding phase to melt.
Each sample should consist of 100 g and a maximum 7 cm3
. For calculations of the samples,
see appendix 1.
The metal powders were weighted with a scale and put into a container together with milling
liquid, milling bodies and compression aid. The calculated amounts of each component are
listed in table 1.
The two containers were then milled for 8h, pressed to receive desired shape and then sintered
for 1 hour at 1520°C.
The microstructure of each sample was investigated with LOM, SEM and EDS mapping.
Hardness was tested with Vickers.
Table 1: Sample composition
Material Type WC-Co-Re WC-Co
wt% wt%
W VM00637 1.097 0.260
WC WC4B003 88.901 89.747
Co CPUUR09 7.795 9.994
Re LOT: G18X001 2.200 0
Compression aid Polyethylene
glycol
2.00 g 2.00 g
Milling bodies PS181 800.00 g 800.00 g
Milling liquid Ethanol 50 ml 50 ml
12. 11
5. Results
5.1 Light Optical Microscopy (LOM)
In figure 5 a light optical photograph can be seen for the WC-Co sample. The darker parts are
WC and are held together by the light binder phase of Co
In figure 6 and 7, the grey parts are WC grains and the lighter parts are the binder consisting
of Re and Co. Parts of concentrated binder, mainly near the edges, can be seen in figure 7.
Figure 6: LOM on WC-Co-Re sampleFigure 5: LOM on WC-Co sample
Figure 7: LOM on edge of the WC-Co-Re Sample
13. 12
5.2 Hardness
The hardness was measured through Vickers hardness test. Ten indents with 2000 g were
made in each sample and the average hardness from these indents was calculated. For
calculations of Vickers hardness, see appendix 2.
The two samples show a difference in hardness. The WC-Co-Re sample has an average
Vickers hardness of 1635 MPa. The WC-Co sample has a Vickers hardness of 1485 MPa.
5.3 Scanning Electron Microscopy (SEM)
A backscatter SEM picture, figure 8 a), shows the WC-Co-Re sample where the light grey
parts are WC grains and the black parts are binder consisting of Re and Co. Figure 8 b) shows
the WC-Co sample. Figure 9 shows another part of the WC-Co-Re sample and this is further
investigated with EDS mapping in 5.4.
Figure 9: SEM BSE picture of WC-Co-Re sample
Figure 8 a): SEM EBS picture of WC-Co-Re sample b): SEM EBS picture of WC-Co sample
14. 13
5.4 Electron-dispersive X-ray Spectroscopy (EDS)
Figure 10 is a 4200x magnification in SEM of the WC-Co-Re sample and is in figure 11
investigated with EDS mapping in order to determine the distribution of W, Re and Co.
Figure 11 further investigates the positions of each element in the sample, where the lighter
parts in the top two and bottom left pictures show where each element is located in the
sample. This shows that Co and Re are located on the same place, indicates that Re together
with Co is the binder. The grey area in the center of figure 9 is a Re-Co alloy consisting
mostly of Re due to the light color that indicates a high density material. It also shows that
there are some WC grains dissolved in the Re-Co alloy.
Figure 10: EDS picture on Re sample
Figure 11: EDS mapping on WC-Co-Re sample
Figure 10: SEM backscatter picture of Re
sample, 4200x magnification
15. 14
5.5 X-ray Diffraction (XRD)
Figure 12 shows XRD results on WC-Co (red) and WC-Co-Re (blue) sample. The intensity
staples show the phases in the sample. Due to technical problems, no conclusions can be
drawn from the results.
Figure 12: XRD on WC-Co sample (red) and WC-Co-Re sample (blue)
16. 15
6. Discussion
The Vickers hardness test showed that the sample with the Re-Co binder was 150 MPa harder
than the sample containing pure Co in the binder. This might indicate that the wanted HCP
structure in Co has increased by addition of Re in the binder or due to deformation hardening
by stacking faults. There could be several reasons for the increased hardness - but further
investigations (studies and techniques) are beyond the scope of this study. This makes it hard
to draw any further conclusions regarding the hardening mechanism of Re in the binder phase.
Investigations of the pictures in EDS mapping showed that Re is dissolved in the Co. The WC
grains are evenly distributed in the binder phase. This indicates that Re worked, as intended,
as a supplement to the binder phase without forming carbides.
The aim of the project was to create a heat resistant cutting tool and determine the mechanical
properties at 800°C. Due to limitations in the extent of this project, no such test was done. On
the other hand, the Re sample was harder at room temperature which is a good indication of a
better starting position for lathing. This does not mean that it has better perseverance during
machining operations.
The WC-Co-Re sample was inhomogeneous. One of the main reason could be that the grain
size of Re was much larger than the size of the WC- and Co-grains. Modifications to receive a
more homogenous microstructure may include smaller Re grains and improved milling
process.
The test results are based on a single specimen where there is one manufacturing process
used. Further studies might alter the manufacturing process such as the sintering temperature,
sintering time, milling and composition in order to receive even better results.
7. Conclusion
• Re addition to the binder increases the Vickers hardness with 150 MPa at room
temperature.
• Re was dissolved in Co but resulted in an inhomogeneous microstructure.
17. 16
8. Acknowledgement
Thanks to Sandvik Coromant for making this project possible. Thanks to Susanne Norgren
and Andreas Blomqvist at Sandvik Coromant for experimental guidance and expertise in the
area of cemented carbides.
Special thanks to Ida Borgh for support, guidance and engagement in this project.
18. 17
9. References
[1] A. F. Lisovskii, “Sintered Metals and Alloys: Cemented Carbides Alloyed with Ruthenium,
Osmium and Rhenium,” Powder Metallurgy and Metal Ceramics, vol. 39, no. 415, pp. 428–
433, 2000.
[2] M. Stender, S. Liu, D. Waldorf, and D. Norgan, “Alternative Binder Carbide Tools for
Machining Superalloys,” in Internation Conference on Manufacturing Science and
Engineering October 7-10, 2008, pp. 1–9.
[3] J. D. Donaldson and D. Beyersmann, “Cobalt and Cobalt Compounds,” Ullman’s Encyclopedia
of Industrial Chemistry. pp. 429–465.
[4] S. Liu, K.-H. Xu, and M. Wang, “Preparation of Co powders for cemented carbides in China,”
International Journal of Refractory Metals and Hard Materials, vol. 24, no. 6, pp. 405–412,
2006.
[5] H. E. Exner, “Physical and Chemical Nature of Cemented Carbides,” International Metals
Reviews, vol. 24, no. 1, pp. 149–173, 1979.
[6] A. Zerr, H. Eschnauer, and E. Kny, “Hard Materials,” Ullman’s Encyclopedia of Industrial
Chemistry. pp. 1–21, 2012.
[7] B. Uhrenius, Pulvermetallurgi. Stockholm: Institutionen för Materialvetenskap, 2000, p. 243.
[8] “Rhenium (Revised),” Chemical Elements: From Carbon to Krypton. [Online]. Available:
http://www.encyclopedia.com/topic/rhenium.aspx.
[9] S. Jonsson, Mechanical Properties of Metals and Dislocation Theory from an Engineer’s
Perspective. Stockholm, Sweden: Department of Material Science and Engineering, 2006.
[10] “Public Health Statement for Tungsten.” [Online]. Available:
http://www.atsdr.cdc.gov/phs/phs.asp?id=804&tid=157.
[11] “Tungsten and Its Environmental Impacts,” 2012. [Online]. Available:
http://news.chinatungsten.com/en/tungsten-information/406-ti-7.
[12] Lenntech, “Cobalt.” [Online]. Available: http://www.lenntech.com/periodic/elements/co.htm.
[13] “Rhenium (Re),” Encyclopaedia Britannica, 2013. [Online]. Available:
http://www.britannica.com/EBchecked/topic/501132/rhenium-Re/.
[14] P. C. Angelo and R. Subramanian, Powder metallurgy: science, technology and applications.
New Delhi: Asoke K. Ghosh, PHI Learning Private Limited, 2008, p. 300.
[15] M. A. McCrae, “Four Leading Conflict Materials,” 2013. [Online]. Available:
http://www.mining.com/infographic-four-leading-conflict-minerals-26308/.
[16] “XRD - X-Ray Diffraction.” [Online]. Available:
http://www.uq.edu.au/nanoworld/index.html?page=160084.
19. 18
[17] “The X-ray Diffraction Small Research Facility: What is XRD?” [Online]. Available:
http://www.sheffield.ac.uk/materials/research/centres/2.4449/whatxrd.
[18] “Bragg’s Law.” [Online]. Available: http://en.wikipedia.org/wiki/Braggs_law.
[19] “Scanning Electron Microscopy (SEM).” [Online]. Available:
http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html.
[20] J. H. Wittke, “Secondary Electrons.” [Online]. Available:
http://www4.nau.edu/microanalysis/microprobe/Interact-SE.html.
[21] “Back scattered electrons.” [Online]. Available:
http://www.emal.engin.umich.edu/courses/sem_lecturecw/sem_bse1.html.
[22] “Energy-dispersive X-ray spectroscopy.” [Online]. Available:
http://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy.
[23] W. D. Callister, Materials Science and Engineering: An Introduction, 7th ed. .
[24] H. Föll, “Partial Dislocations and Stacking Faults.” [Online]. Available: http://www.tf.uni-
kiel.de/matwis/amat/def_en/kap_5/backbone/r5_4_1.html.
20. 19
10. Appendix 1: Calculations of Sample Composition
Table 1: Data of WC-Co-Re sample
wt% atom% Molar mass[23]
C 5.45 42.91 12.01
Co 7.80 12.51 58.90
W 84.55 43.47 183.84
WC 90.00 42.91 195.85
Re 2.20 1.12 186.20
Calculating the wt% of the WC-Co-Re sample
𝑊𝐶:
42.906 ∗ 195.85
42.91 ∗ 195.85 + 1.12 ∗ 186.20 + 12.51 ∗ 58.90 + 0.56 ∗ 183.84
∗ 100 = 88.901wt%
𝑊:
0.564 ∗ 183.84
42.91 ∗ 195.85 + 1.12 ∗ 186.20 + 12.51 ∗ 58.90 + 0.56 ∗ 183.84
∗ 100 = 1.097𝑤𝑡%
𝐶𝑜:
12.51 ∗ 58.9
42.91 ∗ 195.85 + 1.12 ∗ 186.20 + 12.51 ∗ 58.90 + 0.56 ∗ 183.84
∗ 100 = 7.795 𝑤𝑡%
𝑅𝑒:
1.12 ∗ 186.2
42.91 ∗ 195.85 + 1.12 ∗ 186.20 + 12.51 ∗ 58.90 + 0.56 ∗ 183.84
∗ 100 = 2.206𝑤𝑡%
100 g gives a recipe of: 88.9 g WC, 1.097 g W, 7.795 g Co, 2.206 g Re
Controlling the volume does not exceed 7 cm3
:
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+
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+
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+
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= 6.73 𝑐𝑚!
Table 2: Data of WC-Co sample
wt% atom% Molar mass [22]
C 42.13 12.01
Co 10.00 15.60 58.90
W 42.26 183.84
WC 90.00 42.13 195.85
Calculating the wt% of the WC-Co sample
21. 20
𝑊:
0.13 ∗ 183.84
42.13 ∗ 198.85 + 15.60 ∗ 58.90 + 0.13 ∗ 183.84
∗ 100 = 0.260𝑤𝑡%
𝑊𝐶:
42.13 ∗ 195.85
42.13 ∗ 198.85 + 15.6 ∗ 58.9 + 0.13 ∗ 183.84
∗ 100 = 89.747𝑤𝑡%
𝐶𝑜:
58.9 ∗ 15.6
42.13 ∗ 198.85 + 15.6 ∗ 58.9 + 0.13 ∗ 183.84
∗ 100 = 9.944𝑤𝑡%
100 g gives a recipe of: 0.260 g W, 89.747 g WC, 9.944 g Co
Controlling the volume does not exceed 7 cm3
:
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+
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+
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= 6.878 𝑐𝑚!
Table 3: contents of samples, in grams
Material Type WC-Co-Re WC-Co
Calculated Measured Calculated Measured
W VM00637 1.097 1.091 0.260 0.259
WC WC4B003 88.901 88.946 89.747 89.714
Co CPUUR09 7.795 7.788 9.994 9.994
Re LOT:
G18X001
2.200 2.225
Compression
aid
Polyethylene
glycol
2.00 2.00 2.00 2.06
Milling
bodies
PS181 800.00 800.30 800.00 800.02
Milling liquid Ethanol 50 ml 50 ml 50 ml 50 ml
