The document summarizes a study that characterized and compared the properties of sintered WC-Co and WC-Ni-Fe hard metal alloys. Key findings include: 1) The optimal sintering temperatures were determined to be 1350°C for 1 hour for WC-Co alloys and 1400°C for 1 hour for WC-Ni-Fe alloys. 2) Both alloy types underwent effective liquid-phase sintering at these temperatures, resulting in relative densities over 99% and excellent mechanical properties. 3) Corrosion testing found that WC-Ni-Fe alloys sintered at 1400°C had the best corrosion resistance in 0

1. Characterization and properties of sintered WC–Co and WC–Ni–Fe hard
metal alloys
Shih-Hsien Chang ⇑
, Song-Ling Chen
Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608, Taiwan, ROC
a r t i c l e i n f o
Article history:
Received 5 August 2013
Received in revised form 26 September
2013
Accepted 27 September 2013
Available online 8 October 2013
Keywords:
WC–Ni–Fe alloy
WC–Co alloy
Vacuum sintering
Corrosion resistance and KIC
a b s t r a c t
The aim of this study is to explore two different tungsten carbide binders (Co and Ni–Fe) and then impose
various sintering temperature treatments. Experimental results show that the optimal sintering temper-
atures for WC–Co and WC–Ni–Fe hard metal alloys are 1350 °C and 1400 °C for 1 h, respectively. Mean-
while, the WC–Co and WC–Ni–Fe alloys undergo a well liquid-phase sintering and, thus, exhibit excellent
mechanical properties. In addition, the sintered WC–Co and WC–Ni–Fe alloys show that when the rela-
tive density reached 99.76% and 99.68%, the hardness was enhanced to HRA 84.4 ± 0.5 and 85.3 ± 0.5, and
the TRS increased to 2471.2 ± 1.0 and 2524.5 ± 1.0 MPa, respectively. Moreover, the corrosion test results
show that the WC–Ni–Fe alloy sintered at 1400 °C had the lowest corrosion current (Icorr) of 1.11 105
-
A cm2
and the highest polarization resistance (Rp) of 2464.61 X cm2
in 0.15 M HCl solution. Simulta-
neously, the fracture toughness of KIC increased to 15.1 MPa m1/2
. Compared with sintered WC–Co
alloys, the sintered WC–Ni–Fe hard metal alloys possessed much better corrosion resistance and mechan-
ical properties.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
Tungsten carbide composites are widely used in industry for
machining tools, mining tools and wear-resistant parts requiring
superior mechanical properties [1]. Cemented carbide was used
to designate a metal matrix composite constituted by hard cera-
mic particles, normally WC, into a metallic matrix [2]. Conven-
tional tungsten carbide hard metal alloys consist of tungsten
carbide and a ductile binder phase. Although cobalt wets tungsten
carbide well and has good mechanical properties, the corrosion
resistance of conventional tungsten carbide/cobalt hard metal al-
loys is less than satisfactory in certain applications in the chem-
ical and food industries. The substitution of part or all of the
cobalt for nickel or nickel and iron has been investigated in recent
years in an attempt mainly to improve the properties of the bin-
der and at the same time to reduce costs associated with the
short supply and prevailing high market price of cobalt powder
[3–5].
Conventional PM involves mixing the metal powders, compact-
ing of the mixed powders into molds and then sintering of the
compact powders under the different atmospheres [6]. Sintering
is a useful method for manufacturing parts from powders, by
heating the material until its particles adhere to each other.
However, sintering temperature cannot exceed the melting point
of the sintered based materials [7]. In addition, conventional sin-
tered PM-parts usually have more than 5% porosity. Enhanced sin-
tering techniques can be applied to obtain higher densities and
improved porosity in the sintered parts [8,9]. Powder metallurgy
is a good method for fabrication of high melting material with bet-
ter mechanical properties.
As the main binder of conventional cemented carbides, Co is
rare in storage and expensive; thus it has made sense to look for
a substitute. While cobalt has been found to be the widely used
binder metal for most applications, other iron group metals (Ni–
Fe based WC, nickel and iron) are employed for specialized appli-
cations, e.g., where hot hardness and resistance against thermal
cracking or corrosion/oxidation resistance are required [10]. Due
to their high melting point, these hard metal alloys are mostly
manufactured by the power metallurgy method.
In this study, WC–Co and WC–Ni–Fe hard metal alloys were
produced via vacuum sintering using the powder metallurgy tech-
nique. Moreover, the research carried out a series of experimental
tests to explore the characteristics and properties of various sinter-
ing temperatures on hard metal alloys. The effects of the micro-
structural features on mechanical and corrosion resistance were
the main concern. In addition, the feasibility of commercial manu-
facturing of WC–Ni–Fe cement carbides via vacuum sintering was
evaluated.
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.09.188
⇑ Corresponding author. Address: Department of Materials and Mineral
Resources Engineering, National Taipei University of Technology, 1, Sec. 3,
Chung-Hsiao E. Rd., Taipei 10608, Taiwan, ROC. Tel.: +886 2 27712171x2766; fax:
+886 2 27317185.
E-mail address: changsh@ntut.edu.tw (S.-H. Chang).
Journal of Alloys and Compounds 585 (2014) 407–413
Contents lists available at ScienceDirect
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2. 2. Experimental procedure
In the present research, various sintering temperatures were examined in order
to find the optimal parameters of sintered WC–Co (15 wt% Co) and WC–Ni–Fe
(15 wt% Ni–Fe) alloys, as well as to compare the different properties of two binders
(Co and Ni–Fe) in WC materials. In the experiment, the Ni–Fe binder was used
Ni50–Fe50 alloy powders (50 wt% Fe). The particle size of Ni50–Fe50 alloy powders
was about 1–2 lm. Simultaneously, the particle size of Co powders was about 2–
3 lm. Furthermore, the mean particle size of various hard alloys is another concern.
The WC–Co and WC–Ni–Fe powders showed an irregular shape and rough surface.
The mean particle sizes were 66.3 ± 0.5 lm and 1.8 ± 0.5 lm, respectively. During
the forming process, the WC–Co and WC–Ni–Fe alloy powders were put into an al-
loy steel mold (6 6 40 mm) and a vertical force from a hydraulic press was ap-
plied to the mold. The pressure was maintained at 235.3 MPa for 5 min; then, it
underwent a sintering process in which the sintering temperatures were 1250 °C,
1300 °C, 1350 °C and 1400 °C. The vacuum was maintained at 1.33 105
MPa
and the soaking time was 60 min.
To evaluate the microstructural characterization and mechanical properties of
the hard metal alloys via different sintering processes, the porosity, hardness, trans-
verse rupture strength (TRS) tests, fracture toughness KIC, corrosion tests (Potential
Stat Chi 601) and microstructure inspections were performed. Microstructural fea-
tures of the specimens were examined by optical microscopy (Nikon Eclipse Lv150)
and scanning electron microscopy (Hitachi-S4700). Porosity tests followed the
ASTM B311-08 and C830 standards. The hardness of the specimens was measured
by Rockwell indenter (HRA, Indentec 8150LK) with loading of 60 kg, which com-
plied with the CNS 2114 Z8003 standard methods. The Hung Ta universal material
test machine (HT-9501A) with a maximum load of 25 tons was used for the TRS
tests (ASTM B528-05). Meanwhile, Rbm was the transverse rupture strength, which
determined as the fracture stress in the surface zone. F was maximum fracture load,
L was 30 mm, k was chamfer correction factor (normally 1.00–1.02), b and h were
5 mm in the equation Rbm = 3FLk/2bh2
, respectively. The specimen dimensions of
the TRS test were 5 5 40 mm. Moreover, it needs to slightly grind the surface
of the specimen and tests at least three pieces. The toughness of the specimens
can be also express in the term of the crack resistance, which is obtained by the ap-
plied load (294.3N) through the micro-hardness tester (VMT-XT) and calculated the
sum of the dividing crack lengths. The fracture toughness KIC can be calculated by
the following equation [11]:
KIC ¼ 0:15
p
ðHV30=RlÞ
where the HV30 is the hardness (N/mm2
), and Rl is the sum of crack lengths (mm).
In addition, corrosion potential analysis uses three electrodes method: the ref-
erence electrode is a saturated of silver–silver chloride electrode, auxiliary elec-
trode is a platinum electrode, and the working electrode is connected to the test
specimens (ASTM G59-97). The contact area of the specimen was 2.0 cm2
. The cor-
rosive solvent used 0.15 M HCl and was maintained at room temperature. A scan-
ning speed of 0.5 mvs1
, initial potential of 1.5 V, and the final potential of 0.5 V
were controlled [12]. The polarization curve was obtained by Corr-View software
to analyze and compare the corrosion potential (Ecorr) and corrosion current (Icorr)
of different oxidation parameters. Finally, a comparison was conducted for the
polarization resistance (Rp) of sintered WC–Co and WC–Ni–Fe hard metal alloys.
3. Results and discussion
3.1. Effects of sintering temperatures on the microstructure
This study aimed to explore two different binders of tungsten
carbides through various sintering temperatures that were used
to prepare a composite material. Fig. 1 shows the volume shrinkage
ratio of WC–Co and WC–Ni–Fe hard metal alloys after various sin-
tering temperature treatments. The results indicate that the lowest
volume shrinkage ratio (9.0%) appeared in 1250 °C-sintered WC–
Co specimens. It is reasonable to suggest that the WC–Co alloys
sintered at 1250 °C did not achieve full liquid-phase sintering.
Therefore, the 1250 °C-sintered WC–Co alloys represent the small-
est amount of shrinkage. When the temperature was increased to
1300 °C, the volume shrinkage ratio of WC–Co alloys was rapidly
enhanced. The highest volume shrinkage ratio (48.0%) appeared
in 1400 °C sintering for 1 h; this showed that the sintering temper-
ature was close to the liquid-phase sintering temperature [9]. As a
result, the sintering temperature approached the liquid-phase sin-
tering temperature for tungsten carbides, which would enhance
the volume shrinkage ratio and lead to good sintering materials.
In addition, Fig. 1 shows that the trend of the volume shrinkage
ratio for WC–Ni–Fe alloys slowly increased after different sintering
temperature treatments. The highest volume shrinkage ratio
(47.0%) for WC–Ni–Fe alloys appeared in 1400 °C sintering for
1 h. Comparing the sintered WC–Ni–Fe with WC–Co alloys, the
WC–Ni–Fe alloy possesses the small mean grain size; it seems to
easily obtain the optimal sintering driving force. Therefore, the
WC–Ni–Fe alloy can undergo better liquid-phase sintering after a
relatively low sintering temperature. This also agrees with our pre-
vious research [6,9].
Our previous study indicated that when the sintering tempera-
ture increased, the relative density rapidly increased, and the
apparent porosity obviously decreased [7,9]. Fig. 2 shows the rela-
tive density and apparent porosity of WC–Co and WC–Ni–Fe hard
metal alloys after various sintering temperature treatments. The
lowest relative density (60.0%) appeared in 1250 °C-sintered WC–
Co specimens, as shown in Fig. 2a. The relative density was slowly
enhanced to 73.0% as the sintered temperature was increased to
1300 °C. However, when the temperature reached 1350 °C, the rel-
ative density was rapidly enhanced to 99.76%. As the temperature
increased to 1400 °C, the relative density was an insignificant var-
iation, but it should be related to the high-temperature of grain
coarsening and the measurement error. On the other hand, all
the relative densities of sintered WC–Ni–Fe alloys reached more
than 85.0%; the highest value (99.68%) appeared at 1400 °C, sin-
tered for 1 h. Consequently, the relative density of sintered WC–
Co and WC–Ni–Fe alloys can be significantly increased as the sin-
tering temperature is increased. This result also agrees with our
previous findings.
Fig. 2b shows the highest apparent porosity (36.4%) of WC–Co
alloys appearing in 1250 °C sintering for 1 h. The porosity obviously
decreases as the sintering temperature increases. When the sinter-
ing temperature increased to 1350 °C, the apparent porosity of WC–
Co alloys rapidly decreased to about 0.24%. In addition, the appar-
ent porosity of sintered WC–Ni–Fe alloys reached 7.9% after
1250 °C sintering for 1 h. The lowest apparent porosity (0.32%) ap-
peared in 1400 °C sintering for 1 h. Compared with the apparent
porosity of WC–Co and WC–Ni–Fe alloys at the same sintering tem-
perature, the small grain sizes of WC–Ni–Fe powders possess a sig-
nificantly higher driving force. This is one of the main factors that
led to better sintering results. Another chief factor is the melting
point of the Ni50–Fe50 binder (1444 °C) [13] being lower than that
of the Co binder (1495 °C). Therefore, sintered WC–Ni–Fe alloys
possess better microstructure and properties than sintered WC–
Co alloys do under the same sintering conditions.
Fig. 3 shows OM morphology observations of WC–Co alloys
after various sintering temperature treatments. Fig. 3a shows that
Fig. 1. Comparison of the volume shrinkage ratio of WC–Co and WC–Ni–Fe hard
metal alloys after various sintering temperature treatments.
408 S.-H. Chang, S.-L. Chen / Journal of Alloys and Compounds 585 (2014) 407–413

3. the lower sintering temperature (1250 °C) lacks sufficient driving
force; thus, a large number of irregular connected pores can be ob-
served. The relative density only reaches 60.0%. Fig. 3b shows that
the relative density is about 73.0% after being sintered at 1300 °C.
It is assumed that the amount of liquid is insufficient at this sinter-
ing temperature; therefore, it cannot effectively fill the pores be-
tween the tungsten carbide particles. As a result, many internal
pores still remain in the 1250 °C- and 1300 °C-sintered WC–Co al-
loys. However, when the sintering temperature was increased to
1350 °C and 1400 °C, the well liquid-phase sintering resulted in a
densification mechanism, and almost no connected pores existed
in the sintered WC–Co alloys, as shown in Fig. 3c and d. The rela-
tive density reached about 99.68% and 97.10%, respectively. In
addition, the white region (indicated by the arrow) was a Co bin-
der, which was evenly distributed between the gray regions of
the WC grains, as shown in Fig. 3c.
Fig. 4 shows the OM morphology observations of WC–Ni–Fe al-
loys after various sintering temperature treatments. It clearly indi-
cates that the pores relatively decreased as the sintering
temperatures increased, as shown in Fig. 4a–c. Fig. 4a shows that
the relative density of sintered WC–Ni–Fe alloys reaches 83.0% at
the lower sintering temperature (1250 °C). Significantly, there is
a positive relationship between the relative density and sintering
temperature. As a result, the relative density increases and internal
pores obviously decrease as the sintering temperature of WC–Ni–
Fe alloys increases. Compared with Fig. 2, the results also confirm
that the fine grain sizes of WC–Ni–Fe powders could acquire a bet-
ter liquid-phase sintering under lower sintering temperatures.
Fig. 5 shows the BEI morphology observations of WC–Co alloys
after various sintering temperature treatments. There is no signif-
icant amount of precipitated graphite or the brittle g-phases
[14,15] in the microstructure; meanwhile, some refined grains of
WC (as shown by arrows) show scattered distribution within the
specimens, as shown in Fig. 5a. This is representative of the fact
that the sintering mechanism is incomplete. After increasing the
sintering temperature, the refined grains of WC gradually disap-
peared, as shown in Fig. 5b. When the temperature was increased
to 1350 °C, the refined grains of WC obviously disappeared and re-
sulted in a uniform structure, as shown in Fig. 5c. Fig. 5d shows
Fig. 2. Comparison of the properties of WC–Co and WC–Ni–Fe hard metal alloys
after various sintering temperature treatments: (a) relative density, and (b)
apparent porosity.
Fig. 3. OM morphology observations of WC–Co specimens by various sintering temperature treatments: (a) 1250 °C, (b) 1300 °C, (c) 1350 °C, and (d) 1400 °C.
S.-H. Chang, S.-L. Chen / Journal of Alloys and Compounds 585 (2014) 407–413 409

4. that the grain size of WC exhibited a slight coarsening phenome-
non (as shown by arrows) as the sintering temperature increased
to 1400 °C. As the sintering temperature increased, the driving
force of the sintering mechanism was enough to offer the refined
WC grains dissolved and solid-solution to the matrix. However,
increasing the sintering temperature of WC–Co alloys will also lead
to a high-temperature grain growth phenomenon, as shown in
Fig. 5d. Fig. 6 shows the BEI morphology observations of WC–Ni–
Fe alloys after various sintering temperature treatments.
Obviously, the distribution and sizes of Ni–Fe binders are not uni-
form at lower sintering temperatures, as shown in Fig. 6a and b.
Conversely, the uniform distribution and sizes of Ni–Fe binders
appeared in the 1350 °C- and 1400 °C-sintered WC–Ni–Fe alloys,
as shown in Fig. 6c and d. Moreover, the coarsening phenomenon
evidently disappeared in the 1400 °C-sintered WC–Ni–Fe alloys,
as shown in Fig. 6d.
3.2. Effects of sintering temperatures on various properties
Fig. 7 shows the mechanical properties test for WC–Co and WC–
Ni–Fe hard metal alloys after various sintering temperature treat-
ments. Fig. 7a shows that the hardness of sintered WC–Co and
WC–Ni–Fe alloys rapidly increased as the sintering temperature
increased. This can be ascribed to the decreased porosities, which
result in the increasing densification and strengthening
mechanism. Due to the large number of internal pores existing in
Fig. 4. OM morphology observations of WC–Ni–Fe specimens by various sintering temperature treatments: (a) 1250 °C, (b) 1300 °C, (c) 1350 °C, and (d) 1400 °C.
Fig. 5. BEI morphology observations of WC–Co specimens by various sintering temperature treatments: (a) 1250 °C, (b) 1300 °C, (c) 1350 °C, and (d) 1400 °C.
410 S.-H. Chang, S.-L. Chen / Journal of Alloys and Compounds 585 (2014) 407–413

5. the 1250 °C-sintered WC–Co specimens, the hardness is only about
HRA 48.1 ± 0.5. Besides, the greatest hardness of WC–Co specimens
reached HRA 84.4 ± 0.5, which appeared in 1350 °C sintering for
1 h. However, the greatest hardness of WC–Ni–Fe specimens was
HRA 85.3 ± 0.5, which appeared with 1400 °C sintering for 1 h.
The results can be further compared with Fig. 2. The hardness
trend is consistent with the porosity. It is reasonable to suggest
that the porosity of sintered specimens plays an important role
in affecting the hardness.
The TRS test results of sintered WC–Co and WC–Ni–Fe alloys is
shown in Fig. 7b. Owing to the lower relative density and higher
apparent porosity specimen, WC–Co alloys sintered at 1250 °C pos-
sess the lowest TRS value (657.4 ± 1.0 MPa). Conversely, the TRS of
WC–Ni–Fe alloys sintered at 1250 °C reached 1696.6 ± 1.0 MPa be-
cause of the better liquid-phase sintering results. When the sinter-
ing temperature was increased to 1350 °C and 1400 °C, the relative
density of sintered WC–Co and WC–Ni–Fe alloys was over 99.68%.
Generally, internal pores in the matrix easily generated the stress
concentration phenomenon, which results in a lower strength.
However, WC–Co and WC–Ni–Fe alloys sintered at 1350 °C and
1400 °C reached almost full densification, which is effective in
reducing the strain points along the rupture mechanism and
increasing the TRS value. In addition, according to the above men-
tioned results, the smaller grain sizes and lower melting points of
Ni50–Fe50 binders are the major factors that lead the sintered
WC–Ni–Fe alloys to become fully dense under the lower tempera-
ture (Fig. 2). On the other hand, the results can be further com-
pared with Figs. 3c and 4d. Almost no internal pores remained
after 1350 °C-sintered WC–Co and 1400 °C-sintered WC–Ni–Fe al-
loys. Therefore, the TRS achieved the highest values of
2471.2 ± 1.0 MPa and 2524.5 ± 1.0 MPa, respectively. Besides, we
further measured the grain size of 1350 °C-sintered WC–Co and
1400 °C-sintered WC–Ni–Fe specimens by the linear intercept
method (ASTM E112-12). The mean grain sizes of them were
1.4 ± 0.1 lm and 1.0 ± 0.1 lm, respectively. It shows that both of
the grain sizes are no significant difference. Therefore, the TRS re-
sults need to be further examined.
Fig. 6. BEI morphology observations of WC–Ni–Fe specimens by various sintering temperature treatments: (a) 1250 °C, (b) 1300 °C, (c) 1350 °C, and (d) 1400 °C.
Fig. 7. Comparison of the mechanical tests of WC–Co and WC–Ni–Fe hard metal
alloys after various sintering temperature treatments: (a) hardness test, and (b) TRS
test.
S.-H. Chang, S.-L. Chen / Journal of Alloys and Compounds 585 (2014) 407–413 411

6. Fig. 8 shows the TRS fracture surface of WC–Co and WC–Ni–Fe
hard metal alloys after 1350 °C and 1400 °C sintering treatments,
respectively. An increase in the sintering temperatures is useful
for the densification of the sintering mechanism, while high en-
ough to produce large amounts of the liquid-phase to fill the pores
during the sintering process [15,16]. Fig. 8a and b shows good sin-
tering performance; the WC grains are clearly coated with Co bind-
ers. Meanwhile, it can be observed that the Co binders generated
the extensive phenomenon. It is reasonable to suggest that the
fracture toughness of sintered WC–Co alloys is mainly provided
by the Co binders. In addition, according to the above mentioned
results, the smaller grain sizes and lower melting points of Ni, Fe
elements are the major factors that lead the sintered WC–Ni–Fe
alloys to become fully dense under the lower temperature.
Fig. 8c and d show the TRS fracture surface of WC–Ni–Fe alloys
after 1350 °C- and 1400 °C-sintering treatments. Similarly, the
fracture toughness of sintered WC–Ni–Fe alloys is mainly provided
by the Ni–Fe binders. The highest TRS value (2524.5 ± 1.0 MPa)
appeared in WC–Ni–Fe alloys sintered at 1400 °C for 1 h. The
higher bonding can be ascribed to Ni–Fe binders; it results in
optimal fracture toughness.
The fracture toughness KIC is based on fracture mechanics as a
theoretical basis, which mainly measures the material’s ability to
resist cracks’ continuous growth. In the case of a square fracture,
it is usually assumed that the layer of associated plastically de-
formed material extends for a constant distance from the fracture
surface, independent of the plate’s thickness. Also, with the
assumption that the density of plastic deformation energy depends
only upon the distance from the fracture surface, the plastic work
per unit thickness per unit crack extension KIC will be independent
of thickness for a completely square fracture. Fig. 9 shows the OM
images of 1350 °C-sintered WC–Co and 1400 °C-sintered WC–Ni–
Fe hard metal alloys after KIC tests. Significantly, the total crack
length for WC–Co alloys is greater than that for sintered WC–Ni–
Fe alloys.
Comparisons of the KIC between WC–Co and WC–Ni–Fe hard
metal alloys after 1350 °C and 1400 °C sintering for 1 h are shown
in Table 1; the maximum crack length was 0.35 mm which
appeared in 1400 °C-sintered WC–Co alloys, while the KIC value
was the minimum (9.5 MPa m1/2
). Besides, the higher KIC
(10.2 MPa m1/2
) of sintering WC–Co alloys appeared in 1350 °C sin-
tering. However, the minimum crack length was 0.15 mm, which
appeared in 1400 °C-sintered WC–Ni–Fe alloys, while the KIC value
was the maximum (15.1 MPa m1/2
). Thus, the results show that the
WC–Ni–Fe alloys sintered at 1400 °C possess the optimal fracture
toughness and resistance to crack extension.
Fig. 8. SEM images of fracture surface of the various temperature sintering WC–Co and WC–Ni–Fe hard metal alloys after TRS tests: (a) 1350 °C sintered WC–Co, (b) 1400 °C
sintered WC–Co, (c) 1350 °C sintered WC–Ni–Fe, and (d) 1400 °C sintered WC–Ni–Fe alloys.
Fig. 9. OM images of the various temperature sintering WC–Co and WC–Ni–Fe hard metal alloys after KIC tests: (a) 1350 °C sintered WC–Co, and (b) 1400 °C sintered WC–Ni–
Fe alloys.
412 S.-H. Chang, S.-L. Chen / Journal of Alloys and Compounds 585 (2014) 407–413

7. Fig. 10 shows the Tafel slope results of 1350 °C-sintered WC–Co
and 1400 °C-sintered WC–Ni–Fe specimens after the corrosion
tests using 0.15 M HCl. The corrosion current for WC–Co specimens
is obviously higher than for WC–Ni–Fe specimens. Normally, in the
electrochemical reaction, the current value represents the diversi-
fication of equilibrium constants in the oxidation reaction. If the
current value is higher, it will lead to an increase in the equilibrium
constant, and the oxidation rate will be fast. Therefore, the oxida-
tion rate of sintered WC–Co is faster than that of sintered WC–Ni–
Fe alloys. As shown in Table 2, sintered WC–Ni–Fe alloys possess
the minimum corrosion current Icorr (1.11 105
A cm2
) and
highest polarization resistance Rp (2464.61 X cm2
). Significantly,
1400 °C-sintered WC–Ni–Fe alloys had the best anti-corrosion abil-
ity and optimal mechanical properties. According to the results and
discussions above, sintered WC–Ni–Fe alloys obviously possessed
much better properties.
4. Conclusions
In this study, 1350 °C-sintered WC–Co and 1400 °C-sintered
WC–Ni–Fe hard metal alloys were effective in improving the
microstructural and mechanical properties after vacuum sintering.
The relative density of WC–Co alloys reached 99.76% and the
apparent porosity decreased to 0.24% after 1 h of 1350 °C-sintering
treatment. In addition, the relative density of WC–Ni–Fe alloys
reached 99.68%, and the apparent porosity decreased to 0.32% after
1 h of 1400 °C-sintering treatment. The fracture toughness of the
sintered WC–Co and WC–Ni–Fe alloys is mainly provided by the
Co and Ni–Fe binders, respectively. Moreover, the 1400 °C-sintered
WC–Ni–Fe alloys possessed the optimal mechanical properties,
including the greatest hardness (HRA 85.3 ± 0.5) and TRS
(2524.5 ± 1.0 MPa), and excellent fracture toughness of KIC
(15.1 MPa m1/2
).
In addition, 1400 °C-sintered WC–Ni–Fe alloys possessed the
minimum corrosion current Icorr (1.11 105
A cm2
) and highest
polarization resistance Rp (2464.61 X cm2
) in the 0.15 M HCl solu-
tion. Conversely, 1350 °C-sintered WC–Co alloys had the highest
corrosion current Icorr (8.53 105
A cm2
) and a low polarization
resistance Rp (654.04 X cm2
). As a result, the 1400 °C-sintered WC–
Ni–Fe alloys had optimal corrosion resistance.
Acknowledgments
This research is supported by the ASSAB STEELS TAIWAN CO.,
LTD. and the National Science Council of Taiwan under grant
#NSC-101-2622-E-027-023-CC3.
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Table 1
Comparison of the fracture toughness KIC of WC–Co and WC–Ni–Fe hard metal alloys
by 1350 °C and 1400 °C sintered for 1 h.
Specimens Sintering temp (°C) HV30 Rl (mm) KIC (MPa m1/2
)
WC–Co 1350 1417 0.31 10.2
1400 1416 0.35 9.5
WC–Ni–Fe 1350 1463 0.22 12.3
1400 1488 0.15 15.1
WC-Co
WC-Ni-Fe
WC-Co alloys
WC-Ni-Fe alloys
Fig. 10. Tafel results of 1350 °C sintered WC–Co and 1400 °C sintered WC–Ni–Fe
hard metal alloys after the 0.15 M HCl solution of corrosion tests.
Table 2
Corrosion of the corrosion results of the 1350 °C sintered WC–Co and 1400 °C sintered
WC–Ni–Fe specimens in the 0.15 M HCl solution.
Specimens Icorr (105
A cm2
) Ecorr (Volts) Rp (X cm2
)
WC–Co 8.53 0.24 654.04
WC–Ni–Fe 1.11 0.23 2464.61
S.-H. Chang, S.-L. Chen / Journal of Alloys and Compounds 585 (2014) 407–413 413