Controlling Mechanical Properties of Ultra-fineWC-10%Co Cemented Carbides
Moldsfor Precision

精密金型用超微粒 WC-10%Co 超硬合金の機械特性制御
Kenji FUJIMOTO

藤本賢二

Key words ; Hardness, transverse rupture strength, fracture toughness, inhibitor, grain growth
キーワード； 硬さ ,抗折力 , 破壊靱性 , 抑制剤 , 粒成長

Abstract
Mechanical properties of ultra-fine cemented carbides for precision molds are affected by VC andCr3C2 which are added to as grain-growth inhibitors. Therefore, the addition effects of VC and Cr3C2 on
ultra-fine WC-10%Co cemented carbides were investigated. It was confirmed that on the basis ofhardness, the grain growth-inhibiting effect score of Cr3C2 is 0.38 with a score of 1 representing VC. We
also confirmed the influence of Cr3C2 on the transverse rupture strength and the fracture toughness issmaller than that of VC.
精密金型用超微粒超硬合金の機械的性質は粒成長抑制剤として添加される VC と Cr3C2 に大きく影響を受ける。そこで WC-10%Co 超微粒超硬合金における VC と Cr3C2 の添加の影響を調査した。硬さを基準に VC の粒成長抑制効果を 1 とした場合、C3C2 の粒成長抑制効果は 0.38 であることが確認された。また、抗折力や破壊靱性値に及ぼす Cr3C2 の影響は VC より小さいことを確認した。

1.Introduction
With respect to WC of WC-Co cemented carbide, the smaller raw powder of WC is, the more the grains
tend to grow during the sintering. Therefore, to get less than 1 μ m of grain size, some kind of carbides
are usually added as a grain-growth inhibitor. VC and Cr3C2 are widely known as the most effective
inhibitors for WC-Co cemented carbide and many studies on them had been performed , but the1)2)

influence of them is still not clear. Therefore, we investigated the influence of VC and Cr3C2 on themechanical properties of ultrafine WC-10%Co cemented carbide in this paper.

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2.Experimental
Commercial cobalt, tungsten carbide, vanadium carbide and chromium carbide powders were used as
raw powders. Details are shown in Table 1.
Table 1 Properties of raw powders
powdersize[μm]

Total Carbon [%]

WC(a)

0.44

6.09

WC(b)

0.70

6.17

Co

1.5-VC

0.8

17.52

Cr3C2

1.6

13.39

VC and Cr3C2 in the range 0.05-0.60% were added to WC-10% Co (mass %). The mixing was done for96hrs with methanol solvent in a ball mill. The slurry after mixing was dried and green compacts (5×10×30 mm) were prepared under a pressure of 98 MPa. These were finally sintered for 1.5hrs at 1673K invacuum atmosphere and HIP treatment was also applied for 0.7hr at 1613K under a pressure of 40MPa
N2.
Table 2WC[%]
WC(a)

WC(b)

Composition of the specimens

CoVC

Cr3C2

sumC(th)

SG(th)

[%]

[%]

[%]

[%]

[%]

[-]

0.05

5.55

14.49

0.25

5.56

14.45

0.50

5.58

14.41

0.055.57

14.43

5.59

14.40

0.50

5.60

14.35

0.05

5.60

14.38

0.25

5.61

14.35

0.50

5.63

14.30

0.20

Bal.

17.90

10.0

0.40

0.25

0.60

100.0

Sintered specimens were evaluated by specific gravity (SG), relative density (RD), hardness by HRA andHV30, transverse rupture strength (TRS) and fracture toughness by K1C.

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3.Results and Discussion
3.1 Microstructure
Each of microstructures is shown in Fig.1. WC grain size can be measured by these pictures and becomes
smaller as the additive amount of VC and Cr3C2 increases, indicating that VC and Cr3C2 inhibit the grain
growth of WC in WC-Co cemented carbide as reported. In addition, these photographs also show that theaddition effect of VC is larger than that of Cr3C2.

(1) VC: 0.2%, Cr3C2: 0.05%(2) VC: 0.2%, Cr3C2: 0.25%(3) VC: 0.2%, Cr3C2: 0.50%(4) VC: 0.4%, Cr3C2: 0.05%(5) VC: 0.4%, Cr3C2: 0.25%(6) VC: 0.4%, Cr3C2: 0.50%(7) VC: 0.6%, Cr3C2: 0.05%(8) VC: 0.6%, Cr3C2: 0.25%(9) VC: 0.6%, Cr3C2: 0.50%

Fig.1 Microstructures of WC-10% Co cemented carbide
with various additive amounts of VC and Cr3C2
3.2 Effect on hardness
Although it is well known that the effect of VC as a grain-growth inhibitor is larger than that of Cr3C2,
we tried to compare them in detail here in order to grasp the difference quantitatively. We usually
measure a grain size of WC with its microstructure, but the grain boundary is not clear due to its ultrafine
size and the resolution limit of a scanning electric microscope. In this study, therefore, we decided not to
use the grain size for evaluation but tried to evaluate the grain-growth inhibiting effect by hardness of thecemented carbide which is strongly correlated with the grain size.

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Figs. 2 and 3 show addition effects of VC and Cr3C2 on Rockwell hardness of the cemented carbide,respectively. In both cases, the hardness increases as the additive amount increases, and the effect of VCis larger.

Fig. 2 Effect of VC on Hardness

Fig. 3 Effect of Cr3C2 on Hardness

In order to estimate the addition effects of VC and Cr3C2 quantitatively, we defined a value of Z asfollows at first:
Z=x×(additive amount of VC) + y × (additive amount of Cr3C2), x and y are arbitrary units.
Then, we prepared a graph representing hardness on the vertical axis and Z on the horizontal axis when
x=1 (i.e., standardized with the additive amount of VC) and performed a regression analysis changing thevalue of y. Finally, we estimated the effect score of Cr3C2 with a score of 1 representing the grain growthinhibiting effect of VC by calculating a value of y where the squared value of R is the highest.
Fig.4 shows a relation between the grain-growth inhibiting effect of Cr3C2 (y) and the squared value ofR from the regression analysis of Z (x=1, y) and hardness. The value of y in HRA hardness is 0.37 andthat in HV30 hardness is 0.39 where the squared value of R is the highest. Therefore, the grain-growth
inhibiting effect of Cr3C2 is considered to be the average, 0.38.
Fig.5 shows a relation between Z (=VC (%) +0.38Cr3C2 (%)) and HRA hardness when y=0.38.

Fig. 5 Relation between Z (=VC (%)+0.38Cr3C2 (%)) and HRA hardness wheny=0.38Fig. 4 Effect of VC on Hardness

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3.3 Effects on Transverse Rupture Strength (TRS)
We converted the addition effects of VC and Cr3C2 on TRS to the grain growth-inhibiting effect score(Z= VC (%) +0.38Cr3C2 (%)) using the results in Section 3.2 and prepared a graph.
The TRS slightly decreases and the deviation (the error bar shows a standard deviation, n=5) becomes
smaller as Z increases. The reason why the deviation is larger when Z is 0.5 or less is probably because
the grain growth-inhibiting effect is weak and the microstructure of cemented carbide is inhomogeneous.

Fig. 6 Relation of Z (VC+0.38Cr3C2) and TRS (Average)

3.4 Fracture Toughness
Fig.7 is a graph showing the addition effects of VC and Cr3C2 on the fracture toughness (Indentation
fracture method with a load of 30kgf, calculated from Evans’s equation) which are converted to the grain
growth-inhibiting effect scores. In general, fracture toughness is inversely proportional to hardness, but inthis graph showing the hardness as the grain growth-inhibiting effect, the data of 0.20% VC and is greatly
different from 0.40%, suggesting that the addition effect of VC is greater than the grain-growth inhibitingeffect.
To separate the effects of VC and Cr3C2, we prepared graphs representing addition effects of VC andCr3C2 on the horizontal axis, respectively, as shown in Fig. 8 and 9. In Fig. 8, the fracture toughness
decreases as the additive amount of VC increases. On the other hand, the additive amount of Cr3C2
doesn’t affect the fracture toughness in Fig.9, thus Cr3C2 is deemed to have little influence on thefracture toughness.

Fig.7 Relation of Z (VC+0.38Cr3C2) and Fracture Toughness (K1C)– 63 –

Nippon Tungsten Review Vol.40 (2016)

Fig.8 Relation of VC and Fracture
Toughness (K1C)

Fig.9 Relation of Cr3C2 and FractureToughness (K1C)

4.Conclusions
Addition effects of VC and Cr3C2 on mechanical properties of ultrafine WC-10% Co cemented carbide
were investigated.
It was confirmed that on the basis of hardness, the grain growth-inhibiting effect score of Cr3C2 is 0.38
with a score of 1 representing VC. We also confirmed the influence of Cr3C2 on the transverse rupture
strength and the fracture toughness is smaller than that of VC.

5.References[1] Hayashi, Suzuki, et al.: Powder and Powder Metallurgy, 19 (1972), 67-71[2] Hayashi, Suzuki, et al.: Powder and Powder Metallurgy, 19 (1972), 106-112

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