Micro-grained cemented carbide with high strength

Micro-grained cemented carbide with high strength

Refractory Metals and Hard Materials 10 (1991) 57-60 Micro-grained Cemented Carbide with High Strength T a m o t s u F u k a t s u a, Keiichi K o b o...

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Refractory Metals and Hard Materials 10 (1991) 57-60

Micro-grained Cemented Carbide with High Strength T a m o t s u F u k a t s u a, Keiichi K o b o r i b & M i t s u o U e k i a ~'Toshiba Tungaloy Co., Ltd., 1 7 Tsukagoshi, Saiwai-ku, Kawasaki 210, Japan ~'Toshiba Tungaloy Co., Ltd., Iwaki Works, I l 1 Yoshima Industrial Estate, Iwaki-City, 970-11, Japan

Abstract: The present work is concerned with further improvement of the strength

of u l t r a ~ n e grained cemented carbide. It has been reported already that the strength of the alloy is strongly affected by its structural defects, e.g. pores, Co pools, extraordinary coarse carbide grains or impurity compounds. In this work, the effects of carbide grain growth inhibitors, Co contents and impurities of starting powders on the strength of the alloy have been investigated in relation to the structural defects appearing on the fracture surface of transverse-ruptured specimens. The results obtained showed that the amount of impurities of starting powders most closely affected the strength of the alloy, and an extremely high-strength cemented carbide having transverse-rupture strength of over 500 kgf/mm 2 was obtained by using highly purified WC powder.

INTRODUCTION

used as starting materials. Mixed powders were wet-ball-milled, dried, pressed, sintered in a vacuum at 1653 K for 3-6 ks, and finally HIP-ed at 1623 K for 3.6ks (Ar gas, 150MPa). The different WC-VC-Cr3C2-15mass%Co medium carbon alloys with different contents of VC and Cr~C 2 were prepared and used as specimens. The size of the specimens was 4 x 8 x 25 m m 3 according to JIS. At first, transverse-rupture strength tests and analyses of fractured surfaces were carried out for the specimens to determine the suitable amounts of additional VC and Cr3C., The results showed that the structural defects caused by the impurities developed even in the alloy with suitable amounts of additions. Then, an experiment on the specimens prepared by using high purity starting WC (named WC-P, 0.2/tm) was carried out. In this case, the carbon contents of specimens were varied in the two-phase region. The sorts and amounts of impurities in the two sorts of starting WC powders are shown in Table 1. As a result, a micro-grained alloy of an extremely high strength was obtained. Then, a fatigue test of ¢0"3 mm wire and a drilling test with a micro-drill

Recently, the development of fine materials has come to be of great importance in materials research. It is natural that extremely precise control of structure and composition is needed in order to obtain fine materials of excellent properties. In the field of cemented carbides, different new applications such as drills with a small cutting-portion diameter for printed circuit boards (PCBs), pins with a small diameter for dot-printers, slitters for magnetic tapes, and dies for shaping compact disks, for which fine materials are very necessary, are rapidly expanding in Japan. Therefore, the present study is concerned with WC-VC-Cr3C. ~15mass%Co micro-grained alloy of high strength and high quality, paying attention to the influences of structural defects and impurities on the strength of the alloy. EXPERIMENTAL PROCEDURES

The powders of WC, VC, Cr:~C 2 and Co, with grain sizes of 0"2, 3-6, 3"7 and 1-3 ~m, respectively, were 57

RtJ'ractory Metals and Hard Mater&& 0263 4368/91 $3.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain.

Tamotsu Fukatsu, Keiichi Kobori, Mitsuo Ueki

58

Table 1. The sorts and the amounts of impurities in starting WC powders

Amount of impurities (mass ppm) Specimens

Ca

S

AI

Si

Mg

Usual WC WC-P

15 < 5

5 1

10 < 5

90 10

10 < 5

(cutting-portion diameter, 0"3 mm), produced by using the above alloy, were performed. EXPERIMENTAL

RESULTS

The average transverse-rupture strength a(~--mm)of WC-VC-Cr3C2-15mass%Co medium carbon alloy affected by the addition of vanadium carbide (named VC) and Cr3C 2 was first studied, with results as shown in Fig. 1. It is clear that the alloy system containing 2-3mass%VC and 3-4mass%CraC 2 in binder (0.30-0.45mass%VC and 0-454).60mass%CraC 2 in alloy) shows an excellent a-~, as high as about 4.5 GPa. Figure 2 shows an i

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VC in b i n d e r / mass% Fig. 1. The average transverse-rupture strength a(~) of WC-VC-CraC2-15mass%Co medium carbon alloy in relation to the additional amounts of VC and Cr3C 2 in binder.

Fig. 2. An example of SEM microstructure of WCq).45mass%VC_0.60mass% Cr 3C2-15mass%Co medium carbon alloy.

example of the microstructure. It is obvious that the carbide structure is very uniform and very fine (about 0"5/tm), having no crystallized phases such as Cr3C 2 and (W,V)C. The results of analyses on fractured surfaces for the above alloys showed that the coarse WC grains of about 12/~m at maximum and certain defects of about 30/Lm appeared as fracture sources. On the other hand, WC grains > 12/~m acted as the fracture sources, when the additional amounts of VC and CraC 2 were less than the values mentioned above. The shape of certain defects and the results of EPMA analyses on this defect are shown in Fig. 3. Such elements as Ca, S, A1, Si and Mg were detected from the defect, indicating that the defect which develops is directly related to the impurities contained in the starting powder. It was suggested that the strength level of the alloy should further increase by eliminating the defect because the strength of cemented carbides is known to be controlled by structural defects such as micro-pores, coarse WC grains, etc. 1 a The specimens prepared using high purity WC-P powder were examined. The results of measuring a m and defect size on WC-0.45mass%VC0.60mass%Cr3Cz-15mass%Co medium and low carbon alloys are illustrated in Fig. 4. It is clearly known that the strength level of the alloy sharply increases and the strength of medium and low carbon alloys reaches 4.7 and 5.0 GPa, respectively. In this case, all the defects appearing in the specimens were coarse WC grains, and the defect caused by impurities disappeared. The maximum WC grains as fracture sources were 12 and 6 ltm in size for medium and low carbon alloys, respectively. In the next stage of the experiment, a rotating fatigue test of ~b0-3 mm wire produced by using the high strength, low carbon alloy as above was made. The results are shown in Fig. 5. In the figure, the result of ~b0"3mm wire produced by using the usual WC-15mass%Co low carbon alloy with a grain size of about 2/tm is also shown. The fatigue strength of high strength micro-grained alloy is extraordinarily excellent. Figure 6 shows the result of the drilling test of a micro-drill made of high strength micrograined alloy. The breaking resistance is in this case also excellent. The wear resistance of the drill was naturally surpassed, reflecting the increased hardness of the micro-grained alloy. Toshiba Tungaloy Co., Yokohama, has been supplying different products made of high strength micro-grained alloy. Table 2 gives statistics produced by the company, for 1987.

Micro-grained cemented carbide with high strength i

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Fig. 6, The result of drilling test on micro-drills made of high strength micro-grained alloy. A and B are referred to in Fig. 5. Micro-drill:
(d) Fig. 3. An example of (a) the certain defect, and the results of EPMA analyses on the defect: (b) Ca; (c) S; (d) A1.

Micro-drill Pin for dot-printer Others

First

Latter

half 1987

hal~" 1987

Total

3 318 1 507 204

3 548 1235 224

6 866 2 742 428

60

Tamotsu Fukatsu, Keiichi Kobori, Mitsuo Ueki

CONSIDERATION The strength of the micro-grained alloy will first be considered. As is widely known, the strength of cemented carbides is commonly determined by the amount of binder, the carbon content and the carbide grain size of the alloy, and furthermore, by the size and distribution of structural defects. In the author's experiment on WC-15mass%Co low carbon alloy, it was shown that the intrinsic strength of the alloy reached a maximum at a grain size of about 0"7/tin, and it decreased with decreasing grain size. In this connection, the micro-grained low carbon alloy now in question had a grain size of about 0-5/tm, and did not contain any coarser carbides than 6/~m. These coarser carbides are in good contrast to those of 20/zm or more in the usual alloy having a grain size of 0"7-2"0/tm. Then, the increased strength as high as 5-0 GPa in the micro-grained alloy results from the fact that the coarse WC grains acting as the fracture source are so small that these small WC grains compensate the intrinsic strength less than the usual alloy. In this study, the influence of impurities in WC powders on the strength of the alloy was also examined with the following results. Impurities such as Ca, S and A1 coagulated owing to a certain mechanism, probably during ball-milling or drying of the mixed powder, and the defect caused by the coagulated impurities took place after sintering. The influence of impurities of starting powder,

therefore, is also of great importance for getting high strength micro-grained alloy. There is no doubt that the fatigue strength and drilling performance of the products made of high strength micro-grained low carbon alloy were excellent enough. CONCLUSION The strength of WC-VC-CraC2-15mass%Co micro-grained alloy was studied in relation to the additional amounts of VC and Cr3C 2, and also to the impurities in starting WC powders. As the result, it was found that the micro-grained WC-0-45mass%VC-0.60mass%CraC~-15mass%Co low carbon alloy showed an excellent transverserupture strength as high as about 5"0 GPa. It was also found that the products made of micro-grained alloy as above (for instance, the micro-drill having cutting-portion diameter of 0.3 mm) exhibited high resistance to breaking and wear, as expected, proving that the alloy is of great use in its application. REFERENCES 1. Suzuki, H. & Hayashi, K., Trans. JIM, 16 (1975) 353. 2. Suzuki, H., Tanase, T. & Hayashi, K., Planseeber. Pulvermet., 23 (1975) 121. 3. Suzuki, H. & Tanase, T., Planseeber. Pulvermet., 25 (1977) 13.