Research and development of rare-earth cemented carbides

Research and development of rare-earth cemented carbides

International Journal of Refractory Metals & Hard Materials 19 (2001) 159±168 www.elsevier.com/locate/ijrmhm Research and development of rare-earth c...

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International Journal of Refractory Metals & Hard Materials 19 (2001) 159±168 www.elsevier.com/locate/ijrmhm

Research and development of rare-earth cemented carbides Chonghai Xu a

a,*

, Xing Ai b, Chuanzhen Huang

b

Department of Mechanical and Electronic Engineering, Shandong Institute of Light Industry, Jinan 250100, People's Republic of China b School of Mechanical Engineering, Shandong University, Jinan 250061, People's Republic of China Received 27 November 2000; accepted 27 March 2001

Abstract Research, development and application of rare-earth cemented carbides in China are reported and reviewed comprehensively. The adding methods, kinds, forms, contents and acting mechanisms of rare-earth elements are discussed and analyzed in detail. Mechanical properties and cutting performances of these rare-earth-containing cemented carbides are also summarized with detailed analyses and practical application examples in machining various work materials. Problems existing in current research and future research topics in this ®eld are considered further. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Cemented carbide; Rare-earth element; Acting mechanism; Mechanical property; Cutting performance

1. Introduction The application of rare-earth elements in cemented carbides was initiated from the early 1960s. In 1965, a patent was applied and approved in Germany [1] where the ¯exural strength of the cemented carbide is increased by nearly 20% while the hardness is increased by 0.5±1.0 HRA when 0.2% cerium-containing rare-earth metal mixture is added. Two years later, it was reported [2] that the addition of 0.1±2.0% cerium or yttrium can improve both the oxidation resistance and the high temperature strength of the cemented carbide. After that, similar results were discovered in USA [3], South Africa [4] and Japan [5,6] with incorporation of gadolinium or praseodymium, etc., ranging from 0.05 to 5.0 wt%. In the late 1960s, the research and application of rareearth elements in cemented carbides were just started in China. In 1982, a paper was published to analyze systematically the e€ects of rare-earth elements like neodymium and cerium on the properties of YT15 (the corresponding ISO names for the Chinese cemented carbides are given in the appendix) and YG6 cemented carbides [7]. Chen et al. [8] reported their results on WC± Ni cemented carbide in 1984. During the past 10±15 * Corresponding author. Tel.: +86-531-8556865/8616648; fax: +86531-8968495. E-mail address: [email protected] (C. Xu).

years, more and more researches have been carried out and varieties of rare-earth cemented carbides with higher mechanical properties and cutting performances such as YG6R, YG8R, YT5R, YT15R, YS25R and YW1R, etc. have been developed successfully in China [9±18]. In the present work, detailed methods of fabrication, acting mechanisms, mechanical properties and cutting performances of these rare-earth cemented carbides in China are completely summarized and reviewed; latest progresses and future research directions are also discussed.

2. Experimental method of fabrication 2.1. Kinds and forms of the added rare-earth elements According to the literature [9±21], rare-earth elements added in cemented carbides are mainly cerium (Ce), yttrium (Y), praseodymium (Pr), lanthanum (La), scandium (Sc), dysprosium (Dy), gadolinium (Gd), neodymium (Nd) and samarium (Sm), etc. with forms of oxide, pure metal, nitride, hydride, carbide, rare earth± cobalt (R±Co) master alloy, carbonate, nitrate or their mixtures. It suggests that physical and mechanical properties of the cemented carbides vary with the kinds and forms of the added rare-earth elements.

0263-4368/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 3 - 4 3 6 8 ( 0 1 ) 0 0 0 1 8 - X

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The incorporation of active rare-earth-containing additives is a precondition for the strengthening of the cemented carbides. If rare earth is added in the form of oxide, although the slurries and blanks of the cemented carbide can be dried under no vacuum condition because of the high chemical stability of the rare-earth oxide, the escape of oxygen during the sintering process will result in diculties in production. It should be realized that though certain strengthening e€ects can be observed with the direct addition of rare-earth oxide, it is still far from satisfaction [13]. The so-called ``certain e€ect'' implies that the reinforcing e€ect of rare earth on the cobalt binding phase and (W,Ti)C solid solution and its activation of sintering may play an dominant role in the improvement of the mechanical properties of the cemented carbides, while the ``dissatisfaction'' means that the incorporation of inert rare-earth oxides into the cemented carbide is quite di€erent from that of active rare-earth metals. The latter may lead to the more noticeable reinforcement of the cemented carbide. On the other hand, when pure metals or hydrides of the rare-earth elements are added, they will be partially or completely oxidized after the drying process of slurries at 80±100°C [13]. Although measures such as rareearth alloys that are not so easy to be oxidized, raw materials of low oxygen, wet milling media of absolute alcohol and vacuum drying can be taken to control strictly and prevent the oxidation of rare-earth elements, in the ®nal sintering process it is almost impossible to prevent rare-earth elements or alloys completely from being oxidized by the oxygen of minute quantity (approximately 0.2%) existing in the cemented carbide blank before the carbon±oxygen reaction at 600± 1000°C. Furthermore, it costs too much to keep the oxygen content in the blank to be less than 0.2%. Therefore, the key point lies not in the initial and ®nal states of the added rare earth elements but in the transformation of their states in the whole processing period. 2.2. Content of rare-earth element The mechanical properties of the cemented carbides vary sensitively with the amount of the rare-earth elements where an optimum content exists. Generally, the addition of rare earth should be in the range of 0.1±2.0 wt% of the cobalt phase for P series, ISO cemented carbides [7,9,12,20]. But some reports [5,6] give the experimental results till 5 wt%. For K series, ISO, the suitable content of the rare-earth elements is 0.2±1.0 wt% [7,10,12,15]. It appears that the added rare-earth element should be a stable value for the same kind of cemented carbide. But it was found in experiments that the amount of the rare earth varies with the manner in which it is added.

For instance, it is about two times more for the ballmilling dispersing method than that of other incorporating methods [15]. If various factors in the fabrication process are considered, it will then be understandable that agreements can rarely be achieved in the optimum content of rare-earth elements among di€erent researches. 2.3. Adding methods of rare-earth elements Generally speaking, adding methods of rare-earth elements can be divided into three types: direct addition in the wet milling process, indirect addition in the wet milling process and the addition after the wet milling process. The adding method is concerned closely with the forms of the added rare-earth elements. Usually, the oxide, pure metal, hydride, nitride and master alloy, etc. of the rare-earth elements should be incorporated directly into the material during the wet milling process. Some nitrates of rare-earth elements should be added into the material after the wet milling process, while carbonates and some of the nitrates of rare-earth elements should be added indirectly into the material during the wet milling process according to the experiments [19]. Most researchers [16,18,19] thought that rare-earth elements should be incorporated during the wet milling process of the powder mixtures. In this way, the rareearth elements can be weighed accurately and dispersed homogeneously in the raw materials so that cemented carbides with homogeneous compositions can ®nally be fabricated. But recent experiments [15] indicated that the rare-earth elements added in this manner may sometimes be aggregated. Moreover, the dispersion of the rare earth is frequently a€ected by the changing factors in ball-milling which will cause the inhomogeneous distribution of rare-earth elements in the cobalt phase. During the ball-milling process, few of the rare-earth elements stay with cobalt powders, while most of them are mixed with WC and (W,Ti)C particles. Then, a part of them is gathered at the interfaces of carbides. The excessive aggregation of rare earth at the interfaces may then weaken the binding strength of the cobalt/carbide phase boundary and the carbide/carbide interface. Thus, the excessive rare-earth aggregates existing at the interfaces actually act as a kind of harmful impurity. Li's method [15] can guarantee the homogeneous distribution of rare-earth elements in the cemented carbide and enhance the strengthening e€ect of the binding phase. However, the detailed method of the addition of rareearth elements is not clearly pointed out. Additionally, Zhang et al. [12] had fabricated a kind of cemented carbide with the addition of rare-earth oxide mixtures …La2 O3 ‡ Nd2 O3 ‡ Y2 O3 † in the form of rare-earth nitrate.

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2.4. Fabrication process Three methods are currently used in the fabrication of rare-earth-containing raw materials of cemented carbide, i.e., method of the direct addition of rare earth± cobalt master alloy during the wet milling process, method of co-precipitation and method of doping. Typical procedures of the method of co-precipitation and method of doping are described in Figs. 1 and 2, respectively. It appears that the procedure of the coprecipitation method is more complex than that of the doping method. The general procedure for the manufacture of rareearth cemented carbides is introduced here. After the preparation and mixture of the raw materials such as WC, TaC, Co, TiC and rare-earth elements according to certain proportions, the mixtures are subsequently homogenized with absolute alcohol media in a ball mill for 24±96 h. After milling, the slurry is dried in vacuum at a temperature of 80±120°C for 30 min and screened under nitrogen atmosphere. Green bodies are then formed under the pressure of 50 MPa at room temperature. Final densi®cation of the material is accomplished by

Fig. 2. Procedure for the fabrication of rare-earth-containing Co powder by doping method (R means the rare-earth element).

sintering at 1390±1440°C, 1430±1460°C or 1510±1560°C in an N2 or H2 atmosphere for 1±2 h. Details of the procedure and the speci®c processing parameters employed may be di€erent to some extent in di€erent research works for di€erent kinds of rare-earth cemented carbides.

3. Acting mechanisms of rare-earth elements

Fig. 1. Procedure for the fabrication of rare-earth-containing Co powder by co-precipitation method.

Acting mechanisms of rare-earth elements in cemented carbides have not been conclusively decided till now. An early study [10] showed that after the addition of rare-earth elements in YT14, YT15 and YG6 cemented carbides, the densities of these cemented carbides change slightly, while porosities are decreased; the graphite inclusions disappear completely and carbide particles including (W,Ti)C solid solution become ®ner. The grain ®ning e€ect becomes more obvious with the increase in the content of rare earth. The possible reason is that the addition of rare-earth element may cause a decrease in surface tension, and thin ®lms are formed adsorptively at the interfaces of liquid cobalt and carbide phase. As a result, the di€using process is slowed down and the dissolution and separation of carbides are hindered, and the continuous grain growth is inhibited. Thus, ®ne particles can easily be observed in these kinds of cemented carbides. However, the same conclusion can be drawn with the example of WC±Ni cemented carbide except that the density of the material is slightly increased and a small amount of free graphite is observed [19]. It was found in the literature [9,20] that there is little e€ect of rare-earth elements on the size and morphology of the hard phases in WC±TiC±Co cemented carbide. No rare-earth elements are found to exist in WC phase

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while some of them are distributed inside (W,Ti)C particles with spot-like or strip-like shape. Moreover, rareearth elements also exist at the interfaces of hard phases and inside the cobalt phase. XRD analyses indicate that the content of a-Co (fcc) inside the cobalt phase in the rare-earth cemented carbide is more than 95% while that in the corresponding cemented carbide without any addition of rare earth is only equal to about 51.9%. Therefore, the existence of rare-earth elements can inhibit the transformation of a-Co to e-Co (hcp). Since aCo is equipped with more sliding systems than e-Co, the dislocations in a-Co are easier to slide when external forces are exerted, and thus higher toughness and strength can be achieved. Yao et al. [21] studied the e€ect of La2 O3 ; Y2 O3 and CeO2 on the martensitic transformation of cobalt. They thought that the a-Co ! e-Co transformation is realized through the nucleation and extension of the faults with the help of the dislocation movement. The rareearth particles can, on one hand, pin the dislocations and then impede their movement. On the other hand, they also can be pinned at positions of various defects that lead to the decrease of the potential e nucleus. Consequently, the a-Co ! e-Co transformation is hindered, the content of a-Co is increased and the toughness and strength of the cemented carbide are enhanced. Additionally, the added rare-earth elements which are distributed inside the cobalt phase and at the interfaces of WC particles can improve the wettability of cobalt to WC particulates [14]. It is one of the reasons that cause the increase in the ¯exural strength of YG8R rare-earth cemented carbide. It was indicated in the study on the microstructures of YT5R rare-earth-cemented carbide [22] that the strengthening of cobalt phase by rare-earth elements depends mainly on the increase in the fault energy through the interactions among rare-earth elements and dislocations, the hindering of the extensive decomposition of dislocations for the inhibition of a-Co ! e-Co transformation, the solid solution strengthening of cobalt phase by rare earth elements inside, and the improvement of the wettability of cobalt to carbides. Particle strengthening by rare-earth element dominates in the reinforcement of (W,Ti)C solid solution, while obvious strengthening effect can seldom be observed in WC phase. At present, agreement has already been reached in the inhibition of a-Co ! e-Co transformation by rare-earth elements. However, di€erent results are still existing in the solid solution strengthening of the cobalt phase by rare earths. Refs. [22,24] armed it while You et al. [26] just gave a prediction. As a matter of fact, since the radius di€erence between the rare-earth atoms and cobalt atoms amounts to 41.9±46.9% and large di€erence in electronegativity exists between them, it is not suitable for them to form solid solution. According to the XRD

analyses and the accurate measurement of the lattice parameters of cobalt [27], yttrium can rarely be solidly solubilized into the cobalt phase. Even if a minute amount of rare-earth atoms can be solidly solubilized into cobalt, the corresponding solid solution strengthening is extremely weak. Hence, in order to verify the solid solution strengthening mechanism of the rare-earth element, the corresponding researches on other kinds of rare-earth cemented carbides are urgently needed. The distribution and existing state of rare-earth elements in cemented carbides were studied in detail in [9,13,14,23±30]. In early researches, it was realized that rare-earth elements may exist in the form of oxide [13,29] or sulphooxide [26] or carbide with the molecular formula of R2 C3 [9]. Then, it was summarized by Li et al. [28] that atoms of rare-earth metals can be adsorbed at the boundaries of WC/liquid phase (for K series, ISO cemented carbides) or (W,Ti)C/liquid phase (for P series, ISO cemented carbides) at the temperature of liquid sintering. At room temperature, rare-earth elements will be gathered at the phase boundaries of WC/c-phase and (W,Ti)C/c-phase. Sphere-shaped rare-earth-containing particles are separated to exist both in c-phase and at the WC/c-phase boundaries. Accordingly, it is anticipated that rare-earth elements may exist in atomic form while the possibility of the existence in compound state cannot be excluded. In cerium-containing rare-earth cemented carbide [23], however, two kinds of rare-earth compounds, Ce2 O2 S and Ce2 O3 , are formed and distributed both at boundaries of Co/WC or Co/(W,Ti)C and inside the (W,Ti)C solid solution particles. As a result, impurities like calcium and sulfur are gathered by the rareearth elements and the corresponding grain boundaries and phase boundaries are then puri®ed. As rare-earth elements are highly reactive, most of them are found to exist in compound forms like oxide and, possibly, intermetallics [27]. The kind of phenomenon that rareearth compounds are distributed at grain or phase boundaries is just a re¯ection of the segregation of rare earths at the interfaces. It should be noticed, however, that a small part of rare-earth elements have ever been observed to exist in the WC phase with spherical or polygonal shapes [30]. On the other hand, e€ects of rare-earth elements on the grain size of cemented carbides have not yet been decided conclusively. It is mentioned at the beginning of Section 3 that the addition of rare-earth elements can result in the ®ning of carbides, especially (W,Ti)C solid solution particles, and that the ®ning extent increases with increase in the content of rare earth, according to an early study [10]. But later work [31] suggested that rare-earth elements can inhibit the grain growth of WC phase while they have little e€ect on the grain size of (W,Ti)C and cobalt phase in WC±Co cemented carbide. When the rare-earth element is added in the form of

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oxide, the grain ®ning e€ect cannot be observed until the content of rare-earth oxide amounts to 1.0 wt% [21]. At the same time, the abnormal grain growth of WC particulate is also inhibited [14]. During recent years, it was found in more and more researches [27,29±31] that the incorporation of rareearth elements can lead to the ®ning and homogenization of WC particles. Image analyses indicate that the average grain size of WC particles is decreased from 0.83 lm (for YG6 cemented carbide) to 0.76 lm (for YG6R cemented carbide with 0.4 wt% rare earth). Xiong et al. [32] thought that the e€ect of rare-earth element on the WC grain size depends on the combination of two ®ning factors and one coarsening factor. In the cemented carbide made from the coarse-grain raw materials of WC, WC particles will be made ®ne since the growth of coarse grains is relatively easy to inhibit. But, for the cemented carbide consisting of ®ne WC particles, the ®ning e€ect is nearly negligible. However, the ®ning e€ect was rarely observed by Sun et al. [33,34]. Furthermore, the grain size of cobalt binding phase is decreased by approximately 26.9% as a result of the addition of rare-earth elements [27,30,33,34]. Additionally, it was reported [30,35] that the added rare-earth elements can decrease the size of the fracture origins, especially voids. 4. Mechanical property Mechanical properties, especially the ¯exural strength of cemented carbide, can noticeably be improved with the incorporation of a certain amount of rare-earth elements. It was reported in early 1980s by Chen et al. [8]

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that the addition of 0.5% yttrium in WC±Ni cemented carbide can result in the increase of the density from 13.95 to 14.33 g/cm3 , the ¯exural strength from 1735 to 2117 MPa with an increment of about 22% and the hardness from 84.5 to 85.5 HRA. Table 1 gives the mechanical properties of several kinds of rare-earth cemented carbides developed in China. It is suggested that ¯exural strengths of these rare-earth cemented carbides are approximately 10±15% higher than that of the corresponding materials without any addition of rare-earth additives. After a systematic study on the e€ects of La, Ce, Pr, Nd, Gd, Y and rare-earth mixtures on the properties of YT5 cemented carbide, Luo [9] developed a new kind of rare-earth cemented carbide, YT5R. Table 2 indicates the e€ects of varieties of rare-earth elements on the mechanical properties of YT5 cemented carbide. As can be seen, mechanical properties, especially, the ¯exural strength varies di€erently with the kinds of rare-earth elements. The highest ¯exural strength at room temperature is 2303 MPa when Nd is added while the lowest strength, 2038 MPa, corresponds to the incorporation of Ce. But for the ¯exural strength at 800°C, the lowest strength appears when La is added. The addition of Sm, Y, Ce, CmPr or LaCe rare-earth metals or metal mixtures can similarly increase the ¯exural strength of YT15 cemented carbide with the increment ranging from 9.98% to 17.84% according to Yan's study (Table 3) [11]. In fact, mechanical properties of rare-earth cemented carbides are not only in¯uenced by the kinds of rare-earth elements but also sensitive to the content of rare-earth elements. For YT5R cemented carbide [9], the highest ¯exural strength corresponds to the addition of the rare-earth mixture, Ce, Nd and Gd with the contents of 1.2%, 0.8%, 1.1%

Table 1 Properties of rare-earth cemented carbides [9±18,36±38] Density …g=cm3 †

Flexural strength (MPa)

YG6 YG6R YG8 YG8R YT14 YT14R

14.60±15.00 14.60±15.00 14.70 14.71 11.53 11.59

YT5 YT5R YT15 (standard) YT15R YT8R

13.00 13.01 11.0±11.7 12.69±13.00 12.60±13.00

Material

a

Hardness

Fracture toughness …MPa m1=2 †

Increment of ¯exural strength (%)

Increase of tool life

± 2±3 times ± >2 times ± 1.2±1.6 times ± 2±4 times ± >1 times 1.2 times

At room temperature (HRA)

At 800°C (GPa)

1370 1670 2292 2563 1479 1726

89.5 >90 89.6 90.1 90.8 91.0

± ± 6.13 7.13 ± ±

± ± 12.76 15.48 ± ±

± >10 ± >10 ± >15

2009 2303 1150 1680 1373

91.3 91.3 91 ± 91.3

± 8.40 ± ± ±

± ± ± ± ±

± 14 ± >10a >10a

Indicates that the two results are from di€erent sources, and R means the material with rare-earth element.

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Table 2 E€ects of the kinds of rare-earth elements on the mechanical properties of YT5 cemented carbide [9] Kind

No addition Rare-earth mixture La Ce Pr Nd Gd Y

Density …g=cm3 †

Hardness At room temperature (HRA)

At 800°C (GPa)

At room temperature

At 800°C

13.00 13.00 13.01 13.00 13.00 13.01 13.01 12.99

91.3 91.4 91.4 91.4 91.3 91.3 91.4 91.4

± 7.45 8.10 8.30 8.20 8.40 8.10 8.50

2009 2048 2097 2038 2097 2303 2146 2244

± 1587 1431 1591 1735 1803 1656 1597

Flexural strength (MPa)

Table 3 E€ects of the kinds of rare-earth element on the mechanical properties of YT15 cemented carbide [11] Kind

Density …g=cm3 †

Hardness (HRA)

Flexural strength (MPa)

Increment of ¯exural strength (%)

No addition Sm Y SmPr Ce LaCe

11.32 11.33 11.34 11.33 11.34 11.31

92.4 92.4 92.3 92.3 92.3 92.3

1233 1453 1437 1442 1338 1356

0 17.84 16.55 16.95 12.57 9.98

and 0.8%, respectively. Some detailed e€ects of yttrium on the mechanical properties of YT5 cemented carbide are shown in Figs. 3 and 4 [9]. It is implied that the exact control of the rare-earth content is critical for the improvement of the mechanical properties of rare-earth cemented carbides. Additionally, similar conclusions can also be drawn according to the study on the cemented carbides containing rare-earth oxides (Fig. 5) [14].

Fig. 3. E€ects of yttrium on the ¯exural strength and density of YT5 cemented carbide [9].

5. Cutting performance 5.1. 602 MM [10] and 710 MM [10] When YG8 cemented carbide is employed to bore holes in titanium alloy component, only 30 components can be bored since holes are small and deep, and four boring operations are required to ®nish the machining of one hole. Thus, the size accuracy of the hole cannot

Fig. 4. E€ects of yttrium on the hardness of YT5 cemented carbide [9].

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Fig. 5. E€ects of rare-earth oxides on the ¯exural strength of YG8 cemented carbide [14].

be guaranteed precisely. In contrast, 160 components can be bored under the same condition when 602 MM rare-earth cemented carbide is used. Only two boring operations are needed to ®nish a hole and the accuracy can amount to 3 lm. Therefore, the tool life of 602 MM is about ®ve times longer than that of YG8. When YG8 or YG6 cemented carbide is used to cut 2Cr15Mn15Ni2N low magnetic stainless steel, only 20 pieces can be machined with one tool. But for 710 MM rare-earth cemented carbide tool, 34 pieces can be machined and the tool life is increased by nearly 70%. It appears that 602 MM and 710 MM rare-earth cemented carbides are better choices of tool materials for machining titanium alloy and low magnetic stainless steel because of their characteristics of ®ne grain, high hardness and high wear resistance. 5.2. YT5R [9], YT15R [11], YT8R [36] and YT14R [37] Table 4 gives the result of the practical applications of YT5R rare-earth cemented carbide [9]. As can be seen, cutting performances of YT5R are greatly improved in machining various work materials such as 40Cr, 70Mn2Mo, ZG35, ZG20CrMo and ZGM13, etc. Tool lives of YT5R are 1.5±10 times as long as that of the cemented carbides without any addition of rare-earth elements such as YT5, CC135 and S6. Moreover, when machining 38CrNi3MoVA alloy steel, tool life of YT5R is about two times longer than that of YT5. Its fracture resistance is also higher than that of YT5 in the intermittent cutting of the same work material. So, YT5R

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rare-earth cemented carbide can be well used in continuous and intermittent machining of various steels. When cutting carbon steel of 0.45% carbon with a hardness of 85 HRB and the steel of 0.55% carbon with a hardness of 90 HRB under the cutting condition of the cutting speed v ˆ 120±220 m/min, feed rate f ˆ 0:1±0:3 mm/rev and depth of cut ap ˆ 0:8 mm, cutting performances of YT15R rare-earth cemented carbide are obviously enhanced when compared with that of YT15 [11]. The cutting force, the coecient of tool-chip friction and surface roughness of the machined work material are decreased, and wear resistance and tool life of YT15R are increased by approximately 85%. When cutting mild carbon steel containing 0.45% carbon, the cutting force and tool life of YT8R rareearth cemented carbide are approximately 7.5% lower and 20% longer, respectively, than that of YT15 cemented carbide [36]. Under the cutting condition of the cutting speed v ˆ 100±140 m/min, feed rate f ˆ 0:2 mm/rev and depth of cut ap ˆ 1 mm, both the cutting force and the coecient of tool-chip friction of YT14R rare-earth cemented carbide are lower than that of YT14 with no rare-earth incorporation, while wear resistance of YT14R is increased notably, and tool lives are enhanced to some extent (20±30%) when turning the high-strength steel 38CrNi3MoVA with a hardness of 36±40 HRC (Fig. 6) [37]. 5.3. YG6R [16,36] and YG8R [37] The tool life and cutting eciency of YG6R rareearth cemented carbide are 1.5±3.5 times longer and 30% higher, respectively, than that of the corresponding cemented carbide YG6 containing no rare-earth element irrespective of roughing or ®nishing of cast iron [16]. When 8±10 sand holes are existing in the surface of the cast iron workpiece, the fracture resistance of YG6R is extremely higher than that of YG6 and no edge chipping occurs at all. The tool life of YG6R is 1.5±2 times as long as that of YG6 and the surface roughness of the machined surface is decreased when turning, milling and boring stainless steel, cast iron and carbon steel. In machining chilled cast iron, the cutting force and the tool life of YG6R are about 6% lower and 50% longer, respectively, than that of YG6 (Fig. 7) [36]. Therefore, it can be judged that YG6R rare-earth cemented carbide is suitable for both roughing and ®nishing of cast iron and steel. Good cutting performances are found for YG8R rare-earth cemented carbide when machining cast iron HT200 with a hardness of 175±180 HBS. The tool life of YG8R is 1±2 and 2 times, respectively, as long as that of the corresponding YG8 and YG8N cemented carbides (Fig. 8) [37].

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Table 4 Comparisons of cutting performances of YT5R and other cemented carbides [9] Tool material

Work material

Cutting parameters Cutting speed (m/min)

Feed rate (mm/rev)

40Cr cast steel

65

0.8±1.2

10

YT5 YT5R

70Mn2Mo roll with sand inclusions at the surface

24

0.5±0.6

20±30

YT5 YT5R

Anchor frame of cast steel with 0.45% carbon

37.7

0.5±0.8

0±3

1 piece 5 pieces

YT5 YT5R

Four Pillars in the decelerator shell of ZG35 cast steel with sand inclusions

88

0.43

0±7

2.5 pieces 22 pieces

YT5 YT5R

ZG20CrMo cast alloy with ¯ow gate and sink head

47

0.63

8

25 mm 100 mm

YT5 YT5R

ZGM13 cast steel

24

0.36

1±2 3±4

0.2 piece 5 Pieces

S6 (Switzerland) YT5R

45# Forged steel

70

0.8±1.0

4

65 min 100 min

YT5 CC135 (Switzerland) YT5R

Depth of cut (mm)

Machining time or length or pieces of workpiece 57.50 min 96.44 min 112.25 min 10 min 35 min

Note: R means the cemented carbide with rare-earth element.

Fig. 6. Flank wear of YT14 and YT14R cemented carbides when machining 38CrNi3MoVA under two groups of cutting conditions: (1) cutting speed v ˆ 100 m=min, feed rate f ˆ 0:2 mm=rev, depth of cut ap ˆ 1:0 mm; (2) v ˆ 140 m=min, f ˆ 0:2 mm=rev, ap ˆ 1:0 mm [37].

5.4. YS25R [15] and YW1R [38] Irrespective of continuous or interrupted turning or milling of 38CrMnTi and 20CrMnMoA alloy steels and

Fig. 7. Flank wear of YG6 and YG6R cemented carbides when machining chilled cast iron under three groups of cutting conditions: (1) cutting speed v ˆ 10 m/min, feed rate f ˆ 0:2 mm/rev, depth of cut ap ˆ 1:0 mm; (2) v ˆ 15 m/min, f ˆ 0:2 mm/rev, ap ˆ 1:0 mm; (3) v ˆ 10 m/min, f ˆ 0:2 mm/rev, ap ˆ 1:0 mm [36].

carbon steels, tool lives of YS25R rare-earth cemented carbide are 1±4 times higher than that of the corresponding YS25 cemented carbide. Further experiments

C. Xu et al. / International Journal of Refractory Metals & Hard Materials 19 (2001) 159±168

Fig. 8. Flank wear of YG8, YG8N and YG8R cemented carbides when machining cast iron at the cutting conditions of v ˆ 120 m/min, f ˆ 0:21 mm/rev, ap ˆ 1:0 mm [37].

prove that both cutting forces and cutting temperatures are lowered during the cutting process [15]. Practical applications indicate [38] that tool lives of YW1R rare-earth cemented carbide are 1±2 times longer than that of the corresponding cemented carbide YW1 with no addition of rare earth when machining 42CrMo alloy steel with a hardness higher than 350 HBS and a tensile strength of 800 MPa, and surface roughness is decreased from Ra 6.3 to Ra 3:2 lm or from Ra 3.2 to Ra 1.6 lm. Especially in machining high-temperature alloys such as GH15K, GH3128 and K18, etc., cutting performances of YW1R are fairly better than that of YW1, YW2, YT5, YG8 and YT14 cemented carbides and the cutting eciency is increased by approximately 200%. Hence, YW1R rare-earth cemented carbide is capable of machining of dicult-to-cut materials.

adding methods of rare-earth elements, etc. are not consistent with each other and even, sometimes, are in complete contradiction. In fact, the kind of phenomenon is just the re¯ection of the requirement of the systematic and deep researches in this ®eld. It may be said that progress on the acting mechanisms of rare-earth elements will, to a great extent, constrain the future application and development of rare-earth cemented carbides. On the other hand, one of the key problems in the development of rare-earth cemented carbides is the appropriate determination of the adding methods, kinds, forms and contents of the rare-earth elements. New adding methods have to be explored and employed to bring the strengthening function of rare-earth elements fully into play and to increase the dispersing homogeneity of rare-earth elements inside the binding phase, which is urgently needed in the stabilization of the product quality in batch production [40]. Additionally, one of the important research topics in the future lies in the transformation characteristics of the added rareearth elements in forms and states in di€erent stages of production and their ®nal states of distribution and existence. At the same time, future emphases should be placed on the high-temperature mechanical properties and wear and fracture mechanisms [41±43] of rare-earth cemented carbides in order for the better and further applications both in machining process and other areas. Appendix A ISO names for Chinese cemented carbides YT5 ± P30. YT5R cemented carbide containing rare-earth element which is based on YT5, similarly hereinafter. YT8 (YT8R) ± P25. YT14 (YT14R) ± P20. YT15 (YT15R) ± P10. YS25 (YS25R) ± P25. W1 (YW1R) ± M10. YG6 (YG6R) ± K15. YG8 (YG8R) ± K30. YW2 ± M20. YG8N ± K20. 602 MM ± cemented carbide with the composition of WC±TaC±Co±rare earth, K series, ISO. 710 MM ± cemented carbide with the composition of WC±TiC±TaC±Co±rare earth, M series, ISO.

6. Concluding remarks Since the addition of rare-earth elements is of great bene®t to the improvement of the microstructure, mechanical property and cutting performance of cemented carbides, varieties of rare-earth cemented carbides have got wide applications in China in machining various work materials. At present, progress has been achieved in acting mechanisms of rare-earth elements. Especially, agreement has been reached in the understanding of the strengthening of cobalt binding phase by rare earth. It should be noticed [39], however, that results from different laboratories or institutes with di€erent cemented carbides, di€erent kinds and forms and contents and

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