Studies of graded cemented carbides components

Studies of graded cemented carbides components

International Journal of Refractory Metals & Hard Materials 17 (1999) 187±192 Studies of graded cemented carbides components M. Rosso *, G. Porto, A...

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International Journal of Refractory Metals & Hard Materials 17 (1999) 187±192

Studies of graded cemented carbides components M. Rosso *, G. Porto, A. Geminiani Dipartimento di Scienza dei Materiali ± Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy Received 9 March 1998; accepted 18 December 1998

Abstract Graded structures obtained by coupling di€erent WC grades, also containing cubic carbides type (Ta, Nb)C with Co as binder, were produced by vacuum sintering. Di€erent batches of cylindrical samples were compacted by coupling two di€erent grades and were sintered at 1450°C under vacuum. The hardness properties as well as the shrinkage due to sintering process are function of the employed grades and show gradual change between the single values of each of the coupled grades. The residual porosity was always lower than type A02, the observed microstructure shows very good link between the coupled grades, with the presence of a di€usion zone and without any in¯uence of gravity on structure. The results show that it is possible to produce multilayer graded tools with a tough core, constituted by cheap carbide grades with high binder contents, covered by a very hard surface layer, achieving high economical and technical competitiveness for the production of tools for applications in high abrasive as well as in shock loaded environments. Ó 1999 Elsevier Science Ltd. All rights reserved. Keywords: Hardmetals; Graded structures; Shrinkage; Hardness; Tools

1. Introduction Cemented carbides play a key-role in the production of ultra hard components capable to prepare well in high wear environments, such as cutting, machining and mining applications. Di€erent carbides have been studied (e.g. TaC, (Ta, Nb)C, TiC, HfC, VC, Cr3 C2 ) [1±3] and they proved to be e€ective for certain purposes, however the most widely used mixes are straight WC/Co grades [4]. It is well known that varying the content of the binder (cobalt) [5] it is possible to adjust the ®nal hardness of the component: low cobalt mixes are suitable for cutting inserts where the main requirement is elevated hardness, while mixes with a higher content of cobalt are indicated for punching applications where toughness is vital in such shock loaded environment. Moreover, it is proved that ®ne carbide grain mixes [6,7] have a higher hardness compared to coarse grain ones with the same cobalt content, even if their abrasion resistant factor and transverse rupture strength are lower [8].

*

Corresponding author.

In all cases coating ®lms can be deposited via PVD or CVD techniques [9]; the wear resistance of the component will be further enhanced without a€ecting the toughness of the bulk of the tool. Due to the di€erent thermal expansion coecients of the substrate and the coating layer and to the high temperature reached at the top of the tool (800±1000°C), the ®lm necks and splits o€ leaving the substrate `naked'; subsequent rapid wear occurs and the life of the component is therefore dramatically shortened. From here comes the need to develop a hard metal substrate to perform well, even un-coated. The idea is to obtain functionally graded materials, coating techniques (CVD and PVD processes) allow a route, moreover it is possible to obtain gradients in chemical composition, as well as in microstructure and properties [10] by means of proper choice of atmosphere and sintering cycle. In this way tools, having hard surface zone due to poor cobalt content, couple with core of adequate toughness thanks to higher cobalt content, were produced [11]. With the aim to contribute the competitiveness of hard metals, this work will present sintering and structural data of trial samples characterised by graded structures with carbide grain size ranging from 0.8 up to 2.0 lm and the binder content ranging from 4.5 up to 12 wt%; this could be a viable selection to combine suitable

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wear and shock resistance in cemented carbide tools, avoiding the need of a coating ®lm.

Table 2 Samples produced by pairing the di€erent grades and di€erential shrinkage along the diameter for the three batches, after sintering at 1450°C for 30 minutes in vacuum

2. Experimental

Batches

Coupled grades

Di€erential shrinkage %

A±B A±C A±D

A combined with B A combined with C A combined with D

1.0 1.1 4.3

Four grades of WC/Co powders were used, their chemical composition is reported in Table 1 together with their average grain size range. Three batches of cylindrical samples were produced by subsequently pairing two di€erent grades in a single action uni-axial die (diameter ˆ 26 mm) and cold pressing at 200 MPa, as shown in Table 2. The batches were dewaxed and presintered at 800°C in pure nitrogen atmosphere. The presintered samples were placed in the sintering furnace as shown in Fig. 1, in order to determine the in¯uence of gravity on structure and/or composition homogenisation, three presintered samples each batch were placed in the sintering furnace in 3 di€erent positions, that is the ®rst one along the horizontal axis and the other two with vertical axis and with the grade. A part constituting alternatively the top or the bottom of the samples. Sintering was carried out at 1450°C for 30 min under vacuum, the diagram of Fig. 2 shows the complete sintering cycle. After sintering the trial samples were measured to check the di€erential shrinkage. Finally the samples were cut in half along their axis and diamond polished for hardness testing (Vickers and Rockwell methods) and metallographical examination (Standard ISO 4505 for residual porosity and Standard 4499 for metallographic characteristics).

Fig. 3. The highest shrinkage appears for grade type D, containing the highest amount of binder also. However the good compatibility between grades A and D allows well link and di€usion zone at the interface, without the appearance of cracks or microcracks.

3. Results and discussion The di€erential shrinkage of sintered samples, due to the compositional gradient, is reported in Table 2. For batches A±B and A±C the di€erential shrinkage along the diameter is not elevated, but for batch A±D the shrinkage becomes considerable. As a consequence, the observation of the pro®le at the interface between the two di€erent powder grades shows very light variation for samples A±B and A±C, while for samples A±D the pro®le variation is very heavy and evident, also the naked eye, as can be observed in the photographs of Table 1 Chemical composition and average grain size of the experimental grades Grades

WC wt%

(Ta, Nb)C Co wt% wt%

Lubricant Grain size wt% (lm)

A B C D

92.5 94 93 87

3.0 0.5 0.5 1.0

2.0 2.0 2.0 2.0

4.5 5.5 6.5 12.0

0.8 1.2 1.5 2.0

Fig. 1. Placement of the samples on the sintering furnace tray, in order to determine structural and/or compositional homogenisation due to gravity.

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189

Fig. 2. Vacuum sintering cycle.

The residual porosity, observed on the axial section of polished samples and particularly at the interfaces, was related essentially to the presence of a little amount of very small pores and was always lower than type A02, as it is reported in Table 3. After etching, all the samples shown regular microstructure characteristics with the WC grains embedded in the cobalt binder phase, without the presence of g phase and with a transition zone between A grade and the coupled grades. None e€ects due to the in¯uence of gravity on structure and/or composition homogenisation were observed. The microphotographs in Fig. 4 show the microstructure details at the interface zone of the coupled grades, the di€erences in grain size range in case of the coupling of grade A with grade D are particularly evident, however microcracks or other type of defects were never observed. The hardness characteristics are strongly dependent on the powder grades. In Table 3, together with the type of porosity observed on the di€erent samples, the variations of hardness values, Rockwell A and Vickers 50 types, measured on the transverse sections of samples, are reported. Coming up to the expectations the hardness decreases when increasing the grain size and the cobalt content, in fact grade type A showed the highest hardness characteristics, while the lowest were presented by type D grade. However, the hardness of the grade A is in¯uenced by the pairing with the other grades and progressively decreases with a link at the interface with the coupled grade. In particular, when coupled with grade D it seems that grade A can't reach the highest hardness values, as for the other cases, probably the interposition of a medium grade (B or C) between A and D will mitigate the di€erences and will favour higher hardness values to grade A. The above results let see the possibility to produce graded tools with a tough core, constituted by cheap

carbide grades with high binder contents, covered by a very hard surface layer, incorporating di€erent carbides such as TaC, Ta(Nb)C and TiC into the most common micrograin WC/Co mixes. These graded structures can achieve high economical and technical competitiveness for the production of tools for applications in high abrasive as well as in shock loaded environments, combining higher level of hardness and wear resistance with adequate toughness, improving the hardmetal capability to perform well, even uncoated. A signi®cant advantage of graded multicarbide structures with respect to the coated materials is related to the presence of di€usion zones between the coupled layers, that can guarantee full integrity of the surface material during the work, while coated tools, via CVD or PVD processes, can present ¯akiness of coating, because, due to the di€erent thermal expansion coecients coupled to the high temperatures eventually reached at the tool surface during work, the deposited ®lm cracks and splits o€ mainly in the zones where the component performs a great part of its work. Subsequently rapid wearing and cratering of the `naked substrate' shortens the life of the tool. With reference to Fig. 5, where sketches of graded structures and of coated substrate are shown, it is evident that the ®rst one allows an interdi€usion zone between the coupled grades (A and B), with a well structural link between the coupled grades, while in the case of coated material there is a sharp interface between substrate and coating layer and the link without the presence of di€usion zones. Another important aspect is related to residual stress, because if the core of the tool will be constituted by grade type D, with higher shrinkage than the external grade, type A for example, the external layer will be subjected to compressive residual stress, with bene®ts for the fatigue endurance of the tool.

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M. Rosso et al. / International Journal of Refractory Metals & Hard Materials 17 (1999) 187±192 Table 3 Degree of porosity and hardness properties (Vickers and Rockwell A) of the samples as a function of the grades of powders Samples

Grades

Porosity

HV50

HRA

A±B

A

A < A02

1950 1870 1820 1780 1780 1950 1950 1780 1710 1710 1820 1780 1640 1510 1510

93.2 92.9 92.7 92.6 92.5 93.2 93.2 92.9 92.3 92.3 92.2 91.7 91.3 91.0 90.9

Interface A±C

B A

B < A02 A < A02

Interface A±D

C A

C < A02 A < A02

Interface D

D ˆ A00

tools coated with di€erent materials [12] and in particular of WC/Co based coatings sprayed on di€erent substrate materials [13±15] are becoming very important and more and more meeting with the approval for different applications. In this contest the manufacturers of hardmetals must increase the competitiveness of their products toward the development of sintering routes capable of raising the performance/price ratio of new products. 4. Conclusions

Fig. 3. External pro®le variation at the interface region for the di€erent pairs of grades.

Moreover graded multicarbide structures can be competitive with thermal spray coatings, in fact among the di€erent surface modi®cation techniques, spray coatings give a number of alternative industrial processes capable of coating substrate of widely di€erent properties and in this contest the application of steel

Trial samples characterised by graded structures obtained by coupling di€erent WC grades, also containing cubic carbides type (Ta, Nb)C with Co as binder, were produced by vacuum sintering, the aim being to combine suitable wear and shock resistance in cemented carbide tools, avoiding the need of a coating ®lm and contributing the development of hard materials. The hardness properties as well as the shrinkage due to sintering are function of the employed grades and show gradual change between the single values of each of the coupled grades, the highest hardness characteristics being related to the lowest Co content and to the smallest carbides grain size, while increasing the binder content, the sintering shrinkage increases. The residual porosity was always lower than type A02 and all the etched samples show regular microstructure characteristics, with the WC grains embedded in the cobalt binder phase, none e€ects due to the in¯uence of gravity on structure and/or composition homogenisation were observed and at the interface zone of the coupled grades, the observed microstructure show very good link between the coupled grades, with the presence of a di€usion zone, very important for the properties of the multicarbide graded structures.

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Fig. 5. Sketch showing the di€erence between two graded materials with di€usion zone (bold line) at the interface and simply coated components without di€usion phenomena.

carbides such as TaC, Ta(Nb)C and TiC into the most common micrograin WC/Co mixes. These graded structures can achieve high economical and technical competitiveness for the production of tools for applications in high abrasive as well as in shock loaded environments, combining higher level of hardness and wear resistance with adequate toughness, improving the hardmetal capability to perform well, even uncoated. References

Fig. 4. Microstructure details at the interface zone of the coupled grades.

The obtained results show that it is possible to produce graded tools with a tough core, constituted by cheap carbide grades with high binder contents, covered by a very hard surface layer, incorporating di€erent

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