Applied Mechanics and Materials Vols. 423-426 (2013) pp 885-889 Online available since 2013/Sep/27 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.423-426.885
Fabrication of functionally gradient ultrafine-grained WC-Co composites Yigao Yuan1, a, Jianjun Ding1, b, Yankun Wang2, c, Weiquan Sun2, d 1
College of Mechanical Engineering, Donghua University, 201620, Shanghai, China 2
Shanghai tool works Co., Ltd., 200093, Shanghai, China
a
email:
[email protected], bemail:
[email protected] ,
c
email:
[email protected],demail: sunweiquan@ stwc.cn,
Keywords: Ultrafine-grained cemented carbides; Carburization; Compositional gradient; η-phase
Abstract: The carburizing heat treatments of ultrafine-grained WC-Co composites with substoichiometric carbon content were carried out, and the microstructures of ultrafine-grained WC-Co carbides before and after treated were characterized by means of scanning electron microscopy and X-ray diffraction. The results show that the functionally gradient ultrafine-grained WC-Co hardmetals with a Co depleted surface and not comprising the η-phase can be fabricated by carburizing heat treatment. After heat treatment, WC grain sizes in materials are still at the ultrafine grade. Introduction: Functionally gradient WC-Co hardmetals having a near-surface layer with a low Co content have been found to be able to offer exceptional combinations of wear resistant and fracture toughness and thus superior engineering performance [1]. Therefore, process technologies for fabricating functionally gradient WC-Co composites with a Co depleted surface zone have been the focus of extensive research in the hardmetal industry over the past decades. First attempts to produce hardmetals with graded Co composition were based on pressing articles from two graded WC-Co powders with different Co contents [2]. However, this approach had a limited practical importance, as the liquid Co phase will homogenize through migration during the liquid phase sintering. The formation of Co gradients in WC-Co articles can be achieved by carbon gradients within hardmetal articles [3]. Fischer et al. patented a technology based on the carburization of hardmetals with original low carbon contents during liquid phase sintering [4,5]. This technology allows the fabrication of gradient articles with a very low Co content in the near-surface layer. A significant disadvantage of the gradient hardmetals (so-called DP carbide) obtained by this approach is, however, the presence of the very brittle core comprising much η-phase (Co3W3C), which is detrimental to the mechanical properties of WC-Co materials. Recently, a novel process for manufacturing gradient WC-Co hardmetals was developed by Fan Peng et al [6-8]. In this process, the Co gradient is formed by heat treating conventional WC-Co with coarse WC grains in a carburizing atmosphere at temperatures that allow three phase equilibriums among solid WC, solid Co, and liquid Co. Unlike in the DP carbide process, no brittle η-phase exists before the processing or forms during this process. Compared to conventional microstructured WC-Co, ultrafine-grained WC-Co carbides possess higher wear resistance and lower fracture toughness [9]. Such a trade-off between wear resistance and toughness limits the use of ultrafine-grained WC-Co for broader industrial applications. If one All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 218.193.149.114-14/10/13,09:21:59)
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can produce ultrafine-grained hardmetals with a near-surface region with lower Co contents than the average Co content, this region would have a high hardness leading to its better wear resistance, while the core with higher Co contents but not comprising the η-phase would provide higher fracture toughness. It can be expected that such gradient structure would offer strong potential to significantly improve the durability and reliability of industrial tools. For this reason, the carburizing heat treatments of ultrafine-grained WC-Co composites were carried out in this article, and the microstructures of ultrafine-grained WC-Co carbides before and after treated were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The aim of this work is to examine the feasibility for fabricating gradient ultrafine-grained WC-Co composites with a Co depleted surface. Experimental Materials and sample preparation The materials used in this investigation are ultrafine-grained WC-10Co cemented carbides produced by Shanghai tool works Co., Ltd with a nominal 0.4 μm grain size. WC-10 Co specimens with slightly sub-stoichiometric carbon content (about 5.36 wt.%) were prepared. The preparation of specimens was carried out as follows: Tungsten powder was added to the WC-Co mixed powder to reduce the total carbon content. The mixture of the powders was ball milled in heptane for 4 h in an attritor mill. The milled powder was dried at 80℃ and then cold pressed at 200 MPa into green compacts of φ10 mm×200 mm in dimensions. Finally, the green compacts were sintered in vacuum at 1420℃ for 1 h. To avoid the influence of the atmosphere during sintering on surface compositions of specimens, each sintered cylindrical specimen was ground using a centreless grinding machine to obtain a fresh surface. After grinding, the ground samples were ultrasonically cleaned with acetone. Before the heat treatment, the vertical cross sections of ground cylindrical specimens were cut into two halves by using electrical discharge machining (EDM), in order to compare the cobalt concentration profile and WC grain size in specimens after and before treated. The carburizing heat treatments of sintered samples were conducted in atmosphere consisting of mixtures of methane (CH4) and hydrogen (H2). The heat treatments were conducted in the carbon-rich atmosphere with constant PCH / PH2 ratio of 1/42 atm-1 for 2 h at 1300℃ ( PCH and PH 4
4
2
2
expresses the partial pressure of methane and of hydrogen in mixed gas, respectively). 1300℃ were selected based on the ternary phase diagram of W-Co-C at 10wt.%Co[10]. At this temperature, solid WC, solid Co, and liquid Co coexist. PCH / PH2 ratio and holding time were chosen according 4
2
to the calculated results of the thermodynamic and the kinetics of the process. Analysis of the gradient structure After the heat treatment, the vertical cross-sections of both the as-sintered and as-treated specimens were carefully polished with 10, 5 and 1μm diamond paste, respectively. After each sequential polishing step, the surface was ultrasonically cleaned with acetone. To determine the gradient, cobalt concentration profiles perpendicular to the surface were measured using an energy dispersive spectroscopy (EDS) technique. Each data point of the Co composition is an averaged value obtained by scanning a 10μm×60μm rectangular area that was parallel to the surface, the rectangular area for measurement was spaced in 10 μm increments in the
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direction perpendicular to the original surface [11]. It should be noted that the carbon content must be excluded in the quantitative analysis, considering that EDS is not suitable for measuring carbon quantitatively. To evaluate the difference of WC grain size in specimens after and before treated, the average grain size of WC was also measured based on the backscattered electron (BSE) images of the microstructure and IMAGEJ image analysis software [12]. In addition, the vertical cross-sections of both the as-sintered and as-treated specimens were examined by means of X-ray diffraction to see if there was any free carbon or brittle η-phase in the microstructures. Results and discussion SEM micrographs of the polished cross-sections of samples before and after treated are shown in Fig.1. The white crystals are the WC phase, and the black contrast regions between the WC grains are the cobalt binder phase. From the images we can see clearly that the microstructure of the WC-Co specimen before treated (Fig.1a) was uniform. After the heat treatment, the cobalt content in the surface region of the sample is lower than that in the inner region (Fig. 1b). This suggests that a gradient microstructure was developed from the surface inward. Fig.2 shows the X-ray diffraction spectra of samples before and after treated. It can be seen that there was neither free carbon nor η phase in the microstructures.
(a)
(b)
Fig.1 SEM micrographs of the polished cross-sections of the samples, (a) before and (b) after heat treatment.
(a)
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(b) Fig.2 The X-ray diffraction spectra of the samples, (a) before and (b) after heat treatment Fig.3 shows the measured Co concentration profiles of samples before and after treated. For the WC-10Co specimen treated in atmospheres consisting of mixtures of methane and hydrogen at 1300℃, there is a continuous Co gradient as a function of the depth. It is shown that the Co content of the surface zone of the sample is reduced to 5.84 from about 10 wt.%. Deeper into the specimen, the Co content gradually reaches the nominal Co content. On the contrary, the Co content profile of the specimen without treatment is flat. 12 Nominal Co content
11
Co content,wt%
10 9 8 7 Before heat treatment After heat treatment
6 5 4 0
20 40 60 80 100 120 140 160 180 200 220 Distance from surface, μm
Fig.3 EDS profiles of the Co concentration measured in the samples before and after treated The measurement results of WC grain sizes in samples after and before treated are listed in table 1. It can be seen from Table 1 that the WC grain sizes of the treated samples are larger than that of the untreated ones. It illustrates that the WC grains have a tendency to grow up during carburizing heat treatment. Also, the measurements of the mean grain sizes show that there is a little difference between the surface and the interior of the sample after heat treatment, but all the WC grain sizes are still at the ultrafine sized grade. Table 1. Measurement results of WC grain sizes in samples after and before treated average grain size of WC（μm） Samples Surface core Before heat treatment 0.473 0.484 After heat treatment 0.512 0.580
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Conclusions (1) (2)
Functionally gradient ultrafine-grained WC-Co hardmetals with a Co depleted surface but not comprising the η-phase can be fabricated by carburizing heat treatment. WC grains in ultrafine-grained WC-Co hardmetals have a tendency to grow up during carburizing heat treatment, but are still at the ultrafine grade.
Acknowledgement We would like to thank Professor level senior engineer Bai Jiasheng of Shanghai tool works Co., Ltd. (STWC) for experimental guidance. This work was supported by Shanghai Natural Science Foundation (No. 13ZR1401300). References [1] X. Wang, K. S. Hwang, M. Koopman, et al. Int. J. Refractory Met. Hard Mat.,Vol.36(2013), p:46-51 [2] C. Colin, L. Durant, et al. Int. J. Refractory Met. Hard Mat., Vol. 12(1993-1994), p: 145-152. [3] P. Fan, J. Guo, Z. Zak Fang, et al. Int. J. Refractory Met. Hard Mat., Vol. 27(2009), p:256-260 [4] U. Fischer, M. Waldenstrom, T. Hartzell. U.S. Patent 5,856,626.( 1999) [5] U. Fischer, E. Hartzell, J. Akerman. U.S. Patent 4,743, 515. (1988) [6] J. Guo, P. Fan, X. Wang, et al. International Journal of Powder Metallurgy, Vol. 47(2011), p: 55. [7] I. Konyashin, S. Hlawatschek, B. Ries, et al. Int. J. Refractory Met. Hard Mat., Vol. 28(2010), p: 228-237. [8] P. Fan, Z. Zak Fang, J. Guo. Int. J. Refractory Met. Hard Mat., Vol. 36(2013), p:2-9 [9] W. B. Liu, X. Y. Song, J. X. Zhang, et al. Journal of Alloys and Compounds. Vol. 458(2008), p:366-371 [10] J. Guo, Z. Zak Fang, P. Fan, et al. Acta Materialia, Vol. 59(2011), p: 4719-4731 [11] Y. G. Yuan, Y. K. Wang, J. J. Ding, et al. Advanced Materials Research. In press. [12] Y. G. Yuan, X. X. Zhang, J. J. Ding, et al. Applied Mechanics and Materials. Vol. 278-280(2013), p: 460-463.
Applied Materials and Technologies for Modern Manufacturing 10.4028/www.scientific.net/AMM.423-426
Fabrication of Functionally Gradient Ultrafine-Grained WC-Co Composites 10.4028/www.scientific.net/AMM.423-426.885
