Elsevier

Ceramics International

Volume 44, Issue 2, 1 February 2018, Pages 2030-2041
Ceramics International

Effect of Fe/Ni ratio on the microstructure and properties of WC-Fe-Ni-Co cemented carbides

https://doi.org/10.1016/j.ceramint.2017.10.148Get rights and content

Abstract

WC-Fe-Ni-Co cemented carbides with a constant Co content were successfully prepared through low pressure sintering, and the effects of Fe/Ni ratio on the microstructure and properties of the cemented carbides were studied in detail. The X-ray diffractometer and transmission electron microscopy results show that the Fe/Ni ratio is a critical factor that affects the structure of the Fe-Ni-Co binder in the alloys, as it gradually changes from fcc γ-(Fe,Ni) to bcc α-(Fe,Ni) with an increase in the Fe/Ni ratio. Furthermore, the WC grains have a finer and more rounded shape and are more uniformly distributed in the matrix as the Fe/Ni ratio increases. Additionally, both the magnetic saturation (CoM) and coercive force (Hc) increase as the Fe/Ni ratio increases. The relative density of all alloys is greater than 99.93% except for the alloy with the Fe/Ni ratio of 1.67 (99.01%). Ignoring the effect of the lower relative density (Fe/Ni = 1.67), increasing the Fe/Ni ratio can enhance the hardness, transverse rupture strength and wear resistance of the alloys but can reduce the fracture toughness and corrosion resistance at the same time.

Introduction

Conventional cemented carbide processed using powder metallurgy technology is matrixed using refractory carbides (e.g., WC, TiC, Mo2C and TaC) with a soft and ductile metal Co as the binder phase [1], [2], [3], [4]. Due to its excellent wettability, good yield and work hardening behaviour, Co has been the most widely used element for producing the cemented carbide [5], [6], [7]. Co plays an important role in the bonding of WC grains. Since its first use, the WC-Co cemented carbide has been gaining importance for applications in cutting, mining, and drilling tools as well as in machining and wear resistant parts [4], [8], [9].

Currently, because cemented carbides are frequently exposed to severe service environments, alloys with a combination of a high hardness, wear resistance, toughness and corrosion resistance are required. However, the applications of WC-Co cemented carbides under severe conditions are restricted due to the disadvantages of using Co as the binder, namely, the resulting corrosion and oxidation resistance of the alloys. Furthermore, as a crucial yet scarce strategic metal, Co has exhibited increasing and unstable prices, and its adverse effect in terms of pollution should also be taken into consideration [8], [10], [11], [12]. Clearly, it is difficult for such a WC-Co system to meet the requirements for industrial applications due to the disadvantages mentioned above. It is, therefore, of common interest to find more cost-effective and environmentally benign metals to partially or completely replace Co without compromising the properties of cemented carbides [13]. A few studies have found that the corrosion and oxidation resistance of those of Co are inferior to that of Ni, and the hardness and wear resistance of Fe are relatively greater than that of Co [14]. Accordingly, the combination of Fe and Ni is considered to be an ideal alternative for replacing of Co because of its low toxicity, low pollution, good wettability with WC and excellent properties [13], [14].

Environmentally friendly cemented carbide that exhibits good performance has become a popular worldwide research topic. Fe-Ni-Co alloy is a promising binder for WC-based cemented carbides, since it has better ductility, greater strength, better wear resistance, greater corrosion resistance and better sinterability at relatively lower temperatures compared with metallic Co as a binder [8], [15]. Moreover, the properties of Fe-based cemented carbides can be further improved with heat treatment or deep cryogenic treatment [16], [17]. Recently, WC-Fe-Ni-Co cemented carbides have been widely studied due to their promising properties. Polini and co-workers [12] developed good quality and dense cemented carbides using a new binder phase prepared by sinter-hot isotactic pressing (HIP) with medium-grained WC and pre-alloyed Fe/Ni/Co powders. Chang et al. [15] investigated the sintering behaviour and properties of nanostructured WC-Co-Ni-Fe hard metal alloys and found that the sintered alloys possessed much better corrosion resistance and mechanical properties compared with WC-Co alloys. Zhou et al. [18] proposed a thermodynamic model for the C-Co-Fe-Ni-W system, and the calculated results are in good agreement with the experimental results. Gille et al. [19] reported that cemented carbides using the newly developed Fe/Ni/Co binder alloys could provide an alternative for some industrial applications where improved toughness and fatigue strength are required.

In these studies, it is noted that the changes of the binder composition are shown to have a significant effect on the properties of the cemented carbides [12], [19], [20], [21], [22]. Therefore, it is worthwhile to research the effects of different binder compositions. Su [20] and Zhang [21] et al. studied the changes in the properties of WC-Co-Ni cemented carbides with different Co/Ni ratios and found that when Co/Ni was 4:1, better mechanical properties and corrosion resistance were achieved than with the WC-Co alloys. Additionally, WC-Fe-Ni cemented carbides with different Fe/Ni (9:1, 8.5:1.5, 8:2, 7:3 and 5:5) ratios were also investigated by Schubert and his co-workers [22]. However, until now, only a few systematic studies have been conducted regarding the effects of the binder composition (different Fe/Ni ratios) on the microstructure, magnetic properties, mechanical properties, wear resistance and corrosion behaviour of WC-Fe-Ni-Co cemented carbides. Therefore, it is worthwhile to research the binder composition effects to achieve a better understanding of WC-Fe-Ni-Co cemented carbides with different Fe/Ni ratios towards applying them in industrial applications. In this study, WC-20(Fe-Ni-Co) cemented carbides were prepared with different Fe/Ni ratios and a constant Co content via low pressure sintering using a powder metallurgy technique, and the effects of different Fe/Ni ratios on the microstructure, magnetic properties, mechanical properties, wear behaviour and corrosion behaviour of WC-Fe-Ni-Co cemented carbides were investigated.

Section snippets

Preparation of cemented carbides

The characteristics of the raw powders used in this work are listed in Table 1. To investigate the effects of varying Fe/Ni ratios on the properties of WC-Fe-Ni-Co cemented carbides, the content of the binder phase was maintained at 20 wt%, and the ratio of Fe/Ni was altered while maintaining the Co content in the binder. The nominal compositions of these alloys are given in Table 2. The amount of gross carbon was adjusted to obtain the desired alloys with a two-phase region. The mixed powders

Phase constitution and microstructure analysis

Fig. 2 shows the XRD patterns of the WC-Fe-Ni-Co cemented carbides with different Fe/Ni ratios. As shown in Fig. 2, when the Fe/Ni ratio was between 1 and 1.67, the structure of the Fe-Ni-Co binder phase was mainly a face-centred cubic (fcc) γ-(Fe,Ni) solid solution. When the Fe/Ni ratio was approximately 2.2, a heterophase blend consisting of fcc γ-(Fe,Ni) and body-centred cubic (bcc) α-(Fe,Ni) was observed in Alloy 3. When the ratio was equal to or greater than 3, only bcc α-(Fe,Ni) was

Conclusions

In this work, the effects of different Fe/Ni ratios on the microstructure, magnetic properties, mechanical properties and corrosion behaviour of WC-20(Fe-Ni-Co) cemented carbide with a constant amount of Co were investigated, and the following conclusions can be drawn from the experimental results and discussion:

  • 1)

    With the increase in the Fe/Ni ratio, the crystal structure gradually changed from fcc γ-(Fe,Ni) to bcc α-(Fe,Ni) and more rounded WC grains were obtained, while the WC grain size

Acknowledgements

The authors gratefully acknowledge the financial support provided by Nonferrous Research Foundation of Hunan Province (Grant no. 20120619).

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