Comparison of the friction and wear behaviour of WC–Ni–Co–Cr and WC–Co hardmetals in contact with steel at high temperatures
Highlights
► This work compares the wear resistance of WC–Ni–Co–Cr and WC–Co alloys in contact with steel at high temperature. ► For the same metal content, WC–Ni–Co–Cr alloys present higher wear resistance than those based on WC–Co, in spite of their lower hardness. ► This abnormal behaviour is related to the higher oxidation resistance of the Ni–Co–Cr binder phase.
Introduction
Cemented carbide tools used for hot rolling of steels include both plain WC–Co and WC–Ni–Co–Cr grades. For this application, WC grain sizes range from 3.5 to 7 μm and the binder phase contents are typically higher than 24 vol.% [1], [2]. Apart from hardness and mechanical strength, the oxidation and wear resistance of these alloys are the key for ensuring long tool life and good surface quality of the rolled products. A significant amount of information has been published on the oxidation resistance of WC–Co alloys, correlating the passivation properties of the scales generated in different hardmetal grades with their chemical composition [3], [4], [5], [6], [7], [8], [9], [10]. Thus, it is known that, in WC–Co alloys, passivation improves as the Co/WC ratio increases; a phenomenon which has been related to an increasing presence of CoWO4 tungstates in the mentioned oxide layers [7], [8], [9]. Other authors describe a similar effect for compositions including cubic carbides (i.e., TiC, (Ta,Nb)C) [8]. In this case, oxidation resistance increases due to the presence of more stable oxides in the scales (i.e., Ti, Ta and Nb based oxides). With regard to WC–Ni–Co–Cr alloys, very little information is published either on their oxidation resistance or on their friction and wear behaviour in contact with steel at high temperatures [2], [11], [12], [13], [14], [15]. With regard to the role of Ni additions, it has been reported that the partial substitution of Co by this element in hardmetals leads to poorer oxidation resistance due to the predominance of WO3 oxides in the composition of the oxide scales [9]. However, the effect of the combined addition of Ni and Cr still remains unclear [11]. On the other hand, most hardmetal wear studies are focused on metal cutting applications, which are clearly far from the conditions occurring in hot rolling processes or on pin-on-disc tribological tests performed at very low loads and speeds [13], [14], [15]. Under these premises, this work is aimed at analysing the oxidation and wear phenomena produced in several hardmetal grades (i.e., WC–Ni–Co–Cr and WC–Co based alloys) by friction with steel at temperatures in the range of 725–850 °C.
Section snippets
Experimental procedure
The WC–Ni–Co–Cr and WC–Co compositions selected for this work are given in Table 1. Two different metallic contents were chosen: 25 wt.% for alloys 1 and 3 and 15 wt.% for alloys 2 and 4. Both WC–Ni–Co–Cr compositions (alloys 1 and 2) have a Ni/Co atomic ratio close to 2. Alloys 3 and 4 are plain WC–Co grades and were used as reference materials. Nominally, the Cr content of the binder phase in WC–Ni–Co–Cr alloys is 6 wt.%, which according to Ref. [11] is the minimum amount needed for improving
Results and discussion
As described in a previous work [12], the C content of the alloys has been adjusted in order to avoid the presence of secondary carbides or graphite (Fig. 5). The density and hardness values of these specimens are included in Table 2. These data confirm that the compositions with Co binder phases are significantly harder than those based on Ni–Co–Cr alloys. Porosity volume fractions are, in all cases, below 0.06% (according to ISO 4505 standard). WC mean grain sizes are similar for Alloys 1, 2
Conclusions
A new tribometer has been designed to analyse the wear resistance of WC–Co and WC–Ni–Co–Cr cemented carbides. The general trend found in these alloys is that the wear resistance increases as hardness increases, that is as the metallic content of the cermet decreases. However, if the metallic content of the alloys is kept constant, it has been found that WC–Ni–Co–Cr materials exhibit similar wear resistance to those based on WC–Co in spite of their lower hardness, a behaviour related to their
Acknowledgements
El Centro para el Desarrollo Tecnológico e Industrial (CDTI) is gratefully acknowledged for the financial support of this work.
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