Effect of post-treatment of cemented carbide : Microstructural investigation and evaluation of mechanical properties of cemented carbide inserts for rock tool applications
Abstract: Cemented carbides are used in rock tool applications by reason of their beneficial mechanical properties, i.e. a combination of high toughness, wear resistance and hardness. The cemented carbides commonly receive their hardness from tungsten carbides (WC) whereas cobalt (Co) is the dominating binder material. To obtain a beneficial balance of these desired properties, the binder phase can be strengthened by post-sintering treatment to minimize the risk of catastrophic failure. Previous work has shown that the face centered cubic (FCC) cobalt phase is stabilized by a higher degree of dissolved tungsten in the binder. The residual stresses have been related to both further stabilization of the FCC and local transformation to a hexagonal close packed (HCP) phase. By exposing the inserts to mechanical deformation, i.e. inducing stress, the Co-binder material could theoretically transform from FCC to HCP. In this master thesis, five cemented carbides with three different binder phase compositions have been investigated. Additionally, the influence of the carbon content and addition of an alloying element was evaluated. This was done by electron backscatter diffraction (EBSD) analysis of the microstructure of each sample in as-sintered state and after post-treatment to gain better understanding on how the phase stability of the binder is affected by a post-sintering process. In addition, magnetic measurements, hardness-testing and mechanical tests were performed on the materials to evaluate the effect of using different binder compositions. It was found that a low carbon content provides smaller and more rounded WC grains and binder phase dominated by FCC-Co compared to an equivalent material with a high carbon content. A low carbon content furthermore resulted in a more wear resistant material. The addition of alloy MA presented a greater increase in surface hardness after post-treatment and made the materials even more wear resistant compared to the non-alloyed. The non-alloyed material performed better in a test measuring the fracture energy. A material with an Fe-based binder showed the least increase in surface hardness and the highest increase and overall fracture energy. All materials experienced a WC grain size refinement and an increase in their surface hardness after post-treatment. Only one material gave an indication on FCC to HCP phase transformation, no conclusions could be drawn from the other samples.
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