| Hard metals (HM) are widely used in industrial applications since 1920s due to their unique properties such as relative high hardness, toughness, and fatigue resistance. Hard metals consist in their simplest and most common form of a hard phase, for example, tungsten carbide (WC), and a ductile binder phase, for example, cobalt (Co). The properties are mainly dependent on their overall chemistry which itself determines the fraction of tungsten (W) and carbon (C) dissolved in the Co binder phase after sintering. The fraction of W and C solutes in the Co binder phase strongly affects the grain growth and microstructure and, consequently, both the performance as well as the magnetic properties of the hard metal. Therefore, magnetic saturation measurements are used in industry as a fast, reliable and non-destructive method to assess consistency of properties and performance of hard metals. More specifically, magnetic saturation measurement is a means of locating hard metals within the so-called carbon window in the W-C-Co phase diagram, that is, in which only the desired phases exist. These measurements are not only used as quality control in production, but they are equally crucial for the control of several production processes as well as in the development of compositions of new hard metals. However, a well-established relationship between the binder phase composition and the overall properties and performance of hard metals is required to determine the location of samples within the carbon window. The accuracy needed for estimating the C content from the magnetic saturation measurements for samples prepared under closely comparable conditions is about 0.01%.[1] Roebuck et al.[2,3] assessed an empirical equation[4] describing the relationship between the measured magnetic saturation of an insert and that of pure cobalt as a linear function of the tungsten weight fraction dissolved in the Co binder phase. In this empirical equation, it was assumed that the measured magnetic saturation is not affected by the dissolution of C in the Co binder phase.[2,3] This assumption seems realistic since the solubility of C in Co is very limited.[5] The success of the non-destructive measurement of the magnetic saturation and the reliability of the empirical equation by Roebuck for production control is based on decades of experience, the collection and comparison of a tremendous amount of experimental data including those from magnetic saturation measurements, materials analysis and performance testing. Gathering the required amount of information is both expensive and time consuming. Furthermore, the empirical relationship is only valid for cemented carbides consisting of WC-Co without other alloying elements and whenever a new alloying element is added to the Co binder, a new empirical relation must be established, again based on experimental data collection. Due to the ever increasing requirements on quality and performance as well as on replacing Co as binder phase, for example, due to its increased usage in batteries, a more fundamental understanding of the magnetic properties of hard metals is imperative in order to speed up the process of finding alternatives to Co as the binder phase. |
