Seger cones, melting points

The hardness of carbides can only be deterrnined by micro methods because of britdeness, the usual macro tests caimot be used. Neither can the extremely high melting points of the carbides be readily deterrnined by the usual methods. In the so-called Priani hole method, a small hoUow rod is placed between two electrodes and heated by direct current until a Hquid drop appears in the cavity. The temperature is determined pyrometricaHy. When high temperature tungsten tube furnaces are used, the melting point can readily be estimated by the Seger-type cone method. The sample may also be fused in a KroU arc furnace and the solidification temperature determined. 

As a rule of thumb, an insulating material is considered a refractory material if its melting or solidus temperature is well above the melting point of pure iron (1539°C), i.e., if it exhibits a Seger s pyrometric cone equivalent of No. 26 or more (Table 10.19). Moreover, the maximum operating temperature of a refractory material is usually 150°C lower than its pyrometric cone equivalent. 

Because of the heterogeneity of their composition and structure, ceramic refractories do not exhibit a uniform melting point. The refractoriness is characterized by the optical determination of the pyrometric cone equivalent (according to Seger), that is, the temperature at which the tip of a cone made of the sample material softens to the point that it touches the base plate. Reference cones with well-established pyrometric cone equivalents at temperature intervals of 10 °C and above, along with the test cones made from the sample material, are heated in the same furnace so that it is possible to make an accurate comparative temperature determination for the softening point of the refractory material to within approximately 20 °C. 

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