New Filling Technique and Performance Evaluations of the Cr 3 C 2 −C Peritectic Fixed Point

The Cr 3 C 2 −C peritectic fixed point was investigated to test its capability to serve as a practical high-temperature fixed point. An improved filling technique where C/C sheet works as a wick and graphite paper as a hopper was applied successfully, and the long-term stability of the peritectic cell was evaluated by means of radiation thermometry. The repeatability of the melting point in one day was 7 mK with a melting range of approximately 100 mK. The cell was aged for 7 days, and the evaluated 56 melting temperatures during this period all fall within 90 mK, with a standard deviation of 19 mK. X-ray transmission photos showed that the ingot was filled uniformly in the crucible. After the evaluation of long-term stability, no clear degradation of the ingot shape and no leakage of molten metal were observed. From these results, it can be concluded that the Cr 3 C 2 −C peritectic cell has good stability and robustness, and the new filling technique was established. The impurity effect on the Cr 3 C 2 −C peritectic cell was also investigated by adding tungsten powder to another cell as the impurity component. After the observation of melting and freezing plateaux, the cell was cut in half to analyze the microstructure by means of electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). The high concentration of impurity was observed in the area of the chromium-rich domain (eutectic mixture of Cr 7 C 3 and Cr 3 C 2 ), which suggests that impurities were rejected from the Cr 3 C 2 peritectic phase during the peritectic freezing and were accumulated in the Cr 7 C 3 −Cr 3 C 2 eutectic phase. This explains why the impurity effect is more severe for the Cr 7 C 3 −Cr 3 C 2 eutectic point than for the Cr 3 C 2 −C peritectic point.