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The aim of the study was to consider the crystallization interval of dental CoCrMo alloys with different carbon contents as well quantitative and qualitative analysis of changes in their microstructure. The two different batches of Wironit extra-hard alloy (0.413 and 0.806% C by mass.) were tested. The chemical composition has been carried out as well as microstructure characterization (optical, scanning electron microscopy and X-ray diffraction). A quantitative and qualitative image analysis was performed. The process of differential thermal analysis was carried out in the Balzers VSG-02 vacuum induction furnace. The presence of the Co-β solution matrix and M23C6 carbides was demonstrated as well as Co-α solution. Along with the change in the content of carbon in the castings, a qualitative change in the microstructure was noted. Low carbon castings are dominated by blocky eutectic-type precipitates, whereas in high carbon castings, blocky dense precipitates dominate. The results showed that the carbon content influences the crystallization parameters of the alloy (in particular increasing the Tliq and decreasing the Tsol with lower carbon content). It was shown that the melting point of the alloy was higher than indicated by the manufacturer (1533–1578 K). For this study, the range of 1635–1660 K was obtained. This occurrence is very disadvantageous. Using the recommended pouring temperature of 1693 K for casting the material, which usually has a carbon content of about 0.4%, a low degree of superheat is obtained, which significantly limits or even excludes the castability of the alloy.