The structural instability of Fe-Cr-Ni-based superalloys and the mechanism of softening at operating temperatures were studied. We investigated the continuous changes in the structure and kinetics of dissolution of initial phases with formation and dissolution of intermediate phases of the cast superalloy 0,45C-26Cr-33Ni-2Si-2Nb under long-time operation at high temperature. The character and the sequence of phase transformations in Fe-Cr-Ni-based alloys at a temperature of 0,85-0,90 of their solidus, also under external loading, were found out. The structural instability of Fe-Cr-Ni-based superalloys and the mechanism of softening at operating temperatures were studied