Abstract <p>A pseudobinary phase diagram is constructed using computer modeling for a heat-resistant niobium-alloyed Cr–Ni–Si austenitic steel. Based on this diagram and earlier in situ observations of solidification in highly alloyed austenitic steel, the solidification mechanisms of phases are clarified. For carbon contents up to 0.08 wt %, an identical sequence of solid–phase growth mechanisms is established for the solidification of both the weld metal and a ladle sample taken during the melting of Fe–15Cr–9Ni–3Si–Nb electrodes for electroslag remelting. A dendritic growth boundary is constructed on the phase diagram using the concept of discontinuous solid solution solidification, which enables the interpretation of the observed primary structure morphology. Prior to the peritectic reaction, a highly branched framework of skeletal δ ferrite dendrites is shown to develop into a classical dendritic form through layer-by-layer solidification of excess high-temperature δ ferrite. The composite dendrite consists of high-temperature δ ferrite and contains two internal zones separated by a high-angle boundary with different microsegregation characteristics due to dendritic and layer-by-layer solidification mechanisms. Metallographic analysis shows that the “one dendrite–one grain” principle is maintained on a macroscale during solidification for each composite dendrite. This defines the concept of a primary (as-cast) grain for the composition under study. The cubic and blocky niobium-rich micron-sized carbide phases detected in the weld metal are found to be transferred (inherited) from the welding consumables. The identical morphology of the primary solidification structures in the regions between adjacent weld beads is shown to be caused by the same solidification features occurring under far-from-equilibrium conditions.</p>