A mathematical model is provided for phase transformation thermodynamics making comprehensive study possible for the equilibrium phase composition of chromium-nickel welded joints at different temperatures. According to the model, with a chromium content of more than 22 wt.% the temperature for the start of intermetallic precipitation exceeds the solidus temperature for Cr–Ni alloy, i.e., in this case precipitation of primary Ni3Al intermetallic is thermodynamically possible. Primary Ni3Ti precipitates are probable with a chromium content of more than 36 wt.%. Model calculations are confirmed by microstructural studies. In order to improve welded joint strength for the alloy in question, slow joint cooling in the range below 1000°C is recommended, depending on welded structure operating temperature, and ageing in the range of intense γ′-phase precipitation, i.e., 900–600°C. The mathematical model developed for phase transformation thermodynamics may be used for studying the phase composition of other chromium-nickel alloys.