The powder of the Ni-Cr-Si-B system was applied on tubes of low carbon steel (0.2 % C) with the help of air-plasma spraying. The effect of heating temperature on the microstructure of obtained coatings was analyzed in this paper. The coatings were heated up to 500, 600, 700, 800, 900, 1000, and 1100 °C and cooled in the air. It was shown by the method of optical and scanning electron microscopy (SEM) and the X-ray diffraction analysis (XRD) that the same phases (γ-Ni, Ni3B, Ni3Si and Cr2B) were present in the coatings before and after heating. Chromium carbide Cr7C3 was observed in the coating structure after heating to above 1000 °C. Coatings which were heated to 1000 °C consisted of grains of the solid nickel base solution, micro volumes of the Ni+Ni3Si eutectic with inclusions of Ni3B and particles of Cr7C3. The coatings which were crystallized from a liquid state (heated up to 1100 °C) consisted of large areas of the solid nickel base solution and particles of chromium carbide Cr7C3. The space between nickel areas is filled by the Ni3Si+Ni3B eutectic.