Comparative hydraulic soundness tests have been carried out on iron castings with different graphite forms, produced by the in-mold treatment process using 0. 2-1% of master alloy ZhKMK-4R, which contains 8. 3% Mg, 9. 2% Ca, 6. 7% REM, 47% Si, and remainder Fe. The original steelmaking pig iron contained 4. 13% C, 1. 05% Si, 1. 04% Mn, 0. 087% P, 0. 031% S, and melts were made in an acid-lined induction furnace of capacity 60 kg. The bath was heated to 1500 degree C and the molds were poured at 1450 degree C. The cylindrical castings - 30-mm diameter test bars of length 200 mm - were made in baked clay-bonded sand molds. The nodular graphite form ensures maximum hydraulic soundness.