Graphite Spheroid DegenerationIrregular graphite forms (lump, tadpole, vermicular, angular) due to deviated crystal growth. Excess residual magnesium causes tadpole-shaped graphite; high rare earth (RE) leads to "gray spots" (clustered fragmented graphite); insufficient spheroidizer or excessive Ti/Al results in vermicular graphite.

Graphite FlotationA dense graphite zone appears at the top of thick-walled hypereutectic SGI castings, caused by graphite’s lower density floating upward. High residual RE exacerbates this by promoting explosive graphite formation.

Reverse ChillUnlike typical surface chill, carbides form in the core or hot spots of castings. Excess residual spheroidizers (RE more impactful than Mg) increase undercooling during solidification, triggering this defect.

Subsurface PinholesHydrogen-dominated pinholes (with minor CO/N₂) rise when excess residual Mg boosts hydrogen absorption from green sand. Prolonged holding of spheroidized molten iron also increases pinhole count.

Shrinkage Cavities & PorosityCavities form at last-solidified areas (hot spots, riser necks); porosity appears as tiny interconnected holes. Higher residual Mg/RE directly elevates shrinkage tendency—porosity risk grows with spheroidizer content.

Black DrossMgSiO₃-based dross (lumpy, stringy, fine) forms on casting tops via MgO-SiO₂ reactions. Reducing residual Mg controls dross, while RE (strong oxygen affinity) further mitigates it.

Spheroidization DecayResidual Mg decreases with prolonged molten iron holding; unremoved slag reintroduces sulfur—turning graphite irregular/vermicular/flaky. Low RE in spheroidizers or insufficient dosage worsens decay, though over-adding Mg increases slag and carbides.