Gas defects are roughly classified into blowhole defect, pinhole defect, and microporosity defect. Of those defects, blowhole defect is a defect produced by a macrographic factor, and generates comparatively large gas bubbles. Namely, the main causes of this defect are entrainment of air by turbulent flow of molten metal during pouring, and entrainment of gas produced from molds, gas produced from slag and dross, etc. Moreover, although pinhole defects are comparatively small gas bubbles, there are cases where they merge and grow into large gas bubbles, to eventually turn into blowhole defects.
The factors causing pinhole defects are of (A) physical type, (B) oxidation reaction type, (C) dissolution type, (D) slag formation type, and (E) other types. The oxidation reaction type is subclassified into 2 kinds; i.e., (B-1) solidification film oxidation type produced on consolidation skin, and (B-2) remaining (residual) liquid oxidation condensation type produced inside the casting. The slag formation type is subclassified into 4 kinds; i.e., (D-1) manganese silicate based slag, (D-2) inoculant-spheroidizer based slag, (D-3) manganese sulfide based slag, and (D-4) sand inclusion-(foreign matter) inclusion based slag. Microporosity defect is a defect involving extremely small gas bubbles, generating a gas dissolved in the molten metal among the gaps between dendrites.