Subsurface porosity defects have been identified as hydrogen-nitrogen pores or CO-related pores by many researchers. Based on this finding, efforts have been made at the production site to control a wide range of factors necessary for reducing these defects, however subsurface porosity defects could not always be prevented. To resolve such problems, in this study, films on the internal surface of porosities detected after machining were precisely observed and analyzed by using FE-EPMA, after which the causes of the defects were investigated.
The inner surface of the defects was basically composed of oxide and lustrous carbon films, and sometimes additional oxide films were observed on the dual films. The lustrous carbon films exhibited clear folds or creases, and some of them were broken through or were deformed by dendrite growth. The formation mechanism of the dual layer and its characteristic features could not be explained by authorized theories. But it can be explained by assuming that the films on these defects are generated on the surface of the melt and are entrained in turbulent flow. In addition, many small pores in size of 100 - 200 μm were detected around porosity defects, of which inner surface were composed of lustrous carbon films exhibiting clear folds or creases.
From these features, small pores might be entrained in turbulent flow at the gating system, and some of small pores might rise in cavities and remain trapped under the solidified skin. The subsurface porosity might be formed on these small pores if there is enough hydrogen and nitrogen gas in the melt.
