Capped steel is characterized by which of the following properties?

• A. Outer shell relatively high in iron with minimal gas porosity, but less than rimmed steel.
• B. Outer shell has high porosity.
• C. Outer shell is low in iron with significant porosity.
• D. Outer shell is identical to rimmed steel in porosity.

Answer: (A)

This question tests how the casting method affects the surface characteristics of steel, specifically the iron content of the outer shell and the amount of gas porosity. In capped steel, the outer shell tends to be relatively high in iron and has minimal gas porosity, with porosity less than that found in rimmed steel. The cap helps limit gas entrapment during solidification, producing a denser, cleaner surface layer. Rimmed steel, on the other hand, develops a porous outer region due to gases escaping unevenly as the melt solidifies, creating more surface porosity. So the best description is an outer shell that is rich in iron with little porosity, but still less porous than rimmed steel. The other options describe scenarios that don’t match capped steel’s characteristic reduction in porosity or its surface composition.