Gas discontinuities are reduced to flat discs or other shapes parallel to the surface by which process?

• A. Rolling
• B. Machining
• C. Casting
• D. Welding

Answer: (A)

Gas porosity inside metal responds to the way the material is deformed. Rolling applies predominantly compressive and shear forces that flow the metal into a thin sheet, pushing voids to close and flatten along the direction of flow. This causes gas discontinuities to become flat, disc-like shapes with their flat faces parallel to the surface. The constraint in the thickness direction during rolling makes the pores assume that planar form, rather than remaining roughly spherical. Machining, casting, and welding don’t produce that same controlled flattening effect, so they don’t reliably create pores as flat discs parallel to the surface the way rolling does.