Flat flanges have a gasket surface in the same plane as the bolting circle face. It is also known as flat face flange. Applications using flat face flanges are frequently those in which the mating flange or flanged fitting is made from a casting. Flat face flanges are used when the counter-flanges are flat face. This condition occurs mainly on connection to Cast Iron equipment, valves, and specialties. ASME B31.1 says that when connecting flat face cast iron flanges to carbon steel flanges, the raised face on the carbon steel flange must be removed, and that a full face gasket is required.