ASTM A105 is the standard specification for carbon steel forgings for piping applications including flanges, fittings and valve parts, etc.
ASME B16.5 is a standard for Pipe Flanges and Flanged Fittings that covers flanges sizes from NPS ½” to 24”. In this standard, flanges are classified based on their pressure-temperature rating which is also known as a flange class.
A buttweld fitting is a weldable pipe fitting that allows for change of direction of flow, to branch off, reduce pipe size or attach auxiliary equipment.
ASTM A105 is an ASTM specification dealing with forged carbon steel piping components.