A Socket Fusion Joint is distinguished from butt-welding by using custom-shaped and -sized heating plates rather than a basic flat surface. These heads allow for more surface contact, reducing the time needed to heat and fuse the pipe. Socket fusion joins pipe and fittings together, rather than simply joining pipe to pipe. It requires less pressure than butt-welding and is more commonly used on smaller sizes of pipe (4″ or less). Socket welding has additional advantages of requiring less machinery and is more portable than the heavier equipment required for butt fusion.[7]
https://en.wikipedia.org/wiki/Heat_fusion
