Socket weld fittings
To connect one tube to another: socket, nipple, etc.
To change the direction of the pipe fittings: elbow, return, etc.
To change the pipe diameter on the pipe line: reducer, etc.
To increase the pipe branches of the pipe line: tee, cross, etc.
To make an end of the pipeline: plug, cap, etc.
To fix pipes: coupling, hanger, etc.