pipe fitting and welding. Butt weld is the process of welding the ends of pipes or fittings together. It is a weldable steel pipe fitting that allows for branching, changing flow direction, attaching auxiliary equipment, or reducing the pipe size. In the case of stainless steel and carbon steel, the joining components would be butt weld pipe fittings. This would allow pipes, valves, and equipment to be installed on the piping system.