an apparatus and method for forming a fusion welded butt joint between thermoplastic pipe sections . thermoplastic tubular insert sleeves having end flanges are inserted into the ends of the pipe sections with the flanges located in abutting relation . a coupling collar mounting a heating device is slid along one of the pipe sections into overlying relation with the butt joint formed by the engaged flanges . the heating device is energized to fusion weld the coupling sleeve to the pipe sections and to the flanges . the insert sleeves internally support the butt joint area of the pipe sections against thermal sagging during the fusion welding , and the flanges in their molten state tend to flow and compensate for dimensional discrepancies in the pipe sections .