The interior of a straight pipe is clad using a plurality of welding heads. The pipe is carried on support rollers including drive components for rotating the pipe about its axis while holding it fixed against axial movement. There are four welding heads with two carried on a cantilevered support arm of a respective one of a first and a second carriage mounted outside the pipe at opposite ends of the pipe and each movable in a direction longitudinally of the axis of the pipe. The heads are located on the arm at a common angular position at bottom dead center to weld vertically downwardly. The pipe support includes a pair of drive rollers arranged with their axes parallel to the axis of the pipe and adjustable in spacing to change the height of the pipe on the rollers relative to the welding heads.