Abstract: In the manufacture of spiral-welded steel pipe, material strip or plate is fed in its longitudinal direction and continuously formed into spiral pipe form. One side-edge of the strip and the other side-edge of the strip that has already been spirally formed are welded together by the electric-resistance method where they meet. The edges of the strip and formed pipe are held together simultaneously with or immediately after the electric-resistance welding from both inside and outside the pipe using a pair of pressure rolls. The bead formed as a result of the electric-resistance welding is reformed simultaneously with or immediately after the holding by the pressure rolls is accomplished. Then, the electric-resistance welded side-ends are welded again by the arc method.

Abstract: First, a rectangular metal plate having thinner and thicker portions corresponding to a lighter-wall portion and heavier-wall portion of a pipe to be manufactured is prepared. Before bending the plate until both edges thereof meet to form a tubular piece, the plate width is adjusted by cutting off part of the thinner portion so that the desired peripheral lengths are attained in both lighter- and heavier-wall portions of the pipe. The width-adjusted plate is then bent until both edges are butted together to form a tubular piece. By butt-welding the adjoining edges, a metal pipe with longitudinally differentiated wall thickness is obtained.

Abstract: In the manufacturing of a spiral pipe, there is first determined a maximum curvature which should be imparted to a strip of pipe forming material in order that a desired residual moment will be imparted to the finished pipe after the strip bent to the maximum curvature is allowed to spring back from the bent condition, which maximum curvature is determined based on the thickness, Young's modulus and yield stress of the strip, the desired curvature of the finished pipe, and the residual moment to be imparted to the finished pipe. By adjusting the position of three rows of forming rolls positioned at the apexes of a triangle, the flat strip is continuously bent into a spiral that has the thus determined maximum curvature.