in a method of forming tubes or pipes from a flat blank of a plastically deformable material , three spaced apart portions of the blank are first bent to the wanted curvature of the finished tube or pipe , so that two smaller unbent portions are obtained between the curved portions . then , the blank is closed to essentially tubular form by means of forces acting on the curved portions , so that the principal deformation in connection with the closing is obtained in the unbent portions located between the curved portions .