Abstract: For reducing the diameter of any axially limited part of a metal tube, that part is heated externally by an annular heater while at least the adjacent part or parts of the metal tube are being cooled and held at a temperature sufficiently low to limit the free thermal expanison of the heated part. On cooling, the part that has been heated becomes less in diameter than the other portions of the metal tube. The complete length of the metal tube can likewise be reduced in diameter by moving the annular heater and the metal tube relative to each other in the longitudinal direction of the tube from one end to the other while at least the adjacencies of the part being heated are being cooled as by water. The same method is adaptable for the fabrication of wear-resistant dual wall pipe for the transportation of fluids, as is also disclosed herein.