Abstract: An induction heating coil for heat-treating a metallic tube by producing compressive residual stress in the outer surface of the metallic tube. The induction heating coil is adapted to be spaced from and surround the metallic tube and comprises a plurality of donut-shaped hollow electric conductor members for inductively heating the metallic tube and a plurality of donut-shaped hollow water tubes including a large number of nozzles for spouting cooling water toward the metallic tube so as to form a uniform water film thereon with the nozzles being inclined with respect to the longitudinal direction of the metallic tube.

Abstract: The invention provides a vibration damping system in which a vibration damper made of materials which have different degrees of plastic deformation and which are integrally or unitarily joined is interposed between an equipment to be vibration-damped and a support for supporting the equipment.

Abstract: With a metal pipe comprising a double-pipe section connected with a single-pipe section, the double-pipe section in which dead water remains has a lesser degree of heat transfer effect so that it cannot be heated to a steady state. According to the present invention, firstly the single-pipe section starts to be heated and after a predetermined time the double-pipe section starts to be heated. After a predetermined time, the heating of the double-pipe section is stopped and then after a predetermined time the heating of the single-pipe section is stopped, whereby improvement of stresses in the inner wall surface of the pipe can be attained.

Abstract: A welding machine is used to heat the outer surface of a butt welded joint of pipe sections such that the weld heat input condition q/t.sup.2 (where q: heat input and t: the thickness of the wall of a pipe) is higher than approximately 0.01 and the relative heat transfer coefficient ht/.lambda. (where h: the heat transfer coefficient between a coolant and the inner surface of a pipe and .lambda.: the thermal conductivity of a pipe) is higher than approximately 2, whereby the residual stresses at the inner surface of the weld can be relieved by utilizing the temperature difference between the outer and inner surfaces of the pipe.

Abstract: The outer surface of a butt-welded joint is build-up welded such that a ratio h/t is higher than approximate 0.2 and a ratio L/.sqroot.Rt is higher than approximate 1, where h denotes height of a build-up welded portion; t, wall thickness of a pipe; L, width of the build-up welded portion; and R, radius of the pipe. As a result, the residual stresses in the build-up welded portion counterbalances the residual stresses in the welded joint, whereby the residual stresses at the inner surface of the welded joint can be relieved.