WELDED PIPE AND PIPE WELDING METHOD

A bonded pipe produced by using the liquid phase diffusion process in which the pipe ends of pipes to be bonded are butted with an insert material lower in melting point than the pipe material against each other, wherein the thickness is increased in such a manner that the outside diameter of the butted portion is expanded and the inside diameter thereof is reduced. A method for bonding pipes in which pipe ends of the pipes are butted through an insert material lower in melting point than the pipe material against each other, and the butted portion is heated to and held at a holding temperature higher than the melting point of the insert material, whereby the insert material is diffused in liquid phase into the pipe material, characterized in that at the time point when the outer pipe face temperature of the above-mentioned butted portion reaches a value which is not less than the A1 critical temperature of the pipe material and not more than the holding temperature in the process for raising the temperature of the pipe material, the application of a pressure in the pipe axial direction capable of plastically deforming the butted portion to the above-mentioned butted portion is started to increase the thickness of the butted portion.