Residual stress improving method for pipe

Abstract

It is an object to provide a residual stress improving method for a pipe by imparting larger compressive residual stress on the pipe to sufficiently reduce tensile residual stress in order to prevent the stress corrosion cracking. With respect to a stress improving region where the residual stress of a pipe is to be improved, a load in the axial direction of the pipe is made such stress making axial strain of the outer surface of the pipe 0% or above and being yield stress of the pipe or below, and internal pressure of the pipe is raised. The pipe is plastically deformed and is expanded in the radial direction by the internal pressure. After the internal pressure is raised to the degree the pipe is plastically deformed, the internal pressure and the axial load are removed, and thereby compressive residual stress is imparted to a welding section and a heat affected zone which are the stress improving region of the inner surface of the pipe.

1 . A residual stress improving method for a pipe comprising the steps of:
 applying an axial load on a pipe with respect to a stress improving region having tensile residual stress in the pipe filled with fluid inside;   raising the pressure inside the pipe; and performing plastic deformation on the stress improving region,   wherein the axial load on the pipe is in such a range of stress making axial strain at an outer surface of the pipe 0% or above and being yield stress of the pipe or below.   
     
     
         2 . The residual stress improving method for a pipe according to  claim 1 ,
 wherein the stress improving region includes a welding section of the pipe and a heat affected zone affected by the welding section.   
     
     
         3 . The residual stress improving method for a pipe according to  claim 1 ,
 wherein the stress improving region includes a pipe expansion section of the pipe.   
     
     
         4 . The residual stress improving method for a pipe according to  claim 1 , wherein pressure inside the pipe and the axial load of the pipe are removed after the pipe is plastically deformed. 
     
     
         5 . The residual stress improving method for a pipe according to  claim 4 , wherein the pressure inside the pipe is removed at a time point when an outer surface of the pipe starts to be plastically deformed after an inner surface of the pipe starts to be plastically deformed. 
     
     
         6 . The residual stress improving method for a pipe according to  claim 4 , wherein pressure inside the pipe is removed at a time point when circumferential strain of the outer surface of the pipe reaches approximately 0.5%. 
     
     
         7 . The residual stress improving method for a pipe according to  claim 5 , wherein plastic deformation is confirmed by measuring variation of circumferential strain with respect to axial strain at the outer surface of the pipe. 
     
     
         8 . The residual stress improving method for a pipe according to  claim 7 , wherein a detection position of the circumferential strain at the outer surface of the pipe is in the vicinity of the welding section and a detection position of the axial strain is in a region other than the vicinity of the welding section.