Method for improving residual stress in pipe and apparatus

Abstract

An method for converting tensile residual stress on an inner surface of a welded part of a pipe to compressive residual stress with use of ice plugs formed by cooling the pipe from the outer surface, comprises the steps of: placing refrigerant containers for forming the ice plugs in the upstream and downstream of a butt-welded part; cooling the outer surface of the pipe to form the ice plugs; and then cooling the outer surface of the pipe with use of at least one refrigerant container for expanding the pipe arranged between the refrigerant containers for forming the ice plugs. When the method is applied to a pipe with a large inside diameter, the method comprises forming the ice plugs at a curved pipe, an elbow or a branch pipe in the upstream and downstream of the butt-welded part.

1 . A method for improving residual stress in a pipe, comprising:
 cooling a first outer surface of the pipe to form respective ice plugs inside the pipe,   wherein the first outer surface is on both sides of a butt-welded part of the pipe, and   wherein the pipe is a curved pipe, an elbow pipe, or a branch pipe;   cooling a second outer surface of the pipe to expand the butt-welded part and vicinities of the butt-welded part,   wherein the second outer surface is between the ice plugs; and   measuring an amount of strain of a vicinity of the butt-welded part generated during the expansion of the pipe.   
     
     
         2 . The method according to  claim 1 , further comprising:
 stopping the cooling of the outer surfaces of the pipe, based on the amount of the strain measured, and stopping the expansion of the pipe.   
     
     
         3 . The method according to  claim 1 , further comprising:
 measuring a circumferential strain of the butt-welded part and the vicinities of the butt-welded part generated during the expansion of the pipe.   
     
     
         4 . The method according to  claim 1 , further comprising:
 placing a strain gauge on an edge preparation part in the vicinity of the butt-welded part to measure a circumferential strain of the edge preparation part, the edge preparation part having a partially-thin pipe thickness.   
     
     
         5 . The method according to  claim 1 , wherein in the step of cooling the first outer surface of the pipe, a refrigerant including dry ice is used to cool the first outer surface of the pipe on both sides of the butt-welded part to form the ice plugs inside the pipe. 
     
     
         6 . The method according to  claim 1 , wherein the pipe is expanded until the amount of strain in the vicinity of the butt-welded part and in respective circumferential directions becomes greater than or equal to 0.4%