Method for improving residual stress in pipe and apparatus

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.