PURPOSE: To turn the residual tensile stress of the inside surface of a joint to compressive stress and prevent stress corrosion cracking by solidifying and expanding the material sealed in the inside by cooling means disposed in the welded joint part or on the outer circumference of both sides then melting the same.
CONSTITUTION: A material which turns solid and expands when cooled, for example, water 4, is packed in piping. Next, cooling means 5, 5' are disposed on the outer side on both sides of a welded joint part 3, the water 4 is cooled. Then, the water near the inside circumference of the means 5, 5' is cooled to ice and ice plugs 6, 6' are formed. Further when the cooling by the means 5, 5' is continued, the plugs 6, 6' grow gradually so that the internal pressure of the water 4 confined by the plugs 6, 6' is boosted. The pipe diameter is expanded by the internal pressure thereof, whereby a plastic deformation is caused in the joint part 3 and near the same. When the cooling is ceased after the initiation of the plastic deformation and the plugs 6, 6' are melted to release the internal pressure of the water 4, the residual tensile stress in the part 3 turns to compressive stress.
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