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.

Description

CLAIM OF PRIORITY[0001]The present application claims priority from Japanese patent application serial No. 2009-294016 filed on Dec. 25, 2009, the content of which is hereby incorporated by reference into this applicationBACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates to a residual stress improving method for a pipe, and relates specifically to a residual stress improving method for a pipe suitable in improving the residual stress in the vicinity of a welding section of a pipe.[0004]2. Description of the Related Arts[0005]With respect to a pipe used for a power plant, the residual stress occurs in the vicinity of the welding section when welding is performed. Also, a stainless steel pipe or a nickel alloy pipe is commonly used for a pipe for high temperature water considering corrosion resistance. With respect to a pipe using stainless steel, nickel alloy and the like, when it is exposed to high temperature pure water for a long time whi...

AI Technical Summary

Benefits of technology

The present invention is a method for improving the residual stress in a pipe by applying an axial load on a pipe, raising the pressure inside the pipe, and performing plastic deformation on a stress improving region. This method can create large compressive residual stress on the inner surface of the pipe, prevent stress corrosion cracking, and improve the overall condition of the pipe. The method can be easily detected using a strain gauge.

Problems solved by technology

With respect to a pipe using stainless steel, nickel alloy and the like, when it is exposed to high temperature pure water for a long time while being applied with tensile residual stress at the welding section of the pipe, stress corrosion cracking may possibly occur.

Therefore, according to the diameter and thickness of the pipe, the residual stress of the pipe may not be sufficiently improved to the degree stress corrosion cracking is prevented.

In this case, according to the magnitude of the tensile load applied, the residual stress of the pipe may not be sufficiently improved to the degree the stress corrosion cracking is prevented.

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