Heat treatment method and device for piping

Abstract

In a method for heat treatment of an existing pipe constituting a piping system, for converting the residual stress at a welded metallic portion and a welding heat influenced portion of the inside surface of the pipe, into a compressive stress and thereby generate the compressive stress in the inside surface of the existing pipe, a coolant is retained in the pipe; an arbitrary portion of the outside surface of the pipe is heated; thereby a temperature distribution little in temperature difference is produced in the wall surface of the pipe at the heated portion; and then the coolant is allowed to flow. By converting the residual stress at the welded metallic portion and the welding heat influenced portion of the inside surface of the pipe, into the compressive stress, stress corrosion cracking generated from the welded metallic portion and the welding heat influenced portion can be suppressed.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a method for converting a tensile residual stress existing in an inside surface of a pipe constituting a piping system of a plant, into a compressive residual stress to suppress stress corrosion cracking. [0003] 2. Description of the Related Art [0004] In a welded portion of stainless steel or nickel-base alloy steel, chromium carbide is separated out at a crystal grain boundary due to heat of welding. As a result, a chromium-lack layer is produced near the pole of the grain boundary, and sensitization (a phenomenon that the sensitivity to corrosion increases) occurs in the chromium-lack layer. On the other hand, a high tensile residual stress is generally generated in the surface near a welded portion of a pipe constituting a piping system of a plant. Therefore, if the pipe is used in a severely corrosive environment in a state wherein the material has been sensitized, stress corros...

AI Technical Summary

Benefits of technology

[0007] An object of the present invention is to suppress the generation of stress corrosion cracking of an inside surface of a pipe of an existing pipe welded joint for a long time by a simple method.

[0008] According to the present invention to attain the above object, a method for heat treatment of a pipe constituting a piping system of a plant, have a characteristic feature that an arbitrary portion of the outside surface of the pipe is heated in a state wherein a coolant is retained in the pipe, so as to make a gentle temperature gradient in the pipe wall; and then, by allowing the coolant to flow in the pipe, the temperature gradient in the wall of the pipe at the heated portion is changed into a steep temperature gradient near the inside surface of the pipe. After the pipe is heated, a difference in temperature is made in particular between the inside and outside surfaces of the pipe. By rapidly cooling the inside surface of the pipe, the inside surface of the pipe is tensile-yielded and the outside surface of the pipe is compressive-yielded. When the temperatures of the inside and outside surfaces of the pipe are equaled to each other, the tensile residual stress in the inside surface of the pipe is reduced.

[0011] According to the present invention, after a piping system of a plant is assembled, a compressive residual stress can be generated in the inside surface of a pipe constituting the piping system, in particular, near a welded metallic portion. As a result, stress corrosion cracking in the piping system can be prevented.

Problems solved by technology

Therefore, if the pipe is used in a severely corrosive environment in a state wherein the material has been sensitized, stress corrosion cracking occurs.

That is, when three factors of sensitization of the material, a high tensile residual stress, and a corrosive environment, are superposed, the risk of stress corrosion cracking increases.

However, because the pipe is heated without cooling the inside surface of the pipe, it is required to take in / out a coolant on the whole of a piping system when applied to the piping system of an existing plant.

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