KR20140112983A - Fabrication method of stream generator tubing with stress corrosion cracking - Google Patents

Abstract

The purpose of the present invention is to provide a method for manufacturing a stress corrosion crack tube for a steam generator with a nondestructive signal property which is similar to a stress corrosion crack generated in a nuclear power plant field by removing a sensitization tissue by additionally and thermally treating the tube manufactured with an existing room temperature crack manufacturing method. For achieving the purpose, the method for manufacturing the stress corrosion crack tube for the steam generator includes the steps of manufacturing the stress corrosion crack tube for the steam generator by the room temperature crack manufacturing method; removing the sensitization tissue of the tube through a solution treatment or a recovery thermal treatment; and removing a surface oxide film of the tube through a mechanic polishing or chemical cleaning process.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to steam generator cracking,

The present invention relates to a method for manufacturing steam cracking cracked heat transfer tubes, and more particularly to a method for manufacturing a heat transfer tube having a pure grain boundary stress corrosion crack having a shape similar to a field crack occurring on the outer surface of a steam generator heat transfer tube of a power plant, will be.

Currently, various types of stress corrosion cracking defects are occurring in domestic and overseas NPS steam generator tubes.

Development of nondestructive inspection technology for such defect heat pipes and prediction of leakage rate and burst pressure are very important items for establishing safe operation method of nuclear power plant.

Therefore, it is necessary to measure the rupture pressure through leakage and rupture tests based on the results of nondestructive tests during the operation of the nuclear field defect heat pipe, and to establish a prediction model by comparing and analyzing the actual crack information obtained through the rupture inspection of the ruptured pipe.

For this purpose, a large number of samples of natural defects occurred in the heat transfer tubes in the field are required, but there is a limitation that it is very difficult to obtain sufficient sample volume as well as the exposures of researchers during leakage and rupture experiments.

In the case of a defect heat pipe manufactured by a mechanical method such as an electric discharge machining method which is generally easy to manufacture and handle, there is a disadvantage in that it can not represent the nondestructive inspection signal, rupture pressure, and leakage characteristic of a natural defect occurring in an actual site.

Therefore, it is required to develop a technology to implement and manufacture an artificial defect having a nondestructive signal characteristic similar to a natural defect in a heat transfer tube.

In order to simulate the natural defects of the steam generator tube, various experimental methods have been developed. Typical examples are room temperature cracking technology.

At room temperature cracking, the inner or outer surface of the heat transfer tube is exposed to an acidic corrosion solution at room temperature, a high pressure gas is injected into the heat transfer tube, or an external force is applied to apply tensile stress to the inner or outer surface of the heat transfer tube, It is a technique to make cracks.

The patents utilizing the room temperature cracking method include a method of exposing a heat transfer tube to an acidic solution at room temperature, applying a radial denting load, and applying an external force by combining a pressure-proof load method and a tensile load method. Korean Patent Registration No. 10-1063344 discloses a method for locally causing stress corrosion cracking by exposing said acidic solution to a specific position of a heat transfer pipe.

In the above-mentioned conventional invention, there is an advantage that a stress corrosion crack can be produced in a short period of about 1 to 4 weeks. However, in order to induce stress corrosion cracking in an acidic corrosion environment, the heat transfer pipe material is subjected to a sensitizing heat treatment. There is a problem that the nondestructive signal is distorted by the tissue.

Generally, a nickel-based alloy used as a nuclear steam generator heat conduction tube is subjected to a sensitizing heat treatment at a temperature of 600 to 700 ° C. for several hours to several hours in order to produce a stress corrosion crack at room temperature. At this time, Chromium reacts to form a chromium carbide at the grain boundary interface.

Therefore, the concentration of chromium around the interface is lower than the average chromium concentration on the inner surface of the crystal. Therefore, in an acid corrosion solution at room temperature, anodic reaction such as elution of metal ions occurs at a low chromium concentration, A cathodic reaction such as hydrogen generation occurs, so that minute cracks are initiated in the crystal grain boundaries.

In this case, stress corrosion cracking occurs when tensile stress is generated on the inner or outer surface of the heat transfer tube by pressure-proof load method or tensile load method.

Therefore, the heat transfer tube manufactured through the room temperature cracking method finally has a structure that is internally sensitized, that is, the chromium concentration in the grain boundary is low and the nickel concentration is high.

Since nickel is magnetized, it is magnetized during non-destructive testing such as eddy current test, distorting the signal, and crack length and depth predicted from amplitude and phase angle of nondestructive signal are different from actual ones. There is a problem that it may lead to errors in the leak rate and burst pressure prediction formula.

Korea Patent No. 10-0579399 Korean Patent No. 10-1063344

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in order to solve the above-mentioned problems, and it is an object of the present invention to provide a heat transfer tube manufactured by a conventional room temperature cracking manufacturing method by removing the sensitized structure through additional heat treatment, The present invention provides a method of manufacturing a stress corrosion cracking heat transfer tube having a non-destructive signal characteristic.

According to another aspect of the present invention, there is provided a method of manufacturing a stress corrosion cracking heat transfer pipe of a steam generator, the method comprising: a step of manufacturing a steam cracking cracking heat transfer pipe through a room temperature cracking process; A step of removing the annealed tissue by heat treatment or heat treatment; And removing the oxide film on the surface of the heat transfer pipe through mechanical polishing or chemical cleaning.

INDUSTRIAL APPLICABILITY As described above, the method of manufacturing a steam cracking stress cracking heat transfer pipe according to the present invention has the following effects.

First, through the present invention, an artificial defect having a nondestructive signal characteristic similar to a stress corrosion crack generated in an actual nuclear steam generator tube can be manufactured in a heat transfer tube and used for nondestructive inspection.

Second, based on the results of nondestructive tests during the operation of the field defect heat pipe, it is possible to measure the burst pressure through the leakage and rupture test, and to compare the information of the actual crack obtained through the fracture inspection of the ruptured heat pipe, .

Third, it can be used as a standard specimen for the development of nondestructive inspection technique and crack detection ability for inspection during operation of steam generator tube.

1 is a flowchart showing a method of manufacturing a steam cracking stress cracking heat transfer pipe according to the present invention,
FIGS. 2A and 2B are graphs of C scan and Lissajous obtained from the eddy current test for a heat transfer tube manufactured by a normal temperature cracking method after the annealing heat treatment,
FIGS. 3A and 3B are C scan and Rissaju screens obtained through an eddy current test after a heat treatment process of a heat transfer pipe,
FIGS. 4A and 4B are C scan and Risaj views obtained by removing the surface oxide film of the heat transfer pipe by mechanical polishing and chemical cleaning, and by conducting an eddy current test.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

1 is a flowchart showing a method of manufacturing a steam cracking stress cracking transfer tube according to the present invention.

As shown in this drawing, the method for manufacturing a steam generator stress corrosion cracking heat transfer pipe according to the present invention comprises: a step of manufacturing a steam cracking stress cracking heat transfer pipe by a room temperature cracking method; A step of removing the annealed tissue by heat treatment or heat treatment; Mechanical polishing or chemical cleaning to remove the oxide film on the surface of the heat transfer pipe.

That is, a method of manufacturing a steam cracking cracking heat transfer pipe according to the present invention includes: an additional heat treatment step of performing at least one of a solution heat treatment and a recovery heat treatment for removing a sensitized structure from a heat transfer tube manufactured by a conventional room temperature cracking method; Removing the oxide film on the surface of the heat transfer pipe and on the inner surface of the defect.

Here, the solution heat treatment step as one of the means for removing the sensitized structure in the heat transfer tube is performed for a few minutes to over the dissolution temperature of carbon given as [Equation 1] for Alloy 600 material.

The solution heat treatment is intended to dissolve the carbide at the crystal grain interface to recover the chromium concentration close to the average value.

Equation 1

 T = 1,449 + 130.3 x ln (% C)

(T is the minimum temperature for solution heat treatment (° C),% C is the average carbon concentration of the alloy)

Also, the recovery heat treatment step is performed for several hours to several hours at 700 to 750 ° C as one of means for removing the sensitized structure inside the heat transfer tube.

The recovery heat treatment does not dissolve the carbide at the grain boundary, but aims at restoring the chromium concentration around the crystal grain interface to near the average value by increasing the diffusion rate of chromium around the grain boundary interface within the crystal grain.

Although the solution heat treatment and the recovery heat treatment step may be performed sequentially, it is noted that only one step may be performed depending on the carbon concentration of the heat transfer pipe material and the distribution characteristics of the carbide.

Further, in order to confirm the removal of the sensitized tissue, an optical microscope, a scanning electron microscope, a transmission electron microscope or a Huey test can be performed on the cross section of the material.

Since the annealing process for removing the demagnetized tissue is performed at a high temperature, formation of a high-temperature oxidation film on the inner and outer surfaces of the heat transfer tube is accompanied by distortion of the signal during non-destructive inspection such as eddy current inspection, The contact with oxygen should be minimized.

For this purpose, the heat transfer tube may be heat-treated in a sealed glass tube filled with an inert gas such as nitrogen or argon, a reducing gas such as hydrogen, or a composite gas thereof, or the gas may be continuously injected into the heat treatment furnace.

In addition, mechanical polishing or chemical cleaning may be performed to remove the high temperature oxidation film on the inner and outer surfaces of the heat transfer pipe after the above-mentioned annealing removal heat treatment step.

Mechanical polishing as one of the means for removing the high-temperature oxide film can be carried out using a polishing paper containing SiC or diamond fine particles, a polishing slurry or the like, or a sandblasting method may be used.

The chemical cleaning is one of means for removing the high temperature oxidation film. The chemical cleaning is performed by immersing the heat transfer tube in a cleaning liquid containing at least one of ethylenediaminetetraacetic acid (EDTA), ethylenediamine acid (EDA), hydrazine and ammonia water, And circulating the cleaning liquid for several days to several tens of days.

The mechanical polishing and chemical cleaning steps may be performed sequentially, but it is noted that only one step may be performed depending on the oxide film thickness and distribution characteristics of the inner and outer surfaces of the heat transfer tube.

Compared to mechanical polishing, chemical cleaning has the advantage of removing high-temperature oxidation films on the inner surface of stress corrosion cracking defects.

In addition, damage to the heat transfer pipe base material is minimized when the oxide film is removed using the mechanical polishing or chemical cleaning method.

FIGS. 2A and 2B are C scan and Lissajous diagrams obtained through eddy current testing for a heat transfer tube that has been subjected to stress corrosion cracking through an ordinary temperature cracking method and then subjected to an ordinary temperature cracking method.

A noise signal due to the magnetization of the sensitized tissue as well as the crack signal indicated by the arrow appear.

3A and 3B are C scan and Risaju screen views obtained through an eddy current test after the heat transfer tube is subjected to a recovery heat treatment step.

Although the noise signal is generally reduced as the sensitized structure is removed, defective signals are distorted by the surface oxide film.

Figs. 4A and 4B are C scan and Risaju screens obtained by removing the surface oxide film of the heat transfer pipe by mechanical polishing and chemical cleaning, and then performing an eddy current test.

As the surface oxide film is removed, most of the noise signal is lost, and the defective signal is recovered as a normal signal.

Claims (5)

Steam Generator Stress Corrosion Crack through the Cold Crack Manufacturing Process Heat Transfer Tube Manufacturing Stage;
A step of removing the annealed tissue by heat treatment or heat treatment;
And removing the oxide film on the surface of the heat transfer pipe through mechanical polishing or chemical cleaning. The method according to claim 1,
In the case of the Alloy 600 material, the solution heat treatment step, as shown in the following equation,
T (占 폚) = 1,449 + 130.3 占 In (% C)
(T is the minimum temperature for the solution heat treatment, and% C is the average carbon concentration of the alloy) is carried out for a period of several minutes to several hours at a given carbon melting temperature or higher. The method according to claim 1,
Wherein the recovery heat treatment step is performed at 700 to 750 ° C for several hours to several hours. The method according to claim 1,
Wherein the mechanical polishing is carried out using a polishing slurry comprising a SiC or diamond fine particle, a polishing slurry, or a sandblast method. The method according to claim 1,
The chemical cleaning includes a step of circulating the cleaning liquid for a few days to several tens of days at 38 ° C to 199 ° C after immersing the heat transfer tube in a cleaning liquid containing at least one of ethylenediaminetetraacetic acid (EDTA), ethylenediamine acid (EDA), hydrazine and ammonia water Wherein the steam cracking cracking heat transfer pipe is manufactured by a method comprising the steps of:

Images