JP2870034B2 - How to make pipe sample with SCC - Google Patents

Description

The present invention relates to a method for producing an SCC-containing pipe sample.

"Prior art and its problems" Metal tubes such as austenitic stainless steel tubes having excellent characteristics are used in nuclear power plants, various energy plants, chemical plants, thermal power plants and the like. Further, in an austenitic stainless steel, when an environment in which a corrosion factor is present, a tensile residual stress, and a sensitized structure are simultaneously present, stress corrosion cracking (hereinafter, referred to as SCC) tends to progress.

Therefore, the austenitic stainless steel pipes and the like used in each of the above-mentioned plants, at the time of periodic inspection, etc.
For example, it is desirable to check the soundness by ultrasonic inspection, and when stress corrosion cracking is recognized, it is necessary to perform an appropriate process such as repairing the portion.

In order to enhance the reliability of the above-mentioned ultrasonic inspection, the SCC was generated in the internal structure of the metal tube in advance and standardized, and the ultrasonic inspection work procedure was examined using the standardized metal tube. It is considered effective to confirm the sensitivity and to make full use of a method of comparing the standardized metal tube and the metal tube to be inspected by an ultrasonic flaw detector.

However, it is technically difficult to intentionally generate standard SCC on the inner surface of a metal tube.If there is a metal tube that accidentally generates SCC, cut it out as a sample. However, in general, pipe samples with SCC cannot be obtained.

The present invention has been made in view of such a conventional technique, and has as its object to generate a deep and large SCC with a set length direction on the inner surface of a metal tube.

[Means for Solving the Problems] In the present invention, as a means for solving the above problems, a part of a girth welded joint of a metal pipe is formed in a groove along a circumferential direction and in accordance with a length in which SCC is to be generated. Forming a weld groove by removing the weld bead, superimposing a weld bead on the weld groove, performing repair overlay welding, and setting a direction of welding residual stress near the repair overlay weld joint. And filling the inside of the metal tube with a corrosive fluid to grow cracks.

"Action" By removing a part of the girth weld joint of the metal pipe into a groove shape and performing repair overlay welding on the groove portion, when the weld bead solidifies, it cools down to the recrystallization temperature of the welding material. Thermal stress is generated based on the temperature change of This thermal stress is set by the temperature distribution when the weld bead reaches the recrystallization temperature.

If the metal pipe is filled with corrosive fluid in a state where tensile residual stress in the circumferential direction and in the pipe axis direction is applied to the vicinity of the repaired overlay welding joint, that is, to the inner surface of the pipe wall, etc., corrosion may occur. Factors such as tensile residual stress and sensitivity due to welding heat effect are added to the environment where the factor exists, and SCC is generated along the direction orthogonal to the resultant stress of the tensile stress in the circumferential direction and the pipe axis direction, and time SCC grows over time. For this reason, the direction of the SCC is determined by setting the direction of the tensile stress by forming the repair overlay girth weld joint.

Embodiment An embodiment of the method of manufacturing a pipe sample containing SCC according to the present invention will be described below with reference to FIGS. 1 to 5.

In this embodiment, as shown in FIG. 1, a metal pipe (for example, a kind of austenitic stainless steel pipe) is used.
SUS304 steel pipe) 1 in which a girth welded joint 2 is formed, on the inner surface of the metal pipe 1 and near the girth welded joint 2, along the circumferential direction, or along the combined direction with the pipe axis direction Thus, SCC (crack X) is generated.

 Hereinafter, processing steps for generating SCC will be described.

<Formation of Weld Groove> As shown in FIG. 1, a large part (the part shown by oblique lines) of the peripheral weld joint 2 of the metal pipe 1 is formed from the outer surface side by a method such as hanging with a grinder. The welding groove 3 is formed by removing the groove along the direction. The length of the welding groove 3 is set to be substantially the same according to the range in which SCC is to be generated.

<Formation of Repair Overlay Weld Joint> As shown in FIG.
Are sequentially superimposed and overlay welding is performed. By forming such a repair build-up peripheral welding joint 4, in a process where the weld metal in a molten state becomes a solidified state, the structure has a thermal effect on a structure in the vicinity of the welding groove 3 and a temperature difference based on a temperature distribution. Generates thermal stress.

In other words, when the weld metal is in a molten state, the weld metal itself does not generate stress based on the fluidity, although the thermal effect is exerted on the surrounding base metal structure. When it cools down to below, stress is generated in the surrounding structure due to its contraction force and temperature difference, so the thermal stress is based on the temperature change of each part structure below the recrystallization temperature due to the formation of each weld bead and its temperature distribution. Is generated, and a state where a tensile residual stress is applied to the inner surface of the tube wall in the vicinity of the repair overlay welding joint 4 can be obtained.

The uppermost weld beads w to z are deleted by a grinder or the like, and the outer surface is appropriately smoothed (see FIG. 3).

<Applying SCC by Corrosive Fluid> The inside of the metal pipe 1 is filled with high-temperature water having a large amount of dissolved oxygen as a corrosive fluid, and the internal pressure of the metal pipe 1 is kept high.

In the vicinity of the repair overlay welding girth weld joint 4, a sensitization region is given by the welding heat accompanying the formation of the girth welding joint twice, and sensitivity is caused. In addition to this, the aforementioned tensile residual stress is given. Further, when the conditions of the presence of the corrosive fluid and the pressure for permeating the corrosive fluid are established, the inner surfaces of the girth weld joint 2 and the repair overlay cladding girth weld joint 4 are formed at the point of occurrence as shown in FIG. As SCC (crack X)
Occurs and gradually grows over time.

The degree of SCC growth in this case is set by the degree and direction of the application of the residual tensile stress, the magnitude of the internal pressure, and the processing time. The direction of the residual stress is set based on the temperature distribution of each structure when the metal cools to the recrystallization temperature or lower.

The temperature distribution in this case can be displayed dimensionlessly using the Fourier number Fr, and the residual stress distribution is determined from Fr, and the SC
It is possible to set the direction of C. The Fourier number Fr is represented by the following equation (i).

Where C: specific heat (joules / ° C kg) ρ: density (kg / mm) Tr: recrystallization temperature-interlayer temperature (° C) h: plate thickness (wall thickness of tube) (mm) q: net welding heat input (Joule / mm) Using the Fourier number of 0.16 to 32.0, the results of determining the stress distribution in the tube axis direction and the circumferential stress distribution of the 26-inch tube and the 4-inch tube are shown in FIG.

Further, the residual stress on the inner surface of the metal tube 1 is changed in the tensile direction (+
Direction), SC in the direction perpendicular to this
Since C (crack X) occurs, the explanation is as follows.
When the Fourier number is 0.04, the residual stress in the tube axis direction is as shown in FIG. 4 (AI) and the residual stress in the circumferential direction is as shown in FIG. As shown in the drawing, at the inner surface position, based on the fact that the tube axis direction is pulled and the circumferential direction is compressed, the direction perpendicular to the tube axis direction, that is, as shown in the example in FIG. The crack X is generated only in. In this case, since there is almost no difference between the 26-inch pipe shown by the solid line and the 4-inch pipe shown by the broken line, the influence of the diameter of the metal pipe 1 is small, and about 20 % Of cracks X can be generated.

On the other hand, when the Fourier number is 0.32, the residual stresses in the tube axis direction and the circumferential direction are as shown in FIGS. 4 (DI) and 4 (DI).
It is applied as shown in I), and it becomes tensile at the inner surface position, and since the magnitude of the residual stress is both large, the SCC is applied in the combined direction in the circumferential direction and the pipe axis direction.
Will grow.

Therefore, by changing the Fourier number by, for example, setting the heat input at the time of repair overlay welding according to the pipe wall (plate thickness) of the metal pipe 1, the direction of the SCC can be changed in the circumferential direction or its 4 direction.
Set according to the purpose, such as 5 degrees direction, and S
It was confirmed that it was possible to freely manufacture pipe samples with SCC with adjusted CC size.

FIG. 5 is an enlarged view of a microstructure when SCC is generated on the inner surface of the metal tube 1, and a crack X is formed along a crystal grain boundary.
Is developing.

[Effect of the Invention] As described above, according to the method of manufacturing the SCC-containing pipe sample according to the present invention, a part of the girth weld joint is deleted to form a weld groove, and the weld groove part is formed. Since the direction of the residual stress is set according to the conditions when forming the repair overlay welding peripheral weld joint by performing the repair overlay welding, the contact with the corrosive fluid is performed thereafter, not only in the circumferential direction but also in the pipe shaft. A pipe sample containing SCC in which SCC is grown in the synthetic direction can be freely manufactured.

Also, in that case, by adjusting the range of the repair overlay girth weld joint, the range of length to which SCC is given and S
The depth and depth of CC can be set freely.

And so on.

[Brief description of the drawings]

1 to 3 show an example of processing steps of a method of manufacturing a pipe sample containing SCC according to the present invention. FIG. 1 is a perspective view showing a part of an initial state of a metal tube, and FIG. FIG. 3 is a perspective view in which a part of a state in which the repair overlay welding is applied, FIG. 3 is a perspective view in which a part of the state after contact with a corrosive fluid is cut, and FIG. FIG. 5 is an enlarged view of a microstructure after the occurrence of SCC. 1 ... metal pipe, 2 ... girth weld joint, 3 ... welding groove, 4 ... repair overlay cladding girth weld joint, X ... SCC (crack).

Continuation of the front page (56) References JP-A-2-1755091 (JP, A) JP-A-3-82712 (JP, A) JP-A-60-132195 (JP, A) (58) Fields investigated (Int .Cl. ⁶ , DB name) B23K 31/00 C23F 1/00 F16L 13/00

Claims (1)

(57) [Claims] A step of forming a weld groove by removing a part of a circumferential weld joint of a metal pipe along a circumferential direction and in a groove shape in accordance with a length in which SCC is to be generated; First, the weld bead is superimposed and the overlay welding is performed, and the direction of the residual welding stress in the vicinity of the overlay welding peripheral welding joint is set, and the inside of the metal pipe is filled with a corrosive fluid to form a crack. A method for producing an SCC-containing pipe sample having a step of growing.