KR100801404B1 - Apparatus for fomating a thermal-fatigue crack - Google Patents

Abstract

The present invention relates to an apparatus and method for forming a thermal fatigue crack in a specimen for verification of nondestructive testing.

The thermal fatigue crack forming apparatus of the present invention, a heating unit having a conductive member circumferentially attached to one outer peripheral surface of a test piece made of pipe, and an induction heating coil disposed adjacent to the conductive member; A cooling unit having a cooling water pump and a cooling water hose for forcibly injecting cooling water into the inner diameter surface of the tubular specimen from the cooling water storage source; And a control unit controlling the operation of the heating unit and the cooling unit.

Thereby, the present invention has the advantage of ensuring the effectiveness of the non-destructive testing skills verification by forming a thermal fatigue crack in the specimen similar to the characteristics of the thermal fatigue crack generated in the actual operation of the nuclear power plant or equipment industry.

Nondestructive, Specimen, Heat Fatigue, Cracks, Formation

Description

Thermal fatigue crack forming apparatus {APPARATUS FOR FOMATING A THERMAL-FATIGUE CRACK}

1 is a partial perspective view showing the configuration of the heating unit of the thermal fatigue crack forming apparatus according to an embodiment of the present invention.

Figure 2 is a block diagram illustrating a configuration for restraining both ends of the tubular specimen used in the thermal fatigue crack forming apparatus of the present invention.

3 is a block diagram conceptually showing a thermal fatigue crack forming apparatus of the present invention.

4 is a photograph showing a cross section of a pipe taken by breaking a cracked portion of the tubular specimen taken along the line B-B of FIG.

FIG. 5 is a view illustrating an image of a crack photograph of FIG. 4.

Figure 6 is a photograph of the cracks of 85 ° ~ 110 ° region of the tubular specimen.

FIG. 7 is a photograph of the cracked portion of FIG.

Brief description of symbols for the main parts of the drawings

10: tubular specimen 20: heating unit

30: cooling unit

The present invention relates to an apparatus and method for forming a thermal fatigue crack in a specimen for verification of nondestructive testing.

As is well known, safety class devices of nuclear power plants in operation (hereinafter referred to as nuclear power plants) are inspected according to the inspection cycle by applying a non-destructive test method that considers the material, shape and expected defects of the equipment to ensure the integrity of the equipment and the stability of the power plant. Is being carried out.

Since the inspection of these nuclear power plants has limited accessibility and provides important results for life prediction and safety evaluation of the equipment, it is necessary to apply a non-destructive testing method that demonstrates defect detection capability and has high reliability. In particular, the demonstration of the defect detection capability of the ultrasonic test method (UT) and the eddy current test method (RCT) has been used to simulate specimens using machining methods, but many cases have raised doubts about the detection capability of actual defects. Occurred.

As a result, the United States institutionalized and applied the Performance Demonstration to verify the defect detection capability of nondestructive testing applied to the original electric machine using simulated specimens that realized the actual defects expected in the original electric machine since 2000. Since 2003, to apply UT and ECT skills verification, we have been developing a 'Korea skills verification system' centered on regulators and nuclear power owners.

Non-destructive test specimens required by the technical level applied in nuclear power plants (ASME Section XI, Appendix 에는) include geometrical defects, implant defects, weld solidfication cracks and lack of fusion. ), Mechanical fatigue cracks, EDM notch and holes, thermal fatigue cracks, intergranular corrosion cracking (IGSCC).

Among these, there were three methods for forming thermal fatigue cracks in the specimen.

Firstly, cracks are formed by repeatedly heating the specimens at high and low temperatures with appropriate tensile or compressive stress after mounting them in an autoclave,

Secondly, a method of forming cracks by repeatedly applying tensile and compressive stresses while maintaining a constant temperature;

Third, there is a method of forming cracks only by repeated temperature changes without mechanical load on the specimen.

However, the specimen produced by the conventional thermal fatigue crack formation method is different from the characteristics of the thermal fatigue crack generated in the facilities of the nuclear power plant or the equipment industry in actual operation, so that the test for non-destructive testing There was a disadvantage that can not guarantee the effectiveness of.

The present invention has been made in view of the above, by forming a thermal fatigue crack similar to the characteristics of the thermal fatigue crack generated in the actual operation of the nuclear power plant or equipment industry facilities to ensure the effectiveness of the non-destructive testing skills verification The purpose is to provide a thermal fatigue crack forming apparatus that can be.

The thermal fatigue crack forming apparatus of the present invention for achieving the above object,

A heating unit having a conductive member circumferentially attached to one outer circumferential surface of a test piece made of pipe, and an induction heating coil disposed adjacent to the conductive member;

A cooling unit having a cooling water pump and a cooling water hose for forcibly injecting cooling water into the inner diameter surface of the tubular specimen from the cooling water storage source; And

It includes; a control unit for controlling the operation of the heating unit and the cooling unit.

A plurality of notches may be formed on the inner diameter surface of the tubular specimen.

Flanges may be provided at both ends of the tubular specimen, respectively.

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

1 to 3 is a view showing a thermal fatigue crack forming apparatus according to an embodiment of the present invention.

As shown, the thermal fatigue crack forming apparatus according to an embodiment of the present invention is a heating unit 20 for applying heat to one outer peripheral surface of the specimen made of tubular shape (10, hereinafter referred to as 'tubular specimen') and And a cooling unit 30 for forcibly cooling the inner diameter surface 12 of the tubular specimen 10.

The heating unit 20 creates an environment for applying thermal stress required for thermal fatigue crack formation of the tubular specimen 10 by applying predetermined heat to one outer peripheral surface of the tubular specimen 10.

The tubular specimen 10 may be used in actual nuclear power plant or equipment industry pipes, such as STS 304, STS 316, STS 321, STS 347, STS 308, STS 309, Inconel 600, Inconel 690, Inconel 800, Inconel X750, Inconel 718, etc. The material used may be used.

The heating unit 20 includes a conductive member 21 made of a magnetic material circumferentially attached to one outer peripheral surface of the tubular specimen 10 and an induction coil 22 spaced apart from the conductive member 21.

An external high frequency current supply unit is connected to the induction coil 22, and when a high frequency current is applied to the induction coil 22 from the high frequency current supply unit, a high frequency magnetic field is formed in the induction coil 22. The outer peripheral surface of the tubular specimen 10 to which the conductive member 21 is attached is heated to a predetermined temperature by generating heat due to loss of eddy current (eddy current) or hysteresis loss on the conductive member 21 side adjacent to the 22.

On the other hand, a plurality of notches (notch) may be formed on the inner diameter surface of the tubular specimen 10 through lathe machining, and if there are no notches when a plurality of notches are formed on the inner diameter surface of the tubular specimen 10. The formation time of the thermal fatigue crack can be further shortened.

On the other hand, the tubular specimen 10 needs to be constrained at both ends to minimize its external deformation in a state in which thermal stress is applied by the heating unit 20 to one outer peripheral surface thereof.

As illustrated in FIG. 2, the present invention further includes flanges 15 at both ends of the tubular specimen 10 to facilitate the restraint of the tubular specimen 10. Can be.

That is, it is possible to apply a constant external force to the flange 15 of the tubular specimen 10, in particular it may be applied to the repetitive tensile and compressive stress (A) as shown in Figure 2 on the flange 15 side.

By fastening the flange 15 mechanically or by using a hydraulic or pneumatic mechanism or the like, both ends of the tubular specimen 10 may be restrained.

The cooling unit 30 creates a cooling environment required for thermal fatigue crack formation of the tubular specimen 10 by forcibly introducing cooling water into the inner diameter surface of the tubular specimen 10.

As shown in Figure 3, the cooling unit 30 of the present invention is a cooling water pump for supplying the cooling water storage tank 33, the cooling water of the cooling water storage tank 33, the cooling water storage tank 33 to the inner diameter surface of the tubular specimen (10). And a coolant hose 31 in communication with the coolant storage tank 33 and the coolant pump 32 and in communication with the inner diameter surface of the tubular specimen 10, wherein the coolant lake 31 is a tubular specimen ( 10) may be sealingly connected to the flange 15 side via a packing or the like.

In particular, the present invention is a type similar to the thermal layer formed by the temperature difference of the fluid flowing through the inner diameter surface of the tubular specimen 10 by directly introducing the cooling water to the inner diameter surface of the tubular specimen 10 by the cooling unit 30 The thermal stresses (tensile or compressive stresses) generated at the interface can be used to form thermal fatigue cracks similar to those produced in piping in the actual nuclear power plant or equipment industry.

And, the present invention is provided with a control unit 40 for controlling the operation of the heating unit 20 and the cooling unit 30, the control unit 40, such as heating temperature, heating cycle, etc. of the heating unit 20 Control conditions and control conditions such as the amount of cooling water, the cooling cycle of the cooling unit 30, and the like are appropriately adjusted.

The operation relationship of the present invention configured as described above is described as follows with reference to FIG.

As shown in FIG. 3, in a state where the flanges 15 at both ends of the tubular specimen 10 are constrained, the high frequency induction heating is applied to the outer circumferential surface of the tubular specimen 10 by the heating unit 20. After the predetermined high frequency induction heating is performed on the outer circumferential surface of), the fatigue fatigue crack similar to the thermal crack generated in the actual pipe can be realized by repeating the process of cooling the inner diameter surface of the tubular specimen 10 by the cooling unit 30. .

Experimental Example

The specimen used in this test example was applied to STS 304 (OD: 89mm, t: 7.7mm, yield strength: 41.8 kg / mm ² ) widely used as a structural material of nuclear power plants.

The specimen had a length of 500 mm, and an artificial notch was turned to a depth of 0.5 mm at a 55 ° circumferential position of the specimen to control the location of cracking.

In order to understand the propagation rate and propagation pattern of thermal fatigue crack, 25,000 cycles of induction heating for 20 minutes and cooling for 20 seconds were repeated on the tubular specimen, and then the cracks were cut at 30 ° intervals and cooled with liquid nitrogen. The fracture site was broken using a compression tester, and the fracture surface was observed using an optical microscope and a scanning electron microscope (SEM). 4 is a cross-sectional view of a pipe taken after breaking a crack taken along the line BB of FIG. 3, and in FIG. 5, a depth is measured using a CAD program after imaging a crack cross section not clearly identified in the photograph. The pipe cross section imaged is shown. The thermal fatigue crack developed into the specimen at 55 ° to 270 ° of the tubular specimen (75 ° to 295 ° centered on the top of the pipe), and cracks were formed at approximately the same depth at 90 ° to 240 °. This seems to be due to the temperature gradient generated during the thermal fatigue cycle in the 90 ° ~ 240 ° region of the specimen, it can be seen that very similar to the temperature distribution of the cross section of Table 1 below.

Table 1 below is a result table comparing the ultrasonic flaw detection results (conventional control) measured with the specimen compared to the conventional machining and the depth of crack (actual group) generated in the actual pipe at intervals of 30 ° in the circumferential direction of the pipe. .

Table 1

 Angle Crack depth (mm) Ultrasound Exploration (Control) Actual piping (real group) 0 1.950 0 30 2.222 0 60 3.199 4.08 90 3.171 7.7 (through) 120 2.901 4.05 150 2.313 4.05 180 2.851 4.29 210 3.186 5.76 240 2.600 4.64 270 1.293 0 300 1.307 0 330 1.743 0

On the other hand, in Table 1, the upper part of the actual pipe is in a constant compressive stress state because the heating temperature is maintained at a constant level, but in the case of the lower part, the expansion and contraction is repeated due to heating, but the tension is not expanded as the upper part of the pipe. It can be considered that the stress gradient is repeated. Therefore, thermal fatigue cracks seem to be generated and developed almost simultaneously along the same line of the same temperature distribution in the lower part of the pipe, but in the case of the 70 ° ~ 90 ° region where the through crack occurred, the inlet of the induction coil has a larger temperature gradient than the surroundings. The stress gradient was also large, which presumably caused the crack propagation rate to be somewhat faster.

And, as shown in Table 1 above, the results of ultrasonic flaw detection using specimens compared to conventional machining do not predict the initial depth of cracks, and the shape of cracks is different from the thermal fatigue cracks generated in actual piping. It seems that the crack depth of the actual pipe cannot be reliably evaluated.

Figure 6 (b) shows a crack between 85 ° ~ 110 ° in the tubular specimen according to the present invention, Figure 6 (a) is a cross section corresponding to the left side of Figure 6 (b), that is, the cross section of 85 ° Fig. 6 (c) shows a cross section viewed from the right side of Fig. 6 (b), that is, a cross section of 110 ° portion.

As shown in FIG. 6 (a), it was confirmed that the crack propagated through the outer surface of the specimen in the cross section of the 85 ° portion of the tubular specimen, and as shown in FIG. 6 (c), the 110 ° portion of the tubular specimen. It was confirmed that the crack grew to a depth of 3.2 mm in the cross section. The cracked part that is broken by using liquid nitrogen breaks without any deformation and has a very clean fracture surface, but a scale, which is an oxide layer formed on the surface when the metal is heated to a high temperature, is severely contaminated with reddish brown. It is in a state. Fig. 6 (d) is an optical photograph of the crack portion of Fig. 6 (c). The notch portion in which the crack is started has a wedge shape and has a width of about 150 μm, but is relatively wide at several μm at the tip of the crack. This is rapidly decreasing. This is related to the stress gradient generated during the pipe fatigue fatigue test. The shear type crack or slip is initially generated due to the influence of the compressive stress occurring at the upper part of the pipe and the tensile stress occurring at the lower part of the pipe during cooling. It appears that the growth is achieved by transitioning to the stable growth of the opening type, forming a striation, and then rapidly breaking while brittle to soft fracture is crossed.

FIG. 7 shows the fracture surface of FIG. 6 (b) with a scanning electron microscope (SEM), wherein the fatigue wave-specific striation generated by the opening and closing of the crack surface by repeated loading when the crack propagates along the crack surface is shown. A typical form of can be observed. The striation shows the cracks progressing every cycle and can be used locally to determine the direction and speed of crack propagation. As a result of analyzing FIG. 7 (a), where the striation is most clearly identified, a propagation speed of about 1 µm / cycle or more was observed in a place where the crack propagation speed was high, but in most cases, as shown in FIG. The spacing is so tight that it appears to be far below this speed.

The present invention as described above, there is an advantage to ensure the effectiveness of the non-destructive testing skills verification by forming a thermal fatigue crack in the specimen similar to the characteristics of the thermal fatigue crack generated in the actual operation of the nuclear power plant or equipment industry .

Claims (3)

A heating unit having a conductive member circumferentially attached to one outer circumferential surface of a test piece made of pipe, and an induction heating coil disposed adjacent to the conductive member; A cooling unit having a cooling water pump and a cooling water hose for forcibly injecting cooling water into the inner diameter surface of the tubular specimen from the cooling water storage source; And Thermal fatigue crack forming apparatus comprising a; control unit for controlling the operation of the heating unit and the cooling unit. The method of claim 1, Thermal fatigue crack forming apparatus, characterized in that a plurality of notches are formed on the inner diameter surface of the tubular specimen. The method of claim 1, Thermal fatigue crack forming apparatus, characterized in that each end of the tubular specimen is provided with flanges.

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