Ultrasonic testing
The material grade of the heat transfer tube of the steam generator is NC30Fe (690), the outer diameter of the tube is 17.48mm, the wall thickness is 1.02mm, and the state is TT (Thermo Treatment) state.
The GE ROTA40 pulse-reflective ultrasonic inspection system was selected for the test; in order to detect all defects in the vertical and horizontal full-volume range, the selected standard sample tube and the tested tube have the same material, geometric size, manufacturing process and surface condition. The artificial defect of the standard sample tube is the horizontal and longitudinal U-shaped grooves on the inner and outer walls of the pipe. The artificial defect is 12.7mm in length, 0.1mm in width and 0.1mm in depth. The 4 U-shaped grooves should be separated by a certain distance to ensure that the echoes can be clearly distinguished.
The water immersion line focusing method is used for ultrasonic testing, and the couplant adopts Class A deionized water. A 3mm line-focused ultrasound probe is used, the probe frequency is 7MHz, and the refraction angle of the sound wave propagating in the tube wall is 47°~50°.
Adjust the height of the water layer to 20.5~21.5mm according to the outer diameter of the inspected tube and the focal length of the detection probe. Move the standard sample tube, place the notch directly on the probe beam, and adjust the gain to adjust the standard defect amplitude to 80% of the full screen.
Perform axial and circumferential scanning on the inspected tube, and perform scanning from two opposite directions. Select the probe rotation speed to be 6000r/min and the detection speed to be 13.5m/min to ensure two adjacent axial scans. The checked overlap area is at least 20%.
In the inspection process, because the type, location, size and orientation of the defect will affect the inspection result, some defects that meet or exceed the standard defect size will appear, and the signal amplitude generated is lower than the signal amplitude of the standard defect. There may be cases where the signal amplitude of some defects smaller than the standard defect size is higher than the signal amplitude of the standard defect. In order to prevent missed detection and misjudgment in ultrasonic testing, the defect coefficient K=0.70 is set. The K value is defined as: K=scrap limit/80% of the full screen height of the standard defect signal, that is, the standard defect signal is adjusted to 80% of the full screen height, so the pipe with the defect signal amplitude not greater than 56% is judged as qualified.
The various types of defects with signal amplitude exceeding 56% found in the actual ultrasonic inspection process are classified. If a defect produces signals in multiple ultrasonic detection channels, the channel with the largest amplitude is counted. Select the pipes containing each type of defect for ultrasonic testing, and then cooperate with the visual inspection method to accurately locate the defect location and intercept the 20-50mm length sample containing the defect.
The defects identified in the ultrasonic testing of the steam generator heat transfer tube are classified according to the defect type and defect orientation. The statistical results are shown in Table 1. "Longitudinal" means that the direction of the defect is consistent with the rolling direction, and "Horizontal" means the defect. The direction is perpendicular to the rolling direction.
Table 1 Statistic results of defect information detected by ultrasound
Type of defect | Defect orientation | Number of defects | Proportion/% |
Outer surface | Horizontal | 9 | 13.2 |
Outer surface | Vertical | 54 | 79.4 |
Inner surface | Horizontal | 0 | 0 |
Inner surface | Vertical | 5 | 7.4 |
It can be seen from Table 1 that most of the defects identified by ultrasonic testing during the production process of the steam generator heat transfer tube are external surface defects.
Macro observation test
Axio Imager Mzm optical microscope was used to observe the defect surface macroscopically on typical samples containing different types of defects. The surface morphology of different types of defects is shown in Figure 1.
Figure 1 Surface morphology of different types of defects
Defect size measurement
According to ASTM E3-11 (2017) "Metallographic Specimen Preparation Standard Guide" standard, the defect cross-sections are coarsely ground, finely ground and polished to the mirror surface in turn, and then blown dry after ultrasonic cleaning with acetone. , Use the same type of optical microscope for defect observation and defect depth measurement, and use tomography to determine the maximum depth of the defect. The cross-sectional morphology of different types of defects is shown in Figure 2.
Figure 2 Cross-sectional morphology of different types of defects
Statistical analysis of test results
Based on the selected ultrasonic inspection results of typical samples containing different types of defects, statistics of different defect positions, defect orientations, defect types, the full-screen height ratio of the maximum echo amplitude of the ultrasonic signal at the defect, and defect size, etc. are performed, and the results are As shown in Table 2, the relationship between the maximum ultrasonic amplitude of different defects and the depth of the defect is shown in Figure 3.
Table 2 Ultrasonic testing results of typical defect samples
Sample Number | Defect Location | Defect Orientation | Defect Type | Full screen height of the maximum echo amplitude of the defect/% | Depth /μm | Width /μm |
1 | Outer surface | Vertical | Scratch | 56.00 | 9.27 | 218.8 |
2 | Outer surface | Vertical | Roll off | 99.20 | 98.09 | 946.5 |
3 | Outer surface | Vertical | Bump | 58.50 | 82.38 | 371.3 |
4 | Outer surface | Vertical | Bump | 74.50 | 61.78 | 422.2 |
5 | Outer surface | Horizontal | Pit | 107.10 | 116.3 | 1277.0 |
6 | Outer surface | Horizontal | Bump | 56.30 | 55.36 | 275.4 |
7 | Inner surface | Vertical | Roll off | 118.10 | 175.7 | 142.2 |
Figure 3 The relationship between the maximum ultrasonic amplitude of different defects and the depth of the defect
It can be seen from Table 2 and Figure 3:
1
Ultrasonic testing can effectively identify all defects in the longitudinal and transverse full volume of the steam generator heat transfer tube.
2
The depth of the defect does not have a linear relationship with the maximum ultrasonic amplitude at the defect, that is, it is not that the higher the ultrasonic amplitude, the greater the depth of the defect. This is because the actual defect surface is not completely perpendicular to the incident direction of the ultrasonic wave, and the defect wave amplitude is highest when the ultrasonic wave is perpendicular to the defect surface. When there is an inclination angle, the defect wave amplitude decreases sharply as the inclination angle increases.
Cause analysis of defects
Defect No. 1
It is a typical longitudinal scratch defect.
It is formed by a certain sharp corners or particles scratching the outer surface of the pipe during the transportation process. From the macroscopic observation of the surface in Figure 1(a), it can be clearly recognized that the direction of the scratches is along the pipe rolling direction, which is shown from the left to the right, which is consistent with the defect orientation of the ultrasonic inspection. Scratch defects are generally shallow surface defects with shallow depth. The maximum echo amplitude of the ultrasonic signal at No. 1 defect is 56.0% of the full screen height, and the maximum depth is only 9.27μm
Defect #2
It is a typical longitudinal rolling defect on the outer surface.
Some material loss can be observed on the surface of the pipe, and it can be seen from the cross-sectional profile that the defect is at a certain angle to the outer surface. From the cross-sectional morphology in Figure 2(b), it can be observed that the defect is at a certain angle to the inner surface, and a clear fold is formed between the inner and outer layers. The maximum echo amplitude of the ultrasonic signal at No. 2 defect is 99.20% of the full screen height, and the maximum depth is 98.09μm.
Defects 3 and 4
It is a typical longitudinal bump defect on the outer surface.
It is formed by the foreign matter being directly pressed into the surface of the pipe during the transfer process, and the foreign matter is missing in the subsequent circulation process. From the cross-sectional morphology in Figure 2(b), it can be observed that the defect is missing. Unlike defect No. 2, no folds are formed, which can be judged to be formed after the finished pipe is rolled. The maximum amplitude of the ultrasonic signal at defects No. 3 and No. 4 is 58.50% and 74.5% of the full screen height, and the maximum depth is 82.38 μm and 61.78 μm, respectively.
Defect No. 5
It is a typical pit defect.
It is formed by the sharp granular foreign matter pressed into the surface during the production and transportation process (causing part of the metal material to be missing). This defect is a volumetric defect, and only the lateral defect channel is clearly displayed during the ultrasonic inspection process. Through the relationship between wavelength λ, sound velocity c, and frequency f λ=c/f, it can be calculated that the shear wave wavelength is 0.46mm and the longitudinal wave wavelength is 0.84mm (the shear wave speed in steel is 3200m/s, the longitudinal wave speed is 5900m/s, The probe frequency is 7MHz). Since the defect diameter is approximately 1277μm, which is more than twice the wavelength of the transverse wave, the transverse defect channel has good directivity. The maximum echo amplitude of the ultrasonic signal at defect No. 5 is 107.1% of the full screen height, and the maximum depth is only 116.30μm.
Defect No. 6
It is a typical horizontal bump defect.
It is formed by the foreign matter being directly pressed into the surface of the pipe during the transfer process, and the foreign matter is missing in the subsequent circulation process. The causes of No. 6 defect are the same as No. 3 and No. 4 defects. The maximum echo amplitude of the ultrasonic signal at No. 6 defect is 56.30% of the full screen height, and the maximum depth is 55.36 μm.
Defect No. 7
It is a typical longitudinal rolling defect on the inner surface.
There are linear and jagged defects on the inner surface, and the reason for its formation is the same as that of the No. 2 defect. Obvious folding can also be observed in the cross-sectional photograph in Figure 2(g). The maximum echo amplitude of the ultrasonic signal at defect No. 7 is 118.10% of the full screen height, and the maximum depth is 175.7μm.
Preventive measures
In order to obtain high-quality steam generator heat transfer tubes, attention should be paid to the quality control of each production link. In the subsequent production process, prevention and improvement can be made from the following aspects:
1
Strengthen the quality control of the rolled tube, and the surface of the incoming tube is not allowed to have defects such as peeling, scratches, abrasions, and foreign objects.
2
Strengthen the roughness control of the surface of the roller table, rolling tools and dies, straightening rolls, transmission devices, etc., and update the worn tools and dies in time.
3
Control the rolling process to prevent foreign matter from being pressed into the rolling process.
Concluding remarks
(1)
In the actual production process of steam generator heat transfer tubes, most of the defects are longitudinal defects, and transverse defects are extremely rare, and most of them are external surface defects.
(2)
Ultrasonic testing can effectively identify all defects in the longitudinal and transverse full volume of the steam generator heat transfer tube.
(3)
The relationship between the depth of the defect and the maximum ultrasonic amplitude at the defect is not that the higher the ultrasonic amplitude, the greater the depth of the defect.
(4)
Typical defects are scratches, folds, pits and bumps, which are mainly produced during rolling and transportation.
