1. Overview of inspection of austenitic stainless steel overlay on nuclear power outer cylinder
The cylinder body material of the high and medium pressure outer cylinder of nuclear power is ZG17CrMo9-10 low alloy steel. Design requirements for adding a 309L austenitic stainless steel overlay with a thickness of 10mm to 15mm to the side of the gantry stop and tile mouth of the cylinder body. The main function of the overlay layer is to improve the wear resistance, corrosion resistance, and high-temperature performance of these positions. The microstructure of austenitic stainless steel overlay has the characteristics of columnar grains and anisotropic structure, and there is a heterogeneous interface between it and the substrate structure of the cylinder body. During ultrasonic testing, there are heterogeneous interface waves and grain boundary reflection waves of heterogeneous structure, which are easily confused with the welding defect reflection waves of the overlay, making it difficult to evaluate the welding quality of the overlay. There are no inspection standards and technical requirements for the austenitic stainless steel cladding layer of nuclear power outer cylinders at home and abroad. The customer only provided acceptance standards for ultrasonic testing of material and manufacturing weld defects in nuclear electroformed steel parts. How to choose a more reasonable ultrasonic testing method for the detection and quality evaluation of austenitic stainless steel overlay is particularly prominent.
ZHY Casting took the high and medium pressure outer cylinder of nuclear power as an example to introduce the experimental method and quality control points of ultrasonic testing for austenitic stainless steel overlay welding in the outer cylinder’s gantry and tile mouth, and achieved satisfactory control results.
2. Basic requirements and technical difficulties of ultrasonic testing
2.1 Basic requirements for ultrasonic testing
Non destructive testing requirements for high and medium pressure outer cylinders in nuclear power plants, sensitivity FBH for overall straight probe testing of CrMo low alloy steel cylinder body= φ Detection sensitivity FBH with a wall thickness of less than 50mm and a thickness of 3mm= φ 2 mm; Sensitivity SDH for Inspection of Base Metal at Butt Welding and Overlay Layer Positions= φ The acceptance standard for ultrasonic testing of austenitic overlay with a thickness of 1.5 mm is that the defect echo display reaches the SDH of the transverse hole= φ When the DAC curve is 1.5 mm, the defect length is not allowed to exceed 5 mm
2.2 Technical difficulties of ultrasonic testing
The base material of the high and medium pressure outer cylinder of nuclear power is low alloy nuclear electroformed steel parts, and the grains are relatively small and evenly distributed after heat treatment; The austenitic stainless steel overlay is affected by factors such as welding heat dissipation, and the grain structure will exhibit anisotropy, with coarse grains. Coarse grains in the austenitic stainless steel overlay layer can cause severe attenuation of ultrasonic waves and grass like clutter. The grain structure exhibits anisotropy, leading to the deviation of the sound beam. Moreover, changes in sound velocity at the fusion line between the overlay layer and the base metal can cause reflection and refraction of the sound beam, which increases the difficulty of detection and the risk of misjudgment.
3. Testing
3.1 Production of ultrasonic testing comparison for weld overlay
Considering the microstructure characteristics of austenitic stainless steel overlay, increasing the difficulty of ultrasonic testing and bringing uncertainty to defect evaluation. According to the acceptance standards, it is necessary to make a comparison test block for ultrasonic testing of austenitic stainless steel overlay, which is used to adjust the detection sensitivity and sound path calibration. The core of making comparative test blocks is to ensure that the production process of the welding layer on the test block is the same as that of the welding layer on the outer cylinder, so as to have direct reference for comparison as much as possible.
The comparison test block blank consists of two parts: a substrate layer and a weld overlay layer. The substrate layer is CrMo low alloy steel, and the weld overlay layer is austenitic stainless steel made of 309L material. The substrate part should have the same production process as the outer cylinder. When designing and producing the outer cylinder, the number and size of test blocks should be considered, and they should be poured in the same furnace as the outer cylinder; The substrate of the test block and the outer cylinder should undergo the same furnace quality heat treatment to ensure that their internal structure is the same as the outer cylinder; Perform ultrasonic testing on the substrate to ensure that there is no equivalent greater than φ A defect of 1 mm. The overlay layer is welded according to the same overlay welding process as the outer cylinder overlay welding layer, strictly controlling the preheating temperature, interlayer temperature, and post heating temperature during the welding process. The thickness of the overlay welding layer should cover the thickness of all overlay welding layers in various parts of the cylinder body, and the welding process is the same as that of the cylinder body overlay welding layer. After welding, stress relief heat treatment shall be carried out, followed by ultrasonic testing. The welding layer and fusion zone shall not have an equivalent greater than φ A defect of 1 mm.
As shown in Figures 1 and 2, the thickness of the overlay layer on the test block should cover the thickness of the overlay layer on the outer cylinder gantry, tile mouth, and other parts. The thickness of the substrate should be greater than or equal to twice the thickness of the overlay layer; To prevent sidewall interference, the width of the test block should not be less than 50 mm; There are 6 in the test block φ 3 flat bottomed holes and 6 φ 1.5 x 40 horizontal holes, and ensure that one flat bottom hole and one horizontal hole are on the fusion line between the overlay layer and the substrate, using this as the reference line for precision machining of the test block; At one end of the test block, a step is machined for horizontal linear calibration of the straight probe and a circular arc is machined for horizontal calibration of the oblique probe, which increases the functionality of the test block, achieves versatility, and reduces the number and cost of test block production.
The precision machining of test blocks should comply with the basic accuracy requirements of JB/T8428-2015 General Specification for Ultrasonic Test Blocks for Non destructive Testing.
3.2 Determination of detection methods
3.2.1 Detectability of weld overlay
Due to the coarse grain size of the austenitic stainless steel overlay, as the thickness of the overlay increases, the attenuation of the sound beam increases, accompanied by severe grass like waves. Therefore, before ultrasonic testing, the detectability and attenuation of the sound beam of the austenitic stainless steel overlay must be tested. The ultrasonic detectability of the overlay layer can be tested at the location where the maximum thickness of the overlay layer on the cylinder body is located and the upper and lower detection surfaces are relatively parallel. If the echo height of the minimum flat bottom hole or transverse hole measured is not greater than 6 dB of the noise signal, it is considered that the ultrasonic detection of the overlay layer is detectable, and vice versa. If there are no suitable parallel surfaces in the welding layer of the cylinder body, measurements can be made by comparing the welding layer made by the same process with the test block.
The measurement of sound beam attenuation can be carried out using a stepped test block with a weld overlay layer for a reflection of the bottom wave. A 309L austenitic stainless steel overlay layer was deposited on low alloy steel nuclear electroformed steel parts. The USM 36 digital ultrasonic flaw detector and MSB2S-E straight probe produced by General Electric Company were used for testing. When the thickness of the overlay layer reached 40 mm, it was almost impossible to see obvious bottom echo display due to tissue attenuation.
3.2.2 Selection of probes and detection direction
Due to the coarse grain size and severe attenuation of sound waves in the overlay layer, longitudinal waves are longer and have relatively smaller attenuation compared to transverse waves. Therefore, a longitudinal wave probe was chosen. The acceptance requirements for the weld overlay layer are high, and the equivalent should be detected φ A defect of 1.5mm in length is not allowed to exceed 5mm, so it is required that the probe has high resolution and the ability to detect smaller defects. The higher the probe frequency, the better the directionality and resolution, and the higher the accuracy of defect localization and size measurement. However, the higher the probe frequency, the more severe the attenuation, which is not conducive to the detection of thick welding layers; For small thickness overlay layers (such as around 15 mm), in order to improve defect quantification and positioning accuracy, higher frequency twin crystal longitudinal wave straight probes and single crystal longitudinal wave straight probes can be used, such as the MSEB4 and MB4S produced by General Electric
For the austenitic stainless steel overlay welding of low alloy nuclear electroformed steel parts, special consideration needs to be given to the defects in the overlay welding layer and the fusion line of the nuclear electroformed steel parts after overlay welding, in order to reduce the risk of overlay defects during equipment operation. Given this factor, the direction of ultrasonic testing should be chosen to facilitate the discovery of defects on the fusion line between the overlay layer and the nuclear electroformed steel parts after overlay welding, so the testing direction should be perpendicular to the fusion line direction. Considering the detection of fusion points between the end of the overlay layer and the nuclear electroformed steel parts, as well as defects perpendicular to the fusion line, a twin crystal longitudinal wave oblique probe VSY60 ° -4 can be added, as shown in Figure 3.
3.3 Adjustment of scanning speed and distance amplitude curve
3.3.1 Adjustment of scanning speed (baseline)
There is a difference in ultrasonic speed between the austenitic stainless steel overlay layer and the base material of nuclear electroformed steel parts. The welding process of the overlay layer is different, and the anisotropy of the grains can also affect the changes in sound speed. After adjusting the zero position and scanning speed based on conventional ultrasonic standard test blocks, there is a deviation when testing on the actual austenitic stainless steel overlay. After experiments, it has been found that for adjusting the scanning speed of ultrasonic testing on the overlay layer, it is still necessary to make a stepped comparison test block for the overlay layer. By debugging the stepped comparison test block for the overlay layer, it can more accurately reflect the actual sound speed of the overlay layer. The scanning speed of the straight probe and the oblique probe can be adjusted through the step comparison test block shown in Figure 1.
3.3.2 Adjustment of distance amplitude curve
After adjusting the zero position and scanning speed, adjust the distance wave amplitude curve using the ultrasonic comparison test block of the overlay layer. Place the probe on the detection surface of the comparison test block, move the probe, and use the highest reflected echo line on the standard horizontal hole or flat bottom hole at different depths of the test block as the distance wave amplitude curve. When the surface roughness of the actual workpiece is different from that of the reference block, the resulting sound energy transfer loss should be compensated, but compensation is not necessary when the sound energy transfer loss is below 2 dB. Reduce the recording line of defect display by -6 dB according to the product standard distance amplitude curve (DAC curve).
3.4 Application of actual testing
The biggest difference between the detection of austenitic stainless steel overlay and conventional nuclear electroformed steel parts lies in the severe attenuation of acoustic waves and grass like waves in the austenitic structure. Before detection, the measurability and attenuation of the overlay must be measured to determine whether the ultrasonic wave meets the control requirements. The sensitivity of scanning and recording depends on the size, shape, and inclination angle of the defect, as well as the surface roughness of the inspected workpiece. However, the relative sensitivity method used in ultrasonic testing requires scanning sensitivity to be executed according to the customer requirements of the product. After the detection probe detects a defect through scanning, it is necessary to use a quantitative probe for re examination based on the location of the discovered defect, re scan the local area, and quantify and locate the defect.
4. Conclusion
Taking the practical application of ultrasonic testing for austenitic stainless steel cladding in nuclear power outer cylinders as an example, ZHY Casting analyzed and summarized the difficulties of ultrasonic testing for austenitic stainless steel cladding, the design and production of comparative test blocks, the selection of probes and scanning directions, and the judgment of measurability through experiments. The final summary points are as follows:
1) Different thicknesses of welding layers and welding processes have different effects on the measurability and attenuation of ultrasonic waves. Therefore, before conducting ultrasonic testing, it is necessary to make ultrasonic sensitivity comparison test blocks with the same welding process for testing and evaluation. The experiment shows that when the thickness of the austenitic stainless steel overlay is greater than 40 mm, the bottom echo is basically not visible.
2) When the quantitative accuracy of defects in the overlay layer is required to be high, probes with a frequency of 4 MHz should be selected more. However, when the thickness of the overlay layer is large, probes with a frequency of 2 mHz or lower should be selected more to reduce sound beam attenuation.
3) When mainly considering the qualitative analysis of defects at the fusion line between the overlay layer and the base metal, longitudinal wave straight probes are preferred; In order to discover and evaluate defects from multiple dimensions, different angles of twin crystal longitudinal wave oblique probes can be added for detection.
4) When the signal-to-noise ratio of ultrasonic testing on the weld overlay is poor, the ultrasonic testing results can be further verified and evaluated through methods such as radiographic testing.