Abstract
This paper discusses the application of ultrasonic testing technology to detect buried defects in butterfly valve steel casting. During the development of the testing process, the unique acoustic characteristics of steel casting was considered, and appropriate single/dual crystal longitudinal wave probe parameters were selected through comparative testing. The key points of the testing process for both single-crystal and dual-crystal probes are elaborated. Based on testing experience, the waveform characteristics and mechanisms of four typical clustered internal defects commonly found in large butterfly valve steel casting is summarized. Practice has shown that the composite testing process using single/dual crystal longitudinal wave probes is a practical non-destructive testing technique that can ensure that butterfly valve steel casting entering the finishing process meet the corresponding quality requirements.
1. Introduction
Large steel casting, due to their larger size, complex production processes, and variations in production equipment conditions among different manufacturers, often have more types and quantities of internal defects compared to small steel casting. These defects can include voids, cracks, delaminations, and slag inclusions produced during the casting process of steel casting . Excessive defects in steel casting not only affect the quality of the finished product but may also lead to decreased mechanical and physical properties during equipment operation, causing equipment failure and even safety hazards.
Currently, the technical requirements for valve bodies and butterfly plates in the manufacturing process of large butterfly valves mainly include surface macro inspections (VT), mechanical property indicators, material chemical composition, heat treatment requirements, and MT, PT, UT inspection reports. The detection of buried defects in the machined surfaces of butterfly valve steel casting (including flanges, inner chambers, butterfly plates, etc.) primarily employs ultrasonic testing technology [2-3]. Especially before the precision machining of butterfly valve castings, a suitable and stringent ultrasonic testing process should be used to detect defects, identify and evaluate them, perform repairs and re-inspection, and ultimately ensure the quality of the castings meets the requirements before entering the precision machining process.
Steel casting for hydropower butterfly valves exhibit similar acoustic characteristics to steel casting, with coarse grains and non-compact internal structures due to long solidification and cooling times of local molten steel and slower cooling rates [4-5]. The attenuation and scattering of ultrasonic waves due to coarse grains in steel casting is unavoidable issues in the development of flaw detection processes [6-7]. Additionally, the requirement for detecting near-surface defects on machined surfaces makes it often impossible to complete such defect detection using only single-crystal straight probes [8]. Therefore, when developing the testing process, these issues should be fully considered, and probes with appropriate parameters (material, type, frequency) and testing methods should be selected through necessary actual measurements and comparisons to achieve a relatively ideal testing effect.
2. Experimental Equipment and Probe Selection
2.1 Experimental Equipment
This paper utilizes the HS-II digital ultrasonic flaw detector with a vertical linearity error of no more than 3%, a horizontal linearity error of no more than 0.1%, a signal bandwidth of 0.5-30 MHz, and a gain range of 0-120 dB.
2.2 Selection of Single-Crystal Longitudinal Wave Probe Parameters
The attenuation of sound energy during ultrasonic testing of steel casting primarily stems from interface reflections and sound wave absorption caused by coarse grains. The higher the probe frequency, the more severe the ultrasonic attenuation. Simultaneously, grass-like noise interference formed by irregular reflections at grain boundaries during flaw detection also affects the signal-to-noise ratio of ultrasonic testing. Therefore, lower-frequency probes are preferable for ultrasonic testing of steel casting.
In practical inspection, the accuracy and quality of ultrasonic testing of steel casting is influenced by various factors. The best testing effect should be achieved through on-site actual testing and verification. During process debugging, acoustic parameters such as sound velocity, acoustic impedance, and material attenuation coefficients are typically measured to facilitate the selection of appropriate probe parameters (frequency, crystal size). This project employs single-crystal straight probes of different frequencies to conduct comparative tests on simulation blocks within different thickness ranges. By calculating attenuation coefficients, comparing flat-bottomed hole defect equivalents, contrasting signal-to-noise differences during testing, and considering detection equipment conditions, the selection range of single-crystal straight probe parameters for ultrasonic testing of butterfly valve steel casting is established, as shown in Table 1.
Table 1: Single-Crystal Straight Probe for Butterfly Valve Steel Casting Inspection
| Steel Casting Thickness (mm) | Single-Crystal Longitudinal Wave Probe |
|---|---|
| Nominal Frequency (MHz) | |
| 50-100 | 3.0-3.5 |
| 100-200 | 2.5-3.0 |
| 200-400 | 2.0-2.5 |
| 400-600 | 1.5 |
2.3 Selection of Dual-Crystal Straight Probes
The detection of buried defects in the near-surface and thin-walled areas of this butterfly valve steel casting employs dual-crystal straight probes. Dual-crystal straight probes overcome the disadvantages of larger blind zones and wide initial wave widths of single-crystal straight probes, offering significant advantages for detecting near-surface defects in steel casting.
This project uses two different types of dual-crystal straight probes for comparison. Both probes have a focal length of 20 mm, a frequency of 2.5 MHz, and a diameter of ϕ20 mm, but their crystal material differs: lead zirconate titanate ordinary piezoelectric ceramics (PZT type) and polymer piezoelectric composite materials (1-3 type). The two probes are used to scan a dedicated flat-bottomed hole test block for dual-crystal probes, each with a ϕ3 mm flat-bottomed hole (flat-bottomed hole diameter of ϕ3 mm, depths L of 5, 10, 15, 20, 25, 30, 35, 40, 45 mm), as shown in Figure 1.
Figure 1: Flat-Bottomed Hole Test Block for Dual-Crystal Probes
The scanning results show that the 1-3 type dual-crystal probe can well identify flat-bottomed holes with depths of 5, 10 mm, and deeper, as shown in Figure 2. The ordinary PZT dual-crystal probe struggles to identify the 5 mm flat-bottomed hole on the test block but can identify holes with depths of 10, 15 mm, and deeper. However, its signal-to-noise ratio is slightly inferior to that of the 1-3 type dual-crystal probe, as shown in Figure 3. Additionally, compared to the PZT type, the 1-3 type dual-crystal probe exhibits a smaller initial wave width and higher signal-to-noise ratio for near-surface detection, providing a significant advantage for detecting near-surface defects. The reason is that the crystal of the 1-3 type dual-crystal straight probe is composited of piezoelectric ceramics and polymeric materials, featuring low mechanical quality factor, excellent transmission and reception performance, high damping, and high sensitivity. During ultrasonic testing, it offers a high signal-to-noise ratio and can effectively suppress noise waves. Therefore, after comprehensive comparison, the 1-3 type dual-crystal longitudinal wave probe is selected for detecting the near-surface and thin areas of this butterfly valve steel casting.
3. Key Points of the Testing Process
3.1 Measurement of On-Site Attenuation Coefficient of Butterfly Valve Steel Casting
The attenuation coefficient of the steel casting material is calculated using a parallel machined flat surface representing a sound area of the casting. The testing method follows “Non-Destructive Testing of Pressure Equipment” [11]. It is important to measure three areas of each workpiece and take the average value, which is used as a parameter for defect quantification and compensation in the testing results.
3.2 Single-Crystal Longitudinal Wave Probe Testing
When the thickness T of the inspection area is less than 3N (where N is the near-field length), the DAC method (Distance Amplitude Curve method) is used to determine the inspection sensitivity. During debugging, note that the test block material should be consistent or similar to the material, heat treatment state, and attenuation coefficient of the butterfly valve steel casting, and the deepest flat-bottomed hole test block should cover the maximum thickness of the inspection area. When creating the curve, ensure that the reflection amplitude of each flat-bottomed hole reaches 80% of the full screen before recording, and use this as the base sensitivity, increasing the inspection sensitivity by 6 dB during scanning. When the thickness T of the inspection area is greater than or equal to 3N, a parallel machined flat surface representing a sound area of the casting is selected, and the sensitivity is determined using the bottom wave calculation method [5]. This method requires selecting an appropriate flat-bottomed hole diameter based on acceptance criteria to calculate the gain value, which is then increased by 6 dB as the inspection sensitivity. During on-site scanning, the same coupling method used during debugging should be employed, and appropriate surface coupling compensation should be made based on the workpiece surface condition. When evaluating defects, the sensitivity should be reduced by 6 dB, and compensation should be made based on the calculated attenuation coefficient.
3.3 Dual-Crystal Longitudinal Wave Probe Testing
The DAC method is employed to determine the base sensitivity and scanning sensitivity for dual-crystal probe testing. The 1-3 type dual-crystal longitudinal wave probe is used to sequentially scan each ϕ3 mm flat-bottomed hole shown in Figure 1, and the reflection wave height when each reflection amplitude reaches 80% is recorded to create the DAC curve. During scanning, appropriate surface coupling compensation should be made based on the workpiece surface condition.
4. Scanning Methods and Defect Judgement
When scanning the machined surfaces of butterfly valve steel casting, a segmented scanning method is adopted, with scanning performed in mutually perpendicular directions, and each segment overlapping by 15%. When manually scanning with a dual-crystal probe, note that the probe movement direction should be perpendicular to the sound insulation layer, and the scanning speed should not exceed 150 mm/s.
According to differences in the flaw detection contract requirements and acceptance criteria for steel casting, the defect evaluation criteria for various ultrasonic flaw detection projects vary. When using the DAC curve for flaw detection and scanning, the following evaluation method can be referenced: defects are considered unqualified if their reflection waves reach or exceed the DAC line and are not punctiform defects; defects are also considered unqualified if their area exceeds a certain range despite their reflection wave amplitude not reaching the DAC line. When using the bottom wave calculation method to determine sensitivity for scanning, defects should be recorded if the amplitude reduction of the bottom echo caused by the defect is not less than 12 dB. Through repeated practical testing, combining the static and dynamic reflection waveforms of defects is conducive to judging the type and distribution range of defects in steel casting.
5. Typical Defects Detected During Flaw Detection of Butterfly Valve Steel Casting
Due to their structural characteristics of loose organization and coarse grains, steel casting materials are prone to defects such as pores, delaminations, and slag inclusions during the production process, which can significantly affect product quality if they exhibit locally concentrated distributions. Therefore, summarizing the waveform characteristics of such clustered defects provides certain guidance for defect identification and evaluation. Common clustered defects include slag inclusion-type aggregated defects, pore-type aggregated defects, shrinkage cavity-type aggregated defects, and island-like delamination defects. These defects often do not exist independently but accompany other defects, such as pores and slag inclusions, shrinkage cavities and pores, slag inclusions, and delaminations accompanied by cold shuts. Below are the waveform characteristics of four typical defects detected using the composite inspection process with single-crystal/dual-crystal probes for butterfly valve steel casting.
5.1 Waveform Characteristics of Pore-Type Aggregated Defects
Pore defects are concentrated in a local area, and their primary waveform characteristics are low defect waveform amplitudes but sharp pulse peaks that are interconnected. This waveform characteristic arises because pores are primarily globular structures, especially gas evolution pores with smooth inner walls, resembling regular spherical reflectors. When ultrasonic waves reflect on the spherical interface of pores, the reflection wave directions are relatively dispersed, resulting in fewer reflected sound waves received by the probe. Comparing the sound pressure calculation formulas for spherical reflectors and circular plane reflectors, under the same reflector diameter, the sound pressure of spherical defect reflections is proportional to the diameter, while the sound pressure of circular plane reflections is proportional to the square of the diameter. Therefore, the sound pressure of pore reflections is relatively small. When the reflection waveform of pore-type aggregated defects appears, the large flat bottom wave of the steel casting generally does not disappear.
5.2 Waveform Characteristics of Island-Like Delamination Defects
The reflection waveform of delamination defects is steep, sharp, with repeated echoes, and a significant reduction in the bottom wave. During actual testing, more severe large-area delamination defects often lead to the disappearance of the bottom wave. After the emergence of this defect waveform, repeated scanning reveals good defect repeatability. The shape of the delamination distribution area is often irregular, similar to a small island, and the defect waveform within this area is continuous. Combining static and dynamic waveforms for comparative judgment is beneficial for identifying delamination defects. During actual testing, delamination defects may accompany other defects, such as cold shuts, causing certain changes in the defect waveform but overall continuity in the defect waveform.
5.3 Waveform Characteristics of Shrinkage Cavity-Type Aggregated Defects
The waveform characteristics of shrinkage cavity-type aggregated defects are: the main defect wave appears in a bundle with a relatively wide base and branched wave peaks, sometimes with more than one main defect wave, accompanied by small reflection waves nearby. Shrinkage cavity-type aggregated defects are primarily characterized by shrinkage cavity defects in a local area. During the formation of shrinkage cavities, the steel liquid cools and solidifies, contracting, and crystalline growth occurs on the hole inner wall, resulting in an uneven inner wall surface. The smoothness of the inner wall of shrinkage cavities is much worse than that of pores, and their sound pressure reflectivity is relatively high. Although severe scattering also occurs when sound waves reflect on the inner wall of shrinkage cavities, the reflection signals formed by reflectors on the same wavefront are still strong, typically resulting in 1-2 main defect waves. During actual testing, shrinkage cavity-type aggregated defects are often accompanied by small defects such as inclusions and slag nearby. Simultaneously, the reflection sound paths on different wavefronts differ, leading to the appearance of small reflection waves near the main defect wave.
5.4 Waveform Characteristics of Slag Inclusion-Type Aggregated Defects
The reflection waveform characteristics of slag inclusion-type aggregated defects are: the main defect wave appears in a bundle with blunt wave peaks accompanied by branches, a rounded wave head, and many low, small defect waves nearby. The defect wave amplitude fluctuates as the probe moves, with a large echo width. Severe area-type slag inclusion defects are accompanied by significant bottom wave reductions. Unlike pore defects, slag inclusion defects still have filler material, providing a certain degree of sound transmissivity; however, the difference in acoustic impedance between the slag inclusion and the steel casting matrix gives the ultrasonic wave significant acoustic reflection characteristics at this location. The typical waveform of slag inclusion distribution defects.
6. Conclusion
This paper introduces the ultrasonic testing process for large butterfly valve steel casting used in hydropower applications. During the development of the process, considering the structural characteristics of butterfly valve steel casting, appropriate parameters for single-crystal and dual-crystal longitudinal wave probes were selected through comparative testing, and separate testing processes were developed to achieve the best testing effect. The waveform characteristics of four common clustered defects in butterfly valve steel casting is summarized, providing a certain reference for the practical testing of steel casting.