The role of ultrasonic testing in non-destructive evaluation of weld quality

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The role of ultrasonic testing in non-destructive evaluation of welding quality. Download Audo look6.0. Weld inspection methods: 1. Visual inspection. 2. Tightness tests and hydrostatic strength tests. 3. Weld radiography. 4. Ultrasonic testing. 5. Magnetic particle testing. 6. Penetrant testing. Regarding repair requirements: each case should be handled on a case-by-case basis; in any case, efforts should be made to reduce the number of repairs. Steel structures are widely used in factory construction and equipment installation, and the welding quality of such structures is extremely important. Non-destructive testing is an essential method for ensuring the welding quality of steel structures. The conventional methods of non-destructive testing include visual inspection by the naked eye for macroscopic examination, as well as inspections using instruments such as radiographic testing, ultrasonic testing, magnetic particle testing, penetrant testing, and eddy current testing. Macroscopic inspection with the naked eye can be carried out without any instruments or equipment, but the naked eye cannot penetrate the workpiece to detect internal defects; whereas methods such as radiography can utilize various instruments and devices to detect such internal defects that are invisible to the naked eye, and they also help to **improve the accuracy and reliability of inspections. As for the method to be used for non-destructive testing, it needs to be determined based on the condition of the workpiece and the purpose of the testing. So what exactly is ultrasound? Sound waves with frequencies beyond what the human ear can hear, those with frequencies higher than 20 kilohertz, are called ultrasonic waves. Ultrasonic waves used for flaw detection have frequencies ranging from 0.4 to 25 megahertz, with 1–5 megahertz being the most commonly used range. Using sound to determine the quality of objects is a method that has been employed by people for a long time. For example, tap the watermelon with your hand to hear if it is ripe ； The doctor taps on the patient’s chest to check if the internal organs are functioning properly ； Tap on the porcelain bowl with your hand to see if it’s damaged or not, and so on. However, these detection methods that rely on human hearing to judge sounds are more objective and accurate than the sound-based method, and it is also easier to provide quantitative representations. Due to its advantages such as a large detection range, a compact and lightweight inspection device that is easy to carry to the site for inspections, fast detection speeds, and the fact that only coupling agents and worn probes are required during inspection resulting in lower overall costs, this method is currently the most widely used for inspections in the construction industry. Next, we will introduce the application of ultrasonic flaw detection in practical work. Upon receiving a flaw detection task, it is first necessary to understand the technical requirements for welding quality specified in the drawings. Currently, the acceptance standards for steel structures are based on GB50205-95, the \"Code for Construction and Acceptance of Steel Structure Engineering\". The standards specify that when the welding quality grade required for the welds as per the drawings is grade 1, and the assessment grade is grade II, 100% ultrasonic testing is required by the regulations ； When the required weld quality grade per the drawings is grade 2 but the assessment grade is grade III, the code stipulates that 20% ultrasonic testing must be carried out ； When the weld quality grade required by the drawings is level 3, ultrasonic internal defect inspection is not performed. It is worth noting here that ultrasonic testing is used for fully penetrative welds, with the testing ratio calculated as a percentage of the length of each weld, and it must be no less than 200 mm. For welds that are to be inspected locally, if any unacceptable defects are found, the inspection length should be increased along the sections extending from both ends of such defects; this increased length should be no less than 10% of the weld length and no less than 200 mm. If unacceptable defects still exist, a 100% inspection of the weld is required. It is also important to know the appropriate time for inspection: for carbon structural steel, inspection can be carried out after the weld has cooled to ambient temperature, while for low-alloy structural steel, inspection should take place 24 hours after welding is completed. It is also necessary to know the thickness of the base material of the workpiece to be tested, as well as the type of joint and the type of groove. To date, the vast majority of welds that have required flaw detection in my practical work have been of the butt-weld joint type for medium thickness plates; therefore, I will mainly provide a summary focused on the procedures for weld flaw detection. Generally, the thickness of the base material ranges from 8 to 16 mm, and the groove types include I-type, single V-type, X-type, and others. Only after understanding these things can the preparatory work before flaw detection be carried out. Before each flaw detection operation, it is necessary to use standard test blocks (CSK-IA, CSK-ⅢA) to calibrate the overall performance of the equipment and the calibration panel curves, in order to ensure the accuracy of the flaw detection results. 1. Preparation of the testing surface: Spatter, oxide scale, pits, rust, and other impurities on the welding surface must be removed; the surface finish should generally be below ▽4. The trimming width of the inspection surfaces on both sides of the weld is generally greater than or equal to 2KT + 50 mm, where (K is the K-value of the probe, and T is the thickness of the workpiece). Generally, a probe with a K value of 2.5 is selected based on the base material of the welded piece. For example: if the thickness of the base material of the workpiece to be tested is 10 mm, then 100 mm should be ground from each side of the weld. 2. The selection of a coupling agent should take into account factors such as viscosity, fluidity, adhesion, non-corrosiveness to the workpiece surface, ease of cleaning, and cost-effectiveness; based on these considerations, paste is chosen as the coupling agent. 3. Due to the thin thickness of the base material, the detection is carried out in a single-sided and double-sided manner. 4. Since the plate thickness is less than 20 mm, the horizontal positioning method is used to adjust the scanning speed of the instrument. 5. Coarse and fine flaw detection are employed during the flaw detection process. To roughly understand the presence and distribution of defects, as well as to quantify and locate them, this is precisely non-destructive testing. Several scanning methods such as serrated scanning, left-right scanning, front-back scanning, corner scanning, and circular scanning are used to detect various types of defects and determine their nature. 6. Record the detection results; if internal defects are found, conduct an evaluation and analysis of them. The grading of internal defects in welded joints shall comply with the provisions of the current standard GB11345-89 \"Methods for Manual Ultrasonic Testing of Steel Welds and Grading of Test Results\" in order to determine whether the weld is qualified. If defects exceeding the standards are found, a notice for rectification is issued to the workshop, requiring it to make the necessary corrections before retesting until compliance is achieved. Common defects in ordinary welds include porosity, slag inclusions, lack of penetration, lack of fusion, and cracks. To date, there is no mature method for accurately assessing the nature of defects; instead, a comprehensive evaluation of these defects is carried out by considering the shape of the defect waves and the changes in the amplitude of the reflected waves as observed on the fluorescent screen, along with the location of the defect and the welding process. The assessment of the nature of internal defects, as well as the causes of these defects and the measures to prevent them, can be summarized as follows: 1. Porosity: Individual porosity defects have a low echo height, and their waveform is that of a single slit, being relatively stable. When scanned from various directions, the reflected waves are generally similar, but they disappear as soon as the probe is moved slightly. Dense pores give rise to a cluster of reflected waves, with the amplitude of these waves varying depending on the size of the pores. When the probe is rotated in place, an alternating pattern of such waves appears. The main reasons for such defects are the welding materials not being dried at the specified temperature, the coating on the electrodes deteriorating and falling off, the core of the electrodes rusting, the welding wire not being cleaned properly, excessive current being used during manual welding, and the arc being too long ； Excessively high voltage or large fluctuations in the supply voltage during submerged arc welding ； Issues such as low purity of the shielding gas during gas shielded welding. If pores are present in the weld, they not only undermine the density of the weld metal but also reduce its effective cross-sectional area, thereby lowering its mechanical properties. In particular, the presence of chain-like pores leads to a significant reduction in bending and impact toughness. Measures to prevent such defects include: not using electrodes with cracked, flaked, or deteriorated coatings, as well as electrodes with corroded cores; rusted welding wire must be descaled before it can be used. The welding materials used should be dried at the specified temperature, the groove and its surrounding areas must be cleaned thoroughly, and appropriate welding current, arc voltage, and welding speed should be selected. 2. Slag inclusions: The echo signals resulting from punctate slag inclusions are similar to those of punctate pores. The echo signals associated with linear slag inclusions usually have a serrated shape, low amplitude, and a dendritic waveform; small peaks appear beside the main peak. The amplitude changes as the probe is moved, and the reflection amplitude varies when probing from different directions. The causes of such defects include: too low welding current, excessive welding speed, insufficient time for the slag to rise to the surface, inadequate cleaning of the edges to be welded and of the various weld layers, as well as inappropriate chemical compositions of the base metal and welding materials, with high levels of sulfur and phosphorus. Preventive measures include: selecting the appropriate welding current, ensuring that the groove angle of the welded parts is not too small, cleaning the groove thoroughly before welding, and removing slag after each layer in multi-layer welding ； And reasonably select the welding torch angle, welding speed, etc. 3. Incomplete welding: The reflection rate is high, and the wave amplitude is also high. When the probe is moved, the waveform remains relatively stable, and roughly the same reflection amplitude is obtained when inspecting both sides of the weld. Such defects not only reduce the mechanical properties of the welded joint, but also create stress concentration points at the notches and ends in the areas where welding is incomplete; under load, these can lead to cracks, making them dangerous defects. The common causes are: too small a gap at the edge of the groove, too low a welding current or too fast a wire feeding speed, a small groove angle, an incorrect wire feeding angle, and arc deflection. Preventive measures include: properly selecting the groove type, assembly clearance, and using the correct welding process. 4. Lack of fusion: When the probe is moved horizontally, the waveform remains stable; however, when scanning from both sides, the amplitude of the reflected waves differs, and sometimes detection is only possible from one side. The reasons for this include: an unclean groove, too fast welding speed, insufficient or excessive current, incorrect electrode angle, and arc deflection. Preventive measures: Proper selection of the groove and current, thorough cleaning of the groove, and correct operation to prevent welding deviation, etc. 5. Cracks: The echo height is high, the wave amplitude is wide, and multiple peaks appear. As the probe is moved, the amplitude of the reflected waves changes continuously; when the probe is rotated, the wave peaks shift up and down. Cracks are the most dangerous type of defect; in addition to reducing the strength of welded joints, their sharp, pointed ends cause stress concentration when the welded component is under load, thereby becoming the source of structural failure. Cracks are divided into three types: thermal cracks, cold cracks, and reheat cracks. The cause of thermal cracking is the rapid cooling rate of the weld pool during welding, which leads to segregation ； Uneven heating of the weld generates tensile stress. Preventive measures: Limit the content of elements prone to segregation and harmful impurities in the base material and welding materials, primarily by restricting sulfur content and increasing manganese content ； Increase the alkalinity of the electrode or flux in order to reduce impurity levels and improve the degree of segregation ； Improve the welding structure design and adopt a reasonable welding sequence to increase the degree of freedom during weld contraction. Reasons for cold cracking: The weldable material has high hardenability, and it is prone to cracking during cooling under the effect of welding stresses ； During welding, the cooling rate is very fast, preventing hydrogen from escaping and causing it to remain in the weld. Hydrogen atoms combine to form hydrogen molecules, which enter the tiny pores in the metal in gaseous form. This creates high pressure, exerting significant force on the local metal and resulting in cold cracks ； When welding stress, which is a tensile stress, occurs simultaneously with hydrogen accumulation and quenching embrittlement, cold cracks are likely to form. Preventive measures: Preheating before welding and slow cooling after welding enable the austenite decomposition in the heat-affected zone to occur within an appropriate temperature range, thereby preventing the formation of hardened structures and also helping to reduce welding stresses ； After welding, low-temperature annealing and dehydrogenation should be carried out promptly to eliminate the stresses generated during welding and to allow hydrogen to diffuse out of the material ； Use low-hydrogen electrodes and basic fluxes, or austenitic stainless steel electrodes and wires; the welding materials must be dried as specified, and the groove areas must be thoroughly cleaned ； Enhance protection during welding and clean the surface of the area to be welded in order to prevent hydrogen from entering ； Select appropriate welding specifications and adopt a reasonable welding sequence to improve the stress condition of the welded parts.