Reliable Weld Examination Through Modern NDT Technologies

Non-destructive testing (NDT) is a comprehensive method used to assess the integrity of materials and components without causing damage or affecting their performance. This technique employs sound, light, magnetism, and electricity to detect defects or irregularities in the object being inspected. NDT provides crucial information about the size, location, nature, and quantity of any defects, which helps determine the technical condition of the object—such as whether it meets qualifications and its remaining lifespan. Commonly used NDT methods include Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Liquid Penetrant Testing (PT), and Radiographic Testing (RT).

Ultrasonic Testing

Ultrasonic Testing (UT) is one of the non-destructive testing (NDT) methods used in industrial applications. In this process, ultrasonic waves are introduced into an object. When these waves hit a defect within the material, some of the sound waves are reflected back. The transmitter and receiver then analyze these reflected waves to accurately identify the defects, determine their location and size, and measure the thickness of the material.

Advantages of ultrasonic testing:
1. Exceptional penetration capability, allowing for effective detection depths in steel exceeding 1 meter.
2. High sensitivity for identifying planar defects, such as cracks and inclusions, along with the ability to assess their depth and relative size.
3. Lightweight equipment that ensures safe operation and is easy to automate.

Disadvantages:

Not suitable for inspecting CNC milling products with complex shapes; requires a certain level of surface smoothness; and needs a couplant to fill the gap between the probe and the inspected surface for adequate acoustic coupling.

 

Magnetic Particle Inspection

To begin, let’s clarify the principle of magnetic particle inspection. When ferromagnetic materials and components are magnetized, any discontinuities in the material cause local distortions in the magnetic field lines near the workpiece surface, resulting in a leakage magnetic field. This leakage field attracts magnetic particles that are applied to the surface of the workpiece. Under appropriate lighting conditions, these particles form visible traces that reveal the location, shape, and size of the discontinuities present.

The applicability and limitations of magnetic particle inspection are:

1. Magnetic particle inspection is a method used to detect surface and near-surface discontinuities in ferromagnetic materials. It is particularly effective for identifying very small defects that are difficult to see with the naked eye.

2. This inspection technique can be applied to a variety of china CNC parts and under different conditions.

3. It is capable of detecting defects such as cracks, inclusions, hairline fractures, white spots, folds, cold shuts, and porosity.

4. However, magnetic particle inspection cannot be used on austenitic stainless steel materials or welds made with austenitic stainless steel welding rods. It is also ineffective for non-magnetic materials, including copper, aluminum, magnesium, and titanium. Additionally, it is challenging to detect shallow surface scratches, deep holes, and delaminations or folds at angles of less than 20° to the workpiece surface.

Liquid penetrant testing

The basic principle of liquid penetrant testing is as follows: First, the surface of a part is coated with a fluorescent or colored dye. The penetrant then seeps into any surface defects due to capillary action over a designated amount of time. After this, any excess penetrant on the surface is removed, and a developer is applied. Similar to the penetrant, the developer draws out the penetrant that remains trapped in the defects, causing it to seep back into the developer. Under a specific light source—either ultraviolet or white light—the traces of the penetrant in the defects become visible, appearing as yellow-green fluorescence or bright red. This process allows for the detection and evaluation of the morphology and distribution of the defects.

 

Advantages of penetrant testing include:

1. It can test a variety of materials; 2. It offers high sensitivity; 3. The display is user-friendly, operations are convenient, and testing costs are low.

 

Disadvantages of penetrant testing include:

1. Penetrant testing is not appropriate for inspecting workpieces made of porous materials or those with rough surfaces.
2. This method can only identify surface-level defects, making it challenging to assess the actual depth of these defects. Consequently, providing a quantitative evaluation is difficult, and the accuracy of detection results can be significantly influenced by the operator’s skills.

X-ray Inspection

The final method, X-ray inspection, uses X-rays to examine objects. As X-rays pass through the object being inspected, they are absorbed to different degrees depending on the materials and thickness involved. A film positioned on the opposite side of the object captures an image created by the varying intensities of the X-rays. This allows the inspector to identify the presence and nature of any internal defects.

Applicability and Limitations of X-ray Inspection:

1. X-ray testing is highly sensitive to volumetric defects and relatively easy to characterize them.
2. X-ray films are easy to preserve and review.
3. This method visually displays the shape and type of defects.
4. However, there are some disadvantages, including the inability to determine the depth of defects, limited thickness detection, the need for specialized film processing, potential health risks due to radiation exposure, and higher costs.

In summary, both ultrasonic and X-ray testing are effective for detecting internal defects. Ultrasonic testing is recommended for components thicker than 5 mm and with regular shapes. In contrast, X-ray testing cannot accurately determine defect depth and involves exposure to radiation. For surface defects, magnetic particle testing and penetrant testing are suitable. However, magnetic particle testing is only applicable to magnetic materials, while penetrant testing is limited to defects that are open to the surface.

 

 

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