What are the NDT methods for weld quality testing?

Date：2022-01-11 14:31:00Views：1641

Since its birth, welding technology has played an important role in human production and life, and covers almost all industrial industries, such as construction, heavy industry, vehicles, ships, aerospace and so on. However, defects from the inside and outside of the weld will usually lead to welding failure, such as reduced welding strength, poor welding sealing, poor weld aesthetics, etc. Nondestructive testing has a wide range of applications. The development of core technology promotes the development of nondestructive testing equipment. Nowadays, nondestructive testing equipment has gradually changed from completely relying on import to independent research and development. The quality of domestic equipment is not inferior to that of import. Especially in some industries that require product accuracy, product quality is particularly important.

NDT methods for welds, including radiographic inspection (x γ）、 Ultrasonic flaw detection, magnetic particle, penetration and eddy current flaw detection, among which radiographic flaw detection and ultrasonic flaw detection are suitable for the detection of internal defects of welds, magnetic particle, penetration and eddy current are suitable for the inspection of weld surface quality, and the NDT method shall be selected according to the weld material and structural characteristics.

(1) Radiographic inspection (x γ） Method (RT): Yes, using X γ， After the penetrating radiation emitted by the ray source penetrates the weld, the film is sensitized, and the defect image in the weld is displayed on the processed radiographic film. It is a widely used nondestructive testing method at present. It can find defects such as pores, slag inclusion, cracks and incomplete penetration in the weld, and defects such as slag inclusion, cracks and incomplete penetration. Radiographic testing is basically not limited by the thickness of the weld. However, the defect depth cannot be measured, the inspection cost is high, the time is long, and the radiographic inspection operator is damaged.

(2) Ultrasonic testing (UT): it uses the piezoelectric transducer to generate pulse vibration through instantaneous electrical excitation, transmits ultrasonic wave into the metal with the help of acoustic coupling medium, encounters defect reflection during propagation and returns to the transducer, and then converts the acoustic pulse into electrical pulse. The position and severity of defects in the workpiece can be evaluated by measuring the amplitude and propagation time of the signal. Compared with radiographic flaw detection, ultrasonic flaw detection has the advantages of high sensitivity, flexibility and convenience, short cycle, low cost, high efficiency and no harm to human body, but the display of defects is not intuitive and the judgment of defects is not accurate, which has a great impact on the experience and technical proficiency of flaw detection personnel.

(3) Magnetic flaw detection (MT): it refers to the change of magnetic susceptibility caused by surface and near surface defects of ferromagnetic materials, and the leakage magnetic field is generated on the surface during magnetization, and then the defects are recorded and displayed by magnetic particle, magnetic tape or other magnetic field measurement methods. It is mainly used to detect weld surface or near surface defects.

(4) Penetrant testing (PT): the penetrant containing pigment or fluorescent powder is sprayed or coated on the surface of the inspected weld, and the capillary action of the liquid is used to make it penetrate into the defects of the surface opening, then the excess penetrant on the surface is cleaned and removed, the developer is applied after drying, the penetrant in the defect is adsorbed on the weld surface, and the display trace of the defect is observed. This method is mainly used for weld surface detection or root defect detection after gouging.

(5) Eddy current testing (ET): the effect of eddy current can be induced on the weld surface by using the high-frequency current flowing in the probe coil. Defects will change the eddy current magnetic field and cause the change of coil output (such as voltage or phase) to reflect the defects. The control of inspection parameters is relatively difficult, and the surface or near surface defects of conductive material or weld and surfacing layer can be inspected.

In conclusion, the basic purpose of NDT is to evaluate the quality of the object without damaging it. The fundamental reason for this is risk management. Although NDT cannot eliminate the risk, it can significantly reduce or mitigate the risk. Therefore, it can not only check the whole process of raw material manufacturing, intermediate process links and final products, but also check the equipment in use. It can be said that through nondestructive testing, the service life and performance of products in many industries have been greatly improved.