What are the NDT methods for defect detection and welding quality?

Date：2022-07-13 14:58:24Views：755

Welding defect refers to the defect formed in the welding process of the welded joint. Product surface defect detection is an important link in industrial production and a key step in product quality control. With the help of defect detection technology, production quality and efficiency can be effectively improved. In order to help you have an in-depth understanding, this paper will summarize the relevant knowledge of welding nondestructive testing. If you are interested in what this article will cover, then read on.

The internal defects of the weld are:

1. Stomata

Porosity refers to the cavity formed by the residual gas in the weld during welding without overflowing before the metal solidifies. The gas may be absorbed by the molten pool from the outside, or it may be generated by reaction in the process of welding metallurgy.

2. Slag inclusion

Slag inclusion refers to the phenomenon that slag residue exists in the weld after welding.

3. Crack

Cracks are cracks caused by the destruction of atomic bonding in the weld and the formation of new interfaces.

4. Incomplete penetration

Incomplete penetration refers to the phenomenon that the base metal is not melted and the weld metal does not enter the root of the joint.

5. Incomplete fusion

Lack of fusion refers to the defect that the weld metal and the base metal, or the weld metal are not fused together. According to its location, lack of fusion can be divided into groove lack of fusion, interlayer lack of fusion and root lack of fusion.

Inspection method for welding defects

Necessary inspection of welded joints is an important measure to ensure welding quality. Therefore, after the workpiece is welded, the weld should be inspected according to the product technical requirements. Any defects that do not meet the technical requirements should be repaired in time. The inspection of welding quality includes appearance inspection, nondestructive testing and mechanical property test. These three are complementary to each other, and NDT is the main method.

1. Appearance inspection

Visual inspection is generally based on visual observation, sometimes with a magnifying glass of 5-20 times. Through the visual inspection, the weld surface defects can be found, such as undercut, weld beading, surface cracks, pores, slag inclusions and weld penetration. The overall dimension of the weld can also be measured with a crater detector or a template.

2. Nondestructive testing

Inspection of slag inclusions, pores, cracks and other defects hidden in the weld. At present, X-ray inspection, ultrasonic flaw detection and magnetic flaw detection are the most commonly used.

X-ray inspection is to use X-ray to photograph the weld, and judge whether there are internal defects, the number and type of defects according to the negative image. Then evaluate whether the weld is qualified according to the product technical requirements.

The ultrasonic beam is sent from the probe to the metal. When the ultrasonic beam reaches the interface between metal and air, it refracts and passes through the weld. If there are defects in the weld, the ultrasonic beam will be reflected to the probe and accepted, and then the reflected wave will appear on the fluorescent screen. According to the comparison and identification between these reflected waves and normal waves, the size and location of defects can be determined. Ultrasonic flaw detection is much simpler than X-ray photography, so it is widely used. However, ultrasonic flaw detection can only be judged by operating experience, and can not leave an inspection basis. Magnetic flaw detection can also be used for internal defects not deep from the weld surface and extremely small cracks on the surface.

3. Hydrostatic test and pneumatic test

For pressure vessels requiring tightness, hydrostatic test and / or pneumatic test must be carried out to check the tightness and pressure bearing capacity of the weld. The method is to inject clean water 1.25-1.5 times the working pressure or gas equal to the working pressure (mostly air) into the container, stay for a certain time, then observe the pressure drop in the container, and observe whether there is leakage outside. According to these, whether the weld is qualified can be evaluated.

4. Mechanical property test of welded test plate

Nondestructive testing can find the internal defects of the weld, but it can not explain the mechanical properties of the metal in the heat affected zone of the weld. Therefore, sometimes the tensile, impact, bending and other tests should be carried out on the welded joints. These tests are completed by the test panel. The test plate used should be welded together with the longitudinal joint of the cylinder to ensure the consistency of construction conditions. Then carry out mechanical property test on the test plate. In actual production, generally only the welded joints of new steel grades are tested in this regard.

The above is the content related to welding nondestructive testing compiled by the core testing team. I hope it will be helpful to you. Shenzhen Chuangxin Online Testing Technology Co., Ltd. is a well-known professional testing institution for electronic components in China, with 3 standardized laboratories covering an area of more than 1000 square meters. The scope of testing services covers: electronic component testing and verification, IC authenticity identification, product design and material selection, failure analysis, function testing, factory incoming material inspection, component X-ray testing, taping and other test items.