Views: 30 Author: Site Editor Publish Time: 2018-06-08 Origin: www.fuchun-casting.com
Non Destructive Testing is called NDT. It is a technology that uses the principles of ray, ultrasound, infrared, electromagnetic and other principles without damaging or affecting the performance of the detected object, and combining the instruments with the instrument to detect the defects, chemical and physical parameters of the material, parts and equipment.
Non destructive testing can detect defects in materials and workpieces, as well as the surface and inside, and can also measure the geometric features and dimensions of the workpiece, and can determine the internal structure and state of the material or the workpiece.
NDT can be applied in many aspects such as design, material selection, processing, inspection and maintenance.
NDT can play an optimal role in quality control and cost reduction, and also ensure the safe operation and effective use of products.
The commonly used methods of detection are as below:
3、Magnetic particle Testing（MT）；
4、Penetrant Testing （PT）;
5、Eddy Current Testing （ET）.
UT is a nonducted through ultrasonic interaction with the workpiece, and then the reflection, transmission and scattering waves are studied.It is a technique for testing the workpiece, measuring the geometrical characteristics, testing the structure and mechanical properties, and evaluating its specific application.
The pri nciple of ultrasonic work is mainly based on the propagation characteristics of ultrasonic waves in the workpiece.
a. Sound source can generate ultrasonic wave and ultrasonic wave enters the workpiece in certain way.
b. Ultrasonic waves propagate in the workpiece and interact with the material and defects in the workpiece to change the direction or characteristics of the propagation.
c. The modified ultrasonic wave is received through the detection equipment, and can be processed and analyzed.
d. According to the characteristics of the received ultrasound, the defect characteristics of the workpiece are evaluated.
The advantages of ultrasonic testing: it is suitable for a variety of parts such as metal, nonmetal and composite materials. The penetration ability is strong and can be used to detect the internal defects of the specimen in a large thickness range. For metal materials, thin wall tubes and plates with a thickness of 1 ~ 2mm can be detected and steel forgings of a few meters long can be detected. The sensitivity of the flaw detection is high and the depth and relative size of the defects can be measured. The equipment is portable, safe and easy to realize automatic test.
The disadvantage is that it is not easy to check the workpiece with complex shape, and it requires that the surface of the inspected surface be glabrous. For some coarse-grained castings and welds, it is difficult to apply random reflection waves.
RT is a nondestructive testing method for recording information with X - ray or gamma ray. This method is the most basic and the most widely used non destructive testing method.
The principle of RT: the rays can penetrate the unpenetrable substance of the naked eye to make the film photosensitive. When X rays or gamma rays irradiate the film, it can produce the latent shadow of the silver halide in the film, as with the ordinary light, as the number of different absorption lines of different density substances irradiates the radiographs of the film everywhere. The energy will also be different, so we can distinguish defects according to the blackness difference of the negatives processed.
The characteristics of radiography:
a. The visual images with defects can be obtained qualitatively and accurately, and the accuracy of the length and width is also more accurate.
b. The results are recorded directly and can be preserved for a long time.
c. The detection rate of volumetric defects (gas hole, slag, tungsten, burn through, undercut, welding, pits, etc.) is very high. However, if the camera angle is not appropriate, the area type defects (incomplete penetration, non fusion, cracks, etc.) are easy to be missed.
d. It is suitable to test the workpiece with thin thickness, because the thickness of the workpiece requires high energy ray equipment, and with the increase of thickness, the test sensitivity will also decrease.
e. Suitable for checking butt welds, not suitable for inspection of fillet welds, plates, bars, forgings, etc.
f. It is difficult to determine the location and size (height) of defects in the workpiece.
g. The detection cost is high and the speed is slow.
h. With the radiation biological effect, NDT can kill biological cells, damage biological tissues and endanger the normal functions of biological organs.
In general, the characteristics of RT are - more accurate qualitative, visual images that can be preserved for a long time, the overall cost is relatively high, and the rays are harmful to the human body, and the test speed will be slower.
Magnetic particle Testing:
MT is a method to observe defects by using magnetic powder as the display medium.
The workpiece made of iron and steel and other magnetic materials is magnetized, and the magnetic powder is adsorbed by the leakage magnetic energy of the defective parts. Finally, according to the magnetic powder distribution display, the surface defect and the flaw detection method of the near surface are expressed. The characteristics of this method are simple and direct display.
The applicability and limitations of MT:
a. MT is suitable for testing ferromagnetic materials with very small surface size and narrow gap.
b. MT can detect raw materials, semi-finished products, finished products and parts in service, and can also be used for testing sheet, profile, pipe, bar, welding, cast steel and forged steel.
c. Defects such as cracks, inclusions, hairlines, white spots, folding, cold separation and porosity can be found.
d. MT can not detect austenitic stainless steel material and weld seam welded with austenitic stainless steel electrode, and the non magnetic materials such as copper, aluminum, magnesium and titanium can not be detected. For shallow scratches, buried deep holes, and stratification and folding with a workpiece angle less than 20 degrees, it is difficult to find.
Penetrant Testing :
PT is a nondestructive testing method based on the principle of capillary action to check surface defects.
(1) PT can detect the surface openings of (steel, heat resistant alloy, aluminum alloy, magnesium alloy, copper alloy) and non metal (ceramic, plastic) workpiece, such as cracks, porosity, porosity, slag inclusion, cold septum, folding and oxidation spot scars. These surface defects are difficult to detect directly under normal circumstances.
(2) PT is not restricted by chemical composition of the controlled workpiece.
(3) The PT is not restricted by the structure of the workpiece to be inspected. PT can inspect weldments or castings, check calendering parts and forgings, and check machined parts.
(4) PT is not restricted by defect shape (linear defect or volume defect), size and orientation. Only one PT is required to check all defects on the surface at the same time.
However, PT can not or is difficult to examine porous materials, such as powder metallurgy workpieces.It does not apply to inspection of defects caused by external factors, such as shot peening or sandblasting, which may plug the "openings" of surface defects. It is difficult to quantify the quality of the test, depending on the experience of the inspectors, the seriousness and the acuity of the eyesight.
Eddy Current Testing:
ET is a non-contact detection method. It is one of the methods of nondestructive testing in industry to detect the electromagnetic induction between the electromagnetic field and the metal.
In ET, the coil does not need to contact the object directly. It can be detected at high speed and is easy to automate, but it is not suitable for complex shape parts. It can only detect the surface and near surface defects of the conductive material, and the detection results are easily interfered by the material itself and other factors.
Nondestructive testing is a necessary and effective tool for industrial development. It reflects the industrial development level of a country to a certain extent. The importance of nondestructive testing has been recognized. The non-destructive testing equipment is specially designed for the shipbuilding, petroleum, chemical, mechanical, aerospace, transportation and construction industries to inspect the quality of the processing and welding quality of the materials and parts of ships, pipes, high pressure vessels, boilers, aircraft, vehicles and bridges, as well as the quality of all kinds of light metal, rubber, ceramics and other processing parts.