Non-destructive Testing methods for Valves
1 Valve
Valve is a component used to control the medium in the pipeline, which plays a role of cut-off and adjustment. The medium used for the valve includes gas, steam, oil, chemical industry, flammable and explosive, etc. In some pipelines, if the valve fails to operate, it will cause the entire device to stop production, pollute the environment, and seriously endanger the safety of people and equipment.
2 Defects and causes of valve casting
Valve manufacturing involves casting and forging forming, machining, surfacing or welding, heat treatment, non-destructive testing, assembly and test. Given the composition of the steel casting material, the design and shape of the casting as well as the steelmaking process, selection, molding sand, finishing, welding repair, and heat treatment, the casting will experience resistance after cooling and shrinkage, and then stress will occur.
Liquid steel has an ultimate strength and plasticity limit at a certain temperature. If the stress on the casting exceeds the limit at that temperature, cracks will occur. At this time, the casting will undergo a series of solidification and shrinkage reactions, but at the same time, it cannot get sufficient molten steel liquid, that is, the steel liquid is supplemented in time, and shrinkage will occur.
If there is no way to control the raw material of the casting and the process of casting steel, or in the process of pouring the casting, the interaction of the gas in the ladle, the molten steel, and the type of sand and gravel reacts to the remaining metal impurities. Falls inside the steel casting; or because the water content in the molding sand is too high, but the coating for cold iron coating is not properly processed, the poor air permeability of the sand type causes the gas to be inhaled or mixed in the control system and the cavity of the casting during the pouring process but cannot be discharged in time, so pores will appear in the process of casting steel castings.
3 Common testing methods of non-destructive testing
There are many detection methods for valves, including mechanical performance testing, chemical composition analysis, non-destructive testing, etc. Non-destructive testing, in short, is a method of testing the tested items without damaging the tested items. Generally speaking, if a material changes in its internal organization and structure, it will cause physical changes. In this way, we can use this principle to perform non-destructive testing of objects, which is also the basic principle of non-destructive testing. Now we often use radiation, ultrasound, penetration and magnetic powder to detect objects, that is, RT, UT, PT and MT detection. In the process of casting the valve body, there will always be a series of defects such as pores and shrinkage joints in various volume states, and the surface of the valve body is often not so smooth. The sensitivity of radiographic inspection is very high and these defects can be seen directly. At the same time, this technology is minimally affected by artificial factors, so this technology is very suitable for accurate detection of valve body compliance.
3.1 Radiographic testing
Radiographic uses X-rays to carry out non-destructive inspection methods for metal internal defects. There is no essential difference between X-rays and natural light. They are both electromagnetic waves, but the energy of X-rays is much greater than visible light. It can penetrate objects that cannot be penetrated by visible light, and it will have complex physical and chemical interactions with the substance while penetrating the object. It can ionize atoms, cause some substances to emit fluorescence, and make some substances produce photochemistry. reaction. If there is a defect in a local area of the workpiece, it will change the attenuation of the object to the rays and cause the intensity of the transmitted rays to change. In this way, a certain detection method, such as the use of film sensitivity to detect the intensity of the transmitted rays, can determine whether there are defects in the workpiece And the location and size of the defect.
3.2 Ultrasonic Testing
Ultrasonic testing uses ultrasonic waves that enter the material to be inspected to detect surface or internal defects. Sound waves will be refracted and reflected on the interface of two or more different materials. So we can use this principle to carry out ultrasonic testing. When there are some defects inside the valve body, two different material interfaces will be produced, so that we will get different reflected waves or refracted waves, so we can know that there are defects inside the valve, but we can't accurately know the size of the defects and their distribution, so this method is simple and cheaper but not accurate.
3.3 Penetration testing
Penetration detection is based on a principle called "capillary action", which is to use a liquid dye on the surface of the detected object and then wash off the dye after a preset time. However, the dye is then removed due to capillary action. Professional inspectors can analyze the traces left by the inspected object, and then obtain the defect status of the inspected object. This method is mostly applied to items that do not absorb dyes, such as steel, ceramics or non-ferrous metals. This detection method is particularly suitable for field detection.
3.4 Magnetic Particle Testing
After the workpiece of ferromagnetic material is magnetized, due to the existence of discontinuity, the magnetic induction lines on the surface and near the surface of the workpiece are locally distorted to produce a leakage magnetic field, which absorbs the magnetic powder applied on the surface of the workpiece, and forms a visible light under suitable light. Magnetic marks, which show the location, size, shape, and severity of the discontinuity.
4 Summary
In view of the characteristics of valve castings, there may be many defects such as slag inclusions, pores, and shrinkage holes in various shapes in the casting, and the surface of the valve is not smooth. If you choose a suitable detection method, it is possible to accurately detect the internal defects of the valve body, so that you can effectively determine whether the valve meets the requirements of use.