1.Overviews
Magnetic memory testing technology will make an early diagnosis to stress concentration caused by fatigue damage of parts in service in new and fast detection methods. The technology could be used for the detections such as boiler and pressure vessel, pipelines, turbine blades, bearings, gear pair, welded parts and other ferromagnetic metal components.
2.Testing plans
1)Magnetic memory detection of turbine blade
Turbine blade is one of high speed rotating components and bears higher centrifugal force, shear force, torsional force and alternating stress in serving, and high stress concentration points and stress lines are easy to produce in blade root, around tie bar hole. These parts are also main areas of crack arising, if not early check out, blade fracture will be caused. In the event of blade fracture, blades of the stage may be damaged all and causing great economic losses. As a result, it is particularly important for turbine blade to early diagnose. When testing adopting magnetic memory method, put a sensor on blade and make constant speed movement along blade length direction to find blade stress line. After testing 3 ~ 5 blades, you can find out the positions where the stage blade stress line is in. The positions are usually in blade root, around middle tie bar holes and outer tie bar holes. Then using vortex or magnetic particle inspection, focusing on near stress line, thus greatly accelerating testing speed and reducing the labor intensity. When detection, you can also put the unreliable components on record, so that to test them periodically in the future. Through regularly detecting equipment parts and recording results, you can clearly observe, as growth of the running time in service the change conditions of metal structure of parts, in order to timely replace corresponding parts.
2)Magnetic memory testing of pressure vessels
Pressure vessel in service suffers generally harsh working conditions, namely bears certain pressure and temperature or joint action of corrosive medium, serving for a long time can cause containers corrosion thinning, deformation and cracking, and cause the containers leaking, blasting, endangering safe production. When carrying out pressure vessels periodical inspection, often with a certain blindness, namely making magnetic particle inspection and X-ray detector, so that tested parts may not have question, and defective parts are often ignored. When adopting the method of magnetic memory testing, the pen type probe can quickly scan surfaces of pressure vessel, find out ZCP and make marks, and then you can verify that the parts belongs to stress concentrations or defects by using other nondestructive testing means, to reduce the blindness of flaw detection
3)Magnetic memory testing of weld areas
There are many welds in power plants and chemical plants. Due to many joints are welded by hand, it is very difficult to ensure that all welding is qualified. NDT personnel need sampling or 100% for NDT inspections according to certain proportion, this no doubt increases the labor intensity of inspection personnel. If using X-ray detector, it will harm the body of inspection personnel. The magnetic memory testing method can be used for rapidly and accurately pre- detecting of welds. When finding problems, make marks, then adopt other nondestructive testing methods to focus on the parts, so that determine the problems being stress concentration caused by welding or force, or magnetic accumulation formed by stress concentration arisen from defects.
3.Detection technology and characteristics
For components of equipment in service, due to its structure genetic (micro structure formed in manufacture ) and load relationship in operation, the magnetic memory in metal is come out in cumulative way. When operating, the change of magnitude and direction for metal magnetization will be caused by the size and direction of load force of components in running. Metal magnetic memory diagnostic technique is developed according to the principle. Experiments show that under the action of magnetic, defects and inclusions in the ferromagnetic parts in service can produce magnetic domain normalization and leakage magnetic field on it. On the places like defect location and/or the location with concentrated relatively internal stress, have minimum metal magnetic permeability and the maximum tangential component of magnetic field, and the normal component changes its symbols, has zero value. Scanning and testing the normal component of leakage magnetic field of part surface can determine stress concentration area, thereby indirectly judge the possibility of ferromagnetic part defect.
Compared with traditional detection method, magnetic memory testing method has following advantages:
· The method not only detects the macroscopic defects but also the micro defects, and can make dangerous forecast for the future.
· Do not need special magnetizing device for reliable detection of ferromagnetic component.
· The clean or other pretreatment for checked metal surface don't be need
· With small affect of test lift-off effect.
· Portable equipment, simple operation, high sensitivity, good repeatability and reliability.
4.Recommend instruments and accessories
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