Magnetic particle testing of steel pipes
Magnetic particle testing of steel pipes
Magnetic powder detection
Magnetic particle testing is a method of observing material defects by using magnetic powder as the display medium. According to the kinds of magnetic powder medium applied during magnetization, the testing methods are divided into wet method and dry method; according to the time of magnetic powder applied on the workpiece, the testing methods are divided into continuous method and remanence method. Magnetic particle testing is one of the five conventional methods for nondestructive testing.
Purpose of magnetic particle testing
The detectable discontinuity width of magnetic particle detection can reach 0.1 M. Using a variety of magnetization methods, magnetic particle testing is almost independent of the size and geometry of the workpiece, and can detect defects in all directions of the workpiece.
Application scope of magnetic particle testing
1.is suitable for detecting surface and near-surface defects of ferromagnetic materials, such as cracks with very narrow surface and near-surface gaps and other defects that are difficult to see visually. It is not suitable for detecting deep internal defects.
2. It is suitable for testing ferromagnetic materials such as martensitic stainless steel and precipitation hardening stainless steel. It is not suitable for testing non-magnetic materials such as austenitic stainless steel.
3.It is suitable for testing raw materials (such as billets) and semi-finished products, finished products and workpieces in service and used.
4.it is suitable for detecting steel, bar, plate and forged steel castings and weldments.
5. It is suitable for detecting the defect that the direction of extension of the workpiece surface and near surface is as perpendicular as possible to the direction of magnetic line, but it is not suitable for detecting the defect that the angle between the direction of extension and the direction of magnetic line is less than 20 degrees.
6. it is suitable for detecting small defects on the surface and near surface of the workpiece, and is not suitable for detecting shallow and wide defects.
Magnetic particle testing of steel pipes
After the steel tube is magnetized, the magnetic flux leakage field is produced by the local distortion of the magnetic lines on the surface and near the surface of the steel tube due to the existence of discontinuity. The magnetic particles adsorbed on the surface of the steel tube form visual magnetic marks under appropriate light, thus showing the position, size, shape and severity of the discontinuity.
Advantages and disadvantages of magnetic particle testing for steel pipes
Advantages: non-destructive, simple and convenient operation, low detection cost, high sensitivity to steel tube surface and near surface defects detection, is the preferred method of steel tube surface defects detection.
Disadvantages: high requirements for the surface smoothness of the steel pipe, high requirements for the technology and experience of the inspectors, small detection range and slow detection speed.
Magnetic particle testing method for steel pipes
1. wet process and dry process
Magnetic powder suspension in oil, water or other liquid medium is called wet method. It distributes magnetic suspension evenly on the surface of steel pipe in the process of testing. It shows the shape and size of the defect by the flow of liquid and magnetic flux leakage. Due to the dispersion and suspension performance of magnetic suspension, the particle size of magnetic powder can be smaller. Therefore, it has high detection sensitivity. It is especially suitable for detecting tiny surface defects, such as fatigue cracks and grinding cracks.
Dry method is called dry powder method. In some special occasions, when the wet method can not be used for testing, the special dry magnetic powder is applied directly to the magnetized steel pipe according to the program, and the defects of the steel pipe show magnetic marks.
2. continuous method and remanence method
(1) Continuous method, also known as accessory magnetic field method or current magnetic method, is the magnetic particle or suspension applied to the steel pipe under the action of an external magnetic field for magnetic particle inspection. The observation and evaluation of steel tube can be carried out under the action of an external magnetic field or after interruption of magnetic field.
(2) remanence method, remanence method is to magnetize the steel pipe first, and then pour the suspension on the steel pipe, after the magnetic powder aggregation in observation. This is the method of detecting residual magnetism of steel tube, so it is called residual magnetism method.
