Casting surface and near surface defects detection

- Jan 03, 2018 -

1) liquid penetration test

Liquid penetration testing is used to inspect various open defects on the casting surface, such as surface cracks, surface pinholes and other defects that are hard to see with the naked eye. Commonly used penetration testing is coloring inspection, which involves wetting or spraying a colored (generally red) liquid (penetrant) with high penetrating power on the surface of the casting, penetrant penetration into open defects, rapid wiping of the surface permeate Layer, and then the easy-to-dry display agent (also known as imaging agent) sprayed onto the casting surface, to be left in the opening defects of the penetrant out, the display agent is dyed, which can reflect the shape of defects, Size and distribution. It should be pointed out that the accuracy of penetration detection decreases with the increase of the surface roughness of the tested material, that is, the better the surface inspection, the more the grinding surface polished, and even the intergranular cracks can be detected. In addition to the color detection, fluorescence penetration testing is also commonly used liquid penetration detection method, which requires the configuration of UV light irradiation observation, detection sensitivity than color detection.

2) Eddy current testing

Eddy current testing for the following general inspection of the surface is not greater than 6 ~ 7MM deep defects. Eddy current testing points by-pass coil method and through-coil method 2 kinds. When the specimen is placed near a coil with alternating current, the alternating magnetic field entering the specimen induces a current (vortex) flowing in a swirling direction perpendicular to the excitation magnetic field in the specimen, A magnetic field with the opposite direction of the excitation magnetic field is generated, so that the original magnetic field in the coil is partially reduced, thereby causing the change of the coil impedance. If there is a defect on the surface of the casting, the electrical characteristics of the eddy current will be distorted so as to detect the existence of defects. The main disadvantage of eddy current testing is that the size and shape of the detected defect can not be directly displayed, and only the surface location and depth In addition, it is less sensitive to the detection of small openings on the surface of a workpiece than penetration testing.

3) Magnetic particle testing

Magnetic particle inspection is suitable for detecting surface defects and defects several millimeters below the surface. It requires DC (or AC) magnetizing equipment and magnetic powder (or magnetic suspension) for inspection. Magnetization equipment used to produce a magnetic field inside and outside the casting, magnetic powder or magnetic suspension used to display defects. When a magnetic field is generated within a certain range of the casting, a defect in the magnetized region generates a leakage magnetic field, and when the magnetic powder or the suspension is sprinkled, the magnetic powder is sucked up, so that a defect can be revealed. This shows the defects are basically cross-cutting magnetic lines of defects, parallel to the magnetic lines of the long-shaped defects are not displayed, for the operation, the need to constantly change the direction of magnetization to ensure that we can detect the unknown direction of each defect .