Detection Of Surface And Near-surface Defects Of Castings

1) Liquid penetrant testing
Liquid penetrant testing is used to check various open defects on the surface of castings, such as surface cracks, surface pinholes and other defects that are difficult to detect with the naked eye. The commonly used penetrant testing is color testing, which is to soak or spray a colored (usually red) liquid (penetrant) with high penetration ability on the surface of the casting. The penetrant penetrates into the open defect, quickly wipes off the surface penetrant liquid layer, and then sprays the easy-to-dry indicator (also called developer) on the surface of the casting. After the penetrant remaining in the open defect is sucked out, the indicator is dyed, which can reflect the shape, size and distribution of the defect. It should be pointed out that the accuracy of penetrant testing decreases with the increase of the surface roughness of the inspected material, that is, the brighter the surface, the better the detection effect. The surface polished by the grinder has the highest detection accuracy, and even intergranular cracks can be detected. In addition to color testing, fluorescent penetrant testing is also a commonly used liquid penetrant testing method. It requires ultraviolet light for irradiation and observation, and the detection sensitivity is higher than that of color testing.
2) Eddy current testing
Eddy current testing is suitable for checking defects that are generally no more than 6-7mm deep below the surface. Eddy current testing is divided into two types: the placement coil method and the through coil method. When the test piece is placed near a coil with an alternating current, the alternating magnetic field entering the test piece can induce a current (eddy current) in the test piece that flows in an eddy current shape and is perpendicular to the excitation magnetic field. The eddy current will produce a magnetic field in the opposite direction of the excitation magnetic field, which will partially reduce the original magnetic field in the coil, thereby causing a change in the impedance of the coil. If there are defects on the surface of the casting, the electrical characteristics of the eddy current will be distorted, thereby detecting the presence of the defect. The main disadvantage of eddy current testing is that it cannot intuitively display the size and shape of the detected defects. Generally, it can only determine the surface position and depth of the defect. In addition, its detection sensitivity for small open defects on the surface of the workpiece is not as good as that of penetration testing.
3) Magnetic particle testing