The inspection of marine castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical property test. For castings with important requirements or easy problems in casting process, non-destructive testing is also required. The inspection of surface and near surface defects of marine castings includes liquid penetrant testing, eddy current testing and magnetic particle testing;

Liquid penetrant testing

Liquid penetrant testing is used to check various opening defects on the casting surface, such as surface cracks, surface pinholes and other defects that are difficult to be found by the naked eye. The commonly used penetrant testing is dye testing. It is to wet or spray the colored (generally red) liquid (penetrant) with high penetrability on the surface of the casting. The penetrant infiltrates into the opening defect, quickly wipe off the surface penetrant layer, and then spray the easy to dry display agent (also called developer) on the surface of the casting. After the penetrant remaining in the opening defect is sucked out, the display agent is stained, So that the shape, size and distribution of defects can be reflected.

It should be pointed out that the accuracy of penetrant testing decreases with the increase of the surface roughness of the tested material, that is, the brighter the surface, the better the detection effect. The surface polished by the grinding machine has higher detection accuracy, and even can detect intergranular cracks. In addition to dye detection, fluorescent penetrant detection is also a commonly used liquid penetrant detection method. It needs to be equipped with ultraviolet lamp for irradiation observation, and the detection sensitivity is higher than that of dye detection.

Eddy current testing

Eddy current testing is applicable to the inspection of defects below the surface that are generally no more than 6-7mm deep. Eddy current testing is divided into two types: the placement coil method and the through coil method. When the test piece is placed near the coil with alternating current, the alternating magnetic field entering the test piece can induce eddy current (eddy current) flowing in the form of eddy current in the test piece in the direction perpendicular to the excitation magnetic field. The eddy current will generate a magnetic field in the direction opposite to the excitation magnetic field, so that the original magnetic field in the coil is partially reduced, thereby causing the change of the coil impedance.

If there are defects on the surface of the casting, the electrical characteristics of eddy current will be distorted to detect the presence of defects. The main disadvantage of eddy current testing is that it can not visually display the size and shape of the detected defects. Generally, it can only determine the surface position and depth of the defects. In addition, it is less sensitive to detect small opening defects on the surface of the workpiece than penetrant testing.

Magnetic particle testing

Magnetic particle testing is suitable for detecting surface defects and defects several millimeters deep below the surface. It requires DC (or AC) magnetization equipment and magnetic particle (or magnetic levitation liquid) to conduct testing. Magnetization equipment is used to generate magnetic field on the inner and outer surfaces of castings, and magnetic powder or magnetic suspension liquid is used to display defects. When a magnetic field is generated within a certain range of the casting, the defects in the magnetized area will generate leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder will be absorbed, so that the defects can be displayed.

The defects displayed in this way are basically those that cross the magnetic lines of force, but the long defects that are parallel to the magnetic lines of force cannot be displayed. Therefore, the magnetization direction needs to be constantly changed during operation to ensure that all defects in the unknown direction can be detected.