How is gray iron gray surface and near-surface defect inspection conducted?

The inspection of gray iron primarily includes dimensional checks, visual inspections of appearance and surface quality, chemical composition analysis, and mechanical property testing. For castings with critical requirements or those prone to issues during the casting process, non-destructive testing is also conducted. Non-destructive testing techniques suitable for quality assessment of ductile iron castings include liquid penetrant testing, magnetic particle testing, eddy current testing, radiographic testing, ultrasonic testing, and vibration testing.

The detection of surface and near-surface defects in gray iron castings is as follows:

1. Liquid Penetrant Testing

Liquid penetrant testing is used to detect various surface-opening defects in gray iron, such as surface cracks and pinholes—defects that are often difficult to spot with the naked eye. A commonly used penetrant test is dye penetrant inspection, where Gray iron The surface is immersed or sprayed with a highly penetrative, colored (typically red) liquid known as the penetrant. The penetrant seeps into open defects, after which the excess layer on the surface is quickly wiped away. Next, an easily dried developer—also called the revealing agent—is sprayed onto the casting surface. Any penetrant remaining in the open defects is drawn out, causing the developer to become stained. This staining vividly reveals the shape, size, and distribution of the defects. It’s important to note that the accuracy of penetrant testing decreases as the surface roughness of the material being inspected increases. In other words, the smoother the surface, the better the detection results. For instance, surfaces polished to high precision using grinding and polishing techniques can even detect intergranular cracks. In addition to dye penetrant testing, fluorescent penetrant inspection is another widely used liquid penetrant method. This technique requires the use of a UV lamp for observation, and it offers superior detection sensitivity compared to dye penetrant testing.

2. Eddy Current Testing

Eddy current testing is suitable for detecting defects located at depths generally not exceeding 6 to 7 mm beneath the surface. Eddy current testing is divided into two methods: the placement-coil method and the through-coil method. When a sample is placed near a coil carrying an alternating current, the alternating magnetic field entering the sample induces eddy currents within it—these currents flow perpendicular to the direction of the excitation magnetic field. The eddy currents, in turn, generate their own magnetic field that opposes the direction of the excitation field, thereby reducing the original magnetic field within the coil and causing a change in the coil's impedance. If Gray iron Surface defects cause distortions in the electrical properties of eddy currents, enabling the defects to be detected. A primary drawback of eddy current testing is that it cannot directly reveal the size and shape of the detected flaws—typically, it can only determine the defect's surface location and depth. Additionally, its sensitivity for identifying tiny, open-surface defects in the test piece is lower compared to penetrant testing.

3. Magnetic Particle Inspection

Magnetic particle testing is suitable for detecting surface defects as well as flaws located just a few millimeters beneath the surface. It requires DC (or AC) magnetization equipment and magnetic particles (or magnetic suspension) to carry out the test procedure. The magnetization equipment is used to apply a magnetic field, Gray iron Magnetic fields are generated on both the inner and outer surfaces, and magnetic particles or a magnetic suspension fluid is used to reveal defects. When gray iron is magnetized within a certain range, defects in the magnetized areas create leakage fields. As the magnetic powder or suspension is applied, it is attracted to these leakage fields, making the defects clearly visible. However, this method primarily detects defects that are oriented transversely to the magnetic field lines—defects aligned parallel to the lines remain undetected. Therefore, during the inspection process, it’s essential to continuously change the magnetization direction to ensure that all defects, regardless of their orientation, can be identified.