Normal casting defects of ductile iron include some macro-defects, such as air holes, sand inclusion, slag inclusion, shrinkage or shrinkage cavity, and some micro-defects, such as unqualified nodularity rate and too large grain size, etc.
The inspection methods to ductile iron castings include visual inspection, magnetic powder test and ultrasonic test.
1. Visual Inspection
Visual inspection is the most heavy works, they need lots of grinding works accompanied. Normally, the visual inspection require to meet the following requirements:
a) No cracks, no welding treatment, no sand or slag inclusion.
b) No obvious sand holes or air holes. As for allowable sizes and amount for the air and sand holes, you could refer to the other articles in Dandong Foundry Blog. As for the different castings, there are different requirements.
c) No cold flake or mismatch defects.
d) No machining scratch or impression.
During visual inspection, it is hard to distinguish the air holes and sand holes. Air holes normally have smooth inside surfaces, and have regular shapes, but sand holes have irregular shapes and have dirt or sand residues inside. As for the air holes, since they are shallow, the inspector could measure their sizes, and check if they are allowable, but for the sand residues, most of them will cause deeper damages to the castings, so they should be grinded or repaired before acceptance.
Moreover, the inspector should check the sand blasting. The inadequate sand blasting will cause the rough surface quality, or even leave the oxide skin. These situation should be sand blasted again or be grinded further.
2. Magnetic Particle (Powder) Inspection
Magnetic particle inspection is a kind of NDT testing. The florescent magnetic particle testing is suitable for ductile iron castings. According to our experience, both the water base magnetic flaw detection ink and dry magnetic particle are suitable for inspection to ductile iron. Since the surface smoothness has large effects to the inspection results, so you could try to grind the surfaces to improve the inspection sensitivity. Normally, the casting head areas easily have defects.
2. Ultrasonic Testing
Ultrasonic testing is also an inspection method to ductile iron castings. It could inspect not only air holes and sand holes, but also inspect the slag inclusion, cracks and other material defects. However, comparing with the cast steel, forged or welding parts, ultrasonic testing to ductile iron castings has high difficulties, and the results need careful and skillful identification. It is because ductile iron has more complex inside structures. Some structures may cause the wrong judgments, and moreover, the surface quality has some effects to accuracy of inspection results.