Inspection Methods for Defect Detection
There are various test methods that may be employed to evaluate the soundness of a casting. These can be destructive or non-destructive tests. The more common tests employed are magnetic particle inspection, radiography and a method of destructive testing where a part is milled in small layers to reveal defects.
These inspection methods are employed in some way on a majority of castings. Although care must be taken when specifying the defect severity levels for each method. Over specifying will always lead to a more expensive part than what is needed.
All materials that are specified have certain minimum inspection criteria, typically involving chemical analysis and mechanical property verification. Whether to specify added testing can be dependent on some of the variables listed below
- The casting has a proven history with little to no issues
- The design uses large safety factors
- The application is not critical
- The part can be cast with little trouble
Extra testing may be required if:
- The design is new and untested
- There are low safety factors used in the design
Magnetic Particle Inspection
This is a non-destructive test used to detect surface or near-surface defects in ferrous materials only.
An externally applied magnetic field or rectified AC current is passed through the material. A liquid that contains either magnetic iron oxide or finely divided iron particles is applied to the part. These magnetic particles are attracted to the area of the defect and display the defect for an operator.
Experience is needed for this process in order to interpret the results and ensure that magnetic anomalies are not read as defects. The defects that are found in the tested part are compared to reference photographs to determine the severity level of the defect.
Radiography is used if internal inspection of a casting is needed. Internal defects such as shrinkage voids, porosity, and inclusions can be found using this method.
Similar to magnetic particle inspection, the defects are compared to reference radiographs to determine the severity level of the defect. Severity levels range from 1-5, one being small, five being large.
There are limitations here that are described shown in Figure 22. Their will be areas in a casting that an x-ray cannot be obtained. If these areas are critical another method of inspection may need to be employed.
Another method of casting defect inspection, which is more labor intensive, is a destructive test. This method uses a mill to cut away layer by layer in the part and reveal the defects present in the part.
This method will reveal large defects but Level 1 defects and metallic inclusions that are revealed in a radiograph may not be seen using this method.
Figures 23 and 24 shows a comparison between two methods of destructive test-ing. Figure 23 uses a “bread loaf” cutting method. This method is typically used when inspecting a large part that does not have tight control on the castings defect level.
Figure 24 shows a method used for defect inspection of a majority of parts. Figure 24 is a localized inspection for a large shrinkage cavity, although this can be performed on a whole part.
Destructive testing gives a very good three dimensional view of the void, where as a radiograph will only be a two dimensional representation of the defect. This method of testing is also a quick and easy check where radiographic inspection may not be readily available.