Quality control and material testing: from visual inspection to X-ray
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A subject that looks flawless on the outside can easily have a hidden defect inside the material. That is why quality control is something other and more than looking and measuring. The trade uses a range of methods for non-destructive testing (NDT), where the subject is tested without being destroyed in the process.
§Visual control — the first step
Everything starts with thorough visual inspection: are there visible cracks, rough surfaces, poor fill, or other signs of problems? Visual inspection is fast and cheap, but only finds flaws that are visibly on the surface.
§Color penetrant.
With penetrant testing, a colored or fluorescent liquid is applied to the workpiece's surface. The liquid penetrates even very fine cracks by capillary action. After cleaning and developing the cracks become visible as clear markings. The method is used especially on non-porous materials such as metal and requires the surface to be clean and dry.
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On ferromagnetic materials — such as cast iron and many steel alloys — you can create a magnetic field in the workpiece and sprinkle magnetic powder on the surface. The powder collects where the magnetic field is disturbed by a crack, thus revealing surface defects and flaws just below the surface as clear marks.
§Ultrasound and X-rays—look inside the material.
| Method | How it works |
|---|---|
| Ultrasound (UT). | Sound waves are sent into the material, and reflected signals are used to find internal faults — including the depth and size of the fault |
| X-ray/radiography (RT) | X-ray or gamma radiation creates an image of the workpiece's interior and is used e.g. on welds and castings where you need to see through the entire material |
§Destructive testing as supplement
Sometimes NDT is not enough, and a sample from the same casting is sacrificed in a tensile test where it is loaded to failure. This gives concrete numbers for tensile strength and elongation at break that can be compared with the requirements in the relevant material standard – for example DS/EN 1561 or DS/EN 1563.
Together the various test methods form a safety net: some find surface defects, others look into the material, and destructive testing confirms the actual mechanical properties. As a foundry technician, it is your job to know which method is relevant for the specific item you have in front of you.