Non-Destructive Testing
Non-destructive testing (NDT) uses physical effects like ultrasound, X-rays, or magnetic fields to find flaws in the component without damaging it. It immediately shows cracks, pores, and lack of fusion in the weld. This keeps the part serviceable and the test repeatable.
Physical-Chemical Mechanism of Action
The test uses different physical principles. Ultrasonic sends sound waves into the material. At defects, the waves are reflected and become visible as an echo. X-rays or gamma rays penetrate the component. A film or detector shows density differences as shadows. Magnetic particle testing uses a magnetic field. Cracks on the surface disturb the field and attract fine iron particles. Penetrant testing uses capillary action: dye penetrates open cracks and becomes visible after cleaning.
Metallurgical Relevance & Welding Risks
Welds can contain internal defects like porosity, slag inclusions, lack of fusion, or cracks. Surface defects like cracks or notches act as stress raisers and lead to stress concentration. Internal defects reduce the load-bearing cross-section. If a defect goes undetected, the component can suddenly fail in service. Especially dangerous are hydrogen-induced cold cracks and hot cracks, which often appear only after hours.
Workshop Practice & Action Instructions
Choose the test method based on material and defect location. For ferromagnetic steels, magnetic particle testing is suitable for surface cracks. For non-magnetic materials (aluminum, stainless steel), use penetrant testing. For internal defects in thicker plates from approx. 6 mm, use ultrasonic or X-ray. Test only after complete cooling so stress cracks are visible. Keep the test surface clean and free of scale, slag, and oil. Follow the allowable limits from the standard (e.g., ISO 5817). Document every finding with location and size.