The following points highlight the seven main non-destructive tests of weld. The tests are: 1. Visual Inspection 2. Magnetic Particle Inspection 3. Liquid Penetrant Inspection 4. Ultrasonic Inspection 5. X-Ray Inspection 6. Eddy Current Inspection 7. Testing Welds for Leakage by Testing Tanks / Vessels Hydraulically.
Non-Destructive Test # 1. Visual Inspection:
This is suited when appearance is the criterion of checking the quality of weld. Templates are used to check the contour of the welds. It is also used to check welds not requiring high physical strength by inspecting cracks, inclusions, contour, etc. This is, of course, a subjective test.
Non-Destructive Test # 2. Magnetic Particle Inspection:
It is used to check surface flaws in materials which can be magnetised. The surface to be inspected is coated with a liquid (fine oil) solution containing very tiny coloured magnetic particles, and is then subjected to magnetic field created by either passing a current through it or by placing a powerful magnet against it.
Any lack of continuity at or near the surface on magnetisation creates a local north south and magnetic pole, and attracts the metallic particles in the solution. On removal of magnetic field, the flaws are detected by concentrations of magnetic particles. Since best results are obtained when magnetic field flux lines are perpendicular to the crack, the object is tested by magnetising twice, creating magnetic fields at 90° to check all flaws.
Non-Destructive Test # 3. Liquid Penetrant Inspection:
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This method detects surface flaws in all types of metals (It can’t detect subsurface flaws). Either a liquid dye penetrant or a fluorescent liquid is applied to the surface to be inspected, and allowed to penetrate for some time: (3-10 minutes).
The liquid is then removed with a cleaner and surface is dried. In former case a developer is then sprayed on the surface which brings out the colour in the dye penetrant that has penetrated into the flaws. In latter case, a black light source is brought upto the surface so that where fluorescent liquid has penetrated will show up clearly.
Non-Destructive Test # 4. Ultrasonic Inspection:
It can detect surface as well as internal flaws. Ultrasonic waves are passed from a transmitter-receiver unit through the metal for 1-3 micro-seconds and then the selected waves from back surface and flaw surfaces are received back and listened by the same unit.
This action is repeated 0.5 to 5 million times per second. Each wave is visually represented on an oscilloscope which is calibrated to show the distance between searching unit and any flaw. The ultrasonic search unit is traversed in a zig-zag way to detect any flaws indicated by peaks on the oscilloscope screen.
Non-Destructive Test # 5. X-Ray Inspection:
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X-rays can pass through any material and reproduce their image, clearly showing any internal flaws, on film (radiography), or on a fluorescent screen (fluoroscopy), or on a cathode ray tube.
X-ray inspection equipment can be portable one also by utilising radioactive isotope and then field weld can be tested in as installed condition. X-ray inspection equipment selection is dependent on the kind of material to be tested and its thickness, geometry, and access.
Non-Destructive Test # 6. Eddy Current Inspection:
In this method an a.c. coil is brought up close to the conductive metal to be tested so as to induce eddy currents in it. These eddy currents produce their own magnetic field which opposes the field of the a.c. coil, increasing the impedance of a.c. coil which can be measured and provides alarm to indicate presence of flaw in metal.
Non-Destructive Test # 7. Testing Welds for Leakage by Testing Tanks / Vessels Hydraulically:
In this method CO2 is pressurised and a soap and water solution is put on the outside of each weld. Leaks are indicated by the formation of bubbles. In another method, the vessel is pressurised and a pressure gauge is installed to indicate fall in pressure in about 24 hours to indicate the degree of tightness of vessel and leaks.
