As can be seen in the field distribution images, the field strength at the inside surface of hollow conductor carrying magnetic field produced by direct magnetization is very low. Therefore, the direct method of magnetization is not recommended when inspecting the inside diameter wall of a hollow component for shallow defects. The field strength increase rather rapidly as one moves in from the ID,so if the defect has significant depth, it may be detectable.However a much better method of magnetizing hollow component for inspection of the ID and OD surfaces is with the use of a central conductor. As can be seen in the field distribution image to the right, when current is passed through a nonmagnetic central conductor (copper bar), the magnetic field produced on the inside diameter surface of a magnetic tube is much greater and the field is still strong enough for defect detection on the OD surface. After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can.
1) Affect machining by causing to cling to a component.
2) Interfere with electronic equipment such as a compass.
3) Create a condition known as arc blow in the welding process. Arc blow may cause the weld arc wonder or filler metal to be repelled from the weld.
4) Cause abrasive particles to cling to bearing or flying surfaces and increase wear.
Removal of a field may be accomplished in several ways. This random orientation of the magnetic domains can be achieved most effectively by heating the material above its curie temperature. The curie temperature for a low carbon steel is 770 C or 1390 F. When steel is heated above its curie temperature, it will become austenitic and loses its magnetic properties. When it is cooled backdown, it will go through a reverse transformation and will contain no magnetic field. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the earth's magnetic field.
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