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N.D.E. Examinations - Ferrite or Permeability Testing

The Delta Ferrite Indicator is a simple to operate device for measuring the ferrite content of austenitic stainless steel weld metals. The operation of the Indicator is based on the mutual attraction of a permanent bar magnet for a calibrated insert and unknown material. In use, an insert is screwed into the top of the case. The magnet is then attracted to the insert by a force dependent upon the insert's ferrite value. The end of the magnet projecting from the opening in the bottom of the case is then brought into contact with the material being tested. It is essential that the contact surface be clean and free of oxide, scale, or foreign material. The Indicator is then moved away in a direction normal to the contact surface.

If the material being tested has ferrite content higher than that of the insert value, the magnet will first break contact with the insert as the Indicator is moved away. Only full, complete breaks should be considered as indicative of a higher ferrite content than the test material. On the other hand, if the ferrite content of the material being tested is lower than that of the insert value, the magnet will first break contact with the test material as the Indicator is moved away. Thus, by interchanging the inserts, it is possible to bracket the ferrite content of the material under test. Two features of the Indicator deserve special mention. First, the balanced beam to which the magnet is attached permits the use of the Indicator in all positions without correction due to gravity. Second, the hemispherical magnet ends provide point contact with the inserts and the test materials.

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