Volume 4, Number 1
January 2005
Q. On radiographs of thin titanium components, I am finding areas that appear as segregations within the material. My coworker suggests we increase the kilovoltage and use lead screens. The areas disappeared when we changed our process. What was causing the indications on the radiograph? B.C., Arkansas City, Kansas
A. From your description, it appears you experienced mottling, a form of scatter originating from larger grained materials. It’s not a common condition encountered in radiography. Another way to verify this condition is to produce a second radiograph with the component tipped 5 degrees or so. If the component contains segregations, it will change very little. However, if diffraction is causing the indication it will change markedly.
Q. Why are my hands red after performing wet magnetic particle inspection using a stationary unit? E.H., Burbank, California
A. The redness is a form of dermatitis. Your skin is reacting to the suspension fluid used in the machine. A moisturizing lotion or hand cream will offer some relief.
Q. My boss has told me that an AC yoke produces a longitudinal field and a circular field as well. From what I’ve learned during training and in reading, the AC yoke only produces a longitudinal field. What do you say? A.M., Pakistan
A. The magnetic field in a yoke is generated by an electric coil wrapped around a steel bar in the yoke body which is attached to the two legs of the yoke. As a result, the magnetic field from the coil, which is longitudinal, travels down the legs and through the part. Therefore, the magnetic field that is in the part is longitudinal. However, there are some instances when the applied magnetic field can be used in circular applications even though it is a longitudinal field. An example of this would be if the legs of the yoke are applied to a pipe transverse to the axis of the pipe (like you were checking for transverse indications in a round seam weld). In that case, the field flows around the pipe circumference in the same direction that circular magnetization would flow if the pipe were inspected using a head shot or a central conductor. This use of longitudinal magnetization can make it seem like the field is circular, though in fact it isn’t.
The yoke method of magnetization is known as indirect induction, meaning that the magnetic field is created outside of the part and only the magnetic field enters the part. Direct induction, where the electric current is passed through the part (like a prod unit or a head shot) results in circular magnetization in the part.
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hhumphries@asnt.org
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