NDT Solution

Induced Current Magnetization Made Simple

Magnetic particle inspection of components involves magnetizing the part by traditional methods, such as head shot techniques. These methods are often associated with problems, such as damage to the part from arc strikes or burns due to poor surface contact and high amperage requirements. In this NDT solution, a new method of induced magnetization is proposed which is economical and could reduce risks associated with the currently used methods. G.P. Singh Associate Technical Editor

 

Figures 1-3

Problem
A major problem with performing magnetic particle inspection of such items as bevel gears, knife edge seals, and other curved or rounded parts has always been the possibility of burning or arc-striking the part surface due to the combination of poor surface contact and the high amperage required for part magnetization (see Figure 1).

 

Solution
A magnetic field can be established in the item to be examined using a combination of a coil, a soft iron or alloy bar, and a heavy-walled PVC pipe. In this application, place the bar through the PVC pipe, and place the now-insulated bar through the center of the part as you would using a central conductor magnetization technique, supporting the bar on the heads of your horizontal magnetization machine. Next, position the coil over the part as you would when using a coil shot. Energizing the coil will induce a circular field in the part to be tested just as if a head shot had been used, without the associated risk of an arc strike or burning the part. Figure 2 is a side view of the setup illustrating the relative positions of the coil, part, PVC pipe, and bar (wet continuous method).

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This application could save a company thousands of dollars

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The PVC pipe should be at least 6.3 mm (0.25 in.) smaller in diameter than the central hole in the part being examined to allow for rotation of the part during examination, thus ensuring complete inspection (see Figure 3).

A Hall Effect gauss meter may be used to measure the tangential applied field strength to verify that it is within the range of 30 to 60 gauss for examination. Artificial shims or a magnetic field indicator (pie gage) may also be used to verify the field strength and direction.

The PVC pipe insulates the soft iron bar that will, in this application, serve as a transformer core, inducing a magnetic field into the part through the variation in the magnetic field generated by the AC current passing through the coil; it also serves to help support the weight of the part, which would otherwise bend the soft iron bar. The coil serves as the source for the field which induces the impressed current in the iron core, which in turn induces a circular magnetic field within the part surrounding the bar/PVC pipe assembly.

Induced current magnetization has been used in the aerospace industry for many years, and can be easily adapted to industrial and power generation turbines, automotive gears, parts for process machinery, and even fluid control parts such as pipe flanges and valve bonnets. This application is referenced in ASTM E-1444-94a (Paragraph 6.2.9 and Figure 3.3) and MIL-STD-1949.

This application could save a company thousands of dollars by deferring the purchase of an expensive induced current machine while still providing the means for performing part examinations using an induced current technique, and by reducing or eliminating the risk of part damage resulting from arc strikes or burns incurred during circular magnetization from traditional head shot techniques.

 

* Wyman Gordon Forgings, 10825 Telge Rd., Houston TX; 77240-0456; (281) 856-3425;
e-mail jmorris@wyman.com.

Copyright © 1998  by the American Society for Nondestructive Testing, Inc. All rights reserved.

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