The NDT Technician - Vol. 4, Issue No. 1

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Vol. 9, No. 4• October 2010

Magnetic Particle Testing of Plate and Pipe Edges Using Single-Leg AC Yoke by Siddhartha Ghosh

Single-leg AC yoke magnetic particle testing (MT) has been accepted by API RP 2X, Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians¹ primarily as a technique for testing the heel or toe areas of T, K, Y weldments on offshore structures. Conventionally, the electromagnetic yoke is used in a two-leg configuration that produces a strong longitudinal magnetic field between the poles that is ideal for testing wherever accessibility permits. However, many test positions have tight geometries where double-leg placement is not possible and single-leg yoke placement can be substituted and still conform to the required specifications. This article will show other areas where single-leg AC yoke magnetization is applicable.

A single-leg AC yoke configuration produces a radial magnetic field that is primarily symmetrical about the main axis of the activated yoke leg as is demonstrated in the magnetographs shown in Fig. 1. A flexible laminated strip has been affixed to the underside of the paper. The application of dry powder magnetic particles has resulted in linear indications from the flexible laminated strip slots. The field is basically perpendicular to the rectangular side of the yoke leg base in all four directions.

Figure 1. Magnetographs of radial field surrounding activated single-leg yoke.

Testing Edges of Plate and Beveled Pipe

Testing for lamination at the edges of plate or beveled pipe is a common testing requirement. Lamination is a planar discontinuity that is parallel to the surface of plate or pipe. Lamination that extends to an edge will appear as a linear indication in a magnetic particle test of the edge. Welding on a laminated edge can cause the discontinuity to propagate and result in a rejectable weld defect.

Ideal magnetization of a beveled edge area is achieved by placing one leg of a double-leg AC yoke on the top surface in the center of the area to be tested with the other leg on the opposite parallel surface. However, as is most often the case during field testing or during fit up just prior to welding, access to the opposite parallel surface is difficult or impossible. Use of the single-leg AC yoke technique to produce a radial field on the accessible surface close to the beveled edge to be tested can solve the problem.

Establishing Field Direction. To confirm that the radial magnetic field produced by a single-leg AC yoke on the accessible surface extends to the edge section, a flexible laminated strip is attached at the edge area of the plate block or pipe section and verified for indication forming (Fig. 2). Using wet magnetic particle inspection, the strip provides a distinct, clear indication of the magnetic field at the edge of the block.

Figure 2. Visible linear indications in Castrol strip attached to plate edge.

Test Plate Experiment. Figure 3 shows the correct setup of a single-leg AC yoke for magnetization of a carbon steel test plate 6 mm (0.24 in.) thick with one beveled edge containing two laminations of sufficient linear length to be made visible by conventional magnetic particle testing or penetrant testing. Length of the laminations at the beveled edge are specified by the test plate manufacturer as 5 mm (0.2 in.) and 15 mm (0.6 in.) with a distance of 140 mm (5.5 in.) between them. Depth of the lamination begins at 2.5 mm (0.1 in.) from the magnetized surface and extends 5 mm (0.2 in.) into the plate. The radial magnetic field generated by the single-leg AC yoke is capable of detecting both lamination indications with the correct visible lengths as specified (Fig. 4).

Figure 3. Setup for inspection of beveled edge.

Figure 4. Lamination indications at plate edge using visible wet black MT ink.

Sensitivity Factor. It is also observed that the lamination discontinuity indication in Fig. 4 is most distinct when the yoke leg is exactly on top of the region where the lamination discontinuity exists. This demonstrates that the radial field is strongest in the region immediately surrounding the activated yoke leg.

Extent of Radial Field. The extent of the single-leg AC yoke radial magnetic field can be ascertained with the aid of the flexible laminated strip attached to the edge of plate by shifting the position of the yoke leg off center from the strip position (Fig. 5). The result shows that at a distance of 25 mm (1.0 in.) from the yoke leg, the field still exists and can produce a partial indication from the flexible laminated strip.

Figure 5. Partial indication shows extent of magnetic field.

Field Strength Adequacy. Field strength adequacy is determined by attaching a quantitative quality indicator (QQI) to the edge of the plate (Fig. 6). A distinguishable indication is only achieved when the yoke leg is directly aligned with the attached QQI. No indication is produced when the yoke leg is placed slightly off center from the plane of the QQI.

Figure 6. Distinguishable indication produced with yoke leg placed directly above or below QQI.

Conclusion

Based on the above criteria, it can be recommended that overlapping in single-leg AC Yoke magnetic particle testing should be at least half the base length of the yoke leg when conducting continuous scanning of successive areas.

References

API RP 2X, Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians, fourth edition. Washington, DC: American Petroleum Institute (2004).

Bibliography

ASTM E 1444, Standard Practice for Magnetic Particle Testing. West Conshohocken, PA: ASTM International (2005).

Goldberg, L. Magnetic Particle Inspection of Fixed Offshore Structures: Inspector/Diver Training. Columbus, OH: American Society for Nondestructive Testing (1994): p 11, 17-19, 60.

Stanley, R.K. "Burmah-Castrol Strips in NDT." The NDT Technician. Vol. 8, No. 2. Columbus, OH: American Society for Nondestructive Testing (April 2009): p 6-8.

Siddhartha Ghosh, Testconsult Services; Block 365, Clementi Ave. 2, #04-489, Singapore 120365; 97894575; e-mail snahome@singnet.com.sg.

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