Eddy Current Inspection of Ferromagnetic Monel Tubes

EDDY CURRENT INSPECTION OF FERROMAGNETIC MONEL TUBES

The eddy current method is commonly used for inspecting metal tubes during manufacturing. However, in-service inspection because of inherent magnetic "noise" of ferromagnetic materials has been limited to non-ferromagnetic tubes. Both Douglas Point and Pickering generating stations have steam generators with ferromagnetic (Monel-400) tubes. A recently invented eddy current probe made possible non-destructive in-situ testing of the Monel-400 tubes.

INTRODUCTION meters. One is normally looking for flaws such as cracks, corrosion, erosion, wear, porosities or holes A varying magnetic field induces eddy currents in that decrease the local wall thickness. The need is an electric conductor. The magnitude, phase, and therefore to distinguish a delect in the presence of distribution of these eddy currents are determined by irrelevant electrical and magnetic discontinuities. the size, shape, electrical conductivity, magnetic Eddy currents have been used successfully only in permeability, and presence of flaws in the conductor. testing non-ferromagnetic tubes. With ferromagnetic In eddy current testing the magnetic field is obtained tubes, although electrical conductivity is reasonably by applying an alternating current to a lest coil constant, the magnetic permeability of these tubes (inductor). The induced eddy currents create a can vary locally by a factor of one hundred due to magnetic field which interacts with the field of the stress, cold working, heat treatment, temperature or test coil. This interaction produces an impedance alloy composition. This magnetic "noise" can change in the coil. By measuring these impedance completely mask the eddy current signals from changes, as the probe proceeds along a tube, and flaws. Fortunately such tubes can be satisfactorily comparing them with a calibration sample containing inspected if magnetically saturated. In the past this appropriate artificial defects, one can infer the size has been achieved by applying a large magnetic field and location of defects or other perturbing elements with external electro-magnets. This technique is along the tube. applicable to production-line testing but not to in-service inspection (as there is no external access). The eddy current method provides an accurate Eddy current probes, recently developed at Chalk and simple means of inspecting tubes. The probes do River Nuclear Laboratories, are capable of locally not require a couplant and permit 100% inspection. magnetizing Monel tubes (and other ferromagnetic Tubes can be scanned at very high speeds; up to 1000 materials with a low magnetization intensity) to ft/min. When considering in-service eddy current saturation while scanning internally for defects. The inspection of tubes, variations in the electrical and probes can be used directly with many commercial magnetic properties are irrelevant. Thickness and eddy current instruments. integrity of the tube wall are the important para- 0.012" •MAGNETITE SUPPORT BARS ^DEPOSIT iil\\ 0-005" \ V dsap

Figure 1 — Laboratory calibration tube set up

EDDY CURRENT TEST WITH COMMERCIAL EQUIPMENT

EDDY CURRENT TEST WITH MAGNETIC SATURATION PROBE (gainx!5)

(Downward signal represents a loss in wall thickness)

Figure 2 - Eddy current test in a high magnetic permeability MoneWOO tube. Test frequency 50 KHz. MAGNETIC PROPERTIES OF MONEL-400 IN-SERVICE EDDY CURRENT TESTING

The magnetic permeability curve (or B-H curve) In-service eddy current inspection has boon lias the characteristic properties of a typical ferro- successfully demonstrated on a Douglas Point Cene- magnetic material. It has JII initial relative permea- raling Station steam generator which luid developed bility of ju0 = 200 and a maximum permeability of leaks. The steam generator is a hairpin typo, /imax = 400. A magnetic intensity of 3 oersteds will containing 195 Monel-400 tubes, >/i inch O.D. x 0.04') magnetize the material to the "knee" location inch wall and approximately 45 feet in total length. (location of maximum permeability) but over 1000 The defect was believed to be caused by flow-induced oersteds are required to saturate it completely. vibration. However, as the Curie temperature of a material is Before a destructive examination it was decided to approached, the magnetic intensity required for eddy current inspect all tubes. The initial testing saturation decreases. Monel-400 has a Curie tempera- with commercial equipment and conventional piobcs ture close to ambient but is strongly affected by the was unsuccessful; the defect areas were not dis- copper content; a Y'k increase in copper lowers the cernible. The steam generator was heated above the Curie temperature by 20°F. Because the ASTM code Curie temperature but resulted only in minor permits a 12.5% variation in copper content some improvements. A consulting engineer specializing in tubes may be magnetic at room temperature while eddy current inspection was retained to test the others are not. This agrees with the large variation in tubes, but he loo was unsuccessful. These initial magnetic permeability found in the tubes tested. discouraging tests lead to the development of a new probe, capable of saturating the Monel tubes, as described earlier. DESCRIPTION AND PERFORMANCE OF THE SATURATION PROBES' A tube with machined eccentric grooves of various depths and other artificial defects was us-"d as the Two different types of eddy current probes calibration tube. All tests were performed with a capable of saturating ferromagnetic tubes from inside, saturation probe, containing a single coil, at a test have been developed. One probe, the direct current frequency of 100 KHz. At this high frequency, the bias probe, achieves local saturation by superimposing amplitude of the baffle plate eddy current signals is an alternating current test signal on a direct curr .it insignificant compared to that from the wear scars. saturating signal. The degree of saturation depends on The eddy current scans from the defective tube arc the D.C. level. The other piobe, the permanent shown in Figures 3(a) and 3(b). The first figure, 3(a). magnet probe, achieves local saturation from the displays the X-Y coil impedance vector of each baffle plate location and the U-hend. The second figure, magnetic field surrounding a permanent magnet. With 3(b), displays the Y component of the coil impedance these probes the Monel tube responds virtually as a vector. By comparing the coil impedance loci with non-magnetic material. the characteristic defect loci from the calibration Figure 1 illustrates the laboratory calibration tube tube, the wear scar size and depth can be determined. set-up used to simulate the in-service conditions with The steam generator shell was cut longitudinally typical defects. The vertical component of the coil and the upper half removed. The tubes were impedance vector as the probe traversed the tube is examined visually, and in every case the results ol shown in Figure 2. The top trace was obtained with eddy current anomaly have been confirmed. The commercial equipment and the bottom trace with our predictions of wear scar depth were accurate to 15',? own magnetic saturation probe. The greatly improved of wall thickness. Some tubes were cut and moved signal-to-noise ratio of the latter trace is obvious. 2", permitting the defective areas to be photo- Laboratory tests indicate that a 0.010 inch graphed. The leaking tube defect can be seen in diameter through hole can be reliably-identified in Figure 4(a) and some of the other wear scar areas can unsupported tube sections; however, under support be seen in Figures 4(b)-4(d). plates and in the presence of magnetic deposits such as magnetite the defect sensitivity is reduced by a factor of ~ 2. CONCLUSION Knowledge of the magnetic properties of Monel-400 indicated the possibility of magnetic Patent pending saturation to eliminate the permeability variations. TUBE NO 188 1OOkHz SATURATION PROBE (BAFFLE PLATES NO.30-40, 2X SCALE)

38 30 3fc

25 23 21 19

-BENE

J i TUBE SHEET 17 15

Figure 3a — Eddy current scan of tube 188. X-Y display of coil impedance vector.

TUBE NO.188 100KHz SATURATION PROBE

30 25 23 27

U-BEND

13 11 9 7 5 3 1 SHEET 15

Figure 3b - Eddy current scan of tube 188. Y-component of coil impedance vector, (downward signal represents a loss in wall thickness). Two different probes were developed capable of With these probes, improved results should be testing tubes of such material. The eddy current obtainable on all ferromagnetic materials and mate- laboratory and in-service results on the Douglas Point rials containing ferromagnetic inclusions. steam generator tubes, clearly indicate that ferromagnetic Monel tubes can be successfully inspected. V.S.Cecco

Figure 4 - Photographs of wear scar areas. (a) Tube 188, baffle plate 27. (c) U-bend, tube 188 (b) Tube 188, baffle piate 29. (d) Outer tubes, baffle plate 29.