The magnetic leakage field produced by a discontinuity draws and holds the ferromagnetic particles used in magnetic particle inspection (MPI). The particles held by the leakage field then provide the visible evidence to the location of the discontinuity. A change in the leakage field could, potentially, change the detectability of the crack. [1] Typically, the study of magnetic leakage fields has been limited to those emanating from air-filled discontinuities. However, for this investigation, the leakage fields from surface cracks filled with a magnetic iron oxide scale were observed. The goal was to determine if cracks filled with common magnetic oxides could essentially “bridge the magnetic gap” and, therefore, be masked during MPI.</p

CE Betz

JD Fast

R Beissner

Quinn, Mary

Hawkins, Gary

Digital Repository @ Iowa State University (ISU)

DETECTION OF MAGNETIC OXIDE FILLED CRACKS IN STEEL Mary T. Quinn and Gary F. Hawkins The Aerospace Corporation P. O. Box 92957 M2-248 Los Angeles, CA 90009 INTRODUCTION The magnetic leakage field produced by a discontinuity draws and holds the ferromagnetic particles used in magnetic particle inspection (MPI). The particles held by the leakage field then provide the visible evidence to the location of the discontinuity. A change in the leakage field could, potentially, change the detectability of the crack. [1] Typically, the study of magnetic leakage fields has been limited to those emanating from air-filled discontinuities. However, for this investigation, the leakage fields from surface cracks filled with a magnetic iron oxide scale were observed. The goal was to determine if cracks filled with common magnetic oxides could essentially "bridge the magnetic gap" and, therefore, be masked during MPI. A simplistic leakage field model is that of a spherical inclusion, radius (Y, embedded in an infinite medium. Even though this model relies on a number of approximations, extensive experimental results show general agreement between it and measured leakage field data. Derived from the magnetic scalar potential, Eq. (1) is the y component of the magnetic leakage field from a spherical inclusion. If a magnetic field is applied to the medium in the positive x direction and the permeability of the medium is greater than that of the inclusion, then the y component of the resulting leakage field appears as in Fig. 1.[2] PEAK-TO·PEAK x -T" 1- PEAK.TO.PEAK I SEPARATION DISTANCE Fig. 1. Magnetic Leakage Field for a Spherical Inclusion Imbedded in an Infinite Medium Review of Progress in Quantitative Nondestructive Evaluation, Vol. JOB Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1991 1899 (1) The effects of the oxide on the leakage field were determined by measuring changes in the y component of the leakage field. Analysis of the leakage field signatures gives information regarding the perturbation source. The flaw volume is proportional to the peak-to-peak amplitude of the signal. The flaw depth from the surface is related to the peak-to-peak seearation distance. Both peak-to-peak amplitude (PA) and separation distance (PS) were monitored. PROCEDURE A 0.06 in. length fatigue crack was initiated in the center of each of three 0.17 x 1.0 x 3.0 in. D6aC plates. The width of the crack opening was 0.0013 in. and the depth was approximately 0.027 in. The plates were individually magnetized and then scanned with a Hall probe to perform a statistical survey of the variance in the equipment set-up. Scans were made across the center of the crack, normal to the length (see Fig. 2). Magnetization was performed with a cobalt samarium magnet. A permanent magnet was used instead of magnetizing current because it induced a uOlform and easily reproducible magnetic field in the plate. The magnet was removed before a plate was scanned (residual field). Upon completion of scanning, it was noted that the leakage field signature was superimposed on the magnetic field across the plate itself (see Fig. 3a). The background field was removed to allow accurate PA and PS measurements. This was accomplished by mathematically determining the equation of the background (see Fig. 3b), usually a second-order polynomial, and subtracting it from the leakage field signature (see Fig. 3c). 1900 D COMPUTER 10.0454361 00 MUL TIMETER 0 0 LVOT CONDITIONER o 0 X-DRIVE CONTROLLER MICROMETER LVDT 10.045436 1 00 MULTIMETER 0 0 [LJ GAUSSMETER X-DRIVE STEP-IN MOTOR Fig. 2. Diagram of Experiment Set-Up. Fig.3a. t/) t/) o -5 :::I -10 ~ -15 -20.~~~~--~---L---L--~ 0.0 0.2 0.4 0.6 0.8 1.0 1.2 DISTANCE (in.) Signal recorded from a 1.0 in. scan across the center of a 0.125 in. crack. For this scan the residual field technique was used. The leakage field from the crack is superimposed on the background signal from the specimen itself. 0 -5 en en ~ -10 oCt " -15 -20 0.0 Fig. 3b. Portion of the signal from scan attributed to the magnetic field gradient across the 3.0 in. D6aC specimen. Equation of the background signal: y = 1.268 - 16.481 x + 1.744x2. 1901 1.0 0.8 0.6 0.4 0.2 Ul Ul 0.0 :::) c( C) -0.2 -0.4 -0.6 -O.~ -1.0 0.0 DISTANCE (in.) Fig. 3c. The resulting signal when the background gradient is subtracted from the original scan. Peak-to-peak amplitude and separation distance measurements are obtained from this plot. Previous studies have determined that during the tempering of D6aC, magnetic oxides have formed on the interior of surface cracks.[3] The oxide found inside a crack was identified by x-ray diffraction as a combination of Fe304, ),Fe203, FeO, and c¥ Fe (see Fig.4). Fe304 and ),Fe203 are ferromagnetic.[4] The peak for c¥ Fe can be attributed to the D6aC base material. After establishing a baseline, all of the plates were heat treated. One of the plates was used as a control and encapsulated in an evacuated quartz tube before the heat treatment. This was done to prevent the formation of oxides during heat treatment. The heat treatment consisted of a four hour temper at 565°C. The plates were rescanned upon completion of the heat treatment. Again, PA and PS measurements were recorded. The results are listed in Table 1, where the values represent the average measurement obtained from multiple-specimen scans. The corresponding standard deviation a is listed below each average, Table 1. Residual Field Technique Results AS RECEIVED HEAT TREATED CHANGE IN SIGNAL Specimen Peak-to-Peak Peak-to-Peak Peak-to-Peak Separation Amplitude Separation Amplitude Separation Amplitude (in.) (Gauss) (in.) (Gauss) (loss) (loss) 0.06 in. 0.092 1.126 0.081 0.521 NONE 53.7% CRACK (J = 0.002 (J = 0.045 (J = 0.009 (J = 0.022 0.06 in. 0.099 1.029 0.093 0.399 NONE 61.2% CRACK (J = 0.006 (J = 0.034 (J = 0.010 (J = 0.020 CONTROL 0.06 in. 0.094 0.978 0.097 0.457 CRACK (J = 0.006 (J = 0.073 (J = 0.013 (J = 0.022 NONE 53.3% 1902 Fig. 4. Cross-section through an oxide filled crack. The oxide was identified by x-ray diffraction to be a combination of Fe304, ),Fe203, FeO, and Q! Fe. Fe304 and ),Fe203 are ferromagnetic. Magnification 120 x. CONCLUSIONS The presence of magnetic oxide in a crack should not effect the crack's detectability during magnetic particle inspection. There was no difference in the leakage field signal between the 0.06 in. crack plates and the control after heat treatment. This indicates that all signal changes were the result of the heat treatment. The decrease in PA experienced by both may be related to the release of residual stresses around the crack formed during crack initiation and subsequent fatigue-induced growth.[5] The PS remained unchanged for the crack. This was expected. According to Eq. (1), the PS is only dependent on crack depth which went unchanged, whereas the PA is dependent on crack depth, crack volume, crack permeability, material permeability, and the applied magnetic field. It is interesting to note that after heat treatment there was greater variance in the PA measurements and less variance in the PS measurements. Finally, it was determined that the procedure used to measure the magnetic leakage field caused by a surface crack was statistically repeatable. Therefore, the equipment and procedures used for this investigation can be used as a reliable basis for future magnetic leakage field research. The authors would like to acknowledge the help of John Hribar and Paul Adams for x-ray diffraction, and Jeny T#?ndt for the use of his gauss meter. REFERENCES 1. Carl E. Betz, Principles of Magnetic Particle Testing, (Magnaflux Corporation, Chicago, Illinois, 1967), pp. 63-64. 1903 2. R. Beissner, G. A Matzkanin, and C. M. Teller, NDE Applications of Magnetic Leakage Field Methods .. A State-of-the-Art Survey, (Nondestructive Testing Information Analysis Center, San Antonio, Texas 1980). 3. S. Frost, Unpublished Results. 4. J. D. Fast, Interaction of Metals and Gases, l-Vl. 1 Thermodynamics and Phase Relations, (Academic Press, New York, 1965), pp. 61-66. 5. F. N. Kusenberger, G. A Matzkanin, J. R. Barton, and P. H. Francis, Nondestructive Evaluation of Metal Fatigue, (Air Force Office of Scientific Research, Washington, D. C.), AFOSR-TR-0384. 1904 

Detection of Magnetic Oxide Filled Cracks in Steel

Mary T. Quinn

Gary F. Hawkins

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Detection of Magnetic Oxide Filled Cracks in Steel

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