Detection of hidden defects in thin steel plates using GMR sensor arrays

Transcription

1 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic Detection of hidden defects in thin steel plates using GMR sensor arrays More Info at Open Access Database M. Pelkner a, R. Pohl a, Th. Erthner a, M.Kreutzbruck a, C. Commandeur b a BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, Berlin, Germany b Tata Steel R&D, Wenckebachstraat 1, 1970 CA IJmuiden, The Netherlands Abstract At BAM, the Federal Institute for Materials Research and Testing, a group of scientists develops NDTapplications based on GMR sensor technology. In particular, the knowledge gained in the field of automated testing systems based on the magnetic flux leakage (MFL) were combined with GMR sensors to achieve high resolution testing of ferromagnetic materials. In cooperation with Europe s second largest steel producer, Tata Steel Europe, BAM is working on solutions for the detection of small inhomogeneities in thin steel plates. The objective is to incorporate an automated testing facility in a production line. Before setting up an automated testing system, a feasibility study was carried out in order to verify the ability of GMR-MFL-testing and eddy current testing (ET) for the detection of hidden defects in thin steel plates. For this purpose, Tata Steel Europe fabricated in a first step test samples of 0.2 mm thick steel plates in which defects of different depth (5 60 µm) were introduced. Only in case of GMR-MFL-testing, we were able to detect parts of the hidden defects trustworthily with a SNR better than 10 db. The lift off between sensor and surface was 250 µm. On this basis, we investigated different testing scenarios including velocity tests and different lift offs. The achieved results for this special testing problem were presented leading to a practical example for GMR-based testing and paving the way to an automated testing system in a production line. Keywords: magnetic flux leakage, GMR, hidden defects 1. Introduction Small inclusions in thin steel plates could lead to defects in further process steps because of the different material properties. To label areas with inclusions a testing method is needed which detects these inclusions. Since the magnetic material parameters differ between steel and inclusion a magnetic testing method in particular GMR-MFL-testing (GMR: giant magneto resistance; MFL: magnetic flux leakage) could be a solution [1,2]. Therefore, BAM and Tata Steel evaluated the possibilities of GMR sensor arrays adapted to NDT application for an in-situ testing during production. For a validation study Tata Steel provided four test samples 0.22 mm thick steel plates (300x210 mm²) containing three rows with 45 notches (each 15 in one row) which are produced by laser ablation. The profile of the defects is elliptical. Width of the defects ranges between 80 µm and 220 µm. The depth are 5 µm up to 60 µm and the length are 0.5 mm up to 1.3 mm. Figure 1 shows as an example an optical measurement of the geometry of one notch. Depicted are two false color rendering plots and two curves of the profile of the notch.

2 Figure 1: Height profile of an exemplary artificial defect (900x200x50 µm³). 2. MFL-testing using GMR sensors Testing of all four plates was performed using the GMR-testing facility at BAM [2]. Adapted GMR sensor arrays (gradiometer, 16 elements measuring the normal field component related to the surface) are employed [1,2]. A magnetization unit (yoke, coil current 1A) was used to apply a magnetic field inside the plates. Figure 2 shows a C-scan of a GMR magnetic flux leakage measurement (GMR-MFL) for test sample 3/8 in 1:2 scale. Here, we scanned one side in such a way that the notches were at the backside. Figure 2: GMR-scan for buried defects (test sample 3/8, scan area 260x140 mm², 1:2 scale).

3 Besides the signals of the notches MFL-signals of real inclusions were detected. These inhomogeneities were also observed using eddy current testing (EC). The stripes in Figure 2 are caused by different offsets of each GMR element. Using image processing leads to a cancellation of these stripes (see Figure 3, same measurement like in Figure 2). Figure 3: GMR-scan (see Figure 2) after image processing (test sample 3/8, scan area 260x140 mm², 1:2 scale). 2.1 Lift off To investigate signal dependence of increasing lift off (LO, distance sensor-surface) we performed measurements for three different LOs. Figure 4 (a) presents line scans for LO = 0.1 mm (black curve), 0.25 mm (red) and 0.5 mm (blue) starting at the deepest defect (notch 13 at x = 31.5 mm). Figure 4 (b) details notch 13. Figure 4: (a) Line scan along one row for three liftoffs (LO = 100µm, 250 µm and 500 µm). (b) section for line scan along defect 13 (deepest defect). In addition to the decreasing MFL-signal we observed a decline of noisy background signals indicating that this noise is caused by material properties and roughness of the plate itself (see for instance Figure 4 (b) black and red curve). Based on their volume and defined geometry, the MFL signals of the notches can be distinguished from the background signals H noise (noise, roughness, etc.). Table 1 summarizes MFL- and noise signals for the deepest and smallest defect.

4 defect 13 depth 45 µm defect 1 depth 8 µm Table 1: MFL signal and background field signals for defect 13 and 1. Lift off [µm] ΔH [A/m] H noise [A/m] SNR [db] Increasing LO from 100 µm up to 500 µm leads to a MFL signal loss of 20dB. Since, background fields decrease with increasing LO, SNR declines in the range of 5 8 db. Nevertheless, all hidden defects could be resolved with a SNR better than 6 db. 2.2 Angular dependence MFL testing has an angular dependence concerning the direction of the applied magnetic field with respect to the notches. The more cross sectional area of the defect is in the way of the field the more flux will leak. The preferred field direction is therefore perpendicular to the notch. However, during rolling of the steel most of the defects are stretched in the rolling direction. Due to testing principles in a production line the applied field has to be rotated leading to a non-perpendicular testing of inclusions. To investigate the influence of rotating the applied field direction towards the defects we show in Figure 5 a measurement where we rotate the sample 45 degree compared to the measurement of Figure 2. Figure 5: GMR-scan for buried defects. Sample was rotated 45 (scan area 220x220 mm², scale 1:2).

5 The three rows with the artificial defects could be resolved with a good SNR. In addition, natural defects which are likely caused by inclusions are detected using GMR-MFL. To estimate the decline of MFL signals, Figure 6 compares two line scans, (a) 90 and (b) 45 alignment between defect and applied magnetic field. The MFL-signal of the 45 -case is reduced by 30% compared to the 90 -measurement. Figure 6: Left: line scan defect 13 (see Figure 4). Right: Line scan same defect with sample rotated by an angle of 45 compared to the left scan. 2.3 Influence of scan velocity Further, we investigated different scan velocities and their influence on MFL signals. Higher velocities should introduce eddy currents inside ferromagnetic samples. For a testing facility in a production line this has to be considered since production velocities are in the order of some meters per second. For this purpose we used a cylindrical test sample (steel E 295, diameter 229 mm) shown in Figure 7 on the right. This mock-up was fixed in a turning machine to generate the rotation. In this mock-up was introduced an artificial crack with following dimensions: length 15 mm, width 0.5 mm, and depth 1.5 mm. Figure 7: Different views of test setup for velocity measurements (left and middle). Right: Scheme of the test sample. We undertook measurements for a velocity range from 0.9 m/s up to 36 m/s. MFL signals were amplified by a gain factor of 40 db which were charted by using an oscilloscope.

6 Besides a compressed signal due to increased scan velocity sensor signals are also doubled unexpectedly (see Figure 8.) Figure 8: Defect signal for v = 2,7 m/s (left), and v = 36 m/s (right). Table 2 summarizes the voltage signals (peak-peak) for several scan velocities. In addition, the second row shows signal changes relating to the MFL signal for v = 0.9 m/s in %. Table 2: MFL signals for several scan velocities. velocity [m/s] GMR signal [V] GMR/GMR(0.9) [%] Eddy current measurements For further evaluation we performed measurements for the test plates using several eddy current probes (EC). In table 3 we listed these probes, their operating frequency range, and specifications. Table 3: Deployed eddy current probes. Probe Frequency range Specification KDS khz 800 khz Highly resolving and sensitive differential probe AN khz 500 khz Highly resolving absolute probe; excellent resolution SK 25 TA-1 15 khz 100 khz Absolute probe for buried defects; average resolution SK 35 TA 10 khz 80 khz Absolute probe for buried defects; average resolution In general it was observed that no EC-probe detected the hidden notches (see Figure 9). The reason is the used frequency of 100 khz and the skin effect. For ferromagnetic materials skin depth lies in the order of several 10 µm for f = 100 khz. High spatial resolution probes need

7 high frequencies in order to reach their capabilities. Probes for low frequency applications exhibit in general an insufficient detection capability in terms of detection of µm-defects. Figure 9: Comparison of the results (buried defect detection) for test sample 3/8 using different testing methods: GMR-MFL (top), eddy current testing using probe KDS 2-2 (middle), and probe SK 35 TA (bottom), scan area 250 x 90 mm². Figure 9 shows in the top the GMR-MFL measurement and compared to GMR result two EC scans (middle and bottom) using different EC probes. Inhomogeneities could also be detected (depicted by the red markers). The geometrical pattern (also observed optically) is caused by the non-contactless GMR scan. Here, patting rollers were used to control distance. This induces material changes and has to be avoided for testing during production. A contactless testing is compulsory. 4. Magnetic particle inspection For further evaluation of the GMR-MFL testing, we performed magnetic particle inspections (MP). Two different ferromagnetic powders come into operation a fluorescent powder (particle size 4 µm) and a black powder (particle size 0.2 µm). Magnetization was induced by a hand magnet. Figure 10 presents on the top the GMR-MFL scan, in the middle a photo of the fluorescent powder, and at the bottom a photo using the black powder.

8 Figure 10: Comparison between GMR-MFL (top), fluorescent magnetic powder (middle) and black magnetic powder (bottom); scale 1:1. 10 hidden defects of the steel plate could be resolved by MP leading to a detection limit of MP in the order of a depth of 20 µm and, therefore for a remaining wall thickness of 200 µm and ~90%, respectively. GMR-MFL was able to detect all 13 defects (hidden) of the investigated row (max. remaining wall thickness 215 µm, defect geometry from 1300x220x30 µm³ up to 900x170x5 µm³). 5. Conclusion Small hidden defects in thin steel plates (thickness 220 µm) could be resolved with a good SNR using GMR sensors and MFL-testing. Defects down to 5 μm where detected with a SNR of 6 db in the 0.22 mm thick plates. This limit was rather set by the material noise than by the noise of the field sensors. Practical testing situations like the sensor to surface distance (lift off), magnetization angle towards orientation of the defects, and scan velocity were investigated and should be controllable in case of an in-situ testing facility built in a production line. We also carried out alternative testing methods magnetic particle inspection and eddy current testing which can be excluded for a in-situ testing. The inhomogeneities that were detected in addition to the artificial defects will be investigated in more detail as they are likely due to a real inclusions. Further, development and validation of new applicable probes for an in-situ testing during production are being drawn up. Here, challenges are mechanical and electromagnetic conditions. In addition, plates thicker than the presented plates in this contribution shall be investigated in future. References 1. Pelkner, M.; Neubauer, A.; Reimund, V. & Kreutzbruck, M.: Routes for GMR-sensor design in nondestructive testing, Sensors, 2012, 12, Pelkner, M.; Neubauer, A.; Reimund, V.; Erthner, T.; Panke, N. & Kreutzbruck, M.: Automatisierte Streuflussprüfung und Bewertung von Zylinderlagerringen unter Anwendung von GMR-Sensorarrays, DACH-Jahrestagung 2012, Graz (Österreich)

9 3. Pelkner, M.; Neubauer, A.; Reimund, V. & Kreutzbruck, M.: Local magnetization unit for GMR array based magnetic flux leakage inspection, QNDE 2011 (Burlington, Vermont), Review of progress in quantitative nondestructive evaluation, volume 31, American Institute of Physics, 2012,