APSAEM14 Jorunal of the Japan Society of Applied Electromagnetics and Mechanics Vol.23, No.3 (2015) Regular Paper Evaluation of the Material Degradation of Austenitic Stainless Steel under Pulsating Tension Stress Using Magnetic Method Mohachiro OKA *1, Terutoshi YAKUSHIJI *1 and Masato ENOKIZONO *2 To prevent a serious accident by the metal degradation of a structure made of iron-based structural materials, we have already proposed some fatigue evaluation methods for various iron-based structural materials. One of these is a residual magnetization method that uses a thin-film flux-gate magnetic sensor. This method is called the perpendicular residual magnetization method. This fatigue evaluation method had a good correlation between the magnetic sensor output signal and the amount of plane bending fatigue damage in the iron-based structural materials. However, we have not yet evaluated the pulsating tension fatigue damage of iron-based structural material such as the austenitic stainless steels using this method. In this paper, we report the evaluation results of a pulsating tension fatigue damage accumulation in austenitic stainless steels (SUS316 and SUS316L) using the perpendicular residual magnetization method. From our experiment, the maximum value of the Z component residual leakage magnetic flux density clearly depends on the magnitude of pulsating tension and the number of stress cycles. Keywords: fatigue, degradation, SUS316, SUS316L, flux gate magnetic sensor, residual magnetization. (Received: 24 July 2014) 1. Introduction Since austenitic stainless steels show excellent mechanical and chemical strength and corrosion resistance, they are widely used not only in residential but also industrial components. Consequently, it is necessary to prevent accidents that are due to the degradation of these components over time. To prevent accidents caused by metal degradation, many researchers continue to study numerous fatigue evaluation methods. In the field of magnetic material, it is well-known that magnetism and the micro-structure of the metallic material are closely related. Thus, many researchers were attempting to detect degradation of the magnetic material by the change in the Barkhausen noise [1-3]. Moreover, many other researchers were evaluating degradation of the magnetic materials using the change of permeability and other magnetic properties [4]. In the field of the non-magnetic material, both the eddy current testing method [5] and the residual magnetization method [6] are consistently applied. In our case, to determine the amount of fatigue damage in iron-based structural materials such as austenitic stainless steels, we are researching some material degradation evaluation methods using magnetic phenomena. One effective method is residual magnetization, using a thin-film flux-gate (FG) magnetic sensor [7]. This method which we used in a previous study clearly detected material degradation in austenitic stainless steels caused by plane-bending fatigue damage [8,9]. Correspondence: M. OKA, National Institute of Technology, Oita College, 1666 Maki, Oita, 870-0152, Japan email: oka@oita-ct.ac.jp *1 National Institute of Technology, Oita College *2 Vector Magnetic Characteristic Technical Laboratory Another effective method is eddy current testing (ECT), which was also used in our previous study. The ECT method obviously evaluated plane bending fatigue damage of the ferritic stainless steel (SUS430) [10]. More recently, we performed an evaluation of pulsating tension fatigue damage for the mild steel using the eddy current testing method excited by a low excitation frequency under the DC magnetic field [11]. These fatigue evaluation methods had a good correlation between the magnetic sensor s output signal and the amount of fatigue damage in the iron-based structural materials. However, we have not yet evaluated the pulsating tension fatigue damage in the austenitic stainless steels using the perpendicular residual magnetization method. In general, it is said that progress and the distribution of fatigue damage varies greatly between plane bending fatigue damage and pulsating tension fatigue damage. Thus, we believed that we should establish the evaluation method of the pulsating tension fatigue damage of the austenitic stainless steels. In this paper, we evaluated the pulsating tension fatigue damage of two kinds of austenitic stainless steels (SUS316 and SUS316L) utilizing the perpendicular residual magnetization method. 2. The Residual Magnetization Method and the FG Magnetic Sensor 2.1 Principle of the residual magnetization method In the austenitic stainless steels, and in particular, in SUS304, it is well known that the transformation from austenite to magnetic substance such as martensite is induced by plastic deformation caused by fatigue and degradation. In general, the austenitic stainless steels are non-magnetic material under normal conditions. The austenite in an austenitic stainless steel is non-magnetic, 458
and the martensite in an austenitic stainless steel shows magnetism. Moreover, in the case of SUS304, the amount of the martensite is related to the amount of the fatigue damage. In stainless steels such as SUS316 and SUS316L, the generated magnetism caused by fatigue is not specified. However, it is known that there is a good correlation between the accumulation of fatigue damage and strength of the Z component residual leakage magnetic flux density caused by the perpendicular residual magnetization [9]. Accordingly, we attempted to use the perpendicular residual magnetization method to estimate the amount of fatigue damage in the austenitic stainless steels caused by pulsating tension fatigue. The Z component residual leakage magnetic flux density (B z ) in the austenitic stainless steels is very weak. Thus, to clarify the relationship between B z and pulsating tension fatigue, we used a thin-film fluxgate (FG) magnetic sensor (SHIMADZU CORPORATION) that has an extremely high sensitivity. Figure 1 shows the principle of the perpendicular residual magnetization method. Figure 1(a) shows an undamaged specimen made of an austenitic stainless steel and Fig. 1(b) shows a damaged specimen made of the same material shown in Fig. 1(a). In an undamaged specimen, magnetic substance is not perceived. However, in a damaged specimen, magnetic substance is induced in the midrange of the specimen. When the induced magnetic substance is magnetized by the external magnetic field in the Z direction, the residual magnetization appears. Therefore, if the magnitude of B z can be measured, we can determine the amount of fatigue damage caused by pulsating tension fatigue. This is a principle of the perpendicular residual magnetization method. is about 30mV/ T. The FG magnetic sensor head is vertically placed on a specimen keeping 1 mm distance from the surface of the specimen. Fig. 2. The FG magnetic sensor head. 3. Specimen and Experiment 3.1 The specimen Figure 3 shows two types of specimens made of SUS316 and SUS316L used in our experiments. In the case of SUS316, specimens were cut out by the abrasive water-jet machining (AWJM) method. Moreover, in the case of SUS316L, specimens were cut out by the wire electrical discharge machining (WEDM) method. Table 1 shows the chemical compositions of SUS316 and SUS136L used in our experiments. Fig. 3. Dimension of two types of specimens. (a) Undamaged specimen. (b) Damaged specimen. Fig. 1. Principle of the residual magnetization method. 2.2 The thin-film flux-gate (FG) magnetic sensor To estimate the amount of fatigue damage in the austenitic stainless steels, we measured B z at each measured point using the thin-film flux-gate (FG) magnetic sensor [7,8]. Figure 2 shows the FG magnetic sensor head used in this experiment. This FG magnetic sensor can detect B z caused by the perpendicular residual magnetization in a specimen. The Z-axis is taken in the direction of the thickness of the specimen as shown in Fig. 1(b). The sensitivity of this FG magnetic sensor Table 1 The chemical composition of SUS316 and SUS316L in wt%. wt [%] C Si Mn P S Ni Cr Mo SUS316 0.04 0.68 0.99 0.03 0.009 10.1 16.69 2.03 SUS316L 0.021 0.7 1.03 0.024 0.003 12.17 17.44 2.15 3.2 The measurement system Figure 4 shows a setup of experimental equipment. The signal from the FG magnetic sensor was digitized by a multi-meter and was inputted into the computer. The measurement was repeated twice. A smoothing algorithm such as the moving average method was used for better processing of the FG magnetic sensor output and for reducing noise. 3.3 The experimental method This experiment was a fatigue test that impressed the pulsating tension stress to the specimen. To make a 459
clear relationship between the amount of fatigue damage and B z, experiments were carefully carried out in the procedure shown in Fig. 5. At first, specimens were demagnetized using a demagnetizer. Secondly, specimens were magnetized in a static magnetic field (0.3T) by using a DC power supply and the C-shaped exciting coil (as shown in Fig.6.) in the Z direction as shown in Fig. 1. Next, the distribution of B z caused by perpendicular residual magnetization at 1 mm above specimens was measured within the range of 40 mm x 40 mm every 1 mm step as shown in Fig. 3. Partial transformation from austenite to magnetic substance occurred due to fatigue damage at the middle part of a specimen. The area of magnetic substance was magnetized in the excitation magnetic field. This procedure was repeated until the specimen was broken. This experiment was executed when the stress ratio (R) was 0.1. A stress waveform was a sinusoidal wave. The experiments were performed at room temperature. Pulsating tension stress was applied by a tensile and compression tester (V-0674, SAGINOMIYA SEISAKUSHO) which operated at 20 Hz. In this tester, the maximum tensile force was about ±49 kn, and the maximum displacement amplitude was 30 mm. Figure 7 shows the photograph of a specimen set in the tensile and compression tester. happened when the stress was 140 MPa or less in SUS316L. Therefore, fatigue limits of SUS316 and SUS316L were not obtained by this experiment. An impressed a of the following experiments was decided using these S-N curves. We used 140 MPa, 160 MPa, 180 MPa, 200 MPa, and 220 MPa as a value of a. Fig. 6. The C-shaped exciting coil. Fig. 7. Photograph of a specimen set in the tensile and compression tester. Fig. 4. The block diagram of the measurement system. Fig. 8. The S-N curve (SUS316 and SUS316L). 3.5 The background magnetic flux density Fig. 5. Experimental procedure. 3.4 The S-N curves of specimens To determine the mechanical property of our specimens, we measured the relationship between pulsating tensile stress ( a ) and the number of stress cycles (N) (S- N curve). This relationship is shown in Fig. 8. When the stress was 120 MPa or less in SUS316, the specimen was broken at the specimen attaching part due to fretting fatigue. Moreover, the phenomenon similar to SUS316 To exclude the influence of a background magnetic field such as terrestrial magnetism and another magnetic field caused by electrical machineries, we measured the Z component background magnetic field near the FG magnetic sensor head as shown in Fig. 9. Thus, the influence of the background magnetic field was removed from all measurement data, which is shown in this paper. 3.6 Calculation of db z To clarify the relationship between B z and pulsating tension fatigue, we defined db z. Figure 10 shows the 460
distribution of db z at each measurement position when a and N were zero. db z is the difference between the average value (B Zave ) of B z in the black circle shown in Fig. 10 and B z at each measurement point. Moreover, db z was calculated by the following Equation (1). db Z B B. (1) Z Zave In Fig. 10, this specimen was cut out by the AWJM method. Therefore, it is thought that the processing using the AWJM method induced magnetic substance. However, because the specimen was not fatigued, the distribution of db z is smooth at the middle part of a specimen. Moreover, db z is slightly large along the edge of the specimen. The large db z was caused by the AWJM method. On the contrary, in the case of the WEDM method, db z was small at the same part because the influence on the material cut by the WEDM method was small. AWJM method. Figure 11(b) shows the same relation as Figure 11(a). In this case, the specimen was magnetized. In Fig. 11(b), db z is large along the edge of the specimen compared with other parts. db zmax was defined as the maximum value of db z in the range of 20 mm x 40 mm in the vicinity of the middle part of the specimen. In this case, db zmax is about 4.9 T. In the case of Fig. 12, the specimen was SUS316L which was cut out using the WEDM method. Figure 12 shows the same relation as Figure 11. In Fig. 12(b), db z is somewhat large along the edge of the specimen, the same as Fig. 11(b). The value of db zmax in the same middle part of the specimen is about 0.73 T. (a) Degaussed specimen. (b) Magnetized specimen. Fig. 11. The distribution of db z (SUS316 was cut out using the AWJM method., a =200 MPa). Fig. 9. The distribution of the Z component background magnetic flux density near the measurement area. Fig. 10. The distribution of db z (SUS316 was cut out using the AWJM method.). 4. Experimental Results and Discussions 4.1 The distribution of db z (N is equal to 0.) Figure 11(a) shows the distribution of db z at each measurement position when a and N were zero, respectively. In this case, the specimen was SUS316 and it was degaussed. Moreover, it was cut out using the (a) Degaussed specimen. (b) Magnetized specimen. Fig. 12. The distribution of db z (SUS316L was cut out using the WEDM method., a =200 MPa). 4.2 The distribution of db z (N is not equal to 0.) Figure 13 shows the distribution of db z at each measurement position when a was 200 MPa. N was 10,000 in Fig. 13(a) and N was 18,690 in Fig. 13(b). This specimen was magnetized. In Fig. 13(a), db z slightly increased at the position of X=0 mm and Y=-20 mm in the black circle. This increasing part corresponds to the location of the crack generation part in Fig. 13(b). Because pulsating tension fatigue accumulated at this part, it is thought that magnetic substance increased at this part. In Fig. 13(b), the crack of about 4.2 mm in length occurred in this sample at N=18,690. Thus, at the position with the crack, db zmax was 54.9 T. Figure 14 shows the photograph of the damaged specimen with a small crack caused by pulsating tensile fatigue damage. Figure 15 shows the distribution of db z at each measurement position when a was 200MPa. N was 461
50,000 in Fig. 15(a) and N was 165,420 in Fig. 15(b). In this case, the specimen was made of SUS316L. From these figures, db z increased in the same case as SUS316 and db z increased at the position of X=0 and Y=20 mm in the black circle shown in Fig. 15(b). In this case, the length of the crack was 1.4 mm at the position with the crack, and db zmax was 6.34 T. Similarly in SUS316L, this increasing part corresponds to the crack generation part. In this paper, the value of db zmax was used as a parameter that evaluated pulsating tension fatigue in SUS316 and SUS316L. Figure 16 shows the relationship among db zmax, N, and a. In this case, specimens were made of SUS316. Figure 17 shows the same relation as Figure 16 when specimens were SUS316L. From both figures, db zmax hardly increased just before the break of the specimen. However, db zmax has increased greatly by crack generation. Moreover, the value of N to which db zmax increased rapidly was increasing in proportion to a. (a) N=10,000 (a) N=50,000 (b) N=18,690 Fig. 13. The distribution of db z (SUS316, a =200 MPa). (b) N=165,420 Fig. 15. The distribution of db z (SUS316L, a =200 MPa). Fig. 14. Photograph of the damaged specimen with the small crack (SUS316, a =200 MPa, N=18,690). 4.3 Relationship among db zmax, N, and a 5. Conclusion In this paper, we evaluated the pulsating tension fatigue damage of two kinds of austenitic stainless steels (SUS316 and SUS316L) using the perpendicular residual magnetization method. From our experiments, because SUS316 and SUS316L were stainless steels with the stable austenitic phase in the vicinity of the room temperature, the 462
significant change of their magnetism was not seen according to the progress of pulsating tension fatigue. However, a change of their magnetism had been observed immediately before generation of the minute crack. Consequently, it can be said that this method is effective to evaluate the pulsating tension fatigue accumulation in SUS316 and SUS316L. To increase the accuracy of fatigue evaluation in the austenitic stainless steels, improvement of the perpendicular residual magnetization method will be needed in the future. Acknowledgment This work was supported in part by the Japan Society for the Promotion of Science under Grant No. 26420406. Fig. 16. The relationship among db zmax, N, and a (SUS316). [2] S. P. Sagar, N. Parida, S. Das, G. Dobmann, and D. K. Bhattaacharya, Magnetic Barkhausen emission to evaluate fatigue damage in a low carbon structural steel, International Journal of Fatigue, ELSEVIER, Vol. 27, pp. 317-322, 2005. [3] M. Kaplan, C. H. Gur and M. Erdogam, Characterization of Dual-Phase Steels Using Magnetic Barkhausen Noise Technique, Journal of Nondestructive Evaluation, Vol. 26, PP. 79-87, 2007. [4] H. Kikuchi, A. Takahashi, L. Zhang, K. Ara, Y. Kamada and S. Takahashi, NDE for Magnetic Material by Minor Loop Method Using Magnetic Yoke Probe, Electromagnetic Nondestructive Evaluation (IX), IOS Press, pp. 119-125, 2005. [5] T. Teramoto, Accurate Damage Evaluation of Austenitic Stainless Steel by ECT Method, Journal of JSAEM, Vol. 13, No. 2, pp. 87-93, 2005 [6] Y. Nakasone, Y. Iwasaki, T. Shimizu, and S. Kasumi, Electromagnetic non-destructive detection of damage in an austenitic stainless steel by the use of martensitic transformation, Journal of JSAEM, Vol. 9, No. 2, pp. 123-130, 2001. [7] K. Yoshimi, Y. Fujiyama, T. Munaka, H. Nakanishi, T. Yoshida, and Y. Yamada, Microfabricated Thin-Film Flux-Gate Magnetic Sensor and its Applications, SHIMADZU REVIEW, Vol. 56, No. 1&2, pp.19-28, 1999. [8] M. Oka, T. Yakushiji and M. Enokizono, Fatigue Dependence of Residual Magnetization in Austenitic Stainless Steel Plates, IEEE Transactions on Magnetics, Vol. 37, No. 5, pp. 3373-3375, 2001. [9] M. Oka, E. Wada, T. Yakushiji, Y. Tsuchida and M. Enokizono, Evaluation of Fatigue Damage in SUS316 and SUS316L Using the FG Magnetic Sensor, Electromagnetic Nondestructive Evaluation (IX), IOS Press, pp. 159-165, 2005. [10] M. Oka, Y. Tsuchida, T. Yakushiji and M. Enokizono, Fatigue Evaluation for a Ferritic Stainless Steel (SUS430) by the Eddy Current Method Using the Pancake-Type Coil, IEEE Transactions on Magnetics, Vol. 46, No.2, pp. 540-543, 2010. [11] M. Oka, T. Yakushiji, Y. Tsuchida, and M. Enokizono, Examination of the Inductance Method for Non Destructive Testing in Structural Metallic Material by Means of the Pancake-type Coil, Journal of JSAEM, Vol. 21, No. 3. pp. 488-493, 2013. Fig. 17. The relationship among db zmax, N, and a (SUS316L). References [1] Y. Tsuchida, T. Andou and M. Enokizono, Fatigue Evluation Based on Chaotic Attractors of Barkhausen Noise, Journal of the Magnetic Society of Japan, Vol. 26, pp. 756-768, 2002. 463