13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- In-situ monitoring of fatigue of metastable austenitic steels with electromagnetic acoustic transducers, EMATs Andreas Sorich*, Marek Smaga, Dietmar Eifler Institute of Materials Science and Engineering, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany *Corresponding author: sorich@mv.uni-kl.de Abstract In this research work an in-situ method for the characterization of fatigue processes in metastable austenitic steels with electromagnetic acoustic transducers (EMATs) was developed. The austenitic steel AISI 347 (1.4550, X6CrNiNb1810) was investigated in isothermal total strain-controlled tests in the LCF-range at ambient temperature and 300 °C. By means of cross effects between physically based mechanical- and EMAT-measurements, a detailed characterization of cyclic hardening and softening processes as well as fatigue induced phase transformations and changes of specimens topography and crack initiation were performed. Keywords Austenitic stainless steel, fatigue, phase transformation, ´-martensite, NDT, EMAT 1. Introduction In power plants as well as in chemical plants, metastable austenitic steels are subjected to monotonic and cyclic thermo-mechanical loading [1-2]. The cyclic deformation behavior of metastable austenitic steels at ambient temperature (AT) and elevated temperature strongly depends on the deformation induced ´-martensite formation. This phase transformation from paramagnetic austenite (fcc) into ferromagnetic ´-martensite (bcc) leads to cyclic hardening and thus to an increase of the fatigue strength [2-4, 8]. At elevated temperatures no ´-martensite formation was observed in the LCF-range. Therefore the fatigue behavior at elevated temperatures depends basically on cyclic hardening processes due to an increase of dislocation density, followed by cyclic saturation and/or softening processes due to changes in dislocation arrangement and density and finally the formation and propagation of fatigue cracks [5-8]. The investigations are focused on the characterization of the fatigue processes at AT and 300 °C and the development of in-situ monitoring methods on the basis of electromagnetic acoustic measurements. EMATs, which generate ultrasonic waves directly in electrically conductive materials without the use of a couplant, were developed at the Fraunhofer Institute for Non-Destructive Testing (IZFP) Saarbrücken in Germany and calibrated to defined fatigue states in collaboration with the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern in Germany [8]. The activities at WKK are focused on the characterization of the materials science aspects of the fatigue processes, while IZFP develops electromagnetic acoustic transducers. 2. Experimental procedures 2.1. Experimental setup The fatigue tests were performed at different constant total strain amplitudes ( a,t = 0.8, 1.0, and 1.2 %) at ambient temperature and 300 °C on a servo-hydraulic testing system with a strain ratio of R = -1 using triangular load-time functions and a frequency of 0.01 Hz. Figure 1 shows schematically the experimental setup for both temperatures. Besides electromagnetic non-destructive testing methods using EMAT and a Feritescope® sensor, full mechanical strain-stress hysteresis measurements were performed to allow a microstructure-related description of the cyclic deformation behavior of the metastable austenite. Electromagnetic acoustic transducers use electromagnetic fields to excite ultrasonic waves and therefore have the great advantage that
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