Corrosion resistance has been the main scope of the development in high-alloyed low carbon austenitic stainless steels. However, the chemical composition influences not only the passivity but also significantly affects their metastability and, consequently, the transformation as well as the cyclic deformation behavior. In technical applications, the austenitic stainless steels undergo fatigue in low cycle fatigue (LCF), high cycle fatigue (HCF), and very high cycle fatigue (VHCF) regime at room and elevated temperatures. In this context, the paper focuses on fatigue and transformation behavior at ambient temperature and 300 °C of two batches of metastable austenitic stainless steel AISI 347 in the whole fatigue regime from LCF to VHCF. Fatigue tests were performed on two types of testing machines: (i) servohydraulic and (ii) ultrasonic with frequencies: at (i) 0.01 Hz (LCF), 5 and 20 Hz (HCF) and 980 Hz (VHCF); and at (ii) with 20 kHz (VHCF). The results show the significant influence of chemical composition and temperature of deformation induced ´-martensite formation and cyclic deformation behavior. Furthermore, a “true” fatigue limit of investigated metastable austenitic stainless steel AISI 347 was identified including the VHCF regime at ambient temperature and elevated temperatures.

The austenitic steel X6CrNiNb1810 (AISI 347) was investigated in isothermal total strain-controlled tests at ambient temperature and T = 300 °C in the LCF-and HCF-range. The phase transformation from paramagnetic austenite (fcc) into ferromagnetic α´-martensite ́(bcc) leads to cyclic hardening and to an increase in fatigue life. At 300 °C no α´-martensite formation was observed in the LCF-range and the cyclic deformation behavior depends basically on cyclic hardening processes due to an increase of the dislocation density, followed by cyclic saturation and softening due to changes in the dislocation structure. In the HCF-range an increase in fatigue life was observed due to ε- and α´-martensite formation. Measurements of the mechanical stress-strain-hysteresis as well as temperature and magnetic properties enable a characterization of the cyclic deformation behavior and phase transformation in detail. The changes in the physical data were interpreted via microstructural changes observed by scanning-and transmission-electron-microscopy as well as by x-ray investigations. Additionally electromagnetic acoustic transducers (EMATs) developed from the Fraunhofer Institute of Non-destructive Testing (IZFP) Saarbrücken were used for an in-situ characterization of the fatigue processes.

Early detection of fatigue processes in the cyclically loaded metastable austenitic steel AISI 347 was performed by electromagnetic acoustic transducer measurements in total strain-controlled low-cycle fatigue tests at ambient and elevated temperature. The changes in the physical data were interpreted via microstructural changes observed by scanning- and transmission-electron microscopy, as well as x-ray investigations. The application of physically based measurement data, e.g., time-of-flight from electromagnetically activated ultrasonic signals in austenitic fatigue specimens and total strain, enables measurements of a new hysteresis relationship. In analogy to the common stress–strain hysteresis, this hysteresis gives information about the actual state of fatigue and shows significant changes in shape before specimen failure.

In this investigation specimens of the austenitic steels AISI 304, AISI 321 and AISI 348 were investigated in fatigue tests in the temperature range -60°C ≤ T ≤ 550°C. A detailed microstructure-based characterization of the cyclic deformation behavior of austenitic steels was performed by means of stress-strain hysteresis, electrical resistance and magnetic measurements. Up to ambient temperature the occurring deformation induced martensite formation was measured in-situ with a ferritescope during cyclic loading. The temperature range for dynamic strain aging was reliably identified by means of a temperature increase fatigue test with one single specimen.

As effective methods for characterization of temperature and load dependent fatigue behavior, the present paper describes (i) application of strain increase and temperature increase tests for comprehensive quantitative characterization of the dynamic strain aging influence on fatigue properties of cast iron and (ii) analysis of electromagnetic acoustic transducer signals for early detection of fatigue damage of AISI 347 metastable austenitic stainless steel. In case of cast iron, plastic strain and stress amplitude recorded in temperature increase tests allow identification of the onset and peak of dynamic strain aging at frequencies up to 92 Hz. Isothermal strain increase tests and application of Morrow's relationships between cyclic deformation behavior and slopes of a Wöhler curve in the high cycle / low cycle fatigue‐regime allow determining the total strain Wöhler curve in excellent agreement with conventional constant amplitude experiments. In‐situ characterization of fatigue processes in AISI 347 was performed by measurement of ultrasonic signal time‐of‐flight as well as amplitude decay by electromagnetic acoustic transducers which generate ultrasonic waves directly in electrically conductive materials without the use of a couplant. Especially the time‐of‐flight signal reflects the actual specimen loading as well as cyclic hardening / softening and early fatigue damage with excellent sensitivity.