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.

This study presents the results of a detailed investigation of metastability and susceptibility to deformation induced α'-martensite formation of several austenitic steels (AISI 304, AISI 321, AISI 348 and two batches from AISI 347) in the solution-annealed state. Besides conventional characterization of metastability by calculating stacking-fault energy and threshold temperature (designated as M S and M d30 ), the present work introduced a new method for determining susceptibility to α'-martensite formation. The method was based on dynamically applied local plastic deformation and non-destructive micro-magnetic measurement of α'-martensite content. The parameter I ξ was established, which correlated very well with the grade of α'-martensite formation during cyclic loading. The cyclic deformation and phase transformation behavior of cyclically loaded specimens from different metastable austenitic steels were investigated in total-strain and stress controlled fatigue tests with load ratio R = -1 at ambient temperature. The influence of the strain rate on the cyclic deformation and phase transformation behavior was also examined. During the fatigue tests, besides stress-strain hysteresis and temperature measurement, in situ micro-magnetic measurements were performed. Using the compressive measured data, the influence of plastic induced self-heating of the specimen and the strain rate on α'-martensite formation was analyzed.

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To investigate the fatigue behavior of metastable austenite steels in the VHCF-regime, high loading frequencies are essential to realize acceptable testing times. Hence, two high-frequency testing systems were used at the authors’ institute: an ultrasonic testing system with a test frequency of 20 000 Hz and also, a servohydraulic system with a test frequency of 980 Hz. In the present study, two different batches of the metastable austenitic stainless steel AISI 347 were investigated. Fatigue tests on metastable austenitic steel AISI 347 batch A were carried out at an ultrasonic test system at a test frequency of 20 000 Hz, at ambient temperature. Because the test rig acts as a mechanical resonant circuit excited by a piezoelectric transducer the specimen must be designed for oscillation in its vibration Eigenmode at the test frequency to assure maximum displacement at the end and maximum stress in the gauge length center, respectively. For analyzing the deformation behavior during the tests, the change in temperature was measured. Additionally, Feritscope™ measurements at the specimen surface were performed ex-situ after defined load cycles. First results showed a pronounced development of phase transformation from paramagnetic face-centered cubic γ-austenite to ferromagnetic body-centered cubic α‘-martensite. Because formation of α‘-martensite influences the transient behavior and high frequency loadings leads to pronounced self-heating of the material, ultrasonic fatigue tests on metastable austenites represent a challenge in controlling of displacement amplitude and limiting the specimen temperature. First investigations on metastable austenitc steel AISI 347 batch B using a servohydraulic test system at a frequency of 980 Hz and a temperature of T = 300 °C resulted in no fatigue failure beyond N = 107 cycles in the VHCF-regime. However, only specimens with a low content of cyclic deformation-induced α‘-martensite achieved the ultimate number of cycles (Nu = 5·108).

The fatigue assessment of safety relevant components is of importance for ageing management with regard to safety and reliability of nuclear power plants. For reactor internals, austenitic stainless steels are often used due to their excellent mechanical and technological properties as well as their corrosion resistance. During operation the material is subject to loadings in the Low Cycle Fatigue (LCF) regime due to start up and shut down procedures as well as high frequency loadings in the Very High Cycle Fatigue (VHCF) regime induced e.g. by stresses due to fast cyclic thermal fluctuations triggered by fluid dynamic processes. While the LCF behavior of austenitic steels is already well investigated the fatigue behavior in the VHCF regime has not been characterized in detail so far. Accordingly, the fatigue curves in the applicable international design codes have been extended by extrapolation to the range of highest load cycles (Fig. 1). The aim of the cooperative project of the Institute of Materials Science and Engineering (WKK), Materials Testing Institute (MPA) Stuttgart and Framatome GmbH, Germany is to create a comprehensive database up to the highest load cycles N = 2·109 for austenitic stainless steels. For this fatigue tests on metastable austenitic steel AISI 347 / 1.4550 / X6CrNiNb1810 as well as austenitic welds (Fox SAS 2-A) were performed at an ultrasonic testing system at a test frequency of 20 000 Hz to realize acceptable testing times. In addition, an induction generator was implemented in the test system to investigate the influence of operation relevant temperature of 300 °C on the fatigue behavior. The ultrasonic testing system works under displacement control. Therefore, for reliable statements on fatigue life according NUREG/CR-6909 and using of S-N-curve (total-strain amplitude vs. cycle to failure) a fictitious-elastic and elastically-plastic numerical material model was used for calculation of total-strain amplitudes based on experimental data. The results shown, that at ambient temperature (AT) and 300 °C no specimen failure occurred in the VHCF regime for the base material as well as for the welds. Consequently, for these materials a real endurance limit exists. Additionally, in a continuative test a specimen with a pre-autoclaving period in high temperature water (HTW) of 2500 hours was tested in air at a total strain amplitude of 0.1 % in the VHCF-regime up to number of cycle N = 109 using an ultrasonic fatigue testing system. The chemical composition of the HTW for the pre-autoclaving period is comparable to near operation conditions. Afterwards by using of scanning electron microscope no defects or cracks were detected in the oxide layer.