Investigation of the Very High Cycle Fatigue (VHCF) Behavior of Austenitic Stainless Steels and Their Welds for Reactor Internals at Ambient Temperature and 300°C

Abstract

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.

Fatigue behavior and lifetime assessment of an austenitic stainless steel in the VHCF regime at ambient and elevated temperatures

“…Cyclic hardening effects are reproduced with phenomenological approaches and adapted by using mathematical methods and best‐fit algorithms. Therefore, an AFC material model with three backstresses (Chaboche parameters C 1–3 , γ 1–3 ) was applied (see Table 4), and for more details, see literature 23,27,28 . Plasticity initiates when stress passes over σ y0 (yield stress at zero plastic strain).…”

“…For this, the converter transforms an electrical signal from the ultrasonic generator into a high‐frequency mechanical oscillation. Using a booster, which amplifies the oscillation depending on its geometry, the mechanical oscillation is transferred to the specimen 22,23 …”

“…Using a booster, which amplifies the oscillation depending on its geometry, the mechanical oscillation is transferred to the specimen. 22,23 All parts of the system were designed to assure that under resonance conditions, a standing wave develops along the load axis with a maximum displacement amplitude at the specimen ends and maximum stress-strain amplitude in the center of the gauge length of the specimen (see Figure 4). Due to the strong self-heating of the investigated material, the specimens were loaded in pulse-pause mode, which limits the maximum specimen temperature during cyclic loading.…”

Detection of Microstructural Changes in Metastable AISI 347, HSS Z-M4 and Tool Steel Ferrotitanit WFN by Mechanical Loss Coefficient at Ultrasonic Frequencies

“…The fatigue test started with 40 ms pulse and 3000 ms pause duration, which corresponds to an effective test frequency of f eff,1 ≈260 Hz. Since the metastable austenitic steel AISI 347 shows pronounced cyclic hardening caused by deformation induced α′-martensite formation, pulse-pause ratios of up to 60 ms/1800 ms become possible after sufficiently high cycle numbers [ 20 ]. Thus, the effective frequency can be increased to f eff,2 ≈ 390 Hz and eventually to f eff,3 = f eff,4 ≈ 650 Hz.…”

“…Thus, the effective frequency can be increased to f eff,2 ≈ 390 Hz and eventually to f eff,3 = f eff,4 ≈ 650 Hz. To maintain a constant mode and displacement amplitude of the vibration under these circumstances, the generator voltage and the proportional and integral (PI) control parameters have to be adapted during the test [ 20 , 21 ]. After the measurement at N = 10 8 cycles, the PI parameters were adjusted without changing the effective frequency (f eff,3 = f eff,4 ≈ 650 Hz).…”