Influence of PWR Primary Water on LCF Behavior of Type 304L Austenitic Stainless Steel at 300°C: Comparison With Results Obtained in Vacuum or in Air

Abstract

Nowadays, it is well known that the low cycle fatigue (LCF) life of austenitic stainless steels can be affected in specific conditions of temperature, strain rate, strain amplitude or dissolved oxygen concentration by the effect of Pressurized Water Reactor (PWR) primary coolant environment. Nevertheless, questions remain about the best methodology that must be used to consider environmental effects for nuclear power plant licensing and for operating lifetime extensions. These environmental effects are most commonly evaluated from a mean fatigue curve based on tests conducted in air at room temperature. However, it is well established that air is not a neutral environment for metallic alloys and its effect can be highly dependent on the temperature level. Thus, in order to evaluate the intrinsic fatigue resistance of a 304L austenitic stainless steel at 300°C and the importance of complex fatigue – environment interactions in air or in PWR water, LCF tests were performed in both environments and specifically designed ones were conducted in secondary vacuum. Tests were performed on 304L cylindrical specimens at 20 or 300°C in vacuum or in air and only at 300°C in PWR water, under total axial strain control using a triangular waveform at strain amplitudes of ±0.3 or ±0.6% and strain rates of 4 × 10−3, 1 × 10−4 or 1 × 10−5 s−1. It was found that compared with vacuum, air is responsible for a strong decrease in fatigue lifetime in this steel, especially at 300°C and low strain amplitude. The PWR water coolant environment is still more active than air and leads mainly to increased damage kinetics, with slight effects on initiation sites or propagation modes. More precisely, the decreased fatigue life in PWR water is essentially attributed to an enhancement of both crack initiation and “short crack” micropropagation stages. Furthermore, a detrimental influence of low strain rates on the fatigue lifetime at 300°C was observed in PWR water environment or in air, but also in vacuum without environmental effects, and was in the last case exclusively attributed to the occurrence of the dynamic strain aging (DSA) phenomenon. So, the use of data obtained in a neutral environment as a reference allows the evaluation of the intrinsic effect of each environmental or loading condition. Moreover, in an active environment such as air or PWR primary water, damage evolutions as well as fatigue lives cannot be predicted by a simple multiplication of each parameter effect taken separately because they are the result of numerous interactions. The last conclusion is supported by complementary results showing that the PWR water environment effect as well as the ground surface finish effect can be attenuated when LCF tests are performed with a more representative loading signal shape.

Influence of Strain Rate and Waveshape on Environmentally-Assisted Cracking during Low-Cycle Fatigue of a 304L Austenitic Stainless Steel in a PWR Water Environment

“…Therefore, the longer fatigue life obtained with the SIS A signal seems consistent when compared with the results in air presented in the previous section and indicating a less extend of damage with the SIS A signal. These trends are furthermore in agreement with similar data previously obtained on a different grade of 304L [12,17], even though the fatigue resistance of these two alloys are different.…”

“…However, it must be kept in mind that the value of the applied strain In order to examine in more details, the influence of this loading signal and in particular of the slowest part (segment 4), different combinations of these elementary segments have been considered in previous studies [15][16][17]. The results indicated that the signals where the longest segment 4 is applied in tension, that means with a positive deformation, are more damaging than the signals where this slow strain rate segment is applied in the compressive part of the cycle, that means with a negative deformation [12]. In the present study, only the so-called SIS B signal, presented in Figure 3b, will be considered in addition to the SIS A signal.…”

“…The results are given in Table 1. Additional information about the cyclic stress-strain behavior and low-cycle fatigue resistance as a function of temperature and environment can be found in [11][12][13][14]. Fatigue specimens are machined with the loading axis parallel to the rolling direction of the plate.…”

“…Np ( The results obtained in vacuum have evidenced an intrinsic effect of strain rate which induces higher crack growth rates and a reduction in fatigue lives when the strain rate decreases [12]. In a PWR environment, the growth rates and consequently the fatigue lives are additionally conditioned by the action of environmentally-assisted fatigue crack growth mechanisms.…”

Influence of strain range on fatigue life reduction of stainless steel in PWR primary water

“…Fatigue life of stainless steels in high-temperature coolant water of nuclear power plants is shorter than that in air. [1][2][3] To take the reduction in fatigue life into account for component designs, the fatigue life correction factor F en , which is the parameter representing the ratio of fatigue life in air to that in the environment, has been proposed. 1 Through extensive test campaigns, the value of F en has been determined empirically and summarized in assessment guidelines, such as the fatigue assessment guideline issued by the Japan Society of Mechanical Engineers (JSME) 4 (hereafter, called JSME code) and the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers.…”