Fatigue Crack Initiation of 304L Stainless Steel in Simulated PWR Primary Environment: Relative Effect of Strain Rate

Abstract

The French Regulatory Commission insisted on a survey justifying the assumed mechanical behavior of components exposed to Pressurized Water Reactor (PWR) water under cyclic loading without taking into account its effect. In the US and Japan, the fatigue life correlation factors, so called Fen, are formulated and standardized on the basis of laboratory data to take into account the effect on fatigue life evaluation. However, the current fatigue codification, suffers from a lack of understanding of environmental effects on the fatigue lives of stainless steels in simulated hydrogenated PWR environments. Samples tested in a recent study were analyzed to highlight the strain rate effect (within a range 0.4%/s to 0.004%/s) at the early stage of fatigue life in PWR primary environment for a 304L stainless steel. The deleterious effect of PWR primary environment on fatigue crack initiation was observed with a quantitative microscopic approach. Multi scale observations of oxide morphology and microstructure were carried out from common optical microscopy using recent technologies such as 3D oxide reconstruction, and DualBeam observations.

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

“…5 F en value for stainless steels prescribed in the JSME code depends on the strain rate and temperature. [6][7][8][9] The same F en value is applied regardless of the applied strain range when the strain ranges are more than 0.22%, which corresponds to the fatigue limit. When the strain ranges are 0.22% or less, the environmental effect is not considered.…”

“…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) (hereafter, called JSME code) and the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers . F en value for stainless steels prescribed in the JSME code depends on the strain rate and temperature . The same F en value is applied regardless of the applied strain range when the strain ranges are more than 0.22%, which corresponds to the fatigue limit.…”

Environmental effect of PWR primary water on fatigue life of stainless steel (influence of loading rate on fatigue life reduction)

“…The JSME code gives F en of stainless steel for the water environment of a pressurized water reactor (PWR) as where dε / dt (unit: %/s) and T (unit: °C) denote the strain rate and temperature, respectively. Test results revealed that the degree of fatigue life reduction of stainless steels was dependent on the strain rate and temperature 8–11 . Figure 1 shows F en for T = 598 K (325°C).…”

“…Zhang et al 13 pointed out that the hydrogen enhanced localized plasticity (HELP) mechanism caused more damage particularly at low strain rate. Huin et al 11 suggested that the early initiation as well as propagation of micro‐cracks were relevant parameters to explain how the strain rate affects the stainless steel fatigue life in the PWR water environment. However, as far as present author's knowledge, no theoretical or mechanistic explanation has been given for F en having the upper limit at dε / dt = 0.0004%/s.…”

“…Moreover, it is not fully understood why the fatigue life is reduced in the PWR water environment. Although some results showed that hydrogen 13,14 and oxidation 11 could contribute to reduced fatigue life, many factors seem to be involved 15,16 …”