Effect of machining on near surface microstructure and the observation of martensite at the fatigue crack tip in PWR environment of 304L stainless steel

Characterization of IASCC crack tips extracted from neutron-irradiated flux thimble tube specimens in view of a probabilistic fracture model

“…Bogers formalism [68], aggregated bcc α ′ -martensite islands may then nucleate following the shear interactions of two ǫ-martensite bands within the parent austenitic phase. Prior observations of martensite phases at the crack tips of fatigue cracks in SS304L are in support of this reasoning [13] and fully correspond to the findings displayed in Fig. 7.…”

“…Especially following the exposure to a PWR environment, mobile hydrogen may diffuse into the crack and segregate at leading crack tip grain boundaries. Its presence promotes and increases the formation of hcp ǫ-martensite laths while under the influence of straining [13]. In accordance to…”

“…From the microstructural analysis, the change in cracking mechanism under cyclic loading becomes evident from the analysis of IASCC cracks stemming from specimen TTO-1-066 (280 MPa -cyclic loading). Indeed, cyclic crack propagation has been linked to hydrogen-assisted fatigue cracking [13], especially in relation to the formation of susceptible martensite phases adjacent to crack flanks. Microstructural analyses have shown stress-induced austenite to martensite phase transformations adjacent to fatigue cracks in accordance to the Olsen-Cohen model, see Fig.…”

“…7. Martensite phases are typically promoted under high stressed conditions [63,64,65,66] in association to presence of hydrogen [13,67]. Especially following the exposure to a PWR environment, mobile hydrogen may diffuse into the crack and segregate at leading crack tip grain boundaries.…”

“…Influential factors to consider are the high degrees of material stresses and the exposure time to the NPP high-temperature water environment. Additional elements that dominate the material's lifetime include some degree of non-monotonic loading conditions caused by the variability in thermal transients and loading-unloading cycles [12,13,14]. Despite current best efforts, no single indicative failure mechanism for IASCC has been yet agreed upon.…”