Understanding the effect of crack tip deformation on fatigue crack growth behavior in 300-series austenitic stainless steel

Abstract

Fatigue crack growth rates (FCGRs) of Type 304/304L stainless steel at stress ratios above 0.7 in elevated temperature (250–320 °C), high purity water are observed to be accelerated by 10× to 100× (relative to comparable tests in air) during testing at low frequencies (0.167–0.000167 Hz). These effects are most prominent for fatigue tests with applied stress intensity factor amplitudes (ΔK) less than ∼10 MPa√m and has been attributed to a hydrogen-based mechanism. This work evaluates that hypothesis, at ambient temperature, through a combination of fatigue testing (using deuterium pre-charged and uncharged specimens) and materials characterization. Over a range of ΔK values (∼6 to 12 MPa√m), the FCGRs of pre-charged specimens are found to be marginally higher (at most 20–30%) relative to comparable tests of uncharged specimens. However, while the FCGRs between pre-charged and uncharged specimens are comparable, the morphology of the fracture surface of the pre-charged specimens is predominately faceted while the morphology of the fracture surface of the uncharged specimen is quasi-cleavage with some poorly defined faceted regions. Planar slip (and few cell structures) is predominate ahead of the crack tips in which the faceted fracture surface morphology is observed. In contrast, quasi-cleavage and poorly defined faceted fracture surfaces are associated with a cellular structure embedded within a planar slip region ahead of crack tips. These differences in deformation suggest the fracture surface morphology may form by either cracking along slip planes in the case of deuterium pre-charged specimens or by cracking along cell walls in the case of uncharged specimens. The fracture mechanism that operates is controlled by differences in the number of cycles required to evolve the deformation structures ahead of the fatigue crack tip, the grain orientation, and local hydrogen concentration.