Fatigue SIMS a b s t r a c t Secondary Ion Mass Spectrometry (SIMS) analyses were carried out on type 304 austenitic stainless steel. On annealed specimen exposed to hydrogen (10 MPa, 358 K), Element Depth Profiles SIMS mode was able to describe quantitatively the hydrogen profile content computed by the Fick's law. Based on SIMS analyses on the wake of a fatigue crack (propagation in hydrogen gas at 0.6 MPa and RT), it was possible to compute an apparent diffusivity and solubility in the crack tip region. The apparent solubility and diffusivity in the deformed regions were two times and five orders of magnitude higher than the ones on annealed material, respectively. High hydrogen content was found around the crack tip, where the plastic deformation was well developed (pronounced slip activity). The high apparent diffusivity is presumed to result from enhanced hydrogen transport induced by cyclic plastic activity at the crack tip.
A B S T R A C TThe effects of hydrogen charge on cyclic stress-strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS-S45C) were studied. The total strain range of the stress-strain hysteresis loop in the hydrogen-charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen-charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen-charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen-slip band interaction may play an important role in the hydrogen-influenced metal fatigue.Keywords fatigue crack; hydrogen; hysteresis loop; localization; slip bands.
N O M E N C L A T U R E2a = crack length C H = hydrogen content f = test frequency N = number of cycles N f = number of cycles to failure R = stress ratio t = time after hydrogen charge ε t = total strain range ε = strain σ = stress σ a = stress amplitude
I N T R O D U C T I O NIt is known that fatigue failure can occur in the ultra long life regime of N > 10 7 in high strength steels and that the fracture origin is at a non-metallic inclusion contained in the material. Murakami et al. [1][2][3][4] pointed out that a typical morphology, named the optically dark area (ODA), exists around the non-metallic inclusion at the fracture origin and they have shown evidence that the formation of the Correspondence: H. Uyama.ODA is related to the hydrogen trapped by non-metallic inclusions. However, the exact mechanism of the formation of the ODA and the role of hydrogen has not been made clear.Recently, the demonstration and commercialization of fuel cell (FC) systems has led to the realization of their potential for solving the global warming problem. Many metallic components used in FC systems are directly exposed to hydrogen environments. The so-called 'hydrogen embrittlement' has been used to express the degradation of metals due to hydrogen. However, the
This paper has two objectives. The first is to propose a new inclusion rating method based on statistics of extreme. The new inclusion rating method is applied to the evaluation of inclusions contained in two super clean bearing steels with a total oxygen level of 5ppm and 8ppm.
The second objective is to investigate the critical lower bound of inclusion size which influences the fatigue limit. For this purpose, a special steel with extremely low inclusion content, the electron beam remelted super clean bearing steel(EB-CHR), was prepared. It is shown that by decreasing the size of inclusions, the detrimental effect of inhomogeneities larger in size than inclusions, becomes more important. The small inhomogeneity is thought to be a local imperfectly heat- treated structure, bainite.
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