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An austenitic stainless steel (SUS316L) was prepared with and without addition of solute nitrogen. The effect of cold-working and nitrogen addition on hydrogen solubility and hydrogen diffusion were investigated. High-pressure hydrogen gas and thermal desorption techniques were used. Increasing dislocation densities were related to a higher hydrogen content and higher nitrogen content related to a lower hydrogen content. Both dislocations and nitrogen had a negligible effect on hydrogen diffusion. The different hydrogen contents in the dislocations and the metal lattice, as well as trapping and diffusion activation energies explained the lack of effect of dislocations on hydrogen solubility.

Martensitic transformation is a pressing issue in austenitic steels in the context of hydrogen embrittlement; strain-induced martensite formation may increase the level of hydrogen embrittlement in metastable austenite. Therefore, the effect of hydrogen on phase stability was investigated. An austenitic stainless steel with several grain sizes was charged with various hydrogen contents, and the mechanical stability of austenite under cold-rolling was studied. Grain size did not affect the amount of martensite formed in the material for any condition. Increasing hydrogen contents decreased the amount of strain-induced martensite at each tested strain. The results were confirmed with magnetization saturation, X-ray diffraction, and electron backscattered diffraction measurements. Hydrogen appears to both change the deformation mechanism and the martensite forming rate. The stabilization is caused by a lower population of martensite nucleation locations.

The fatigue properties of ultra-fine grain austenitic steel (UFG16-10), which has a 1 μm average grain size, were studied as part of the project aimed at the development of high-strength low-cost stainless steels for hydrogen service. The fatigue properties of the UFG16-10 were compared with that of a coarse grain material with the same chemical composition (CG16-10) and two kinds of commercial steels, JIS SUS316 and JIS SUH660. The fatigue strength of the UFG16-10 was 2.8 times higher than that of the CG16-10. The effect of hydrogen on the fatigue limit of the UFG16-10 was not significant. However, the fatigue life of the UFG16-10 was reduced by hydrogen in the short life regime. In the fatigue crack growth test, the UFG16-10 showed a good crack growth resistance that was equivalent to that of the SUH660 and significantly higher than that of the SUS316. However, the crack growth rate was significantly accelerated by hydrogen. The cause of the hydrogen-assisted fatigue crack growth of the UFG16-10 was transformation of the microstructure at the crack tip from austenite to strain-induced martensite. This was also the cause of the reduced fatigue life of the hydrogen-charged UFG16-10.

The metastable austenitic stainless steel is Fe-16Cr-10Ni, of which the grain size can be controlled between 1 and 21 μm. Hydrogen precharging causes a critical drop in ductility during tensile tests for the largest grain size (21 μm). In order to understand how efficient grain refinement is against hydrogen-induced ductility reduction, by varying the heat treatment conditions, it was possible to manufacture six different grain sizes and pinpoint the grain size at which the drop of ductility is critical. This change in ductility was associated with a transition from fully ductile fracture surface to a fracture surface composed of dimples, quasi-cleavages and intergranular fracture.

The present paper deals with hydrogen embrittlement (HE) susceptibility of a weld thermal simulated heat affected zone of X70 structural steel in high pressure hydrogen gas at 20 °C. Fracture mechanics Single Edge Notched Tension tests at various hydrogen pressures (0.1, 0.6, 10 and 40 MPa H 2 ) have been carried out. The HE susceptibility was quantified through the measurement of the fracture toughness K Q and J (the effect of hydrogen pressure was addressed through linear load increase conditions till failure was obtained). The results show that hydrogen causes a strong decrease in the fracture toughness with increasing hydrogen pressure. The critical hydrogen pressure for the onset of HE was observed to fall between 0.1 MPa and 0.6 MPa. These results were supported by Scanning Electron Microscope (SEM) investigations of the fracture surfaces which showed a clear shift in the fracture mode at 0.6 MPa H 2 . Moreover, constant load tests were carried out in order to investigate the influence of hydrogen exposure time. The results imply that fracture always occurs within 8 h and that longer time to failure is related to stronger toughness reduction. This effect is more pronounced for test at 40 MPa than at 0.6 MPa hydrogen pressure levels. 3D Finite Element calculations of hydrogen diffusion have been performed and the results are discussed in relation to the experiments, in order to attempt to identify the hydrogen populations (diffusible or trapped) which act predominantly on the embrittlement of the material.

To ensure the safety of hydrogen systems, materials must be selected according to strength levels and susceptibility to hydrogen. Austenitic steels are notoriously resistant to hydrogen embrittlement, but usually suffer from relatively low strength levels. In this study, the material used is Fe-16Cr-10Ni, a metastable stainless steel. A slab was cold-rolled to 1.5 mm thickness and annealed to produce two different austenite grain sizes (1 μm, ultra-fine grain, and 50 μm, coarse grain). The plate material was charged in hydrogen gas (10 MPa, 270 °C, 72 hours) and the mechanical properties were evaluated by tensile testing in air. Thermal desorption spectrometry was used to confirm that there is no significant difference in hydrogen content between the two materials and that therefore grain boundaries do not play a significant role in trapping hydrogen. Tensile testing showed that the strength was increased by grain refining but that the increase in strength did not enhance the material's susceptibility to hydrogen embrittlement. Ductility, evaluated by elongation and reduction of area, decreased in both ultra-fine-grained material and coarse- grained material. Cold-rolling coupled with saturated magnetization measurements of BCC phase content showed little difference in phase transformation for hydrogen-charged and uncharged material.

Experiments to investigate the effect of hydrogen pressure and test frequency on the fatigue crack growth properties of a Ni–Cr–Mo steel for the storage cylinder of a 70 MPa hydrogen storage station were conducted. Compact tension specimens were cut out from the storage cylinder. The crack growth properties obtained in hydrogen gas were compared with those obtained in air. Higher hydrogen pressures and lower loading frequencies lead to faster crack growth. However, there is an upper limit to the acceleration of the fatigue crack growth rate in hydrogen gas, which can be used for the design of the hydrogen cylinder. The effect of long and large inclusions present in the steel was also verified. The observations carried out on specimen fracture surfaces showed that the low population of inclusions did not influence the fatigue crack growth rate.

SNCM439 steel specimens cut out from an 80 MPa storage cylinder prototype for a 70 MPa hydrogen station were used for inclusion rating analysis, hydrogen desorption analysis and tension–compression fatigue tests. The fatigue tests were conducted for hydrogen gas-exposed and unexposed specimens. Some specimens contained a small artificial hole, while the others were smooth. The testing frequencies were f = 0.2, 2, 20 and 500 Hz, and the stress ratios were R = −1, 0, 0.3, 0.5 and 0.7. In the smooth specimens, Al2O3 inclusion clusters were the fracture origins. There was an approximately 10-fold acceleration of the fatigue crack growth rates for the hydrogen gas-exposed specimens compared to the unexposed specimens. There was however no clear effect of hydrogen on the fatigue limit. The area parameter method was used to estimate the fatigue strength, using the upper limit of the fatigue crack growth acceleration with hydrogen.

Solution nitriding was applied to a metastable austenitic stainless steel plate (Fe–18Cr–8Ni alloy) to stabilize the austenite near the plate surface. After cold rolling, the interior austenite almost entirely transformed to martensite, but the stabilized surface austenite remained unchanged. The interior deformation-induced martensite contributes to the material’s high strength and the surface stable austenite-layer plays an important role in preventing hydrogen permeation. As a result, the resistance to hydrogen embrittlement was significantly improved for the cold-rolled specimen.