Unified interpretation of hydrogen-diffusion and various fracture properties in high-pressure hydrogen gas based on trap-site occupancy of hydrogen by dislocation-austenitic stainless steels and low-alloy steels
The fracture toughness and toughening mechanism of six aluminum alloys, 5083-O, 6061-T6, 6066-T6, 7N01-T5, 7N01-T6 and 7075-T6, were examined in air and 115 MPa hydrogen gas. In both the environments, tentative fracture toughness, KQ, was about twice higher in 7N01-T5 and 7N01-T6 than in 5083-O, 6061-T6, 6066-T6 and 7075-T6. Stretch zone, dimples and inclusions at the dimple bottom were observed on the fracture surface by means of scanning electron microscopy. All the critical stretch zone width, SZWC, had a good correlation with tentative J integral, JQ, regardless of environments and materials. The dimple diameter was nearly equivalent in 6061-T6, 6066-T6, 7N01-T5 and 7N01-T6, although KQ was about twice higher in 7N01-T5 and 7N01-T6 than in the others. Among these four aluminum alloys, the existence probability of large inclusions (diameter ≥ 6 μm) at the dimple bottom was lower in 7N01-T5 and 7N01-T6, which had less impurity elements of Fe and Si. These results inferred that higher values of KQ were attributed to the delay in the initiation and growth of microvoids in 7N01-T5 and 7N01-T6. Taking into account the tentative fracture toughness in combination with slow strain rate tensile (SSRT), fatigue life and fatigue crack growth properties, a design by analysis of thick-walled cylinder was performed. The result suggested that 7N01-T5 and 7N01-T6 having higher KQ enabled the design of larger-sized cylinders with thicker wall.
Four types of strength tests, slow strain rate tensile (SSRT), fatigue life, fatigue crack growth (FCG) and fracture toughness tests, were performed on six types of aluminum alloys, 5083-O, 6061-T6, 6066-T6, 7N01-T5, 7N01-T6 and 7075-T6, in air and 115 MPa hydrogen gas at room temperature. All the strength properties were not deteriorated in every alloy in 115 MPa hydrogen gas. In all the alloys, FCG rates were lower in 115 MPa hydrogen gas than in air. This was considered to be due to a lack of water-or oxygen-adsorbed film at crack tip in hydrogen gas. Relative reduction of area (RRA) of 5083-O, 6061-T6 and 6066-T6, and fracture toughness of all the alloys were higher in 115 MPa hydrogen gas than in air. These improvements were attributed to a hydrostatic pressure produced in 115 MPa hydrogen gas. RRA of 7N01-T5, 7N01-T6 and 7075-T6, and fatigue life of 6061-T6 in 115 MPa hydrogen gas were almost the same as those in air. These results suggest that aluminum alloy components used in high-pressure hydrogen gas can be designed based on the strength properties in air.
This paper deals with the material and fracture surface examination of T-type joint for piping that caused hydrogen leakage in a high pressure hydrogen facility (Service pressure: 99 MPa). T-type joint is made of precipitation hardening martensitic 17-4PH stainless steel. The design fracture pressure of T-type joint is as high as 1050 MPa. Nevertheless, Ttype joint caused hydrogen leakage in 150 MPa hydrogen gas whose pressure was 14 % of the design fracture pressure. Chemical composition analysis, microstructure observation and Vickers hardness measurement proved that 17-4PH of T-type joint was sound. Fracture surface observation showed that semi-elliptical crack surface was covered with quasicleavage and that peel-like defects were formed on taper surface. It was presumed from these points that hydrogenleakage of T-type joint was caused by hydrogen-induced crack growth from peel-like defect in 150 MPa hydrogen gas.
Three types of strength tests, slow strain rate tensile (SSRT), fatigue life, and fatigue crack growth (FCG) tests, were performed using six types of aluminum alloys, 5083-O, 6061-T6, 6066-T6, 7N01-T5, 7N01-T6, and 7075-T6, in air and 115 MPa hydrogen gas at room temperature. All the strength properties of every material were not deteriorated in 115 MPa hydrogen gas. In all the materials, FCG rates were lower in 115 MPa hydrogen gas than in air. This was considered to be due to a lack of water- or oxygen-adsorbed film at crack tip in hydrogen gas. In 5083-O, 6061-T6 and 6066-T6, relative reduction in area (RRA) were remarkably higher in 115 MPa hydrogen gas than in air. These differences were attributed to a hydrostatic pressure produced in 115 MPa hydrogen gas. In contrast, in 7N01-T5, 7N01-T6 and 7075-T6, the values of RRA in 115 MPa hydrogen gas were nearly the same as those in air. Observation of fractured specimens inferred that the degree of such a hydrogen-induced improvement was determined by the fracture mode (e.g. cup-and-cone or shear fracture), which is dominated by the microstructure morphology.
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