Publications

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Applications requiring the containment and transportation of hydrogen gas at pressures greater than 70 MPa are anticipated in the evolving hydrogen economy infrastructure. Since hydrogen is known to alter the mechanical properties of materials, data are needed to guide the selection of materials for structural components. The objective of this study is to characterize the role of yield strength, microstructural orientation, and small concentrations of ferrite on hydrogen-assisted fracture in two austenitic stainless steels: 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-SMn (22-13-5). The testing methodology involves exposure of tensile specimens to high-pressure hydrogen gas at elevated temperature in order to precharge the specimens with hydrogen, and subsequently testing the specimens in laboratory air to measure strength and ductility. In all cases, the alloys remain ductile despite precharging to hydrogen concentrations of ∼1 at. %, as demonstrated by reduction in area values between 30% and 60% and fracture modes dominated by microvoid processes. Low concentrations of ferrite and moderate increases in yield strength do not exacerbate hydrogen-assisted fracture in 21-6-9 and 22-13-5, respectively. Microstructural orientation has a pronounced effect on ductility in 22-13-5 due to the presence of aligned second-phase particles. Copyright © 2008 by ASME.

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Tubular specimens of the nitrogen-strengthened alloy 21Cr-6Ni-9Mn were instrumented with thermocouples and inertia welded using a wide range of axial forces and kinetic energies. It was determined that a linear relationship exists between upset and kinetic energy for a given axial force. Furthermore, the peak temperatures are inversely related to the applied axial force. Microstructural characterization was performed using optical and electron microscopy techniques. Ferrite was observed locally at the weld interface, and it was determined that the width of the ferrite zone could vary widely depending on the process parameters. Electron backscattered diffraction analysis revealed that the ferrite and austenite at the weld interface exhibit the Kurdjumov-Sachs orientation relationship, and suggests that a very large amount of ferrite is present during the welding process that subsequently transforms to austenite during cooling. The fracture toughness of inertia welds thermally charged in gaseous hydrogen was also measured. It was found that the hydrogen-assisted fracture susceptibility of the inertia welds was greater than that of the base metal, but less than that of 21Cr-6Ni-9Mn gas tungsten arc welds. Copyright © 2006 ASM International®.

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The objective of this study was to quantify the hydrogen-assisted fracture susceptibility of gas-tungsten arc (GTA) welds in the nitrogen-strengthened, austenitic stainless steels 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-5Mn (22-13-5). In addition, mechanisms of hydrogen-assisted fracture in the welds were identified using electron microscopy and finite-element modeling. Elastic-plastic fracture mechanics experiments were conducted on hydrogen-charged GTA welds at 25 C. Results showed that hydrogen dramatically lowered the fracture toughness from 412 kJ/m{sup 2} to 57 kJ/m{sup 2} in 21-6-9 welds and from 91 kJ/m{sup 2} to 26 kJ/m{sup 2} in 22-13-5 welds. Microscopy results suggested that hydrogen served two roles in the fracture of welds: it promoted the nucleation of microcracks along the dendritic structure and accelerated the link-up of microcracks by facilitating localized deformation. A continuum finite-element model was formulated to test the notion that hydrogen could facilitate localized deformation in the ligament between microcracks. On the assumption that hydrogen decreased local flow stress in accordance with the hydrogen-enhanced dislocation mobility argument, the finite-element results showed that deformation was localized in a narrow band between two parallel, overlapping microcracks. In contrast, in the absence of hydrogen, the finite-element results showed that deformation between microcracks was more uniformly distributed.

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