The technical barriers identified in this division include the limited understanding of hydrogen-surface interactions, hydrogen uptake, and material degradation mechanisms; the need to develop next-generation materials having improved resistance to hydrogen embrittlement at higher strength levels; and the need to improve fatigue and fracture property measurements in hydrogen gas.
● RESEARCH HIGHLIGHTS AND ACCOMPLISHMENTS
A combined experimental-modeling effort amplified understanding of the effects of oxygen impurities on hydrogen-gas accelerated fatigue crack growth in low-strength steels, and hydrogen uptake was numerically modeled at the crack tip for a low-strength steel in hydrogen gas. An analytical model based on oxygen diffusion in the crack channel was developed that predicts the effects of variables such as oxygen concentration and load ratio on hydrogen-accelerated fatigue crack growth. In addition, density functional theory (DFT) modeling provided new insights on the mechanisms of competitive co-adsorption for oxygen and hydrogen on iron surfaces. These activities involved international collaboration among multiple institutions (Sandia National Laboratories, UIUC, University of Göttingen) and collaboration across I2CNER division boundaries (Hydrogen Structural Materials-Hydrogen Production).
Experiments of cyclic contact between steel surfaces revealed that hydrogen uptake was enhanced by cyclic normal contact but suppressed by cyclic contact with sliding. Surface analysis suggested that the permeation of hydrogen was affected by the formation and removal of oxide films on the steels. In another steel surface effort, a multilayer oxide film developed by an original method was applied to austenitic stainless steel alloy 304 in an interdisciplinary research program between mechanical engineering and surface science that elucidated the mechanism of hydrogen inhibition of the developed film.
A new inclusion rating method by the positive use of hydrogen embrittlement phenomenon was proposed. This new method is the most reliable and efficient method among the existing inclusion rating methods in the world.
As a result of joint studies conducted with manufacturers of materials and hydrogen-supply components, a high-strength austenitic stainless steel was identified that is resistant to hydrogen embrittlement. The steel has minute amounts of N, Nb and Mo added to the chemical composition of Type 304, and the novel stainless steel has both high strength and excellent hydrogen embrittlement resistance
● FUTURE DIRECTIONS
Among our future plans are to simulate hydrogen adsorption with hydrogen transport and elastoplastic deformation to develop a fracture criterion that is predicated on the underlying fracture mechanism(s) acting at the microscale. In this framework, the extent by which surface oxidation affects the uptake process and how the competition between hydrogen and oxygen for surface adsorption sites controls the amounts of hydrogen that reaches the fracture initiation sites will be explored. We expect to continue studies on hydrogen-affected friction and wear, and explore the role of nonmetallic inclusions on hydrogen embrittlement of high strength steels. In particular, we hope to clarify the roles of trace elements in promoting embrittlement resistance in a high-strength stainless steel. Finally, we will quantify effects of hydrogen on tensile deformation and fracture behavior, as well as deformation-induced martensite transformation, in stainless steels having grain sizes from 1 to 60 μm.