Micromechanics of hydrogen transport and embrittlement in pipeline steels

The technology of large scale hydrogen transmission from central production facilities to refueling stations and stationary power sites is at present undeveloped. Among the problems which confront the implementation of this technology is the deleterious effect of hydrogen on structural material properties, in particular at gas pressure of 1000 psi which is the desirable transmission pressure suggested by economic studies for efficient transport. To understand the mechanisms of hydrogen embrittlement our approach integrates mechanical property testing, TEM observations, and finite element modeling. In this work a hydrogen transport methodology for the calculation of hydrogen accumulation ahead of a crack tip in a pipeline steel is outlined. The approach accounts for stress-driven transient diffusion of hydrogen and trapping at microstructural defects whose density evolves dynamically with deformation. The results are analyzed to correlate the level of load in terms of the applied stress intensity factor with the time after which hydrogen transport takes place under steady state conditions. The transient and steady state hydrogen concentration profiles are used to assess the hydrogen effect on the mechanisms of fracture as they depend on material microstructure.