Continuum Damage Model for Hydrogen Embrittlement in Ferritic Steels

Hydrogen embrittlement of metals and alloys, particularly steels, has been an important scientific and engineering challenge in the Oil and Gas industry for many years. It impacts the integrity and performance of a wide range of structures and equipment such as downhole tubulars and pipelines in sour service in the Upstream (U/S) and hydro-processing reactors in the Downstream (D/S). In addition, the rapidly growing interest in hydrogen as an energy carrier for fuel cells and mobility or as a clean fuel/heat source for hard to decarbonize industrial processes, draws attention to this key challenge of materials integrity in handling hydrogen. The fundamental understanding of failure mechanism(s) and the capability to model material behavior is important for managing the integrity and for repurposing existing infrastructure for transporting hydrogen as well as for extending the life of structures. To that extent, the present work develops a robust mathematical model to estimate the strength degradation and embrittlement due to hydrogen in steels. The model incorporates hydrogen affected constitutive response of material, within the framework of finite element method. The modified constitutive response is a Gurson plasticity based continuum damage model and incorporates two vital aspects of NVC failure theory. These key aspects are (i) hydrogen enhanced localized dislocation plasticity, and (ii) hydrogen enhanced vacancy stabilization forming nano-voids. The deformation and damage in the material is coupled with trap mediated hydrogen diffusion. Calibration of damage model parameters is performed for X65 commercial linepipe steel. Finally, capability of the damage model is demonstrated with numerical simulation of round bar tensile tests on X65 steel under hydrogen exposure. The numerical simulations are shown to be in excellent agreement with experimental results.