Effect of Hydrogen on Mechanical Behaviour of Steel

Steel is an essential material that is utilized in virtually every industry, from the manufacturing of automobiles, ships, and aircraft, in construction of buildings, bridges, and even in the manufacture of everyday appliances like washing machines and refrigerators. The strength, ductility and toughness of steel is drastically altered on exposure to hydrogen-rich environment (such as marine atmosphere, and oil/gas pipelines). This undermines the safety and integrity of structural components utilized in the aviation, petroleum, and nuclear sectors. The annual cost of operations in an aggressive environment for India is close to 3% of the GDP. The hydrogen assisted degradation of mechanical properties of steels takes place across several length/time scales, and presents a challenge to research aimed at understanding the underlying and development of mitigation strategies.

There is also a lack of comprehensive understanding on the role of plasticity (which is the quality of being easily shaped and molded), during the mechanical degradation of steels when exposed to a hydrogen-rich environment. The knowledge of this is necessary as it provides insight into the microscopic level failure and serves as an input for engineering next-generation environmentally resistant steels. These issues cannot be answered by solely relying on experimental characterization techniques, due to the inability to visualize hydrogen distribution, and the difficulty in accurately quantifying the effect of hydrogen on the deformation behavior.

Recent advances in computational tools/resources have enabled the development of several multi-scale approaches to assess such multi-scale and multi-physics problems in a consistent manner. In this study conducted by Dr. Pranav Kumar and Prof. Ilaksh Adlakha from the Department of Applied Mechanics, Indian Institute of Technology Madras (IITM), Chennai, India, Mr. Mohit M. Ludhwani and Prof. Anand Kanjarla from the Department of Metallurgical and Materials Engineering, IITM, Chennai, India, Dr. Sambit Das and Prof. Vikram Gavini from the Department of Mechanical Engineering, University of Michigan, Ann Arbor, USA, a new Fe-H EAM based interatomic potential was developed using a computational multi-scale study to quantify the effect of interstitial hydrogen concentration on the dislocation-based plasticity in α-Fe. This is the first time such a study has been carried out. The computational tools used in this study include DFT (Density-functional theory), DFT-FE (DFT with finite elements), molecular dynamics, molecular statics (MS), and crystal plasticity. Interestingly, it was found that the presence of hydrogen increased the yield strength, due to the changes in the non-glide stress contributions towards dislocation glide, which was in agreement with several past findings. Overall, the novel insights of this study could serve as a foundation for future multi-physics efforts to properly understand the hydrogen assisted mechanical degradation of steel.

A multi-scale framework was developed to accurately examine the influence of hydrogen on plasticity in steels. Several properties of the influence of hydrogen on α-Fe were noted.

Dr. Lucas Hale, a Materials Scientist from the National Institute of Standards and Technology, U.S. Department of Commerce, Gaithersburg, Maryland, United States, gave his analysis of the work done by the authors with the following comments: “This article provides a necessary and important step in improving our understanding of mechanical properties in iron materials. While we have long known of the importance of crystalline defects on mechanical properties, we currently have an incomplete understanding of the atomistic mechanisms of how many of those defects behave and interact. This paper successfully builds upon previous work of dislocations in pure iron to investigate how hydrogen affects dislocation slip at the atomic scale. The resulting models capture this atomic level physics to provide a means of better predicting how iron is structurally impacted by a hydrogen-rich environment.”

Article by Akshay Anantharaman
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