Abstract
Hydrogen embrittlement (HE) results in loss of ductility and, consequently, a loss of strength. Preventing HE failure is a fundamental concern implicating the entire supply chain from design, to engineering, and to manufacturing. High strength steel, both martensitic and bainitic, used to make critical structural components such as high-tensile fasteners and aircraft landing gear where a failure can have catastrophic consequences, is the area of greatest concern. This research emphasises the concept of material susceptibility to hydrogen embrittlement. Each investigation begins by measuring the HE susceptibility of a material, which is characterised by a rapid sigmoidal ductile-brittle transition. Susceptibility is a function of the material condition, which is comprehensively described by its metallurgical and mechanical properties. Increasing material strength and hardness have a first order effect on increasing susceptibility. High strength steel above 1200 MPa tensile strength becomes very susceptible. However, the specific critical strength above which the ductile-brittle transition occurs varies by alloy and heat treatment. This paper focuses on studying these second order effects on HE susceptibility. More precisely, the objective is to better define the relationship between chemical composition, microstructural characteristics, and susceptibility by employing innovative characterisation techniques and advanced data analysis methods. We identify second order effects on HE susceptibility that are dependent on chemistry, heat treatment, and thus fundamentally on microstructure characteristics that control hydrogen trapping and transport. Finally, we introduce a mathematical model being developed to simulate the experimental conditions and predict susceptibility of any given high strength steel.