Control of Hydrogen Environment Embrittlement of Ultra-High Strength Steel for Naval Application

Abstract : The overarching objective of this research, conducted from October of 2002 through March of 2006, was to develop the foundation & understanding necessary to mitigate hydrogen environment embrittlement (HEE) of ultra-high strength steels (UHSS) relevant to the Navy. We seek to develop the fundamental knowledge that clarifies, as a coating/UHSS/environment system, how to deploy ultra-high strength steels that resist hydrogen embrittlement when stressed in marine environments. Specific aims are to understand the electrochemical and chemical details that govern H supply to crack tip damage sites, as well as the detailed processes that govern crack tip H damage in UHSS. By controlling H supply to crack tips through control of electrochemical and chemistry factors, as we as by understanding factors such as trapping that control intrinsic HEE resistance, we seek to identify viable mitigation strategies for resistance to HEE. The class of secondary hardened martensitic steels is emphasized in an approach that centered on two tasks: (a) fracture mechanics based measurement and modeling of HEE coupled with (b) electrochemical studies of H production and uptake in occluded geometries plus H interaction with microstructural trap sites. The results of this work are detailed in this report & include experimental data on the severe transgranular HEE produced in this class of steels, the central importance of H trapping at martensite interfaces and nano-scale carbide strengthening precipitates, the strong effect of electrochemical polarization on H production/uptake & crack growth, and avenues to mitigate such cracking by electrochemical control derived from tailored coatings. This research sets the stage for next generation modeling to control HEE in UHSS>.