Hydrogen Assisted Cracking of Ultra-High Strength Steels

Modern martensitic ultra-high strength steels (UHSS) are used in aerospace structural and other high performance applications due to their exceptionally high combination of tensile strength and fracture toughness. High strength martensitic microstructures are susceptible to severe stress corrosion cracking (SCC) via a hydrogen environment assisted cracking (HEAC) mechanism when exposed to service environments that produce atomic hydrogen near ambient temperature. Hydrogen-driven degradation of fracture and fatigue properties in UHSS alloys experiencing SCC is typically severe, limiting operating stress levels and component life. Mechanism-based micromechanical models of HEAC properties allow the accurate prediction of component life. However, the predictive accuracy of such models is severely limited by a lack of quantitative crack growth kinetics in modern UHSS. Further, the microstructural dependences of key variables in these micromechanical models have not been sufficiently investigated and tied to physical quantities. The objective of this research is to: (a) quantitatively measure high-resolution subcritical crack growth kinetics for modern UHSS, and (b) further the fundamental understanding of the electrochemical and microstructural basis for key variables in micro-mechanical models of SCC-HEAC properties.
The stress corrosion cracking resistance of a systematically selected, multi-generational, range of UHSS and martensitic stainless UHSS (UHSSS) is characterized via slow-rising stress intensity (K) loading in full immersion 0.6 M NaCl solution for a wide range of applied-cathodic potentials (Eapp). Quantitative crack growth kinetics and HEAC fracture morphology are correlated with a detailed high resolution EBSD analysis of the complex sub-micron martensitic microstructure for each steel, including feature size/morphology and interface crystallographic characteristics. Results are interpreted in the context of quantitative micromechanical models of the threshold stress intensity for HEAC growth (KTH) and diffusion-limited Stage II crack growth rate (da/dtII). The results establish that all alloys are susceptible to severe HEAC, characterized by low KTH (~10 MPa√m) when polarized at substantial cathodic Eapp in aqueous chloride immersion. However, all modern steels are highly resistant to HEAC when stressed within a limited range of Eapp, mildly cathodic to the open circuit condition, which confers elevated KTH and reduced da/dtII. Decohesion based models of HEAC provide consistent and accurate predictions of the observed Eapp dependence of KTH and da/dtII in legacy 300M, AerMet 100, and Ferrium M54. Modeling efforts establish the dominating-beneficial effect of increased alloy purity and precipitation of nano scale carbides in reducing HEAC susceptibility of modern UHSS. The HEAC fracture path in modern UHSS is dominated by an identical transgranular morphology established as cracking along {110}α’ martensite block interface planes. HEAC of UHSS(S) is consistent with a modern and microstructurally sensitive, interactive hydrogen enhanced localized plasticity (HELP) – hydrogen enhanced decohesion (HEDE) mechanism.
This dissertation provides: (a) a comprehensive database of HEAC kinetics for modern UHSS(S) with sub-micrometer microstructure and interface characterization, (b) fundamental understanding gained from semi-quantitative mechanism-based crack tip damage modeling, and (c) guidance pertaining to the important roles of alloy purity and strengthening precipitates in metallurgical design of next generation UHSS.