Microstructure and strength affect hydrogen embrittlement (HE) susceptibility of martensitic steels. In order to understand their respective roles, two martensitic steels with the same chemical composition, but different strength (and or hardness) levels were selected. Incremental step load (ISL) technique was used to evaluate the environmental hydrogen embrittlement susceptibilities (EHE) of the materials by in-situ charging of hydrogen at a cathodic potential of −1.2VSCE. Microstructural characterization was performed using TEM. Stress and hydrogen concentration distributions at the time of failure were estimated from a stress coupled hydrogen diffusion finite element analysis (FEA). It was primarily observed that microstructure controlling strength has a more significant effect on HE failure, as compared to microstructure affecting hydrogen diffusion in case of EHE. This observation was further corroborated with fractographic analyses and qualitative discussion based on linear elastic fracture mechanics (LEFM).
Quench and tempered (Q & T) martensitic steels are widely used to manufacture fasteners. These frames, bridges and other structural components. However, premature failures of these fastener components due to hydrogen embrittlement (HE) can lead to total loss of structural integrity and catastrophic failures. For instance, the HE cracking of high strength anchor rods and structural bolts in the San Francisco–Oakland Bay Bridge is a notable incident.1 However, HE in martensitic steels is a complex phenomenon and has been a topic of research for decades.2,3 The identification of the factors responsible for HE, involves fundamental considerations implicating the entire fastener supply chain, starting from the steel mill, the fastener manufacturer, the coater, all the way to the end user. The selection of heat treatment process could be considered as a decisive factor towards the prevention of HE failure of threaded fasteners. In general, fastener specifications place broad restrictions on material selection, without any mandate on the selection of a particular grade of steel. As a result, it is necessary to develop a better understanding on the susceptibility of these materials to HE. Based on the sources of hydrogen ingress into the steel structure, there are two broad categories of HE: a) internal hydrogen embrittlement (IHE) which is a delayed failure, caused by residual hydrogen from steelmaking, surface cleaning, electroplating in particular.4,5 And, b) environmental hydrogen embrittlement (EHE), which occurs due to the introduction of hydrogen by sacrificial corrosion during service such as subsea applications. It is also generally accepted that strength (and or hardness) have a first-order effect on HE susceptibility in high-strength martensitic steels, with an increase in hardness and/or yield strength leading to a decrease in resistance to HE failures.6–8 However, the underlying mechanism(s) that make strength a significant factor in influencing HE susceptibility needs further understanding.
