Alloy and Composition Dependence of Hydrogen Embrittlement Susceptibility in High Strength Steel Fasteners

Book excerpt: "High strength steel fasteners characterised by tensile strengths above 1100 MPa are often used in critical applications, where a failure can have catastrophic consequences. Preventing Hydrogen Embrittlement (HE) failure is a fundamental concern implicating the entire fastener supply chain. Hydrogen embrittlement results in loss of ductility and, consequently, a loss of strength, caused by atomic hydrogen in combination with tensile stress that can lead to sudden and unexpected brittle fracture after certain time. This research aims to better define the relationship between chemical composition, microstructural characteristics and HE susceptibility of high-strength steel used for manufacturing mechanical fasteners. HE susceptibility is a function of the material condition, which is comprehensively described by the metallurgical and mechanical properties. Hardness/material strength have a first order effect on HE susceptibility which increases significantly above 1200 MPa and is characterised by a ductile-brittle transition. At a given hardness and for a given concentration of hydrogen, the critical strength above which the ductile-brittle transition begins can vary due to second order effects. In this work, testing and observations made on nine alloys, each quenched and tempered to four hardness levels, illustrate second order differences in susceptibility that are dependent on chemistry and heat treatment, and ultimately on microstructure." --