Hydrogen-induced Fractures in Large Fasteners

Abstract Wind power plants and heavy steel construction require the use of large-scale bolts of property class 10.9. Due to their high strength, however, these fasteners are subject to an increased susceptibility to hydrogen-induced stress corrosion cracking (SCC). Under tensile stress, this degradation mechanism leads to brittle, time-delayed failure—without any prior visible warning. This poses a severe risk to structural integrity and, in the event of failure, incurs significant costs due to unplanned downtime and extensive repair efforts. This contribution presents selected failure cases involving large-scale bolts with dimensions ranging from M42 to M64, which fractured within a few weeks or months after installation as a result of hydrogen-induced stress corrosion cracking. The root causes of failure can be categorized into susceptible material conditions, coating-related factors, and installation-related factors. In manufacturing, impermissible surface carburization—which leads to hardness values significantly exceeding the permitted limit—as well as defects in the hot-dip galvanizing process pose a substantial risk. An often-underestimated influencing factor is the installation practice. Weather-related impacts on the thread surfaces drastically alter the coefficients of friction. In torque-controlled tightening procedures, this can result in uncontrolled preload forces that exceed the design criteria. Selected failure cases are presented based on fractographic, microanalytical, and microstructural investigations. Furthermore, the presentation highlights the degradation process underlying hydrogen-induced stress corrosion cracking, the normative framework, and the correlation between material susceptibility and hydrogen content.