Use case: Hydrogen-induced fractures in bolts and bolted joints

In bolted joints using high-strength bolts (typically strength class 6.8 or higher), seemingly unexplained, spontaneous fractures can occur, usually with a time delay — that is, not immediately after assembly, but during service. In such cases, hydrogen embrittlement is suspected as the cause of crack initiation, which ultimately leads to fracture.

Hydrogen-induced fractures must be distinguished from fatigue fractures, in which the fracture is caused by operational stress with alternating loads, as well as from impact fractures, which result from overloading. Hydrogen embrittlement is generally caused by atomic (not molecular) hydrogen dissolved in the iron lattice, where it can diffuse easily even at room temperature and cause embrittlement. Based on the fracture surface morphology, hydrogen embrittlement can be easily distinguished from other fracture mechanisms. Hydrogen-induced fractures often follow an intergranular path. Pores and microplastic deformation lines (“crow’s feet”) are frequently visible on the grain boundaries.

Various factors can serve as sources of atomic hydrogen and there are several ways to determine whether bolts are susceptible to hydrogen embrittlement. Various measures can be taken to prevent hydrogen embrittlement:

• Steel production (hydrogen is highly soluble in molten iron),
• Electrolytic processes in which hydrogen is released as a byproduct (e.g., galvanizing), or upstream pickling processes
• Carburizing using gaseous carburizing media containing H2 or CH4,
• Sulfuric gas (media containing H2S),
• Corrosion (development of H at the cathode).

• Measurement of hydrogen content using carrier gas hot extraction
• Stress testing
• Slow Strain Rate Tests (SSRT, slow tensile test with strain rates < 10⁻⁵ s⁻¹)

• Avoidance of hydrogen sources. This is not always possible, e.g., in electrogalvanizing. Appropriate process measures during pickling or coating can reduce hydrogen ingress.
• Driven-out of dissolved hydrogen through suitable hot annealing (starting at approx. 200°C, duration depending on screw diameter and any coating layer).

Our research and development services for identifying and preventing hydrogen embrittlement in fasteners and bolted joints.

• Analysis of fracture surfaces and identification of the cause of damage
• Measurement of hydrogen diffusion and hydrogen content
• Tensile tests or other mechanical tests in a hydrogen atmosphere
• Development of recommendations for damage prevention

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