Failure Analysis to Clarify Unambiguous Hydrogen Fracturing in Steels

Hydrogen can get into metallic components and damage them in very different ways. Liquid-phase metallurgical processes such as casting and welding can lead to the formation of “fish-eyes” and “flakes”. Apart from metallurgical sources, semi-finished and finished components can also absorb hydrogen from galvanic processes or as a result of corrosion. In these cases hydrogen diffuses into regions of higher stress and strain causing cracking, often unexpectedly and after significant periods of time, which is referred to as “delayed fracture” and “cathodic stress corrosion cracking”. In failure analysis the clarification of hydrogen-induced component damages is based on fractographic examinations in combination with H-analysis. However, a systematic approach of consecutive steps is required in order to clarify hydrogen-induced fractures unambiguously. In failure analysis a delayed fracture is a strong evidence for a damage caused by hydrogen. If a sample taken from the as-received component breaks delayed during a clamping test, this can only be caused by hydrogen which has been present in the component. If too much of the hydrogen has escaped in the space of time from failure to investigation, a clamping test with simultaneous hydrogen uptake can be performed. The fractographic comparison of a sample broken in a clamping test and the primary fracture of the component, as well as the final fracture is necessary. As these three fractures arise from the identical material, the comparison should lead to an unambiguous result. Finally there is the possibility to selectively detect the diffusible (damaging) hydrogen using the HCA method.