This paper presents a novel TFT (tuning fork test) stress corrosion testing method, which was developed for classifying martensitic high‐strength steels. The novel method was developed by applying finite element calculations to optimize a tuning fork geometry to enable accurate stress adjustment with simple inexpensive equipment. Different steels were exposed to cathodic hydrogen charging conditions with various elastic tensile stress levels that were achieved by displacement control. All steels exhibited hydrogen‐induced stress corrosion cracking after exceeding a material‐specific threshold stress level that decreased linearly with increasing hardness.

Hydrogen embrittlement is a well-known problem with high-strength steels. An important aspect of hydrogen embrittlement research is the effect of the prior austenite grain (PAG) structure on hydrogen-induced fracture. The microstructural anisotropy of PAG structure depends on the steel manufacturing process. In this study, 500 HBW martensitic steels with different PAG structures are investigated with a novel tuning-fork test that utilizes an integrated loadcell system. The loadcell clamping system is used during hydrogen charging allowing tracking of the applied force throughout the tests, which enables detection of separate phases of cracking and time-to-fracture. The elongated PAG morphology produces different results depending on the crack path direction in relation to the rolling direction, whereas the equiaxed PAG morphology does not manifest an orientation dependence. Depending on the PAG shape, also the fracture morphology differs. Time-to-fracture results show that elongated grain morphologies with transgranular quasi-cleavage crack propagation are more beneficial against hydrogen-induced fracture than equiaxed grain structure with intergranular crack propagation. Our results demonstrate that the shape of the PAG structure plays an important role in the crack propagation mechanism and that it is important to consider the possible direction of hydrogen-induced cracks in the final structural applications.

A novel tuning-fork test method was developed to study hydrogen-induced stress corrosion cracking of high-strength steels. Special tuning-fork specimens are designed to enable accurate stress adjustment via constant displacement under cathodic hydrogen charging conditions. In this study, the testing method is further developed, making the potentiostatic hydrogen charging possible through the modifications of the corrosion cell. Different direct-quenched, lowand medium-carbon steel grades, with a hardness range of 300-550 HBW, are investigated with both galvanostatic and potentiostatic hydrogen charging techniques. For each steel grade, the lowest fracture stress and highest nofracture stress are determined. Both hydrogen charging techniques produce similar results, and it is observed that the fracture stress decreases with the increase in hardness. However, the potentiostatic technique produces larger differences between the lowest fracture stress results, thus having a better resolution.

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