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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 model for static recrystallization by Zurob et al. [1] has been fitted to experimental stress relaxation [2] data obtained on a low-alloyed steel using a Gleeble thermomechanical simulator. The model has been implemented as an algorithm that calculates the stress relaxation as a function of time, including physical descriptions of the recovery and recrystallization processes. The activation energy and volume were used as fitting parameters for recovery, and the activation energy of diffusion and nucleation site density were used as the fitting parameters for recrystallization. The four fitting parameters were determined from the experimental data by applying the Nelder-Mead algorithm within Matlab software. It can be concluded from the preliminary results that Zurob’s model can be successfully fitted to the stress relaxation data in order to illustrate the static restoration characteristics and kinetics in carbon steels using these fitting parameters.