Influence of hydrogen on the Phase Transformation behaviour of Low Transformation Temperature (LTT) alloys investigated using ED-XRD

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Hydrogen uptake in steels can take place during manufacturing, processing and during service. This can cause a dramatic decrease of the steel's mechanical properties. Especially newly developed and high strength steels are prone to hydrogen-assisted cracking. Several models describe the mechanisms behind hydrogen-assisted cracking phenomenologically. Yet, these models are inconsistent and due to the lack of suitable measurement techniques its experimental validation is often poor. Furthermore, materials consisting of more than one phase are not considered within the established models. The planned research project focuses on two two-phase high strength steels of industrial relevance (Low Transformation Temperature Steel and Supermartensitic High Grade Steel). With help of time- and space-resolved energy-dispersive X-ray diffraction experiments, the influence of hydrogen on the phase transformation from austenite to martensite under load and the hydrogen diffusion in the two phase microstructure will be investigated in tensile tests. The varied parameters are the hydrogen concentration and the strain rate. The data analysis allows identifying the correlation between the applied load and phase-specific properties like grain size distribution, crystal structure and micro stresses. These findings allow identifying the hydrogen diffusion paths under mechanical load in the two-phase microstructure and the role of hydrogen during strain-induced phase transformation and crack initiation. The results will be compared with the existing models from literature and the models will be extended regarding the applicability for multiphase microstructures.