Influence of microstructure on hydrogen embrittlement in Ni-Cr-Mo high strength steel and on hydrogen-related effects in API 5L X65Q steel

The energy transition increasingly relies on hydrogen as a low-carbon energy carrier, yet safe and economical deployment will likely require partial use of existing natural-gas infrastructures. This thesis investigates how microstructural features control hydrogen uptake, transport and hydrogen-assisted damage in engineering steels used in turbomachinery and pipeline applications. Two complementary material systems were studied: a Ni–Cr–Mo high-strength martensitic steel and API 5L X65Q pipeline steel. For the high-strength alloy, heat treatments were optimized to produce distinct microstructures, and a tailored testing protocol combining physical characterization (SEM, EBSD, TEM) and mechanical tests in hydrogen environments was used to correlate microstructural parameters with susceptibility to hydrogen embrittlement. For the pipeline steel, comparative short- and long-term electrochemical hydrogen-permeation experiments were performed to quantify the effects of temperature, charging conditions and microstructure on hydrogen uptake and diffusion. The results provide insight into microstructure–hydrogen interactions for safer integration of hydrogen into existing infrastructures.