Influence of the Local Hydrogen Concentration in Multi-Phase Steels on their Tendency to Hydrogen Induced Cracks
Final Report Abstract
Hydrogen is considered a green energy source for reducing CO2 emissions. Particularly highstrength steels with a tensile strength above 800 MPa are, however, at risk of hydrogen-induced cracking or hydrogen embrittlement. The susceptibility of steels to hydrogen embrittlement depends not only on the material strength but also on the intrinsic hydrogen content. In this regard, it is most relevant that hydrogen concentration gradients can emerge due to locally different stress states. Material regions subjected to higher elastic strains, for example, caused by notches, shoulders, or surface defects, exhibit increased hydrogen solubility (Gorsky effect), which might lead to critical local hydrogen concentrations where subsequent cracking can occur. Additionally, specific microstructural phases such as martensite, bainite, pearlite, or ferrite influence local hydrogen content due to different strengths, solubilities, and diffusion coefficients. All of this means that the risk of hydrogen-induced cracking cannot be determined solely by hydrogen analyses on large volume samples. Rather, the local hydrogen content must be determined considering the applied stress and the respective microstructural phases. In the present project, samples of quenched and tempered steel C60E (material no. 1.1221) were subjected to different heat treatment routes, resulting in a total of seven material modifications with varying strengths and microstructures. The samples were charged with hydrogen and elastically stressed. By introducing a defined notch, stress gradients within the samples were established, resulting in a varying hydrogen concentration profile within the stressed samples. The local hydrogen concentrations were analyzed by means of electrochemical microcapillary measurements, with the measurement spot having a diameter of 2 to 4 µm. This allowed measurements to be performed within individual grains to verify both stress-dependent hydrogen concentration and, in theory, the microstructural dependence. The described investigations demonstrated the dependence of local hydrogen concentration on applied stress. In the quenched and tempered martensitic state, a local "concentration factor" was determined, which can be up to five times higher than the baseline hydrogen content. These results are of high technical relevance as they allow defining material safety factors for hydrogen applications. However, the microstructural-specific hydrogen concentration could not conclusively be demonstrated. This is due to the signal being captured from a volume with a diameter of approximately 100 µm, significantly exceeding the average grain size of about 15 µm. Therefore, the signal is unavoidably gained from multiple grains as well as intergranular and interphase boundaries.
Publications
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Differenzierende Wasserstoffanalytik zum Nachweis von Wasserstoffversprödung. In: Tagungsband zur „Tagung Werkstoffprüfung 2021 – Werkstoffe und Bauteile auf dem Prüfstand“. Hrsg.: S. Brockmann & U. Krupp
Jürgensen, J. & Pohl, M.
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Impact and Detection of Hydrogen in Metals. HTM Journal of Heat Treatment and Materials, 78(5), 257-275.
Jürgensen, J. & Pohl, M.
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Local Hydrogen Measurements in Multi-Phase Steel C60E by Means of Electrochemical Microcapillary Cell Technique. Metals, 13(9), 1585.
Jürgensen, Jens & Pohl, Michael
