Experimental and numerical assesment of the influence of hydrogen and microstructure on the fatigue behavior of duplex steels

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In the context of the energy turnaround, duplex steels are increasingly used in the field of offshore technology, where they are exposed to the influence of hydrogen through cathodic polarization. This can lead to cracking in duplex steels as a result of hydrogen embrittlement. The effect of hydrogen in the two-phase structure of duplex steel is extremely complex. Due to the different elastic-plastic properties of the two phases, internal stresses occur at the phase boundaries, which influence hydrogen diffusion. In the preceding DFG project "Experimental and Numerical Analysis of Work Hardening Effects in Single- and Polycrystals during Cyclic Loading (Bauschinger Effect) ", various factors related to the microstructure influencing the cyclic plasticity behavior of the duplex steel 1.4462 were analyzed and quantified both experimentally and numerically. However, the influence of these factors on hydrogen embrittlement in duplex steel under cyclic loading is not considered in the current state of research. It can be assumed that the transient inhomogeneous stress distribution under cyclic loading affects the hydrogen diffusion in the two-phase duplex microstructure and leads to a fatigue damage evolution dependent on the cycle time and the global and local hydrogen concentration. Therefore, the objective of this project is to gain an understanding of the interaction of material and loading history on hydrogen embrittlement of duplex steels. With an understanding of the interaction, material properties can be specifically tailored to application conditions by optimizing the two-phase microstructure, and component life increases can be achieved. In this project, the investigation of hydrogen embrittlement in duplex steel is achieved by a combined approach of experimental and numerical analyses. The experimental investigations at RWTH Aachen University include quantification of hydrogen embrittlement and analysis of failure mechanisms for different microstructural conditions. Influencing factors are systematically analyzed and evaluated with respect to their relevance. In addition, the causes of the observed effects are investigated on a microstructural size scale and advanced analysis methods are applied. Here, computational analyses based on microstructure-sensitive models of Offenburg University of Applied Sciences make an essential explanatory contribution. The models include statistical information on the polycrystalline microstructure and use a single crystal plasticity model for cyclic loading. The coupled calculation of mechanics and diffusion under cyclic loading implemented in the project enables the local analysis of internal stresses and hydrogen concentrations, from which critical local states in the microstructure regarding hydrogen embrittlement can be identified.