PSI - Issue 54

Magdalena Eškinja et al. / Procedia Structural Integrity 54 (2024) 123–134 M. Eškinja et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction In light of recent global efforts to replace fossil fuels and reduce emissions, hydrogen has emerged as a promising green energy source. As a result, the demand for hydrogen is expected to grow excessively in the coming years. For hydrogen to become a viable energy carrier, a secure supply chain must be accessible, which includes developing new and improving existing materials for transport and storage [1]. Hydrogen can have a detrimental effect on metallic materials due to the mechanism of hydrogen embrittlement (HE). HE is triggered by the penetration of hydrogen atoms into the steel. Once the hydrogen atoms are adsorbed in the steel, diffusion into areas of high stress and accumulation in stress concentration regions and segregation at lattice defects occur. The latter leads to a deterioration of mechanical properties, including a decrease in ductility, ultimate tensile strength, and fracture elongation and fracture toughness [2]. Ingress of hydrogen into the metal can occur during the service (cathodic electrochemical reactions) or during various processing and fabrication procedures (e.g. melting, pickling, electro-plating, welding, casting, electrochemical machining) [3]. High-strength martensitic steels are used in various industries such as automotive, tooling, and construction due to their high strength and adequate ductility. Low and medium martensitic steels usually have lath martensite microstructure. The lath microstructure comprises several units: laths, blocks, packets and prior austenite grain boundaries (PAGBs). These steels appear to be an economically viable choice for the material of hydrogen pipelines, as the high strength allows a reduction in pipe thickness and thus steel cost. However, it is known that susceptibility to HE increases with strength. Before using high-strength martensitic steels in the hydrogen sector, this susceptibility to HE must be overcome [4]. There are several studies dealing with the reduction of HE susceptibility of high-strength martensitic steels. The following approaches are proposed to achieve this goal: reduction of hydrogen at crack tips, suppression of hydrogen ingress, improvement of intrinsic fracture resistance or precipitation of various carbides in tempered martensitic steels [5]. Carbides are used to maintain high strength of steel after tempering treatment. Metal carbides can act as efficient hydrogen trapping sites, promoting the accumulation of hydrogen atoms and reducing their diffusivity. The amount and size of carbides play an important role in trapping behavior, and these properties can be altered by changing the heat treatment of the material. The best known crystal structure of Mo carbides is hexagonal Mo 2 C or MoC carbides, with Mo 2 C carbides often formed in the microstructure of the steel. The influence of Nb, Ti and V carbides is well studied in the literature, while the influence of Mo-based precipitates in pipeline steels has not been investigated in detail. In addition, the influence of Mo additions has been reported to be beneficial in the case of sulfide stress cracking, as the formation of a sulfide layer on the material surface can influence hydrogen-induced crack propagation [6]. However, in sulfide-free environment, there is limited knowledge about the HE behavior of Mo-containing martensitic pipeline steels in sulfide-free environments. The objective of this study is to clarify the role of varying Mo content and Mo carbides in the HE susceptibility of martensitic steels. We will investigate the joint influence of heat treatment and Mo addition on the change of microstructural features, which may play a positive role in suppressing HE. In addition, the hydrogen-trapping capability, the change of mechanical properties in the hydrogen environment and microstructural evolution were investigated by hydrogen-related experiments. 2. Experimental procedure 2.1. Materials investigated and microstructure characterization Two materials used in this study were High Mo Steel (HMoS) and Low Mo steel (LMoS) with detail chemical composition presented in Table 1. Both materials are medium-carbon steels alloyed with similar content of Cr and different content of Mo. HMoS alloy was austenitized at 880 °C for 1 h, followed by quenching in water and tempering at 670 °C for 110 min, while LMoS alloy was subjected to the identical heat treatment with the change of