ADVANCED MATERIALS & PROCESSES | MAY/JUNE 2025 14 Monopiles are a crucial component of wind turbines in offshore power plants, serving as the foundation for these structures. Fabricated from heavy steel plates welded together, these structures are built to withstand cold, oceanic environments with a minimum service life of 20 years. Hence, the mechanical properties and crack-resistance are of utmost importance to guarantee sufficient product properties[1]. Electron beam welding, a technique investigated in this study as an alternative to submerged arc welding, has seen limited industrial application in monopile construction to date, primarily due to concerns regarding the fracture toughness of the resulting material and the vacuum conditions required for welding. Due to the necessity to weld the steel plates for monopile construction, thermal influence is generated on the microstructure of the base material, which can generally worsen the mechanical properties, particularly the fracture toughness[2]. Thus, a tough microstructure must be generated to guarantee high crack resistance. Acicular ferrite as a microstructure constituent is well established as a crack-resistant phase promoting the fracture toughness and strength of welds[3]. Its formation is based around particles as nucleation sites, which necessitate an adapted chemical alloy composition to facilitate suitable particle nucleation as not all particles are as likely to nucleate acicular ferrite, for example in the case of vanadium nitride and vanadium carbide[4]. To better understand the formation of acicular ferrite, an investigation of acicular ferrite nucleation sites in electron beam welding was performed and a suitable approach for the evaluation of acicular ferrite is proposed. This characterization was performed as a scale-bridging approach, to facilitate the macroscopic assessment of the present phase and combine it with higher resolution imaging and information. Scale-bridging in this context refers to the use of measurements that span multiple orders of magnitude, which is then correlated. Once the nucleation site is identified, the particle of interest can be investigated. Here both chemical information as well as crystallographic information were obtained, further enabling a nanoscopic analysis using a transmission electron microscope and a scanning transmission electron microscope. EXPERIMENTAL PROCEDURES The material investigated was a low-alloyed steel of the type S355 ML. Steel plates with a thickness of 8 cm were used for the study, which were welded using electron-beam welding with a nickel foil of 0.1 mm thickness in a horizontal position. Then samples were sectioned and manually ground using P80, P180, P360, P600, and P1200 grit papers. Subsequently, electro- polishing was carried out using A2 electrolyte with the LectroPol-5 system by Struers for contrasting. These preparation steps enabled observations with the light microscope and various electron microscopy techniques. The laser-scanning microscope of the type OLS4100 by Olympus, was operated in optical mode to obtain light microscope images of the investigated area. An overview of the selected parameters for the electron microscopy techniques and their abbreviations are provided in Table 1. TABLE 1 — ELECTRON MICROSCOPY TECHNIQUES, ABBREVIATIONS, AND PARAMETERS EMPLOYED IN THIS STUDY Microscopy technique Abbreviation Parameters Scanning electron microscopy SEM 5 kV Backscatter electron detector BSE 5 kV Energy dispersive x-ray spectroscopy EDS 10 kV Electron backscatter diffraction EBSD 20 kV, 0.3 µm Scanning transmission electron microscopy STEM 30 kV Transmission Kikuchi diffraction TKD 30 kV, 0.015 µm Transmission electron microscopy TEM 200 kV SEM and BSE images as well as EBSD measurements were taken using a Helios G4 PFIB CXe, while STEM and TKD analyses were performed with a FEI Helios Nanolab600 Ga-FIB/REM. Additionally, a TEM Jeol F200 was used to acquire TEM images. The evaluations of EBSD measurements were conducted with the program EDAX OIM Analysis 8, and EDS evaluations were attained using APEX EDX (2.5.1001.0001). TEM results, especially selected area diffraction patterns (SAED patterns), were furthermore evaluated using the program SingleCrystal version 5.1 and prior by CrystalMaker Software Ltd[5]. RESULTS AND DISCUSSION Figure 1a showcases an electron beam weld seam within the steel with its melt zone highlighted within blue lines as well as its heat affected zone highlighted in black. The contrasted microstructure obtained by performing an experimental routine is illustrated in Fig. 1b and was found in the melt zone. The characteristic microstructure of acicular ferrite, namely the laths radiating from a central particle, is revealed in Fig. 1b. The observed laths are elongated in three different growth directions to around 50 µm each and a thickness of about 10 µm, resulting
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