PSI - Issue 68

Adam Ståhlkrantz et al. / Procedia Structural Integrity 68 (2025) 1051–1058 Ståhlkrantz et al./ Structural Integrity Procedia 00 (2025) 000–000

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order to design more robust materials for a given application, it is important to understand how and to what degree hydrogen influences the mechanical properties of the material. In particular, there is an increased need to understand the hydrogen/metal interactions at a microstructural level in AHSS, especially with respect to microconstituents that have a significant effect on formability, such as retained austenite (RA), Takashima et al. (2024), Li et al. (2023) and Smith (2021) . This is especially valid from the perspective of how much hydrogen is absorbed and how much of the absorbed hydrogen is trapped in RA, Hai et al. (2024) and Malitckii et al. (2016). It is well established that the hydrogen solubility in austenite is much higher compared to other steel phases. In addition, RA is also known for its capability to act as a strong trap for hydrogen. However, although RA has the possibility to strongly trap hydrogen in large amounts it may also bring a high risk to HDC, HIC or HE due to hydrogen releasing from the RA because of strain induced transformation to martensite. This phenomenon must be taken into consideration to understand and correlate the effect of hydrogen on plasticity in steels with RA, Wagner and Larour (2018). In this work, the influence of hydrogen on the RA in AHSS with different amounts of RA has been studied. The aim of this study is to better understand how hydrogen affects the formability and plasticity of AHSS. To this end, an AHSS material composition was chosen and heat-treated in a laboratory setup. To obtain different amounts of RA, five different heat treatment cycles were performed which yielded five different complex microstructures with varying amounts of RA. 2. Materials and methods Table 1 presents the composition of the AHSS used in this study. The material was supplied as 1,2 mm thick sheet in a cold rolled state and was heat treated in five different cycles to produce specimens with different microstructures and RA contents. All five heat treatment cycles were performed in a Gleeble 3800, simulating heat treatments possible to perform in an industrial annealing and hot dip galvanizing line. The material was heat treated in the Gleeble as 100 by 10 mm strips, cut from the cold rolled sheet by electrical discharge machining (EDM). The heat treatments included quenching and tempering, quenching and partitioning (Q&P), austempering, as well as Q&P and austempering following intercritical annealing. The parameters were designed in such a way that the specimens would have a roughly similar ultimate tensile strength (UTS), but different yield strengths, uniform elongations and total elongations. For the mechanical testing, miniature tensile test specimens were EDM cut from the heat-treated Gleeble strips, with a gage length of 6 mm and a gage width of 2 mm. The specimens were tested to fracture with a strain rate of 0.105 mm/min in an Instron 4505 servoelectric tensile testing machine with a capacity of 100 kN, equipped with a contact extensometer. The leftover material from EDM cutting the gage section gave two specimens that could be used for characterization of the material microstructure. To verify the heat treatment results, the excess materials from the production of the tensile specimens were used for material characterization using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM) and X-ray diffraction (XRD). The samples for EBSD were first mechanically ground and polished down to the final step with 0.25 µm. This was followed by vibration polishing in a Buehler Vibromet 2 with 0.2 µm colloidal silica suspension for 2 hours. The SEM analysis was performed in a field emission gun (FEG) SEM Zeiss Gemini 450, operated at 15 kV with 15 nA and a working distance of approximately 15 mm. Prior to the XRD measurement, the samples were ground to remove the surface layer and etched with a 3:1 mixture of 10-% sulfuric acid and hydrogen peroxide (the “Piranha” etchant) to remove any surface deformation. The XRD measurements were conducted using a Bruker D8 Discover with Cu-Kα radiation and a LynxEye 1D energy dispersive detector. Data were recorded from 35 to 110 degrees 2θ with angular intervals of 0.05 degrees and an acquisition time per step of 2 seconds. The XRD measurements were used to determine the RA content by Rietveld fitting. The cathodic hydrogen charging of the samples was done in accordance with Vennet et al. (2023) , where the Table 1: Material composition of the investigated steel alloy. C Al Mn Ni Ti + Nb + V Si + Cr + Mo Balance 0.19 0.56 2.5 0.04 0.019 0.434 Fe and traces of Cu, P, S