PSI - Issue 68

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V. Javaheri et. al , Structural Integrity Procedia 00 (2025) 000–000

Vahid Javaheri et al. / Procedia Structural Integrity 68 (2025) 1098–1104 © 2025 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of ECF24 organizers Keywords: Retained Austenite; Medium Manganese Steel; Mechanical Properties; Hydrogen Embrittlement; Transformation-Induced Plasticity; Microstructural Characterization 1. Introduction The mechanical properties of medium manganese steels are significantly influenced by the presence and behavior of retained austenite–RA(γ), particularly when considering the influence of hydrogen. RA(γ), as a metastable phase, can transform into martensite under mechanical strain, a phenomenon known as the Transformation-Induced Plasticity (TRIP) effect (Sadeghpour et al., 2022). This transformation enhances the strength and ductility of the steel, making it highly desirable for applications requiring both high strength and formability (Zou et al., 2016) . RA(γ), which can exist or can form in steel through different heat treatments like intercritical annealing or quench and partitioning, has been shown to contribute both positively and negatively to the overall performance of steel under various conditions. On one hand, RA(γ) can enhance ductility and toughness in steel. Its transformation into martensite during deformation can lead to increased strain hardening rate, which is beneficial for applications requiring high strength and toughness (Jacob et al., 2020). This transformation is particularly favorable in medium manganese steels, where the balance between strength and ductility is crucial for structural applications. The stability of RA(γ) under mechanical stress can also provide additional work hardening, improving the material's ability to withstand deformation without catastrophic failure (Kumar et al., 2024; Lee & Han, 2015). Conversely, RA(γ) can also pose challenges. Its inherent instability may lead to premature phase transformations under certain conditions, such as exposure to hydrogen. The interface between RA(γ) and the matrix (typically tempered martensite in medium manganese steel) is a critical aspect that influences the mechanical properties of the material. This interface plays a significant role in determining the overall performance and damage tolerance, particularly at the presence of hydrogen. The presence of hydrogen introduces additional challenges at the interface between RA(γ) and the (tempered martensite) matrix. Hydrogen embrittlement is a well-documented phenomenon that can significantly degrade the mechanical properties of steels by several mechanisms depending on the materials structure, in case of BCC-FCC steels by facilitating the transformation of RA(γ) into brittle phases or promoting crack initiation (Oriani, 1978). Understanding the interplay between RA(γ) stability and hydrogen exposure is essential for developing BCC-FCC steels with improved resistance to hydrogen embrittlement (Chen et al., 2024; Zhang et al., 2024). This study aims to investigate the fracture surface of pre-charged medium manganese steels containing 40% RA(γ) after slow strain rate tensile test (SSRT) to provide a better understanding of how RA(γ) contributes to the performance of medium manganese steels with and without presence of hydrogen. 1099

2. Materials and Methods 2.1. Material Preparation

The medium manganese steel used in this study, with its nominal composition detailed in Table 1, was developed and produced by our research team using a vacuum induction furnace. Post-casting, the material underwent austenitization at 1200°C for 2 hours. This was followed by hot rolling, achieving a 90% reduction in thickness. The resulting 4 mm thick sheet, with a finish rolling temperature of 900°C, was subsequently air-cooled. The sheet then underwent intercritical annealing at 700°C for 1 hour.

Table 1. The chemical composition of studied materials

Alloy element

C

Si

Mn

Al

Nb

wt.%

0.4

0.9

6

2

0.05