PSI - Issue 68

Atef Hamada et al. / Procedia Structural Integrity 68 (2025) 581–587 Atef Hamada et al/ Structural Integrity Procedia 00 (2025) 000–000

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The experimental material used in this study was high-Mn TWIP steel with the composition (wt.%): 0.59C, 22.3Mn, 0.22Si, and 0.012Al. The steel was laboratory cast via vacuum induction melting, after which the cast ingots were homogenized at 1200 °C for 2 hours. The homogenized ingots were then hot rolled to 5 mm thick sheets at Centro Sviluppo Materiali (CSM, Rome). Following the hot rolling process, the sheets underwent heavy cold rolling, reducing the thickness to 1 mm, corresponding to an 80% reduction (true strain ~1.7). Flash heating (FH) cycles were subsequently applied to the cold-rolled steel using a Gleeble 3500 simulator. The FH cycles involved of rapid heating at 500 °C/min to the target annealing temperature, followed by a short holding time of 5 s, and rapid cooling at approximately 400 °C/min. The microstructures obtained by the FH cycles were characterized using electron backscatter diffraction (EBSD) in a field-emission scanning electron microscope (FE-SEM, Zeiss Ultra Plus). EBSD measurements were conducted at an accelerating voltage of 15 kV with varying step sizes. Quasi-static mechanical properties were evaluated at room temperature via uniaxial tensile tests, performed at a strain rate of 10⁻³ s⁻¹ using a Zwick Z100 tensile machine (Zwick Roell, GmbH). Tensile specimens were prepared according to ASTM E646–98 standards, with dimensions of 1 mm thickness, 25 mm gauge length, 6 mm width, and a total length of 180 mm.

Fig. 1. A schematic illustration of the time vs temperature cycles of the high-frequency flash induction heating in comparison with the conventional furnace heating treatment 4. Results and discussion The applied FH cycles resulted in significant variations in grain structures depending on the heating temperature at soaking duration, as depicted in Fig. 2. At 1000 °C, the promoted grain structure is a fully recrystallized fine-grained austenitic structure, characterized by a significant fraction of S 3 CSL boundaries ~ 40 %, red, which represents annealing twins as illustrated in Fig. 2(a). The average grain size of this grain structure is 2.5 um. The corresponding phase map shown in Fig. 2(b) displayed unified blue color that confirms the fully austenitic structure. As the heating temperature increased to 1100 °C during the FH cycle, the recrystallized grains grow associated with S 3 boundaries (red) with average grain size of 10 μm, as shown in Fig. 2(c). This increase in grain size can be attributed to the high thermal energy, which facilitates the movement of grain boundaries and the coalescence of smaller grains into larger ones (Rollett et al., 2004). The corresponding phase map of that grain structure, Fig. 2(d), shows unified blue color, i.e., one phase structure g -fcc, similar to Fig.2(b). At the highest temperature of 1200 °C, a coarse grain structure developed, with an average grain size of 23 μm, as depicted in Fig. 2(e). It is evident that there no phase transformation even during FH at 1200 °C, since phase map in Fig.2(f) displayed a unified blue color, one g -fcc phase structure. This is in agreement with our previous work (Hamada and Karjalainen, 2011) on the same steel. The differential thermal analysis of the steel showed that the austenite phase becomes stable at RT and over the heating temperatures until the melting temperature 1378 °C, i.e., no phase transformation during the FH tests. Thus, in the FH temperature range of 1000–1200 °C, the ferrite phase is not predicted to have been present in the promoted microstructure of the studied steel.