PSI - Issue 69

Haofei Zhu et al. / Procedia Structural Integrity 69 (2025) 113–120

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Fig. 1 The equilibrium phase diagram of the studied steel calculated using Thermo-calc software.

The microstructures were characterized using a scanning electron microscope (SEM, ZEISS SUPRA55), X-ray diffraction (XRD, Bruker AXS D8 Advance diffractometer with Cu-Kα radiation), and a field emission transmission electron microscope (TEM, FEI Talos F200X) equipped with energy-dispersive X-ray spectroscopy (EDS). The samples for SEM analysis were mechanically ground, polished, and then etched using 4 % nital. XRD patterns were collected over a 2θ range of 40° to 105° with a scanning step size of 0.01°. The modified Miller’s equation was employed to calculate the volume fraction of austenite by integrating the intensity of (200) α , (211) α , (200) γ and (220) γ peaks. ! = 1.4 ! " + ! (1) where " and ! are the integrated intensities of martensite and austenite peaks, respectively. TEM foils were mechanically polished down to 50 μm thickness and then twin-jet electropolished in a solution of 10 vol. % HClO 4 + 90 vol. % C 2 H 5 OH at -30 °C. Uniaxial tensile and Charpy impact tests were conducted at room temperature to evaluate the mechanical properties. The tensile specimens were machined with a gauge diameter of 5 mm and a gauge length of 30 mm. Uniaxial tensile testing was performed in accordance with GB/T 228.1-2021 using an Instron 8501 mechanical testing machine at an initial strain rate of 5 × 10 -4 s -1 . Charpy impact testing was carried out according to GB/T 229-2020 using 55 × 10 × 10 mm 3 V-notch specimens and an MTS ZBC 230 impact testing machine. 3. Results and discussion Fig. 2 shows the SEM images of the samples after solid-solution treatment and the corresponding changes in PAGs at different temperatures. After etching, the PAG are clearly visible. The average size of the PAGs was measured using the intercept method, as shown in Fig. 2(e). It can be observed that with an increase in solid-solution temperature, the value of the PAGs increases, especially above 930 °C, where a significant growth in PAGs is evident. After solid solution treatment at 960 °C, the PAGs increase from 14.22 ± 2.10 μm at 870 °C to 17.85 ± 3.11 μm. To characterize the phase composition of samples solid-solution treated at different temperatures, the XRD analysis was performed, as shown in Fig. 3. The results indicate that with increasing solid-solution temperature, the retained austenite content increases. Specifically, as the solid-solution temperature increased from 870 °C to 960 °C, the retained austenite content increased from 1.52 ± 0.4 vol.% to 1.98 ± 0.5 vol.%. This trend is consistent with changes observed in other martensitic steels with increasing solid-solution temperature [11]. This phenomenon can be attributed to the enlargement of PAG and the coarsening of martensite grains after quenching at higher solution temperatures. These changes result in an increase in internal stresses and constrained volume expansion during the martensitic transformation, thereby reducing the extent of the austenite-to-martensite transformation and leading to an increase in retained austenite [11].