PSI - Issue 42

Liese Vandewalle et al. / Procedia Structural Integrity 42 (2022) 1428–1435 Vandewalle et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Materials and Methods The ULC steel used in this study has a chemical composition given in Table 1. The material is subjected to cold rolling to a thickness reduction of 60% after hot rolling. This cold rolled condition is considered to be the starting condition. Subsequent annealing in an air resistance furnace at various temperatures is performed to gradually annihilate different types of defects. Recovery annealing is done at 350°C (623K) for 10 min followed by furnace cooling. Recrystallization is obtained by annealing at 760°C (1033K) for 30 min, followed by furnace cooling. Furthermore, annealing at intermediate temperatures, i.e. 77°C (350K) and 177°C (450K), for 10 min followed by furnace cooling are performed in order to study the evolution of the H-CW peak. Additionally, the recovered material is re-deformed by tensile straining to 10% elongation as well as by torsional deformation of the IF samples. Via this re-deformation, an attempt is made to reintroduce the defects responsible for the H-CW peak in a semi-controlled way. The IF measurements are performed in an inverted torsion pendulum, operating in free vibration mode. Samples with dimensions of 1.3 x 1.3 x 30 mm³ are prepared by mechanical cutting and grinding. The IF spectra were measured in the temperature range of -170°C (100K) to 330°C (600K), with a heating rate of 2 °C/min, and at a frequency of about 2 Hz. The samples are excited every 15 s, with a strain amplitude of 10 -4 . From the free decay signal the IF coefficient, Q -1 , is determined. The measurements are performed in a He atmosphere with a pressure of about 5 mbar, assuring a good heat transfer while minimizing external friction effects. During the TDS measurements, disc shaped samples with a diameter of 20 mm and a thickness of 1.7 mm are subjected to a constant heating rate, 20 °C/min and the H desorbed from the samples is measured by a quadrupole mass spectrometer. Samples are charged electrochemically in an 0.5 MH 2 SO 4 solution containing 1g/l thiourea at a current density of 0.8 mA/cm². To minimize H losses, the samples are tested immediately after charging or stored in liquid nitrogen until testing. Table 1: Chemical composition of the ULC steel, given in wt% C N S P Mn Ti Al Fe 0.0214 0.0088 0.0038 0.0073 0.25 0.002 0.047 Balance 3. Results The full IF spectrum of the H-charged cold rolled material is shown in Fig. 1.a. Clearly, a well-developed H-CW peak is present in the spectrum as well as three higher temperature peaks imposed on an exponential background. These higher temperature peaks are in detail discussed elsewhere (Vandewalle et al. (2022)). As this work focuses on the H-CW peak, it suffices to state that these peaks are related to the movement of microstructural defects such as dislocations and C-vacancy clusters. The H-CW peak is shown in more detail in Fig. 1.b, where it can be seen that the peak appears to consist out of two sub-peaks at -144°C (129K) and -116°C (157K), which are further referred to as PH1 and PH2, respectively. Successive annealing at low temperatures results in a gradual decrease of the H-CW peak intensity, together with the background level, and after annealing at 350°C even complete annihilation of the H-CW peak is observed. Based on the calculation method described in Vandewalle et al. (2021), Vandewalle et al. (2022), the relaxation strengths, which represent the peak intensity, together with E a -values can be extracted. The E a are found to be 37 kJ/mol (0.388 eV) and 46 kJ/mol (0.474 eV) for PH1 and PH2, respectively, being in fair agreement with literature values. No significant changes are observed regarding the E a of the peaks upon annealing. The change in the relaxation strength upon annealing is visualized in Fig. 1.c. It can be clearly seen that initially PH2 decreases more rapidly than PH1, even though PH2 corresponds to the higher E a relaxation process. Clearly, the H-CW peak involves two different, metastable H-dislocation interactions, with the more difficult relaxation/movement being annihilated first. To further study these H-dislocation interactions, an attempt is made to reintroduce the H-CW peak by re deformation of the material showing no H-CW peak. The influence of tensile straining is visualized in Fig. 2. No significant changes in hardness as well as in the TDS spectrum are observed after tensile straining. This may be related to the rather limited effect of the additional dislocations introduced during tensile straining compared to the high amount of dislocations still present after recovery. However, a slight increase of the dislocation peaks in the IF spectra is visible, indicating the introduction of a small amount of new dislocations. Despite, H-CW peak does not reappear in the IF spectrum.