PSI - Issue 51

Kamila Kozáková et al. / Procedia Structural Integrity 51 (2023) 145–151 K. Kozáková et al. / Structural Integrity Procedia 00 (2022) 000–000

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3. Experiment Fatigue tests of two specimens (no. 2 and 3 from Fig. 1) using a resonance fatigue testing machine with a loading frequency of 20 kHz (ultrasonic tests) were performed in the range 10 4 – 10 10 cycles. 3.1. Description of experimental material The investigated materials 1.4306 and 1.4307 stainless steels were delivered in the form of a hot-rolled sheet of 20 mm thickness, after basic solution annealing at 1050 °C for 1 hour. The chemical composition of experimental materials delivered by the producer is listed in Table 2. The most significant difference between these materials is nickel content, which is higher in the case of 1.4306 steel. 1.4306 steel is more highly alloyed and therefore can provide higher corrosion resistance than 1.4307 steel. Table 2: Chemical composition of experimental material AISI 304L (in wt. %, provided by a supplier) (Klusák et al., 2022) Material C Mn Si P S Cr Ni Cu N ����� ���� ���� ������ ������ 18.0–20.0 10.0–12.0 - ���� 1.4307 ����� ���� ���� ������ ������ 17.5–19.5 8.0–10.0 ���� ����� Metallographic samples were mechanically ground by SiC papers, and mechanically polished with diamond paste. Subsequently, the sample surfaces were electropolished by a solution of 600 ml methanol, 360 ml ethylene glycol monobutyl ether, and 60 ml perchloric acid under a voltage of 35 V for 40 s at a temperature of 10 °C. As the final step to reveal the microstructure, the samples were etched using Beraha II. Solution Beraha II consists of 800 ml distilled water, 400 ml hydrochloric acid, and 48 g ammonium bifluoride. Observed microstructural features and material texture (grain orientation) were examined by TESCAN LYRA 3XMU field emission gun scanning electron microscope (SEM), equipped with Ultim Max100 EDS detector and Symmetry electron backscatter diffraction (EBSD detector). Documentation of grain orientation was carried out before etching. The microstructure of both materials consists of equiaxed austenitic grains with numerous annealing twins and -ferrite stringers aligned along the rolling direction, see Figs. 5 and 6. -ferrite stringers are indicated by black arrows, annealing twins are indicated by white arrows. Grains are oriented randomly, and the structure does not show any preferential grain orientation, regardless of the rolling direction. The average grain size determined from EBSD, considering the grain misorientation angle of 15°, was set as the equivalent circle diameter. The value is 48 ± 20  m in the transversal and 54 ± 24  m in the longitudinal direction for 1.4306 and 40 ± 23  m in the transversal and 41 ± 22  m in the longitudinal direction for 1.4307. Twin grain boundaries were not taken into account for the grain size determination. 1.4306

Fig. 6. 1.4306, longitudinal cut, SEM, BSE.

Fig. 5. 1.4307, longitudinal cut, SEM, BSE.