PSI - Issue 68

Takanori Ito et al. / Procedia Structural Integrity 68 (2025) 420–424 Takanori Ito et al. / Structural Integrity Procedia 00 (2025) 000–000

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holding at 723 K for 100 seconds. Then, fatigue tests were conducted under fully reversed cyclic axial loading with the round-bar specimens extracted from the simulated HAZ in both conventional and developed steels. Figure 1 shows the dimensions of the fatigue test specimens. The diameter of the parallel section of the fatigue test specimens was set to 4 mm, and the length was 10 mm. The average grain size was analyzed by SEM/EBSD. Vickers hardness tests were also conducted with a load of 98 N. 2.2. Fatigue property evaluation using cruciform fillet welded joints. The cruciform fillet joint was fillet welded using the 100 % CO 2 gas shielding welding method on 12 mm thick plates (conventional or developed steels). The welding material used was gas metal arc welding flux-cored wire for mild steel (MX-200), the welding current was set to 270 A, and the welding voltage to 31 V. Figure 2 shows the appearance of fatigue test specimen obtained from the cruciform fillet welded joints. Fatigue tests were conducted by applying the load in the direction indicated by the white arrows in the figure. To induce a fatigue crack in the HAZ, grinding was performed at all weld toes. The curvature radius of the grinder bar tip was set to 3mm, and grinding was performed to a depth of 0.3mm from the surface. We conducted fatigue tests under axial loading conditions. In this test, stress ratio was set to 0. After the fatigue tests, the specimens were wet-polished, etched with a Nital solution, and then observed using an optical microscope.

Fig. 1. The shape of round-bar-type specimen in fatigue test obtained from simulated heat cycle test specimens.

Fig. 2. The geometric dimensions of fatigue test specimen obtained from the cruciform fillet welded joints.

3. Results and discussion 3.1. Fatigue property evaluation using simulated HAZ specimens.

Figure 3(a) illustrates S-N curves of simulated HAZ specimens. It was found that the fatigue limit of the developed steel was superior to that of the conventional steel. In this study, the strength corresponding to one million cycles was defined as the fatigue strength (σ w ). Figure 3(b) shows the relationship between fatigue strength and Vickers hardness. Variations in the conditions of the simulated thermal cycles tests resulted in differences in hardness. It was observed that increased t c correlated with a tendency for hardness to decrease. In general, it’s known that the fatigue strength is proportional to the strength. In fact, the fatigue strength is proportional to Vickers hardness. It’s clear that the fatigue strength relative to the hardness of the developed steel is higher than that of the conventional steel. The differences in fatigue properties between the two steel plates cannot be explained solely by hardness. Figure 4 shows the relation between the fatigue strength normalized by Vickers hardness and average grain size. The influence of changes in grain size due to thermal effects on fatigue properties is negligible. The fatigue improvement effect (solid solution strengthening) observed in the ferritic matrix is also effective in the HAZ microstructure.