PSI - Issue 68

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

421

2

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

propagation of fatigue cracks [Suzuki et al. (1979), Wasynczuk et al. (1984), Mutoh et al. (2007) ]. However, resistance to fatigue crack initiation is important to improve the fatigue life of materials as well as improve crack propagation resistance. In our previous study, a new 490 MPa strength class thick steel was developed, which exhibited high resistance to crack initiation by strengthening the ferritic matrix through solid solution strengthening elements such as Si and Cu [Ito et al. (2024)]. The microstructure of the developed steel is mainly ferrite, but in welded joints, it transforms into bainite or martensite due to the welding heat. This microstructure affected by welding heat is commonly referred to as the Heat Affected Zone (HAZ). It is unclear whether the fatigue improvement effect of the developed steel can still be expected when such microstructural change. In addition, fatigue cracks usually initiate either in the weld metal or HAZ in welded structures subjected to cyclic loading. Therefore, it is essential to comprehend the fatigue properties within the HAZ due to its significant impact on the overall performance and durability of welded structures. This study has investigated the effect on fatigue strength of welded joints using the developed steel. The fatigue strength of the HAZ was examined by means of a simulated HAZ specimen. Subsequently, the fatigue improvement effect was confirmed by using fatigue tests of cruciform fillet welded joints. 2. Experimental procedure 2.1. Evaluation of fatigue properties and microstructure observation using simulated heat cycle test specimens. The 490 MPa class steel plates with the chemical composition shown in Table 1 were prepared. Specimens with dimensions of 11.5 mm × 33 mm × 70 mm were extracted. A microstructure equivalent to the HAZ was produced by simulating the heat conditions of single-pass fillet welding. It subjected to heat treatment under the conditions specified in Table 2. The test conditions were carried out under four different conditions to change the grain size and hardness in HAZ microstructures. In the simulated thermal cycle test, the specimen was heated at a rate of 50 K/s using high frequency induction heating until it reached the maximum temperature ( T max ). After that, it was held at the maximum temperature for 0 to 10 seconds before being cooled. When the cooling time from 1073 K to 773 K is defined as t c , it was varied to 15, 30, and 60 seconds. In condition D, an additional heat treatment was performed by Table 1. Chemical composition of conventional and developed steels (mass%).

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