PSI - Issue 75

Jérémie BOUQUEREL et al. / Procedia Structural Integrity 75 (2025) 442–449 Author name / Structural Integrity Procedia (2025)

447

6

fatigue limit, shortening the lifespan of the part. Interestingly the local misorientation assessed by KAM mapping is not connected with the carbides distribution and it is located naturally around LAGBs. Nevertheless, not every LAGB is associated by increased local misorientation and lattice distortion. Complementary TEM investigations have been carried out and the corresponding observations are displayed in Figure 5. In addition to the grain growth and modification of the precipitation state, the presence of beach marks, zebra and fish scale structure (twist sub-boundaries) may be noticed, pointed in red and irregular dark lines, pointed by blue arrows.

Figure 5: TEM micrographs attesting of damage structuration within the HDEU1 microstructure

The new valve grains interior is often filled with texture comparing to mostly plain grains of the used valve. Dark lines marked by blue arrows are assumed to be dislocation accumulation inside the grains of the used valve, similar to the observations made by Argillier (1998), Vogt et al (2000) as well as Gariboldi and Dudova (2022). No similar lines were found in the new valve structure. In the used valve, dislocations arranged into sub-grain boundaries in the frame of grain recovery referring to sub-grain growth. When internal dislocation content is observed, this suggests coalescence of martensite laths and deformation within them. Beach marks and twist sub-boundaries or fish scale structure were observed by Argillier (1998) for low alloyed 2.25Cr-1Mo steels after around 150 000 h at 565 °C referring to fatigue damage of power plant parts, subjected to thermo-mechanical strain. This morphology is described as recovered dislocation substructure. Fish scale structure is also shown by Bhadeshia (2024); describing annealing sub-grains in low alloyed steel, held at elevated temperature (730 °C, 168 h), also referring to as recovered dislocation substructure. This structure with high number of carbides also refers as tempering resistant. The fish scale structure in high alloyed AISI 316 steel was found by Bernard, 1984. Their presence is associated with cyclic strain at elevated temperatures: 650 °C, represented by incomplete recovery, screw dislocation overproduction and marks it as sub-grain boundaries, a network of dislocations (Bernard et al. (1984)). Moreover, the twist sub-boundaries or fish scale structure formation might be connected with high temperature and stress, noting that the stress can be relatively low. Dislocation construction in the form of straight lines or beach marks might be edge dislocation structure formed the same way, under high temperature and under some amount of stress. Different appearance might be cause by their orientation, due to extinction contrast, while in another beam angle, they might not be able to be observed at all (Vogt (1991)). Those multi-scale displayed microstructural features at the area of the valve fillet point out a damage related with high-temperature, long-term acting fatigue, probably not high stress levels. In addition, repeated warming and cooling cycles must also be considered. Nevertheless, taking into account these parameters and the in-depth analysis of the microstructure, it is reasonable to