PSI - Issue 75

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

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hardness measurement in the fillet aeras might attest of a material degradation. Hardness decrease can be corelated with yield stress decrease and yield stress decrease means increased possibility of fatigue damage meaning cyclic softening and an area sensitive to crack initiation. It can also be noted that the operating temperature range must be compared with the tempering temperature commonly observed for the Silchrome 1 steel ranging from 550-650°C to 720-820°C depending of the standards (eg: EN 10090 1998).

Figure 2: left: exhaust gasses flow around the valve (yellow) and zone of the maximum degradation in the fillet (red arrow), right: Vickers Hardness Profile along both HDEN1 and HDEU1 valves (HV – 1 kg, 5 mm step size)

3.3. Microstructure observations Valve fillet is in general the most loaded part of the valve, as it is mechanically tensioned by the closing springs, heated by exhaust gases directly and not cooled by the contact with the valve guide or valve seat by contrast to the valve stem and face. As mentioned previously, the highest microstructure degradation evidence is expected in this area, which has been indicated by strong hardness decrease. To assess a clear view of the microstructure, representative SEM-EBSD maps are correlated with Forescatter Electron Detectors micrographs. A first step in the microstructure identification reveals the presence of mainly (Fe,Cr) 7 C 3 carbides imaged by the bright dot zones observed SEM (Figure 3b) correspond (Figure 3d). Here, several phenomena resulting from mechanical loading and elevated temperature during thousands of engine hours and many service cycles, followed by many stand-by periods meaning temperature fluctuations, can be stated. Firstly, the IQ-LSM EBSD maps (Figures 3c, 3d) attest of a noticeable structure coarsening. The carbides growth is clear as well. Nevertheless, the ratio between LAGB (Low Angle Grain Boundaries or sub-grain boundaries) in grain interiors and GB (Grain Boundaries) remains similar. In addition to the previous maps, Inverse Pole Figure and KAM maps were also considered (Figure 4). Compared to the fine microstructure before any use, the exhaust valve microstructure evolves into much larger grains (up to 10 µm diameter). Moreover, while misorientation gradients are widespread throughout HDEN1 grains, the situation appears to be quite different in HDEU1. Here, the misorientation gradients within grains tends to disappear as coalescence occurs. This aspect could attest the presence of less geometrically necessary dislocations within the grains that can be, for example, linked to coalescence / recovery and dynamic recrystallisation occurring during the valves use at high temperature (Tarasiuk et al. (2002)).