PSI - Issue 75

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

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state that the modified microstructure, considered as degraded, results from the high temperature, coupled with possible very low loadings but of cyclic nature. Indeed, as mentioned above, based on available information, the valve is expected to be submitted to 1.7 10 8 to 4.1 10 8 load cycles. It is therefore reasonable to conclude that the valve fillet was subjected to high temperature very high cycle fatigue (VHCF). Papers on VHCF at high temperature are rather scarce and recent except some ones, e.g. (Himmelbauer et al. (2022), Li et al. (2023) or Bortoluci-Ormastroni et al. (2024)). Such lab experiments have not been yet conducted on Silchrome 1 but the present investigation based on a case study reveals the VHCF indicators in a fillet valve made of Silchrome 1. 4. Conclusions An historical exhaust valve coming from a motorcycle used during the second world war (WLA/WLC Harley Davidson) has been investigated by means of the microstructure characterization tools and approaches available at the beginning of the 21 st century (SEM-EBSD, TEM). While valve degradation is generally studied in terms of surface degradation (erosion, wear…) linked with the contact with fuel/gas, the present work focused on the degradation occurring in the valve bulk. To eval uate such kind of damage, a valve in “used” condition has been compared with a virgin one, referred as as-new and product at the same period. The investigated H-D engine valves are made of Silchrome 1 steel. The present work enlarges the knowledge on the Silchrome microstructure which was well described at the beginning of the 20 th century (up to the 1940s) but very scarce after that period. It also documents how the microstructure evolves with the in-service temperature and loadings. Matrix structure coarsening, grain size increase, grain coalescence and grain boundary shifting can be linked with long-term elevated temperature exposure of the fillet in the first place, accompanied by mechanical loading and repeating warming and cooling cycles. It is difficult to consider whether such conditions can be simulated by accelerated tests. One thousand hours exposition or more is required for appropriate fatigue test. Large grains chromium depletion, probably linked with secondary carbides formation at the GBs, identified at the used valve fillet, is assumed to be connected with long-term elevated temperature exposure in the first place as well, due to faster diffusion. Two to three times (Fe,Cr) 7 C 3 blocky carbides growth and secondary (Fe,Cr) 23 C 6 carbides precipitation on the grain boundaries, found in the valve fillet may be linked with long-term acting elevated temperature. The particular dislocation structuration is connected with high-temperature fatigue and according to literature comparison, the mechanical load does not have to be very high. Observed beach marks, twist sub-boundaries fish scale and zebra morphology are linked to grain recovery process under the cyclic load. Studying a real part allows to obtain observations attesting of degradation related to VHCF thermo-mechanical fatigue that there not described before for this kind of steel. Finally, this study shows that it is possible to obtain very good information on microstructural modifications with historical materials even if the history is not fully known. The vehicle's engine may have been disassembled and reassembled several times for various reasons, and despite this, a VHCF damage indicator has been pointed out. Acknowledgments The SEM and TEM national facility in Lille (France) is supported by the Conseil Régional du Nord-Pas de Calais, the European Regional Development Fund (ERDF), and the Institut National des Sciences de l’Univers (INSU, CNRS). The authors would like to thank Mr. A. Addad for their help in carrying out the work on the Electron microscopy facility (TEM) of the Advanced Characterization Platform of the Chevreul Institute References DIN EN 10090-98; Valve Steels and Alloys for Internal Combustion Engines . European Committee for Standardization: Brussels, Belgium, 1998. S. Argillier, Analyse des mécanismes de plasticité cyclique à chaud d’aciers de type 10CD910 : détermination d’un indicateur d’endommagement de fatigue, Université des Sciences et Technologie de Lille, 1998.