PSI - Issue 17

D.G. Papageorgiou et al. / Procedia Structural Integrity 17 (2019) 532–538 D.G. Papageorgiou, H. Bravos, C. Medrea / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 6. (a) Engraved chemical analysis on the surface of the U-bolts : 42CrMo4, (b) Chemical composition of the first two pieces the typical chemical composition limits, (c) Chemical composition of third piece and typical chemical composition limits. hardness specified for the annealed condition (159HV30-15 HRC-82HRB). Nevertheless, it is insufficient for the specific application (ASM Aerospace Specification, 304L, 2019). The core exhibits lower hardness related to the surface hardness indicating that the stainless steel round may have been cold worked (Standard Cr-Ni Stainless Steels 2006). The specific material is prone to cold working as hardness is increasing and toughness decreasing during service life. All in all, the 304L stainless steel is improper for the specific application. The microstructure of the first two holders comprised of tempered martensite and low percentage of retained austenite as expected (Fig. 7a). High density of inclusions on the grain boundaries is observed. The presence of inclusions impairs the fatigue strength of the material (Fig. 7b). On the thread, the grains have been extensively deformed due to the forming procedure (Fig. 8a). Uneven plastic deformation on the two opposite sides of the thread is observed (Fig. 8b). Uneven forming of the threads has negative influence to the fatigue resistance of the holders. Cracks were detected on the tip of the thread during cold forming (Fig. 8b). These areas are a potential origin of cracks during working.

Fig. 7. (α) The microstructure of the first piece, (b) Various inclusions observed on the second piece.

Fig. 8. (a) Detail of a thread, (b) Crack and uneven plastic deformation on the top of the thread