PSI - Issue 5

George Pantazopoulos et al. / Procedia Structural Integrity 5 (2017) 476–483 G. Pantazopoulos and A. Vazdirvanidis / Structural Integrity Procedia 00 (2017) 000 – 000

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SEM fractography provided sufficient evidence of the occurrence of brittle intergranular fracture (IGF) found at the initiation area close to the original brass surface which is roughly estimated to occupy a depth of the order of 100 μm (Fig. 3a-b). Crack evolved through a quasi-brittle mixed fracture mechanism, consisting of brittle-IGF and shallow dimpled rupture accommodating pronounced molten metal (Pb) pool formation (Fig. 3c-d). The damaging aspects of the liquid metal induced embrittlement were elucidated in Fig. 3e-f, where grain boundary decohesion and intergranular sensitization effects are highlighted. The acquired experimental evidence is suggestive of the operation of hot-shortness as the root-source of the brittle failure mechanism. Hot shortness, could be due to chemical segregation and localized melting at high temperatures or extrusion speeds during the extrusion process. Surface and subsurface IG cracking can be facilitated by the flow of low melting point constituents (such as Pb) serving as weakening agent reducing the grain boundary cohesive strength. Moreover, liquid metal diffuses in the tip of the crack and fracture areas leaving small voids behind and forming molten metal films or pools, see also Lynch (2008). Two bolts, manufactured by machining of Ø19 extruded CW614N brass bar, presented severely damaged threads (Fig. 4). SEM examination of the fracture surfaces revealed the intergranular fracture mode and grain boundaries wetting by Pb, related with grain boundary sensitization and decohesion (Figs. 5, 6). Secondary cracking, extending below the surface was also observed. Longitudinal section on the bolt revealed that even non-fractured threads suffered from severe root- and crest-cracks (Fig. 7). The collected evidence suggests strongly that thread fractures could be attributed to metal-induced embrittlement mechanism. The extent, location and morphology of primary and secondary cracking advocate that it was caused by hot shortness likely due to overheating during fabrication process. The presence of the above weakness produces a detrimental effect on thread cutting integrity, leading to poor thread surface condition and/or complete thread detachment. Lack of metallurgical abnormalities in the examined samples rules out the possibility of pre-existing harmful conditions that could be related to the damage of the final components. 3.2. Threaded components fracture

Fig. 4. Macrograph of the as-received bolts with the damaged threads after machining operation.