Abstract: Early breaking failure occurred during the use of diesel engine exhaust bolts made of IN718 alloy. The causes of the bolt fracture were investigated by means of chemical composition testing, tensile testing, metallographic analysis, observation of fracture morphology, and transmission electron microscope observation of precipitates from two aspects of material microstructure and manufacturing process. The results show that the main reason for the bolt fracture is that the surface of the rolled thread is in a high-energy state, and a large amount of nano phases precipitate at the root of the thread during subsequent aging treatment, resulting in embrittlement of the surface layer at the root of the thread.

Key words: bolt, nanophase, embrittlement, fracture