The bolts in the tune door of a 660 MW ultra supercritical unit were broken after short-term service.These bolts were made of GH783 alloy

an anti-oxidized Ni low expansion superalloy.The fracture bolts were investigated by macroscopic examination

metallographic examination

SEM equipped energy spectrum analysis composition analysis and mechanical properties test.The results show that:in the bolt hot tightening process

three heating pipes were high temperature damaged and remained in the bolt holes which burned and damaged the bolt materials near the bolt holes.Cracks initiated in the burning sits and propagated under normal service conditions leading early fracture of the three bolts.After this

the size and distribution of the stress in the left bolts was changed and additional flexure stress was introduced.Some bolts were fatigue fractured under alternating stress due to the environmental temperature and loading change in the different order.Other bolts failed in the disposable brittle fracture.Some suggestions were also put forward such as regulating the hot tightening process

enhancing the machining quality of bolt outer and inner surface and re-assessing the service reliability of GH783 at 600 ℃.