Fracture of Alloy783 bolts in medium-pressure valve bonnets of USC units will cause a significant threat to personal and equipment safety. In order to study the main failure factors

the whole data statistics

macroscopic inspection

microstructure analysis and mechanical performance tests were carried out for a fractured Alloy783 bolt. It was found that

all the fracture positions lacked regularity and longitudinal cracks were observed

so it is impossible that the excessive preload during installation will cause the cracks. The segregation of primary β phases was very serious

showing a clear band distribution. The precipitation of grain-boundary secondary β phases was less

which was mainly due to insufficient solution treatment and subsequent β aging heat treatment. The fatigue cracks of all broken bolts were mainly intergranular

since the stress-induced grain boundary oxidation(short for SAGBO) led to fatigue crack propagation within oxidation damage zone. Thus the crack propagation rate was controlled by the intergranular oxidation rate. The poor resistance against SAGBO was attributed to lack of secondary β phases along grain boundaries.