Failure Analysis on Fracture of Flange Sealing Bolts

Through macrographic examination, chemical composition analysis, tensile test, haedness test, metallographic examination, fracture analysis and other methods, the fracture failure reasons of a batch of 20Cr13 steel flange sealing bolts were analyzed. The results show due to the low quenching temperature, there were a lot of granular carbides in the microstructure of the flange sealing bolts and a lot of banded undissolved ferrite which led to local stress concentration in some areas. At the same time, it also resulted in lower strength, uneven hardness and inhomogeneous microstructure. When the bolts were under normal working lood, cracks would appear and propagate along the banded ferrite at the stress-concentrated thread root, and eventually fracture failure of the bolts happened.