Abstract:

In a dismantling repairmen of a steam turbine of a power plant, a hightemperature bolt made of 20Cr1Mo1VNbTiB was found broken. The fracture causes were analyzed by fracture analysis, metallographic test, mechanical properties test at room temperature and chemical composition analysis. The results show that the stress concentration caused by the mechanical damage on the screw surface is the direct cause of the bolt breakage. At the same time, coarse crystal grain of bolt, the carbide precipitating under longterm high temperature/highpressure operation, and reduce of  impact toughness  are the root causes of cleavage brittle fracture of the bolt.

Key words:

20Cr1Mo1VNbTiB, bolt, stress concentration, impact toughness, cleavage brittle fracture