Abstract: To study the crucial reason of fracture of the shaft of torque gauge, chemical composition, fracture morphology, metallographic analysis and hardness were investigated and analyzed. The results show that the major factor of fracture of the shaft is abnormal microstructure. The coarse tempered martensite and the upper bainite which propagates from grain boundaries towards inside of grains bring about the initiation and propagation of crack, and form the morphology along the grain boundaries and quasi-cleavage fracture. The banded structure accelerates the heterogeneity of microstructure and hardness distribution, which results in anisotropism of material and weakens the service performance of the part. In addition, due to the tensile stress, the stress concentration effect at the root of the keyway is also a vital factor of the fracture of shaft.

Key words: torque gauge shaft, microstructure, intergranular crack, brittle fracture, failure analysis