Abstract: A small shaft of the aircraft engine control box broke during installation. The fracture properties and causes of the small shaft of control box were determined by means of fracture morphology, microstructure, hardness and chemical composition, contrast test and simulation test. The results show that the intergranular brittle fracture occurs in the small shaft after tempering in the tempering brittle zone of 570 ℃, the reason of which is not only due to the reduction the impact performance of the small shaft caused by the tempering at the temperature in the brittleness zone, but mainly because there is no ferrite in the matrix. The loss of ferrite reduces the total area of grain plane and phase interface, resulting in more weakened particles generates at grain boundary during tempering brittle zone, which intensifies the high temperature tempering brittleness, and finally leads to intergranular brittle fracture of the small shaft. The absence of ferrite in the matrix is due to the high content of austenite forming elements(Ni) and low content of ferrite forming elements(Cr,Si) in the small shaft.

Key words: small shaft of control box, 1Cr17Ni2 steel, intergranular fracture, temper brittleness, no ferrite