Abstract: Cracks were found in the spline part of 40CrNiMoA steel transmission shaft used in a aero engine during normal inspection. Macro analysis, scanning electron microscopy and energy spectrum analysis, metallographic analysis, hardness analysis and temperature and color contrast experimental analysis were carried out on the fracture site. The results show that the structure of spline crack zone of the 40CrNiMoA transmission shaft is consistent with that of the matrix, and no decarburization phenomenon is observed. The crack is intergranular fracture, and the oxidation of crack surface is obvious. Therefore, it can be inferred that the crack is generated during the heating stage of high-frequency quenching, which results from excessive heating current or uneven heating. It is suggested to reasonably control the high frequency quenching process of transmission shaft.

Key words: drive shaft spline, 40CrNiMoA steel, quenching cracks, high frequency quenching