Abstract: Fractured specimen of a 38CrMoAl steel piston rod of hydro-pneumat spring was detected by means of metallographic microscope, scanning electron microscope and energy spectrum. The results show that the fracture is fast fracture with radial fringe features. There are several arc-shaped surface cracks on the outer circle of the piston rod end, it is inferred that the high-speed impact from the external force causes the flange end to bear great bending stress and finally produces the fracture in the groove where the stress concentration is greatest. Microstructure examination of the fracture shows that there are two grooves at the flange end of the piston rod, and the fracture is caused by the first groove near the rod. At the second groove where the rounded corner is, the radius R is very small and the surface crack appears. The nitriding layer deepens obviously on both sides of that surface crack, indicating that the surface crack is formed before nitriding and belongs to the quenching stress concentrated crack formed in the quenching and tempering process. Through that, it is concluded that the stress concentration cracking tendency also exists in the first groove. There are surface cracks and surface spalling in the fracture source area, and the surface nitrided layer is brittle. The surface decarburization layer exists in the outer circle of the rod, and brittle acicular nitrides are produced. The microstructure is tempered sorbite with coarse martensite orientation, and the matrix is coarse superheated microstructure, which cause the strength and toughness of the material to be reduced significantly.

Key words: 38CrMoAl steel, cleavage fracture, quench cracking, acicular nitride, surface decarburized layer, coarse superheated structure