The transmission electron microscopy was used to study the microstructure of steel 40Cr for virgin, fatigue damaged, and
recovered after fatigue damageing by
intermediate annealing. The results indicate that the stability of temper is raised due to the existence of chromium in high temperature tempering. The original martensite lathes is remained. There are M_7C_3 and M_ 23 C_6 carbides particle along the boundaries of lathes or inside the lathes. In addition, thin rod-like M_3C carbides are distributed inside some broader lathes. Configuration of
dislocation in the specimens endured fatigue damage is different from that of virgin specimens. Some dislocations tangle and become dislocation walls in specific zones, and dislocation cells are observed in some regions. After the fatigue damage, the size of M_ 23 C_6 carbides particle becomes larger, the quantity of rod-like M_3C carbides increases and the length of rod shrinks. Through the recovery by intermediate annealing, there are fewer dislocation tangles exists around carbides and the wall thickness of dislocation cells thinned. So it is considered that the density of dislocation is reduced. Only big clump M_ 23 C_6 carbides, which is spherical and evenly distributed, can be observed in all region of the recovered specimens. There are some differences in microstructures among above three kind of specimens, so their mechanical properties and total fatigue life times are different.