Fig.9 Microcrack morphology(Dotted area in Fig. 10)
As shown in Fig. 8, a large number of cracks (1#,2#,3#) were found
in the scanning area at low magnification. All the three cracks
originate from the corrosion pit. However, the crack initiation and
propagation direction are different. Both 1 # and 2 # cracks originate
from the corrosion pits on the lower surface of the blade, and the
cracks propagate to the upper surface, forming many directional
dendritic cracks near the crack end. 3# cracks are quite special. On
the one hand, they start to expand upward from the origin of the lower
surface, and on the other hand, many secondary cracks originate from the
corrosion pits on the upper surface and expand downward. The reason of
the secondary cracks on the upper surface is inferred: because the main
cracks are 40mm long and the opening is 0.8mm large, and they have
already shown penetrability, the actual stress area of the blade is
reduced, the local stress increases, and the alternating stress
amplitude value will inevitably increase after the deterioration of the
operation condition (the vibration of the shafting increases sharply
after the occurrence of the first broken piece). In addition, there are
corrosion pits on the upper surface to form the stress concentration,
which are the total of these factors. Under the same action, a large
number of secondary cracks are inevitable. The upper and lower surface
cracks finally intersect and form a through crack. No matter the crack
originates from the upper surface or the lower surface of the blade, it
starts from the corrosion pit and extends in the opposite direction.
Most of the cracks are along the original austenite grain boundary.
2.5 Fracture surface
analysis