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