Fig. 11 The fracture after cleaning
2.5.1 Macro Fracture Analysis
Three kind of cracks were found in the 4#blade: near the root, middle and near the variable cross-section of the top. 4-6 cracks were taken as the research object.
As shown in Fig. 10, the crack length of sample 4-6 is 56mm, showing penetrability, and the crack direction is perpendicular to the main stress direction. 4-6a and 4-6b fractures were obtained after opening along the crack, among which 4-6b fractures were analyzed macroscopically after cleaning corrosion products. Through the observation of stereoscope and magnifying glass, the origin area, expansion area and instantaneous fracture area of the cleavage pattern can be clearly defined. The whole fracture surface is relatively flat and smooth, without obvious macroscopic plastic deformation.
As shown in Fig.11, the fracture crack origin area is located on the back arc surface about 20 mm away from the steam inlet side.The surface is rough and grainy, and there are a lot of small corrosion pits on the outer surface of the metal in the source area at low magnification. The crack propagates radially from the center of the source region to the center of blade, forming a relatively bright propagation region. It is found that there are many concentric arc lines (shell pattern) perpendicular to the crack propagation direction. It is the trace of plastic deformation along the line before the crack transient, and it is also the most basic macroscopic feature of the fatigue fracture. It can be concluded that there is no obvious stress concentration in the fracture. With the local change of crack growth direction, there are many radial fatigue steps which are perpendicular to the fatigue arc, which are the macroscopic plastic deformation traces left by the intersection of two different height fatigue cracks. Before fracture cleaning, it was observed that there was obvious corrosion product coverage in the source area and expansion area (as shown in Fig. 13). When the crack propagates to the vicinity of the inner arc through the center, an arc instantaneous fracture zone with a length of 50 mm and a width of less than 3 mm appears. The surface is gray and rough fibers, which is shown as a shear oblique fracture generally in the instantaneous fracture zone of ductile materials. It is worth noting that the area of the instantaneous fracture zone is small, which does not cause the blade fracture to fly off.The crack propagation direction is generally from the back arc to the inner arc surface along the thickness direction.The instantaneous fracture zone is generated when the critical size in the thickness direction is reached.Because the area of the extended zone is much larger than that of the transient breaking area(Fig.11) , it can be inferred that the nominal stress of the fracture is relatively low.
2.5.2 Micro fracture analysis
The micro analysis of 4-6b fracture was carried out by SEM / EDS after confirming the basic situation of low power macro fracture.