Fig. 4 A:Cracks on the top of blade,B:Cracks in the middle of blade,C:Cracks at blade root
Through inspection, it is found that there is corrosion of different severity on the surface of blades, and there are a large number of obvious corrosion pits, the second and last stage blades are the most serious. Water erosion was also found on the last stage blade (Fig. 3). The water erosion on the edge of the blade has both physical and chemical effects. The defect appears near the top of the inlet side and the root of the outlet side, with a depth of less than 1mm. There are 80 cracks near the root, near the top and in the middle of the blade back arc in 98 moving blades of the last stage of low pressure rotor. According to the location and general shape of the cracks, it can be divided into three types: the first type cracks of near root(Fig. 4 C): mainly distributed in the back root of the blade opposite to the steam face, the cracks are relatively obvious, some cracks are visible to the naked eye after sandblasting, and the trend is vertical to the direction of the maximum normal stress. The second kind of cracks in the middle of blade back arc(Fig. 4 B): the cracks are relatively thin, short and shallow, all appear on the back arc surface, which can be found through magnetic particle inspection, and the direction is perpendicular to the direction of the maximum normal stress. The third kind of near blade top crack(Fig. 4 A): mainly distributed in the variable cross-section of blade tip, and the direction of crack is generally perpendicular to the direction of maximum normal stress. This kind of crack is mainly found in the forward blade of the second last stage.
In this paper, the broken 4 # blade is taken as the research object.