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.