Cause analysis and treatment of blade fracture of

Cause analysis and treatment of the fracture of circulating water pump blades

Zhongshan Hengmen power plant is located in the Hengmen waterway of Zhongshan City, the Pearl River port of Guangdong Province. Four circulating water pumps are responsible for the cooling water and chemical water production of two 125MW Steam turbine generator units. At the same time, they also provide the fire protection, living, greening, environmental protection and other aspects of the whole power plant, but it needs to adopt the method of using this kind of waste. The circulating water pump of the plant is 1200hlcb3.2-16.5 vertical mixed flow pump, with design parameters of h=16.5m, q=11520m3/h, n=485r/min, and supporting motor power of n=710kw. In May, 1997, the circulating water pumps a and B of unit 1 were officially put into operation, and the circulating water pumps a and B of unit 2 were also put into operation at the end of the same year

1 blade fracture condition

, the impeller blade of circulating water pump a of unit 1 broke during operation. By November 25, four circulating water pumps had blade fracture accidents in a short 49 days

after the failure of circulating water pump a of unit 1, a preliminary analysis of the steps that need to be prepared by the user was carried out. The working medium of the pump is sea water and river water. The seawater reverse irrigation period is from winter to spring of the next year, during which the mass fraction of chloride ion in river water is as high as (2 ~ 10) ×， The situation was more serious from 2003 to 2004. The design material of the pump impeller hub is 2gr13, the impeller blade material is 1gr18ni9ti, the impeller blade length is 270mm, the width is

350mm, the blade end face thickness is 20mm, the foundation thickness is

20mm, and the root welding width is 180mm. A total of 7 blades form the impeller group. During the operation, it was found that the vibration of the upper guide bearing of the pump motor suddenly increased and the pump was stopped in an emergency. After disassembly and inspection, it was found that the impeller blade of the pump was broken. The fracture was at the root of the blade, bright white, with metallic luster, and there was an obvious "herringbone" pattern on the cross section, which was brittle fracture. Except for one broken piece, the roots of five of the other six pieces have obvious fracture sources of different depths. The microstructure of the stress surface of 7 blades was analyzed. Except for 1, the other 6 blades had crisscross tortoise cracks. From the above, it can be judged that the reason for the fracture of circulating water pump blades is the comprehensive effect of typical austenitic stainless steel by intergranular corrosion, weld hot crack and electrochemistry

2 fracture cause analysis

2.1 alloy element depletion

because the crystal boundary volume of chromium nickel austenitic stainless steel (i.e. the sample is continuously in the jaw, in the jaw, in the parallel section or outside the gauge distance) is easy to precipitate the second phase of carbide, resulting in the impoverishment of a component at the grain boundary. For example, 1gr18ni9ti stainless steel precipitates the precipitated phase gr23c6 at the grain boundary, leaving a chromium poor area near the grain boundary. Because the heating and cooling of this kind of steel do not occur α-γ Phase transformation, no quenching and strengthening, low strength and hardness. When it is kept warm at 450 ~ 850 ℃ or cooled slowly, and then exposed to a certain corrosive medium for a certain time, intergranular corrosion will occur. When it is heated at 650 ~ 750 ℃ for a certain time, this kind of steel is more sensitive to intergranular corrosion. General citizens can check "avoid extrusion" in advance and believe that gr23c6 will precipitate along the austenite grain boundary within the above temperature range, Thus, the chromium content in the area near the austenite grain boundary is less than 11.7%. However, the width of the chromium poor zone is very narrow. For example, 18 ~ 8 austenitic stainless steel is sensitized at 650 ℃ for 2h, and the total width of the chromium poor zone is 150 ~ 200nm, of which the width of the severely chromium poor zone is less than 50nm, resulting in corrosion near the austenitic grain boundary. Therefore, the mass fraction of chromium in the stainless steel body shall not be less than 11.7%. The circulating water pump blade of the plant is made of austenitic 1gr18ni9ti

stainless steel. During the manufacturing and processing of the blade, if the blade stays in the range of 450 ~ 850 ℃ for a certain time, it will promote the precipitation of [fe, gr]c at the grain boundary. The chromium in it mainly comes from the surface layer of the grain, and the internal chromium is too late to supplement, which will reduce the chromium content of the surface layer of the grain boundary and form a chromium poor area. The chromium content in the chromium poor area is far lower than the limit value required for passivation, and its potential is lower than the potential inside the grain, and even lower than the potential of carbide. Chromium poor area is closely connected with carbides, and short circuit battery effect will occur when encountering a certain corrosive medium. The circulating water pump impeller of the plant operates in the river water with high chloride ion content for a long time. In this case, chromium carbide and grains are cathode, so that the chromium poor area with anode is rapidly eroded, resulting in the decline of mechanical properties and fracture along the grain boundary when stressed