2 Test results and analysis 2.1 Macroscopic fracture analysis The blade fracture location is at the root of the impeller, 1.2 is the macroscopic photograph of the fracture of the fractured blade, and several fatigue arcs can be clearly seen. There is a clearly distinguishable characteristic region fatigue fracture source on the fracture. In the zone extension zone and the instantaneous zone, the middle arrow refers to the starting position of the fatigue crack. The expansion area accounts for about 90 of the fracture area, and the area of ​​the instantaneous fault area is the smallest. Combined with the high speed analysis during operation, it is known that the fracture form is high cycle fatigue fracture 0, and the stress level it receives is low, and no short-term additional large is found. stress. 3 is the partial enlargement of the source region, the fatigue arc is clear, and the center of the arc is the starting position of the fatigue crack. At this time, the crack is about 6 mm long and 2.5 mm deep. So far, the fatigue crack has formed, and then the crack growth rate will be accelerated.
2.2 Micro-fracture analysis makes the base surname to microscopic observation of 4 regions, respectively, 5, 7.5 is the instantaneous fault zone, the dimple region 6 is the extended region fatigue band region 2. The source region is enlarged 7, the region 3 is found There is an inherent crack in it and it is exposed to the surface, and the fatigue crack starts between the two cracks. For the 7-position magnification, the dense tissue 8 is not dense, and there is no fracture feature.
The composition analysis of cracks and normal areas of fractures is not carried out. The results show that the oxygen content of the dense tissue in the Mingyuan area is higher than that in the normal area. The carbon and oxygen content in the crack is much higher than that in the normal area, and the chromium content is very low. It can be judged that corrosion has occurred in the source area.
The fracture sample was tilted to observe the outer shape of the crack initiation position in the source region. It was found that there were a large number of corrosion pits 9 near the source region, and the energy spectrum analysis was performed on the normal region and the etch pit region respectively. As a result, the etch pit contained a large amount of oxygen. , indicating corrosion products. The etch pit is formed during use and is related to the blade material itself and the environmental medium.
Observation of the fracture near the source region revealed that there was a built-in crack 10 of about 0.5 mm on the left side of the source region, and there were a large number of microcracks and voids. The secondary source 12 is observed on the right side of the source region, and there are also metallurgical defects such as voids and microcracks, which are inherent to the material itself and have nothing to do with use.
There are cracked voids and undensified structures in the source region of the fatigue fracture to form a defect group. Such defects are often allowed in the position where the internal force of the impeller is not strong, and such defects are located at the root of the blade with the greatest force. It is very dangerous and fatigue cracks often sprout in this area. There are a large number of pitting pits on the side of the source area, and the pitting pits are mixed with the inherent defects of the material to form a defect group. The defect group seriously destroys the integrity of the blade surface and generates stress concentration.
2.3 Metallographic analysis A metallographic sample was cut at a distance of about 10 mm, 20 mm and 70 mm from the fracture. It was observed that the crystal grains were all visible to the naked eye, the grain size was 23, the crystal grains were coarse, and the grain boundaries were coarse. Most of the organization is coarse martensite, as well as ferrite and austenite 13, and there is microscopic inhomogeneity.
SEM observation of the metallographic samples revealed that there were a large number of chain-like bubbles l4 in the leaves, and the maximum diameter of the bubbles was about 15 μm. The minimum spacing of the bubbles was about 20, and grain boundary cracks and segregation 15 were also found.
2.4 Surface analysis The side of the sample was observed, and it was found that in addition to the machined tool marks and the marks being hit, cracks 16 and a large number of corrosion pits were present.
The etch pit is roughly divided into two types, which are shallower in corrosion, 17 is the initial stage of pitting corrosion; the other type of deep pitting is an electrochemical corrosion process. Pitting corrosion must have a condition 1 local failure of the stainless steel passivation film; 2 has a relatively high degree of shame, the pitting resistance of the child is not related to the uniformity of the surface of the surface. The metallographic structure of the material plays a key role in the pitting resistance of the impeller. Various defects in the material, such as non-metallic inclusions and metal structure inhomogeneity, have a significant impact on pitting corrosion. Pitting corrosion and environmental relationship The concentrated water collected on site is chemically analyzed, and the test results are as follows 2.
For martensitic stainless steel, the concentration of 1 is 0, which is a very low value. In the air of the city after rain, the concentration of the tap water is generally greater than 10. Under normal conditions, the impeller blade will not be pitting. 3. Working environment In the medium, some common anions have a corrosion inhibition effect on pitting. 47 Because of their existence, the stainless steel is pitting and the concentration is increased. If these anions are present in the dish 33, 8, 42, 032, The effect of increasing the pitting initiation potential, prolonging the incubation period and slowing the pitting rate. Therefore, the presence of 8,42 has an inhibitory effect on pitting corrosion of the impeller blades. At the same time; there is little effect on the pitting of 5 pairs of materials.
Only the stage impeller blades that have been in operation for a long time have produced obvious pitting corrosion. If the cause is not in the environment, it is sure to ask the pitting resistance of the material of the impeller blade.
3.3 The relationship between pitting and material of the blade The martensitic stainless steel 841500 has good comprehensive properties, moderate strength and toughness, good high temperature stability and good corrosion resistance. It is the first choice for the manufacture of impeller materials. However, in addition to the strict requirements on the composition of steel, martensitic stainless steel has strict specifications for the control of smelting thermal processing including heat treatment parameters.
Porosity is a common defect of steel in smelting. Their existence reduces the compactness of the material. If it is on the surface of the impeller, it becomes the anode of pitting corrosion. It is dissolved and becomes an etch pit. The mixture of metallurgical defects of the pit becomes a defect group. Great damage to the integrity of the blade surface. If the position of this defect group is at the maximum force at the root of the blade, it becomes the initiation zone of fatigue damage.
The grain size of the blade material is controllable. If the blade is expected to have good high temperature durability and high temperature creep properties, it is often desirable to have larger grains. The working temperature of the impeller is about 1001351. For the martensitic stainless steel, about 13,1 is not high temperature. If the process specification requires the impeller to be processed into 23 grades, it is not necessary.
The coarse grain itself does not have much influence on the pitting resistance of the impeller, but the grain coarseness is accompanied by grain boundary coarsening. The grain boundary is the position where the precipitation phase of the impurity defects in the material gathers. In particular, 306 is segregated at the grain boundary, which makes the periphery of the grain boundary lean in chromium, and reduces the corrosion resistance of the material. The grain boundary exposed on the blade surface becomes the anode, which is surrounded by a cathode, and the anode dissolves to form a pitted pit.
In summary, the reason for the pitting corrosion of the 1 blade is that there are dense metallurgical defects and hot processing defects in the material; 2 the concentration is at a normal value relative to the pitting resistance of the 841,500 martensitic stainless steel.
3.4 Fatigue crack initiation The occurrence of fatigue cracks in the impeller blades depends on the magnitude of the applied load and the load carrying capacity of the blade. The maximum applied stress of the impeller blades is at the root of the blade. From the point of view of force, crack initiation occurs mostly at the root of the impeller blade. + Impeller blade load carrying capacity depends on the material's basic mechanical properties tensile strength and material surface integrity. Scratches on the blade surface machining tooling marks material metallurgical defects, thermal processing defects and other defects will destroy the integrity of the impeller blade surface, and form local stress concentration. The magnitude of the stress concentration depends on the shape and size of the defect and the composition of the defect group. The distribution of defects on the blades is random. When the defect is located at the maximum stress at the root of the blade, it becomes the best position for fatigue crack initiation.
Pitting has a gestation process and requires a fixed period of time. In many cases, the incubation period is very long. The defects of the exposed materials will participate in and affect the cracks earlier and more directly. The formation and development of pitting will expand the defects and form defect groups, which will accelerate the formation of fatigue cracks.
4 Conclusion The blade fracture is fatigue fracture.
Metallurgical defects and thermal processing defects in materials are the main causes of pitting corrosion and fatigue fracture.
Xiao Jimei. The metal of stainless steel is awkward. Beijing Metallurgical Industry Press, 2006.
Zhang Dekang. Local corrosion of stainless steel. Beijing Science Press, 1982.
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