Analysis of the failure of heat exchanger bolts
Natural fracture observation and analysis The fracture position of the failure bolt is distributed in the center of the plug without thread. The fracture is basically perpendicular to the axis of the bolt, and the section is slightly inclined without plastic deformation marks. There is a clear source of cracks on one side of the fracture and a higher shear lip on the other side (about 6 mm high). The entire fracture has a distinct brittle fracture feature. The fracture observation and analysis after washing are the front and side topography of the fracture after acid cleaning. The distribution of the fracture surface and the fracture side is clearly visible. The crack is initiated by the source of the crack on the outer surface of the bolt and extends as a dead branch toward the core of the bolt. There is a pattern in the whole fracture surface that is radially expanded from the surface crack source to the inside (the arrow in the figure is the surface crack source), which makes the fracture into three parts: crack initial formation zone, rapid expansion zone and instantaneous fault zone.
No original material defects were found in the transverse cross section and the longitudinal outer surface of the bolt after acid cleaning. Based on the results of the comprehensive macro-low test, the preliminary analysis shows that the bolt break time has been longer. The cracks causing the fracture originate from the outer surface of the bolt and expand radially toward the center of the bolt at a relatively fast speed. Chemical composition analysis was performed on the failed bolts for chemical composition analysis. The results of chemical analysis indicate that the chemical composition of the failed bolt does not meet the standard value of 25CrMo steel and meets the standard of 40Cr steel. Therefore, it can be confirmed that the steel for manufacturing bolts is 40Cr steel. The hardness test is performed at the fracture of the failed bolt and at the end of the test to test the Rockwell hardness.
It can be seen from the results of the hardness test that the hardness of the matrix of the failed bolt is too high as a bolt type fastener. This will reduce the toughness of the material. Metallographic examination will break the failure bolt in the central fracture table 2 Rockwell hardness test results of different parts of the test bolt test position near the fracture near the bolt end Rockwell hardness average HRC44.0 and the end of one end intercept the transverse and longitudinal specimens Metallographic examination. A and b of Fig. 5 are the transverse and longitudinal microstructures at the fracture, respectively, which are coarse tempered troostite structures. Plastic inclusions are distributed in the longitudinal direction.
The results of comprehensive metallographic examination show that the microstructure of the failure bolt is a coarse tempered troostite, the longitudinal structure contains a certain amount of strip-shaped plastic inclusions, and a large number of intergranular cracks are distributed near the fracture and on the section. These cracks originate from the surface of the bolt. Fracture micro-area energy spectrum analysis The X-ray energy spectrometer was used to analyze the micro-area composition of the natural fracture surface and crack of the failed bolt. Fig. 12 is a spectral line diagram of the surface energy spectrum scanning of the bolt fracture surface. It can be seen that the deposition material on the fracture surface is mainly iron and oxygen, but at the same time there are many elements such as S, Cl, Al and Si. This indicates that the surface of the bolt fracture surface has been severely rusted, and there is a large amount of iron oxide. A certain amount of sulfur and chloride ions must exist in the liquid medium that invades the fracture.
The result of the metallographic examination of the failed bolt is that the microstructure of the various parts of the bolt (both lateral and longitudinal) is a uniform tempered troostite structure. The longitudinal structure also contains a small amount of plastic inclusions that have been elongated and deformed in the rolling direction. There are a lot of cracks from the surface to the core at the fracture. These cracks are generated on the outer surface of the bolt. Microscopic observation shows that the crack originates from the bottom of the pitting pit on the surface of the bolt, and is radially or from the surface to the core. Wear crystal to stretch. The width of the crack is narrower, the angularity is more obvious, and there are no obvious decarburization and oxidation traces on both sides of the crack. The crack is filled with a gray medium. According to comprehensive observation, the crack has obvious characteristics of stress corrosion cracking.
The fracture scanning electron microscopy analysis of the failed bolt indicates that the fracture surface has been severely rusted and oxidized. This shows that the bolt breaks before the disassembly, and the fracture is a distinct intergranular fracture, which is a brittle fracture. The results of the energy spectrum analysis of the fractured micro-regions prove that the deposits on the bolt fractures and the gray matter filled in the surface cracks contain different amounts of oxygen, sulfur, chlorine, silicon, calcium, aluminum and other elements. This shows that in addition to the oxides of iron oxide and silicon, calcium, aluminum, etc., a large amount of sulfur and chlorine components are present in the crack. These substances are obviously deposited after the plucking oil in the shell invades the bolt crack. If harmful elements such as oxygen, sulfur, chlorine, etc. exist in the form of ions, it will become the main cause of corrosion of the bolt.
Conclusions (1) The shell bolts of the heat exchanger system are broken after serving for 2 years because the bolts are immersed in the oil of the extracting oil containing corrosive components for a long time, resulting in stress corrosion. The large tightening tensile stress and heat treatment quenching residual stress on the bolt and the S, Cl, OH- and NO3- ions contained in the environmental medium are the main factors causing stress corrosion on the bolt surface. (2) The tempered troostite structure of the bolt base itself and the higher hardness and brittleness reflected may increase the sensitivity of stress corrosion crack propagation. The operating temperature above 200 °C in the environment is also an important factor in promoting the development of corrosion cracks.
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