There are parallel fatigue steps 608 bolts on the grain boundary to open the burn marks of the new fracture source area. The fracture originates from transgranularity, followed by the intergranular features, fatigue fringes along the crystal plane, and the near-source region is relatively flat and unstable. The undulations are larger. The burnt trace of the new fracture source area is 410. The bolt break is a double source fatigue fracture. The small and delicate source area (right side) is macroscopically characterized by the crystal, and the source area fracture characteristic with the outer surface is under the slow alternating stress. It is destroyed as a fatigue platform, the secondary surface of the source region is characterized by crystal, and some of the crystal features become insufficiently clear in the fatigue expansion. The subsurface has fatigue fringes along the grain boundary surface, and the fatigue fringes along the crystal dimple are clearly visible. The walls of the strips are elongated, the instability and the instantaneous break zone are intergranular and secondary cracks; the cracks in the larger source area (left side) originate from the external surface, and the fatigue platform can be seen at the burn site. The wear damage marks and the intergranular features, the extended area is the intergranular fracture, and no fatigue streaks and corrosion marks are found.
According to the comprehensive analysis, the 410 bolt fracture is double source fatigue, and the small source area is first cracked. Then, the second fatigue source area is generated due to stress concentration in the burned area, and no surface defects such as burns are found in the small source area. The metallographic analysis is the metallographic structure of the 608 bolt associated crack sample, which is C+C' phase structure. The twinning characteristics are obvious, and the associated cracks develop along the grain boundary, which is consistent with the results of scanning electron microscopy. 608 bolt metallographic structure 410 bolt metallographic structure, C + C ' tissue, crystal clear, normal organization.
Chemical composition analysis Chemical composition analysis of 608, 410 bolt samples, the test results show that the bolt components meet the standard requirements. Mechanical performance analysis The hardness of the broken bolt is tested and basically conforms to the standard. Bolt normal temperature mechanical performance test results show that it meets GH4145/SQ performance indicators. GH4145/SQ does not require impact energy, and its value is low from the test results.
With reference to the requirements of the R-26 alloy, the two bolts were subjected to a combined long-term test. The test temperature was 566 ° C, the stress was 603 MPa, and the fracture was 101 h. The sample was subjected to a high temperature tensile test at 566 ° C for the test results. It can be seen that the tensile strength Rb and the yield strength Rs are higher than the room temperature result, which is higher than the 570 °C test data of the SAIC plant, but the elongation profile shrinkage rate is lower than the 570 °C high temperature mechanical property test data of the SAIC plant, which may have experienced with the sample. Relevant to the enduring test.
The results of energy spectrum analysis show that the content of elements in the fracture parts is not much different. Corrosive elemental sulfur is slightly higher than the 4,5 parts of the 410 bolt fine area, and the other parts are about 1.5%, near the fine area where corrosion may occur. No corrosion characteristics were observed in the analysis, and the fatigue characteristics were typical, and the corrosion effects were excluded. Scanning electron microscopy analysis shows that surface burns promote the generation of fatigue, which provides conditions for the formation of fatigue crack sources, which may be arc spots generated during spectral analysis.
Conclusions and recommendations (1) The cause of bolt breakage is fatigue damage. (2) It is recommended to further improve the supervision and inspection of the unit vibration and the strengthening of the bolt fastening process, and change the spectrum and hardness inspection parts to the end of the bolt. (3) Because bolt breakage involves materials, fastening, inspection, etc., further research is recommended.
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