A nuclear power fastening bolt is used to connect the drum filter and the spoke of the drum filter. The material is austenitic stainless steel, the specification is M24mm×140mm, and the mechanical performance grade is GB/T 3098.6-2014. Screws and studs are required for grades A4-70. During the overhaul, when the equipment was inspected, five bolts were found missing, two bolts were broken, and most of the bolts were loose. The location of the broken bolt on-site assembly is shown in Figure 1. In order to find out the cause of the failure of the connecting bolt and prevent the breakage from happening again, the author has inspected and analyzed it.
1 Physical and chemical testing
1.1 Macro inspection
Figure 2 shows the macroscopic shape of the broken bolt. It can be seen that the nut has been lost. Compared with the complete bolt, both bolts are broken at the joint between the bolt head and the polished rod. The surface of the fracture and the polished rod are covered with yellow-brown corrosion products. Figure 3 shows the macroscopic appearance of the bolt fracture after cleaning. It can be seen from Fig. 3 that the two fractures are similar in shape, the section is flush, and almost no plastic deformation is observed; the middle part of the fracture is a short-cut zone, which is small in area, slightly convex and elongated; the source of the fracture is outside the bolt On the surface, there are more radial steps distributed on the edge of the fracture, and the whole fracture has obvious multi-source bidirectional fatigue fracture characteristics [1].
1.2 General physical and chemical testing
Samples were taken on the fracture bolts for chemical analysis, room temperature stretching, non-metallic inclusions, microstructure, etc. Tables 1 and 2 are the results of chemical composition analysis and room temperature tensile test, respectively. The content of non-metallic inclusions is evaluated according to the method A of GB/T 10561-2005 "Determination of the content of non-metallic inclusions in steel--the standard rating chart microscopic test method".
It can be seen from Table 1 and Table 2 that the chemical composition of the fracture bolt and the tensile properties at room temperature are in compliance with the technical requirements of GB/T 3098.6-2014.
Figure 3 Macroscopic appearance of the fracture
Figure 4: Non-metallic inclusion morphology of broken bolts
It can be seen from Fig. 4 that there are many non-metallic inclusions in both bolt structures. Among them, the black strip-shaped inclusions are silicate inclusions, and the results of non-metallic inclusions are: A1, B0, C3e, D2. Figure 5 shows the microstructure of the fracture bolt, the etchant is aqua regia, and the inspection equipment is the Zeiss Axiovert 200MAT inverted universal material microscope. It can be seen from Fig. 5 that the bolt microstructure is austenite + a small amount of ferrite, and no obvious tissue abnormality is observed. Samples were taken at the joint of the unbroken bolt head and the polished rod of the same batch to make a metallographic sample and observed under a microscope, as shown in Fig. 6. It can be seen that the transition fillet of the bolt head and the polished rod is obtuse, and the transition is smooth, the processing flow line shape is good, and no obvious structural abnormality is found.
Figure 5 Microstructure of broken bolts
1.3 Fracture micro analysis
The fracture profiles of the two failure bolts were similar. The failure bolt 1 fracture was placed under a TESCAN VEGA 5136 scanning electron microscope (SEM), as shown in Fig. 7. Radial steps can be seen in the crack initiation zone of the fracture edge, which is a multi-source fault, see Figure 7a); there are obvious fatigue bands in the extension zone on both sides, showing obvious fatigue fracture characteristics [2], see Figure 7b); Small, microscopic appearance of dimples, see Figure 7c). Figure 8 shows the results of the yellow-brown corrosion product energy spectrum (EDS) analysis near the fracture. It can be seen that the corrosion products are mainly iron oxides, and no corrosive elements are present, indicating that corrosion is not the main cause of fatigue fracture of bolts [3].
2 Comprehensive analysis
It can be seen from the above physical and chemical test results that the chemical composition of the failed bolt and the tensile properties at room temperature are in accordance with the technical requirements of GB/T 3098.6-2014; the microstructure of the bolt is austenite + a small amount of ferrite, and there are more non-metals in the structure. Inclusions, especially silicate inclusions, are high in content and some inclusions are oversized. Silicate inclusions are brittle inclusions, and there are large differences in physical properties and deformability between steel matrix and steel, which destroys the uniform continuity of the steel matrix and reduces the mechanical properties of the steel, especially the reduction. The plasticity, toughness and fatigue limit of steel [4].
When the head of the bolt is formed, the structure and inclusions at the joint between the head and the polished rod tend to be laterally distributed as the head is thickened, which reduces the overall mechanical properties of the joint, and the deformation ability of the inclusion is poor. Micro-cracks are formed at the interface between steel and inclusions. These micro-cracks become the source of fatigue failure when the bolts are stressed [5]. If dense inclusions are exposed to the surface of the bolt, the formation and expansion of fatigue cracks will be accelerated. The energy spectrum analysis of the corrosion products near the fracture showed that no corrosive elements were present, indicating that corrosion is not the main cause of bolt fracture.
Bolt fracture analysis results show that the bolt fracture form is multi-source two-way fatigue fracture. Combined with on-site inspection and maintenance, most of the bolts of the equipment have been loosened. When the equipment rotates, the spokes of the drum filter slide down, causing large shear stress on the bolts, and the cycle reciprocates, eventually leading to two-way fatigue fracture of the bolts [6] . There are many possible causes of loose bolts. Vibration, high and low load changes, impact during installation, low preload force during installation, improper anti-loose measures, improper assembly methods, etc. may cause loose bolts. [7].
Fig. 8 SEM morphology and EDS spectrum of corrosion products on fracture surface
3 Conclusions and recommendations
The bolt fracture form is multi-source two-way fatigue fracture. The main cause of bolt breakage is that after the bolt is loosened, the drum filter spoke slides up and down during the rotation of the equipment, which causes the bolt to withstand large shear stress, and finally leads to two-way fatigue fracture of the bolt; a large amount of ultra-sized silicic acid in the bolt structure Salt inclusions accelerate the formation and expansion of fatigue cracks to some extent. It is recommended to improve the quality of bolt raw material production, and control the content and size of non-metallic inclusions in the raw material structure to a reasonable level; the bolts should be installed in accordance with certain tightening order principles, and take anti-loose measures, such as adding lock washers, and regularly Torque check and tightening.
references:
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[2] Meng Wenhua, Zeng Weichuan, Gu Jingqing, et al. Failure analysis of early fatigue fracture of 10.9 bolts [J]. Physical and Chemical Testing (Physical Section), 2017, 53(4): 365-367.
[3] Jiang Aihua, Chen Liang, Shi Hongqi, et al. Failure analysis of bolt fatigue fracture [J]. Thermal Processing Technology, 2013, 42(2): 222-223.
[4] Wang Lei, Chen Xueguang, Yan Haixiang. Failure analysis of high strength bolts for diesel engines [J]. Physical and Chemical Testing (Physical Section), 2016, 52(6): 431-434
[5] Ding Huilin, Jin Rongfang. Mechanical parts defects, failure analysis and examples [M]. Beijing: Chemical Industry Press, 2013.
[6] Bao Xigui. Analysis of the cause of high-strength bolt fracture of an excavator [J]. Physical and Chemical Testing (Physical Section), 2017, 63(3): 204-207.
[7] Xie Yancheng. Analysis of the reasons for loosening of assembly bolts and anti-loose measures [J]. Equipment Manufacturing Technology, 2015(4)
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