40. Made from 10cr steelGrade 9 wheel bolts, after being assembled into the automobile wheel hub, it was found that the wheel was shaking for about 1 month, and one bolt was found to be broken after being removed. The cause of fracture was analyzed by chemical composition analysis, metallographic analysis, electron microscopy analysis and other methods.
The results show that the fatigue fracture of the bolt may be related to the existence of small cracks in the cusp and the bottom of the thread on the surface of the bolt, and the crack defects in the tip and bottom of the thread may be the crack defects generated in the bolt machining process (such as wire rolling).
Bolts play the role of connection, fastening, positioning, sealing and other functions in the mechanical structure, in addition to the effect of axial preload tensile load, the working process may also be subject to additional axial tensile (alternating load), transverse shear (alternating) load or the effect of the bending load resulting from the compound, sometimes impact load.
Generally, the axial alternating load will lead to fatigue fracture of bolts, and the research points out that the fatigue failure process is the process of fatigue crack occurrence and propagation, and the occurrence and development of fatigue crack is first carried out in the place where the local stress of the material is concentrated.
40. Made from 10cr steelGrade 9 wheel bolts, assembled into the automobile hub after use for about 1 month after the wheel found shaking, after removing a bolt found to be broken, the bolt manufacturing process: raw materials pickling and drawing, spheroidization annealing, pickling and drawing, cold heading, wire rolling, heat treatment, electroplating, dehydrogenation.
The causes of fracture were analyzed by chemical composition analysis, metallographic analysis, electron microscopy analysis and other methods.
01. Physical and chemical inspection
1. Macroscopic morphology of bolt fracture.
The fracture position of the bolt is located at the thread position of the screw, the section of one end of the bolt fracture is yellow-brown because of serious wear and pollution, the fracture morphology can not be observed basically, the fracture section at the other end of the fracture is relatively flat, there is also wear and slight rust, but the shell-like stripes are still visible, which is a typical fatigue fracture morphology, and no other metallurgical defects are found, see Figure 1 and Figure 3. 
Figure 1 Schematic diagram of the location of the bolt break (the arrow indicates the bolt break).
Figure 2 One of the macroscopic morphologies of the bolt fracture fracture
Fig. 3 The second macroscopic morphology of the fracture (shell patterns can be seen in the cross-section).
2. Macroscopic morphology of bolt fracture.
After ultrasonic cleaning, the fracture of the fracture bolt was observed under the ZEISS EVO18 scanning electron microscope, and there was obvious shell-like fatigue glow morphology on the cross-section, the bolt fracture originated at the bolt teeth, there were obvious tear edges in the fracture source area, the microscopic fracture morphology of the fracture source area and the crack propagation zone was cleavage or quasi-cleavage fracture, and the micromorphology of the instantaneous fault area was dimple fracture, which were the basic characteristics of the fracture, as shown in Fig. 4 Fig. 5. 
a) The location of the source area of the bolt fracture.
b) There are tear ridges in the fracture source area.
Figure 4 One of the microscopic morphologies of the bolt fracture surface
a. Morphology of the fault source area;b. Morphology of crack propagation area;c. Morphology of the instantaneous fracture area) Figure 5 The second microscopic morphology of the bolt fracture surface.
On the longitudinal surface near the fracture of the fracture bolt, tear-like cracks are found on the thread surface at the fracture source, and small cracks are found at the intact thread teeth nearby, and the small cracks occupy most of the thread circumference, as shown in Fig. 6. 
a) Morphology of the longitudinal thread teeth in the source area of the bolt fracture.
b) There are small cracks at the bottom of the thread.
Fig. 6 Micromorphology of the longitudinal threads in the source area of the bolt fracture.
3. Chemical composition analysis.
Samples were taken on the fracture bolts and the chemical composition was analyzed using the QSN750 direct-reading spectrometer from the German superspectral company, and the results are shown in Table 1.
It can be seen that the chemical composition of the fractured bolt conforms to the chemical composition specification of 40CR steel in the GB T3077-2015 "Alloy Structural Steel" standard. 
Table 1 Chemical composition (wt%) of broken bolts
4. Metallographic structure and hardness inspection.
Longitudinal samples were taken near the fracture surface for metallographic examination, and it was found that there were about 2The 6mm long crack (crack 1 at the base of the tooth) corresponds to Figure A (crack 1) in Figure 6, there is no gray oxide in this crack, and there is no decarburization around it after being corroded by nitric acid alcohol
A small crack was found at the base of the second thread (crack 2), which corresponds to Figure A (fine crack 1) in Figure 6, and there is also a small crack at the tip of the thread, with a maximum depth of about 006mm, there are gray oxides in and around the small cracks at the bottom of the thread and the tip of the teeth, and the gray oxides in the cracks mainly contain "O", "Al" and "Fe" elements, which are actually iron oxides
After the corrosion of nitric acid alcohol, it was found that the small cracks at the base of the tooth and the small cracks at the tip of the tooth existed in the decarburization layer, and there were obvious tissue deformation streamlines around the cracks, and the matrix structure was tempered sostenite, as shown in Fig. 7 and Fig. 13. The Rockwell hardness test of the bolt shows that the hardness of the bolt thread is between 34 and 35HRC, and the hardness of the bolt matrix is between 36 and 39HRC.
Figure 7 Schematic diagram of a crack in the longitudinal section of a bolt.
a) Floor crack 1 magnified morphology.
b) Enlarged morphology of undertooth crack 2 Fig. 8 Microscopic morphology of bolt undertooth crack.
a) One of the cusp crack morphology.
b) Morphology of cusp crack 2 Fig. 9 Microscopic morphology of bolt cusp crack.
Fig. 10 Gray oxide spectroscopy analysis results in threaded cusp cracks.
Fig. 11 Structural morphology of the longitudinal section of the bolt.
a) Tissue morphology at the crack at the base of the tooth
b) Tissue morphology at the cusp crack.
Fig. 12 Morphology of bolt threads.
Figure 13 Microscopic morphology of the bolt matrix.
02. Technical analysis
1. The chemical composition analysis shows that the chemical composition of the bolt meets the chemical composition specification for 40Cr steel in the GB T 3077-2015 "Alloy Structural Steel" standard.
2. The macroscopic inspection and analysis shows that the fracture position of the bolt is located in the thread position of the screw, and the yellow-brown section at one end of the bolt fracture is due to serious wear and pollution, and the fracture morphology is basically unobservable, and the fracture section at the other end of the fracture is relatively flat, and there is also wear and slight rust, but the shell-like stripes can still be seen, which is a fatigue fracture feature, and no other metallurgical defects are found, indicating that the bolt belongs to fatigue fracture.
3. Scanning electron microscopy (SEM) examination shows that the bolt fracture originates from the bolt teeth, there are obvious tear edges in the fracture source area, the microscopic fracture morphology of the fracture source area and the crack propagation area is cleavage or quasi-cleavage fracture, and the micromorphology of the instantaneous fracture area is dimple fracture, which are the basic characteristics of fractureOn the longitudinal surface near the fracture of the fracture bolt, it was found that there were tear-like cracks on the thread surface at the fracture source, and small cracks were found at the thread teeth, and the small cracks occupied most of the thread circumference.
It can be shown that there are small defects at the bolt thread teeth, and in the process of bolt stress, the crack belongs to the stress concentration point, and with the use of the bolt, the fatigue crack is slowly formed here, and finally the fatigue fracture is caused, and the small crack at the thread is an important cause of the bolt breakage.
4. Metallographic examination and analysis show that there are about 26mm long crack, no gray oxide in this crack, after being corroded by nitric acid alcohol, there is no decarburization phenomenon around, this crack may be a stress cracking crack;Gray oxides exist in and around the small cracks found at the bottom and cusp of the thread, and the gray oxides in the cracks mainly contain "O", "Al" and "Fe" elements, which are actually iron oxides, indicating that the small cracks here have existed before the heat treatment of the bolt, and then after heat treatment, the cracks are oxidized to form iron oxideThere are obvious microstructure deformation streamlines around the crack, indicating that the processing deformation is large here.
5. The hardness test and analysis show that the hardness of the bolt thread is between 34 and 35hrc, and the hardness of the bolt matrix is between 36 and 39hrc, which is in line with 10The hardness requirements of grade 9 bolts indicate that the strength of the bolts is qualified.
6. To sum up: the bolt fracture is fatigue fracture, the fracture source originates from the surface, and the existence of small crack defects at the tip and bottom of the thread on the bolt surface is an important reason for the bolt fracture, and the crack defect causes stress concentration in the crack defect during the stress process, and the crack continues to expand and eventually leads to fracture.
Because the main cause of fatigue is the alternating load, the insufficient preload is the main cause of the increase of the alternating load, and the material defect is the cause of stress concentration, which is an important cause of early fatigue failure.
03. Screw Jun's experience and summary
1) The fracture of the bolt is fatigue fracture, the fracture source originates from the surface, the existence of fine crack defects in the thread tip and tooth bottom on the surface of the bolt is an important reason for the bolt fracture, the crack defect in the process of stress is caused by stress concentration, and the crack continues to expand and eventually leads to fracture.
The crack defects in the tip and bottom of the bolt thread may be the crack defect generated during the bolt thread rolling process.
2) It is recommended to improve the accuracy and quality of the bolt processing process to avoid the damage of the thread thread caused by the bolt thread rolling process.
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