Metallographic, Hardness Testing and Impact Toughness Although the specific heat treatment process of the failed bolt is not clear, it should be used as a high strength bolt depending on the material and condition of use. The heat treatment process should be quenched and tempered (quenched high temperature tempering), which is a tempered sorbite structure, in order to make the material have good toughness. Therefore, the metallurgical structure, hardness test and impact toughness test are performed on the failed bolt to check whether the heat treatment process of the failed bolt is appropriate. The metallographic sample was directly taken on the failed bolt and the microstructure was examined. The results show that the metallographic structure of the bolt is tempered sorbite. It is a photo of its metallography. It can be seen that the impact absorption work is much higher than the technical requirements, and the hardness value is at the lower limit of its technical requirements. The results show that the bolt material is very good. It is the macroscopic and microscopic appearance of the impact fracture. It can be seen that the entire section of the impact fracture is dark and fibrous, consisting of dimples. The impact fracture morphology is completely different from the fracture bolt fracture morphology, showing the characteristics of ductile fracture.
Based on the above analysis, the chemical composition of the broken bolt meets the standard composition of 35CrMoA steel. The metallographic structure is tempered sorbite. The hardness value and impact toughness are in line with the technical requirements, indicating that the heat treatment process is normal. Since the crack propagation zone of the fracture has typical characteristics of beach line and striation, etc., and the energy spectrum analysis does not find any corrosion elements at the fracture, it can be ruled out that the fracture is caused by corrosion, that is, no corrosion fatigue fracture occurs. It is simply a fatigue fracture caused by stress concentration. According to the service state of the bolt described herein, the bolt is always subjected to alternating tensile stress and is long (8 years). Moreover, the stress at the root of the bolt itself is concentrated, and many factors may become sources of cracks (such as machining defects, inclusions, etc.) and form multi-source fatigue fracture.
The breaking position of the four bolts is on the thread of the joint of the tightening nut and the device. This is because the axial tension distribution on the bolt is uneven after the nut is tightened, and the axial force is screwed in each thread. The distribution between the two is a hyperbolic cosine function, and the axial force of the first thread is the largest, and then decreases. This just shows that the two fractures of the bolts A and B are different in axial force due to the fact that the bolts are finally disconnected from A first. Statistics show that 65% of the bolt breakage occurs on the thread where the fastening nut and the device are joined.
The twelve bolts used for the cylinder head fastening should theoretically be uniform in stress, but in practice it is impossible. There is always a certain stress that is greater than the other roots. At the same time, although the material and heat treatment process of the bolt are the same, there are also differences (machining defects, inclusion distribution, etc.). Therefore, those bolts with large or defective forces are prone to break first, and their fractures tend to increase. The load on other unbroken bolts, which in turn causes other bolts to break. Moreover, experience has shown that when the device is started and stopped, the bolt is broken due to the sharp change of the stress.
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