Fatigue fracture of vacuum pump connecting rod bolt and circulating machine piston rod

Type vacuum pump connecting rod bolt failure analysis. A vacuum pump that is purchased by a factory. After the use of the pump, there will be a breakage of the connecting rod and the side connecting rod bolt. After a month, the connecting rod and connecting rod bolt of the vacuum pump will occur. Break, for which failure analysis is performed.

1.1 The macroscopic form of the bolt breakage of the connecting rod bolt occurs in the middle, that is, there is no obvious plastic deformation at the joint of the big head cover bush of the connecting rod, and the macroscopic shape of the fracture is a brittle flat fracture.

The crack source occurred at six places near the surface where there were scratch marks. There are two obvious steps in the crack source area. At 18 places, there is a distinct bead-shaped arc. The shell pattern spreads forward in a wave shape centering on the crack source. This is the fatigue fringe, so it is a fracture caused by fatigue. Radial ridges appear from the fatigue zone, and the radial direction is parallel to the crack propagation direction and reversely points to the crack source. This indicates that the crack is transformed from slow expansion to rapid unstable expansion, and finally, when the cross-sectional area is about 20, The crack spreads close to the surface, at which point the plane stress state predominates, and two pairs of shear lip shadow regions appear instantaneously.

As the bolt begins to deform and causes deformation, the stress suddenly rises, so the fatigue breaks the people smaller, and the instantaneous damage is large.

1.2 Micro-morphology of the connecting rod bolt fracture The fatigue source area is the fatigue pattern. The fatigue expansion area has the radiation mark with the focus of the fatigue source. You can see the brittle fracture of many long-shaped tearing ridges, similar to the quasi-cleavage fracture. Morphology.

The fracture morphology of the rapid expansion zone has many fracture facets and secondary cracks. In the termination zone, the stress amplitude meets or exceeds the yield limit of the material, and the microscopic features of the fracture are tear-type micro-pits.

1.3 The hardness and metallographic analysis of the components of the screw inspection. The chemical composition of the bolt contains carbon 4 containing chromium 93, the corresponding material is 40, the alkyne is captured, the material is quenched and tempered, and the metallographic structure is Body, in some areas, there is less flaky ferrite. There are few non-metallic inclusions in the material, and there is no quenching crack on the surface. Fatigue cracks ± if the crystal is expanded. The ends of the cracks are relatively round and blunt.

1.4 The connecting rod big head cover and connecting rod bolts were observed firstly. It was found that the bolt fracture was a flat fracture without macroscopic plastic deformation, and there was a shell pattern on the top. The final fracture part was formed by shearing, and it was 45 with the flat fracture. It is a typical fatigue fracture. However, the macroscopic deformation of the broken cap of the connecting rod is not large, similar to the impact fracture.

Therefore, the bolt is broken first, causing the connecting rod cover to loosen, causing the connecting rod cover to hit the cylinder and the like to be broken.

Fracture failure analysis of 1.5 èžµ bolts The fatigue strength of the material is insufficient. The connecting bolts are subjected to alternating stress such as tension and shear, and the surface is concentrated by the stress of the dam to form a fatigue core. The fatigue occurs in the region where the local stress is the highest and the strength of the circumference is the lowest. The cyclic loading causes the metal surface to produce a slip zone. The micro-crack develops along the main slip of the slip belt, which makes the effective section of the bolt gradually decrease. When the remaining section can not bear the action stress, the bolt is broken. Therefore the fatigue strength of the material is highly dependent on the yield strength. When the yield strength of the material is low, it is easy to prematurely slip the belt during the fatigue process, and initiate fatigue cracks at the residence slip zone or the extrusion extrusion.

Mast bolts are often treated with 40 times of quenching and tempering, which is considered to have the best overall mechanical properties, but is often ineffective due to fatigue fracture. According to the measurement, the chemical composition and hardness of the bolt bolts basically conform to the fatigue limit of the quenched and tempered parts. 7 Whether the surface strength and hardness of the material can be appropriately increased by heat treatment. If the hardness is increased from æ›°, 23 to æ›°, 0, it can also increase the left heart. Because the fatigue limit of steel is 70.287 when pulling, and the fatigue limit of 0.24 when twisting, the corresponding fatigue limit can be increased by about double.

Of course, the strength and toughness index of the material usually has 1 shield, so the fatigue resistance of the material must be improved as much as possible while ensuring the necessary toughness. In addition, the surface-carburizing surface thin shell quenching surface rolling and surface, light treatment and other strengthening methods can also improve the fatigue resistance of the bolt.

It may be that the bolt is too tight and the pre-tightening force is too large to cause the bolt to act as a fastener. The pre-tightening force should normally be the load of the bolt when the equipment 1 is used to ensure the fastening state of the coupling body; the pre-tightening force should generally be the yield strength of the small bolt. To pre-tighten the household. Tighten the bolts to withstand tensile stresses and torsional shear stresses. When the pre-tightening force is too large, etc. 1 increases the applied load. If it is a bolt subjected to axial alternating load, fatigue damage is likely to occur at this time.

If the breakage of the connecting rod bolt is only for material reasons, the two bolts on the connecting rod cover will be damaged to varying degrees. Now it is only broken, this bolt is too tight, causing an average stress of 7, increase, the strength of the bolt is insufficient, the possibility of fatigue fracture is the most.

Therefore, the nut should be tightened to the specified torque. When the residual elongation exceeds 0.20. Bolt degree, the connecting rod bolt must be replaced.

It may be improperly installed by the D-Bolt, which may cause additional bending stress when the eccentric load is improperly installed or if there is an eccentric load in the process, causing excessive stress. If the head of the bolt head and the end face of the nut hook are not evenly contacted, the bolt can be subjected to additional bending than the number of normal axial loads and the axial contact color can be used to check the contact between the bolt head and the end face of the nut and the bearing surface. The length of the arc where the contact point is broken must not exceed 18.

If the gap is not suitable when the connecting rod is installed, such as the phenomenon of overheating and burning the shaft, the working stress of the connecting rod bolt will increase, causing the bolt to fatigue fracture.

1.6 Conclusion According to the macroscopic and microscopic features of the fracture, the fracture of the connecting rod bolt is a high-cycle fatigue fracture.

The reason for the fracture is that the strength reserve of the butyl material is insufficient. However, it is subject to heavy impact fatigue load. Therefore, if the pre-tightening force is too large and there may be eccentric load, it is easy to cause the 1 stress to be greater than the fatigue limit, and thus the early fatigue fracture occurs.

The method of improving the heat treatment conditions to increase the surface strength can be used to improve the fatigue resistance of the material; at the same time, it is necessary to pay attention to proper preload and prevent eccentric load during installation, and also pay attention to the gap of the connecting rod during installation.

2 Failure Analysis of Piston Rod Fracture Accident of No.5 Cycle Machine The piston rod of the head of the No.5 cycle machine of a factory has suddenly broken, and the high-pressure airflow rushed out to cause fire, resulting in local short-term shutdown. The piston rod is made of 380 and the actual use time is 365 days. Qualified by magnetic particle inspection before use. According to the production operation record, the production operation before the fracture was normal, and no abnormality such as pressure or fluctuation of the medium composition was found. Failure analysis of the broken piston rod is now carried out.

2.1 Fracture morphology and electron microscopy analysis The fracture site is in the threaded part of the piston rod piston. The root fracture of the last tooth has no obvious plastic deformation, and the macroscopic shape is brittle flat fracture. The basic shape is caused by mechanical buffing caused by multiple times of lower cracks at the initial stage of the fracture; there are obvious corrugated shells at 8 places; the area is a stable extension of cracks, the surface is flat, and the fracture is gray The radiant ridge line is faintly visible on the upper side, and the two sides are more obvious than the middle, showing the direction of the crack; there are several obvious arcs F at the instantaneous break zone, which is about 43 of the section, and the surface is rough and dark gray.

The fracture morphology was analyzed by scanning electron microscopy with 0, 5, and 2. The metal structure of the crack source area is severely broken, and the fatigue expansion area of ​​the crack source has obvious shell-like arc. The existence of the fatigue groove line can be seen in the crack propagation zone, and the micro fracture morphology is a quasi-cleavage fracture with a small number of secondary cracks. The microscopic feature of the fracture of the fast-breaking zone is the deformation of the initial pit, which is microscopically fractured.

2.2 Metallographic examination A sample was taken for metallographic analysis near the fracture of the piston rod. After etching, the metallographic structure was a tempered sorbite with orientation.

The metallographic structure of the crack source area is more complicated, mainly tempered sorbite, but also a small amount of white ferrite, and the grains are finer.

2.3 Thread projection test Using a 1-blade six large-scale microscope to project the thread, it was found that the root of the thread was smooth, not sharply cut and scratched. The measured half angle of the tooth shape is 57. Since the complete thread has only the tooth ring, the other threads are completely damaged, so the diameter of the thread and the pitch of the thread cannot be measured.

2.4 Failure Analysis of Piston Rod Fracture A characteristic area of ​​fatigue fracture occurs in the fracture morphology, crack propagation zone and instantaneous fracture zone in the fatigue source zone. The extension zone has obvious shell-like arcs, and it also has a radiant ridgeline, which has obvious fatigue characteristics, and therefore belongs to high-cycle fatigue fracture.

The piston rod is a variable-section shaft with a shoulder on the broken thread edge. The thread has a small diameter and a threaded tip groove, so the stress concentration is most likely to occur. When the piston rod is subjected to long-term tensile and pulsating impact loads, the metal surface is caused to slip, and the sliding belt is sprouted to form microcracks. Ding Gao Zhou fatigue has a strong notch sensitivity, so the thread valley after thread gold processing is easy to become the origin of fatigue crack. The microcracks expand along the main slip of the slip zone under the action of alternating stress. When the effective section of the piston rod is gradually reduced until the residual section can not bear the action stress, rapid instability and expansion occur, causing the piston rod to break.

The metallographic structure of the piston rod material of 380 hot rolled state is fine pearlite + ferrite. The metallographic structure of 600 fire treatment after quenching is basically consistent with the metallographic analysis of the fired sorbite fracture sample. No abnormalities were found in the thread projection, and the magnetic powder was inspected before use. Therefore, the material and the added state are not the main causes of the fracture, and the main cause of the fracture should be the fatigue fracture caused by the stress concentration at the root of the thread.

2.5 Conclusion According to the macroscopic and microscopic morphology analysis of the fracture, the fracture of the piston rod is a high-cycle fatigue fracture.

Stress concentration at the root of a variable-section shaft is the leading cause of early fatigue failure.

The following improvement measures can be taken to avoid or reduce the occurrence of such accidents to increase the surface strength to improve the resistance to fatigue; 2 increase the thread diameter and strict heat treatment conditions, pay attention to the addition, quality, and prevent the stress concentration caused by the nicks such as knife marks. Because fatigue has a strong gap sensitivity.

1 Tu Zhi. Wu Pei published a fracture analysis of the fatigue failure analysis mechanical mechanics Du, 卯7 2 Hai Jiao University Gold Heat Treatment Teaching and Research Group. Jiaotong University. 976 3 China Equipment Maintenance Professional Institute equipment, analysis. Mechanical Industry Press. 989 4 Chen Nanping. Mechanical failure analysis of Beijing, Beijing University Press. 984

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