1. Introduction
Connecting rod bolts play a crucial role in the operation of engines. They are responsible for securely fastening the connecting rod bearing cap, connecting rod bearing shell, and connecting rod shank, ensuring the stable connection between the piston and the crankshaft. In an engine, the connecting rod transfers the gas pressure received by the piston to the crankshaft, enabling the conversion of the piston’s reciprocating motion into the rotational motion of the crankshaft for power output. Any failure of the connecting rod bolts can lead to severe consequences, such as engine stalling, damage to the engine cylinder block, and even complete engine breakdown.
This article focuses on the analysis of the fracture of SCM435 steel connecting rod bolts in a certain engine. Through a series of physical and chemical tests, including hardness testing, metallographic examination, and fracture analysis, the root cause of the bolt fracture is identified. Additionally, corresponding improvement measures and preventive suggestions are proposed to enhance the reliability and durability of the connecting rod bolts.
2. Function and Importance of Connecting Rod Bolts
2.1 Mechanical Function
The connecting rod bolt serves as a key component to maintain the structural integrity of the connecting rod assembly. It withstands significant tensile, shear, and torsional forces during the engine’s operation. The torque applied during installation determines the pre – load of the bolt, which is essential for ensuring a tight connection between the components. A proper pre – load can prevent relative movement between the connecting rod bearing cap and the connecting rod shank, reducing the risk of fatigue failure and ensuring smooth power transmission.
2.2 Impact on Engine Performance
A well – functioning connecting rod bolt is directly related to the normal operation and performance of the engine. If the bolt fractures, the connecting rod assembly will malfunction, disrupting the connection between the piston and the crankshaft. This can cause uneven force distribution, abnormal vibrations, and a significant loss of engine power. In severe cases, it can lead to engine seizure, resulting in costly repairs and downtime.
| Engine Malfunction Caused by Bolt Fracture | Impact on Engine |
|---|---|
| Disrupted piston – crankshaft connection | Power loss, abnormal vibrations |
| Loosening of connecting rod bearing cap | Wear of bearing shell, potential damage to cylinder wall |
| Engine seizure | Complete engine failure, high repair costs |
3. Fracture Case Introduction
During the bolt torque sampling inspection of a particular engine, a connecting rod bolt was found to be fractured. The fractured bolt, made of SCM435 steel and subjected to quenching and tempering treatment, had a specified hardness requirement of 34 – 38 HRC. The appearance of the fractured bolt is shown in Figure 1.
4. Physical and Chemical Testing
4.1 Fracture Analysis
4.1.1 Macroscopic Fracture Features
The fracture of the fractured connecting rod bolt exhibited a neck – shrinking phenomenon, indicating that the bolt had undergone plastic deformation. The local area of the fracture was abraded, with an uneven surface. The microstructure was fine, showing a dark gray and fibrous appearance, which is a typical characteristic of a ductile fracture. Macroscopically, the fracture of the bolt had a counterclockwise torsional feature, as shown in Figure 2.
4.1.2 Microscopic Fracture Features
Using a ZEISS EVO 18 scanning electron microscope to analyze the fracture morphology of the connecting rod bolt, it was found that both regions A and B on the fracture had shear dimple characteristics, and region C had equiaxed dimple characteristics. The crack propagated in a counterclockwise torsional manner, as shown in Figure 3.
| Fracture Region | Dimple Feature | Crack Propagation |
|---|---|---|
| A | Shear dimple | Counterclockwise torsion |
| B | Shear dimple | Counterclockwise torsion |
| C | Equiaxed dimple | Counterclockwise torsion |
4.2 Hardness Testing
Transverse samples were taken from the fractured connecting rod bolt at a distance from the tail equal to the bolt diameter. After grinding and polishing, the hardness was measured, and the result was 35 HRC, which met the requirement of 34 – 38 HRC. This indicates that the hardness of the bolt is within the specified range and is not the cause of the fracture.
4.3 Metallographic Examination
Transverse samples were taken from the fractured connecting rod bolt at a distance from the tail equal to the bolt diameter to prepare metallographic specimens. After etching with a nitric acid solution, the examination showed that the edge of the bolt had no decarburization phenomenon, and the core structure was tempered sorbite, as shown in Figure 4. This indicates that the microstructure of the bolt is normal and there is no problem with the heat treatment process.
5. Analysis and Discussion
5.1 Relationship between Fracture and Heat Treatment Quality
The results of hardness testing and metallographic examination show that the hardness and microstructure of the bolt meet the technical requirements. This indicates that the fracture of the connecting rod bolt is not related to the heat treatment quality. The normal hardness and microstructure ensure that the bolt has the required mechanical properties under normal conditions.
5.2 Fracture Mechanism
The presence of shear dimples and equiaxed dimples on the bolt fracture is a typical characteristic of ductile fracture. According to the macroscopic and microscopic morphology of the fractured bolt, it can be inferred that the fracture of the connecting rod bolt is caused by the action of overload torque. That is, the actual stress borne by the critical section of the bolt exceeds the yield strength or ultimate strength of the material, resulting in rapid shear fracture and a counterclockwise torsional shape. Overload torque may be generated during the assembly process due to improper operation, such as excessive torque applied by the assembly tool or incorrect assembly sequence.
6. Improvement Measures
6.1 Optimization of Assembly Process
It is recommended to optimize the installation process of SCM435 alloy bolts. This can be achieved by using torque – controlled assembly tools with high precision to ensure that the torque applied during the installation of the connecting rod bolts is within the technical requirements. Additionally, training should be provided to assembly workers to improve their operation skills and awareness of correct assembly procedures.
6.2 Quality Control during Production
During the production process, strict quality control measures should be implemented. This includes strengthening the inspection of raw materials to ensure that the SCM435 steel used meets the required standards. In addition, the heat treatment process should be closely monitored to ensure that the bolts have consistent and stable mechanical properties.
7. Conclusion
The fracture of the SCM435 steel connecting rod bolt is mainly due to the action of overload torque, resulting in a ductile fracture. The hardness and microstructure of the bolt meet the requirements, indicating that the heat treatment quality is not the cause of the fracture. To prevent similar failures from occurring in the future, it is necessary to optimize the assembly process of the bolts, ensure that the applied torque is within the specified range, and strengthen quality control during production. By taking these measures, the reliability and durability of the connecting rod bolts can be effectively improved, ensuring the normal operation of the engine.
8. Future Research Directions
Although the cause of the connecting rod bolt fracture in this case has been identified, there are still some areas worthy of further research. For example, the long – term fatigue performance of SCM435 steel connecting rod bolts under complex working conditions needs to be studied. In addition, the development of new materials and manufacturing processes for connecting rod bolts can also be explored to improve their performance and reliability. Future research can also focus on the development of more advanced non – destructive testing methods to detect potential defects in bolts at an early stage, reducing the risk of bolt fractures.
