This article focuses on two critical issues in mechanical components: the failure of engine connecting rod bolts and the appearance of black spots on gear surfaces. Through in – depth experimental research and analysis methods such as chemical composition testing, hardness measurement, and morphology observation, the root causes of these problems are identified. For connecting rod bolts, inappropriate post – heat – treatment rolling forming and rough machining are the main culprits; for gear surfaces, cleaning solution residue is the key factor. Based on these findings, corresponding improvement strategies are proposed to enhance the quality and reliability of mechanical components.
1. Introduction
In the field of mechanical engineering, the reliability and durability of components are of utmost importance. Engine connecting rod bolts and gears are two crucial components in engines and mechanical transmission systems. The failure of connecting rod bolts can lead to severe engine malfunctions, even catastrophic accidents. Meanwhile, the appearance of black spots on gear surfaces not only affects the aesthetic appearance but also may have an impact on the performance and service life of gears. Therefore, it is necessary to conduct in – depth research on the failure mechanisms of these components and propose effective improvement measures.
2. Failure Analysis of D20 Engine Connecting Rod Bolts
2.1 Experimental Materials and Methods
The fatigue strength test of D20 engine connecting rod bolt specimens was carried out on an Amsler HFP250kN high – frequency fatigue testing machine. The average load of the test was 80 kN, and the frequency was 98 Hz. The bolts were vertically installed on the test fixture, and the un – screwed thread length was 3 threads. The specified number of cycles was 1×, and the specimen was judged to have failed when the system frequency dropped and the bolt was found to be broken. Samples were taken from the failed bolts, and instruments such as an ARL3460 direct – reading spectrometer, an HR – 150A Rockwell hardness tester, a 2000 – C stereomicroscope, and an A1m metallographic microscope were used to study the chemical composition, hardness, macroscopic morphology, and microstructure respectively.
2.2 Results and Discussion
2.2.1 Fatigue Strength Test Results
A total of 24 specimens were tested. The test results are shown in Table 1.
| Serial Number | Load (kN) | Fatigue Amplitude (kN) | Fatigue Life () | Fracture Location |
|---|---|---|---|---|
| 1 | 8.5 | 1.0 | Not Broken | – |
| 2 | 8.5 | 0.47 | Thread | – |
| 3 | 8.5 | 0.24 | Under the Head | – |
| … | … | … | … | … |
| 24 | 8.0 | 1.0 | Not Broken | – |
Among them, 12 specimens were qualified, and 12 specimens were broken. Among the 12 broken specimens, 9 had fractures at the thread and 3 had fractures at the head bearing surface.
2.2.2 Chemical Composition Analysis
Samples were taken from bolts No. 3 (broken at the head bearing surface), No. 6 (not broken), and No. 10 (broken at the thread) for chemical composition analysis using the spectral analysis method. The results are shown in Table 2.
| Item | C | Si | Mn | S | Cr | B | |
|---|---|---|---|---|---|---|---|
| 32CrB4 Steel Standard | 0.30 – 0.34 | 0.30 – 0.90 | 0.60 – 0.90 | ≤0.025 | ≤0.025 | 0.90 – 1.20 | 0.0008 – 0.005 |
| Bolt No. 3 | 0.34 | 0.16 | 0.83 | 0.016 | 0.006 | 0.99 | 0.00082 |
| Bolt No. 6 | 0.34 | 0.17 | 0.79 | 0.011 | 0.002 | 1.03 | 0.00081 |
| Bolt No. 10 | 0.34 | 0.19 | 0.85 | 0.013 | 0.003 | 1.05 | 0.00083 |
The results show that the materials of the three bolts meet the chemical composition requirements of 32CrB4 steel in DIN_EN_10263 – 4 “Steel Rod, Bars and Wire for Cold Heading and Cold Extrusion”.
2.2.3 Hardness Analysis
In order to truly reflect the hardness value of the bolts, transverse cross – section specimens were cut from the rod parts of bolts No. 3, No. 6, and No. 10 for hardness testing. The test results are shown in Table 3.
| Sample Serial Number | Test Values (HRC) | Average Value (HRC) | |||
|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | ||
| Bolt No. 3 | 39.0 | 38.0 | 36.0 | 39.0 | 38.0 |
| Bolt No. 6 | 36.5 | 36.5 | 37.0 | 36.0 | 36.5 |
| Bolt No. 10 | 36.0 | 38.0 | 36.0 | 38.0 | 37.0 |
The hardness of the surface layer and core of the connecting rod bolts is between 36.5 – 39.0 HRC, which meets the requirements of 34 – 41 HRC for 11.9 – grade bolts specified in ISO 6508 – 2 – 2005 “Metallic materials – Rockwell hardness test”.
2.2.4 Fracture Macro – analysis
The photos of the broken connecting rod bolts No. 10 and No. 3 are shown in Figure 1.
[Insert Figure 1: Macrograph of the failed bolt here]
Bolt No. 10 broke at the 11 – thread position from the tail bearing surface of the thread. As can be clearly seen from Figure 2(a, b), there are wear – damage marks on the surface near the fracture of bolt No. 10. The fracture is perpendicular to the bolt axis, and there is no obvious plastic deformation near the macroscopic fracture. Clear fatigue stripes parallel to the fracture surface can be seen on the fracture, and the fatigue zone of the fracture accounts for about 2/3 of the fracture. There is large plastic deformation in the middle of the fracture, and an extrusion groove appears. Finally, the instantaneous fracture zone is small.
[Insert Figure 2: Fracture morphologies of the failed bolts here]
Bolt No. 3 broke at the head bearing surface. As can be clearly seen from Figure 2(c, d), there is also no obvious plastic deformation on the surface near the fracture. The fatigue zone of the fracture accounts for about 1/2 of the fracture. There is large plastic deformation in the middle of the fracture, and an extrusion peak appears. Finally, the instantaneous fracture zone accounts for about 1/4 of the cross – section.
2.2.5 Metallographic Analysis
Metallographic inspection found that there were no obvious non – metallic inclusions and other defects in the core of the connecting rod bolts. The microstructures of the cores of bolts No. 3 and No. 10 were both fine and uniform normal quenched structures, indicating that the cores of the bolts were fully quenched and the microstructures met the requirements. However, there were serious surface processing defects on the top and near – root surfaces of the threads of bolt No. 10. Under high magnification, irregularly shaped micro – protrusions could be seen on the top and near – root surfaces of the threads.
2.3 Fracture Cause Analysis
For bolt No. 10, the fracture position is at the 11 – thread position from the tail bearing surface of the thread, which is exactly above or near the thread – nut joint surface. Wear marks can be seen at the top of the thread at the fracture crack source. These wear marks are caused by the small relative movement between the micro – protrusions on the top of the thread and the nut during the fatigue test. The micro – protrusions on the top of the thread are formed during the rolling – forming process after heat treatment. These relative movements generate many micro – cracks, causing stress concentration at this location. Under the action of alternating loads, when the stress cycle reaches a certain number of times, micro – cracks initiate and continue to expand. When they reach a certain extent, the micro – cracks expand and meet to form a fatigue crack source, and then plastic tearing occurs, macroscopically manifested as an extrusion groove. When the effective bearing surface of the bolt is too small to bear the test load, the bolt breaks completely. Bolt No. 10 is a typical fatigue fracture.
For bolt No. 3, the intersection of the head bearing surface and the screw should be a fillet transition with a fillet radius of R1.8. However, there is no fillet transition at the intersection of bolt No. 3, but rather sharp tool marks. In addition, the head bearing surface of the bolt is rough. Sharp tool marks and rough bearing surfaces are prone to causing stress concentration. Under the action of alternating loads, the stress – concentrated area develops into an internal crack, which becomes the fatigue fracture source, which is the main cause of the early fatigue fracture of the connecting rod bolt.
3. Analysis of Black Spots on Gear Surfaces
3.1 Experimental Process and Results
3.1.1 Pre – heat Cleaning Test Process
63×8 machined finished gear parts made of 8620H steel were prepared for the experiment. They were divided into 8 groups, with 63 parts in each group loaded into a tray for the test. The parts went through the processes of pre – heat cleaning, AICHELIN continuous furnace carburizing and quenching, post – heat cleaning, tempering, and shot peening. Surface quality inspections were carried out before and after shot peening after carburizing and quenching heat treatment.
3.1.2 Experimental Results
Eight groups of tests were carried out, starting from factors such as the temperature and concentration of the cleaning solution before heat treatment, the service life of the cleaning solution after heat treatment quenching, and the cycle time of shot peening cleaning. The test results of 1 – 4 in Table 1 show that the temperature of the cleaning solution before heat treatment affects the cleaning effect of the parts. When the concentration of the cleaning solution before heat treatment is 0.1%, the optimal cleaning temperature of the cleaning solution should be higher than 60°C. The test results of 5 – 7 in Table 1 show that when the concentration of the cleaning solution before heat treatment reaches 1.0%, the cleaning effect of the cleaning solution at a temperature of 60 – 80°C can meet the surface quality requirements, and the cleaning temperature can be appropriately reduced to 60°C.
3.2 Cause Analysis of Black Spots
The black spots on the gear surface are caused by the residue of the cleaning solution. During the production process, if the cleaning process is not thorough, the cleaning solution will remain on the gear surface. After heat treatment and subsequent processes, these residues will react with the gear material or the surrounding environment, resulting in the appearance of black spots.
4. Improvement Strategies
4.1 For Connecting Rod Bolts
- Strictly Control the Heat Treatment Process: Ensure that the heat treatment parameters are accurate, such as heating temperature, holding time, and cooling rate. This can prevent the formation of abnormal microstructures that may affect the mechanical properties of the bolts.
- Improve Machining Precision: Optimize the rolling – forming process after heat treatment to avoid the formation of surface micro – protrusions. At the same time, ensure that the intersection of the head bearing surface and the screw has the correct fillet transition and that the bearing surface is smooth, reducing stress concentration.
4.2 For Gear Surfaces
- Optimize the Cleaning Process: Adjust the temperature and concentration of the cleaning solution according to the experimental results. Ensure that the cleaning process before and after heat treatment is thorough to completely remove the cleaning solution residue on the gear surface.
- Regularly Replace the Cleaning Solution: To prevent the accumulation of impurities in the cleaning solution, regularly replace the cleaning solution to ensure its cleaning effect.
5. Conclusion
Through the in – depth analysis of the failure of D20 engine connecting rod bolts and the appearance of black spots on gear surfaces, it can be concluded that inappropriate processing and cleaning issues are the main reasons for these problems. By implementing the proposed improvement strategies, such as strictly controlling the heat treatment process, improving machining precision, and optimizing the cleaning process, the quality and reliability of mechanical components can be effectively improved, reducing the occurrence of failures and defects, and ensuring the normal operation of mechanical equipmen.