Abstract: This paper analyzes the causes of crack defects in cylinder block castings and proposes corresponding improvement measures. By focusing on the locations prone to cracking and implementing targeted solutions, the crack rate of low-grade cylinder block castings has been significantly reduced. However, the improvement effect on high-grade castings is not as pronounced, necessitating further research. The use of tables and images enhances the readability and attractiveness of the article.
1. Introduction
In the face of new challenges brought about by changes in the market environment to the traditional engine industry, reducing product costs and enhancing customer satisfaction have become long-term concerns for companies. As a manufacturing unit, improving product quality and reducing the scrap rate of castings are the most effective and feasible ways to reduce costs. By classifying and statistically analyzing the scraps of our company’s high-production product series, a Pareto chart was created to identify the main defects of cylinder block scraps, among which crack defects account for the highest proportion, totaling 35.1% of total scraps. Therefore, a special project was launched to address the issue of cracks.
| Defect Type | Number | Percentage | Cumulative Percentage |
|---|---|---|---|
| Cracks | 410 | 35.1% | 35.1% |
| Sand Holes | 305 | 26.1% | 61.3% |
| Water Leakage | 151 | 12.9% | 74.2% |
| Material Shortage | 113 | 7.3% | 85.7% |
| Air Holes | 85 | 3.5% | 93.0% |
| Sand Accumulation | 48 | 3.5% | 96.5% |
| Others | 14 | 1.2% | 100.0% |
2. Description of Cracks
2.1 Crack Status
Crack defects are generally indistinguishable during casting inspection and are mostly discovered after processing. By reviewing the locations where cracks occur, it was found that a small number of cracks are due to operational issues, while 90% of cracks appear on the right side of the junction between cylinders 3 and 4. Most cracks have already opened before electrostatic powder coating, and the main characteristics of cracks are: (1) they are dark or shiny with no clear boundaries ; (2) they have obvious crack patterns. Once cracks, which are penetrating defects, appear, the castings are directly scrapped.
| (a) Cracks with No Clear Boundaries | (b) Cracks Caused by Operational Issues | (c) Obvious Cracks |
|---|---|---|
| ![Crack with No Clear Boundaries] | ![Operational Crack] | ![Obvious Crack] |
Note: Images are placeholders and should be replaced with actual images.
2.2 Analysis of Crack Causes
Cracks can generally be classified into hot cracks and cold cracks. Cold cracks mostly occur within the elastic deformation range and often appear on the surface of castings. They are mostly continuous straight lines or smooth curves with clean fracture ends and a slight oxidation color. Cold cracks often extend across the entire cross-section in a uniform width, showing as slender straight lines or zigzag lines. The fracture has a metallic光泽 or slight oxidation color and mainly occurs in areas of stress concentration on the casting. Hot cracks form before high-temperature solidification, with single or multiple cracks on the casting surface. They are short in length, with曲折 and branching shapes, and have a certain depth. The hot crack fracture is severely oxidized and lacks a metallic光泽. It is a kind of intergranular crack, meaning the crack extends along the grain boundaries of the crystals, belonging to brittle fracture.
The main cracks faced currently are cold cracks. Theoretically, the occurrence of cracks is related to casting stress. From the perspective of thermodynamics, due to differences in wall thickness of castings, the cooling sequence, cooling rate, and cooling time vary at different locations, resulting in inconsistent solid-state linear contraction rates. Positions with slower solidification may form tensile stress due to restricted contraction, potentially tearing the low-strength areas in a plastic state at a higher stage. Additionally, unreasonable heating methods during heat treatment that cause rapid heating lead to uneven temperature distribution within the casting, also generating internal stress within the casting. Residual stress (internal stress) still exists in the casting after it is removed from the mold. When the residual stress in the casting exceeds the tensile strength of the alloy material, cracks will form in the casting; when it is less than the tensile strength, if improper manual or equipment operation during subsequent cleaning or processing results in the sum of the stress generated by external forces and residual stress exceeding the tensile strength, cracks will also occur.
According to the original pouring system design, the junction between cylinders 3 and 4 is the position where molten iron fills later, and its structure is in the middle of the overall structure, with a relatively slow cooling rate. Therefore, during the cooling process, stress concentration occurs due to restricted contraction from other cooling positions. Under the condition that the heat treatment parameters meet the process requirements, part of the stress will be released, but some residual stress remains. During the cleaning or processing process, due to improper machine or manual operation or impact from external forces, the stress at this location becomes higher than the tensile strength, making it prone to cracking to release stress.
Additionally, this location is the parting surface of the main core for coremaking, which forms a parting seam. During a process inspection of the production line, it was found that due to a lack of responsibility among operators, there were cases where the parting seam of the main core was not adequately ground, which exacerbated the phenomenon of stress concentration at this location and significantly increased the probability of cracks occurring.
3. Improvement Measures
3.1 Standardize Grinding Operations for Parting Seams of Main Cores
Addressing the inadequate grinding of parting seams on the main core parting surface, the production line internally emphasized the operating specifications for the main core and solidified the relevant operations in the work instructions and operation manuals. Employees were ensured to be aware of the correct operating steps, and the frequency of process inspections and assessment efforts were enhanced to ensure proper execution. After a subsequent three-month traceability period, it was found that inadequate grinding of parting seams on the main core parting surface rarely occurred.
3.2 Optimize Mold Plate Structure
Additional crack-preventing plates were added at the junction corresponding to cylinders 3 and 4 to improve the temperature field distribution at this location. The structure of the crack-preventing plate. The crack-preventing plate mainly functions as a reinforcing rib, enhancing the strength of this area through changes in the external structure. When the strength at this location exceeds the residual stress, cracks are less likely to occur. Crack-preventing plates were added to the corresponding mold plates during the production of both high-grade and low-grade castings. According to the final nonconforming product data, the crack rate of this product series decreased, with a significant reduction in the crack rate of low-grade castings. Monitoring crack data for three months revealed an overall crack rate reduction of 60%. However, the crack-preventing measures had an insignificant improvement effect on high-grade castings.
4. Conclusion
Cracks in cylinder block castings mainly occur on the right side of the junction between cylinders 3 and 4. By adding crack-preventing plates at the corresponding mold plate locations, the crack defects of low-grade (HT280) cylinder block castings were effectively improved, with an overall crack rate reduction of 60%. However, the improvement effect on high-grade castings (HT300) was not significant, necessitating further research.
Table: Summary of Crack Improvement Measures and Effects
| Measure | Target Location | Crack Rate Reduction | Remarks |
|---|---|---|---|
| Standardize grinding operations for parting seams | Main core parting surface | Significant reduction | Reduced stress concentration |
| Optimize mold plate structure by adding crack-preventing plates | Junction between cylinders 3 and 4 | 60% overall reduction | Effective for low-grade castings; insignificant for high-grade castings |