Abstract:
This article delves into the casting process design for a large ductile iron bearing cover, addressing the common issues of shrinkage defects and slag inclusion. Through detailed analysis, simulation, and experimental verification, a series of improvements were implemented, including the adoption of a built-in sand core with cold iron, adjustment of material composition, and enhancement of the filter slag removal system. These measures effectively eliminated defects, improved casting yield, and reduced processing costs. The article presents a comprehensive approach to optimizing the casting process for large ductile iron bearing covers, highlighting the importance of defect analysis and process optimization in ensuring product quality.
1. Introduction
The bearing cover, as a crucial component in diesel engines, serves to fix and support the crankshaft, enduring cyclic alternating loads during engine operation. Therefore, it demands exceptional quality standards. Recently, our company developed a large diesel engine bearing cover characterized by its substantial size. Initially, the casting process was designed based on past experience, but it resulted in defects such as shrinkage porosity and slag inclusion. To address these issues, a specialized technical project was initiated.
2. Original Casting Process and Defect Analysis
2.1 Casting Structure and Technical Requirements
The bearing cover has dimensions of 700mm × 450mm × 150mm, with a raw casting weight of approximately 190kg. It is made of QT400-15 material, featuring a maximum thickness of 150mm. The main bolt holes are machined later, and no defects are allowed within a 10mm radius of these holes.
2.2 Initial Process Design
The bolt holes were not cast out but machined later. To mitigate shrinkage porosity, insulation risers were placed at the thickest points, accompanied by chill plates at the corresponding lower sections. Two ceramic filter plates were used to enhance slag removal. the initial process design. This approach reduced the risk of shrinkage porosity and slag inclusion, eliminated material shortage for bolt hole machining, but increased the hot spot volume, leading to incomplete elimination of shrinkage porosity and lower casting yield.
2.3 Defect Characteristics and Analysis
After cleaning, the castings exhibited surface defects resembling slag inclusion . Microscopic examination and energy dispersive spectroscopy (EDS) analysis confirmed the presence of slag inclusion . Dissection revealed dispersed porosity defects at the bolt hole locations, indicative of shrinkage porosity .
2.4 Defect Cause Analysis
Several factors contributed to the defects:
- Compact Gating System: The tight arrangement of the gating system, with a short distance between the sprue and the filter plates, hindered optimal slag removal.
- Vertical Filter Placement: Vertical ceramic filters did not achieve maximum filtration efficiency during the initial pouring stage.
- Large Hot Spot: The uncast bolt holes created a significant hot spot, and despite insulation risers, the limited feeding distance could not fully eliminate shrinkage porosity.
3. Improved Casting Process Design
3.1 Adjustment of Gating System
The gating system was modified by extending the cross gate length and changing the filter plates from vertical to horizontal placement. This allowed iron to flow through the filters from bottom to top, significantly enhancing slag removal efficiency.
3.2 Optimization of Casting Structure
The bolt holes were cast out, significantly reducing the hot spot size. Steel cores were inserted into the bolt hole sand cores to prevent core breakage and deformation.
3.3 Cooling System Optimization
Contour-following chill plates replaced the insulation risers, fully covering the hot spot areas. This increased the cooling rate of the molten iron. Based on the principle of proportional solidification, the casting utilized graphitization expansion for self-feeding, and the chill plates provided additional feeding during the initial solidification stage.
3.4 Verification Results
3.4.1 Simulation Analysis
The results from MAGMA software simulations indicated a stable filling process during the casting of the bearing cover, with a significant reduction in the tendency for shrinkage porosity at the bolt hole locations. The simulation results are illustrated.
3.4.2 Physical Inspection of Castings
Upon visual inspection of the casting surfaces, no obvious slag inclusion defects were found. Furthermore, dissection inspections revealed that the shrinkage porosity was located far away from the bolt holes (greater than 76mm), meeting the application requirements .
3.4.3 Small-Batch Verification
Twenty castings were produced for small-batch verification, and none of them exhibited defects such as shrinkage porosity or slag inclusion. It was determined that the issues had been effectively resolved. Additionally, the casting process yield rate increased from the original 56% to 82%.
4. Conclusion
(1) Extending the length of the cross gate and placing the filter plates horizontally can effectively improve the slag blocking and removal efficiency of the gating system.
(2) For thick ductile iron castings, by increasing the sand cores, reducing the hot spot size, and reasonably setting chill plates, the risk of shrinkage porosity can be effectively lowered.
The above improvements in the casting process design for large ductile iron bearing covers have not only eliminated defects such as shrinkage porosity and slag inclusion but also significantly improved the casting yield rate and reduced processing costs.