Abstract
This comprehensive article delves into the intricate process of box casting, specifically focusing on the design, simulation, and optimization of the casting process for a lifting box component used in oil drilling equipment. The lifting box, constructed from ZG25CrNiMo material, features an uneven wall thickness with critical hot spots prone to shrinkage porosity defects. Through a meticulous analysis of various casting schemes, the article outlines the selection of an optimal casting process, the design of the mold and cores, the positioning of risers, and the utilization of numerical simulation tools to predict and mitigate defects. The emphasis on ‘box casting’ and ‘casting process’ highlights the critical steps and considerations involved in achieving high-quality castings.
1. Introduction
Box castings are a crucial component in various industrial applications, particularly in heavy-duty equipment such as oil drilling machinery. The complexity of these castings, often characterized by irregular shapes and uneven wall thicknesses, poses significant challenges during the casting process. Shrinkage porosity, a common defect in such castings, arises due to inadequate feeding of molten metal into the last solidifying regions, leading to reduced mechanical properties and shortened service life.
This article presents a detailed study of the casting process for a lifting box used in oil drilling equipment. The material of choice is ZG25CrNiMo, a cast steel renowned for its strength and corrosion resistance. The study employs a multifaceted approach, combining theoretical analysis, mold and core design, riser positioning, and numerical simulation using the HZCAE11.0 software to ensure the production of defect-free castings.
2. Part Analysis and Casting Challenges
2.1 Part Description
The lifting box is a large-scale component with dimensions of 1040mm × 776mm × 933mm and a net weight of 1058kg. The primary shape feature is a rotary body with a wall thickness ranging from 30mm to 165mm (excluding external bosses). The uneven wall thickness and the presence of thick sections with high heat concentrations (hot spots) pose significant challenges during the casting process.
2.2 Casting Challenges
- Shrinkage Porosity: The thick sections and hot spots are prone to shrinkage porosity due to insufficient feeding of molten metal during solidification.
- Uneven Wall Thickness: The variable wall thickness complicates the mold design and increases the risk of defects such as misruns and cold shuts.
- Complex Shape: The intricate shape of the lifting box necessitates a precise mold and core design to ensure dimensional accuracy and surface finish.
3. Casting Process Design
To address the challenges mentioned above, two primary casting schemes were evaluated: horizontal molding with vertical pouring and vertical molding with vertical pouring.
3.1 Horizontal Molding with Vertical Pouring
In this scheme, the mold is divided into two halves, facilitating ease of molding and core assembly. However, positioning the risers and pouring basin presents difficulties due to the uneven wall thickness and the need for precise placement to ensure effective feeding. illustrates the horizontal molding layout.
3.2 Vertical Molding with Vertical Pouring
Vertical molding offers advantages in terms of achieving better sequential solidification and improved feeding efficiency. Two variations of this scheme were evaluated: large face up and large face down.
3.2.1 Large Face Up
With the large face facing upwards, four cylindrical risers were positioned on the upper surface to feed the hot spots. However, numerical simulations revealed significant shrinkage porosity in the lower hot spots, making this scheme ineffective.
3.2.2 Large Face Down
Positioning the large face downwards ensures that the thick sections solidify last, allowing the risers to effectively feed the molten metal into these regions. This scheme proved to be more effective in eliminating shrinkage porosity.
4. Further Measures for Defect Elimination
Despite the implementation of the optimized casting process and the use of numerical simulation tools, some defects such as porosity and looseness still persist in specific areas of the lifting box casting. To further improve the quality of the casting, additional measures are necessary.
4.1 Application of External Chills
As mentioned earlier, porosity and looseness were observed at the intersection of the two walls in the middle of the casting and at the junction between the bottom lifting ring shaft hole and the inner side wall. These defects cannot be eliminated solely through the expansion of the riser system. Therefore, external chills were introduced at the bottom of the casting to achieve better sequential solidification and feeding.
External chills are metallic or non-metallic inserts placed in the casting mold to accelerate the cooling rate of specific areas. By placing chills at the critical locations, the solidification front can be manipulated to move towards the risers, ensuring that the last areas to solidify are adequately fed by the molten metal in the risers.
4.2 Use of Special Sands
In addition to external chills, the use of special sands with higher thermal conductivity than conventional silica sand was also considered. These special sands can enhance the heat transfer rate during the solidification process, further promoting sequential solidification and reducing the likelihood of porosity and shrinkage defects.
4.3 Optimization of Gating System
The gating system plays a crucial role in controlling the flow of molten metal into the mold cavity and ensuring uniform filling. To further optimize the casting process, the gating system was reviewed and refined. The location and size of the ingates were adjusted to minimize turbulence and ensure smooth metal flow, reducing the risk of entrapped gases and other defects.
4.4 Post-Casting Inspections and Defect Repair
Post-casting inspections, including visual inspections, radiographic testing, and ultrasonic testing, are essential to identify any defects that may have escaped detection during the casting process. If defects are found, appropriate repair methods such as welding, brazing, or machining can be employed to restore the casting to its intended quality.
5. Conclusion
The casting process design and numerical simulation of the lifting box, made from ZG25CrNiMo material, have been thoroughly analyzed and optimized. By selecting the appropriate casting position (“the large side of the lifting ring shaft hole facing down, the lifting lug and the small end face of the thick side wall facing up”) and implementing a three-part mold (upper, middle, and lower) with strategically placed risers, significant improvements in casting quality have been achieved.
The use of numerical simulation tools like HZCAE11.0 has been instrumental in predicting and mitigating defects, particularly in the thick and large hot joints. While some residual porosity and looseness remain in specific areas, the introduction of external chills, special sands, and optimized gating systems has further enhanced the casting’s quality.
Future work could focus on refining the casting process further, exploring advanced materials and technologies, and implementing stricter quality control measures to ensure consistently high-quality castings for critical applications such as oil drilling equipment.