1. Component structure analysis
The lifting box casting is mainly used in petroleum drilling and production equipment, made of ZG25CrNiMo, with an external dimension of 1040 mm x 776 mm x 933mm and a net weight of 1058 kg. It is a vulnerable part. The three-dimensional diagram of the lifting box casting is shown in Figure 1, with the main shape feature being a rotating body. The thickness of most of the box walls is 45 mm, with a local thickest point of 165 mm (excluding the outer wall protrusion) and a thinnest point of 30 mm. The structural wall thickness of the part is uneven, and there are large hot spots in local positions, which will inevitably produce shrinkage and porosity defects during the solidification of the steel liquid. Therefore, when designing the sand casting process, risers must be set up in these thick and large hot spots for shrinkage filling.
2. Formulation of process plan
2.1 Plan 1: Horizontal modeling and vertical pouring process
This plan divides the model into upper and lower halves, as shown in Figure 2. The model is relatively simple to make and is also convenient for modeling and core insertion operations. After horizontal molding and core placement, the mold is erected vertically for pouring. As analyzed earlier, the structure of the piece is uneven in thickness, and there are large hot spots in the thicker parts, which must be supplemented by setting a riser. To ensure a dense structure of the box casting, the riser must be placed at the left or right end of the sand mold casting for shrinkage, and poured at the corresponding other end, as shown in Figure 3.
When using this design, regardless of which end the riser and runner are set, the placement of the riser is particularly difficult, and it is also difficult to achieve precise positioning, which is not convenient for workers to operate, so it is not suitable to use.
2.2 Option 2: Standing, sitting, and pouring
There are two options for standing, sitting, and pouring, as shown in Figure 4, which are facing upwards (on the left) and downwards (on the right). According to the principle of metal solidification, sand castings begin to solidify from the thin-walled area where heat dissipation is most easily achieved, and finally solidify from the thick and large hot spots.
2.2.1 Option 2A: Standing, sitting, and pouring – with the large side facing upwards
This solution requires a three box design of “top, middle, and bottom”, as shown in Figure 5. To ensure effective shrinkage of the sand casting as a whole by the riser, especially in the thick and large hot spot areas, the preliminary size specification of the riser is determined to be ø 300 mm x 500 mm, with a quantity of 4 evenly distributed on the upper end face, as shown in Figure 5. The HZCAE11.0 software developed by Huazhong University of Science and Technology was used for numerical simulation of sand casting process. Solidification simulation was carried out using a pure temperature field without considering flow and instantaneous filling of steel liquid. The results are shown in Figure 6.
Through the simulation results in the above figure, it is found that when the steel liquid solidifies to 572 seconds, there are two obvious isolated areas in the middle and lower parts of the sand mold casting of the lifting box, and the riser does not play a significant role in filling and shrinking these two areas. When it solidifies to 3552.41 s, the upper part of the sand mold casting for the lifting box has completely solidified, and the steel liquid in the riser is still relatively abundant. However, significant concentrated shrinkage holes have formed at the lower hot spot, which will inevitably affect the fatigue strength and service life of the sand mold casting for the lifting box. Therefore, this approach is not suitable for use.
2.2.2 Option 2B: Standing, Sitting, and Pouring – Face Down
This process scheme places the thick and large hot spot at the top (as shown in Figure 7), so that the lower thin-walled part of the sand casting solidifies first, and the upper hot spot can be filled and shrunk through the riser, achieving sequential solidification and ultimately obtaining a dense structure.
Through the above comparative analysis, the final decision was made on the vertical pouring process plan with the large side facing downwards and the small side facing upwards.
3. Model and sand core design
3.1 External mold
The external mold is shown in Figure 8, and the corresponding sand mold is divided into three parts: upper, middle, and lower. The upper part is the riser box, the middle box is the main body of the model, and the lower box is the core positioning and runner entrance. The model is disconnected from the middle and removed from the middle box upwards and downwards, respectively.
There are three areas on the outer mold that need to be treated separately. A movable pad needs to be made between the top end face of the model and the surface of the convex platform, as shown in Figure 9. When making the middle box, the base plate and outer mold are combined together. When making the upper box, remove the cushion plate and connect the molding sand here to the upper box after molding.
A convex model with a diameter of 260 mm x 210 mm is used as a live block, as shown in Figure 10. After the main model is removed downwards, the live block is taken out from the cavity inward.
The other two are raised parts with a flange. When taking the outer mold downwards, these two areas must be made into “dovetail grooves” movable blocks, with the “dovetail” facing upwards and a slope of 5 °. The thinnest part of the movable block should have a thickness of 20 mm, as shown in Figure 11. After removing the outer mold, these two active blocks are taken out radially facing inward, while retaining the sand inside the square hole. This prescription pore sand is aligned with the main body 1 # core square pore sand, forming a complete convex square hole shape.
3.2 Sand Core
3.2.1 Main core
The main core is shown in Figure 12, with a core head height of 80 mm and a slope of 5 °. Due to the requirement of convex platform position on the outer side of the core, this core head has a step positioning, with a height of 40mm and a thickness of 40mm. The inner hole of the convex platform is made on the core, and after the core is lowered, it is connected to the outer wall of the sand mold. Special attention should be paid to aligning the sand mold formed by the protruding method and the core head with the active block.
3.2.2 Lifting ring shaft hole core
The core of the lifting ring shaft hole is shown in Figure 13. This core has two pieces, only one core box is made. The core head has a height of 40mm and a slope of 5 °.
3.2.3 Lug core
The hanging ear core is shown in Figure 14. This core is made in one piece or two, with only one core box. The thinnest part of each side wall of the sand core is 30 mm thick. The outer mold is disconnected at the junction of the core and the conical surface of the box sand casting, as shown in Figure 8, for easy demolding.
4. Riser design
The correct position of the riser directly affects the feeding efficiency of the riser and the quality of the sand casting. Several parts that need to be fed should be divided according to the characteristics of the sand casting structure, and then the size and quantity of the riser for each part should be determined. The riser should be set in the highest or last solidified part of the sand casting. If necessary, measures such as changing the position of the inner gate, adding subsidies, using cold iron or special sand for quenching should be taken to form a sequential solidification towards the riser. The methods for determining the size of the riser include modulus method, cubic equation method, filling and shrinking liquid method, proportion method, shape factor method, hot spot circle method, and shrinking tube method. The structure of this component is quite complex, making it difficult to determine the size of the riser using a single method. This plan is determined by a combination of modulus method and computational solidification simulation.
Install one waist shaped open riser “240 mm x 360 mm x 400 mm” and two cylindrical open risers “ø 240 mm x 400 mm” on the top of the sand mold casting for coupled simulation of “filling+solidification”. The results are shown in Figure 15.
From the simulation results, it can be seen that the final shrinkage cavity is formed at the upper part of the sand mold casting and the root of the riser. However, there is still looseness at the junction of the two walls in the middle of the sand mold casting and the junction of the bottom lifting ring shaft hole and the inner wall, which cannot be eliminated by increasing the size of the riser. During molding production, an external cold iron is installed at the junction of the lifting ring shaft hole and the inner wall. The special sand with a larger thermal storage coefficient than silica sand is used as the molding sand. The sequential solidification of the sand casting is controlled through the quenching effect, and the feeding distance of the riser is increased, forming a dense structure on the surface of the sand casting. The final designed sand casting, pouring system, riser, and external cold iron are shown in Figure 16.
The process yield under this process condition is: sand casting weight/(sand casting weight+riser weight+pouring system weight) x 100%=1186.4/1720 x 100% ≈ 69%.
5. Conclusion
(1) Choose the vertical pouring process plan with the lifting ring shaft hole facing downwards and the lifting ears and thick side walls facing upwards. The upper, middle, and lower unboxing shapes are set with risers, model bodies, core heads, and runners respectively. Wooden model, including 1 external mold (with the middle part broken), 3 core boxes, 1 movable block cushion plate, 1 movable block, and 2 dovetail groove movable blocks.
(2) The riser is designed using the modulus method and combined with HZCAE11.0 software for solidification simulation under pure temperature field and coupled filling heat transfer conditions. One waist shaped open riser and two cylindrical open risers are determined to be used for feeding on the thick wall end face. The specifications and dimensions of the two types of risers are “240 mm x 360 mm x 400 mm” and “ø 240mm x 400 mm”, respectively. Through multiple simulation results, it has been shown that the shrinkage cavity in the thick and large hot spot area is basically eliminated. There is still looseness at the junction of the two walls in the middle of the sand mold casting and the junction of the bottom lifting ring shaft hole and the inner wall, but it can no longer be eliminated by feeding through the riser. To eliminate the above defects, an external cold iron is installed at the bottom to achieve better sequential solidification and shrinkage.