This article mainly introduces the lost foam casting process of nodular iron vacuum pump housing. Through 3D simulation analysis of the casting process, a reasonable gating system is designed. The foam cutting program is used to cut the white pattern of the vacuum pump housing to ensure the smooth implementation of the pattern assembly. The wire feeding nodularization process is used to nodularize the molten iron, and the quality of the casting meets the technical requirements. The article also points out the feasibility and advantages of the lost foam process, the wire feeding nodularization process, and the top gating pouring and riser feeding process.
1. Introduction
The water ring vacuum pump housing used in the author’s company is formed by welding and processing steel plates. During use, it is affected by the long-term erosion of water and sand particles, resulting in water leakage of the water ring vacuum pump housing, which causes the pressure of the entire vacuum system to be low, affecting the production efficiency and quality of castings. Therefore, it is decided to use nodular iron to produce vacuum pump housing castings to improve the service life of the vacuum pump housing.
2. Casting Process
The overall dimension of the vacuum pump housing casting is relatively large. The maximum overall dimension of the casting is φ864 mm × 637 mm, the wall thickness of the casting barrel is 35 mm, and the single weight of the casting is 425 kg. The material grade is QT600 – 3, and the technical requirement is that the casting shall not leak. The sand box size is 1200 mm × 1000 mm × 1300 mm, with one casting per box. The pouring weight of the casting box is 560 kg, and 700 kg of molten iron is tapped from the nodularization ladle. The wire feeding method is used for nodularization treatment. The pouring temperature of the molten iron is set at 1430 – 1450 °C, and the vacuum negative pressure is controlled at – 0.05 – – 0.06 MPa. To prevent the casting from deforming, the holding time after pouring the molten iron is set at 1 h.
A top gating and semi – open gating system is adopted, and the gating ratio is ∑F_straight:∑F_pattern:∑F_inner = 1:1.4:1.2. The cross – sectional dimension of the straight runner is φ50 mm, the cross – sectional dimension of the cross runner is 75 mm × 75 mm, and the cross – sectional dimension of the inner runner is 40 mm × 140 mm. The size of the riser is 120 mm × 120 mm × 140 mm, and the risers are evenly distributed on the top of the casting. Since it is inconvenient to arrange external chills in the lost foam process and internal chills may cause poor fusion, the riser feeding process is used to eliminate shrinkage holes or shrinkage porosity in the casting. Six risers are placed on the top of the casting, of which two are hot risers. The molten iron enters the mold cavity from the hot risers, first falls to the bottom of the mold cavity, and then the liquid level of the molten iron rises. When the molten iron fills the top of the mold cavity, the leading edge of the molten iron enters the other four risers. After the molten iron is filled, the six risers jointly feed the casting.
3. Simulation Analysis
To verify the rationality of the top gating process and ensure the smooth pouring of the molten iron and the effective feeding of the shrinkage holes by the gating system, the MAGMA numerical simulation software is used to simulate the filling and solidification processes of the casting process.
From the simulation results of the molten iron filling process, it can be seen that when the top gating process is used for pouring, the molten iron enters the mold cavity from the gating system, falls to the bottom of the mold cavity, and then gradually rises from the bottom. The filling is stable, and the lower molten iron gradually cools down during the filling process. The hot molten iron entering from the upper part feeds the solidification of the lower molten iron. Finally, the solidification of the upper molten iron of the casting is fed by the six risers on the top, realizing the orderly feeding of the molten iron.
From the simulation results of the solidification process, it can be seen that the risers solidify last during the solidification process of the casting, playing a good feeding role, and no large isolated liquid phase area is found throughout the process. The slight shrinkage porosity can be compensated by the graphitization expansion in the local tiny isolated liquid phase area.
4. Lost Foam Process and On – site Control
4.1 White Pattern Making
Due to the large size of the vacuum pump housing casting, the white pattern of the casting cannot be made completely at one time, and can only be made by splicing. First, the vacuum pump housing casting is decomposed into several pieces with CAD software, and then cut with a fully automatic CNC cutting platform. The barrel part of the vacuum pump housing is decomposed into 12 pieces, and the flanges at both ends of the vacuum pump housing are decomposed into 8 pieces. The fillet radius of the barrel is R10 mm, which is bonded with a thin foam strip.
4.2 White Pattern Assembly
The cut foam blocks are manually spliced. First, the center position of the circle is aligned on the glass plate, and then the cut foam blocks are aligned and fixed with fiber rods. Then, glue is applied to the gaps of the white pattern and paper tape is wrapped to eliminate the gaps of the white pattern and prevent the coating from entering the gaps during the coating dipping process. To prevent the deformation of the white pattern of the vacuum pump housing during the coating dipping process, two wooden strips with a cross – sectional dimension of 15 mm × 15 mm are bonded to the upper and lower ends of the white pattern for reinforcement. To prevent shrinkage holes or shrinkage porosity in the casting, the gating system is used to feed the casting. Finally, the riser of the white pattern is bonded and reinforced with fiber rods to prevent damage to the white pattern during the production process.
4.3 Coating Dipping
Since the overall dimension of the assembled vacuum pump housing increases, it is required to dip the coating to every position of the white pattern when dipping the coating. First, the operator slowly rotates the white pattern when dipping it in the coating pool, and then uses a small container to pour the coating onto the places where the white pattern is not easy to dip. After the coating is dipped, the yellow pattern is placed on a special drying rack truck and pushed into the drying room for dehumidification and drying. To ensure that the molten iron is not damaged during the pouring process, the coating is required to be dipped 4 times, and the Baume degree of the coating is controlled at 69 – 71 °Bé.
4.4 Boxing and Pouring
First, the bottom sand is scraped flat, and then two people cooperate to load the yellow pattern into a sand box. The sand box is filled with sand manually and flexibly, and the three – dimensional vibration time of the sand box is controlled at 90 s.
5. Melting Process and On – site Control
The chemical composition of the molten iron is shown in Table 1. The tundish cover ladle nodularizing process with double wires is adopted. The addition amount of FeSiMg25Re3 nodularizer is 0.7%. The ladle inoculation adopts the method of 0.3% pretreatment agent and 0.2% 75FeSi composite inoculation. Since the nodularizer is wrapped in steel sheet to avoid oxidation, the wire feeding length and wire feeding speed are controlled by PLC to directly enter the bottom of the molten iron, resulting in high alloy absorption rate and good nodularization effect. After the molten iron is nodularized, the slag is quickly removed and transported to the pouring station by forklift.
To reduce the defects of shrinkage holes or shrinkage porosity in the casting, the pouring temperature of the molten iron is set at 1430 – 1450 °C. Clean cold iron blocks are placed at the pouring site to prevent the molten iron temperature from exceeding 1450 °C. When the temperature is too high, the clean cold iron blocks are immediately put into the ladle for cooling. After pouring, the mold cavity is replenished with molten iron. The vacuum negative pressure at the pouring site is controlled at – 0.05 – – 0.06 MPa, and the holding time after pouring is set at 1 h.
6. Production Results
According to the above process, trial production is carried out. The metallographic structure and mechanical property test results of the attached test bar are shown in Table 2. The metallographic structure photo is shown in Figure 9.
The vacuum pump housing casting after sand falling is sandblasted with a 5810 shot blasting equipment, and the risers and runners are removed by gas cutting. No shrinkage holes or shrinkage porosity are found by observing from the riser neck. The overall dimension of the casting is measured and meets the design requirements of the vacuum pump housing casting. The vacuum pump housing casting is sent to the processing unit for processing, and no shrinkage holes and sand slag holes are found on the processing surface, which can meet the use requirements of the vacuum pump.
7. Conclusion
(1) The scheme of producing thick and large nodular iron vacuum pump housing by the lost foam process is feasible. The white pattern is cut into pieces by numerical control and manually assembled and bonded, resulting in high casting dimension accuracy and is suitable for small – batch production.
(2) The molten iron adopts the tundish cover ladle nodularizing process with double wires, which ensures the wire feeding quality of the molten iron and realizes the material requirements of the casting.
(3) The casting process adopts the top gating pouring and riser feeding process, which realizes the stable and orderly filling process and sequential solidification. The casting is dense without shrinkage porosity and leakage, meeting the customer requirements.
In conclusion, the lost foam casting process of nodular iron vacuum pump housing has good application prospects and can provide a reference for the production of similar castings.
Table 1 Chemical composition of liquid iron
| Element | C | Si | Mn | S | P | Mg | Sn |
|---|---|---|---|---|---|---|---|
| Content | 3.7 | 2.4 | 0.5 | 0.015 | 0.05 | 0.05 | 0.06 |
Table 2 Metallurgical structure and mechanical properties test results of attached test specimen
| Project | Nodularization level/grade | Graphite size/grade | Pearlite volume fraction/% | Cementite volume fraction/% | Phosphorus eutectic volume fraction/% | Tensile strength/MPa | Elongation/% |
|---|---|---|---|---|---|---|---|
| Numerical value | 2 | 7 | 65 | ≤1.0 | ≤0.5 | 641 | 3.5 |
