Nodular cast iron, also known as ductile iron, is a type of graphite cast iron with excellent mechanical properties. It has been widely used in various industries due to its high strength, good toughness, and excellent wear resistance. In this article, we will focus on the casting technology of spiral series thick and large nodular cast iron castings and explore how to solve the common defects such as shrinkage porosity and slag in the casting process.
1. Introduction
Spiral series products are a kind of small-batch and multi-model series products, with various product types and weights ranging from 500 to 2000 kg. They belong to thick and large nodular cast iron castings, with a main wall thickness of 20 mm and a maximum wall thickness of 100 mm. The uneven distribution of the wall thickness and the complex distribution of the hot spots make the casting process quite challenging. The requirement that the castings should not have any visible casting defects after processing further adds to the difficulty of the casting process.
2. Casting Structure and Requirements
The contour dimensions of the spiral series products are 1450 mm × 850 mm × 380 mm, and the weight ranges from 200 to 1500 kg. The material is QT400 – 18 – LT, and the castings are required to undergo UT (according to DIN EN 12680 – 1 Grade 1) and MT (according to DIN EN 1369 Grade SM 3) flaw detections. There should be no visible appearance defects on the castings after processing.
3. Experimental Scheme
To address the shrinkage porosity and slag defects in the spiral series products, a bottom pouring process scheme using cold iron and risers was adopted. The specific experimental scheme includes the following aspects:
3.1 Optimization of Process Design to Simplify Molding and Reduce Cost
(1) For different models of spirals within the weight range of 200 – 1500 kg, a unified gating system of one specification was used (as shown in Table 1). This not only reduces the number of molds, further lowering the mold production cost, but also simplifies the molding operation and avoids the repeated switching of the gating system.
| Gating System Specifications | Details |
|---|---|
| Inlet Section | 4 × φ40 mm |
| Cross Runner Section | 2 × (40/50) mm × 40 mm |
| Sprue Section | 660 mm |
(2) Due to the high cost of new sand boxes, most substitute sand boxes were used in the production of spiral products, which are of small batch and multi – model. During the process design, the gating system and the outer mold were designed as movable structures, so that by adjusting the positions of the gating system and the outer mold, a smaller – sized sand box could be selected to reduce the amount of sand consumed and lower the molding cost.
3.2 Optimization of Mold Structure to Reduce Mold Cost
(1) Considering the low mold utilization rate and the relatively low requirement for mold life of the spiral series products, a wood mold structure with the lowest cost was adopted for the mold structure, which includes metal mold structure, steel – wood structure, and wood mold structure.
(2) A non – plate outer mold was used, which is expected to save 50% of the mold cost compared to the outer mold with a plate. The solid pattern and the expansion movable block were placed on the mold (as shown in Figure 1). The mold was used to achieve the upper and lower parting. The molding steps of the non – plate outer mold are as follows: place the solid mold and the expansion movable block on the mold – place the solid mold, the expansion movable block, and the mold as a whole on the molding plate – place the sand box and fill it with sand to make the lower mold – after the lower mold is cured, flip it over – take out the mold – sprinkle the parting dry sand with a particle size of 100 – 140 mesh on the parting surface – place the sand box and fill it with sand to make the upper mold – after the upper mold is cured, demold to obtain the cover box – take out the solid mold and the expansion movable block – demold to obtain the bottom box.
[Insert Figure 1 here]
(3) Considering the large number of product models and the frequent occurrence of mold borrowing, the mold was split into a basic mold pattern and an expansion mold movable block. When borrowing a mold, only the expansion mold movable block needs to be replaced without the need to make a new basic mold, greatly reducing the mold production cost.
3.3 Optimization of Casting Process to Solve the Inclusion Problem in the Spiral
On the one hand, the source of inclusions was controlled to improve the purity of the molten iron. The main measures include: (1) controlling the time from tapping to pouring within 10 minutes to prevent inoculation decay and reduce the generation of oxidized slag; (2) placing a piece of asbestos on the surface of the molten iron in the ladle before pouring to absorb the oxidized slag on the surface of the molten iron and reduce the slag in the ladle from entering the pouring basin; (3) installing a filter sheet (as shown in Figure 2) in front of the inner runner to filter and block the slag of the molten iron entering the mold cavity. The specification of the filter sheet is 150 × 150 × 20, and the number is 2 pieces.
On the other hand, the impact of inclusions was reduced by controlling the loose sand in the mold cavity or the gating system and improving the cleanliness of the mold cavity. The specific measures include: (1) cleaning and grinding the local virtual sand or sharp – corner sand in the mold cavity before closing the box to avoid being brought into the molten iron during pouring; (2) using a ceramic tube for the inner runner (as shown in Figure 2) to reduce the risk of the runner sand being brought into the mold cavity during pouring; (3) adopting a bottom pouring gating system, so that the molten iron flushes the mold smoothly from bottom to top, and even if there is oxidized slag in the molten iron or loose sand in the mold cavity, it is easy to float up and be discharged through the overflow of the riser vent.
[Insert Figure 2 here]
3.4 Optimization of Casting Process to Solve the Dispersed Shrinkage Porosity Problem in Local Positions of the Spiral
By analyzing the structure of the casting, risers and cold irons were placed at the hot spots to reduce or eliminate the shrinkage porosity defect of the casting. The specific measures are as follows:
(1) Insulating risers were added to the thick parts on the cover box surface of the casting (as shown in Figure 3) to feed the thick parts of the casting. There are 4 insulating risers on the top circular platform, with the size of ø200 mm × 200 mm for the insulating riser and ø100 mm × 50 mm for the riser neck. There is 1 insulating riser on the top surface of the flange, with the size of ø100 mm × 100 mm for the insulating riser and ø50 mm × 20 mm for the riser neck.
(2) Cold irons were placed at the hot spots and important processing positions of the casting (as shown in Figure 3) to quench the casting and reduce the tendency of shrinkage porosity.
(3) A low – temperature open pouring scheme was adopted. The gating system selected an inner runner section of 4 × φ40 mm, a cross runner section of 2 × (40/50) mm × 40 mm, and a sprue section of 660 mm. The pouring temperature was set at a low temperature of (1330 ± 10) °C.
[Insert Figure 3 here]
3.5 Verification of the Spiral Process
(1) The bottom pouring casting process optimized with cold iron and risers effectively reduced the porosity tendency of the casting. Through solidification simulation of the shrinkage porosity distribution, it was found that the hot spots of the casting were widely and dispersedly distributed, and the shrinkage porosity rate of the casting was controlled at about 1%, as shown in Table 2.
| Simulation Results | Details |
|---|---|
| Shrinkage Porosity Rate | About 1% |
(2) A total of three batches of experiments, namely “1 + 2 + 4”, were conducted. The experimental pieces met the technical requirements of the customer after MT and UT flaw detections, and there were no any casting defects on the appearance of the processed experimental pieces.
(3) In actual production, this process has been solidified and mass production has been realized. Using this casting process plan, more than 150 similar – structured castings of 35 types have been produced, and the casting quality is stable and has been delivered as finished products.
4. Results and Analysis
4.1 Effect of Optimization of Cold Iron and Riser on Solving Shrinkage Porosity
The optimization of the cold iron and riser placement significantly reduced the shrinkage porosity in the spiral castings. The insulating risers provided effective feeding to the thick sections of the casting, reducing the risk of shrinkage. The cold irons at the hot spots helped to quench the casting and minimize the shrinkage tendency. This combination of cold iron and riser effectively controlled the solidification process of the molten metal, resulting in a more uniform and dense microstructure.
4.2 Effect of Control of Molten Iron Purity and Mold Cavity Cleanliness on Solving Slag Defects
The measures taken to control the purity of the molten iron and the cleanliness of the mold cavity had a significant impact on reducing the slag defects in the castings. By controlling the time from tapping to pouring within 10 minutes, placing asbestos on the surface of the molten iron in the ladle, and installing filter sheets in the inner runner, the amount of oxidized slag and impurities in the molten iron was minimized. Additionally, the cleaning and grinding of the local virtual sand or sharp – corner sand in the mold cavity, the use of ceramic tubes for the inner runner, and the adoption of the bottom pouring gating system all contributed to reducing the risk of slag inclusion in the castings.
4.3 Economic Benefits of Process Optimization
The optimization of the process design, mold structure, and casting process not only improved the quality of the castings but also brought significant economic benefits. The reduction in the number of molds, the use of substitute sand boxes, and the optimization of the mold structure all contributed to lowering the production costs. The improved casting quality also reduced the scrap rate and rework costs, further enhancing the economic efficiency of the production process.
5. Conclusion
In conclusion, the optimized bottom pouring process scheme with cold iron and risers can greatly solve the problems of shrinkage porosity and slag defects in the spiral series thick and large nodular cast iron castings. The combination of risers and cold irons is beneficial in reducing the shrinkage tendency of the castings. Additionally, controlling the purity of the molten iron and the cleanliness of the mold cavity can significantly address the slag defect problem in the castings. These findings have important implications for the production of high – quality nodular cast iron castings and can provide valuable references for similar casting processes in the future.
It is worth noting that continuous research and innovation are essential in the field of casting technology to further improve the quality and performance of castings and to meet the increasingly demanding requirements of various industries. Future studies can focus on further optimizing the casting process parameters, exploring new materials and technologies, and improving the efficiency and sustainability of the casting production process.