This paper focuses on the common defects in the lost foam casting process of steel castings and presents corresponding prevention measures. Through in-depth analysis of defects such as carbon increase, porosity, slag inclusion, backfire, and negative pressure cutting, it provides valuable guidance for improving the quality of steel castings produced by the lost foam casting process.
1. Introduction
Lost foam casting (EPC), also known as expendable pattern casting, has witnessed significant development since its invention in 1956. It has been widely applied in industrial production, ranging from small-batch large-part production to large-batch small-part assembly line production, and from simple non-machined part production to complex machined part production. In recent years, the number of domestic factories adopting the lost foam casting process has been increasing, but there are still some technical challenges. Due to the incomplete understanding of the casting characteristics and laws, especially the metal filling characteristics, coating properties, and pattern pyrolysis characteristics, steel castings are prone to various defects. This paper aims to analyze these defects and propose effective prevention methods.
2. Common Defects and Prevention Measures
2.1 Carbon Increase Defect
Carbon increase is a common defect in the lost foam casting of steel castings. The foam pattern material mainly consists of carbon and hydrogen. Under high-temperature molten steel, it decomposes rapidly, generating hydrogen and free carbon. Since hydrogen has a stronger affinity for oxygen than carbon, the hydrogen decomposed at high temperature first combines with the oxygen in the pattern gap to form steam and is discharged, while a large amount of free carbon remains in the mold and corrodes the surface of the molten steel, resulting in surface carburization of the casting.
| Prevention Measures | Details |
|---|---|
| Select high-quality foam plastic | High-quality foam plastic has low carbon content and high molecular weight. Under the same foam strength, the pattern density is lower. Choosing raw beads with less gas generation and low carbon content and molded foam with low bulk density and light weight is the preferred condition for lost foam casting of steel castings and the most effective way to solve the problem of carbon increase in steel castings. |
| Choose a reasonable pouring process | The design of the pouring process should accelerate the gasification of the pattern material, reduce and stagger the contact and reaction time between the liquid and solid phases in the decomposition products and the molten steel, and reduce or avoid the carburization of steel castings. This includes controlling the pouring temperature and speed, ensuring good coating and dry sand mold permeability, setting an appropriate negative pressure on the mold wall, and using anti-carburization coatings. |
| Set risers | Utilize the characteristic that the farther away from the ingate, the more serious the carbon increase. Set risers at the farthest end from the ingate or at the highest point of the casting. This allows the severely carburized molten steel that enters the mold cavity first to enter the riser, ensuring that the casting body obtains relatively pure molten steel. |
| Adopt “burn first, pour later” method | Burn the foam plastic in the mold cavity to form a cavity before pouring the molten metal. This can effectively reduce the carbon source in the mold and thus reduce the carbon increase in the casting. |
2.2 Porosity Defect
Porosity defects in steel castings produced by lost foam casting can be classified into the following four types according to their causes:
| Type | Cause | Prevention Measures |
|---|---|---|
| Porosity caused by the entrainment of foam pattern pyrolysis products in molten steel | During the pouring and filling process, turbulence occurs, or during the gasification process, part of the pattern is surrounded by molten metal and decomposes. The generated gas cannot be discharged from the molten metal, resulting in the formation of large and numerous pores with carbon black on the inner surface. | Improve the process to ensure the smooth filling of molten steel without turbulence; reasonably increase the pouring temperature and negative pressure (if the porosity is caused by turbulence, reduce the negative pressure); improve the permeability of the coating and molding sand. |
| Porosity caused by poor drying of foam pattern and coating | When the foam pattern coating is not dried properly and the foaming agent content is too high, a large amount of gas will be generated during pouring, causing porosity. | Fully dry the foam pattern according to the characteristics of the pattern foaming and molding process; ensure that the coating is completely dried and strictly control the addition amount of the foaming agent. |
| Porosity caused by excessive pattern binder | Excessive use of pattern binder with high gas generation and slow gasification makes it easy for the gas to be entrained in the molten steel to form pores. | Select a low-gas-generating model binder and use as little adhesive as possible while ensuring firm adhesion. |
| Porosity caused by air entrainment during pouring | When the sprue is not filled during pouring, air is easily entrained. If these gases cannot be discharged in time, they are likely to cause porosity. | Design a reasonable pouring system to ensure the smooth flow of molten metal without air entrainment during filling; when using a closed pouring system, ensure that there is a certain amount of molten metal in the pouring cup to maintain the full state of the sprue; use a hollow sprue pattern to reduce gas generation and prevent porosity. |
| Porosity caused by problems in molten steel melting quality | During the melting process of carbon steel, if it contains gas and is not fully deoxidized, oxygen will remain in the molten steel, forming pores. | During the melting process, strictly follow the melting process and perform deoxidation treatment before pouring to purify the molten steel. |
2.3 Slag Inclusion Defect
Slag inclusion defect refers to the defect formed when dry sand grains, coatings, and other inclusions enter the casting along with the molten iron during the pouring process. It is a common defect in lost foam casting production. On the surface of the machined casting, white or black-gray inclusion spots can be seen, which are distributed singly or in patches. The white ones are quartz sand particles, and the black-gray ones are slag, coatings, residues after the pyrolysis of the foam model, and other inclusions.
| Prevention Measures | Details |
|---|---|
| Select coatings with good comprehensive properties | Steel casting coatings should have a series of properties such as high strength, high refractoriness, good thermal shock resistance, good coating ability, suspension, and non-flowability in addition to the general requirements. To prevent slag inclusion defects, the coating should first have high strength and refractory properties, and when the molten steel enters the mold, the sprue should be tightly closed, and the coating layer on the surface of the casting and the pouring system should not peel off or crack. In addition, the coating applied to the surface of the white pattern should have sufficient room temperature strength and not crack during drying and transportation. Moreover, the coating layer should have good high-temperature strength, that is, under the long-term scouring of high-temperature metal during the pouring process, the coating does not peel off, crack, and ensure that the coating layer has a certain thickness. |
| Standardize boxing operations | When boxing, the pattern group should be placed stably on the bottom sand of the sand box. It is not allowed to start sanding and vibrating when it is suspended to avoid cracking the coating layer. Do not add sand violently directly to the pattern. First, use flexible sanding to prevent the material layer from breaking. Tightly seal the sprue to prevent sand from entering. The entire boxing and molding operation process should be very careful to ensure that there is no peeling, cracking, or cracks in the pattern group coating layer before pouring. Before pouring, it should be confirmed again that there is no floating sand, dust, or debris in the pouring cup. |
| Reasonably set pouring head, temperature, and time | The higher the pouring head, the greater the scouring of the pouring system and the mold, and the greater the possibility of damaging the coating and causing sand to enter. For different sizes of castings, the pouring head should also be different. Select a ladle with an appropriate capacity and try to reduce the pouring height as much as possible. The ladle nozzle should be as close as possible to the pouring cup and avoid using a large ladle to pour small castings. Select an appropriate pouring temperature. Because the higher the pouring temperature, the higher the requirements for the coating performance, and the more likely it is to produce defects such as sand sticking and slag inclusion. |
| Reasonably determine the negative pressure | The pouring process of lost foam casting is generally carried out under vacuum conditions. The role of negative pressure is to compact the dry sand, accelerate exhaust, and improve the filling ability. The magnitude of the negative pressure has a great impact on the quality of the casting. Excessive negative pressure increases the possibility of the molten metal sucking in dry sand and inclusions when passing through cracks and fissures and also increases the sand sticking defects of the casting. At the same time, excessive negative pressure leads to too fast filling speed, increasing the scouring strength of the metal on the runner and the mold, making it easy for the coating to peel off and enter the metal and also easy to damage the coating layer and cause sand to enter. For steel castings, the appropriate negative pressure is generally -0.030 – 0.045MPa. |
| Set slag retaining, slag skimming, and slag collecting risers | When designing the pouring system, consider setting slag retaining and slag skimming functions. At the same time, setting slag collecting risers on the casting helps to improve the defects of sand entering and slag inclusion. |
| Adopt molten steel purification technology | Molten steel melting must pay attention to the purification problem, which is one of the key technologies in lost foam casting. The whole process from molten steel melting, overheating, deoxidation to pouring into the mold should be considered for purification treatment. |
2.4 Backfire
During the pouring process, if the amount of gas generated by the thermal decomposition of the gas pattern is too large and cannot be discharged in time, the gas pressure in the mold cavity will rise sharply, which may cause fire spitting or molten metal spraying, resulting in the scrapping of the casting.
| Prevention Measures | Details |
|---|---|
| Control the density of EPS pattern | The density of the EPS pattern should be controlled within 0.015 – 0.020 g/cm³. The pattern should be dried, and after coating, it should also be dried to reduce the water content and gas generation. |
| Select coatings with good permeability | Select coatings with good permeability and adjust the coating thickness to 1.0 – 2.0 mm to ensure that the gas generated after the pattern is cracked can escape in time. |
| Control the permeability and particle size of dry sand | Design the sand box scientifically, reasonably, and applicable. Control the negative pressure (vacuum pump suction) to make the pattern gasify under vacuum and anoxic conditions and reduce gas generation. |
| Control the pouring temperature and speed | Use the heat of the molten steel to ensure the gasification of the pattern. At the same time, when the pattern generates a large amount of gas, control the pouring speed to avoid excessive pouring speed causing a large amount of pyrolysis gas to burst out. |
| Design a reasonable pouring system | Ensure that the molten metal fills the mold smoothly, balanced, and quickly to ensure that the gas generated by the pattern escapes from the mold cavity and is sucked and discharged. |
2.5 Negative Pressure Cutting
The main reason for negative pressure cutting is that during the pouring process, due to the damage of the mold in the vacuum state of the sand box, outside air is sucked into the mold; the strong air flow penetration process has a penetrating scouring force on the unsolidified molten metal, forming a cutting phenomenon.
| Prevention Measures | Details |
|---|---|
| Control the negative pressure during pouring | Generally, the negative pressure in the sand box during pouring should be within the appropriate range of -0.020 – 0.035 MPa. Avoid excessive negative pressure to reduce the risk of air being sucked into the mold. |
| Ensure the coating thickness | The coating thickness should be guaranteed to be 1.0 – 2.0 mm. A proper coating thickness can prevent the mold from being damaged and reduce the occurrence of negative pressure cutting. |
| Control the pouring height and time | During pouring, the pouring height of the ladle should be as low as possible, and the ladle nozzle should be as close as possible to the pouring cup. After pouring, the holding time should be controlled within 3 – 7 minutes to avoid excessive air flow and ensure the solidification of the molten metal. |
3. Conclusion
Lost foam casting of steel castings has great development potential in China’s casting industry. By effectively controlling the production operation process, strictly controlling the melting process of castings, reducing and eliminating the process conditions of carbon increase during pouring, combining actual production conditions, strengthening the training of skilled technicians, and improving the overall level of the team, strict control of each process in the technology can produce fully qualified steel casting products. Continuous research and improvement in the lost foam casting process will contribute to the development of the casting industry and meet the increasing demand for high-quality steel castings in various fields.
In the future, with the development of technology and the improvement of understanding of the lost foam casting process, it is expected that more effective methods will be found to further improve the quality of steel castings and promote the wider application of lost foam casting technology. At the same time, it is also necessary to pay attention to environmental protection and energy conservation in the production process to achieve sustainable development of the casting industry.
