yvonne@nydcasting.com 
When using lost foam casting to produce steel castings, surface carbonization of castings has always been a controversial topic. When using EPS pattern materials, low carbon steel (such as ZG25) castings are prone to surface carbonization, but when using STMMA pattern materials, there is almost no surface carbonization on the surface of steel castings. For medium carbon steel (such as ZG45 ~ ZG60), the use of EPS materials basically does not cause carbonization. For high carbon steel (such as ZG60 and above), the use of EPS pattern materials basically does not produce carbonization. We have conducted a lot of experiments to determine whether lost foam casting is suitable for the production of steel castings, especially low carbon steel castings.
1. The phenomenon and mechanism of carbonization
The form of surface carbonization of cast steel is manifested in several aspects: surface carbonization, volume carbonization, local carbonization and surface decarburization.
Surface carbonization:
During steel liquid filling, hydrogen accumulates in the air gap between the molten steel and the solid pattern, indicating the formation of solid-phase carbon. Gaseous byproducts escape under negative pressure, but solid carbon adheres to the coating wall, increasing surface carbon. Additionally, styrene and benzene vapors condense in the coating and surrounding sand. These organic substances decompose during solidification and cooling, further contributing to carbon increase. For various steel castings, the carbon-enriched layer is 0.1–0.3 mm deep, with a carbon increase of about 0.01%.
Volume carbon increase:
During pouring, a large heat gradient forms between the molten steel front and the mold, and heat mainly transfers by thermal radiation. The temperature near the molten steel front is the highest, close to the molten steel temperature. Carbon generation is significant in this area, and the thermodynamic conditions for carbon increase are sufficient, making it easy to cause volume carbon increase in the casting. Volume carbon increase is secondary to surface carbon increase.
Local carbonization:
When the method of introducing molten steel into the casting mold is inappropriate, the liquid product is drawn into the molten steel during the pouring process, and then decomposed into solid carbon and gas. If the gas fails to overflow the molten steel and remains inside, pores will be generated: the solid carbon is directly absorbed by the molten steel, resulting in an increase in the local carbon content of the casting, forming local carbonization of the casting.
Surface decarburization:
Lost foam casting uses dry sand molding, and the cooling rate of the casting is slow. The carbon content on the surface of the casting after solidification will continue to change. During the cooling process, there is not only surface carbonization, but also surface decarburization. Surface decarburization terminates at a higher temperature, and decarburization is mainly formed during the cooling process of the casting. In terms of mechanism, decarburization is an oxidation reaction, mainly the oxidation of matrix iron and carbon.
In the actual process, the form of carbonization of the casting mainly depends on the thermal decomposition state of the mold under the rapid thermal shock of the molten steel and the way in which its products interact with the molten metal.
2. Measures to reduce carbon increase
The factors affecting carbon increase on the surface of lost foam castings include the composition of molten steel, wall thickness of castings, cooling rate of castings, thickness and air permeability of coating layer, negative pressure, pattern material, pattern density, pouring and riser process, exhaust method, pouring position, pouring temperature, pouring speed, etc.
1）. Balance of carbon concentration between original molten steel and pattern
The distribution of carbon concentration on the surface of steel castings mainly depends on the concentration gradient of carbon in the decomposition products of the pattern and carbon in the molten steel. With the increase of the original carbon content in the molten steel, the concentration difference between the two is reduced, which can effectively inhibit carbon increase on the surface of steel castings. Alloy elements in molten steel have a great influence on carbon increase on the surface of steel castings. For example, Cr can increase carbon increase on the surface of cast steel, but can reduce the depth of carbon layer.
2）. Select low-density or low-carbon pattern materials
Changing pattern materials can reduce carbon concentration in decomposition products, thus lowering surface carbon increase in castings. Low-density patterns produce less gas and solid-phase carbon, reducing carbon enrichment. These patterns gasify easily with minimal residual liquid, causing less cooling of molten steel and reducing local carbon increase. For general steel castings, optimal pattern density is 0.016–0.025 g/cm². Copolymer patterns minimize residual carbon from EPS and reduce gas emissions from PMMA, preventing casting defects like back spray, choking, and porosity. They are ideal materials for steel casting patterns.
3）. Use highly permeable coatings
In lost foam steel casting, coatings play a key role in surface carbon increase. High air permeability helps reduce carbon buildup. Coatings with round, coarse aggregates and porous structures improve permeability. Thinner coatings also enhance this effect. Permeability impacts carbon increase more than coating composition. Low-permeability coatings trap carbon, raise air pressure, increase pattern gasification temperature, and promote more solid-phase carbon. Adding oxidizing agents changes the reducing atmosphere, effectively limiting carbon enrichment. Among quartz, corundum, and zircon coatings, quartz powder results in the least carbon increase.
4）. Reasonable casting process
The casting process significantly affects surface carbon increase in steel castings. Key factors include inner runner placement, exhaust channels, and slag risers. Bottom casting causes more carbon increase at the top; side or top casting leads to irregular carbon buildup, often at the bottom. Proper exhaust and slag risers help discharge pyrolysis products, reducing carbon defects. For uneven wall thickness, placing the inner gate at thinner sections helps control carbon increase. Higher casting temperatures and proper casting speeds also aid molten steel flow and efficient gas discharge, minimizing carbon-related defects.
5）. Appropriate mold wall negative pressure
Properly increasing the negative pressure is beneficial to reduce the degree of carbon increase on the surface of the casting. Negative pressure can reduce the concentration of gas generated in the gap and reduce the amount of solid carbon in the pyrolysis products of the mold: at the same time, negative pressure is also conducive to the solid carbon that has been generated to be discharged from the mold with the gas, thereby reducing the carbon increase of the casting. For general steel castings, the negative pressure can be controlled at 0. 03~0. 05Mpa.
In addition, in order to reduce the further diffusion of pyrolysis carbon in the coating layer to the surface of the high-temperature casting, the casting should be sanded as soon as possible.
3. Conclusion
3.1. Lost foam casting is suitable for the production of steel castings, and low-carbon steel castings can be produced by taking certain measures;
3.2. Using EPS pattern materials can produce steel castings with grades above ZG45; using STMMP copolymer pattern materials is the most effective measure to solve the problem of steel castings. At the same time, controlling the pattern density at 0.016~0.022g/cm2, and using hollow non-pattern material runners, opening exhaust channels at appropriate parts of the pattern can effectively inhibit the carbon increase of castings;
3.3 The selection of coatings is an effective measure to reduce the carbon increase of steel castings, and adding appropriate additives to the coating can effectively control the carbon increase of castings;
3.4 In the process measures, the design of the pouring and riser is very important, and the pouring and riser system should be reasonably set according to the structure of the casting. Properly increasing the casting temperature is conducive to reducing the carbon increase of castings; properly controlling the negative pressure is also an effective measure to reduce the carbon increase of cast steel.