In the production of steel castings by EPC, the surface carburization of castings has been a controversial topic. When EPS pattern material is used, the surface carburization of low carbon steel (such as ZG25) castings is easy to occur, but when stmma pattern material is used, the surface carburization of steel castings is almost impossible. For medium carbon steel (such as ZG45 ~ZG60), the EPS material does not appear carburization. For high carbon steel (such as zg60 or above), EPS pattern material basically does not produce carburization. Whether EPC is suitable for steel castings, especially for low carbon steel castings, we have done a lot of experiments.
The forms of surface carburization of cast steel are as follows: surface carburization, volume carburization, local carburization and surface decarburization.
Surface carburization: during the filling process of molten steel, there is a large amount of hydrogen in the air gap between the molten steel front and the solid pattern. It shows that there is solid carbon formation. The gas product can penetrate into the coating under the negative pressure and discharge from the mold. However, the solid carbon is adsorbed on the surface of the coating wall casting, resulting in the surface carburization of the casting. In addition, when styrene and benzene vapor are discharged under negative pressure, they are condensed in the coating and surrounding sand. The organic liquid substances adsorbed on the coating layer will continue to decompose during the solidification of molten steel and the cooling of castings, which will also cause the carbon increase of castings. For different steel castings, the depth of carburized layer is 0.1 ~ 0.3mm, and the amount of carburization is 0.01%.
Volume carburization: during the pouring process, there is a large heat gradient (from room temperature to 1550 ℃) in the dynamic gap between the front edge of molten steel and the pattern, and the heat transfer from the front edge of molten steel to the pattern decomposition is mainly completed by heat radiation. The temperature near the front of molten steel is the highest, near the temperature of molten steel, and the amount of carbon is large. Therefore, the dynamic and thermodynamic conditions required for the carburization of liquid steel in the filling process are very sufficient, and it is easy to form the volume carburization of castings at this time. Volume carburization is less important than surface carburization.
Local carburization: when the method of introducing the molten steel into the mold is not right, the liquid product is involved in the molten steel during the pouring process, and then it is decomposed into solid carbon and gas. If the gas does not overflow the molten steel and stay in the interior, it will cause porosity: solid carbon is directly absorbed by the molten steel, resulting in the increase of local carbon content of the casting, forming local carburization of the casting.
Surface decarburization: dry sand casting is used in EPC, the cooling rate of the casting is slow, and the carbon content of the casting surface will continue to change after solidification. During the cooling process, there are not only surface carburization, but also surface decarburization. The decarburization of the surface is terminated at higher temperature, and the decarburization is mainly formed in the cooling process of the casting. In mechanism, decarburization is an oxidation reaction, mainly the oxidation of matrix iron and carbon.
In the actual process, the form of carburization depends on the hot shock of the molten steel, the thermal decomposition state of the pattern and the way of the interaction between the product and the molten metal.