The carburizing defects of steel castings can be divided into surface carburization, volume carburization and local carburization.
Surface carburizing: during the filling process, due to the pyrolysis and gasification of the foam pattern, an air gap layer is formed at the front of the molten steel, and the air gap layer contains a large amount of hydrogen and solid carburizing. Under the negative pressure, the gas enrichment products are generally discharged through the coating layer, but a part of the solid carburization is still adsorbed on the inner wall of the coating layer, resulting in Carburization on the casting surface. In addition, a part of the steam increased benzene and styrene condense in the coating layer and the surrounding dry sand during the discharging process. These liquid products will continue to decompose during the solidification and cooling process of the casting, which will also lead to surface carburization of the steel castings.
Volume carburizing: during the filling process, there is a great thermal gradient between the air gap of the molten steel front and the foam pattern. The transfer of heat from the front of the molten steel to the foam pattern is mainly achieved by thermal radiation. The higher the temperature near the front edge of molten steel, the more carbon generated by pyrolysis. The thermodynamic and kinetic conditions required for carburization at the liquid surface are relatively sufficient, and it is easy to form volume carburization of steel castings.
Local carburization: when the introduction of molten metal is improper or the process parameters are set unreasonably, it is easy to cause turbulence and soft phenomenon. When soft, the liquid products of foam pattern are easily wrapped in steel water, and then continue to decompose solid carbon and gas. If the gas can not be discharged, it will produce porosity defects. When solid carburization is absorbed by liquid metal, local carburization will occur.