Mechanism and prevention of carbon increase on the surface of lost die casting steel

Surface carburization of steel castings by EPC has always been a controversial topic.  A lot of experiments have been carried out on whether EPC is suitable for producing steel castings, especially low carbon steel castings.

(1) the phenomenon and mechanism of carburization

Surface carburization of cast steel is manifested in several aspects: surface carburization, volume carburization, local carburization and surface decarburization.

Surface carburization: in the molten steel filling process, there is a large amount of hydrogen in the air gap between the molten steel front and the solid shape.  That means there’s solid carbon.  The gas product can penetrate into the coating under the action of 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 the styrene and benzene vapor is discharged under negative pressure, it condenses in the coating and the surrounding sand.

Volume carburization: in the pouring process, there is a large heat gradient in the dynamic gap between the molten steel front and the profile, and the heat in the gap is transferred from the molten steel front to the profile decomposition mainly by thermal radiation.  The outlet temperature near the molten steel front is the highest, close to the molten steel temperature, where the amount of carbon generation is large, so the dynamic and thermodynamic conditions required for the liquid steel surface carburization in the mold filling process are sufficient, and it is easy to form the volume carburization of the casting at this time.

Local carburization: when the molten steel is introduced into the casting mold, the liquid product in the pouring process is involved in the molten steel, and then decomposed into solid carbon and gas. If the gas fails to overflow the molten steel to stay inside, it will lead to the formation of pores.  The solid carbon is directly absorbed by the molten steel, resulting in the increase of the local carbon content of the casting and the formation of local carburization of the casting.

Surface decarburization: Lost mold casting adopts dry sand molding, casting cooling rate is slow, solidified casting surface carbon content will continue to change, the cooling process is not only surface carburization, but also surface decarburization.  Surface decarburization is terminated at higher temperature, and the decarburization is mainly formed in the cooling process of casting.

(2) Measures to reduce carburization

The influencing factors of surface carburization of lost die casting steel include composition of molten steel, thickness of casting wall, cooling rate of casting, thickness and permeability of coating layer, negative pressure, shape material, shape density, pouring riser process, exhaust method, pouring position, pouring temperature and pouring speed.

1. Balance between original liquid steel and shape carbon concentration

The distribution of carbon concentration on the surface of steel castings mainly depends on the concentration gradient between the carbon in the molded steel and the carbon in the molded steel. With the increase of the original carbon content in the molded steel, the concentration difference between the carbon and the molten steel decreases, which can effectively inhibit the surface carburization of steel castings.  The alloying elements in molten steel have a great influence on the surface carburization of cast steel. For example, Cr can increase the surface carburization of cast steel, but can reduce the depth of carbon layer.

2. Choose low density or low carbon shape materials

The change of shape material can significantly reduce the carbon concentration in the shape decomposition products, thus reducing the surface carburization of the casting.  The lower the density of the shape, not only the amount of gas is reduced, but also the corresponding reduction of solid carbon generation, so the carbon concentration in the decomposition products of the shape is reduced.  The appearance of low density is easy to gasify, and the residual liquid phase is less, so the cooling effect on molten steel is small, which is conducive to reducing the local carburization of cast steel.  It can be used as the first shape material of steel castings to produce defects such as back spray, choking fire and air hole when casting.

Add decarburizing agent in the shape material, wait like boric acid, borax, prevent its combustion;  Can also add caustic soda, ammonia hydrogen phosphate salts, improve the shape of the ignition temperature, delay delay combustion, shorten the action time of molten metal and shape pyrolysis products.

3. Use high breathable paint

Coating is another important factor affecting surface carburization of steel castings in EPC production.  The air permeability of the coating layer is good, which can reduce the amount of carburization.  The aggregate is round, the strength is coarser and the distribution is concentrated. The coating with multi-empty structure is beneficial to improve the air permeability of the coating.  In addition, reducing the thickness of the coating can also significantly enhance the air permeability of the coating.  Experience shows that the influence of coating thickness on carburization is greater than the composition of coating materials.  In the case of low permeability of the coating layer, the carbon has been formed difficult to remove, the air pressure in the gap increases, improves the gasification temperature of the shape material, thus forming the solid phase carbon increases correspondingly, resulting in the tendency to increase the surface carburization of the casting.  The addition of oxidizing materials in the coating can change the reducing atmosphere in the casting process and effectively restrain the surface carburization of parts.

4. Reasonable casting process

The influence of casting process on surface carburization of steel castings is complex, and the key is to choose the location of inner runner, set up the necessary exhaust passage and slag collecting riser.  When the bottom casting is used, the carburization of the upper part of the casting is large.  When side casting or top casting is used, the regularity of carburization at the top of the casting is poor, and local carburization is easy to occur at the lower part of the casting.  After setting the necessary exhaust passage and slag collecting riser;  It can guide the smooth discharge of pyrolysis products and help to reduce the defect of carburization.  For the casting with uneven wall thickness, opening the inner runner in the thin wall is beneficial to reduce the carburizing defect.  Higher casting temperature and proper casting speed can be used to fill the molten steel and discharge the pyrolysis products smoothly.

5. Appropriate negative pressure of the mold wall

Appropriate increase of negative pressure is beneficial to reduce the degree of carburization on the surface of casting.  The negative pressure can reduce the concentration of gas generated in the gap and reduce the amount of solid carbon in the pyrolysis products.  At the same time, the negative pressure is also conducive to the generated solid carbon with the gas out of the mold, and then reduce the casting carburization.  For general steel castings, the negative pressure degree can be controlled in 0.03 ~ 0.05MPa.  In addition, in order to reduce the pyrolytic carbon in the coating layer to further diffuse to the surface of high temperature castings, castings should be treated as soon as possible.

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Post time: Jun-04-2021