Measures to reduce surface carburization of EPC steel castings

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

1. the balance of carbon concentration between the original liquid steel and the pattern

The distribution of carbon concentration on the surface of steel casting mainly depends on the concentration gradient of carbon in the pattern decomposition products and carbon in the molten steel. With the increase of the original carbon content in the molten steel, the concentration difference between the two decreases, which can effectively inhibit the carbon increase on the surface of steel casting. For example, Cr can increase the carburization of steel casting surface, but can reduce the depth of carbon layer.

2. Choose low density or low carbon pattern material

The change of pattern material can significantly reduce the carbon concentration of pattern decomposition products, thus reducing the carburization of casting surface. The lower the density of the pattern, the less the gas generation and the corresponding reduction of the solid carbon, so the carbon concentration in the pattern decomposition products can be reduced. The low density pattern is easy to gasify and has less residual liquid phase, so it has little cooling effect on the molten steel, which is beneficial to reduce the local carburization of the cast steel. For the production of general steel castings, the pattern density is suitable to be controlled between 0.016 and 0.025g/cm2. The production practice shows that the copolyester pattern material can not only overcome the large amount of residual carbon in the thermal decomposition products of EPS pattern material, but also overcome the defects of PMMA pattern, such as back spraying, choking, air hole, etc., which can be used as the first pattern material of steel casting.

Decarburizing agents, such as boric acid and borax, can be added to the pattern material to prevent its combustion. Caustic soda and hydrazine phosphate can also be added to increase the ignition temperature of the pattern, delay the combustion, and shorten the reaction time between the metal liquid and the pyrolysis products of the pattern.

Flame retardant is added to the pattern to prevent cracking and burning at high temperature, so that no or less carbon containing solids are produced. For example, chlorinated paraffin, triphosphate, pentabromodiphenyl ether, antimony trioxides, etc. can be added, as well as diphenyl peroxide, lauroyl oxide, etc. can be added to accelerate the transformation of flame retardant containing pattern into gas.

3. Use high permeability coating

In EPC production of steel castings, coating is another important factor that affects the surface carburization of steel castings. The air permeability of the coating layer is good, and the carbon increment can be reduced. The aggregate is round, the strength is coarse and concentrated, and the coating with multi empty structure is conducive to improving the permeability of the coating. In addition, reducing the thickness of the coating can also significantly enhance the permeability of the coating. Experience shows that the influence of coating thickness on carburization is greater than the composition of coating materials. When the permeability of the coating layer is low, the formed carbon is difficult to be removed, the air pressure in the gap is increased, the gasification temperature of the pattern material is increased, so the solid-phase carbon formed is increased correspondingly, and the tendency of carbon increase on the casting surface is increased. The addition of oxidizing materials in the coating can change the reducing atmosphere in the casting process and effectively inhibit the surface carburization. Three kinds of coatings, quartz powder, corundum powder and staggered quartz powder, are used to increase the carbon in the casting, but the phenomenon of carbon increase in the casting is the least.

4. Reasonable casting process

The influence of casting process on the surface carburization of steel castings is complex. The key is to choose the location of the internal sprue, set the necessary exhaust channel and slag collecting riser. When the bottom casting is used, the amount of carburization in the upper part of the casting is large; when the side casting or top casting is used, the regularity of carburization in the top part of the casting is poor, and local carburization is easy to occur in the lower part of the casting. After the necessary exhaust channel and slag collecting riser are set, the pyrolysis products can be discharged smoothly, which is beneficial to reduce the carburization defects. For the castings with uneven wall thickness, it is beneficial to reduce the carburization defects when the sprue is set in the thin wall. High casting temperature and proper casting speed are beneficial to the smooth filling of molten steel and the smooth discharge of pyrolysis products.

5. Appropriate negative pressure of profile wall

Proper increase of negative pressure is beneficial to reduce the carburization degree of casting surface. Negative pressure can reduce the gas concentration in the gap and the amount of solid-phase carbon in the pyrolysis products of the pattern. At the same time, negative pressure is also conducive to the generated solid-phase carbon being discharged out of the pattern with the gas, thus reducing the carburization of the casting. For general steel castings, the negative pressure can be controlled within 0.03 ~ 0.05Mpa.

In addition, in order to reduce the further diffusion of pyrolytic carbon from the coating layer to the surface of high-temperature castings, the castings should be sand shed as early as possible.

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