Knowledge

1. Overview

(1) Characteristics: There are numerous small pitted holes on the surface of the casting, with diameters ranging from 0.3 to 0.8 mm and depths of 0.3 to 0.5 mm. This defect often occurs on alloy steel castings with low carbon content and w(Cr) ranging from 5% to 18%. Before cleaning, the shallow pits are filled with slag-like substances.

(2) Causes: The pitting is mainly caused by chemical reactions between metal oxides and oxides in the shell material.

(3) Locations: On the local or entire surface of the casting, with more occurrences in thin-walled areas.

(4) Illustration: See Figure 1.

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Figure 1 Pitting

 

2. Causes

(1) Excessive metal oxides in the molten metal

① Excessive oxides in the charge. When using induction furnaces for melting, if the charge is severely rusted or a large proportion of recycled materials is used with many recycling times, the amount of oxides in the molten metal will increase.

② Inadequate deoxidation of the molten metal. Improper selection of deoxidizers fails to achieve the goal of thoroughly deoxidizing the molten metal while ensuring that the formed oxides have a low melting point, are easy to aggregate and float. Insufficient addition of deoxidizers affects the deoxidation effect, leaving excessive oxides in the molten metal.

③ Improper melting process or operation. This may result in incomplete removal of oxides from the molten metal, etc. During the melting process of the charge, if the molten metal surface is exposed for a long time, the oxidation opportunity of alloy elements will increase. After pre-deoxidation, if the power-off holding time is short, the oxides in the molten metal will not float up in time and completely. Inadequate final deoxidation may cause secondary oxidation during pouring.

 

(2) There are too many metal oxides in the surface layer refractory materials. Zircon sand powder has the advantages of good thermal conductivity, large heat storage capacity and high refractoriness. It is used as the surface layer refractory material of the shell in the production of stainless steel investment castings. The refractoriness of pure zircon sand ZrO2·SiO2 is above 2000℃. However, with the increase of impurity content, the refractoriness decreases accordingly. When zircon sand contains Ca and Mg oxide impurities, the decomposition temperature will drop to about 1300℃; when it contains K and Na oxides, the decomposition temperature will drop to about 900℃. The amorphous SiO2 precipitated during the decomposition of zircon sand has high activity and can react chemically with alloy elements such as Cr, Ni, Ti, Mn and Al in the metal at high temperatures, resulting in pitting on the surface of the casting.

(3) Poor shell baking. Poor shell baking leaves residual moisture, organic matter and volatile substances in the mold cavity. During pouring, these substances cause secondary oxidation of the molten metal, generating a large amount of gas or new oxides. These oxides react chemically with the oxides in the shell, easily forming pockmarks on the surface of the casting.

(4) Improper pouring process. When pouring, if the temperature of the molten metal and the mold shell is too high and the cooling is too slow, it is conducive to the interaction between the molten metal and the surface layer material of the mold shell, which is likely to cause the casting to have pockmarks.

(5) Insufficient protection during the pouring and solidification process. During the pouring and solidification process, the molten metal reacts with the mold shell and the gases from the outside, resulting in pitting on the castings.

 

3. Prevention and Control Measures

(1) Improve the quality of molten metal and reduce the content of oxides in it

① Select dry and clean furnace materials, control the usage of recycled materials and the number of times they are reused; avoid increasing the amount of oxides in the furnace materials.

② Adopt complete deoxidation: add ferromanganese first, then ferrosilicon for deoxidation, followed by calcium silicate for deoxidation. Then, cut off the power and let it stand for 2 minutes, and finally add aluminum for final deoxidation. In production, the final deoxidation can also be carried out by the double addition method. The first addition is made in the furnace, with an addition amount of 0.10% to 0.12% (the addition amount of aluminum is related to the rust of the furnace materials. When the rust is severe, take the upper limit, otherwise take the lower limit). The second addition is made in the ladle as supplementary deoxidation, with an addition amount of 0.02% to 0.05%.

③ Develop reasonable smelting and casting processes and strictly follow them. For example, the holding time before tapping should be sufficient to allow oxides to float up fully. The slag removal should be thorough, and the generated oxides should be removed in time. Add covering agents such as plant ash in the ladle.

(2) Select shell materials reasonably and ensure their quality.

① Select shell materials reasonably. For example, the quality of zircon sand/powder must meet the process requirements, and the content of Fe2O3 and impurities should be strictly controlled to be less than 0.05%. When necessary, the surface layer of the shell should be made with neutral corundum powder and coated with corundum sand. Excessive Fe2O3 content will aggravate the occurrence of pitting.

② Inspect the raw materials upon arrival at the factory. Only those that pass the inspection can be stored in the warehouse. Regularly re-inspect the raw materials in the warehouse. Surface layer materials that do not meet the standards cannot be put into use.

(3) Select a reasonable shell baking process and strictly follow it. The purpose of baking the shell is to remove volatile substances (such as residual moisture, organic matter, mold material, etc.) from the shell and further improve the quality of the shell. Shell baking process: The baking temperature for sodium silicate shell is 850~950℃, and the time is 0.5~2 hours; the baking temperature for silica sol shell is 950~1200℃, and the time is 30 minutes.

(4) Select a reasonable casting process. Strictly control the temperature of the molten metal and the shell within a reasonable range during casting. Avoid prolonged interaction between the molten metal and the surface layer material of the shell to prevent the formation of pockmarks.

(5) Protective treatment should be carried out during the pouring and solidification process. Cover the mold with sand grains or form a protective atmosphere around the mold; pour under vacuum conditions if necessary.

 

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