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Causes of pitting defects in castings

Released on Dec. 05, 2019

The pits are also called pits, pits, and oxidized pits, which are one of the common surface defects of stainless steel investment castings. Generally, such defects cannot be repaired but can only be scrapped. Not only increased production costs, but also affected normal production schedule and delivery. Therefore, how to reduce and eliminate pitting defects in castings is one of the main tasks of large steel castings manufacturers.

The pitting is mainly caused by the chemical reaction between the metal oxide and the oxide in the shell material, especially the inappropriate selection of the refractory material of the shell surface layer, or the refractory material of the shell surface layer that does not meet the quality requirements, The reason for poor management of the refractory material of the shell surface layer after the factory is that it is easier to produce pitting defects of different degrees on the surface of a batch of castings after pouring. The main reasons are as follows:

Casting Pump Frame Part

Casting Pump Frame Part

(1) Too much oxide in the molten metal: Too much oxide in the charge. When using an induction furnace for smelting, the charge is more rusted and heavier, or the proportion of regrind used is larger, and the reuse frequency is more, which will increase the oxides in the molten metal. Insufficient deoxidation of molten metal. The choice of deoxidizer should not only make the metal liquid fully deoxidize, but also achieve the purpose of low melting point of oxides formed after deoxidation, easy to aggregate and float. Adding a small amount of deoxidizer, metal processing really affects the deoxidation effect, metal processing really does make excessive oxides remain in the metal liquid. Improper smelting process or improper operation, the oxides in the molten metal have not been removed, etc .; during the melting process of the furnace, the surface of the molten metal is exposed for a long time, which increases the chance of oxidation of alloy elements. After pre-deoxidation, the blackout time is short, and the oxides in the metal liquid do not float up in time and completely; the final deoxidation is insufficient, and secondary oxidation occurs during the pouring process.

(2) Too much oxide in the shell material. According to related information, because zircon sand has the advantages of good thermal conductivity, large heat storage capacity, high fire resistance, etc., in the production of stainless steel castings, zircon sand is used as the shell surface refractory material. Zircon sand is the only compound in the ZrO2 · SiO2 binary system. The amorphous SiO2 precipitated during decomposition has a high activity and can occur with alloy elements such as Cr, Ni, Ti, Mn, and Al at high temperatures. Chemical reaction causes pitting on the surface of the casting.

(3) It is related to the calcination quality of the shell. Full roasting of the shell can remove the moisture, residual wax, saponification, organic matter and volatile matter in the shell, as well as the residual ammonium chloride hardening solution and sodium salt in the shell. If these substances remain in the mold shell, the metal liquid will undergo secondary oxidation during pouring, generating a large amount of gas or new oxides. These oxides react chemically with the oxides in the mold shell, which is easy to be on the surface of the casting. Generate pitting defects.

(4) Related to the binder. At present, water glass, silica sol and ethyl silicate hydrolysate widely used by stainless steel casting parts factory in investment casting production are all acidic binders, and their common feature is that they can generate acidic oxide SiO2 (that is, silicate sol). This kind of adhesive is suitable for casting of carbon steel, copper and aluminum alloy castings. If casting of titanium, chromium alloy steel castings, it is easy to form pitting defects on the surface of the casting.

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