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Causes of Pitting Defects in Casting

Sep. 08, 2020

Cast iron weights manufacturers share this article for you.

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

1. The characteristics of pitting

Pockmarks usually appear on stainless steel castings with wCr<20% and wNi<10%. There are many gray-black circular shallow pits on the surface of the casting, the diameter of the pits is 0.3 to 1.0 mm, and the depth of the pits is 0.3 to 0.5 mm.

According to relevant information, the pit was filled with slag material before the casting was cleaned. The petrographic analysis shows that there are chemicals such as iron silicate, manganese silicate and chromium silicate in the slag material at the defect. The electron diffraction results show that the black pits are composed of magnetite (Fe3O4) and iron chromium spinel (FeO·Cr2O3). According to the results of spectroscopic analysis, the silicon content in the metal component of the defect increased, while the manganese content was extremely small.

After the casting is cleaned by shot blasting and sand blasting, the surface of the casting will have gray-black pits, as shown in Figure 1.

Figure 1







FIG. 1 Pitting on casting

Pitting defects often occur on local thick sections, corners, inner holes, and even on the whole surface of the casting.







2. Cause

It can be seen from the above-mentioned petrographic, electron diffraction and spectroscopic analysis that the pitting is mainly caused by the chemical reaction between the metal oxide and the oxide in the shell material, especially the improper selection of refractory materials for the shell surface layer, or The surface layer refractories of the factory do not meet the quality requirements, or the surface layer refractories after entering the factory are not well managed. After pouring, it is easier to produce different degrees of pitting defects on the surface of batches of castings. The main reasons are as follows:

(1) Too much oxide in molten metal

① Too many oxides in the charge. When using induction furnace for smelting, the charge is more rusty and heavier, or the proportion of recycled charge is larger, and the number of reuse is more, all of which will increase the oxide in the molten metal.

②Insufficient deoxidation of molten metal. The choice of deoxidizer should not only fully deoxidize the molten metal, but also achieve the purpose of low melting point of the oxide formed after deoxidation, and easy aggregation and floating. The amount of deoxidizer added is small, and the metal processing is really good, and it affects the deoxidation effect. The metal processing is really good, leaving too much oxide in the molten metal.

③Improper smelting process or improper operation, the oxides in the molten metal are not cleaned, etc.; during the melting of the charge, the surface of the molten metal is exposed for a long time, which increases the chance of oxidation of alloy elements. After the pre-deoxidation, the power failure and the standing time are short, the oxides in the molten metal do not float up in time and completely; the final deoxidation is insufficient, and secondary oxidation occurs during the pouring process.

(2) There are too many oxides in the shell surface material. According to relevant information, zircon sand has the advantages of good thermal conductivity, large heat storage capacity and high refractoriness. When producing stainless steel investment castings, zircon sand is used as the mold. Refractory for the shell surface. The refractoriness of pure ZrSiO4 is above 2000 ℃, but as the impurity content increases, the refractoriness decreases accordingly. When the zirconium sand contains Ca and Mg oxide impurities, the decomposition temperature will drop to about 1300°C; when it contains K and Na oxides, the decomposition temperature will drop to about 900°C. Zircon sand is the only compound in the binary system of ZrO2·SiO2. The amorphous SiO2 precipitated during its decomposition has high activity and can interact with Cr, Ni, Ti, Mn, Al and other alloy elements in the metal at high temperatures. The chemical reaction causes pitting on the surface of the casting.
For example, a certain investment foundry produces stainless steel castings, and the original shell surface material is made of quartz sand/powder. In severe cases, the pitting defects of the castings account for about 80%. Later, when the zircon sand/powder was replaced (without incoming material inspection) and ethyl silicate hydrolysate as the binder, the pitting defects of the castings were reduced, but still around 50%. When the incoming inspection is adopted to control the oxides in the zircon sand/powder, not only the number of pitting defects in the casting is greatly reduced, but the degree of pitting defects is also significantly reduced.

(3) It is related to the quality of mould shell roasting. Full roasting of mould shell can remove moisture, residual wax, saponification, organic matter and volatile matter in mould shell, as well as residual ammonium chloride hardening liquid and sodium salt in mould shell. If these substances remain in the mold shell (see Figure 2), the molten metal will undergo secondary oxidation during pouring, generating a large amount of gas or new oxides. These oxides chemically react with the oxides in the mold shell. It is easy to generate pitting defects on the surface of the casting.

 Figure 2





Figure 2 residue in the cavity

Saponification salt




 


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

3. Preventive measures

(1) Reduce and reduce the oxides in the molten metal and improve the quality of the molten metal.

First, select dry and clean charge, control the number of times or usage of the recharge charge, and avoid the increase of oxides in the charge.

Second, take complete deoxidation, first add ferromanganese, add ferrosilicon for deoxidation, then add silicon calcium for deoxidation, and then stand for 2 minutes after power failure, then add aluminum for final deoxidation, out of furnace, and pouring. It is also possible to select only the final deoxidation of aluminum, and then supplement the deoxidation. That is to say, the final deoxidation in the production adopts the secondary addition method, the first final deoxidation in the furnace, the addition amount is 0.10% ~ 0.12% (the amount of aluminum added is related to the corrosion of the furnace charge, and the upper limit of the amount of addition is taken when the furnace charge is severely rusted; Lower limit). For the second time, add supplemental deoxidation to the ladle, it is advisable to use aluminum 0.02%~0.05%;

Third, formulate reasonable smelting and pouring processes, and strictly implement them. For example, the sedation time before tapping must be able to fully float the oxides, slag slag must be cleaned, and the generated oxides must be removed in time; add them to the ladle Plant ash, when necessary, use vacuum pouring to prevent secondary oxidation of molten metal.

Some materials suggest that charcoal powder should be added to the shell sand filling material, or carbonaceous materials such as graphite and pitch can be added to the shell making material, or carbon tetrachloride should be added to the shell before pouring, or the mold should be molded immediately after pouring. The shell is placed in a reducing atmosphere, such as adding sawdust or waste wax and then sealing the box for cooling, or reducing the melting temperature and other measures to reduce or reduce pitting. The author believes that when selecting these measures, we must fully consider their feasibility and rationality in mass production, as well as their pollution to the working environment and their impact on the health of the operators.

(2) Reasonably select shell materials and ensure their quality.

First, the selection of shell materials, such as the quality of zirconium quartz sand/powder, must meet the process requirements, especially the oxides contained in the surface refractories must be within the process range. The metal processing is really good, especially the oxides such as iron oxide. The content can not exceed the standard, too high Fe2O3 content will aggravate the occurrence of pitting.

Second, the incoming raw materials must be inspected, and only after they are qualified can they enter the factory and warehouse. The stock of raw materials shall be re-inspected regularly, and unqualified surface materials shall not be used.

(3) Choose a reasonable shell roasting process and strictly implement it.

The purpose of baking the shell is to remove volatile substances (such as residual moisture, salt, mold material, etc.) from the shell, and further improve the quality of the shell. Shell roasting process often used in production: temperature is 850-900C, time is 2-4h (time is related to furnace load).

The baking quality of the shell can be judged from the color of the appearance of the shell. If the shell is dark gray, it means that there is more carbon in the shell, indicating that the shell is not sufficiently roasted; if the inner surface or section of the shell is white, pink or pink, it means that the roasting quality is good.

The baked shell should be poured in time. If the storage time is too long, it should be baked again (not roasted) before pouring. The shell cannot be fired multiple times. Each firing will increase or enlarge the cracks on the shell.

(4) Use neutral or alkaline binder.

The main reasons for the pitting of stainless steel investment castings are: firstly, there are more oxides in the molten metal and the refractory material of the shell surface layer, the second is the poor firing of the shell, and the third is the type of binder.

When the first three measures are still unsatisfactory, only neutral or alkaline binders can be considered. Neutral binders include aluminum, chromium, zirconium and other metal salts, such as aluminum nitrate Al(OH)2NO3, basic aluminum chloride Al(OH)2Cl, and basic chromium nitrate Cr(OH)2NO3. Alkaline binders are mainly magnesium, calcium and other metal salts, such as Ca(NO3)2, Ca3(PO4)2, Mg3(PO4)2 and so on.

4. Conclusion

(1) Pockmarks are one of the common surface defects of stainless steel investment castings.

(2) To reduce or prevent pitting defects, the first step is to improve the quality of molten metal and ensure the quality of refractory materials for the shell surface; secondly, to ensure the roasting quality of the shell.


luckykong@sjfr-machinery.cn 602589453