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Measures to prevent pitting in castings

Released on Dec. 12, 2019

Pores are one of the common surface defects in OEM stainless steel casting. The first step to reduce or prevent pitting defects is to improve the quality of the molten metal and ensure the quality of the refractory material on the surface of the shell; the second is to ensure the baking quality of the shell.

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

First, choose dry and clean charge materials to control the number of times or the amount of repeated use of recharge materials to avoid the increase of oxides in the charge materials.

Second, complete deoxidation, first add ferromanganese, add ferrosilicon to deoxidize, then add silicon calcium to deoxidize, and then leave to stand for 2 minutes after power failure, and then add aluminum to deoxidize, and then take out and cast. It is also possible to use only aluminum final deoxidation and then supplement deoxidation. That is to say, the final deoxidation in production adopts the second addition method. The first final deoxidation in the furnace is 0.10% to 0.12% (the amount of aluminum added is related to the rust of the charge. When the rust of the charge is severe, the upper limit is taken; otherwise Lower limit). The second time to add supplemental deoxidation to the ladle, it is appropriate to use aluminum 0.02% to 0.05%.

Third, large steel castings manufacturers formulate a reasonable melting process and pouring process, and strictly implement it. For example, the sedation time before tapping should be able to fully float the oxides, the slag must be cleaned, and the oxides that have been generated should be removed in time; Plant ash, if necessary, vacuum casting to prevent secondary oxidation of molten metal.

(2) Reasonably select the shell material and ensure its quality.

First, the quality of the selected shell material, such as zirconium quartz sand / powder, must meet the process requirements, especially the oxides contained in the surface refractory must be within the process range. The metal processing is really good, especially the oxides such as iron oxide. The content should not exceed the standard, too high Fe2O3 content will aggravate the generation of pits.

Second, the raw materials entering the factory must be inspected, and they can be entered into the warehouse after passing the inspection. The raw materials in stock should be re-inspected regularly, and the unqualified surface layer materials cannot be used.

Casting Pump Body

Casting Pump Body

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

The purpose of firing 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. The shell roasting process often used in production: the temperature is 850 ~ 900C, and the time is 2 ~ 4h (time is related to the furnace capacity).The quality of the shell baking quality can be judged by the color of the appearance of the shell. If the shell is dark gray, it means that there is more carbon in the shell, which means that the shell is not fully 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 it is stored for too long, it should be baked again (not roasted) before pouring. The shell cannot be fired multiple times, and each time it fires, it will increase or enlarge the cracks on the shell.

(4) Use neutral or alkaline binders.

The main causes of pitting in stainless steel investment castings such as custom stainless steel bracket are: firstly, the metal liquid and the shell surface layer refractory contain more oxides; secondly, the shells are poorly baked, and again the type of binder.

When the first three measures are still not effective, you can consider using a neutral or alkaline binder. Neutral binders include metal salts such as aluminum, chromium, and zirconium, such as aluminum nitrate Al (OH) 2NO3, basic aluminum chloride Al (OH) 2Cl, and basic chromium nitrate Cr (OH) 2NO3. Basic binders are mainly metal salts such as magnesium and calcium, such as Ca (NO3) 2, Ca3 (PO4) 2, Mg3 (PO4) 2 and so on.

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