Control method for surface pitting of precision castings

Control method for surface pitting of precision castings

Pitting, also known as pitting, pitting, and oxidation pitting, is one of the common surface defects in stainless steel investment castings. This defect cannot be repaired and can only be scrapped. This not only increases production costs, but also affects normal production progress and delivery. Today, we will analyze how to reduce and eliminate pitting defects in castings.

1、 Cause of formation

1. Metal oxide inclusions brought in by poor deoxidation of molten steel;

2. Related to the quality of shell roasting, shell roasting can remove moisture, residual wax, saponification substances, organic matter, volatile substances, as well as residual ammonium chloride hardening solution and sodium salts from the shell. If these substances remain in the mold shell (see Figure 2), it will cause secondary oxidation of the molten metal during pouring, generating a large amount of gas or new oxides. These oxides react chemically with the oxides in the mold shell, which can easily generate pitting defects on the surface of the casting.

① When the shell is not fully baked, a small amount of gas will be generated at high temperatures during casting, which will promote the occurrence of interfacial reactions and form pitting.

② Currently, water glass, silica sol, and ethyl silicate hydrolysate widely used in investment casting production are all acidic binders, with the common feature of generating acidic oxide SiO2 (i.e. silica sol). This type of binder is suitable for pouring carbon steel, copper, and aluminum alloy castings. If titanium and chromium alloy steel castings are poured, it is easy to form pitting defects on the surface of the castings.

2、 Preventive measures

1. Reduce and lower the oxides in the metal liquid, and improve the quality of the metal liquid.

2. The operation should be strictly carried out according to the smelting process. The process is carried out by following the steps of bottom slag cushion, covering during melting, pre deoxidation after melting, power outage and static slag removal, and final deoxidation before pouring.

3. The selection of deoxidizer should achieve the goal of both deoxidizing the steel liquid and reducing the melting point of the oxide formed after deoxidation, making it easy to aggregate and float. The amount of aluminum added as the final deoxidizer must be strictly controlled, as excessive aluminum will promote the formation of pitting. (The residual amount of aluminum should be controlled between 0.015 and 0.02. If it is too low, it is easy to produce pores and pitting, and if it is too high, there will be white and black spots after processing.)

4. Develop reasonable smelting and pouring processes, and strictly implement them. For example, the static time before tapping should be able to make the oxide float, the slag should be cleaned, and the generated oxide should be removed in a timely manner; Add grass and wood ash to the pouring bag, and if necessary, use vacuum pouring to prevent secondary oxidation of the metal liquid.

5. Due to the temperature of the mold shell being above 800 ℃ -900 ℃ during pouring, and the temperature of the molten steel being poured being around 1600 ℃, the temperature in contact with the casting surface is extremely high at this time, and the metal will not solidify instantly, especially for thick walled parts. At this time, oxygen in the atmosphere can easily react with the metal surface through the mold shell, form oxides, and aggregate with inclusions in the steel to form pitting. Therefore, when pouring large parts, wood chips (or charcoal) are added to the insulation. After pouring, the module immediately adds waste wax blocks to cover the box for insulation, which is an important measure to prevent the generation of pitting.

6. The roasting temperature of the shell shall not be lower than 1180 ℃, and the insulation time shall not be less than 45 minutes. The partially burned shell will have a small amount of gas generation during pouring, and will participate in the interface reaction between the steel liquid and the shell, bringing oxygen into the steel liquid and causing secondary oxidation. Carbon steel, on the other hand, has a high content of C and does not contain Cr. Therefore, when cooled in air, the interface reaction can only form a decarburization layer without producing pitting; If there are spots, the shell is not burned properly.

3、 Summary

1. Pitting is one of the common surface defects in stainless steel investment castings.

Precision machining