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Structural Influencing Factors and Performance Requirements of Stainless Steel Precision Castings

Generally speaking, the dimensional accuracy of precision castings is affected by many factors, such as casting structure, casting material, mould making, shell making, roasting, pouring, etc. Any unreasonable link setting and operation will change the shrinkage rate of castings, resulting in deviation of the dimensional accuracy and requirements of castings. The following factors can cause dimensional accuracy defects of precision castings:

(1) Casting structure: a. Casting wall thickness, shrinkage rate is large, casting wall is thin, shrinkage rate is small. B. The free shrinkage rate is high and the hindrance shrinkage rate is small.

(2) The influence of casting material quality: a. The higher the carbon content in the material, the smaller the linear shrinkage, the lower the carbon content, the larger the linear shrinkage. B. The casting shrinkage of common materials is as follows: casting shrinkage K=(LM-LJ)/LJ*100%, LM is cavity size, LJ is casting size. K is affected by the following factors: wax mould K1, casting structure K2, alloy K3, pouring temperature K4.

(3) The influence of mould making on the linear shrinkage of casting: a. The influence of wax injection temperature, wax injection pressure and holding time on the size of melting die is more obvious than that of wax injection pressure. The holding time has little effect on the final size of melting die after ensuring the melting process. B. The linear shrinkage of wax (mould) material is about 0.9-1.1%. C. When the melt is stored, further shrinkage will occur, the shrinkage value is about 10% of the total shrinkage, but after 12 hours, the size of the melt is basically stable. D. The radial shrinkage of wax mould is only 30-40% of the length shrinkage. The effect of wax injection temperature on the free shrinkage is much greater than that on the hindered shrinkage (the better wax injection temperature is 57-59, the higher the temperature is, the larger the shrinkage is).

(4) The influence of shell-making materials: zircon sand, zircon powder, Shangdian sand and Shangdian powder can be neglected because of their small expansion coefficient, which is only 4.6 *10-6/(?)

(5) The influence of shell roasting: because the expansion coefficient of shell is small, when the shell temperature is 1150 C, it is only 0.053%, so it can also be ignored.

(6) The influence of casting temperature: the higher the pouring temperature, the larger the shrinkage rate, the lower the pouring temperature and the smaller the shrinkage rate, so the pouring temperature should be appropriate.

If there are more impurities in the oxide film of stainless steel investment casting, its purity will be low and its transparency will be reduced. The impurities in the oxide film mainly come from stainless steel castings themselves, and some of them come from oxidation bath liquid. Therefore, if the content of alloy elements in the castings is reduced properly, the impurity content can be reduced as far as possible.

The equipment needed for the basic process of stainless steel precision casting is: precision casting wax ejector, ice water ejector, air conditioner; slurry-barrel, sand floater, air conditioner, dewaxing furnace, dehumidifier, dewatering barrel and static barrel; mechanical arm and automatic centralized wax feeding system can also be used for slurry immersion.

Gold melting-sintering furnace, high cycle furnace, shell shaking machine; post-processing-cutting machine, grinding machine, sandblasting machine, cleaning machine, heat treatment furnace, punch; overhaul-pneumatic grinding machine, argon welding machine, grinding machine; quality assurance-spectrometer, tertiary element and so on. These are relatively common, but also need vacuum casting furnace or vacuum heat treatment furnace or X-ray and so on.

Stainless steel precision castings require tool rod materials in the manufacture. Because of its large cutting force in use, the tool rod must have sufficient strength and rigidity to avoid chattering and deformation in the cutting process. This requires the selection of appropriate large cross-sectional area of the tool rod and the use of high strength materials to manufacture the tool rod.

During the tempering process of stainless steel investment castings, the stress produced during quenching can be effectively eliminated to a certain extent. After quenching, the metastable structure of martensite and retained austenite can be decomposed and transformed. The tempering temperature of products is usually below the eutectoid transformation temperature, which is higher than 650 C, and the holding time is not less than 2 hours.

As the tempering temperature of stainless steel investment castings increases, the martensite of stainless steel investment castings needs to precipitate carbon from the supersaturated state of carbon, which will effectively form dispersed and fine carbides. According to the requirements of casting performance, tempering temperature can be divided into three types: low, medium and high.

In the process of processing materials for stainless steel precision castings, impurities such as dirt and oil need to be removed. The product's return charge must effectively spray out its sand and oxide scales. After cleaning, it can be put into the furnace for smelting. After the steel melting pool is completely melted out, low carbon ferromanganese and low carbon ferrosilicon are put into alloying and pre-deoxidation as required.

The addition duration, time and method of various alloying elements in stainless steel investment castings are closely related to their recovery rate. The order and time of alloying elements are determined according to the following requirements: the alloying elements should be melted as soon as possible to make the composition uniform.

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