In general, the dimensional accuracy of precision castings is affected by many factors such as casting structure, casting material, mold making, shell making, baking, pouring, etc. Any setting or unreasonable operation of any link will change the shrinkage rate of the casting, resulting in The dimensional accuracy of castings deviates from the requirements. The following are the factors that can cause defects in the dimensional accuracy of precision castings:
(1) The influence of casting structure: a. Casting wall thickness, large shrinkage rate, thin casting wall, small shrinkage rate. b. The free shrinkage rate is large, and the hindered shrinkage rate is small.
(2) Influence of casting material: a. The higher the carbon content in the material, the smaller the linear shrinkage rate, and the lower the carbon content, the greater the linear shrinkage rate. b. The casting shrinkage rate of common materials is as follows: casting shrinkage rate K=(LM-LJ)/LJ×100%, LM is the cavity size, and LJ is the casting size. K is affected by the following factors: wax mold K1, casting structure K2, alloy type K3, pouring temperature K4.
(3) The influence of mould making on the linear shrinkage rate of castings: a. The influence of wax injection temperature, wax injection pressure and pressure holding time on the size of investment mold is obvious in the wax injection temperature, followed by the wax injection pressure, and the pressure holding time is guaranteed After the investment is formed, it has little effect on the final size of the investment. b. The linear shrinkage rate of wax (mold) material is about 0.9-1.1%. c. When the investment mold is stored, there will be further shrinkage, and its shrinkage value is about 10% of the total shrinkage, but when stored for 12 hours, the investment mold size is basically stable. d. The radial shrinkage rate of the wax mold is only 30-40% of the lengthwise shrinkage rate. The wax injection temperature has a far greater influence on the free shrinkage rate than the hindered shrinkage rate (the preferred wax injection temperature is 57-59℃, The higher the temperature, the greater the shrinkage).
(4) The influence of shell-making materials: zircon sand, zircon powder, Shangdian sand, and Shangdian powder are used. Because of their small expansion coefficient, only 4.6×10-6/℃, they can be ignored.
(5) The effect of shell baking: Because the expansion coefficient of the shell is small, when the shell temperature is 1150℃, it is only 0.053%, so it can be ignored.
(6) The influence of casting temperature: the higher the casting temperature, the greater the shrinkage rate, the lower the casting temperature, and the smaller the shrinkage rate, so the casting temperature should be appropriate.
If there are more impurities in the oxide film in stainless steel precision casting, its purity will be low and transparency will decrease. The impurities in the oxide film are mainly derived from the stainless steel casting itself, and part of it comes from the oxidation bath. Therefore, appropriately reducing the content of alloying elements in the casting can minimize the content of impurities.
The equipment required for the basic process of stainless steel precision casting is: precision casting wax injection-wax injection machine, ice water machine, air conditioner; slurry dipping-slurry bucket, floating sand machine, air conditioner, dewaxing furnace, dehumidifier, dewatering bucket, standing bucket , The dipping slurry can also choose mechanical arm and automatic centralized wax supply system;
Gold melting-sintering furnace, high frequency furnace, shell shaker; post-processing-cutting machine, smoothing machine, sandblasting machine, sand cleaning machine, heat treatment furnace, punching machine; maintenance-pneumatic grinder, argon welding machine, smoothing machine; Quality assurance-spectrometer, three-dimensional, etc. These are relatively common and common, and vacuum casting furnace or vacuum heat treatment furnace or X-ray and so on are needed.
Stainless steel precision castings require the toolholder material during production. When stainless steel is processed, the toolholder must have sufficient strength and rigidity due to its high cutting force during use, so as to avoid chattering and deformation during the cutting process. This requires the selection of an appropriately large cross-sectional area of the tool holder, and the use of higher-strength materials to manufacture the tool holder.
In the process of tempering, stainless steel precision castings can effectively eliminate the stress generated during quenching to a certain extent. After quenching, the product decomposes and transforms the martensite and retained austenite metastable structure, and the product is tempered The temperature is usually below the eutectoid transformation temperature, the higher is about 650℃, and the holding time is not less than 2h.
Stainless steel precision castings will effectively increase with the increase of its tempering temperature, and its martensite needs to precipitate carbon from the supersaturated state of carbon, which will effectively form dispersed and fine carbides, according to the requirements for casting performance , Tempering temperature is divided into three types: low, medium and high.
In the process of processing the processing materials of stainless steel precision castings, it is necessary to remove impurities such as dirt and oil. The reheating material of the product must effectively spray off its sand and oxide scale, and then it can be put into the furnace for smelting after being cleaned. After the bath is completely melted, low-carbon ferromanganese and low-carbon ferrosilicon are added as required for alloying and pre-deoxidation.
The duration, time and method of adding various alloying elements in the production of stainless steel precision castings have a great relationship with its recovery rate. The order and time of adding alloy elements are determined according to the following requirements: the added alloy elements must be melted as soon as possible , Make the composition uniform.