a. Formulation of molding materials
Impeller investment casting sand casting molding material is the most important factor in casting production. It is related to the form of thermal action and chemical reaction when molten steel contacts it, so as to determine the surface quality of blank. In the casting process of stainless steel, due to the high casting temperature, it is easy to produce casting surface defects such as sand sticking, which seriously affects the surface roughness of castings, so it puts forward high requirements for the fire resistance of molding sand.
Chromite ore has high fire resistance (> 1700 ℃) and high-temperature thermal stability. Its chemical properties are slightly neutral. It is not easy to react with molten steel and stick to sand, and has the advantages of good thermal conductivity. Therefore, in the process design, chromite CO2 hardened sodium silicate sand is used as surface sand and core sand, quartz sand is used as back sand, and zircon powder coating is applied. The ratio (mass ratio) and properties of sodium silicate CO2 hardened sand (casting surface sand and core sand of stainless steel parts) are shown in the table.
b. Process description
(1) Except that quartz sand is used to replace chromite sand, the ratio of back sand and other processes are the same as that of surface sand.
(2) In order to ensure the quality of the mold, we should first select the appropriate raw sand, and choose the natural chromite sand with round particle shape as much as possible. The particle size of the raw sand should be selected according to the size and wall thickness of the impeller investment casting sand casting. The sand with fine particle size and dispersed sieve number is conducive to the improvement of the strength of the sand mold and the reduction of the surface roughness of the impeller investment casting sand casting, In addition, it is required that the mud content of raw sand should be less than 2%, and the water content of raw sand must be controlled. Generally, drying treatment is required. In the experiment, 100% 40 / 70 mesh chromite sand was used as surface sand, with emphasis on 50 mesh.
(3) Select high-quality sodium silicate and reasonable modulus. At present, the supply of sodium silicate is relatively conservative. The sodium silicate provided by different manufacturers cannot be exchanged and the quality varies greatly. Colorless and transparent liquid should be selected as far as possible. In summer, the low modulus should be 2.0 ~ 2.3; In spring and autumn, the modulus can be 2.4 ~ 2.6; In winter, when the ambient temperature is low, high modulus 2.7 ~ 3.2 should be selected.