RU1808463C - Method of casting large-size thin-walled special-purpose castings using investment-casting process - Google Patents

Abstract

Usage: the invention relates to foundry, in particular, to the manufacture of large-sized thin-walled castings of special purpose for smelted models. Summary of the invention: before placing mold shells made on the basis of crystalline quartz in the flask, the flask bottom and its side walls are lined with heat-insulating refractory filler, and after installing the molds in the flask, the gap between the sprue cups of the molds and the flask is sealed from above with a plate made of the same refractory filler, gating apertures under the cup, and the thickness of the filler was calculated from the formula in dependence on the thermal filler material, the exponent of the parabola of the temperature distribution in the bed floor filler and from time unloading the flasks with forms of calcination furnace .Before pouring.

Description

The invention relates to foundry, in particular to the manufacture of large-size, thin-walled castings for special purposes for smelting models.

The purpose of the invention is to improve the quality of castings by reducing the rejects of castings according to the loosening and shrinkage porosity while reducing the time of calcining the molds.

Achieving the goal is ensured by the fact that before placing shells of molds made on the basis of crystalline quartz in the flask, the flask bottom and its side walls are lined with heat-insulating refractory filler, and, after the molds are placed in the flask, the gap between the gate

with mold bowls and a flask, they are sealed from above with a plate made of the same refractory filler, with openings for sprue bowls. The thickness of the filler is calculated by the formula:

H 1, (n + 1),

: where H is the thickness of the layer of refractory filler in the flask, m ;.

n is an exponent of the parabola of the distribution of the temperature field in the refractory filling layer about the flask (n 1 ... 3); the numerical values of n depend on the nature of the material and the configuration of the refractory coating layer, determined experimentally;

a is the coefficient of thermal diffusivity of the material of the refractory filler, m / s2;

d - from unloading flasks with molds from the calcining furnace to pouring them with metal, s.

, PRI me R. From the model composition paraffin-ceresin synth. - Lignite wax-still residue from thermal cracking of paraffin 70-12-13-5 (P-3), low-melting models of thin-walled (wall thickness 2-10 mm) large-sized (length 200- 400 mm) case-type castings. The ten-layer shells of the molds were made of quartz sand of grades K016 and K04 with a clay content of less than 0.5% (by weight). As a binder suspension, a hydrolyzed solution of ethyl slickate-40 was used. Dust quartz of grade B was used as a filler in the suspension ( GOST 9077-82). Each layer of the shell was dried in an air-ammonia medium. Removing the models from the shell, the mold was carried out in a medium superheated to 160 ° -180 ° steam at a pressure of 5-6 bar for 12-20 minutes.

Before installing the mold shells in the flasks, the inner surface of the flask was lined with a layer of refractory filler 35–40 mm thick. A mixture of fireclay chips with a particle size of 0.5-1.5 mm with 2–3% (by weight) of liquid glass was used as a refractory filler. Drying of the filler layer was carried out in air at 25-30 ° C for 24 hours (plates of foam diatomite and foam chamotte were also used as refractory filler for flasks).

After installing shells of molds in the lined flask, the latter were sealed on top with a slab of the same refractory filler with openings for the sprue cups to eliminate the gap between the sprue mold cups and the flask.

Flask with molds was loaded into an H-75 type calcination electric furnace at a temperature of 730-750 ° C, kept at this temperature for 2 hours, followed by heating the molds to 940-980 ° C at a rate of 150 ° C / h.

During heating of the molds, their temperature was controlled at 15 points with chromel-alumel thermocouples complete with a KSP-4 potentiometer. It was found that the temperature anisotropy of the forms disappears when they are kept in the calcination furnace for 0.5 h.

After removing the flasks from the calcining furnace, the molds were poured with steel of grades 10X18N9L

and 08X14H7ML at a metal temperature of 1580 ° -1640 ° C. Temperature control of the molds from unloading to pouring with metal showed that the temperature gradient after 8 minutes. after removal from the oven amounted to 30-40 ° C.

The results of the control of the calcination duration and quality of the castings are shown in the table.

Using the proposed method for the manufacture of molds allows:

- reduce the duration of calcination of the shells of the forms, and, consequently, the energy consumption for this operation;

- reduce the temperature gradient over the body of the mold shell, and, therefore, reduce the thermal stresses in it;

- improve the quality of critical thin-walled castings by reducing defects in shrinkage defects, oxide films and pitting corrosion.

Claims (1)

SUMMARY OF THE INVENTION A method for manufacturing large-size thin-walled castings for special purposes by investment casting, including applying a refractory coating on the basis of crystalline quartz to the model, drying it, removing the model from the mold shell, installing the shell in a flask with a gap, filling the gap between the mold shell and the wall of the flask with refractory filler, calcining the mold and pouring molten metal into it, characterized in that, in order to improve the quality of castings by reducing casting rejects lumps and shrinkage porosity while reducing the time of calcining the molds, the flask, before placing the mold shells in it, is lined with heat-insulating refractory filler along its bottom and side walls, and after the molds are installed, the gap between the sprue cups of the molds and the flask is sealed on top with a plate made of the same refractory filler, with holes for sprue cups, and the thickness of the filler is determined by the formula H-MMn-Mja7 0 5, where H is the thickness of the layer of refractory filler in the flask, m; n is an exponent of the parabola of the distribution of the temperature field in the refractory filler layer in the flask (n 1 ... 3); a-coefficient of thermal diffusivity of the material of the refractory filler, m / s2; t is the time from unloading the mold boxes from the calcining furnace to pouring, s.