RU2427441C1 - Fluid mix with increased thermochemical stability for pattern casting 3d moulds - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to metallurgy. Proposed mix comprises the following components in wt %: finely dispersed quartz sand - 55.00-60.00, dispersed Portland cement clinker - 12.00-18.00, sodium salt of carboxymethyl cellulose - 0.30-0.45, and water.

EFFECT: ruling out gas defects in casting.

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Description

The invention relates to the field of foundry and can be used in the manufacture of molds and cores in the production of artistic and precise technical castings, mainly from cast iron.

Currently, for the production of three-dimensional molds and cores for investment casting in art and jewelry casting, bulk mixtures based on gypsum and cement binders are most widely used.

The prior art mixture (Patent RF 2175902, IPC ⁷ В22С 1/00. A mixture for the manufacture of molds and cores on a gypsum binder in the manufacture of castings from non-ferrous and precious alloys, publ. 20.11.2001), including gypsum, fine quartz sand, dust sublimation of chamotte production, water, sodium dihydrogen phosphate as a gypsum setting retarder. However, molds made from this mixture do not allow castings from ferrous metals with a high pouring temperature to be obtained. Gypsum is unstable at temperatures above 1200 ° C, and it actively interacts with cast iron carbon with the decomposition and release of a large number of gaseous reaction products (SO ₃ , SO ₂ ), which hit the castings with gas shells over the entire surface.

The closest in technical essence is a self-hardening mixture with cement as a binder (RF Patent 2252103, IPC ⁷ В22С 1/00. Self-hardening liquid mixture for the manufacture of molds and cores in the manufacture of investment castings, publ. 05/20/2005). In accordance with the specified prototype, the mixture includes quartz sand, Portland cement, aluminum nitrate and water in the following ratio of ingredients, wt.%:

Portland cement 12.72 ... 17.90 fine quartz sand 53.00 ... 58.80 aluminum nitrate 1.79 ... 3.18 water rest.

This molding mixture allows you to get large castings from non-ferrous alloys, as well as small and thin-walled castings from cast iron and steel. However, the main disadvantage of the prototype mixture is that Portland cement also contains calcium sulfate hydrate (gypsum) in the amount of 3 ... 6%, which, according to the manufacturing process, is introduced in the milling process of the clinker to increase the cement setting time. Therefore, upon receipt of large castings from cast iron in massive nodes, the gypsum contained in Portland cement decomposes, and due to the very low gas permeability of the mold, the casting is damaged by gas shells.

The invention is based on a technical problem - the creation of a self-hardening liquid mixture for the manufacture of investment castings, including large cast iron, ensuring the absence of gas defects in them, due to the use of a binder with increased thermochemical resistance.

The specified technical problem is solved in that the bulk mixture with increased thermochemical stability for the manufacture of molds and cores in the production of investment castings, including finely divided filler based on silica, a crystalline hydrate binder, water, according to the invention, the mixture further comprises a sodium salt of carboxymethyl cellulose, and as a crystalline hydrate the binder contains dispersed Portland cement clinker in the following ratio of ingredients, wt.%:

Dispersed Portland cement clinker 12.00 ... 18.00 Fine quartz sand 55.00 ... 60.00 Carboxymethyl Cellulose Sodium 0.30 ... 0.45 Water rest.

The proposed technological solution allows to prevent gas shells from casting iron with massive nodes, since the inventive mixture does not contain calcium sulfate (gypsum), which decomposes during high-temperature interaction with carbon to form a large number of gaseous products.

As a binder, it is recommended to use Portland cement clinker, for example, the TU 5739-002-00284339-94 commodity clinker manufactured by a cement plant, which must be dispersed, for example, in a ball rotating or vibration mill to a specific surface of 300 ... 500 m ² / kg for subsequent high-grade the course of hydration processes and the formation of strength of molds.

The content of the proposed clinker mixture of less than 12.00 wt.% Does not provide the necessary strength of the molding mixture and, therefore, the surface quality of the casting. On the contrary, the clinker content of more than 18.00 wt.% Leads to an excessive increase in strength and deterioration of knockout, as well as the cost of the molding sand.

Dispersed Portland cement clinker differs from Portland cement produced by cement plants in the absence of calcium sulfate, which slows down the hydration of aluminate phases and thus regulates the setting and hardening of cement, which is a positive moment for the bulk molding sand, since the setting time for building mortars is too long for it. A long period from filling of the flask to the beginning of setting of the bulk sand mixture leads to sedimentation of the filler, cracking of the forms during drying and calcination, and the formation of defects “fouling” and “blockage” on the castings. The absence of calcium sulfate reduces the start time of setting compared with Portland cement from several hours to 35 ... 50 minutes, but for high forms this is not enough to prevent sedimentation of the filler, especially if there is a fraction of more than 100 microns in it. For this purpose, the carboxymethyl cellulose sodium salt is introduced into the proposed bulk molding sand mixture.

The sodium salt of carboxymethyl cellulose (grade KMTs 85/400 TU 6-55-39-90) is introduced by dissolving it in tap or distilled water during the preparation of the molding mixture. The additive, dissolving in water, envelops the filler grains and creates a “polymer skeleton” in the volume of the mixture, which prevents the settling of filler particles.

The amount of sodium salt of carboxymethyl cellulose in the composition of the mixture of 0.3 ... 0.45 wt.% Is optimal from the standpoint of sedimentation stability of the suspension. Its content in the mixture of less than 0.3 wt.% Does not provide the necessary sedimentation stability, leads to delamination of the mixture, an excessive increase in the onset time of the solidification of the mixture, its delamination, cracking of the forms during drying and calcination, and the formation of defects “breakdown” and “blockage” on castings . The content of sodium salt of carboxymethyl cellulose 0.45% completely prevents sedimentation and a further increase in its amount, leads only to an increase in the cost of the molding mixture.

The dispersion of quartz sand particles to a grain size of 50 ... 160 microns provides high quality imprint forms on a clinker binder, as well as their required physical and mechanical characteristics. In this case, dispersion can be carried out by simple grinding of quartz sand, for example, grades 2K ₂ O ₂ O3 or 3K ₃ O ₂ O3 (GOST 2138-91) in a ball rotating or vibration mill.

The aggregate (water) has a direct effect on the rheological properties of the suspension (fluidity, formability, sedimentation stability) and the finished form (strength, shedding, crack resistance). The optimal amount of water is in the range of 35 ... 38 mass.h. per 100 mass. parts of the dry components of the mixture. Such a content of the grout makes the mixture a suspension of consistency suitable for the manufacture of bulk bulk molds for casting both small and large-sized hollow products.

The preparation of the inventive mixture for the manufacture of molds and cores on a clinker binder in the manufacture of castings from non-ferrous, ferrous and precious alloys is as follows. Estimate the amount of Portland cement clinker and dispersed silica sand. Then measure the required amount of water and dissolve in it the right amount of sodium salt of carboxymethyl cellulose.

Before making molds and cores, a Portland cement clinker and quartz sand mixed with each other are poured into a pre-prepared grout, at the rate of 350 ... 380 ml of grout per 1 kg of dry components. Next, the mixture is thoroughly mixed at an impeller rotation speed of 3000 ... 6000 rpm for 1 ... 2 minutes. After that, the suspension is used to make molds and cores. After manufacturing, the mold is dried, then the model composition is extruded (removed) with hot air at a temperature of 120 ... 160 ° C and calcined at 550 ° ... 600 ° C. The temperature of the molds before pouring the metal is reduced to 400-500 ° C.

The proposed composition of the mixture for the manufacture of molds and cores on a clinker binder is illustrated by the following example.

Example. A dry mixture is prepared from Portland cement clinker grade TU 5739-002-00284339-94 and dispersed quartz sand. Then the sodium salt is weighed and dissolved in a hardener - distilled water, then the dry mixture itself is introduced, followed by stirring for 1 ... 2 minutes.

In parallel, to obtain comparative data, the mixture is prepared on a cement binder according to the prototype. The compositions of the mixtures are given in table 1.

Table 1 Mixtures Name of ingredients The content of ingredients, wt.% prototype one 2 3 Portland cement 14.50 - - - Dispersed Portland cement clinker - 12.00 15.00 18.00 Fine quartz sand 56.90 60.00 57.50 55.00 Aluminum Nitrate 2,50 - - - Carboxymethyl Cellulose Sodium - 0.45 0.37 0.30 Water rest rest

Prepared mixtures are used for the manufacture of molds and cores on a cement binder with fixation of their physical and mechanical properties.

The molds are filled with gray cast iron SCh20 at a temperature of 1300 ° C. For each option, 12 castings were made.

Indicators for comparison are the coefficient of thermal linear expansion (CTE) of each mixture in the temperature range of 20 ... 700 ° C, the compressive strength of the molds after drying and after calcining, the tearability of the molding mixture after calcining the molds, the surface roughness of castings obtained in these molds, the presence of gas defect castings.

CTLR is determined on a PAULIK dilatometer using samples with a diameter of 5 mm and a height of 30 mm.

The strength of the molds is determined by testing standard samples with a diameter and a height of 50 mm for compression after exposure to air after 24 and 72 hours, after calcination (700 ° C, 3 hours) and cooling to room temperature.

The tearability of the molding sand is evaluated according to the standard procedure on the model 056 instrument to determine the trituration of the molding sand.

The roughness of gray castings SCh20 is determined on the model 296 profilometer.

The presence of defects of gas origin on the surface of the castings is assessed visually, in the body of the castings - using x-ray control.

The test results of the mixtures (prototype and declared) are given in table.2.

table 2 Test results Indicators Test results of mixtures by composition. prototype one 2 3 1. CTLR of the mixture in the temperature range of 20 ... 800 ° С, · 10 ^-5 , 1 / ° С 0.91 0.91 0.90 0.89 2. The strength of the molds in compression, MPa: a) exposure to air for 24 hours 2,8 2.6 2,8 3,1 b) exposure to air for 72 hours 3,5 3,1 3.4 3.6 c) after calcination 2.2 2,4 2.5 2.6 3. Flaking form after calcination,% 1,03 0.82 0.78 0.71 4. The surface roughness of the casting R _z , microns 20 ... 80 20 ... 80 20 ... 80 20 ... 80 5. The presence of gas sinks in the massive part of the cast iron mass of 72 kg there is no no no

The test results show that, compared with the prototype, the claimed composition of the mixture is practically inferior in all basic physical and mechanical characteristics, while preventing the formation of gas defects on cast iron with massive parts.

The proposed mixture has been tested at CJSC Ural Bronze (Chelyabinsk). High-quality artistic castings from gray cast iron weighing up to 72 kg without gas defects were obtained.

Claims (1)

A bulk mixture for the manufacture of three-dimensional molds in the production of investment castings, including a finely divided silica-based filler, a crystalline hydrate binder, water, characterized in that the mixture further comprises a sodium salt of carboxymethyl cellulose, and as a crystalline hydrate binder contains a dispersed Portland cement clinker, in the following ratio wt.%:
Dispersed Portland cement clinker 12.00-18.00 Fine quartz sand 55.00-60.00 Carboxymethyl Cellulose Sodium 0.30-0.45 Water Rest