RU2631930C1 - Modifier - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: modifier contains a mixture of powders, wt %: silicon carbide 30-40; aluminium 20-30; the rest is boron carbide.

EFFECT: strengthening modifying effect and increasing mechanical properties of malleable iron products.

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Description

The invention relates to metallurgy and can be used to modify malleable iron castings

Known modifier for cast iron and graphitized steel [SU No. 1148888 from 04/07/1985], containing mass. %:

titanium powder 35-40 aluminum 35-40 ultrafine silicon carbide powder rest

The modifying effect of the components of such a modifier is explained by the predominant role of the formed heterophase graphite nuclei, which determines the existence of a certain time limit for graphitization. As a result of this, during the period of primary crystallization of cast iron, emissions of free carbon (sinter) are observed in massive sections of the castings, which reduces the strength properties of the alloy.

The closest technical solution to the present invention is a ductile iron modifier containing a mixture of particles of 15-30 weight. % boron carbide and 70-85 weight. % silicon carbide [SU No. 616325 of 07.25.1978].

The main disadvantages of its use are the poor wetting of the particles by the melt and their rapid oxidation, which lead to poor assimilation of their particles by the melt and reduced, therefore, modifying ability, leading to a slight increase in the strength and ductility of castings from cast iron.

The objective of the invention is to enhance the modifying effect and increase the mechanical properties of castings from cast iron. The most active elements in small quantities, intensifying the course of the graphitization process, primarily include aluminum and boron introduced into the melt together with silicon and graphite [Goldstein Y.E., Mizin V.E. Inoculation of iron-carbon melts. M .: Metallurgy, 1993. - S. 309-310]. The introduction of the above additives into the melt, changing such crystallization conditions as the melt supercooling temperature, the number of nucleation centers, the nature of the structure of the graphite rosette, allows one to influence the microstructure and service-important characteristics of cast iron.

The technical result of the task is achieved in that the proposed malleable cast iron modifier containing a mixture of powders of silicon carbide and boron carbide, additionally contains aluminum in the following ratio of components, mass. %:

silicon carbide 30-40 aluminum 20-30 boron carbide rest

The introduction of silicon carbide in the composition of the mixture is due to its ability to create a modifying effect, which is manifested in a small supercooling of liquid cast iron with a large number of generated grains in a reduced volume, to reduce the force interaction of the hardened castings with the casting mold, shrinkage of the alloy during crystallization, and increase the resistance to the formation of hot crystallization cracks, which leads to a decrease in the level of thermal stresses in castings and an increase in the mechanical properties of cast iron. When the concentration of silicon carbide in the modifier is up to 30%, the casting properties of modified cast iron are insufficient, and with an increase in the content of silicon carbide over 40%, the stability of the modification process and technological properties are reduced.

Boron carbide enhances the modifying effect, intensifies graphite formation and improves the distribution of graphite inclusions. When the content of boron carbide is less than 30%, the increase in the modifying effect is negligible. With an increase in its amount over 50%, the modifying effect decreases, and the hardness of the matrix increases, which affects the mechanical properties of malleable cast iron.

Aluminum is a strong reducing agent for iron-based alloys and is the most active graphitizer. An additional introduction of aluminum into the modifier enhances the inoculant effect due to an increase in the number of aluminum oxides and nitrides, which are active centers of crystallization. Upon modification, an aluminocarbothermic process proceeds directly in the casting ladle, accompanied by the release of a significant amount of heat. The resulting gaseous products of aluminocarbothermic reactions sparge through the melt, increasing its degree of mixing, uniform distribution of alloying particles over the melt volume, which makes it possible to obtain a uniform pearlite-ferrite matrix in cast iron microstructure without structurally free cement with a uniform distribution of hardness in different sections of the castings. Along with this, the enrichment of the melt with aluminum increases the survivability of the modifying effect of the modifier. When the aluminum content is less than 20 wt.%, The inoculating effect of the modifier is insufficient, since it has a negligible effect on the microstructure of cast iron. With an increase in its amount more than 30 wt. % the shape of graphite in castings worsens, the formation of “sinter” is possible, captures appear that reduce the mechanical properties of cast iron.

When such a modifier is dissolved in the microvolumes of the melt, local supersaturations with carbon, silicon, boron, and aluminum are created, which are retained during crystallization, which prevents “blocking” in massive parts of the castings.

The heat treatment of such modified cast iron proceeds much faster, since along with the formation of heterophase graphitization centers, the diffusion of carbon in cast iron also increases sharply. In addition, local supersaturation of γ - Fe contributes to an increase in the rate of decomposition of austenite into ferrite and graphite during the second stage of graphitization. At the same time, the local supersaturation of austenite with carbon, silicon, boron and aluminum simultaneously, when introduced into the liquid metal, favorably affects the shape and size of annealing carbon inclusions, which causes an increase in the strength properties of ductile iron and at the same time reduces the graphitization time.

The modifier is prepared by pressing a mixture of powders into a briquette at a pressure of 500-700 MPa. The components used are powders: silicon carbide (GOST 3647-71), boron carbide (GOST 5744-85), aluminum grade PA-4 (GOST 5494-95).

Melting of the source iron is carried out in an induction furnace with a main lining. The modification is carried out in a casting ladle at a melt temperature of 1380-1420 ° C. The modifier is introduced in an amount of 0.02-0.05% by weight of the casting to the bottom of the ladle heated to 500-600 ° C. Cast iron is subjected to graphitizing annealing according to the regime: heating to 950 ° C, holding for 16 hours, cooling with an oven to 670 ° C and cooling in air.

Example. The proposed modifier was tested in the manufacture of castings of cast iron chemical composition, mass. %: carbon 2.81; silicon 1.28; manganese 0.42; chrome 0.1; sulfur 0.08; phosphorus 0.04 and the rest iron.

To conduct comparative tests, 1 composition of the known modifier was made, 3 compositions of the proposed concentrations for the lower, middle and upper levels of the content of components and 2 composition of transcendental ratios. The modifier consumption in the experimental melts was 0.03% of the mass of the processed melt. The modifying effect was determined by the number of large graphite inclusions formed during the period of secondary crystallization.

The chemical compositions of the modifiers are given in table. one.

The properties of the alloys obtained by introducing modifiers are given in table. 2.

As can be seen from the table, the best properties are possessed by alloys obtained using modifiers 2, 3 and 4 of the compositions.

The introduction of modifiers of these compositions into the melt leads to an increase in the modifying effect by 40-70%, an increase in the tensile strength of ductile iron by 50-80 MPa, and relative elongation by 3-7%.

Claims (2)

Modifier for ductile iron containing a mixture of powders of silicon carbide and boron carbide, characterized in that it additionally contains aluminum powder in the following ratio, wt. %: silicon carbide 30-40 aluminum 20-30 boron carbide rest