RU2510306C1 - Method of making thin-wall casts of cast iron with ball-like graphite - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to metallurgy and can be used for production of containers for transportation of used nuclear fuel and other radioactive wastes. Proposed method comprises smelting of iron, spheroidising it in teeming ladle and graphitising modification in the mould injection bowl. Said spheroidising is performed in teeming ladle reaction chamber wherein alloys Si-Ba, Ni-Mg and Si-Mg in amount of 1.8-2.2 wt % of initial iron weight are laid in layers and coated with the layer of iron-carbon chips. Modification is made by quick-dissolving moulded inserts and mix of foundry alloys in amount of 0.4-0.7 wt % composed of Si-Mg and Si-Ba at 1:1 ratio.

EFFECT: higher quality of microstructure and its isotropy, better mechanical properties.

3 cl, 2 dwg

Description

The invention relates to foundry, in particular to methods for the manufacture of large-capacity and thick-walled castings from spheroidal graphite mainly of container bodies for transporting spent nuclear fuel and other radioactive waste.

A known method of manufacturing thick-walled cases of containers of cast iron with spherical graphite [1], including the smelting of the initial melt, spheroidizing treatment with magnesium, modification in the sprue bowl by dissolving the granular modifier in the melt and pouring molten iron into a casting mold, consisting of an external side chill mold, top ( overlapping) and the lower (supporting) half-molds and the central core forming the cavity for casting, the gate-feeding system and the filling bowl.

The disadvantages of this method are:

- obtaining in a thick-walled casting a significant proportion of inclusions of graphite of irregular spherical and compact shape due to a significant reduction in the spheroidizing effect of magnesium, over a long period of time from the moment of spheroidizing treatment in the ladle to the start of crystallization of the metal in the mold;

- an increased number of non-metallic inclusions in the metal, which are the products of the interaction of the modifier with the components of cast iron and coming directly with the metal into the mold.

The closest adopted for the prototype is a method of manufacturing thick-walled castings of spheroidal graphite iron [2], including the smelting of the initial melt, spheroidizing treatment with magnesium, modification in the sprue bowl by dissolving granular modifier in the melt and pouring molten iron into a casting mold consisting of external lateral chill mold, the upper (overlapping) and lower (supporting) half-molds and the central rod, which form the cavity for casting, the gate-feeding system and the filling cup, which holds the entire liquid cast iron necessary for pouring into the mold and equipped with feeders overlapped by locking devices, while the modification is carried out simultaneously with a mixture of microcrystalline ligatures in an amount of 0.21 ... 0.6%, consisting of Si-Mg, Si-Ba and Si-REM alloys in the ratio 1: 1: (0.5 ... 1.0), and cast iron is poured into the mold cavity at the same time through all feeders after homogenizing exposure in the casting bowl for 1 ... 5 minutes.

As microcrystalline ligatures, mainly Si-Mg alloys with a content of 5 ... 9% Mg, Si-Ba with a content of 22 ... 30% Ba and Si-REM with a content of 25 ... 35% REM are mainly used.

The filling cup is partitioned with plates made of material dissolved in liquid iron, for example, from a steel sheet, and forming cavities into which the modifying mixture is filled.

The disadvantages of this method is to obtain in a thick-walled casting a significant proportion of inclusions of spherical graphite of irregular and compact shape due to the long period of time elapsing from the moment of spheroidizing treatment in the ladle to the start of casting and crystallization of cast iron in the mold, which substantially reduces the spheroidizing effect of magnesium and the number of effective centers of crystallization of graphite decreases due to insufficiently effective graphitizing modification in a bowl and the required reduced temperatures of molten iron.

The technical result of the proposed method for the manufacture of thick-walled castings from nodular cast iron is to eliminate microstructure defects, to obtain a predominantly ferritic metal base of high-strength cast iron in a cast state, and to increase the isotropy of the structure and mechanical properties of the metal over the cross-section of a thick-walled casting.

The technical result is achieved in that the spheroidizing treatment in the casting ladle is carried out with a mixture of crushed ligatures in an amount of 1.8 ... 2.2%, consisting of alloys 0.30 ... 0.45% Si-Ba, 0.2 ... 0.3% Ni -Mg and 1.4 ... 1.6% Si-Mg, which are placed sequentially in even layers in the reaction chamber at the bottom of the casting ladle, formed by a refractory wall between the walls, while the first layer is filled with Si-Mg alloy to the bottom of the chamber, the second layer - from a Ni-Mg alloy, the third layer is from an Si-Ba alloy and is covered with a protective layer of shavings of an iron-carbon alloy, and graphitized The modification is performed when the metal is drained from the casting ladle into the pouring bowl with simultaneously rapidly soluble modifying casting inserts uniformly distributed and fixed at the bottom of the pouring bowl, and a mixture of ligatures in an amount of 0.4 ... 0.7%, consisting of alloys, Si- Mg and Si-Ba in a 1: 1 ratio.

The proposed combination of spheroidizing treatment technology and graphitizing modification, the ratio of the components of the modifying alloys and casting technique enhance the spheroidizing and graphitizing effect during the casting solidification period and ultimately increase the isotropy of the metal properties over the cross section of the cast thick-walled workpiece.

Spheroidizing treatment in the casting ladle with a mixture of crushed ligatures, consisting of Si-Ba, Ni-Mg and Si-Mg alloys, placed sequentially in even layers in the reaction chamber at the bottom of the ladle and coated with a protective layer of shavings of the iron-carbon alloy, provides the necessary delay in the interaction start time magnesium with molten iron during the sequential filling of the ladle with a melt from several furnaces and the subsequent calmer and more efficient process of dissolving magnesium-containing alloys to obtain higher magnesium content of the melt. Moreover, each layer of the alloy in the reaction chamber performs its role.

After the protective layer of the iron-carbon alloy shavings is dissolved, the Si-Ba alloy layer comes into contact with the melt from the first furnace first, which retains the contact of the molten iron with magnesium-containing alloys for a certain time, acting as a protective layer and at the same time as an active deoxidizer and desulfurizer. Then, when the ladle is completely completely filled from other furnaces, the Ni-Mg alloy layer comes into contact, which briefly activates the release of magnesium into the melt without the alloy particles floating up onto the metal mirror. Then, the Si-Mg alloy gradually reacts layer-by-layer with liquid iron, providing a high degree of absorption of magnesium during the passage of its vapor in a column of liquid iron of maximum height.

It is preferable to use Si-Ba alloys with a content of 22 ... 30% Ba, Ni-Mg with a content of 14 ... 17% Mg and Si-Mg with a content of 5 ... 7% Mg.

To achieve maximum effect during spheroidizing treatment in a casting ladle, magnesium ligatures should be filled in an amount ensuring the introduction of at least 0.12% Mg into the cast iron.

During the graphitizing modification in the casting bowl, rapidly soluble modifying alloy inserts provide additional centers of graphite crystallization, and in the modifying mixture, the Si-Mg alloy enhances the spheroidizing effect and the Si-Ba alloy provides a longer graphitizing effect.

Figure 1 shows a diagram of a casting ladle with a reaction chamber and layers of crushed modifying alloys and a protective layer made of shavings of an iron-carbon alloy placed therein. Figure 2 presents a diagram used in the inventive method of the mold.

When a spheroidizing treatment is carried out in the casting ladle shown in FIG. 1, having a reaction chamber 1 formed by a bottom 2, a refractory wall 3 and walls 4, in which a layer 5 of a Si-Mg alloy, a layer 6 of a Ni-Mg alloy, are placed, a layer of Si-Ba alloy covered by a protective layer 8 of shavings of the iron-carbon alloy, after dissolution of which, as a result of contact with the melt coming from the first furnace, the first layer of contact with the melt comes into contact with the melt, the Si-Ba alloy, which holds for a certain time cast iron melt contact with magnesium-containing alloys, acting as a protective layer and at the same time an active deoxidizer and desulfurizer. Then, when the ladle is completely completely filled from other furnaces, the Ni-Mg alloy layer comes into contact, which briefly activates the release of magnesium into the melt without the alloy particles floating up onto the metal mirror. Then, the Si-Mg alloy gradually reacts layer-by-layer with liquid iron, providing a high degree of absorption of magnesium during the passage of its vapor in a column of liquid iron of maximum height.

Then the melt from the casting ladle enters the casting mold shown in figure 2. The mold contains a metal pan 9, a lower half-mold 10 mounted on it, forming the side surface of the casting, a central rod 11 of a high-heat-forming molding mixture with an iconic part and a hollow frame in the form of a perforated pipe 12 with gas outlets, an upper sand mold 13 with gating elements -the feed system 14 and the upper metal refrigerator 15 located therein, forming the outer surface of the bottom of the casting, as well as a pouring bowl 16 with l mounted on the upper half-mold tnikovymi channels overlapping the locking device 17 and accommodating the entire volume required for casting the molten iron. In the pouring bowl 16, on the side of the metal drain from the ladle, using a soluble partition wall plate 18, a cavity is separated for accommodating the modifying mixture 19 and quickly soluble modifying cast inserts 20. The wall of the lower half-mold is made in the form of a side metal refrigerator 21 with an external heat-insulating sand layer 22. The mold is mounted on metal supports 23 for ease of installation and maintenance. The proposed method is as follows. The initial molten iron smelted simultaneously in different furnaces is successively poured from each furnace into a casting ladle, at the bottom of which a mixture of crushed alloys consisting of Si-Ba, Ni-Mg, and Si-Mg alloys, which are filled in series with even layers and coated with protective coating, is placed in the reaction chamber a layer, for example, of cast iron shavings. The magnesium cast iron thus treated from the casting ladle is poured into the casting thicket 16 of the assembled and prepared for casting mold (see figure 2). As it enters the casting bowl 8, molten iron flows over the rapidly soluble modifying cast inserts 20 and the modifying mixture 19, dissolving in layers and interacting with them.

By the end of the filling process of the filling bowl 8 with molten iron, the partition plate 18 is completely dissolved. Next, the molten iron is held in the pouring bowl 16, in which all reactions of the interaction of the modifying additive with the components of molten cast iron proceed most fully, the melt is homogenized in its entirety and the reaction products float to the slag. After reaching the set pouring temperature, the locking devices 17 are simultaneously lifted, and molten iron fills the mold cavity through the gate channels. With this casting method, subsequent directional crystallization of the casting is provided from the bottom up due to the receipt of the last hot portions of the metal directly in profit and efficient feeding of the crystallizing metal, eliminating the formation of shrink defects.

During the graphitizing modification in the casting bowl, rapidly soluble modifying alloy inserts provide additional centers of graphite crystallization, and in the modifying mixture, the Si-Mg alloy enhances the spheroidizing effect and the Si-Ba alloy provides a longer graphitizing effect.

After cooling the casting to a predetermined temperature, the mold is taken apart, the casting is removed and sent for finishing operations.

The proposed method was used for the manufacture of castings of the container body of cast iron with spherical graphite grade VCh40 for transport packaging.

The container body had the following dimensions (mm):

outer diameter - 1920,

inner diameter - 1210,

height (length) - 4100,

wall thickness - 405.

The casting mass is 55 tons, the mass of the required molten iron for casting is ≈70 tons.

Cast iron was melted for the production of pilot casting simultaneously in three IChT-25 furnaces, with a crucible capacity of 25 tons. Cast iron, popular in accordance with TU 0812-042-50254094-2001 of the LN grade, of the following composition (wt.%) Was used as charge materials: carbon 4.15; silicon 1,2; manganese not more than 0.04; phosphorus 0.03; sulfur 0.015 and steel scrap grade 1A.

Spheroidizing treatment was carried out in a casting ladle with a capacity of 75 tons by pouring the initial molten iron into it from 3 ovens in turn. As a spheroidizing additive, a mixture of crushed ligatures was used in an amount of 2.1% of the total mass of the original molten iron, consisting of 0.40% Si-Ba (22% Ba), 0.3% Ni-Mg (15% Mg) alloys, and 1.4% Si-Mg (6% Mg) placed sequentially in even layers in the reaction chamber at the bottom of the ladle and coated with a protective layer of cast iron chips.

After the end of the spheroidizing treatment in the ladle and removal of the resulting slag, magnesium molten iron was poured into the casting pan of the assembled and prepared for casting mold. The pouring cup from the falling side of the bucket of metal stream was blocked with a steel plate and cast modifying inserts from Germalloy alloy and a modifying mixture of ligatures of fractions 1 ... 3 mm in an amount of ~ 0.5%, consisting of 6% Si-Mg, were placed in the cavity formed ligatures (0.25%) and SiBaR22 ligatures (0.25%). The modification process took place calmly, without pyroeffect and smoke emission and ended almost by the time the casting ladle was completely emptied. After homogenizing for 3 minutes and reaching the melt temperature in the pouring bowl of ~ 1320 ° C, the mold was cast through profitable cavities by simultaneously raising 2 stoppers. The total duration of the process from the moment of pouring the metal from the first furnace into the casting ladle and to pouring molten iron into the mold was ~ 30 min.

The magnesium content in cast iron at different stages of sampling was (wt.%):

- in the casting ladle after spheroidizing treatment - 0.10;

- in the pouring bowl before pouring cast iron into the mold - 0.08;

- in a tidal sample - 0.07;

- in the body of the casting 0.05 ... 0.055.

The mechanical properties of nodular cast iron in the wall of the casting have the following meanings:

- temporary tensile strength in tension (Km) - 385 ... 410 MPa,

- elongation (A5) - 19 ... 23%.

BIBLIOGRAPHY

1. Description of the invention to patent DE 3324929, publ. 01/17/1985.

2. Description of the invention to patent RU 2440214 C1, publ. 01/20/2012.

Claims (3)

1. A method of manufacturing thick-walled castings of nodular cast iron, comprising smelting the initial melt, spheroidizing the treatment of the melt in the casting ladle, graphitizing the modification in the casting mold bowl by dissolving the modifier in the melt, and pouring molten iron into the casting mold containing the outer side chill, the upper the overlapping and lower supporting half-molds and the central rod forming the cavity for casting and the gate-feeding system, and the filling cup with feeders, locking devices containing all the liquid cast iron necessary for pouring into the mold, characterized in that the spheroidizing treatment of the melt in the casting ladle is carried out with a mixture of crushed ligatures in an amount of 1.8 ... 2.2% of the mass of the original cast iron containing Si-Ba 0 alloy 30 ... 0.45%, Ni-Mg alloy 0.2 ... 0.3% and Si-Mg alloy 4 ... 1.6%, which are placed successively in even layers in the reaction chamber at the bottom of the casting ladle, formed by a refractory partition between the walls of the ladle while the first layer of Si-Mg alloy is poured onto the bottom of the chamber, the second is the layer and Ni-Mg alloy, the third is a layer of Si-Ba alloy and covered with a protective layer of shavings of the iron-carbon alloy, and the graphitizing modification is performed when the metal is poured from the casting ladle into the casting bowl with the simultaneous use of instant modifying casting inserts evenly distributed and fixed to the bottom of the casting bowl, and a mixture of crushed ligatures in an amount of 0.4 ... 0.7%, consisting of Si-Mg and Si-Ba alloys in a ratio of 1: 1. 2. The method according to claim 1, characterized in that Si-Ba alloys with a content of 22 ... 30% Ba, Ni-Mg with a content of 14 ... 17% Mg and Si-Mg with a content of 5 ... 7% Mg are used. 3. The method according to claim 1 or claim 2, characterized in that during the spheroidizing treatment in the casting ladle, magnesium ligatures are poured in an amount ensuring introduction of not less than 0.12% Mg into the cast iron.

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