RU2458995C1 - Cast-iron melt processing method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves cast-iron melting in melting unit, or charging of liquid cast-iron to melting unit, heating of cast-iron to 1460-1560°C. To the melt heel there supplied are granules of carbonic acid diamide (carbamide) in quantity of 0.1-0.5% of the melt weight at simultaneous melt mixing. Diadime decomposition products are dissolved in cast-iron melt. As original cast-iron for such processing in melting unit there can be used liquid cupola iron the carbon content in which is usually 2.9-3.3 wt %. In addition, carbonisation of cast-iron melt supplied to the melting unit from cupola furnace is performed prior to processing. Melt is carbonided till carbon content in cast-iron is 3.4-4.2 wt %.

EFFECT: higher hardness, ultimate strength, impact strength and relative wear resistance of cast-iron.

2 cl, 1 dwg, 1 tbl, 1 ex

Description

The invention relates to the field of metallurgy and can be used to improve the properties of cast iron castings.

Known methods for treating cast iron melt, including the melting of charge materials and introducing nitrogen and hydrogen-containing components into the resulting molten iron, by blowing with nitrogen or by blowing together with nitrogen and steam [RF patent No. 2130081] in order to improve the properties of cast iron.

For the prototype, a method was selected for alloying cast iron with nitrogen, including melting of cast iron in an induction furnace, overheating of molten cast iron to 1360 ° C, processing of molten cast iron with urea, with preliminary heating of urea without air access to 600-850 ° C [a.c. USSR No. 1786088].

The disadvantage of this method is the low mechanical properties - temporary resistance and hardness.

The objective of the invention is to improve the properties of cast iron, in particular at the same time hardness, temporary resistance (tensile strength), impact resistance and relative wear resistance.

This object is achieved in that in a method for processing cast iron melt, including melting cast iron in a melting unit, or loading molten iron into a melting unit, overheating of the molten iron is carried out to temperatures of 1460-1560 ° C, at this temperature and mixing the melt, coal diamide is introduced into the melt acid (urea) in an amount of 0.1-0.5% by weight of the melt.

As a source of cast iron for such a treatment, liquid cupola iron, the carbon content of which is usually 2.9-3.3 wt.%, Can be used in the melting unit. At the same time, an increase in the productivity of the method is achieved (reduction of energy costs and time spent on the melting of a solid charge), and in order to provide increased hardness and wear resistance of cast iron castings, carbonization of the cast iron melt received in the melting unit from the cupola is carried out before processing. The melt is carburized at temperatures of 1460-1560 ° C to a carbon content of cast iron of 3.4-4.2 wt.%.

The introduction of carbonic diamide in an amount of more than 0.5% by weight of the melt causes the appearance of porosity in cast iron castings and reduces the strength and impact resistance of castings. The introduction of carbonic diamide in an amount of less than 0.1% by weight of the melt does not lead to the necessary increase in mechanical properties. The use of overheating with a melt temperature of less than 1460 ° C does not provide increased mechanical properties. A melt temperature of more than 1560 ° C should not be used due to an increase in energy and time costs (additional energy is consumed and time to cool the melt before casting), as well as due to accelerated wear of the furnace lining at elevated temperatures. The exclusion of the operation of carburization of the melt of low-carbon cuproduct iron to carbon contents of 3.4-4.2 wt.% Leads to lower values of hardness and relative wear resistance of castings. The carbon content of cast iron in excess of 4.2 wt.% Does not provide the level of strength of cast iron σ _in more than 390 MPa. The lack of melt mixing with the involvement of urea decomposition products inside the melt does not provide sufficient nitrogen uptake (the decomposition products of carbonic acid diamide burn out to a greater extent without being involved in the melt).

The essence of the proposed method is as follows. The treatment of cast iron melt is carried out by introducing carbonic diamide into heated to elevated temperatures - 1460-1560 ° C - and mixed with the cast iron melt. Heated carbonic diamide at 600-850 ° C decomposes thermally into nitrogen, hydrogen and carbon monoxide [a.c. USSR No. 1786088]. Mixing of the melt is carried out in any technologically possible way.

With increasing melt temperature, diffusion processes intensify, and the solubility of gases in the melt (nitrogen and hydrogen, which are part of the decomposition products of urea and the atmosphere) increases. Mixing a cast iron melt heated to elevated temperatures enhances these effects. As a result of this, there is an increase in the nitrogen content in cast iron, which increases the strength characteristics of cast iron castings [Levy L.I. Cast iron nitrogen for castings. - M.: Mechanical Engineering, 1964. - 231 p.]. Nitrogen is an element that can increase the stability of carbides in cast iron. With increasing melt temperature, the solubility of nitrogen in cast iron gradually increases [Guide to iron casting. - 3rd ed. reslave. and add. / Ed. N.G. Girshovich. - L .: Mechanical engineering. Leningra. Department, 1978. - 758 p.]. The interstitial elements — nitrogen, hydrogen, and carbon — can, to one degree or another, cause a general increase in mechanical properties, located in the crystal lattice of the structural components of cast iron — cementite and ferrite.

Example. Melting of cast iron and processing of the melt was carried out in an industrial induction double-crucible furnace with a capacity of one crucible of 3 tons (for cast iron). Carbonic acid diamide was used in the form of granules with a size of 1-3 mm, a density of 1.25-1.32 g / cm ³ . The temperature control of the molten iron was carried out by thermocouple immersion TBP (tungsten-rhenium). An increase in the melt temperature to 1460–1560 ° C was carried out by increasing the power of the induction furnace. An increase in power also led to intensive mixing of the cast iron melt and the convexity of the “mirror” of the melt with the involvement of the molten and decomposed carbonic acid diamide (into nitrogen, hydrogen, and carbon monoxide) by the flows of molten iron into the thickness of the melt, as shown in the figure. The molten urea and its decomposition products were partially captured by streams of molten iron, and in larger quantities on areas of the surface of the melt near the walls of the crucible (lining). Mixing cast iron flows moved in the crucible of the switched on (with high power) induction furnace along trajectories (see figure), which facilitated the equalization of the metal chemical composition in the bath volume and faster dissolution of the elements located on the melt surface. In this case, diffusion processes in the melt in such an overheated state occurred more actively than in a sedentary melt with a low temperature. The treatment of cast iron superheated to 1460-1560 ° C with urea decomposition products involved from the surface of the cast iron melt into the thickness of the cast iron melt led to an increase in mechanical properties.

In one crucible, 2 tons of charge cast iron with a chemical composition in wt.% Were melted: carbon - 3.55; silicon - 1.21; Manganese - 0.55; phosphorus - 0.083; sulfur - 0.101; iron is the rest. After melting the cast iron and removing the slag, the melt temperature was brought to 1460 ° C. With an increased furnace capacity (about 0.9 MW), providing intensive mixing of the melt, urea granules in an amount of 0.3 wt% were gradually poured onto the convex “mirror” of the melt, which quickly rolled down to the crucible walls (as shown in the figure) and areas of the melt near the walls of the crucible were finally melted and decomposed with the involvement of urea decomposition products in the melt. Partially decomposition products burned out. At the end of the treatment, the melt temperature was 1560 ° C.

In another crucible, 2 tons of liquid cupola iron with a chemical composition in wt.% Were poured with a drum ladle: carbon - 3.2; silicon - 0.8; manganese - 0.5; phosphorus - 0.08; sulfur - 0.1; iron is the rest. Thus, the second heat was an element of the duplex process. Slag was removed from the melt surface and the melt temperature was adjusted to 1460 ° C. With an increased furnace capacity providing intensive mixing of the melt, a carburizer was gradually poured onto the convex “mirror” of the melt until 4.15 wt.% Carbon content was obtained in the cast iron samples, and then urea granules were introduced in an amount of 0.35 wt.% Under stirring conditions molten iron. As a carburizer used crushed electrode battle. At the end of the treatment, the melt temperature was 1560 ° C.

After processing, the melt from each crucible was cooled to 1460 ° C and poured into cast iron molds and into the ground. The obtained castings — balls with a diameter of 60 mm and cylinders with a diameter of 30 mm and a height of 300 mm — determined the mechanical properties, wear resistance, and nitrogen content. In addition, a metallographic analysis of the obtained castings was performed.

The results of this and other examples are shown in the table.

As can be seen from the above results, the effectiveness of this method of processing the molten iron is higher compared to the known method.

Due to the proposed treatment of molten iron increased iron properties, in particular hardness up to 523 HB, tensile strength σ _in up to 470 MPa, impact resistance up to 214-302 punches relative abrasion resistance of 5-6.

As is known, graphite worsens the strength characteristics of Fe-C alloys [Gulyaev A.P. Metallurgy. - M.: Metallurgy, 1986. - 544 p.]. The proposed melt processing allows to prevent the appearance of graphite in iron castings, despite the increased carbon content in this cast iron - 4.2 wt.%. Due to the proposed melt processing, cast iron acquires a ledeburite structure without graphite, in addition, casting defects in castings are minimized - shrinkage shells decrease in size or completely disappear.

The figure shows a diagram of the electrodynamic circulation of metal in a crucible of an induction furnace with a feed of urea (F - electrodynamic forces), where:

1 - cast iron melt, 2 - directions of cast iron flows, 3 - urea granules, 4 - direction of urea movement, 5 - zone of involvement of the molten and decomposed urea in the cast iron melt, 6 - zone of the most intense saturation of the melt with decomposed urea products.

Table Cast iron melt processing σ _in , MPa HB Shock resistance (number of blows to failure) ¹ Relative wear resistance ² The nitrogen content in cast iron, wt.% By the proposed method: including melting of cast iron ³ in an induction furnace, overheating ^{4 of} molten cast iron to 1460-1560 ° C with intensive stirring ⁵ and the introduction of carbonic acid diamide at these temperatures and mixing in the amount of the mass of molten cast iron,%: 0.05 366 240 40 1,64 0.0070 0.1 398 458 181 2.02 0.0085 0.3 450 480 302 2.11 0.0150 0.5 505 515 214 2,34 0.0216 0.6 386 512 36 2,30 0.0233 including loading liquid cupola iron into the crucible of an induction furnace, overheating this cast iron to 1460-1560 ° C with vigorous stirring ⁵ , carbonization to carbon content ⁶ in cast iron 4.0 wt.% and the introduction of carbonic diamide in an amount of 0.35% by weight molten iron at these temperatures and stirring 470 523 231 2,51 0.0171 According to a known method: including the melting of cast iron in an induction furnace, overheating of the molten iron ⁷ to 1360 ° C and the introduction of carbonic acid diamide at these temperatures in an amount of 0.32% by weight of the molten iron. 391 241 - 0.38 ^{1) It was} determined on a pendulum head (the weight of the striker is 200 kg, the height of the striker is 85 mm, the impact energy is 166 J, the test specimens are кок60 mm cast iron balls). ²⁾ The wear resistance of white cast iron with a chemical composition, wt.%: C 2.5; Si 1.0; Mn 0.5; S 0.1; P 0.1. ^{3) The} chemical composition of the source of iron (averaged over each element from several samples), wt.%: C 3,55; Si 1.21; Mn 0.55; S 0.101; P 0.083. ^{4) The} introduction of carbonic diamide in an amount of 0.15% at a melt temperature of 1450 ° C leads to values of strength σ _in = 375 MPa, hardness - HB 235. ⁵⁾ The lack of mixing does not lead to the necessary assimilation of the urea decomposition products and the achievement of the required mechanical properties. ⁶⁾ The carbon content of cast iron in excess of 4.2 wt.% Does not provide the level of strength of cast iron σ _in more than 390 MPa. ^{7) The} chemical composition of the source of cast iron, wt.%: C 3,5; Si 2.01; Mn 0.45; S 0.100; P 0.038.

Claims (2)

1. The method of processing the molten iron, including the melting of cast iron in the melting unit, overheating of the molten iron and the introduction of carbonic acid diamide, characterized in that the introduction of carbonic acid diamide in an amount of 0.1-0.5% by weight of the melt is carried out at a melt temperature of 1460- 1560 ° C, with melt stirring. 2. The method according to claim 1, characterized in that as the source of cast iron, liquid cupola cast iron is used, while the cast iron is subjected to carburization in a smelting unit of up to 3.4-4.2 wt.%.

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