RU2307875C1 - Cast iron and method for heat treatment of its castings - Google Patents

Abstract

FIELD: ferrous metallurgy, namely production of high-strength cast irons with spheroidal graphite, possibly production of cast products having high strength, ductility and impact viscosity.

SUBSTANCE: cast iron contains, mass%: carbon, 3.28 - 4.03; silicon, 2.34 - 3.62; manganese, 0.22 - 0.53; copper, 1.16 - 2.34; molybdenum, 0.21 - 0.52; magnesium, 0.02 - 0.05; barium, 0.03 - 0.08; rare-earth metal, 0.02 - 0.06; iron and inevitable impurities, the balance. Cast pieces of such cast iron are subjected to heat treatment comprising stepped austenization at heating up to 820 - 830°C; soaking for 0.5 h and further heating till 870 - 900°C and soaking for 0.5 - 1.5 h. Then castings are cooled till temperature less than 500°C. Cooling rate is controlled depending upon wall thickness of cast piece. If wall thickness is less than 20 mm, cast product is cooled in air; if wall thickness is 25 -40 mm cast product is cooled in water for 4 - 5 s; if wall thickness exceeds 40 mm cast product is cooled in water for 6 - 10 s. Then casting are subjected to thermocycling in temperature range 270 - 390°C for 1.5 -3 h and to air cooling.

EFFECT: improved stable mechanical properties of castings with different wall thickness.

2 cl, 2 dwg

Description

The invention relates to the field of metallurgy, in particular to the production of high-strength cast iron with spherical graphite, and can be used in the manufacture of cast products characterized by high strength, ductility and toughness.

A combination of high mechanical properties, including increased ductility and toughness, in nodular cast iron is obtained by choosing their necessary chemical composition and method of heat treatment of castings.

Known cast iron [1], containing, wt.%:

Carbon 3.0-3.8 Titanium 0.01-0.1 Silicon 1.6-2.8 Magnesium 0.04-0.08 Manganese 0.06-0.4 Sulfur 0.006-0.02 Copper 0.5-1.8 Phosphorus less than 0.1 Chromium 0.05-0.15 Iron rest Vanadium 0.04-0.2

This cast iron crystallizes in castings without structurally free cementite, possesses a stable pearlite-ferrite structure in the cast state with increased mechanical properties.

The disadvantage of cast iron is the instability of mechanical properties (in the cast state, the tensile strength σ _in from 500 to 1000 MPa).

Closest to the proposed is cast iron [2], containing, wt.%:

Carbon 3.0-4.0 Chromium less than 0.1 Silicon 1.5-3.0 Phosphorus less than 0.06 Manganese 0.3-0.6 Sulfur less than 0.02 Copper 0.3-1.0 Magnesium 0.02-0.06 Molybdenum 0.3-0.6 Iron rest Nickel up to 1.5

For this cast iron, a heat treatment method is proposed, which includes heating (austenitization) to 850–950 ° C, holding for 0.5–4 h, cooling to 250–450 ° C at a rate that suppresses pearlite transformation, holding for 1–4 h at 250 -450 ° C to complete the bainitic transformation and cooling in air.

After such a heat treatment, the structure of cast iron consists of bainite, residual austenite and spherical graphite. Cast iron has increased mechanical properties (σ _in = 950-1050 MPa).

The disadvantages of cast iron include the preservation in the structure of a large amount of residual austenite and the resulting limitation of the upper limit of strength properties by the above value of σ _in .

A known method of heat treatment of ductile iron, providing a combination of high mechanical properties, in the form of isothermal hardening, including austenitization at 850-900 ° C, cooling in an alkaline or salt bath with an isothermal exposure of 0.5-1 h at 350-400 ° C and cooling in the air [3]. Such processing provides obtaining bainitic-austenitic structure and high strength properties with a relatively high ductility and viscosity of cast iron.

The disadvantage of this method is the need to use special equipment and liquid cooling media in the form of molten salts and alkalis with harmful emissions.

Closest to the proposed method is a heat treatment [4], including austenitization at 870-900 ° C, short-term (for 4 s) cooling in water ("soaking"), isothermal aging in an oven at 350-380 ° C and final cooling on air.

This method allows in many cases to obtain the necessary structure and high mechanical properties of cast iron, but this does not take into account possible differences in the chemical composition of cast iron and the thickness of the casting wall, which can lead to the formation of unfavorable structures with insufficiently high mechanical properties of cast iron.

The objective of the invention is to create a cast iron dispersed structure consisting of bainite, a limited amount of austenite (up to 30-35%) and spherical graphite.

EFFECT: obtaining a complex of high and stable mechanical properties of cast iron (strength, ductility and toughness) in castings with different wall thicknesses.

This is achieved by the fact that:

1. Cast iron containing carbon, silicon, manganese, copper, molybdenum, magnesium, impurities and iron, additionally contains barium and rare-earth metals in the following ratio, wt.%:

Carbon 3.28-4.03 Magnesium 0.02-0.05 Silicon 2.34-3.62 Barium 0.03-0.08 Manganese 0.22-0.53 REM 0.02-0.06 Copper 1.16-2.34 Iron and impurities rest Molybdenum 0.21-0.52

As impurities are allowed, wt.%: Phosphorus up to 0.04, sulfur up to 0.02, chromium up to 0.08.

2. In the method of heat treatment of castings from cast iron, including austenitization, cooling to temperatures below 500 ° C, holding in a furnace and final cooling in air, the cast iron is subjected to heat treatment according to claim 1, while the austenitization is carried out stepwise according to the 820-830 ° mode C, 0.5 h → 870-900 ° C, 0.5-1.5 h, the cooling rate is regulated depending on the wall thickness of the casting: up to 20 mm in air, at 25-40 mm in water for 4 -5 s, more than 40 mm in water for 6-10 s, and the exposure in the furnace is carried out for 1.5-3 hours with thermal cycling of 270-390 ° C.

Changes in the chemical composition of cast iron have been introduced in order to consistently produce bleached castings with different wall thicknesses, increase the hardenability of cast iron, simplify the heat treatment of castings, and ensure the necessary properties of cast iron after heat treatment.

The composition of cast iron is selected based on the following considerations.

The content of silicon in cast iron is increased, which is the main graphitizing element and contributes to the production of a bainitic structure during heat treatment. When the silicon content is less than 2.34% in the structure of cast iron in thin-walled castings, the appearance of structurally free cementite is possible (partial bleaching); this also makes it difficult to obtain a crushed bainitic structure that provides high mechanical properties of heat-treated cast iron. With an increase in silicon content of more than 3.62%, a significant amount of ferrite (in particular, silicoferrite) appears in the structure of heat-treated cast iron, which leads to a decrease in its mechanical properties.

The manganese content should not exceed 0.53%, because with a higher content, the tendency of cast iron to bleach in castings increases, which complicates complete graphitization, increases the heterogeneity of the structure during heat treatment, and leads to a decrease in ductility and toughness of cast iron. The minimum amount of manganese in cast iron, amounting to 0.22%, corresponds to its content as a technical impurity; a further decrease in the manganese content is difficult when using conventional charge materials.

The copper content in the composition of cast iron is increased in order to reduce the tendency of cast iron to bleach, increase its hardenability and increase the strength properties of cast iron due to additional dispersion hardening during heat treatment. Copper is also a substitute for more expensive nickel and can reduce the cost of cast iron compared to the prototype. With a copper content of less than 1.16%, this complex effect of copper is not sufficiently manifested, which affects the mechanical properties of cast iron. A copper content of more than 2.34% does not increase the properties of cast iron, but causes it to become more expensive.

Molybdenum is used to increase bainitic hardenability and eliminate the temper brittleness of cast iron. At a content of less than 0.21 wt.% Molybdenum, this role is not sufficiently manifested in castings with a wall thickness of more than 25 mm, and at a content of more than 0.52 wt.%, The cost of cast iron increases and additional components appear in the structure that increase its hardness.

The magnesium content is recommended in the range of 0.02-0.05 wt.%. If the residual magnesium content is less than 0.02 wt.%, The results of the modification are unstable. An increase in magnesium content of more than 0.05 wt.% Is impractical, since this does not increase the properties of cast iron.

REMs are introduced in order to neutralize elements that have a desferoidizing effect on graphite (for example, copper and various trace elements). When the content is less than 0.02 wt.% REM, complete spheroidization of graphite is not provided. An increase in the content of rare-earth metals more than 0.06 wt.% Is impractical, since it does not have a positive effect, but the cost of cast iron.

Additionally, barium was introduced into the composition of cast iron (in the form of silicobarium as part of a complex modifier). Together with other components of the complex modifier, it provides deep refining of cast iron, increases the stability of the modifying treatment of cast iron and completely eliminates bleaching even in thin-walled castings. For this, the barium content in the claimed limits is sufficient. With a residual barium content of more than 0.08%, its modifying effect is not enhanced, but the cost of cast iron increases. With a barium content of less than 0.03%, its effect is not significant.

The accepted carbon content provides the necessary structure and properties of cast iron. When the carbon content is less than 3.28 wt.%, The tendency of cast iron to graphitization decreases and the formation of bleached sections of the structure with increased hardness becomes possible. If cast iron contains more than 4.03 wt.% Carbon, the amount of graphite increases in its structure, and the likelihood of the formation of graphite inclusions of an unfavorable shape (with an insufficient degree of spheroidization) and their localization in the form of spells increase, which can manifest itself in a decrease in all mechanical properties cast iron.

Cast iron heat treatment consists of three stages. The first stage is carried out with the aim of complete austenitization, while ensuring the tendency of austenite to partial ferritization upon cooling (up to 20-30%) and homogenization of the rest of the austenite, which is achieved by stepwise heating to 820-830 ° C with a holding time of 0.5 h (first stage ) and further heating to 870-900 ° C with a shutter speed of 0.5-1.5 hours, depending on the wall thickness of the casting (second stage).

The second stage consists in cooling the cast iron to 450-400 ° C at a speed higher than critical (in order to prevent the formation of pearlite structures), for which thin-walled castings (up to 20 mm) are cooled in air, and more massive castings (with a wall thickness of 25 mm or more ) "soaked" in water with different exposure times, depending on the wall thickness of the casting (with a wall thickness of 25-40 mm for 4-5 s, and with a thickness of more than 40 mm - for 6-10 s).

The third stage of the heat treatment is carried out in a conventional thermal furnace by thermal cycling in the temperature range 270-390 ° C with a total duration of 1.5-3 hours. This stage is carried out with the aim of forming a crushed bainitic ("ausferritic") structure and its hardening by artificial aging. After the third stage, the product is cooled in air to room temperature.

Cast iron was melted in induction crucible furnaces with a capacity of 50 and 150 kg with an acid lining. Used a mixture consisting of cast iron, ferroalloys (ferrosilicon and ferromolybdenum) and copper waste. The modification was carried out with a mixture of complex ligatures and silicobarium in casting ladles with a capacity of 50 to 100 kg at a temperature of 1390-1430 ° C. For each variant of the chemical composition of cast iron, plates with a thickness of 20, 30, and 50 mm were cast into dry sand-clay molds. After heat treatment, standard samples for mechanical testing were cut from the plates.

The chemical compositions of cast irons for all options are given in Table 1, and the results of mechanical tests are shown in Table 2.

It can be seen that the proposed combination of the chemical composition of cast iron and the heat treatment method provides, in comparison with the prototype, higher values of the tensile strength of cast iron, especially in thick-walled castings, while maintaining sufficiently high values of ductility and toughness; It is also important that the hardness of cast iron is not excessively high, which allows the necessary machining of castings with a blade tool.

When the chemical composition of cast iron goes beyond the proposed limits (alloys No. 5 and 6), the properties of cast iron deteriorate significantly. The deviation of the heat treatment method from claim 2 of the claims (for example, during heat treatment according to the prototype mode [4]) also leads to a decrease in some properties of cast iron (elongation and toughness) and increase hardness, especially in thin-walled castings.

Table 1 Chemical compositions of cast irons Alloy The content of elements, wt.% FROM Si Mn Cu Mo Mg Wa REM one 3.28 3.62 0.22 1.81 0.52 0.02 0,03 0.06 2 3.56 3.34 0.41 2,34 0.21 0.05 0.08 0,03 3 3.88 2.83 0.53 1.16 0.46 0,03 0.05 0.02 four 4.03 2,34 0.39 1,63 0.30 0,03 0.06 0.04 5 3.05 4.07 0.81 2,57 0.14 0.01 0.01 0,07 6 4.12 2.26 0.20 0.75 0.69 0.08 0.10 0.01 Famous ^* 3.44 2.21 0.53 0.58 0.56 0.05 - - * Also contains 1.2% Ni and 0.1% Cr table 2 Mechanical properties of cast irons (medium) Alloy Plate thickness mm Tensile strength σ _in , MPa Elongation δ,% Impact strength KS, J / cm ² Hardness HB one twenty 1230 7 59 310 thirty 1220 7 56 310 fifty 1190 5 fifty 290 2 twenty 1220 7 52 320 thirty 1210 8 49 310 fifty 1180 6 45 300 3 twenty 1190 8 fifty 310 thirty 1170 7 47 300 fifty 1160 5 45 290 four twenty 1220 7 55 290 thirty 1200 6 48 300 fifty 1170 6 46 280 5 twenty 910 2 32 290 thirty 890 2 27 280 fifty 850 2 23 270 6 twenty 1060 four 46 330 thirty 1050 four 42 330 fifty 1030 3 40 310 Famous ^* twenty 1080 5 40 370 thirty 1060 5 44 360 fifty 1030 four 41 320 ^* Heat treatment carried out according to the mode [4]

Information sources

1. RF patent №2138576, cl. C22C 37/10, claimed 12/18/1998, publ. 09/27/1999.

2. Japanese application No. 60-106946, cl. C22C 37/08, C21D 5/00, claimed 11/15/1983, publ. 06/12/1985.

3. Cast Iron: Ref. ed. / Ed. A.D.Sherman and A.A. Zhukov. - M.: Metallurgy, 1991 .-- 576 p.

4. Zhukov A.A. Some questions of the theory and practice of bainitic hardening of cast irons // Metallurgy and heat treatment of metals, 1995, No. 12. - S.26-29.

Claims (2)

1. Cast iron containing carbon, silicon, manganese, copper, molybdenum, magnesium, iron and impurities, characterized in that it additionally contains barium and rare-earth metals in the following ratio, wt.%: carbon 3.28-4.03 silicon 2.34-3.62 manganese 0.22-0.53 copper 1.16-2.34 molybdenum 0.21-0.52 magnesium 0.02-0.05 barium 0.03-0.08 REM 0.02-0.06 iron and impurities rest 2. The method of heat treatment of castings from cast iron, including austenization, cooling to temperatures below 500 ° C, holding in a furnace and finally cooling in air, characterized in that the castings of cast iron according to claim 1 are subjected to heat treatment, and austenization is performed stepwise according to mode: heating to 820-830 ° C with a shutter speed of 0.5 h and subsequent heating to 870-900 ° C with a shutter speed of 0.5-1.5 h, the cooling rate is regulated depending on the wall thickness of the casting: up to 20 mm - air, at 25-40 mm - in water for 4-5 s, more than 40 mm - in water for 6-10 s, and rzhka in the furnace is carried out for 1.5-3 hours with thermal cycling in the range of 270-390 ° C.