Cast iron is a hard, corrosion-resistant, but brittle, iron-carbon alloy with a carbon content of from 2.14 to 6.67%. Despite the presence of characteristic deficiencies, it has a variety of species, properties, and applications. Malleable cast iron is widely used.
History
This material was known from the 4th century BC. e. Its Chinese roots are in the VI century. BC e. In Europe, the first mention of industrial production of the alloy dates back to the XIV, and in Russia - to the XVI century. But the malleable iron production technology was patented in Russia in the 19th century. After developed by A. D. Annosov.
Since gray cast iron is limited in use due to its low mechanical properties, and steel is expensive and has low hardness and durability, the question arose of creating a reliable, durable, solid metal, at the same time having increased strength and a certain ductility.
Forging of cast iron is not possible, but due to its ductile characteristics, it lends itself to certain types of pressure treatment (for example, stamping).
Production
The main method is smelting in blast furnaces.
Source products for blast furnace processing:
- A mixture is an iron ore containing metal in the form of ferum oxides.
- Fuel is coke and natural gas.
- Oxygen - is blown through special tuyeres.
- Fluxes - chemical formations based on manganese and (or) silicon.
Blast furnace stages:
- Pure iron reduction by chemical reactions of iron ore with oxygen supplied through tuyeres.
- Coke combustion and the formation of carbon monoxide.
- Carburization of pure iron in reactions with CO and CO 2 .
- Saturation of Fe 3 C with manganese and silicon, depending on the required output properties.
- Drain the finished metal into molds through cast-iron notches; drainage of slag through slag tap holes.
At the end of the working cycle, the blast furnaces receive cast iron, slag and blast furnace gases.
Blast-furnace metal products
Depending on the cooling rate, microstructure, carbon and additives saturation, several types of cast irons are possible:
- Converted (white): carbon in bonded form, primary cementite. Used as raw materials for the smelting of other iron-carbon alloys, processing. Up to 80% of all blast furnace alloy produced.
- Foundry (gray): carbon in the form of fully or partially free graphite, namely its plates. Used for the production of low-profile case parts. Up to 19% of blast furnace production.
- Special: saturated with ferroalloys. 1-2% of the considered type of production.
Malleable cast iron is obtained through heat treatment of the pig iron.
Theory of Iron Carbon Structures
Carbon with ferum can form several different types of alloys according to the type of crystal lattice, which is displayed on the microstructure variant.
- A solid solution of penetration into α-iron - ferrite.
- A solid solution of penetration into γ-iron - austenite.
- The chemical formation of Fe 3 C (bound state) is cementite. Primary is formed by rapid cooling from liquid melt. Secondary - a slower decrease in temperature, from austenite. Tertiary - gradual cooling, from ferrite.
- The mechanical mixture of grains of ferrite and cementite is perlite.
- A mechanical mixture of perlite or austenite and cementite grains is ledeburite.
Cast iron is characterized by a special microstructure. Graphite can be in a bound form and form the above structures, and can remain in a free state in the form of various inclusions. Both basic grains and these formations influence the properties. The graphite fractions in the metal are plates, flakes or balls.
The lamellar form is characteristic of gray iron-carbon alloys. It determines their fragility and insecurity.
Flake-like inclusions have malleable cast irons, which positively affects their mechanical properties.
The spherical structure of graphite further improves the quality of the metal, affecting the increase in hardness, reliability, and the ability to withstand significant loads. High-strength cast iron has these characteristics. Malleable cast iron determines its properties by ferritic or pearlitic substrates with the presence of flaky graphite inclusions.
Getting ferritic malleable cast iron
It is produced from a white conversion hypereutectoid low-carbon alloy by annealing ingots with a carbon content of 2.4-2.8% and the corresponding presence of additives (Mn, Si, S, P). The wall thickness of the annealed parts should be no more than 5 cm. For castings of significant thickness, graphite has the form of plates and the desired properties are not achieved.
To get malleable cast iron with a ferritic base, the metal is placed in special boxes and poured with sand. Tightly closed containers are placed in heating furnaces. The following sequence of actions is carried out during annealing:
- The structures are heated in furnaces to a temperature of 1,000 ° C and left to withstand at constant heat for a period of 10 to 24 hours. As a result, primary cementite and ledeburite decompose.
- The metal is cooled to 720 ° C with the furnace.
- At a temperature of 720 ° C they are kept for a long time: from 15 to 30 hours. This temperature ensures the decay of secondary cementite.
- At the final stage, it is again cooled together with the working stove to 500 ° C, and then taken out into the air.
Such technological annealing is called graphitizing.
After the work, the microstructure of the material is a ferrite with flocculent grains of graphite. This type is called "black-hearted", since the kink is black.
Getting pearlitic malleable cast iron
This is a type of iron-carbon alloy, which also originates from white hypereutectoid, but its carbon content is increased: 3-3.6%. To obtain castings with a pearlitic base, they are placed in boxes and sprinkled with powdered iron ore or scale. The annealing procedure itself is simplified.
- The temperature of the metal is increased to 1000 ° C, can withstand 60-100 hours.
- Constructions are cooled with an oven.
Due to languishing under the influence of heat, diffusion occurs in the metal environment: the graphite released in the cementite decay partially leaves the surface layer of the annealed parts, settling on the surface of the ore or scale. A softer, viscous and ductile upper layer of "white" malleable cast iron with a solid middle is obtained.
Such annealing is called incomplete. It provides the decomposition of cementite and ledeburite into lamellar perlite with the corresponding graphite. In case granular pearlitic malleable cast iron with higher indices of impact strength and ductility is required, additional heating of the material to 720 ° C is used. In this case, perlite grains with flocculent graphite inclusions are formed.
Properties, labeling and use of malleable cast iron
Long “languishing” of the metal in the furnace results in the complete decomposition of cementite and ledeburite into ferrite. Thanks to technological tricks, an alloy with a high carbon content is obtained - a ferritic structure characteristic of low-carbon steel. However, carbon in itself does not disappear - it passes from a state bound to iron to a free state. The temperature effect changes the shape of graphite inclusions to flocculent.
The ferritic structure causes a decrease in hardness, an increase in strength values, and the presence of such characteristics as impact strength and ductility.
Marking of ductile iron of ferritic class: 30-6, 33-8, 35-10, 37-12, where:
KCH - designation of a variety - malleable;
30, 33, 35, 37: σ in , 300, 330, 350, 370 N / mm 2 - the maximum load that it can withstand without breaking;
6, 8, 10, 12 — elongation, δ,% — plasticity index (the higher the value, the more metal lends itself to pressure treatment).
Hardness is about 100-160 HB.
This material in terms of performance occupies a middle position between such as steel and iron-carbon alloy gray. Malleable cast iron with a ferritic base is inferior to pearlitic in terms of wear resistance, corrosion and fatigue strength, but higher in mechanical endurance, ductility, and casting characteristics. Due to its low price, it is widely used in industry for the manufacture of parts operating at low and medium loads: gears, crankcases, rear axles, and plumbing.
Properties, labeling and use of pearlitic ductile iron
Due to incomplete annealing, primary, secondary cementites and ledeburite have time to completely dissolve in austenite, which at a temperature of 720 ° C turns into perlite. The latter is a mechanical mixture of grains of ferrite and tertiary cementite. Actually, part of the carbon remains in bound form, determines the structure, and part is “released” into flocculent graphite. In this case, perlite can be lamellar or granular. Thus pearlitic malleable cast iron is formed. Its properties are due to a saturated harder and less pliable structure.
These, in comparison with ferritic, have higher anticorrosive, wear-resistant properties, their strength is much higher, but the casting characteristics and ductility are lower. Resistance to mechanical stress is increased superficially, while maintaining the hardness and viscosity of the core of the product.
Marking of malleable cast iron of pearlite class: 45-7, 50-5, 56-4, 60-3, 65-3, 70-2, 80-1,5.
The first digit is the strength designation: 450, 500, 560, 600, 650, 700 and 800 N / mm 2, respectively.
The second is the designation of plasticity: elongation δ,% - 7, 5, 4, 3, 3, 2, and 1.5.
Pearlitic malleable cast iron has found application in mechanical engineering and instrumentation for structures operating under heavy loads - both static and dynamic: camshafts, crankshafts, clutch parts, pistons, connecting rods.
Heat treatment
Material obtained as a result of heat treatment, namely annealing, can be re-exposed to methods of temperature influences. Their main goal is an even greater increase in strength, wear resistance, resistance to corrosion and aging.
- Hardening is used for structures requiring high hardness and viscosity; It is produced by heating to 900 ° C, the parts are cooled at an average speed of about 100 ° C / s using machine oil. It is followed by a high tempering with heating to 650 ° C and cooling in air.
- Normalization is used for medium-sized simple parts by heating in an oven to 900 ° C, standing at this temperature for a period of 1 to 1.5 hours and subsequent cooling in air. Provides troostite granular perlite, its hardness and reliability during friction and wear. It is used to obtain anti-friction malleable cast iron with a pearlitic base.
- Annealing is repeated in the manufacture of antifriction: heating - up to 900 ° C, long-term exposure to this heat, cooling along with the furnace. Provides a ferritic or ferritic-pearlitic structure of malleable cast iron.
Heating of cast iron products can be carried out locally or comprehensively. For local, high-frequency currents or an acetylene flame are applied (hardening). For complex - heating furnaces. With local heating, only the top layer is hardened, while its hardness and strength increase, but the ductility and viscosity of the core are preserved.
It is important to point out that forging of cast iron is impossible not only due to insufficient mechanical characteristics, but also because of its high sensitivity to a sharp temperature drop, which is inevitable during quenching with water cooling.
Anti-friction malleable cast irons
This variety applies to malleable and alloyed, they are gray (ASF), malleable (AFC) and high strength (AFC). For the production of AFC, malleable cast iron is used, which is subjected to annealing or normalization. The processes are carried out in order to increase its mechanical properties and the formation of a new characteristic - wear resistance during friction with other parts.
It is marked: -1, -2. It is used for the production of crankshafts, gears, bearings.
Effect of additives on properties
In addition to the iron-carbon base and graphite, they also include other components that also determine the properties of cast iron: manganese, silicon, phosphorus, sulfur, and some alloying elements.
Manganese increases the fluidity of molten metal, corrosion resistance and wear resistance. It helps to increase hardness and strength, the binding of carbon with iron in the chemical formula Fe 3 C, the formation of granular perlite.
Silicon also has a positive effect on the fluidity of a liquid alloy, contributes to the decomposition of cementite and the release of graphite inclusions.
Sulfur is a negative but inevitable component. It reduces mechanical and chemical properties, stimulates the formation of cracks. However, a rational ratio of its content with other elements (for example, manganese) allows you to adjust microstructural processes. So, with a Mn-S ratio of 0.8-1.2, perlite is retained for any period of temperature effects. By increasing the ratio to 3, it becomes possible to obtain any necessary structure depending on the given parameters.
Phosphorus changes fluidity for the better, affects strength, reduces toughness and ductility, and affects the duration of graphitization.
Chromium and molybdenum impede the formation of graphite flakes; in some contents, they contribute to the formation of granular perlite.
Tungsten increases wear resistance when working in high temperature zones.
Aluminum, nickel, copper contribute to graphitization.
By adjusting the number of chemical elements that make up the iron-carbon alloy, as well as their ratio, you can affect the final properties of cast iron.
Advantages and disadvantages
Ductile iron is a material that is widely used in engineering. Its main advantages:
- high rates of hardness, wear resistance, strength along with fluidity;
- normal characteristics of impact strength and ductility;
- manufacturability in pressure processing, in contrast to gray cast irons;
- various options for correcting properties for a particular part by methods of thermal and chemical-thermal treatment;
- low cost.
The disadvantages include individual features:
- fragility;
- the presence of graphite inclusions;
- low cutting performance;
- significant weight of castings.
Despite the existing shortcomings, malleable cast iron occupies an important place in metallurgy and mechanical engineering. It produces important parts such as crankshafts, brake pad parts, gears, pistons, connecting rods. With a small variety of brands, malleable cast iron occupies an individual niche in the industry. Its use is typical for those loads under which the use of other materials is unlikely.