Ductile Iron: Properties, Marking and Applications

Cast iron is a solid, corrosion-resistant, but brittle, iron-carbon alloy with a carbon content of C ranging from 2.14 to 6.67%. Despite the presence of characteristic shortcomings, there is a variety of types, properties, applications. Widely cast iron is widely used.

History

This material was known from the IV century BC. E. His Chinese roots are in the VI century. BC. E. In Europe, the first mention of the industrial production of the alloy is dated XIV, and in Russia - the 16th century. But the technology of production of malleable cast iron patented in Russia in the XIX century. After developed AD Annosov.

Since gray cast iron is limited in use because of its low mechanical properties, and steel is expensive and has low hardness and durability, the question arose about the creation of a metal that is reliable, durable, solid, at the same time having increased strength and a certain ductility.

Forging of cast iron is impossible, but due to its plastic characteristics, it is susceptible to certain types of pressure treatment (for example, stamping).

Production

The main method is melting in blast furnaces.

Initial products for blast furnace processing:

• Shihta - iron ore, containing metal in the form of oxides of a ferum.
• Fuel - coke and natural gas.
• Oxygen is blown through special tuyeres.
• Fluxes are chemical formations based on manganese and (or) silicon.

Stages of blast furnace smelting:

1. Recovery of pure iron by chemical reactions of iron ore with oxygen fed through the tuyeres.
2. Combustion of coke and formation of carbon oxides.
3. Carburization of pure iron in reactions with CO and CO ₂ .
4. Saturation of Fe ₃ C with manganese and silicon, depending on the required properties at the outlet.
5. Draining of finished metal into molds through cast iron tapholes; Slag discharge through slag tapes.

At the end of the working cycle, the blast furnaces receive cast iron, slag and blast furnace gases.

Metallic products of blast-furnace production

Depending on the rate of cooling, microstructure, carbon saturation and additives, it is possible to obtain several types of cast iron:

1. Redistribution (white): carbon in a bonded form, primary cementite. Used as raw materials for smelting other iron-carbon alloys, processing. Up to 80% of all produced blast-furnace alloy.
2. Foundry (gray): carbon in the form of completely or partially free graphite, namely its plates. Used for the production of low-case body parts. Up to 19% of the produced blast casting.
3. Special: saturated with ferroalloys. 1-2% of the considered type of production.

Ductile iron is produced by heat treatment of the pig iron.

Theory of iron-carbon structures

Carbon with a ferum can form several different types of alloys by the type of crystal lattice, which is displayed on the microstructure version.

1. A solid solution of penetration into α-iron-ferrite.
2. A solid solution of penetration into γ-iron is austenite.
3. The chemical formation of Fe ₃ C (bound state) is cementite. The primary is formed by rapid cooling from the liquid melt. Secondary - a slower decrease in temperature, from austenite. Tertiary - gradual cooling, from ferrite.
4. The mechanical mixture of grains of ferrite and cementite is perlite.
5. Mechanical mixture of grains of perlite or austenite and cementite - ledeburite.

For cast iron, a special microstructure is characteristic. Graphite can be in a bound form and form the above structures, but can remain in a free state in the form of different inclusions. The properties are influenced by both the main grains and these formations. Graphite fractions in a metal are plates, flakes or spheres.

The plate form is characteristic for gray iron-carbon alloys. It causes their fragility and insecurity.

Inclusions floccous have ductile cast iron, which positively affect their mechanical performance.

The spherical structure of graphite further improves the quality of the metal, influencing the increase in hardness, reliability, aging of significant loads. These characteristics are high-strength cast iron. Ductile iron properties of its causes ferrite or pearlite basis with the presence of floccous graphite inclusions.

Production of ferritic malleable cast iron

It is produced from a white, pre-eutectic low-carbon alloy by annealing ingots with a carbon content of 2.4-2.8% and the corresponding additives (Mn, Si, S, P). The thickness of the walls of annealed parts should not be more than 5 cm. For castings of considerable 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 over with sand. Tightly closed containers are placed in heating furnaces. The following sequence of actions is performed during annealing:

1. The structures are heated in furnaces to a temperature of 1,000 ° C and left to stand at constant heat for a period of 10 to 24 hours. As a result, primary cementite and ledeburite decompose.
2. The metal is cooled to 720 ° C together with the furnace.
3. At a temperature of 720 ° C, they are maintained for a long time: from 15 to 30 hours. This temperature ensures the decomposition of the secondary cementite.
4. At the final stage, they are again cooled together with the working stove to 500 ° C, and then they are taken to air.

Such a technological annealing is called graphitizing.

After the work, the microstructure of the material is a ferrite with flake grains of graphite. This type is called "black hearted", since the fracture is black.

Production of pearlite malleable cast iron

This is a kind of iron-carbon alloy, which also originates from the pre-eutectoid white, but the carbon content in it is increased: 3-3.6%. To obtain castings with a pearlite base, they are placed in boxes and poured with crushed powdered iron ore or scale. The annealing procedure itself is simplified.

1. The temperature of the metal is raised to 1 000 ° C, maintained for 60-100 hours.
2. The structures are cooled with an oven.

Owing to the languor under the influence of heat in the metallic environment, diffusion occurs: the graphite released in the cementite decomposition partially leaves the surface layer of the annealed parts, settling on the surface of the ore or scale. Get a softer, viscous and ductile top layer of "heart-shaped" malleable cast iron with a solid middle.

Such annealing is called incomplete. It ensures the decomposition of cementite and ledeburite into plate-like perlite with the corresponding graphite. In the event that granular pearlite malleable cast iron is required with higher impact strength and ductility, additional material preheating is used up to 720 ° C. In this case, perlite grains are formed with flake graphite inclusions.

Properties, marking and application of ferritic malleable cast iron

The prolonged "languishing" of 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 ferrite structure characteristic of low-carbon steel. However, the carbon itself does not go away anywhere - it passes from the iron-bound state to the free state. The temperature influence changes the shape of the graphite inclusions to the flocculent.

The ferritic structure causes a decrease in hardness, an increase in strength, the presence of characteristics such as impact strength and ductility.

Marking of pig iron malleable ferritic class: KCh30-6, KCh33-8, KCh35-10, KCh37-12, where:

KC - designation of the 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 - relative elongation, δ,% - plasticity index (the higher the value, the more metal is subject to pressure treatment).

The hardness is about 100-160 HB.

This material in its parameters occupies a middle position between such as steel and iron-carbon gray alloy. Ductile cast iron with ferritic base is inferior to pearlite in terms of wear resistance, corrosion and fatigue strength, but higher in mechanical holding, plasticity, casting characteristics. Due to its low price it is widely used in industry for manufacturing parts that work under small and medium loads: gears, crankcases, rear bridges, sanitary ware.

Properties, marking and application of pearlite malleable cast iron

Due to incomplete annealing, the primary, secondary cementites and ledeburite have time to completely dissolve in austenite, which at 720 ° C turns into perlite. The latter is a mechanical mixture of grains of ferrite and cementite of tertiary. In fact, part of the carbon remains bound, determines the structure, and part of it is "released" into flaky graphite. In this case perlite can be lamellar or granular. Thus, pearlite malleable cast iron is formed. Its properties are caused by a saturated, harder and less pliable structure.

These, in comparison with ferrite, have higher anticorrosive, wear-resistant properties, their strength is much higher, but lower casting characteristics and plasticity. The compliance to mechanical influences is increased superficially, while maintaining the hardness and toughness of the core of the product.

Marking of ductile cast iron pearlite class: KCh45-7, KCh50-5, KCh56-4, KCh60-3, KCh65-3, KCh70-2, KCh80-1.5.

The first digit is the strength designation: 450, 500, 560, 600, 650, 700 and 800 N / mm ^2, respectively.

The second is the plasticity designation: relative elongation δ,% - 7, 5, 4, 3, 3, 2 and 1.5.

Pearlitious malleable cast iron is used in mechanical engineering and instrument making for structures operating under heavy loads, both static and dynamic: camshafts, crankshafts, clutch parts, pistons, connecting rods.

Heat treatment

The material obtained due to heat treatment, namely annealing, can be repeatedly subjected to the methods of temperature influences. Their main goal is an even greater increase in strength, wear resistance, resistance to corrosion and aging.

1. Hardening is used for structures requiring high hardness and toughness; Is produced by heating to 900 ° C, the parts are cooled at an average speed of about 100 ° C / s with machine oil. Following it follows a high tempering with heating to 650 ° C and cooling in air.
2. Normalization is used for small simple details by heating in an oven up to 900 ° C, standing at this temperature for a period of 1 to 1.5 hours and then cooling in air. Provides troostite granular perlite, its hardness and reliability in friction and wear. It is used for the production of antifrictional malleable cast irons with a pearlite base.
3. Annealing is repeated in the production of antifriction: heating - up to 900 ˚С, long-term holding at this heat, cooling with the furnace. A ferritic or ferritic-perlite structure of antifrictional malleable cast iron is provided.

Heating of cast iron products can be carried out locally or in a complex manner. For local, high-frequency currents or acetylene flame are used (hardening). For complex - heating furnaces. With local heating, only the top layer is hardened, while its hardness and strength increase, but the plasticity and core viscosity remain.

Here it is important to point out that forging pig iron is impossible not only because of insufficient mechanical characteristics, but also because of its high sensitivity to the sharp temperature drop, which is inevitable during quenching with water cooling.

Antifrictional malleable cast irons

This variety applies to malleable and alloyed, they are gray (ASF), forging (AFC) and high-strength (AChV). For the production of AFC, ductile cast iron is used, which is annealed or normalized. Processes are carried out with the purpose of increase of its mechanical properties and formation of a new characteristic - wear resistance at friction with other details.

It is marked: АЧК-1, АЧК-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 have other constituents in their composition, which also determine the properties of cast iron: manganese, silicium, phosphorus, sulfur, and some alloying elements.

Mangan increases fluidity of liquid metal, corrosion resistance and wear resistance. It promotes hardness and strength increase, binding of carbon and iron to the chemical formula Fe ₃ C, the formation of granular perlite.

Silicium also positively affects the fluidity of the liquid alloy, promotes the breakdown 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 correlation of its content with other elements (for example, with manganese) makes it possible to correct microstructural processes. So, at a ratio Mn-S of 0.8-1.2 perlite is retained for any periods of temperature influences. With an increase in the ratio to 3, it becomes possible to obtain any necessary structure depending on the specified parameters.

Phosphorus changes the fluidity for the better, affects the strength, reduces the toughness and plasticity, affects the duration of graphitization.

Chromium and molybdenum make it difficult to form graphite flakes, in some of the contents contribute to the formation of granular perlite.

Tungsten improves wear resistance when working in high temperature areas.

Aluminum, nickel, copper promote graphitization.

Correcting the number of chemical elements that make up the iron-carbon alloy, as well as their ratio, it is possible to influence the final properties of cast iron.

Advantages and disadvantages

Ductile iron is a material widely used in engineering. Its main advantages are:

• High indicators of hardness, wear resistance, strength, along with fluidity;
• Normal characteristics of impact strength and ductility;
• Processability in the treatment of pressure, in contrast to gray cast irons;
• A variety of options for correcting properties for a certain detail by methods of thermal and chemical-thermal treatment;
• low cost.

The disadvantages include individual features:

• Fragility;
• Presence of graphite inclusions;
• Low performance in machining;
• Significant weight of castings.

Despite the existing shortcomings, malleable cast iron occupies a critical place in metallurgy and machine building. It produces such important details as crankshafts, brake pad parts, cogwheels, pistons, connecting rods. Having a small variety of brands, an individual niche in the industry is malleable cast iron. Its application is typical for those loads in which the use of other materials is unlikely.