White cast iron: application, marking, composition, properties, types

Initially, cast iron technology was first mastered in China back in the 10th century, after which it became widespread in other countries of the world. The basis of cast iron is an alloy of iron with carbon and other components. A distinctive feature is that cast iron contains more than 2% carbon in the form of cementite, which is not found in other metals. A prominent representative of such an alloy is white cast iron, which is used in mechanical engineering for the manufacture of parts, in industry and in everyday life.

Composition and types

The carbon in white cast iron forms a cementite structure. Depending on its content, alloys are distinguished:

Classification by crystal structure gives the following types:

Carbide-forming elements are used as alloying elements:

When these elements are introduced into the hypoeutectic structure, the strength and stability of austenite and ledeburite increase. The degree of alloying and physical and mechanical properties also depend on the size of the castings and cooling conditions.

Features of receiving

When producing white cast iron, it is important to exclude the graphitization process during crystallization of the melt, which is done:

The degree of wear resistance of castings is largely determined by the nature and composition of carbides. Alloying the metal with nickel, manganese and chromium gives a martensitic carbide structure. When their total concentration is equal to the carbon content, the hardest structure is formed.

Most often, chromium is used as the main alloying additive. It gives the alloy high corrosion resistance, which is maintained even in aggressive environments. After normalization, these blanks are resistant to acids at temperatures up to 1000 degrees. Additional alloying with nickel (0.1%), titanium (0.5%) and copper (0.5-2.0%) gives the parts the ability to maintain their geometric shape and original dimensions under prolonged heating conditions.

Products made of high-chromium white cast iron, called sormite, are capable of operating at temperatures of 800-900 degrees. It contains:

The blanks are first fired to relieve internal stress. During the annealing process, the temperature slowly rises to 850 degrees, followed by gradual cooling. The resulting parts are characterized by:

Mechanical properties

Due to the fact that white cast iron contains carbon in the form of carbides, it exhibits good strength characteristics:

Alloys with reduced carbon content are more resistant to intense thermal effects. This feature is used to reduce the number of cracks in the metal. When alloyed with nickel, chromium, and vanadium, a heat-resistant alloy is formed that has high wear resistance.

The compositional features determine the negative properties of white cast iron:

The material is very difficult to weld even when heated - microcracks form at the seam, which increase even more when cooled.

Explanation of markings

The alloy is marked with an alphanumeric code, which indicates its main properties and alloying additives:

Decoding the grade of white cast iron ChN20D2KhSh gives the following information - this is a high-alloy heat-resistant material, along with iron and carbon containing:

Grades X28, X34 are stainless cast iron with high electrical resistivity.

Application area

The high strength characteristics of the alloy and the ability to maintain a given shape under thermal influence have found application:

High hardness combined with brittleness makes machining difficult by cutting or milling methods and requires the use of tools made of special steel grades. Therefore, the use of white cast iron as a structural material is limited. Special conversion types with low silicon content are used for steel smelting and foundry production.

Half-and-half or bleached varieties contain carbon in the form of carbides and in a free state. They have high wear resistance and are used for the manufacture of friction mechanisms subject to dry friction:

Some products require high surface hardness of the cementite structure. It is created artificially by rapidly cooling the workpiece to a depth of 5 mm. This operation is called bleaching. It is required:

Preparation of malleable cast iron

White cast iron serves as a raw material for the production of other types of metallurgical products. It is used for the production of malleable cast iron using heat treatment, during which the process of graphitization and decarbonization of the workpieces occurs. The ratio of the starting elements in the raw materials is:

It contains the structure of ledeburite, which is a mechanical mixture of cementite and perlite. Workpieces on special pallets are slowly passed through chambers with a given temperature regime. The speed of movement is calculated in such a way that the metal is exposed to heat for a certain time.

Annealing takes place in several stages with different temperature conditions:

The annealing time can be reduced by pre-hardening the castings and subsequently using a higher temperature regime. But at the same time, stresses and cracks form in the metal structure. Therefore, the method is used to a limited extent - only for small parts of simple shape.

Source

Structure, properties and application of cast irons

White cast iron

. The structure of white cast irons is presented in the iron-cementite diagram, according to which white cast irons are divided into: hypoeutectic - containing carbon from 2.14 to 4.3%, eutectic - C = 4.3%, hypereutectic - C>4.3% . In white cast iron, all carbon is in the form of cementite, i.e. the degree of graphitization is zero. White cast iron has high hardness, brittleness, and practically cannot be processed with cutting tools, therefore it has limited use (for castings that do not require machining and operate under conditions of abrasive wear during dry friction). It is also used as a starting material for producing malleable cast iron.

Gray cast iron

. In cast iron, all or part of the carbon is present in the form of plate-shaped graphite. The structure of cast iron consists of a metal base and graphite inclusions. Based on the structure of the metal base, gray cast iron is divided into ferritic; structure – ferrite and graphite. Ferrite-pearlite: structure – ferrite + pearlite + graphite. Pearlitic; structure pearlite + graphite.

The mechanical properties of gray cast iron depend both on the properties of the metal base, as well as the number and nature of graphite inclusions. Cast irons are divided into grades depending on the values of mechanical properties. Cast iron is marked with the letters SCH (gray cast iron) and a number indicating the minimum value of tensile strength in MPa·10-1. For example, cast iron grades SCh10 and SCh35 have a tensile strength of at least 100 and 350 MPa, respectively. The use of gray cast iron is determined by its mechanical properties.

Ferritic and ferritic-pearlite cast irons SCh10...SCh15 are used for building columns, foundation slabs, lightly loaded parts of agricultural machinery, etc.

Pearlitic cast irons (SCh18...) are used for the manufacture of castings, for example machine beds, engine blocks, cylinders, compressor and reinforcement castings, for metallurgical equipment, etc. Cast irons of the SCh30, SCh35, SCh40, SCh45 grades are obtained by modification (adding ferrosilicon to liquid cast iron before casting or silicocalcium in an amount of 0.3-0.8%) in order to reduce graphite inclusions, which provide higher strength values.

For parts operating at elevated temperatures, alloyed gray cast irons are used: heat-resistant (additionally contain Cr, Al), heat-resistant (Cr, Ni, Mo). Non-magnetic chromium-nickel cast irons with an austenitic structure are also used.

Gray cast iron castings are subjected to heat treatment. Low annealing (5600C) is used to relieve internal stresses and stabilize dimensions, normalization or hardening with tempering to increase mechanical properties and wear resistance. Gray cast irons have better casting properties compared to other cast irons; castings made from gray cast iron are cheaper than those made from other cast irons and are 1.5 times cheaper than steel castings. Gray cast iron is the most common casting alloy, accounting for 64% of all castings by weight.

Ductile nodular cast iron

. High-strength cast iron is produced by adding magnesium or cerium to liquid cast iron (modification) in an amount of 0.03-0.07%. In terms of the content of other elements, high-strength cast iron does not differ from ordinary gray cast iron. Under the influence of magnesium (or cerium), graphite during crystallization takes not a plate-like, but a spherical shape. Nodular graphite weakens the metal base of cast iron much less than lamellar cast iron, so these cast irons have higher mechanical properties that are not inferior to cast carbon steel, while maintaining good casting properties. High-strength cast irons, like gray ones, are divided according to the structure of the metal base into ferritic, ferrite-pearlitic, and pearlitic.

Cast iron is marked with the letters HF (high-strength cast iron) and a number indicating the minimum tensile strength in MPa·10-1. For example, cast iron grades VCh35 and VCh100 have a tensile strength of 350 and 1000 MPa, respectively. Grades of high-strength cast iron according to GOST 7293-85 from VCh35 to VCh100.

High-strength cast iron castings are used in the automotive and diesel industries (crankshafts, cylinder covers, etc.), in heavy engineering (rolling mill parts, press cross-arms, rolling rolls, etc.), chemical and oil industries (pump housings, etc.). To improve the mechanical properties, high-strength cast irons are subjected to special heat treatment, consisting of heating to 9500C, cooling to 6000C and heating to 7250C with long-term exposure at this temperature, which ensures the production of granular perlite.

Malleable iron

are obtained by prolonged heating at high temperatures (annealing) of white cast iron castings. As a result of annealing, flake-shaped graphite is formed. Compared to lamellar graphite, such graphite reduces the strength and ductility of the metal base of the cast iron structure less. In addition, cast iron has a low carbon and silicon content. The metal base of malleable cast iron: ferrite (ferritic malleable cast iron), ferrite and perlite (ferrite-pearlite malleable cast iron), perlite (pearlitic malleable cast iron). Ferritic malleable cast iron, which is most widely used in mechanical engineering, has the greatest ductility.

The cross-sectional thickness of a ductile iron casting should not exceed 40 mm. With a larger casting size, lamellar graphite is formed in the core and the cast iron becomes unsuitable for annealing. The duration of annealing is 70-80 hours. To speed up annealing, various measures are used: cast iron is modified with aluminum, the heating temperature of cast iron is increased before casting, and the graphitization temperature is increased.

Malleable cast iron is marked with the letters KCH (malleable cast iron) and numbers. The first two digits indicate the minimum value of tensile strength in MPa·10-1, the second - the relative elongation in percent. For example, cast iron grade KCh30-6 has a tensile strength of 300 MPa (no less) and a relative elongation of 6% (no less). Some grades of malleable cast iron according to GOST 1215-79 - KCh 30-6; KCh35-10, KCh60-3, KCh80-1.5, etc.

Ductile iron castings are used to make parts that operate under shock and vibration loads (gearbox housings, hubs, hooks, cardan shaft forks, couplings, brake pads, etc.). To increase the hardness, wear resistance and strength of malleable cast iron, normalization is used at 800-8500C or hardening at 850-9000C and tempering at 450-7000C.

White cast iron

Gray cast iron

Ferritic and ferritic-pearlite cast irons SCh10...SCh15 are used for building columns, foundation slabs, lightly loaded parts of agricultural machinery, etc.

Ductile nodular cast iron

Malleable iron

White cast iron

White cast iron is a type of cast iron that contains carbon compounds. In this alloy they are called cementites. This metal got its name due to its characteristic white color and shine, which is clearly visible at the fracture. This shine is manifested due to the fact that such cast iron does not contain large inclusions of graphite. In percentage terms, it is no more than 0.3%. Therefore, it can only be detected by spectral or chemical analysis.

Types of annealing

To form white cast iron, industry uses rapid cooling of the alloy. Today, the following main types of carbon alloy annealing are actively used:

softening annealing is used primarily to increase the ferrite content of cast iron;
annealing to relieve internal stresses and minimize phase transformations;
graphitizing annealing, as a result of which it is possible to obtain malleable cast iron;
normalization at a temperature of 850-960 degrees, resulting in graphite and perlite, and also increases wear resistance and strength.

Composition and types of white cast iron

White cast iron consists of the so-called cementite eutectic. In this regard, it is divided into three categories:

In addition to the above classification, it is divided into ordinary, bleached and alloyed.

The internal structure of white cast iron is an alloy of two elements: iron and carbon. Despite high-temperature production, it retains a fine-grained structure. Therefore, if you break a part made of such metal, a characteristic white color will be observed. In addition, in the structure of a hypoeutectic alloy, for example, hard grades, in addition to pearlite and secondary cementite, cementite is always present. Its percentage can approach 100%. This is typical for a eutectic metal. For the third type, the structure is a composition of eutectic (Ep) and primary cementite.

One of the varieties of such alloys is the so-called bleached cast iron. Its basis, that is, the core, is gray or high-strength cast iron. The surface layer contains a high percentage of elements such as ledeburite and perlite. The whitening effect up to 30 mm deep is achieved using the rapid cooling method. As a result, the surface layer is white, and then the casting consists of an ordinary gray alloy.

White cast iron structure

Depending on the percentage of alloyed additives, the following types of metal are distinguished:

Quite common elements are used as alloying additives. The alloyed white cast iron obtained in this way acquires new, predetermined properties.

White cast iron (page 1 of 2)

In white (marginal) cast iron, almost all carbon is contained in the form of cementite. White cast iron has the following properties:

— light gray, almost white color;

- cannot be machined.

White cast iron is used only for converting parts into steel, but not for their manufacture.

Marginal open hearth cast irons contain (%): carbon 3.5-4; silicon 0.3-1.5, manganese 1.5-3.5; phosphorus 0.15-0.3, sulfur 0.03-0.07; the rest is iron

Cast irons are iron-carbon alloys with a carbon content of more than 2%. There are white (marginal), gray (foundry) and malleable cast irons.

White cast irons are iron-carbon alloys in which all the carbon is chemically bonded to iron. As a result, they have increased hardness and fragility and are therefore rarely used in technology. They are used primarily as an intermediate product for processing into steel and for producing malleable cast iron.

White cast irons are rarely used in the national economy as structural materials, since due to the high cementite content they are very brittle and hard and difficult to cast and process with tools. They are used to make parts of hydraulic machines, sand blowers and other structures that operate under conditions of increased abrasive wear. To increase wear resistance, white cast iron is alloyed with chromium, vanadium, molybdenum and other carbide-forming elements. The marking of white cast iron is not established.

A variety of white cast iron is bleached cast iron. The surface layers of products made from such cast iron have the structure of white (or half-cast) cast iron, and the core is of gray cast iron. Chilling to a certain depth (12.30 mm) is obtained by rapidly cooling the surface (for example, casting cast iron into metal or sand molds). To relieve structural stresses that can lead to the formation of cracks, castings are heated at 500–550 °C. The high oxidation resistance of bleached cast iron is due to the surface hardness reaching 400-500 HV. Rolls of sheet mills, wheels, balls for mills, etc. are made from bleached cast iron.

Limit plastic and elastic deformations of white cast iron

11/14/2009 | Author: admin

The limiting plastic and elastic deformations of white cast iron in the temperature range 700–1150° C were determined by tensile testing of three samples at each temperature point at 50° C on a high-temperature microscope NM-4 “Union”. The formation of a hot crack in the casting is indicated by obtaining a negative ek value. The more positive the criterion is, the less prone the metal is to cracking.

Based on experimental data obtained when testing samples (2.80% C; 1.20% Si; 0.14% S; 0.40%q Mn; 0.12% P) for tension in the temperature range 700-1150° C, and the results of processing oscillograms of changes in free and hindered shrinkage with decreasing temperature of samples cooling after pouring, graphs were constructed. A noticeable increase in the ultimate elastic deformation in white cast iron is observed at 900° C.

Cold cracks in white iron castings

Cold cracks occur at temperatures below 600-650° C, when elastic deformations predominate in the casting material. The mechanism of their formation is almost no different from the mechanism of formation of hot cracks. In appearance, cold cracks have a small width and regular outlines, since when they form, destruction occurs along the grain, with a light or tarnished granular fracture surface. The main reasons for the formation of cold cracks are residual stresses that arise in the casting during its cooling, as well as temporary stresses.

Properties of white cast iron

Any cast iron alloy, on the one hand, is very strong, but at the same time it is quite brittle. Therefore, the main positive properties of white cast iron include:

White cast irons, with a reduced percentage of carbon, are more resistant to high temperatures. This property is used to reduce the number of cracks in castings.

Appearance of white cast iron

The disadvantages include:

Another disadvantage is poor weldability. Problems in welding parts made of this material are caused by the fact that during welding, cracks form, both during heating and cooling.

Properties

In comparison with other metals, iron-carbon alloy has the following characteristics and properties:

high fragility;
increased hardness;
high resistivity;
low casting properties;
low machinability;
good heat resistance;
high shrinkage (up to 2%) and poor filling of molds;
low impact resistance;
high wear resistance.

The metal mass has great corrosion resistance in hydrochloric or nitric acid. If there are free carbides in the structure, then corrosion will occur when cast iron is placed in sulfuric acid.

White cast irons, which contain a lower percentage of carbon, are considered more resistant to high temperatures. Due to the increased mechanical strength and toughness that appear when exposed to high temperatures, the formation of cracks in castings is minimized.

Marking of white cast iron

To mark white cast iron, letters of the Russian alphabet and numbers are used. If it contains impurities, then the marking begins with the letter “H”. The composition of the available alloying additives can be determined by the following letters P, PL, PF, PVK. They indicate the presence of silicon. If the resulting metal has increased wear resistance, then its marking will begin with the letter “I”, for example ICHH, ICH. For example, the presence of the designation “Ш” in the marking means that the alloy structure contains spherical graphite.

The numbers indicate the amount of additional substances present in white cast iron.

Brand CHN20D2ХШ is deciphered as follows. This is a heat-resistant high-alloy metal. It contains the following elements: nickel - 20%, copper - 2%, chromium - 1%. The remaining elements are iron, carbon, and spherical graphite.

Application area

This alloy is used in the following industries: mechanical engineering, machine tool building, shipbuilding. Some elements of household products are produced from it. In mechanical engineering, it is used to make parts for trucks and cars, tractors, combines and other agricultural machinery. The use of alloying additives makes it possible to obtain specially specified properties. For example, they are used in the manufacture of slabs with different surface shapes.

White iron casting

Bleached cast iron has a fairly limited scope. It is used to produce parts of simple configurations. For example: balls for mills, wheels for various purposes, parts for rolling mills.

It is widely used in the production of parts for such large units as hydraulic and molding machines, and other industrial mechanisms in this area. A specific feature of their work is that they are constantly exposed to abrasive material.

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Application

Based on the above properties, we can conclude that it makes no sense to practice thermal and mechanical treatment of white cast iron. The alloy found its main application only in the form of casting. Consequently, white cast iron obtains its best properties only if all casting conditions are met. This processing method is actively used if it is necessary to produce massive products that must have high surface hardness.

In addition, white cast iron is annealed, resulting in malleable cast iron, which is used for the production of thin-walled castings, for example:

automobile parts;
products for agriculture;
parts for tractors, combines, etc.

The alloy is also used for the manufacture of plates with a ribbed or smooth surface, and is also actively used for the production of steel and gray cast iron.

The use of white cast iron in agriculture as a structural metal is quite limited. Most often, iron-carbon alloy is used for the manufacture of parts for hydraulic machines, sand throwers and other mechanisms that can operate under conditions of increased abrasive wear.

White cast iron

Useful

See what “White cast iron” is in other dictionaries:

White cast iron - Phases of iron-carbon alloys Ferrite (solid solution of interstitial C in α iron with a body-centered cubic lattice) Austenite (solid solution of interstitial C in γ iron with a face-centered cubic lattice) Cementite (iron carbide; Fe3C ... Wikipedia

White cast iron is cast iron in which all the carbon is in the form of iron carbide or cementite. See also: Metallurgy Financial Dictionary Finam ... Financial Dictionary

white cast iron - cast iron in which all the carbon is chemically bound in cementite; Received its name from its matte white fracture. White cast iron has high hardness and brittleness, and is practically impossible to machine with cutting tools. White cast iron is widely... ... Encyclopedic Dictionary of Metallurgy

White cast iron - (named for the type of fracture that has a dull white color) cast iron in which all the carbon is in the form of cementite. The structure of white cast iron at normal temperatures consists of cementite and perlite (Fig. B 7). White cast iron has high hardness and ... Encyclopedic Dictionary of Metallurgy

WHITE CAST IRON - (named for the type of fracture that has a dull white color) cast iron in which all the carbon is in the form of cementite. The structure of white cast iron at normal temperatures consists of cementite and perlite (Fig. B 7). White cast iron has high hardness and ... Metallurgical Dictionary

White cast iron - White iron White cast iron. Cast iron, which is essentially free of graphite, and carbon is present in the form of individual grains of solid Fe3C. White cast iron has a white, crystalline fracture surface running along the edges of iron carbide.... ... Dictionary of metallurgical terms

white cast iron - baltasis ketus statusas T sritis chemija apibrėžtis Ketus, turintis 3.5–3.6% C, 0.6–1% Si, 0.5–0.6% Mn, 0.5–1.75% Ni , white cast iron0.2% V, white cast iron0.5% P. atitikmenys: angl. cast iron; white iron rus. white cast iron … Chemijos terminų aiškinamasis žodynas

White cast iron - see Art. Cast iron ... Great Soviet Encyclopedia

WHITE CAST IRON - see art. Cast iron ... Big Encyclopedic Polytechnic Dictionary

WHITE - WHITE, about color, suit, paint: colorless, opposite to black. | In a comparative sense, light, pale. White wine, white beer, honey, plums; white face, white bread, so called to distinguish it from red (wine, honey), black (beer, plums, bread) ... Dahl's Explanatory Dictionary

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Cast iron contains carbon

“Cast iron” is an alloy of iron with carbon (and other elements). content in cast iron is at least 2.14% (the point of limiting solubility of carbon in austenite on the state diagram): less - steel. Carbon gives iron alloys strength and hardness, reducing ductility and toughness. Carbon in cast iron can be contained in the form of cementite and graphite. Depending on the form of graphite and the amount of cementite, there are: pale, colorless, malleable and high-strength cast irons. Cast irons contain permanent impurities (Si, Mn, S, P), and in some cases also alloying elements (Cr, Ni, V, Al, etc.). Typically, cast iron is brittle.

Cast iron is an alloy of iron with carbon (and other elements). content in cast iron is at least 2.14% (the point of limiting solubility of carbon in austenite on the state diagram): less - steel. Carbon gives iron alloys strength and hardness, reducing ductility and toughness. Carbon in cast iron can be contained in the form of cementite and graphite. Depending on the form of graphite and the amount of cementite, there are: pale, colorless, malleable and high-strength cast irons. Cast irons contain permanent impurities (Si, Mn, S, P), and in some cases also alloying elements (Cr, Ni, V, Al, etc.). Typically, cast iron is brittle.

Malleable cast iron is produced by long-term annealing of white cast iron, which results in the formation of flake-shaped graphite. The metal base of such cast iron is ferrite and, less commonly, pearlite. Malleable cast iron got its name due to its increased ductility and viscosity (despite the fact that it is not subjected to pressure treatment). Malleable cast iron has increased tensile strength and increased impact resistance. Parts of complex shapes are made from malleable cast iron: car rear axle housings, brake pads, tees, angles, etc.

At normal temperatures, the structure of cast iron consists of perlite, secondary cementite and ledeburite. Ledeburite after eutectoid transformation is a mechanical mixture of pearlite and cementite. Cast irons with a carbon content of up to 4.3% are called hypoeutectic cast irons. such cast irons are called white cast irons if the carbon is in the cast irons in a chemically bonded state with iron, i.e. in cementite. The microstructure of alloy IV, which is hypoeutectic white cast iron, is shown in Fig.

carbon content, are divided into hypoeutectic - 2.14 ... 4.3% C, eutectic - 4.3% C and hypereutectic - 4.3 ... 6.67% C carbon. Hypoeutectic cast irons containing 2.14 ... 4.3% C, after final cooling, have the structure of pearlite, ledeburite (perlite + cementite) and secondary cementite. Eutectic cast iron (4.3% C) at temperatures below + 727 ° C consists only of ledeburite (perlite + cementite). Hypereutectic, which cannot be canceled 4.3 ... 6.67% C, at temperatures below + 727 ° C consist of primary cementite and ledeburite (perlite + cementite). In practice, the most widely used are hypoeutectic cast irons containing 2.4 ... 3.8% C carbon. The actual value of the carbon content in cast iron is determined by its technological characteristics during casting - ensuring good fluidity. Fluidity is the ability of metals and alloys in a molten state to fill a mold cavity and accurately reproduce the shape and size of a casting. carbon content above 3.8% C leads to a sharp increase in hardness and brittleness. Fluidity is determined by a spiral test, and its value is determined by the length of filling part of the spiral. Shrinkage is a decrease in the linear and volumetric dimensions of metal, submerged into a figure during its crystallization and cooling.

In the German standard DIN 1693 - 506 - 50 in the name of the brand, the letters indicate: G - “gegosen” (cast), G - “gubeisen” (cast iron), G - “globular” (ball), 50 - the lowest value of the strength limit in MPa 10 - 1 (for example, GGG - 50). In most national standards for high-strength unalloyed cast iron, which regulate mechanical properties, the chemical composition of cast iron is not specified. Inevitable for control are tensile strength, yield strength, and elongation. The samples of all parties, with the exception of German and US standards, provide controlled limits for hardness values.

Including the slight resistance of gray iron castings to tensile and impact loads, this material should be used for parts that are subject to compressive or bending loads. In the machine tool industry these are basic, body parts, brackets, gears, drives; in the automotive industry - cylinder blocks, piston rings, camshafts, clutch discs. Gray cast iron castings are also used in electrical engineering and for the manufacture of consumer goods.

High-strength cast irons (GOST 7293) can have a ferritic (VCh 35), ferritic-pearlite (VCh45) and pearlitic (VCh 80) metal base. These cast irons are made from colorless ones, as a result of modification with magnesium or cerium (0.03...0.07% of the casting weight is added). Compared to gray cast iron, mechanical properties are increased, this is generated by the lack of unevenness in the distribution of forces due to the spherical shape of graphite.

White cast iron

Ferrite (interstitial solid solution of C in α-iron with a body-centered cubic lattice) Austenite (solid solution of interstitial C in γ-iron with a face-centered cubic lattice) Cementite (iron carbide; Fe3C metastable high-carbon phase) Graphite stable high-carbon phase

Ledeburite (eutectic mixture of cementite and austenite crystals, which turns into pearlite upon cooling) Martensite (highly supersaturated solid solution of carbon in α-iron with a body-centered teragonal lattice) Pearlite (eutectoid mixture consisting of thin alternating plates of ferrite and cementite) Sorbite (dispersed perlite) Troostite (highly dispersed pearlite) Bainite (ustar: acicular troostite) - an ultradispersed mixture of crystals of low-carbon martensite and iron carbides

White cast iron (brittle, contains ledeburite and no graphite) Gray cast iron (graphite in plate form) Ductile cast iron (graphite in flakes) Ductile iron (graphite in the form of spheroids) Half cast iron (contains both graphite and ledeburite)

White cast iron

- a type of cast iron in which carbon in a bound state in the form of cementite, when fractured, has a white color and a metallic sheen. In the structure of such cast iron there are no visible inclusions of graphite and only a small part of it (0.03-0.30%) is detected by subtle methods of chemical analysis or visually at high magnifications. The main metallic mass of white cast iron consists of cementite eutectic, secondary and eutectoid cementite, and alloyed white cast iron consists of complex carbides and alloyed ferrite.

MICROSTRUCTURE OF CAST IRONS

GOAL OF THE WORK

To investigate metallographically the microstructure of white and graphitized cast irons. Study the markings and practical applications of graphitized cast irons.

DEVICES AND MATERIALS

Metallographic microscopes. Collection of microsections of cast iron.

BASIC POINTS

Cast irons include alloys of iron and carbon containing more than 2.14% C (Fig. 1). Cast irons with a carbon content of up to 4 – 4.5% are found in practical use. With a larger amount of carbon, the mechanical properties deteriorate significantly.

Industrial cast irons are not double alloys, but contain in addition to Fe and C,

the same impurities as carbon steels Mn, Si, S, P, etc. However, in cast iron there are more of these impurities and their influence is different than in steels.
If all the carbon present in cast iron is in a chemically bound state, in the form of iron carbide (F3C -
cementite), then such cast iron is called white. Cast irons in which all or most of the carbon is in a free state in the form of graphite inclusions of one form or another are called graphitized.

White cast iron

Microscopic analysis of white cast iron is carried out using the Fe – Fe3C phase diagram (Fig. l). Due to the presence of a large amount of cementite, white cast iron has high hardness (HB = 4500 - 5500 MPa), is brittle and practically cannot be machined by cutting. Therefore, white cast iron has limited use as a structural material.

A common structural component of white cast iron is ledeburite. Ledeburite

is a mixture of austenite and cementite formed by a eutectic reaction when a liquid of composition point C (4.3% carbon) is supercooled below a temperature of 1147 °C.

eutectic (ledeburite)

Cast iron containing 4.3% C (point C) is called white eutectic cast iron. To the left of point C are hypoeutectic, and to the right are hypereutectic white cast irons. In hypoeutectic white cast irons, austenite crystallizes from the liquid phase, then the eutectic, ledeburite.

When cast iron is cooled in the temperature range from 1147 °C to 727 °C, austenite is depleted in carbon, its composition changes along the ES line and secondary cementite is released. With slight hypothermia below 727 ° C

authenite of point S composition decomposes into pearlite (F + C) by eutectoid reaction

Rice. 1. Structural diagram of the state of the iron-cementite system (in a simplified form)

Secondary cementite, released along the boundaries of austenite grains, merges with ledeburite cementite. Under a microscope, it is difficult to distinguish inclusions of secondary cementite.

Thus, at room temperature, hypoeutectic white cast iron contains three structural components - pearlite, ledeburite and secondary cementite (Fig. 2).

Eutectic white cast iron at room temperature consists of one structural component - ledeburite. The latter, in turn, consists of pearlite and cementite and is called converted ledeburite.

In hypereutectic white cast irons, primary cementite crystallizes from the liquid in the form of flat needles, then ledeburite is formed.

Rice. 2. Microstructure of white cast iron (schematic representation on the left): a) hypoeutectic; b) eutectic; c) hypereutectic

At room temperature, eutectic white cast iron contains two structural components: primary cementite and ledeburite. The phase composition of white cast irons at room temperature is the same as in carbon steels in equilibrium; they all consist of ferrite and cementite.

Graphitized cast irons.

Depending on the shape of the graphite inclusions, gray, high-strength, malleable cast iron and cast iron with vermicular graphite are distinguished.

Gray cast iron is produced at a lower casting cooling rate than white cast iron. They contain 1 – 3% Si

– having a strong graphitizing effect.

Gray cast iron is widely used in mechanical engineering. It can be processed well with cutting tools. It is used to produce machine beds, cylinder blocks, foundation frames, cylinder liners, pistons, etc.

Gray cast irons are marked with the letters SCh and followed by the value of tensile strength (in kgf/mm2),

for example SCh 15, SCh 20, SCh 35 (GOST 1412-85).

Graphite in gray cast iron is observed in the form of dark inclusions on a light background of an unetched section. The shape and dispersion of graphite, on which the mechanical properties of gray cast iron largely depend, are assessed from an unetched section.

Gray cast irons are divided according to the microstructure of the metal base, depending on the completeness of graphitization. The degree or completeness of graphitization is assessed by the amount of freely released (unbound) carbon.

The completeness of graphitization depends on many factors, the main ones being the cooling rate and the composition of the alloy. With rapid cooling, the formation of cementite rather than graphite is kinetically more favorable. The slower the cooling, the greater the degree of graphitization. Silicon acts in the same direction as slowing down cooling, i.e. promotes graphitization, and manganese, a carbide-forming element, hinders graphitization.

If graphitization in the solid state is complete, then cast iron contains two structural components - graphite and ferrite. This alloy is called gray cast iron with a ferritic base (Fig. 3a). If the eutectoid decomposition of austenite took place in accordance with the metastable system

eutectoid (pearlite)

then the structure of cast iron consists of graphite and perlite. This alloy is called pearlite-based gray cast iron. Finally, an intermediate option is possible when austenite partially decomposes by eutectoid reaction into ferrite and graphite, and partially with the formation of pearlite. In this case, cast iron contains three structural elements - graphite, ferrite and pearlite. This alloy is called gray cast iron on a ferrite-pearlite base.

Ferrite and pearlite in the metal base of cast iron have the same microstructural characteristics as in steels. Gray cast irons contain an increased amount of phosphorus, which increases fluidity and produces a triple eutectic.

In the metal base of gray cast iron, phosphide eutectic is found in the form of light, well-defined areas.

Ductile iron with nodular graphite is produced by modifying gray cast iron with alkaline earth elements. More often, magnesium is used for this, introducing it into the liquid melt in an amount of 0.02 - 0.03%. Under the influence of magnesium, graphite crystallizes in a spherical shape (Fig. 3b). Spherical graphite inclusions in a metal matrix are not as strong stress raisers as graphite plates in gray cast iron. Nodular cast irons have higher mechanical properties that are not inferior to cast carbon steel.

High-strength cast iron is marked with the letters HF, followed by the tensile strength values (in kgf/mm2)

HF 40, HF 45, HF 80 (GOST 7293-85). Just like gray cast irons, they are subdivided according to the microstructure of the metal base depending on the completeness of graphitization and are available on ferritic, ferrite-pearlite, and pearlite bases. High-strength cast iron is used in many areas of technology to replace cast and forged steel, gray and ductile cast iron. High mechanical properties make it possible to widely use it for the production of critical-use castings, including in marine engineering: cylinder heads, turbochargers, pressure pipes, crankshafts and camshafts, etc.

Malleable iron is produced by annealing white iron castings. The production of malleable cast iron is based on the fact that instead of the unstable cementite of white cast iron, annealing graphite of white cast iron is formed at elevated temperatures. Small products of complex configuration, cast from white cast iron, are annealed (malleable cast iron is obtained) to impart sufficient ductility necessary for their use in work. Malleable cast iron is marked with the letters KCH and is followed by the tensile strength values (in kgf/mm2)

and relative elongation (in%), for example, KCh 35-10, KCh 60-3 (GOST 1215-79).

Graphitization occurs through the dissolution of metastable cementite in austenite and the simultaneous separation of more stable graphite from the austenite. The longer the holding time during annealing and the lower the cooling rate, the more complete the graphitization occurs. Depending on the graphitization, the same three main types of structures are found as in gray cast iron with flake graphite: malleable cast irons on ferritic, ferrite-pearlitic and pearlitic bases (Fig. 3v). Ductile cast irons differ from gray (foundry) cast irons in microstructure only in the form of graphite.

If in thin sections (Fig. 3) of gray cast iron graphite has the form of tortuous veins, then in malleable cast iron graphite, called annealing carbon, is in the form of more compact flake-like inclusions with torn edges. The more compact form of graphite provides increased mechanical properties of ductile iron compared to gray cast iron with flake graphite. Having mechanical properties similar to cast steel and high-strength cast iron, high resistance to impact loads, wear resistance, and machinability, ductile cast iron finds its application in many industries. Pistons, gears, connecting rods, brackets, porthole rings, etc. are made from it.

Cast irons with vermicular graphite are produced like high-strength cast irons by modification, only a smaller amount of spheroidizing metals is introduced into the melt. Cast iron with vermicular graphite is marked with the letters CHVG and then followed by a number indicating the value of tensile strength ( kgf/mm2),

for example, ChVG Z0, ChVG 45 (GOST 28394-89). Vermicular graphite, like lamellar graphite, is visible on a metallographic section in the form of veins, but they are smaller, thicker, with rounded edges (Fig. 3g). The microstructure of the metal base of ChVG, like that of other graphitized cast irons, can be ferritic, pearlitic and ferrite-pearlite.

In terms of mechanical properties, cast irons with vermicular graphite are superior to gray cast irons and are close to high-strength cast irons, and the damping capacity and thermophysical properties of CCG are higher than those of high-strength cast irons. Cast irons with vermicular graphite are more technologically advanced than high-strength cast irons and compete with gray cast irons. They are characterized by high fluidity, machinability, and low shrinkage. Cast irons with vermicular graphite are widely used in the global and domestic automotive industry, tractor manufacturing, shipbuilding, diesel engineering, power and metallurgical engineering for parts operating under significant mechanical loads under conditions of wear, hydrocavitation, and variable temperature increases. For example, ChVG is used instead of midrange for the production of cylinder heads for large marine diesel internal combustion engines.

CONTENTS OF THE REPORT

1. Title of the work. 2. Purpose of the work. 3. Fragment of the diagram Fe – Fe3C (2.14 – 6.67% C ).

4. Diagrams of the microstructures of the studied samples, indicating their brands. 5. Conclusions.

CONTROL QUESTIONS

1. What alloys are classified as cast iron? 2. What groups are cast irons divided into? 3. Which phase diagram is used when analyzing the microstructure of white cast iron? 4. Why is white cast iron of limited use? 5. What is ledeburite called? 6. What process occurs in white cast iron when the melt is supercooled below 1147 °C? 7. How much carbon is contained in eutectic white cast iron? 8. How many structural components can be seen at room temperature in white hypoeutectic cast iron? 9. How many structural components can be seen at room temperature in white eutectic cast iron? 10. How many structural components can be seen at room temperature in white hypereutectic cast iron? 11. By what method are gray cast irons produced? 12. By what method is malleable cast iron produced? 13. What method is used to produce high-strength cast iron? 14. What method is used to produce cast iron with vermicular graphite? 15. How are cast irons marked? 16. On what factors does the degree of graphitization depend? 17. How many structural components does cast iron contain if graphitization in the solid state is complete? 18. How do the microstructures of graphitized cast irons on the same base differ? 19. What is the essence of the eutectic transformation in cast iron?

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Physical and mechanical properties

White iron castings are wear-resistant, relatively heat-resistant and corrosion-resistant. The presence in part of their cross-section of a structure different from the structure of white cast iron reduces these properties. The strength of white cast iron decreases with increasing carbon content in it, and therefore carbides. The hardness of white cast iron increases with increasing proportion of carbides in its structure, and consequently with increasing carbon content.

White cast iron with a martensitic structure of the main metal mass has the highest hardness. Coagulation of carbides sharply reduces the hardness of cast iron.

When impurities dissolve in iron carbide and form complex carbides, the hardness of them and white cast iron increases. According to the intensity of their influence on the hardness of white cast iron, the main and alloying elements are arranged in the following sequence, starting with carbon, which determines the amount of carbides and increases the hardness of cast iron more intensely than other elements.

The effect of nickel and manganese, and partly chromium and molybdenum, is determined by their influence on the formation of the martensitic-carbide structure and their content in quantities corresponding to the carbon content in cast iron ensures maximum hardness of white cast iron.

Cast iron containing 0.7-1.8% boron has a particularly high hardness HB 800-850. White cast iron is a very valuable material for parts operating under wear conditions at very high specific pressures and mainly without lubrication.

There is no direct relationship between wear resistance and hardness; Hardness does not determine wear resistance, but must be taken into account in conjunction with the structure of cast iron. The best wear resistance has white cast iron with a thin structure of the main metal mass, in which carbides, phosphides, etc. are located in the form of separate small and evenly distributed inclusions or in the form of a fine mesh.

The structure of the main metal mass also determines the special properties of alloy cast iron - its corrosion resistance, heat resistance, and electrical resistance.

Depending on the composition and concentration of alloying elements, the main metallic mass of alloyed white cast iron can be carbide-austenitic, carbide-pearlitic and, in addition, contain alloyed ferrite.

The main alloying element in this case is chromium, which binds carbon into chromium carbides and complex chromium and iron carbides.

Solid solutions of these carbides have a high electrode potential, close to the potential of the second structural component of the main metal mass of cast iron - chromium ferrite, and the resulting protective oxide films determine the increased corrosion resistance of high-chromium white cast iron.

In the presence of chromium as an additional component, the temperature resistance of carbides increases significantly due to a significant slowdown in diffusion processes during complex alloying.

These characteristic features of alloyed white cast iron have determined its areas of use, depending on the structure, as stainless steel, magnetic cast iron and high electrical resistivity cast iron.

Source

Stainless alloys

To increase the resistance of white cast iron to corrosion, large amounts of chromium are added to it. This leads to the formation of an oxide film on the surface and further cessation of oxygen access. In addition, high-chromium white cast iron becomes resistant to alkaline solutions, sulfuric and nitric acid.

Additionally, the process of alloying with chromium prevents the possibility of coagulation of carbides when the alloy is heated strongly. This allows you to obtain high-quality welded joints of parts made of white cast iron. If nickel and molybdenum are added along with chromium during the alloying process, the resulting stainless alloy can be compared in strength to the best heat-resistant steels, which are much more expensive.

Chromium-containing white cast iron is used in cases of severe operating conditions, the presence of alkalis and oxidizing agents, and the need for high electrical resistance.

White cast iron - structure, composition, properties, marking

Metal alloys of iron and carbon, where the content of the second element exceeds 2.14%, are called cast iron. White cast iron includes alloys in which carbon is presented in the form of iron carbide Fe3C (cementite). It is because of the light color on the fracture that they are called white.

The manufacturing conditions for white grade castings are given in GOST 1215-79 and GOST 26358-84. They indicate technical requirements, procedures for acceptance, testing, transportation and storage of cast iron alloys. Marked with the letters BC.

Types of produced white cast iron

Depending on the crystalline structure, as well as the presence and ratio of the constituent elements, white cast iron is divided into:

A separate type is cast iron alloys with high electrical resistivity.

The internal structure of ordinary white cast iron contains carbon in the form of cementite grains. The amount of carbon affects the melting point and, depending on this, cast iron is divided into:

The effect of cast iron bleaching is achieved by rapidly cooling the casting, which as a result is heterogeneous in composition. The top layer, up to 30 mm thick, becomes white, and the rest of the core is ordinary gray cast iron.

Bleached cast irons

This alloy is considered a type of white cast iron. It is possible to achieve a chill of 12-30 mm by rapidly cooling the surface of the iron-carbon alloy. Material structure: the surface part is made of white, gray cast iron in the core. Wheels, balls for mills, and rolling rolls are made from this material, which are mounted in machines for processing sheet metal.

Features of producing white cast iron

In the process of producing white cast iron of a given structure, it is necessary to suppress the graphitization process during the entire time of crystallization of the liquid mass. In this case, both the competent selection of source materials and adherence to the technology for cooling cast iron in the mold are important.

When castings are made from unalloyed cast iron in wet sand molds, there is a need to maintain the proportion of carbon and silicon: C (Si + lg R) Advantages and Disadvantages

Like all cast iron alloys, white ones are distinguished by great strength combined with brittleness under strong mechanical shocks. Among the main positive qualities of white cast iron are:

An important quality of white cast iron is considered to be very good resistance to high temperatures, which is used to reduce the number of cracks in initial castings.

The main disadvantages include such qualities as:

Formation of defects during welding due to the rapid burnout of carbon and the formation of pores.

GRAY CAST IRON

In the alloy of such cast iron, carbon - all or partly - is in the form of lamellar graphite. Following the example of light cast iron, gray got its name because of the color of the fracture. Gray cast iron also contains silicon and sometimes magnesium. The amount of carbon in gray cast iron is from 2.9 to 3.7%.

Gray cast iron, like white cast iron, is brittle, but at the same time it has high casting properties, fluidity and low shrinkage. The bases of machine tools, cylinders of various mechanisms, and pistons are made from gray cast iron.

Cast iron (white, gray, high-strength, malleable). Preparation, structure, labeling, scope

White cast irons: composition, properties, scope.

Carbon is in the form of cementite Fe3C. The break will be white if broken. In the structure of hypoeutectic cast iron HB 550, along with pearlite and secondary cementite, there is brittle eutectic (ledeburite), the amount of which reaches 100% in eutectic cast iron. The structure of hypereutectic cast iron consists of eutectic (Ep) and primary cementite, which is released during crystallization from the liquid in the form of large plates. High hardness, difficult to cut. Ch. property: high wear resistance. Cast iron is brittle. Rarely used in mechanical engineering. It is used in the manufacture of millstones in mills, rolling rolls in rolling machines, and fences are made from this cast iron. If the casting is small (up to 10 kg), then white cast iron is formed during rapid cooling.

Preparation: Three types of white cast iron are smelted in blast furnaces: foundry coke iron, pigment coke iron and ferroalloys.

The structure does not affect the ductility; it remains extremely low. But it does affect hardness. Mechanical strength is mainly determined by the number, shape and size of graphite inclusions. Small, swirl-shaped graphite flakes reduce strength less. This form is achieved through modification. Aluminum, silicocalcium, and ferrosilicon are used as modifiers.

Gray cast iron is widely used in mechanical engineering, as it is easy to process and has good properties.

Depending on the strength, gray cast iron is divided into 10

brands (GOST 1412).

Gray cast irons, with low tensile strength, have fairly high compressive strength.

Gray cast irons contain carbon - 3,2…3,5 %

;
silicon – 1.9…2.5%
;
manganese – 0.5…0.8%
;
phosphorus – 0.1…0.3%
; sulfur –

They are designated by the index SCh (gray cast iron) and a number that shows the value of the tensile strength multiplied by 10 -1 SCh 15.

Preparation: Graphite is formed in gray cast iron as a result of the decomposition of brittle cementite. This process is called graphitization. The decomposition of cementite is caused artificially by introducing silicon or special heat treatment of white cast iron.

Application area

Ordinary white cast iron is used very limitedly, since it is poorly applicable to mechanical and heat treatment. For the production of products it is often used in the form of unprocessed or partially processed castings.

The alloy is most widely used in the manufacture of large parts of simple configuration. These are housings and parts of machine tools and rolling mills, balls for mills, drive and support wheels. In addition, white cast iron is used for the manufacture of assembly units that are constantly exposed to abrasive materials.

An important point is the use of ordinary cast iron as a raw material for the manufacture of malleable grades of iron-carbon cast iron and steel alloys.

HIGH STRENGTH CAST IRON

Malleable iron contains carbon in the form of graphite in the form of isolated flakes. Thanks to this, the material has greater ductility and toughness than other types of cast iron. Malleable cast iron is made from white cast iron by treating it with prolonged exposure to high temperatures. As a result of such processing, graphitization processes occur in the alloy - cementite disintegrates, forming graphite.

In addition to its high malleability characteristics, from which it gets its name, this type of cast iron is also characterized by increased tensile strength and impact resistance. The malleability of malleable cast iron to machining allows it to be used to make products of complex shapes. Brake pads, angles and other parts for machines and mechanisms are made from it.