Types of cast iron. Great encyclopedia of oil and gas

The most common types of cast iron are gray and white. What does each one represent?

What is gray cast iron?

The corresponding type of cast iron is one of the most common in the field of mechanical engineering. This metal is characterized by the presence of plate-shaped graphite in the thin section. Its content in gray cast iron may vary. The larger it is, the darker the metal becomes at the fracture, and also the softer the cast iron. Castings from the type of metal in question can be produced in any thickness.

Main features of gray cast iron:

1. minimum relative elongation - as a rule, not exceeding 0.5%;
2. low impact strength;
3. low plasticity.

In gray cast iron there is no large percentage fixed carbon - no more than 0.5%. The remaining part of the carbon is presented in the form of graphite - that is, in a free state. Gray cast iron can be produced on a pearlitic, ferritic, or mixed ferrite-pearlitic basis. The metal in question usually contains a significant percentage of silicon.

Gray cast iron is quite easy to process using cutting tools. This metal is used for casting products that are optimal in terms of compression resistance. For example, various support elements, batteries, water pipes. The use of gray cast iron is also widespread in mechanical engineering - most often in the manufacture of parts that are not characterized by shock loads. For example, housings for machine tools.

What is white cast iron?

This type of cast iron is characterized by the presence of carbon, which is almost completely represented in the metal structure in a bound state. The metal in question is hard and at the same time quite fragile. It is resistant to corrosion, wear and temperature. White cast iron is quite difficult to work with hand tools. When fractured, this metal has a light tint and a radiant structure.

The main area of ​​application of white cast iron is subsequent processing. As a rule, it is converted into steel, in many cases - just in gray cast iron. In industry, its use is not very common due to its fragility and difficulty in processing.

The percentage of silicon in white cast iron is significantly less than in gray cast iron. The metal in question may also have more high concentration manganese and phosphorus (note that their presence is largely predetermined chemical composition ore from which pig iron is smelted). Actually, an increase in the amount of silicon in a metal is accompanied by a decrease in the volume of bound carbon in its structure.

Comparison

The main difference between gray cast iron and white is that the former contains a small percentage of fixed carbon, while the latter, on the contrary, contains mainly fixed carbon. This feature predetermines the difference between the metals under consideration in the aspect:

• hardness;
• colors on the break;
• wear resistance;
• fragility;
• machinability with hand tools;
• scope of application;
• percentage of fixed and free carbon;
• percentage of silicon, manganese, phosphorus.

To more clearly study the difference between gray and white cast iron in these aspects, a small table will help us.

Table

Gray cast iron	White cast iron
Less hard	More solid
Darker at the break	Lighter on the break
Less resistant to wear	More resistant to wear
Less fragile	More fragile
Easy to process with hand tools	Not very easy to work with hand tools
Actively used in various fields industry	Mainly used for the purpose of making steel, gray cast iron
Has a large percentage of free carbon - in the form of graphite	Includes mostly fixed carbon
Characterized by a large percentage of silicon, a smaller percentage of manganese and phosphorus	Characterized by a lower percentage of silicon, a higher percentage of manganese and phosphorus

With a face-centered cubic lattice)
Cementite (iron carbide; Fe 3 C metastable high-carbon phase)
Graphite stable high carbon phase

Structures of iron-carbon alloys Cast iron

White cast iron(brittle, contains ledeburite and does not contain graphite)
Gray cast iron (graphite in plate form)
Malleable cast iron (graphite flakes)
Ductile iron (graphite in the form of spheroids)
Half cast iron (contains both graphite and ledeburite)

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 martensite-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 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 individual 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 alloyed cast iron - its corrosion resistance, heat resistance, and electrical resistance.

Depending on the composition and concentration of alloying elements, the main metal mass of alloyed white cast iron can be carbide-austenitic, carbide-pearlite 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.

Notes

See also

Links

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

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

Based on the color of a fresh fracture, two types of cast iron are distinguished: a) gray and b) white. Both of these types clearly differ from each other both in physical and in chemical properties; namely, gray cast iron (also called soft) is somewhat malleable and has toughness... Encyclopedic Dictionary F.A. Brockhaus and I.A. Ephron

white cast iron- cast iron, in which all 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 widely... ...

White cast iron Encyclopedic Dictionary of Metallurgy

WHITE CAST IRON- (named after the type of fracture, which has a matte white) cast iron, in which all the carbon is in the form of cementite. The structure of white cast iron at normal temperature consists of cementite and perlite (Fig. B 7). White cast iron has high hardness and... Metallurgical Dictionary - 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) ... Dictionary Dahl

White cast irons: composition, properties, scope.

The carbon is in the form of cementite Fe 3 C. The fracture 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.

Gray cast iron.

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 – < 0,12 % .

The structure of the metal base depends on the amount of carbon and silicon. With increasing carbon and silicon content, the degree of graphitization and the tendency to form a ferrite structure of the metal base increase. This leads to softening of cast iron without increasing ductility.

Pearlitic gray cast iron has the best strength properties and wear resistance.

Given the low resistance of gray cast 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, guides; 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.

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.

High-strength nodular cast iron.

High-strength cast irons (GOST 7293) can have a ferritic (VCh 35), ferrite-pearlitic (VCh 45) and pearlitic (VCh 80) metal base.

These cast irons are obtained from gray cast irons as a result of modification with magnesium or cerium (added 0,03…0,07% from the mass of the casting). Compared to gray cast irons, mechanical properties increase, this is caused by the lack of unevenness in stress distribution due to spherical graphite

Cast irons with a pearlitic metal base have high performance strength with a lower ductility value. The ratio of ductility and strength of ferritic cast iron is the opposite.

High-strength cast irons have a high yield strength,

which is higher than the yield strength of steel castings. Also characterized by fairly high impact strength and fatigue strength,

,

with pearlite base.

High-strength cast irons contain: carbon – 3,2…3,8 %, silicon – 1,9…2,6 % , manganese – 0,6…0,8 % , phosphorus – up to 0,12 % , sulfur – up to 0,3 % .

These cast irons have high fluidity, linear shrinkage is about 1%. Casting stresses in castings are slightly higher than for gray cast iron. Due to the high modulus of elasticity, the machinability is quite high. They have satisfactory weldability.

High-strength cast iron is used to make thin-walled castings (piston rings), forging hammers, beds and frames of presses and rolling mills, molds, tool holders, and faceplates.

Crankshaft castings weighing up to 2..3 t, instead of forged steel shafts, they have higher cyclic toughness, are insensitive to external stress concentrators, have better anti-friction properties and are much cheaper.

They are designated by the index HF (high-strength cast iron) and a number that shows the value of the tensile strength multiplied by HF 100.

Preparation: High-strength cast iron (GOST 7293-79) is a type of gray cast iron that is obtained by modifying it with magnesium or cerium. The graphite inclusions in these cast irons are spherical in shape.

Malleable iron

Produced by annealing white hypoeutectic cast iron.

Good properties for castings are ensured if during the process of crystallization and cooling of the castings in the mold the graphitization process does not occur. To prevent graphitization, cast irons must have a reduced carbon and silicon content.

Malleable cast irons contain: carbon – 2,4…3,0 % , silicon – 0,8…1,4 % , manganese – 0,3…1,0 % , phosphorus – up to 0,2 % , sulfur – up to 0,1 % .

The formation of the final structure and properties of castings occurs during the annealing process, the diagram of which is presented in Fig. 11.4. The castings are kept in an oven at a temperature 950…1000С during 15…20 hours. Cementite decomposes: Fe 3 C → Fe y (C) + C .

The structure after exposure consists of austenite and graphite (annealing carbon). With slow cooling in the range 760…720 o C, decomposition of cementite, which is part of pearlite, occurs, and the structure after annealing consists of ferrite and annealing carbon (ferritic malleable cast iron is obtained).

With relatively rapid cooling (mode b, Fig. 11.3), the second stage is completely eliminated, and pearlitic malleable cast iron is obtained.

Structure of cast iron, annealed according to the regime V, consists of pearlite, ferrite and annealed graphite (ferrite-pearlite malleable cast iron is obtained)

According to mechanical and technological properties malleable iron occupies intermediate position between gray cast iron and steel. The disadvantage of malleable cast iron compared to high-strength cast iron is the limited wall thickness for casting and the need for annealing.

Ductile iron castings are used for parts operating under shock and vibration loads. Gearbox housings, hubs, hooks, brackets, clamps, couplings, and flanges are made from ferritic cast iron.

Pearlitic cast iron, characterized by high strength and sufficient ductility, is used to make driveshaft forks, links and rollers of conveyor chains, and brake pads.

They are designated by the index KCh (high-strength cast iron) and two numbers, the first of which shows the value of the tensile strength multiplied by , and the second - the relative elongation - KCh 30 - 6.

Preparation: Malleable cast iron is a type of gray cast iron obtained by long-term (up to 80 hours) keeping white cast iron at high temperature. This heat treatment is called simmering. In this case, cementite disintegrates and the graphite released during its decomposition forms flocculent inclusions. Depending on the temperature and duration of aging, malleable cast irons are produced on ferritic and ferrite-pearlite bases.

Alloys of iron and carbon (> 2.14% C) are called cast iron. The presence of eutectic in the structure of cast iron determines its use exclusively as a casting alloy. Carbon in cast iron can be in the form of cementite or graphite, or both in the form of cementite and graphite. Cementite gives the fracture a specific light shine, so cast iron, in which all the carbon is in the form of cementite, is called white. Graphite imparts fracture to cast iron gray. Depending on the form of graphite and the conditions of its formation, there are the following groups cast iron: gray, high-strength nodular and malleable.

Gray cast iron. Gray cast iron (commercial) is essentially an alloy of Fe - Si - C, containing Mn, P and S as inevitable impurities. In the structure of gray cast iron, most or all of the carbon is in the form of graphite. Feature The structure of gray cast iron, which determines many of its properties, is that graphite has the shape of plates in the field of view of a microsection. Most wide application hypoeutectoid cast irons containing 2.4 - 3.8% C were obtained. The higher the carbon content in cast iron, the more graphite is formed and the lower its mechanical properties. In this regard, the amount of carbon in cast iron usually does not exceed 3.8%. At the same time, to ensure high casting properties (good fluidity), carbon must be at least 2.4%.

Gray cast iron is marked with the letters C - gray and Ch - cast iron (GOST 1412 - 70). The letters are followed by numbers. The first digits indicate average value tensile strength, and the second - the average value of the tensile strength when tested in bending. The flexural strength is used to assess the ductility of cast iron, since the relative elongation of all gray cast irons is practically zero.

White and bleached cast iron. White cast iron, due to the presence of cementite in it, has high hardness, is brittle and practically cannot be machined, therefore it has limited use. Bleached iron castings are those in which the surface layers have the structure of white (or half-cast) cast iron, and the core has the structure of gray cast iron. Between these zones there may be a transition layer. Chilling to a certain depth (12 - 30 mm) is a consequence of rapid cooling of the surface resulting from casting cast iron into metal molds (molds) or into a sand mold. High surface hardness (HB 400-500) provides good resistance to wear, especially abrasive wear. Hollow bleached cast iron is used to make sheet mill rolls, wheels, balls for mills, etc. In this case, cast iron with a low silicon content is used, which tends to to bleaching. Its approximate composition: 2.8-3.6% C; 0.5-0.8% Si; 0.4-0.6% MP. Due to different cooling rates across the cross section and the production of different structures, the casting has large internal stresses, which can lead to the formation of cracks. To relieve stress, castings are subjected to heat treatment, i.e. they are heated at 500-550 C.

It was first mastered in China back in the 10th century, after which it was found widespread in other countries of the world. The basis of cast iron is an alloy of iron with carbon and other components. 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.

Appearance

The alloy has a white color when fractured and a characteristic metallic luster. The structure of white cast iron is fine-grained.

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;
• large shrinkage (up to 2%) and poor filling;
• low impact resistance;
• high wear resistance.

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

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

Compound

Iron-carbon alloy is considered a cheaper material compared to steel. White cast iron contains iron and carbon, which are in a chemically bonded state. Excess carbon, which is not present in the solid solution of iron, is contained in a combined state in the form of iron carbides (cementite), and in alloyed cast iron in the form of special carbides.

Species

Depending on the amount of carbon content in white cast iron is divided into the following types:

1. Hypoeutectic contains from 2.14% to 4.3% carbon and, after complete cooling, acquires the structure of pearlite, secondary cementite and ledeburite.
2. Eutectic contains 4.3% carbon and has a structure in the form of a light background of cementite, which is dotted with dark pearlite grains.
3. Hypereutectic has from 4.3% to 6.67% carbon in its composition.

Application

Based on the above properties, we can conclude that practicing thermal and mechanical processing white cast iron makes no sense. The alloy found its main application only in the form of casting. Hence, best properties white cast iron is obtained only if all casting conditions are met. This method processing 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 slabs with a ribbed or smooth surface, and is also actively used for gray cast iron.

The use of white cast iron in agriculture in the form of 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.

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 and balls for mills are made from this material, which are mounted in machines for processing sheet metal.

Alloying elements of the alloy

Specially introduced alloying substances added to the composition of white cast iron can impart greater wear resistance and strength, corrosion resistance and heat resistance. Depending on the amount of added substances, the following are distinguished:

• low alloy alloy (up to 2.5% excipients);
• medium alloyed (from 2.5% to 10%);
• highly alloyed (more than 10%).

Alloying elements can be added to the alloy:

• chromium;
• sulfur;
• nickel;
• copper;
• molybdenum;
• titanium;
• vanadium,
• silicon;
• aluminum;
• manganese.

Alloyed white cast iron has improved properties and is often used for casting turbines, blades, mills, parts for cement and conventional furnaces, pumping machine blades, etc. The iron-carbon alloy is processed in two furnaces, which allows the material to be brought to a certain chemical composition:

• in a cupola;
• in electric melting furnaces.

Castings made of white cast iron are annealed in furnaces to stabilize the required dimensions and relieve internal stress. The annealing temperature can increase to 850 degrees. The heating and cooling process in mandatory must be done slowly.

The marking or designation of white cast iron with impurities begins with the letter H. Which alloying elements are contained in the alloy can be determined by the subsequent letters of the marking. The name may contain numbers that indicate the amount in percentage terms of additional substances that are contained in white cast iron. If the marking contains the designation Ш, this means that the alloy structure contains spherical graphite.

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;
• normalization at temperature conditions 850-960 degrees, resulting in graphite and perlite, and also increases wear resistance and strength.

Additional information

Today it has been proven that there is no direct relationship between wear resistance and hardness of a carbon alloy. Only due to the structure, namely the arrangement of carbides and phosphides in the form of a regular grid or in the form of uniform inclusions, increased wear resistance is achieved.

The strength of white cast iron is most strongly influenced by the amount of carbon, and the hardness depends on the carbides. The greatest strength and hardness are those cast irons that have a martensitic structure.