It was first mastered in China back in the century, after which it was widely disseminated in other countries of the world. The base of cast iron is an alloy of iron with carbon and other components. A distinctive feature It is that the cast iron contains more than 2% of carbon in the form of cementite, which is not in other metals. A bright representative of such an alloy can be called white cast iron, which is used in mechanical engineering for the manufacture of parts, in industry and in everyday life.

Appearance

The alloy has white color on a break and characteristic metal glitter. The structure of white cast iron is fine-grained.

Properties

Compared to other metals, iron-carbon alloy has such characteristics and properties:

• high fragility;
• increased hardness;
• high resistivity;
• low casting properties;
• low workability;
• good thermal resistance;
• great shrinkage (up to 2%) and poor filling;
• low shock stability;
• high wear resistance.

Metal mass has a large corrosive resistance in hydrochloric or nitric acid. If there are free carbides in the structure, then corrosion will be observed when placing the cast iron in sulfuric acid.

White cast iron, as part of which there is a smaller percentage of carbon, are considered more resistant alloys to high temperatures. Due to the increased mechanical strength and viscosity, which appear when exposed to high temperatures, the formation of cracks in castings is minimized.

Structure

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

Views

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

1. Doevertectic accommodates from 2.14% to 4.3% of carbon and, after complete cooling, acquires the perlit structure, secondary cementite and ice-terita.
2. Eutectic contains 4.3% carbon and has a structure in the form of a light background of cementite, which is littered with dark grains of perlite.
3. Zaletectic has from 4.3% to 6.67% of carbon in its composition.

Application

Based on the above properties, it can be concluded that practicing thermal and mechanical processing White cast iron does not make sense. Alloy found its main application only in the form of castings. Consequently, the best properties of white cast iron receives only when all the casting conditions are met. This method Processing is actively used if it is necessary to make massive products that must have a high surface hardness.

In addition, there is annealing of white cast iron, as a result of which dyeing cast iron is obtained, which serve for the manufacture of thin-walled casting, for example:

• car details;
• agricultural products;
• parts for tractors, combines, etc.

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

The use of white cast iron in agriculture In the form of structural metal is quite limited. Most often, the iron carbon alloy is used to make parts of hydromachins, sandboxes and other mechanisms that can function in conditions of increased abrasive wear.

Bleached cast iron

This alloy is considered a variety of white cast iron. To achieve a beat of 12-30 mm possible by quick cooling of the surface of the iron-carbon alloy. The structure of the material: the surface part is made of white, gray cast iron in the core. From such a material, wheels are made, balls for mills that are attached in machines to process sheet rolled products.

Alloying alloy elements

Specially entered alloying agents added to the composition of white cast iron are able to give greater wear resistance and strength, corrosion resistance and warm-heartability. Depending on the number of added substances, they distinguish such:

• low alloy alloy (up to 2.5% auxiliary substances);
• secondary degrees (from 2.5% to 10%);
• high-alloy (more than 10%).

The alloy can add alloying elements:

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

Alloyed white cast iron has improved properties and is often used to cast turbines, blades, mills, parts for cement and ordinary stoves, blades of pumping machines, etc. It is processed by an iron carbon alloy in two ovens, which allows the material to a certain chemical composition:

• in Vagranka;
• in electrollobal furnaces.

Castings made of white cast iron are exposed to an annealing in the furnaces to stabilize the necessary sizes and removal of internal voltage. The annealing temperature can increase to 850 degrees. The process of heating and cooling in obligatory Must be done slowly.

Marking or designation of white cast iron with impurities begins with the letter C. What exactly the alloying elements are accommodated in the composition of the alloy, it is possible to determine the following labeling letters. In the title, there may be numbers that indicate the amount in percentage terms of additional substances that accommodate in white cast iron. If there is an indication of the mark in the marking, it means that in the structure of the alloy there is graphite of the spherical shape.

Types of annealing

For the formation of white cast iron in the industry, the heat cooling of the alloy is used. Today, such basic types of carbon alloy hill are actively used:

• mitigating annealing applies mainly to increase the ferrite cast iron;
• annealing to remove the internal stresses and minimizing phase transformations;
• graphitizing annealing, according to the result, what can be obtained;
• normalization temperature mode 850-960 degrees, resulting in graphite and perlite, and also increases wear resistance and strength.

Additional information

Today it is proved that direct dependence does not exist between the wear resistance and the hardness of the carbon alloy. Only due to the structure, namely the location of carbides and phosphides in the form of the correct grid or in the form of uniform inclusions, increased wear resistance is achieved.

The strength of the white cast iron is most intensively affected by the amount of carbon, and the hardness depends on carbides. Thunderstorms and hardness are distinguished by those cast irons that have a martensitic structure.

Iron alloy with carbon (\u003e 2,14 % C) Call cast iron. The presence of eutectic in the structure of the cast iron (see Fig. 87) determines its use exclusively as a foundry alloy. Carbon in cast iron can be in the form of cementite or graphite, or at the same time as cementite and graphite. Cementite gives a sneakless light glitter. Therefore, cast iron in which the entire carbon is in the form of cementite, is called white. Graphite gives a heavy cast iron gray, so cast iron is called gray. Depending on the shape of graphite and the conditions for its formation, the following cast iron distinguishes: gray, high-strength and macked (see Fig. 101 and 102).

Gray and white cast iron

The gray cast iron (technical) is essentially an FE-SI-C alloy containing Mn, P and S permanent impurities in the structure of gray cast irons, most or all carbon is in the form of graphite. Characteristic feature The structures of gray iron, determining many of its properties, is that graphite has a microclife form of plates in the field of view (see Fig. 88). The most widespread use was obtained by deetectic cast iron containing 2.4-3.8% C. The higher the content in the cast iron of carbon, the greater the graphite forms and the lower its mechanical properties. At the same time, to ensure high foundry properties (good fluctuations) should be at least 2.4 % FROM.

The incision of the triple diagram of the state of Fe - Si-C for a constant silicon content (2%) is shown in Fig. 99. In contrast to the stable FE-C diagram (see Fig. 87) in the FE-SI-C system peritectic (w +

Fig. 99.

Z - liquid phase; And austenite; G * graphite

F-6-ferrite-? A), eutectic (zh- * a + g) and eutectoid (a -? F + d) transformations occur at a constant temperature, and in some temperature range.

The magnitude of the temperature range in which the equilibrium with the liquid alloy contains austenite and graphite, depends on the content of silicon. The greater the silicon content, the wider the eutectic temperature range.

Cooling cast iron B. real conditions Makes substantial deviations from equilibrium conditions. The structure of the cast iron in the castings depends primarily on the chemical composition (carbon and silicon content) and crystallization rate.

Silicon contributes to the graphitization process, acting in the same direction as the slowdown in the cooling rate. Changing, on the one hand, the content in cast iron of carbon and silicon, and on the other - the cooling rate, you can get a different structure of the metal base iron. The structural chart for cast iron showing what should be the structure in the casting with a thickness of the wall 50 mm, depending on the contents

Fig. 100.

but - influence with si; Neither the structure of the cast iron: b - the effect of cooling rate (casting thickness) and the amount of C + Si on the structure of the cast iron; I. - white cast iron; // -V. - Gray chops

Fig. 101.

but - white cast iron; b - pearl gray cast iron: in - ferrite-pearl gray cast iron; g. - ferrite gray cast iron

silicon and carbon cast iron is shown in Fig. 100, but.With this carbon content, the larger the silicon cast iron, the fuller the graphitization flows. The more carbon cast iron, the less silicon is required to obtain a given structure.

Depending on the carbon content associated with cementitis, distinguish:

• 1. White cast iron (Fig. 100, but, /), in which all carbon is in the form of cementite Fe 3 C. The structure of such cast iron - perlite, iceburitis and cementite (Fig. 100, a, I. and 101 but).
• 2. Half ch\u003e Tun (Fig. 100, but, //), Most of the carbon (\u003e 0.8%) is in the form of Fe 3 C. The structure of such cast iron is perlite, iceburt and lamellar graphite with
• 3. pearl gray cast iron (Fig. 100, a, III) The structure of the cast iron (Fig. 101, b) - perlite and plate graphite. In this cast iron, 0.7-0.8 ° B C is in the form of Fe 3 C, which is part of the perlite.
• 4. Ferritic-pearlit (Fig. 100, A, / V) Gray cast iron. Structure of such cast iron (Fig. 101, in) - perlite, ferrite and plate graphite (forms, see Fig. 100, and, Iii). In this cast iron, depending on the degree of decay of eutectoid cementite in the bound state, 0.7 to 0.1% C.
• 5. Ferritic gray cast iron (Fig. 100, a, V.). Structure (Fig. 101, d) - Ferrite and plate graphite. In this case, the whole carbon is in the form of graphite.

With this carbon and silicon content, graphitization flows the more fully than slower cooling. IN production conditions Cooling speed is conveniently characterized by the thickness of the casting wall. The thinner the casting, the faster the cooling and to a lesser extent graphitization flows (Fig. 100, b).

Consequently, the silicon content should be increased in a small cross section, coolant, or in cast iron with a lower carbon content. In thick sections of castings, cooling slower, graphitization flows fully and the silicon content may be less. The amount of manganese in the cast iron does not exceed 1.25-1.4 %. The manganese prevents gravity, i.e. it makes it difficult to release graphite and increases the ability of cast iron to bleaching - the appearance, especially in the surface layers, the structure of white or halm cast iron. The sulfur is harmful admixture, worsening the mechanical and casting properties of the cast iron. Therefore, its content is limited to 0.1-0.2 %. In the gray cast iron, sulfur form sulphides (FES, MNS) or their solid solutions (Fe, Mp) s.

The mechanical properties of the cast iron are due to its structure, mainly by the graphite component. Cast iron can be viewed as steel, permeated with graphite, which plays the role of incisions that weaken the metal base of the structure. In this case, the mechanical properties will depend on the amount, the magnitude and nature of the distribution of graphite inclusions.

The smaller graphite inclusions than they smaller and more degree of insulance of them, the higher the strength of the cast iron. Cast iron O. large quantity Straight large graphite discharges separating its metallic base has a coarse-grained break and low mechanical properties. Cast iron with small

and currency graphite discharge has higher properties.

The plates of graphite reduce the resistance of the separation, the time resistance and especially the plasticity of the cast iron. Relative lengthening of gray cast iron tensile, regardless of the properties of the metal base, is almost equal to zero (- "0.5%). Graphite inclusions affect the decrease in the strength of compression and hardness, the amount of them is determined mainly by the structure of the metal base of iron. When compressed the cast iron undergoes Significant deformations and destruction has the character of a cut at an angle of 45 °. The destructive load during compression, depending on the quality of cast iron and its structure, 3-5 times greater than when tensile. Therefore, the cast iron is recommended to be used mainly for compression products.

The plates of graphite is less significant than when tension, reduce strength and bending, as part of the product is experiencing compressive voltages. Bending strength limit has an intermediate value between the tensile strength limit and compression. Cast iron hardness 143-255 HV.

Graphite, disrupting the solidness of the metal base, makes the cast iron at least sensitive to all sorts of voltage concentrators (surface defects, cutters, tonsings, etc.). As a consequence, the gray cast iron has about the same design strength in the castings of a simple form or with smooth surface and complex shapes with cuts or with a poorly treated surface. Graphite increases the wear resistance and antifriction properties of the cast iron due to its own "lubricating" action and increase the strength of the film lubricant. It is very important that graphite improves the workability by cutting, making the chips with breaking.

The metal base in the gray cast iron provides the greatest strength and wear resistance, if it has a pearlite structure (see Fig. 100, b). The presence in the structure of ferrite, without increasing the plasticity and viscosity of the cast iron, reduces its strength and wear resistance. Ferritic gray cast iron has the smallest strength.

The gray cast iron is marked with the letters with gray and ch - cast iron (GOST 1412-85). After letters follow the numbers indicating the minimum value of the time resistance of 10 "1 MPa (kgf / mm 2).

Gray cast iron by properties and use can be divided into the following groups.

Ferritic and ferrite-pearlic cast iron (Sch 10, sch 15, sch 18) have a temporary resistance of 100-180 MPa (10-18 kgf / mm 2), bending strength of 280-320 MPa (28- 32 MPa). Their approximate composition: 3.5-3.7 % FROM; 2.0-2.6% Si; 0.5--0.8%;

Sch 15). These cast irons are used for inappropriate parts experiencing small loads in operation with a thickness of the casting wall of 10-30 mm. So, the cast iron MC 10 is used for construction columns, foundation plates, and cast iron sch 15 and sch 18 -st cast low-loaded parts of agricultural machinery, machine tools, cars and tractors, reinforcements, etc.

Perlitic cast iron (Sch 21, sch 24, sch 25, sch 30, sch 35) are used for responsible castings (mills of powerful machines and mechanisms, pistons, cylinders, details working on large pressures, compressors, reinforcement, diesel cylinders, engine blocks , Metallurgical equipment, etc.) with a wall thickness up to 60-100 mm. The structure of these cast iron is a fine-plane perlite (sorbitol) with small currency graphite inclusions. The so-called pearlite belongs to stalisty and modified cast iron.

When smelting of stal walls, Schi 24, MF 25 in the mixture is added 20-30 % steel scrap; Castings have a reduced carbon content, which provides a more dispersed pearlite base with a smaller number of graphite inclusions. Sample composition: 3,2-3,4 % FROM; 1,4-2,2 % Si; 0.7-

1,0 % MP; % R;

Modified cast iron (sch 30, sch 35) is obtained by adding in liquid cast iron before the casting of special additives - modifiers (graphite, 75% ferrosilica, silicolization in an amount of 0.3-0.8 % etc.). Modifying is used to obtain in cast-iron castings with different thickness of the walls of the pearlite metal base with the inclusion of a small number of isolated medium graphite plates.

The modification is subjected to low-carbon cast iron, containing a relatively small amount of silicon and an increased amount of manganese and having without the introduction of a modifier, the structure of the half of the cast iron, that is, the icebell, perlite and graphite. Exemplary chemical composition Cast iron: 2.2-3.2% C; 1.0-2.9% Si; 0.2-1.1% MP;

To remove casting cast iron stabilization and stabilization, cast iron castings are ignited at 500-600 ° C. Depending on the shape and sizes of the casting, the exposure at an annealing temperature is 2-10 hours. Cooling after annealing slow, together about the furnace. After such treatment, the mechanical properties change little, and the internal stresses decrease by 80-90%. Sometimes, the natural aging of the cast iron is used to remove stresses in cast iron castings - their exposure in stock for 6-10 months; Such exposure reduces the voltage by 40-50%.

Antifriction cast iron Apply for the manufacture of bearings of sliding, bushings and other parts that work with friction on metal, more often in the presence of lubricant material. These cast iron should provide low friction (small friction coefficient), i.e. antifriction. The antifriction properties of the cast iron are determined by the ratio of perlite and ferrite based, as well as the amount and form of graphite. Antifriction cast irons manufactures the following brands :

ACS-1 (3.2-3.6% C; 1.3-2.0 % Si; 0.6-1.2 % MP; 0.15-0.4% p; % SG; 1.5-2.0% C); ACS-2 (3.2-3.8% C; 1.4-2.2% Si; 0.3-1% MP; 0.15-0.4 % R; % Ti; 0.2-0.5% SI) and ACS-3 (3.2-3.8 % FROM; 1.7-2.6% Si; 0.3-0.7 % MP; 0.15-0.4% p; 0.2-0.5% si;

Parts working in a pair with hardened or normalized steel shafts are manufactured from pearline gray CHSS-1 and AHS-2; For work in a pair with thermally untreated shafts, pertly ferrite cast iron AHS-3 is used.

A pearlic cast iron containing an increased amount of phos ^ Fora (0.3-0.5%) is used to make piston rings. The high wear resistance of the rings is provided by a metal base consisting of a thin perlite and evenly distributed phosphide eutectic in the presence of isolated discharge of plate graphite.

• Graphite crystallizes in the form of pretty complex shapes (See Fig. 88, b, o), but the cross section of their microclife plane gives the kind of plates.
• 2 In white cast iron, eutectic is possible (Fe + Fes) and solving sulfur in Feac.
• The greater the thickness of the casting walls, the lower the mechanical properties. 149.
• A - antifriction, ch - cast iron, s - gray.

In mechanical engineering, castings are used from gray, forged and high-strength cast iron. These cast iron differ from white cast iron by the fact that they have all the carbon or most of it is in its vigorous state in the form of graphite (and the white cast iron has all the carbon in the form of cementite).

The structure of these castons consists of a metal base similarly to steel (perlite, ferrite) and non-metallic inclusions - graphite.

Gray, mappy and high-strength cast iron They differ from each other in the main form of graphite inclusions. This determines the dimensions of the mechanical properties of the specified cast iron.

W. Gray cast iron graphite (when examined under micro-hop) has the form of plates.

Graphite has low mechanical properties. It dismisses the solidness of the metal base and acts as an incision or a small crack. The larger and straightforward shape of the graphical inclusions, the worse the mechanical properties of gray cast iron.

The main difference high-strength chu-gun It is that graphite in it has a spherical (oc-crushed) form. Such a form of graphite is better than a lamellar, since it is significantly less disturbed by the continuity of the metal base.

Malleable cast iron It is obtained by a long annealing of white cast iron castings, as a result of which graphite of flakes-like form is formed - carbon annealing.

The mechanical properties of the candidates under consideration can be improved by thermal processing. At the same time, it is necessary to remember that significant internal stresses are created in the cast iron, therefore heat the cast-iron castings during thermal processing, it slowly to avoid the formation of cracks.

Casting castings are subjected to the following thermal processing.

Low-temperature annealing. To remove the inner suspension and stabilize the size of cast-iron castings from gray cast iron, natural aging or low-temperature forth is used.

Older way is elder At which the casting after complete cooling is undergoing long-term departure - from 3-5 months to several years. Natural aging is used in the case when there is no required equipment for annealing. This method is currently not used; They produce mainly low-temperature annealing. For this, the castings after complete solidification are laid into a cold furnace (or a furnace with a temperature of 100-200 ° C) and together with it slowly, with a speed of 75-100 ° C per hour heated to 500-550 ° C, at this temperature them Withstand 2-5 hours and cool-give up to 200 ° C at a rate of 30-50 ° per hour, and then in air.

Graphitizing annealing.

When casting products, it is possible for hours-tireless gray cast iron from the surface or even throughout the cross section. To eliminate bleach and improve the processability of cast iron, a high-temperature graphitizing annealing is performed with you-kept at a temperature of 900-950 ° C for 1-4 hours and cooling products up to 250-300 ° C with the furnace, and then in air. With such annealing in the bleached plots, FE 3 cementing, the smith and graphite, as a result of which the white or half cast iron goes into gray.

Normalization.

Normalization is subject to casting simple forms and small sections. Normalization is carried out at 850-900 ° C with an excerpt of 1-3 hours and the subsequent cooling of the castings on the road. With this heating, part of the carbon-graphite dissolves in austenite; After cooling in the air, the metal base is semi-taps the structure of the Trootovoid perlite with a higher solidity and better wear resistance. For gray cast iron, normalization is applied relatively rarely, more widely use quenching with leave.

Quenching.

You can enhance the strength of the gray cast iron by hardening it. It is produced with heating to 850-900 ° C and cooling in water. Overcake can be subjected to pearlite and ferritic chugu-us. The hardness of the cast iron after hardening reaches HB 450-500. In the structure of the hardened cast iron there are martensite with a significant amount of residual austenite and graphite allocation. The effective method of improving the strength and wear-resistant gray cast iron is an isothermal hardening, which is performed by an ana logically hardening steel.

High-strength cast iron With spherical graphite, you can refer to a flame or high-frequency surface hardening. Cast iron parts after such treatment have a high surface hardness, a viscous core and well resist shock loads and abrasion.

Alloyed gray cast iron and high-strength magnesium cast iron Sometimes subject to nitrogen. Surface hardness azoty cast iron products reaches HV600-800 ° C; Such details are high wear resistance. Good results give sulphidation of cast iron; For example, sulfidated porsche-non-rings are quickly developing, the use of α-tiracy is well, and their service life increases several times.

Vacation.

To remove the hardening voltages, after quenching makes a vacation. Details designed to work on the Istra - Low leave at a temperature of 200-250 ° C. Casting castings that do not worry on abrasion are exposed to high leave at 500-600 ° C. When the hardened cast iron leaves, hardness decreases significantly Less than when the steel is vacation. This is explained by the fact that in the structure of the hardened cast iron, a large co-nutity of residual austenite, as well as the fact that it contains a large amount of silicon, which increases the release of martensite.

For annealing, white cast iron approximately the following chemical composition is used to annealing for dake cast iron: 2.5-3.2% C; 0.6-0.9% Si; 0.3-0.4% μη; 0.1-0.2% ρ and 0.06-0.1% S.

There are two ways to annealing for making cast iron:

graphitizer annealing in a neutral medium based on the decomposition of cementite on ferrite and carbon annealing;

decarbing annealing In an oxidizing medium, based on carbon burning.

Annealing for a mappy cast iron according to the second method takes 5-6 days, so currently forging cast iron is obtained by the primary formation of graphitization. Castings, peeled from sand and sprues, are packaged in metal boxes or laid on the pallet, and then annealing in the methodical, chamber and other ilatory furnaces.

The annealing process consists of two grades' stages. The first stage consists in uniform heating of castings up to 950-1000 ° with reduction of 10-25 hours; Then the temperature is reduced to 750-720 ° C at a cooling rate of 70-100 ° C per hour. On the second sty-diya at a temperature of 750-720 ° C, an excerpt is given 15-30 hours, then the castings are cooled together with the furnace to 500-400 ° C and at this temperature are removed to the air, where they are cooled at a free velocity. With such a step of annealing in the area of \u200b\u200bthe temperature of 950-1000 ° C, there is a decay (graphitization) of cementite. As a result of annealing for such a mode, the structure of the forging cast iron prevents the grains of ferrite with the inclusions of the carbon sockets of the ECC - Graphite.

The pearlock of the cast iron is obtained as a result of incomplete-alone: \u200b\u200bafter graphitization at 950-1000 ° C, the cast iron is cooled with the furnace. The structure of pellite forging cast iron consists of perlit and carbon annealing.

In order to increase the viscosity, the pearlock of the cast iron is exposed to spheroid at a temperature of 700-750 ° C, which creates the structure of the grainy perlite.

To accelerate the annealing process on a mackey cast iron, white cast iron products are quenched, then a graphitization is carried out at 1000-1100 ° C, the simulatization of the hardened cast iron during annealing is explained by the presence of a large number of graphitization centers formed when quenching. This makes it possible to reduce the annealing time of hardened castings to 15-7 hours.

Thermal Dake cast iron treatment.

To increase other things and wear resistance, maversea cast irons are subjected to normalization or quenching with vacation. The normalization of the forging cast iron is produced at 850-900 ° C with shutter speed at this temperature of 1-1.5 hours and air cooling. If the blanks have increased hardness, they should be subjected to high leave at 650-680 ° C with an excerpt of 1-2 hours.

White cast iron is a variety of cast iron, which in its composition contains carbon compounds. In this alloy, they are called cements. Its name is similar to the metal, thanks to the characteristic white color and gloss, which is clearly visible on the break. This shine is manifested due to the fact that there are no large inclusions of graphite as part of such cast iron. In percentage, it is not more than 0.3%. Therefore, it is possible to detect it only by spectral or chemical analysis.

Composition and types of white cast iron

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

• Doeevtectic. These are such alloys in which carbon does not exceed 4.3% of general composition. It turns out after a complete cooling. As a result, it acquires the characteristic structure of such elements as perlite, secondary cementite and iceburitis.
• Eutectic. They have a carbon content equals 4.3%.
• Zaletectic white cast iron. The content exceeds 4.35% and can reach 6.67%.

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

The internal structure of the white cast iron is an alloy of two elements: iron and carbon. Despite the high-temperature production in it, a structure with shallow grain is preserved. Therefore, if you break a detail of such a metal will be observed a characteristic white color. In addition, in the structure of the deetectic alloy, for example, solid grades, except perlite and secondary cementite is always present cementite. Its percentage can approach 100%. This is characteristic of eutectic metal. For the third type, the structure is the composition of eutectic (l n) 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 ice and perlite. The bleaching effect of a depth of 30 mm is achieved using the rapid cooling method. As a result, the surface layer is obtained from white colorAnd then the casting consists of an ordinary gray alloy.

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

• low allocated (they contain allocating elements of no more than 2.5%);
• the total degrees (the percentage of such elements reaches 10%);
• high-alloyed (in them the number of alloying additives exceeds 10%).

Rightly common elements are used as alloying additives. The doped white cast iron thus obtained becomes new, predetermined properties.

Properties of white cast iron

Any cast-iron alloy, on the one hand, is very durable, but at the same time it has sufficient fragility. Therefore, as the main positive properties of white cast iron, you can allocate:

• High hardness. This greatly complicates the processing of parts, in particular, cutting.
• Very high resistivity.
• Excellent wear resistance.
• Good resistance to increased thermal exposure.
• Sufficient corrosion resistance, including various acids.

White cast iron, with a reduced carbon percentage, have greater resistance to high temperatures. This property is used to reduce the quantity of cracks in castings.

The disadvantages include:

• Low casting properties. It has a bad filling of casting forms. During the fill, internal cracks can form.
• Increased fragility.
• Bad workability of the castings themselves and the parts made of white cast iron.
• Great shrinkage that can reach 2%.
• Low resistance to shock influences.

Another disadvantage is bad weldability. Problems in welding parts from similar material caused by the fact that at the time of welding, cracks occurs, both when heated and when cooled.

White cast iron marking

For marking white cast iron, the letters of the Russian alphabet and numbers are used. If there are impurities in it, the marking begins with the letter "h". The composition of the existing alloying additives can be determined by the subsequent letters of P pl, PF, PVC. They testify to the presence of silicon. If the resulting metal has increased wear resistance, then its marking will begin with the letter "and", for example, ICH, ICH. For example, the presence in marking the designation "W" means that in the structure of the alloy there is graphite of the spherical shape.

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

The brand CN20D2HSH is decoded 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, graphite of the spherical shape.

Application area

This alloy is used in the following industries: mechanical engineering, machine-tooling, shipbuilding. It produces some elements of household products. In mechanical engineering, it is made of: parts of freight and passenger cars, tractors, combines and other agricultural machinery. The use of alloying additives allows you to obtain specially specified properties. For example, used in the manufacture of plates with different shape of the surface.

Bleached cast iron has a sufficiently limited area of \u200b\u200bapplication. It produces details of a simple configuration. For example: Mill balls, various destination wheels, parts for rolling mills.

It received widespread use in the production of details of such large units, such as hydraulic and molding machines, other industrial mechanisms of this direction. Specific feature Their work lies in the fact that they are constantly exposed to abrasive material.

Structure and properties of cast iron.

Castings are called iron carbon alloys containing more than 2.14% carbon. In engineering, cast iron is one of the main foundry, which is explained primarily by its good foundry and strength properties. It is not subjected to pressure processing. The main factor determining properties, and, consequently, the scope of cast iron is its structure that can be diverse.

The structure of cast iron is divided into white, gray, masting and high strength.

9.1. White cast iron.

White is called cast iron, in which the entire carbon is in a chemically bound state in the form of cementite Fe 3 C, which gives a white brilliant iron.

Phase transformations in these castles flow according to the metastable diagram Fe - Fe 3 C (see Fig.23). By structure, white cast irons are divided into:

a) DEEVETTECTIC, comprising from 2.14 to 4.3 s. They consist of perlite, ice-terita and secondary cementite, released from austenite grains in the temperature range from 1147 ° (EU line) to 727 ° (SK line). Secondary cementitis merges with cementitis of the iceburitis and may not be visible on a microclife as an independent structural component (Fig. 51, a);

b) eutectic, containing 4.3% C. It consists of eutectic - labelite, which is a mechanical mixture of cementite and perlite (Fig. 51, b);

C) zevertectic containing from 4.3% to 6.67 % C. They consist of primary cementite, released in the form of large plates and laburtite (Fig. 51, B).

Fig. 51. The structure of the white cast iron: a) the deepetectic b) eutectic c) of the zevertectic

The white cast iron microstructure contains a lot of cementite, so it is very hard and fragile, but it is well resisting wear. It is almost non-cutting cutting (with the exception of abrasive), therefore white cast irons do not find direct use in mechanical engineering, they are rarely used, only for the manufacture of parts operating under conditions of increased abrasive wear (parts of hydromachins, sand and dr.). Being the main product of the domain smelting, this cast iron is used by interlelnergia for redistribution to steel (anterior cast iron). In a minor quantity, white cast iron is also used to obtain a forging cast iron.

9.2. Gray cast iron.

The gray is called the cast iron in which the carbon is in the form of a grapple having a slightly curved plates or scales, or branched outlets with lamellar petals. Due to the large number of graphite in the structure of such cast iron in the breakfast is gray.

Silicon contributes to the graphitization process, reduces the shrinkage, silicon is part of ferrite, forming a solid substitution solution with α-iron.

The manganese increases the leaning of cast iron to the preservation of cementite, and therefore, and increases the hardness of the cast iron.

Sulfur - harmful admixture Castows, it increases their hardness and fragility of 5-6 times more than amn and significantly worsens the casting properties.

Phosphorus in small quantities in cast iron is a useful admixture (in contrast to steels), improves the casting properties of gray chunks, as phosphorus forms Eutectic FE + Fe 2 P, melting at a temperature of 983 ° C, which is valuable for the production of a thin-walled blast . Chemical composition of gray castoffs: 2.5 ... 4% C; 1.0 ... 4.8% Si; 0.5 ... 0.7% Mn; up to 0.12% s; 0.2 ... 0.5% P.

According to the structure of the metallic base, gray cast irons are divided mainly into the following groups;

1. Perlit. The structure of P + GG (plate graphite), a metal base - n, and the amount of associated carbon (Fe 3 C) is equal to eutectoid concentration of 0.8% (Fig. 52, a).

2. Ferritic-pearlity. The structure F + P + PG, the metal base consists of F + P, and the amount of Fe 3 C is less than eutectoid concentration (Fig. 52, b).

3. Ferritic. Structure F + GG. The foreign base consists of F, and Fe 3 C \u003d 0 (Fig. 52, B).

Fig.52. Structure of gray cast iron: a) pearlite b) ferrite-pearlite c) ferrite

The mechanical properties of the cast iron depends on the properties of the metal base, the number and size of graphite inclusions. When designing machine parts, it should be borne in mind that gray cast irons work on compression better than tensile. They are little sensitive to cuts during cyclic loading, well absorb oscillations in vibrations, have high antifriction properties due to the lubricity of graphite. Gray cast iron is well processed with cutting, cheap and easy to manufacture. Along with these positive properties, they have relatively low strength and extremely low plasticity.

The gray cast iron brand consists of the SC letters (gray cast iron) and the numbers showing the value reduced 10 times (in the megapascals) of the time resistance during tension (Table 7).

The strength of the cast iron significantly depends on the thickness of the casting wall. The value indicated in the mark value σ B corresponds to the castings with a wall thickness of 15 mm. With an increase in the wall thickness of 15 to 150 mm, the strength and hardness of the cast iron decrease almost twice.

Graphite, worsening mechanical properties, at the same time gives cast iron a number of valuable properties. It grinds the chip in the processing of the reference, it has a mitigating effect and, therefore, increases the impetence of castors, gives them a damping ability. In addition, the plate graphite provides a small sensitivity of the iron to the surface defects. Thanks to this resistance to fatigue and steel parts are commensurate.

According to GOST 1412-85, the castings are made of gray cast iron of the following marks: sch10, sch15, sch18, sch20, sch25, sch30, sch35. The figures in the designation of the brand correspond to the minimum limit of tensile strength (σ in, kgf / mm 2). Country Country - ferritic, and starting with sch25 and more - pearlit, intermediate - ferritic-pearlite.

From ferritic castoffs, they are mainly made by the invisputable details, which are mainly imposed by the requirements of good cutting machines, and not durability, such as slabs, loads, trough, covers, casing, etc.

Carter, brake drums, lids, pistons, piston rings, large pulleys, gears, etc. are manufactured from ferrite-pearlite cast iron in the automotive industry.

From pearlite - blocks of cylinders, sleeves, flywheels, etc. In the machine tool, gray cast iron is the main structural material (stannes of machines, tables and upper sleds, spindle grandmothers, columns, carriages, etc.), to wear-resistant, the bleached gray cast iron belongs to wear-resistant (0 ) having a thin surface layer with a white cast iron structure. It is used for the manufacture of castings of rolling rolls, carriage wheels, etc.

Dovety cast iron.

The name "Dake-up cast iron" is conditional, since products from it, as from any other cast iron, are made not forging, but casting. The name "Dusty" This cast iron was obtained due to higher, compared with gray cast iron plastic properties.

The schematic diagram of the technology of obtaining parts made of ductile cast iron consists of two operations. First, by casting from a white milking cast iron, the parts are obtained (the recommended chemical composition of the alloy poured into the shape: 2.4 ... 2.9% C; 1.0 ... 1.6% Si; 0.3 ... 1, 0% mn; ≤ 0.1% s; ≤ 0.2% p, then the resulting castings are subjected to a special graphitizing annealing (tension). Annealing consists of two stages (Fig. 53).

Initially, the castings from white cast iron (more often packed in sandboxes) slowly heated for 20 ... 25 hours to a temperature of 950 ... 1050 ° C. And at the same temperature, they are kept (for 10 ... 15 hours). During this period, the first grades stage flows, i.e. The decay of cementite, which is part of the Larburt (A + Fe 3 C), and establishing a stable equilibrium of austenite + graphite.

As a result of the decay of cementite, flaky graphite (carbon anneal) is formed.

The metal base of the cast iron is formed at the second stage of annealing with eutectoid transformation. In the case of continuous cooling of castings (in air) in the eutectoid (727 ° C), the austenite temperature decays to perlite and the graphitization process will not have time to cover cementitis perlite. Cast iron takes the structure: perlite plate + flake graphite (xg) It has high hardness, strength and small plasticity (HB 235 ... 305, σ B \u003d 650 ... 680 MPa, Δ \u003d 3.0 ... 15%) . To increase the plasticity while maintaining a sufficiently high strength, a short (2 ... 4 h) isothermal extract of pig iron or slow cooling at temperatures of 690 ... 650 ° C. This is the second stage of annealing, which is in this case an annealing on grainy perlite.

Fig. 53. White cast iron annealing schedule

In mechanical engineering, ferrite carpet cast iron is widely used, characterized by high plasticity (δ \u003d 10 ... 12%) and relatively low strength (σ B \u003d 370 ... 300 MPa). The ferritic base of the cast iron is formed with a very slow passage of the interval of 760 ... 720 ° C or in the process of isothermal exposure at 720 ... 700 ° C. Here Austenit and cementitis, including cementitis perlite, if Perlit managed to appeal, disintegrates on ferrite + flake graphite. The flake shape of graphite is the main cause of higher strength and plasticity of the forging cast iron compared to gray cast iron (see Table 7).

The duration of annealing in general is 48 ... 96 h (the duration of stage II is about 1.5 times greater than I). To reduce the duration of annealing to the melt before it is casting in forms (is introduced (modified) aluminum (less often boron, bismuth, etc.), which creates additional artificial education centers of graphite. According to GOST 1215-79, the following brands of forging cast iron KCh30-8, KCh35 -10, kch37-12, kch45-7, kch50-5, kch55-4, kch60-3, kch65-3, kch70-2, kch80-1,5. The first two digits correspond to the minimum limit

tensile strength (σ B, kgf / mm 2); Figures after dash - relative elongation (δ, % )

Dovety cast irons are used for parts operating at shock vibration loads (hubs, brake pads, crankshafts, hooks, gearboxes, etc.).

The main disadvantage of the receipt of KC is a long annealing of castings and limiting the thickness of their walls (up to 50 mm). In passive parts as a result of slow cooling during crystallization, a plate graphite arises (instead of flakes), which reduces the strength and plasticity of cast iron.

Table 7.Mechanical properties of cast iron.

Gray cast iron (GOST 1412 - 85)

Sch 10.		-	-	-190	F.
Sch 15.		-	-	163-210	F.
Sch 25.		-	-	180-245	F + P.
Sch 35.		-	-	220-275	P

High-strength cast iron (GOST 7293 - 85)

HF 35.				140-170	F.
HF 45.				140-225	F + P.
HF 60.				192-227	F + P.
HF 80.				248-351	P
HF 100.				270-360	B.

Matchy cast iron (GOST 1215 - 79

KC 30 - 6		-		100-163	F + up to 10%
KCh 35 - 8		-		100-163
KCh37 - 12.		-		110-163
KCh45 - 7.		-		150-207
KC 60 - 3		-		200-269	P + up to 20% f
KC 80-1.5		-	1,5	270-320

9.4. High-strength cast iron.

High-strength cast iron is obtained by modifying (micro-linking of liquid cast iron by magnesium (0.1 ... 0.5%) or cerium (0.2 ... 0.3%). In this case, the influence of magnesium graphite in the crystallization process takes not a plate. And a spherical form. The microstructure of the modified cast iron on ferritic and perlit-based base is shown in Fig. 54, a, b.

Fig. 54. Structure of high strength cast iron: a) ferritic b) pearlite

The main cause of high mechanical properties of high-strength cast iron (Table 7) is a spherical form of graphite. A ball-shaped graphite having a minimum surface with a given volume, significantly less weakens the metal base of cast iron than lamellar graphite. Unlike the latter, it is not an active voltage concentrator.

According to GOST 7293-85, the castings are made from high-strength cast iron of the next brands: RF35, RF40, RF45, RF50, RF60, RF70, RF80, RF100 (numbers in the designation correspond to the minimum limit of tensile strength σ in, kgf / mm 2)

High-strength cast iron has high mechanical characteristics, has good foundry and technological properties. It is used as new Material And as a substitute for steel, forging and gray cast iron with lamellar graphite. Compared to steel, it has greater wear resistance, the best antifriction and anti-corrosion properties, the best processed cutting, due to the smaller casting density is easier than steel by 8 ... 10%. From high-strength cast iron, unlike dwarf, you can cast parts of any section, mass and sizes.

Application areas: in machine-tooling - caliper, cutting holders, heavy tables, spindles, levers, etc.; For rolling and forging equipment - Rolling rolls, steel rolling mills and forging hammers, swabs, traverts of presses; For other types of equipment - drums of telphelters of excavators, crankshafts, etc.

9.5. Alloyed cast iron.

Requirements for alloyed cast iron for castings with an increased heat resistance, corrosion resistance, wear resistance or heat-resistance are granted by GOST 7769-82. The brands of alloyed iron and their properties are shown in Table. eight.

Alloyed cast iron is subject to heat treatment to provide the necessary properties and structures.

An important property of doped iron is to wear resistance.

Castings are used as antifrictions according to GOST 1585-85. They are intended for the manufacture of parts running in friction nodes with lubricant. The standard defines the brands of antifriction cast iron, their chemical composition, characteristics, appointment, shape, size and distribution of graphite, perlite dispersion, the nature of the distribution of phosphide eutectic, hardness and limit modes of operation of parts from these cast iron. Their base is iron, permanent components,%: 2.2-4.3 s; 0.5-4.0 Si; 0.3-12.5 Mn. Impurities are allowed,%: 0.1-1 p; 0.03-0.2 S.

The brands of antifriction castnesses, their characteristics and value are presented in Table. nine.

Table 8.

Stamps and properties of doped castoffs (GOST 7769-82)

Mark Chuguna	Properties
Чх1, hp2, hp3	Castings with increased corrosion resistance in gas, air and alkaline media under friction and wear conditions, heat-resistant in the air, withstand temperatures from 500 to 700˚. Designed for the manufacture of parts of metallurgical production, Kokile glassform, details of chemical equipment, etc.
Ч3Т, Чх9Н5, Чх22, Чх16m2, Чх28Д2	Castings with high resistance against abrasive wear and abrasion
CX22S.	This cast iron is characterized by increased corrosion resistance at 1000 ° C
CHC13, EFS15, CHC17, CHC15MA, CHS17M3	Resistant to concentrated and diluted acids, alkali solutions, salts
CG6S3SH, CHG7X4	Cast iron with high resistance in abrasive environment
CG8D3.	Non-magnetic wear-resistant cast iron
CNHT, CNHMD, CHN2X, CHNMSH	Castings with high mechanical properties, well resist wear and corrosion
CN15D3SH, CN19X3SH, CN11G7SH, CHN20D2SH, CN15D7	Cast iron possessing high mechanical properties, high corrosion and erosion resistance in alkalis, weak solutions Acid, in sea water. Cast iron CHN20D2SH can be plastically deformed in cold condition

Table 9.

Antifriction cast iron brands, their properties and purpose

(GOST 1585-85)

Mark Chuguna	Properties and destination
ACS-1.	Pearlic cast iron doped with chromium (0.2-0.5%) and copper (0.8-1.6%); Designed for the manufacture of parts running in a pair with hardened or normalized shaft
ACS-2.	Pearlic cast iron doped with chromium (0.2-0.5%), nickel (0.2-0.5%), titanium (0.03-0.1%) and copper (0.2-0.5% ); Purpose - the same as the cast iron of the ASF-1 brand
ACS-3.	Pearly ferrite cast iron doped with titanium (0.03-0.1%) and copper (0.2-0.5%); Details of such cast iron can work in a pair, both with "raw" and with a thermally processed shaft
AHS-4.	Perlite cast iron doped with antimony (0.04-0.4%); Used for the manufacture of parts running in a pair with hardened or normalized shaft
ACS-5	Austenitic cast iron doped by manganese (7.5-12.5%) and aluminum (0.4-0.8%); From this cast iron, details are made in particularly loaded friction nodes in a pair with hardened or normalized shaft
ACS-6	Pearly porous cast iron doped with lead (0.5-1.0%) and phosphorus (0.5-1.0%); It is recommended for the production of parts operating in friction nodes with a temperature of up to 300 ˚ with a pair with a "raw" shaft
ACV-1.	Pearlic cast iron with spherical graphite; Details of such cast iron can operate in friction nodes with elevated circumferential speeds in a pair with hardened or normalized shaft
ACV-2.	Perlit-ferrite cast iron with spherical graphite; The parts made from this cast iron work well in friction conditions with elevated circumferential speeds in a pair with "raw" shaft
ACHK-1.	Pearlity cast iron with cade-shaped graphite, doped with copper (1.0-1.5%); Designed for the manufacture of parts operating in a pair with a thermally treated shaft
ACHK-2.	Ferrite-pearlic cast iron with flourishing graphite; Details from this cast iron work in a pair with "raw" shaft

Letters in the notation of the brands of cast iron means: Ah - antifriction cast iron, C - gray cast iron with lamellar graphite, in - high-strength cast iron with spherical graphite, k - macked cast iron with flaky graphite. The hardness of castings from antifriction cast iron (from 100 to 290 HV) depends on the content of elements and thermal processing conditions.

Limit modes of operation of parts from these castles in friction nodes: Specific pressure (50 - 300) 10 4 Pa \u200b\u200b(5-300 kgf / cm 2), Circuit Speed \u200b\u200b0.3-10 m / s.