Cast iron types. Cast irons (white, gray, high strength, malleable). Receiving, structure, marking, scope

An alloy of iron with carbon is called cast iron. The presence of eutectic in the structure of cast iron (see Fig. 87) determines its use exclusively as a casting alloy. Carbon in cast iron can be in the form of cementite or graphite, or simultaneously in the form of cementite and graphite. Cementite imparts a specific light sheen to the fracture. Therefore, cast iron, in which all the carbon is in the form of cementite, is called white. Graphite imparts fracture to cast iron grey colour, therefore cast iron is called gray. Depending on the shape of graphite and the conditions for its formation, the following cast irons are distinguished: gray, high-strength and ductile (see Fig. 101 and 102).

1. GRAY AND WHITE CAST IRON

Gray cast iron (technical) is essentially an alloy containing and as permanent impurities. In the structure of gray cast irons, most or all of the carbon is in the form of graphite. Salient feature the structure of gray cast irons, which determines many of its properties, lies in the fact that graphite has the form of plates in the field of view of a microsection (see Fig. 88). The most widely used are hypoeutectic cast irons containing. The higher the carbon content in cast iron, the more graphite is formed and the lower it mechanical properties... At the same time, to ensure high casting properties (good fluidity), there should be no less.

Silicon, the content of which in gray cast irons is within the limits, has a great effect on the structure, and, consequently, on the properties of cast irons, therefore, when studying structure formation in technical cast iron, it is necessary to use not a state diagram but a triple diagram

Rice. 99. Diagram of the state of liquid faea; A - austenite; G - graphite

A section of the ternary phase diagram for a constant silicon content is shown in Fig. 99. In contrast to the stable diagram (see Fig. 87) in the system, peritectic eutectic and eutectoid transformations proceed not at a constant temperature, but in a certain temperature range.

The value of the temperature range in which austenite and graphite are in equilibrium with the liquid alloy depends on the silicon content. The higher the silicon content, the wider the eutectic temperature range.

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

Silicon promotes the graphitization process by acting in the same direction as slowing down the cooling rate. By changing, on the one hand, the content of carbon and silicon in cast iron, and on the other, the cooling rate, it is possible to obtain a different structure of the metal base of cast iron. Structural diagram for cast irons, showing what the structure should be in a casting with a wall thickness of 50 mm, depending on

Rice. 100. Structural diagrams for cast irons: a - the effect of C and on the structure of cast iron; b - the effect of the cooling rate (thickness of the casting) and the amount on the structure of cast iron; white cast irons; - gray cast iron

Rice. 101. The structure of cast iron, a - white cast iron; b - pearlitic gray cast iron; c - ferrite-pearlitic gray cast iron; ferritic gray cast iron

Depending on the content of carbon bound in cementite, a distinction is made between:

1. White cast iron (Fig. In which all carbon is in the form of cementite The structure of such cast iron is perlite, ledeburite and cementite (Fig. 100, a, I and 101, a).

2. Half cast iron (Fig. Most of the carbon is in the form of the structure of such cast iron -1 perlite, ledeburite and lamellar graphite.

3. Pearlitic gray cast iron (Fig. 100, a, III) the structure of cast iron (Fig. 101, b) - pearlite and lamellar graphite. In this cast iron, 0.7-0.8% C is in the form of pearlite, which is part of the composition.

4. Ferrite-pearlite (Fig. 100, a, IV) gray cast iron. The structure of such cast iron (Fig. 101, c) is pearlite, ferrite and lamellar graphite (see the compositions in Fig. 100, a, III). In this cast iron, depending on the degree of decomposition of eutectoid cementite in the bound state is from 0.7 to 0.1%.

5. Ferritic gray cast iron (Fig. 100, a, V). The structure (Fig. 101, d) is ferrite and lamellar graphite. In this case, all of the carbon is in the form of graphite.

At a given content of carbon and silicon, the slower the cooling, the more fully the graphitization proceeds. V working conditions it is convenient to characterize the cooling rate by the thickness of the casting wall. The thinner the casting, the faster the cooling and the less the graphitization proceeds (Fig. 100, b).

Consequently, the silicon content must be increased in a casting of a small cross-section, which is cooled at an accelerated rate, or in cast iron with a lower carbon content. In thick sections of castings, cooling more slowly, graphitization proceeds more completely and the silicon content may be lower. The amount of manganese in cast iron does not exceed Manganese prevents graphitization, that is, it makes it difficult to release graphite and increases the ability of cast iron to bleach - the appearance, especially in the surface layers, of a structure of white or half cast iron. Sulfur is harmful impurity deteriorating the mechanical and casting properties of cast iron. Therefore, its content is limited to 0.1-0.2%. In gray cast iron, sulfur forms sulfides or their solid solutions

The phosphorus content in gray cast iron is more often but sometimes allowed even up to.With an increased phosphorus content in the structure of cast iron, solid inclusions of phosphide eutectic are formed: in gray cast irons - double austenite), and in white cast iron - triple austenite). Eutectic improves the casting properties of cast iron.

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

The fewer graphite inclusions, the finer they are and the greater the degree of their isolation, the higher the strength of cast iron. Cast iron with big amount rectilinear large graphite precipitates separating its metal base, has a coarse-grained fracture and low mechanical properties. Cast iron with fine

and swirling graphite precipitates has higher properties.

Graphite plates reduce the pull-off resistance, temporary resistance and especially strongly the ductility of cast iron. The tensile elongation of gray cast iron, regardless of the properties of the metal base, is practically zero. Graphite inclusions have little effect on reducing the ultimate strength in compression and hardness, their value is determined mainly by the structure of the metal base of cast iron. When compressed, cast iron undergoes significant deformation and destruction has the character of a cut at an angle of 45 °. The breaking load in compression, depending on the quality of cast iron and its structure, is 3-5 times higher than in tension. Therefore, cast iron is recommended to be used mainly for products working in compression.

Plates of graphite, less significantly than under tension, reduce strength in bending, as part of the product experiences compressive stresses. Flexural strength is intermediate between tensile and compressive strength. Cast iron hardness

Graphite, breaking the continuity of the metal base, makes cast iron insensitive to all kinds of stress concentrators (surface defects, notches, grooves, etc.). As a result, gray cast iron has approximately the same structural strength in simple castings or with flat surface and complex shape notched or poorly finished. Graphite increases the wear resistance and antifriction properties of cast iron due to its own "lubricating" action and increased film strength lubricant... It is very important that graphite improves the machinability by making the chips breakable.

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

Gray cast iron is marked with the letters C - gray and H - cast iron The letters are followed by numbers indicating the minimum value of the ultimate tensile strength

Gray cast irons can be divided into the following groups according to their properties and applications.

Ferritic and ferritic-pearlitic cast irons have ultimate tensile strength in bending Their approximate composition: The structure of cast irons - pearlite, ferrite and graphite, more often in the form of large precipitates

These cast irons are used for unimportant parts that experience light loads when working with a casting wall thickness of 10-30 mm. So, cast iron is used for building columns, foundation slabs, and cast irons and - for cast lightly loaded parts of agricultural machines, machine tools, automobiles and tractors, fittings, etc.

Pearlitic cast irons are used for critical castings (beds of powerful machine tools and mechanisms, pistons, cylinders, parts that wear out under high pressure conditions, compressors, valves, diesel cylinders, engine blocks, parts of metallurgical equipment, etc.) with a wall thickness of up to 60-100 mm. The structure of these cast irons is small-lamellar pearlite (sorbitol) with small swirling graphite inclusions. The so-called steel and modified cast irons belong to pearlitic ones.

When smelting steel cast iron, steel scrap is added to the charge; cast irons have a reduced carbon content, which provides a more dispersed pearlite base with a smaller amount of graphite inclusions. Approximate composition:

Modified cast irons are obtained by adding special additives - modifiers (graphite, ferrosilicon, silico-calcium in the amount of graphite, ferrosilicon, silico-calcium) to liquid iron before pouring.The modification is used to obtain in cast iron castings with different wall thicknesses a pearlite metal base with a small amount of isolated medium-sized graphite plates interspersed.

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 half cast iron, i.e. ledeburite, pearlite and graphite, is subjected to modification. Exemplary chemical composition cast iron:

To relieve casting stresses and stabilize dimensions, cast iron castings are annealed at 500-600 ° C. Depending on the shape and dimensions of the casting, exposure at the annealing temperature is. Cooling after annealing is slow, together with the furnace. After such processing, the mechanical properties change little, and the internal stresses decrease by. Sometimes To relieve stresses in cast iron castings, natural aging of cast iron is used - keeping them in a warehouse for 6-10 months; this delay reduces the voltage by 40-50%.

Antifriction cast irons are used for the manufacture of plain bearings, bushings and other parts operating under friction against metal, more often in the presence of a lubricant. These cast irons must provide low friction (low coefficient of friction), i.e. antifriction. Antifriction properties of cast iron are determined by the ratio of pearlite and ferrite in the base, as well as the amount and shape of graphite. Antifriction cast irons are manufactured in the following grades:

Parts, working in tandem with hardened or normalized steel shafts, are made of pearlitic gray cast iron for operation in tandem with thermally untreated shafts, pearlite-ferritic cast iron is used

Pearlitic cast iron containing an increased amount of phosphorus is used for the manufacture of piston rings. High wear resistance of the rings is provided by a metal base consisting of fine pearlite and uniformly distributed phosphide eutectic in the presence of isolated precipitates of lamellar graphite.

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 luster, which is clearly visible at a break. This gloss is manifested due to the fact that there are no large inclusions of graphite in the composition of such cast iron. In percentage terms, it is no more than 0.3%. Therefore, it can only be detected by spectral or chemical analysis.

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:

Hypoeutectic. These are alloys in which carbon does not exceed 4.3% of general composition... It is obtained after complete cooling. As a result, it acquires the characteristic structure of such elements as perlite, secondary cementite and ledeburite.
Eutectic. They have a carbon content of 4.3%.
Hypereutectic 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 doped.

Internal structure white cast iron is an alloy of two elements: iron and carbon. Despite the high-temperature production, it retains a fine grain structure. Therefore, if you break a part made of such a 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 be close to 100%. This is typical for eutectic metal. For the third type, the structure is a composition of eutectic (Lp) and primary cementite.

One of the varieties of these alloys is the so-called chilled cast iron. Its base, that is, the core, is gray or nodular 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 obtained from white, and then the casting consists of an ordinary gray alloy.

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

low-alloyed (they contain no more than 2.5% alloying elements);
medium-alloyed (the percentage of such elements reaches 10%);
highly alloyed (the amount of alloying additions in them exceeds 10%).

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

Properties of white cast iron

Any cast iron alloy, on the one hand, is very strong, but at the same time has sufficient fragility. Therefore, the main positive properties of white cast iron are:

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

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.

The disadvantages include:

Low casting properties. It has poor mold filling. Internal cracks may form during pouring.
Increased fragility.
Poor machinability of the castings themselves and of white iron parts.
Large shrinkage, which can reach 2%.
Low impact resistance.

Another disadvantage is poor weldability. Problems in welding parts from similar material caused by the formation of cracks at the time of welding, both during heating and cooling.

White cast iron marking

For marking white cast iron, letters of the Russian alphabet and numbers are used. If there are impurities in it, then the marking begins with the letter "CH". The composition of the available alloying additives can be determined by the subsequent 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, ICH, 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.

The CHN20D2HSH brand stands for the following. It is a heat-resistant high-alloy metal. It contains the following elements: nickel - 20%, copper - 2%, chromium - 1%. The rest of the elements are iron, carbon, spherical graphite.

Application area

This alloy is used in the following industries: machine building, machine tool building, shipbuilding. Some elements of household products are made from it. In mechanical engineering, it is used to manufacture: parts for cargo and passenger 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 plates with various surface shapes.

Bleached cast iron has a fairly limited field of application. Parts of a simple configuration are made from it. 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 direction. Specific feature their job is that they are constantly exposed to abrasive material.

Gray cast iron has low mechanical characteristics. sv-in during tensile tests. Graphite inclusions act as stress concentrators. The hardness and strength in compression tests, depending on the properties of the metal base, are quite high in cast iron. Gray cast iron with lamellar graphite has a number of advantages. It allows you to get cheap casting, because at a low cost, it has good fluidity and low shrinkage. Fur. Holy Islands of gray cast irons depend on the metal base, as well as the shape and size of the graphite inclusions. The most durable are gray cast irons on a pearlite basis, and the most ductile are gray cast irons on a ferritic basis. Gray cast iron is obtained by adding substances to the molten metal that promote the decomposition of cementite and the release of carbon in the form of graphite. For gray cast iron, silicon is the graphitizer. When about 5% of gray cast iron cementite is introduced into the silicon alloy, it almost completely decomposes and a structure of a plastic ferrite base and graphite inclusions is formed. With a decrease in the silicon content, cementite, which is part of the pearlite, partially decomposes and a ferrite-pearlite structure with graphite inclusions is formed. With a further decrease in the silicon content, a structure of gray cast iron on a pearlite base with inclusions of graphite is formed.

Graphite inclusions make the chips brittle, hence, cast iron is well cut. Due to the lubricating effect of graphite, cast iron has good antifriction properties. Cast iron has high damping properties, well dampens vibrations and resonant vibrations. Marked gray cast iron with letters СЧ and numbers characterizing the value of the ultimate strength during tensile tests. Nr, SCH10 contains (3.5 ... 3.7)% C, (2.2 ... 2.6)% Si, (0.5 ... 0.8)% Mn, P<0,3% и S<0,15%, d В =100МПа, твёрдость <190НВ. SCH35 d B = 350MPa, hardness<275НВ.

Gray cast irons - it is foundry iron. Gray iron goes into production in the form of castings. Gray cast iron is a cheap construction material. It possesses good casting properties, is well machined by cutting, resists wear, has the ability to dissipate vibrations under vibration and alternating loads. The vibration damping property is called damping ability. The damping rate of cast iron is 2-4 times higher than that of steel. High damping sp-th and wear resistance led to the use of cast iron for the manufacture of beds for various equipment, crankshafts and camshafts of tractor and automobile engines, etc. The following grades of gray cast irons are produced (in parentheses, the numerical values of HB hardness are indicated): SCh 10 (143-29), SCH 15 (163-229), SCH 20 (170-241), SCH 25 (180-250), SCH 30 (181-255), SCH 35 (197-269), SCH 40 (207-285), SCH 45 (229-289).

According to their physical and mechanical characteristics, gray cast irons can be conditionally divided into four groups: low strength, increased strength, high strength and with special properties.

Alloyed gray cast iron has a fine-grained structure and a better structure of graphite due to the addition of small amounts of nickel and chromium, molybdenum and sometimes titanium or copper.

Modified gray cast iron has a uniform structure over the section of the casting and a finer swirling shape of graphite. Modifiers - ferrosilicon, silicoaluminium, silicocalcium, etc. - are added in an amount of 0.1-0.3% of the mass of cast iron directly into the ladle during its filling.

Gray and white cast irons differ sharply in properties. White cast irons very hard and brittle, poorly processed with a cutting tool, they are melted into steel and are called pig irons. Part of the white cast iron is used to produce ductile iron.

White cast irons are used as wear-resistant materials of construction. In such cast irons, all carbon is bound to carbide-forming elements (chromium, manganese, boron, titanium). With the introduction of 5-8% Cr, a cementite type carbide (Fe, Cr) 3 C is formed, and with a content of more than 10% Cr, complex and hard carbides (Fe, Cr) 7 C 3 and (Fe, Cr) 23 C 6 are formed. To make cast iron more viscous, heat- or corrosion-resistant, nickel is introduced into its composition.

Alloys of iron with 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 simultaneously in the form of cementite and graphite. Cementite imparts a specific light luster to a fracture; therefore, cast iron, in which all the carbon is in the form of cementite, is called white. Graphite gives the cast iron a gray color. Depending on the shape of graphite and the conditions of its formation, the following groups of cast irons are distinguished: gray, high-strength with nodular graphite, and malleable.

Gray cast iron. Gray cast iron (technical) is essentially an Fe - Si - C alloy containing Mn, P and S as inevitable impurities. In the structure of gray cast irons, most or all of the carbon is in the form of graphite. A characteristic feature of the structure of gray cast irons, which determines many of its properties, is that graphite has a plate-like shape in the field of view of a microsection. The most widely used are hypoeutectoid cast irons containing 2.4 - 3.8% C. 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 H - cast iron (GOST 1412 - 70). The letters are followed by numbers. The first numbers indicate the average tensile strength and the second the average flexural strength. Flexural strength is used to evaluate the ductility of cast iron, since the 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, brittleness and practically does not lend itself to cutting, therefore it has limited application. Cast iron castings are called bleached, in which the surface layers have the structure of white (or half) cast iron, and the core is gray cast iron. There may be a transition layer between these zones. Bleached to a certain depth (12 - 30 mm) is a consequence of the rapid cooling of the surface resulting from the casting of cast iron into metal molds (chill mold) or into a sand mold. High surface hardness (HB 400-500) provides good resistance against wear, especially abrasive wear; hollow of chilled cast iron is used to make rolling rolls of sheet mills, wheels, balls for mills, etc. In this case, cast iron with a low silicon content is used, which is prone to to whitening. Its approximate composition: 2.8-3.6% C; 0.5-0.8% Si; 0.4-0.6% Mp. Due to the different cooling rates over the section and the production of different structures, the casting has high internal stresses that can lead to cracking. To relieve stresses, the castings are subjected to heat treatment, i.e. they are heated at 500-550 C.

Gray, ductile and ductile irons are materials in which all or part of the carbon is in the form of graphite. The fracture of these cast irons is gray, matte. Their structure is distinguished: the structure of the metal base and the precipitation of graphite. They differ from each other only in the form of graphite precipitates.

In gray cast irons, graphite is released in the form of plates (veins, flakes); in high-strength - in the form of balls; in malleable - in the form of flakes (Fig. 4.2).

Lamellar graphite. In ordinary gray cast iron, graphite forms in the form of petals; such graphite is called lamellar. In fig. 4.2, a shows the structure of a conventional ferritic cast iron with streaks of graphite; The spatial view of such graphite inclusions is shown in Fig. 4.3, a(you can see the intersection of lamellar inclusions by the plane of the thin section).

Nodular graphite... In modern so-called high-strength cast irons, smelted with a small amount of magnesium (or cerium) additives, graphite takes on the shape of a ball. In fig. 4.2, b shows the microstructure of gray nodular cast iron, and Fig. 4.3, b- photograph of a spherical graphite inclusion in an electron microscope.

Flaky graphite. If white cast iron is obtained during casting, and then, using the instability of cementite, by annealing it is decomposed, the resulting graphite acquires a compact, almost equiaxial, but not round shape. Such graphite is called flake or annealed carbon. The microstructure of flaky graphite iron is shown in Fig. 4.2, v... In practice, flake graphite iron is called ductile iron.

a B C D

Rice. 4.2. The shape of graphite in cast irons:

a- lamellar (ordinary gray cast iron), × 100; b- spherical (high-strength cast iron), × 200; v- flaky (malleable cast iron), × 100; G- vermicular, × 100

Rice. 4.3. Graphite inclusions in cast iron (× 2000):

a- lamellar; b- spherical

Vermicular graphite- in the form of worm-like veins (Fig.4.2, G).

Thus, cast irons are called:

- with lamellar graphite, ordinary gray cast iron;

- with worm-like graphite - gray vermicular cast iron;

- nodular cast iron - ductile iron;

- cast iron with flaky graphite - ductile iron.

According to the structure of the metal base, all cast irons are classified:

1) for ferritic ones - with the structure of ferrite and graphite (the amount of bonded carbon C bond = 0.025%);

2) ferrite-pearlite - with the structure of ferrite, pearlite and graphite (the amount of C bond = from 0.025 to 0.8%);

3) pearlite - with the structure of pearlite and graphite (the amount of C bond = 0.8%).

Hence, we can conclude that the metal base in this group of cast irons is similar to the structure of eutectoid and hypoeutectoid steel and iron and differs only in the presence of graphite inclusions (carbon in a free state), which predetermine the specific properties of cast irons.

a B C

Rice. 4.4. Microstructure of gray cast iron:

a- pearlite, × 200; b- ferrite-pearlite, × 100; v- ferritic, × 100

The structure of pearlitic cast iron consists of pearlite with graphite inclusions (Figure 4.4, a- graphite in the form of streaks; typical for gray cast iron). Pearlite contains 0.8% C, therefore, this amount of carbon in gray pearlitic cast iron is in a bound state (that is, in the form of Fe 3 C), the rest is in free form, that is, in the form of graphite.

Ferrite-pearlitic cast iron (Fig.4.4, b) consists of ferrite and pearlite + inclusions of fusiform graphite. In this cast iron, the amount of fixed carbon is less than 0.8% C.

In ferritic cast iron (Fig.4.4, v) the metal base is ferrite, and all of the carbon in the alloy is in the form of graphite (shown as spindle-shaped graphite in the photograph).

The structure diagrams (Table 4.1) summarize the above-described classification of cast iron according to the structure of the metal base and the shape of graphite.

Gray cast irons. Gray cast irons, like white ones, are obtained directly during casting (during crystallization from a liquid melt). Since the formation of graphite from a liquid is a slow process (the work of nucleation is large: a significant diffusion of carbon atoms and the removal of iron atoms from the crystallization front of graphite are required), it is possible only in a narrow temperature range. Consequently, the cooling of gray cast iron is carried out slowly, and cementite, released from a liquid or solid solution, being an unstable chemical compound, especially at high temperatures, decomposes to form graphite:

Fe 3 C ® Fe γ (C) + C g at temperatures above 727 ° C

Fe 3 С ® Fe α (С) + С g at temperatures below 727 ° С (below the PSK line).

With the acceleration of the cooling of cast iron, the probability of the formation of graphite in it decreases, and at a certain cooling rate, part of the alloy can crystallize in accordance with the stable, and part, for example, the surface layer, with metastable diagrams. Iron castings, in which the surface layers have the structure of white iron, and the core is gray, are called bleached. I bleached them to a certain depth - a consequence of the faster cooling of the surface. Therefore, a prerequisite for producing gray cast iron is a very low melt cooling rate.

Graphite in gray cast iron is precipitated in the form of plates. Lamellar inclusions of graphite in gray cast irons can be considered as cracks, notches, creating high stress concentrations in the metal base. Therefore, the properties of these cast irons are very different from those of steel.

To determine the presence of graphite and the shape of its inclusions, an un-etched microsection is examined using a metallographic microscope. Graphite looks like a dark phase against a light background of a polished metal base, then the microsection is etched (with a 3-5% solution of HNO 3 in alcohol) and the structure of the metal base is established.

According to the degree of graphitization, several types of gray cast irons are distinguished: pearlite, pearlite-ferritic and ferritic cast iron. If the amount of fixed carbon is more than 1%, such cast iron is called half iron. Its structure is composed of ledeburite, pearlite and graphite.

Table 4.1

Cast iron structure diagrams

However, in addition to the cooling rate, the amount of impurities, alloying elements and crystallization centers (modifiers) present has a significant effect on the graphitization process.

All elements introduced into cast iron are divided:

1) elements that prevent graphitization (Mn, Cr, W, Mo, S, O 2, etc.), which contribute to the production of carbon in a coherent state in the form of alloyed cementite and other carbides and prevent its decomposition at elevated temperatures;

2) graphite-forming elements (Si, C, Al, Ni, Cu, etc.), which contribute to the production of carbon in a free state in the form of graphite.

Impurities Mn, Si, S, P, present in cast iron, mainly affect the graphitization process, and, consequently, the structure and properties of cast iron.

To determine what structure should be expected depending on the total content of carbon and silicon, as well as depending on the cooling rate (wall thickness of the casting), use the structure diagram (Fig. 4.5).

Rice. 4.5. Effect of cooling rate and total silicon content

and carbon in cast iron on its structure:

I - white cast irons; II - gray pearlitic cast irons; III - gray ferritic cast irons

Therefore, in order to avoid chilling of cast iron, thin-section parts are cast from cast iron with a high content of graphite-forming elements (Si, Ni, C). For the casting of large section parts, cast iron with a lower content of these elements can be used.

The size and shape of the precipitated graphite inclusions also depend on the presence of crystallization centers in liquid iron.

Crystallization centers can be the smallest particles of oxides Al 2 O 3, CaO, SiO 2, MgO, etc. The impact on the graphitization process through the formation of additional crystallization centers is called modification, and the elements themselves are called modifiers. Modifiers are introduced into the liquid iron prior to pouring it.

Gray cast iron has poor mechanical properties, since the graphite plates cut through the metal base.

Depending on the strength of the base metal and the amount of graphite, gray cast irons can have a tensile strength of about 100 to 400 MPa with virtually zero elongation. In compression, gray cast irons work much better than in tension, since under compressive loads, the notching effect of graphite plates is insignificant.

According to GOST 1412-70, there are 11 grades of gray cast iron: SCh00 (not tested); SCH12-28; SCh15-52; SCH18-36; SCH21-40; SCh24-44; SCh28-48; SCH32-52; SCHZ6-56; SCH40-60; SCh-44-64.

The first number shows the tensile strength and the second shows the flexural strength in kg / mm 2.

Cast iron grade SCh12-28 is characterized by a ferritic metal base.

Cast iron grades SCH15-52, SCH18-36 - ferrite-pearlite metal base.

Cast irons of these grades are used for unimportant parts with light loads (building columns, foundation slabs, brackets, flywheels, gear wheels).

The rest of the grades have a pearlite metal base with a reduced carbon and silicon content. Cast irons with a pearlite base are used for critical parts that wear out at high pressures (machine beds, pistons, cylinders, parts for compressor, turbine and metallurgical equipment). Gray cast iron of the indicated grades is necessarily modified with silicocalcium or ferrosilicon, which contains about 2% calcium, or other additives in order to prevent primary crystallization according to the metastable diagram.

Ductile iron. Ductile iron is produced by modifying a liquid melt with magnesium or cerium. Magnesium and cerium are introduced in relatively small amounts: 0.1 - 0.2% by weight of the liquid iron being modified. Magnesium and cerium contribute to the formation of nodular graphite inclusions (Fig.4.2, b, 4.3, b).

Spheroidal graphite can be formed during primary crystallization, as well as during the annealing of white modified cast iron. Of course, the most desirable is the formation of nodular graphite directly during primary crystallization, since in this case high-temperature annealing is not required. In addition, the formation of graphite in the structure during primary crystallization sharply reduces the shrinkage of the alloy. And this, in turn, greatly simplifies the casting technology.

Ductile cast irons are marked with the letters HF and subsequent numbers.

The first two digits of the brand show the average value of the tensile strength in kg / mm 2, the second - the relative elongation in percent. For example, cast iron grade VCh60-2 has a tensile strength σ = 600MPa; elongation δ = 2%.

According to GOST 7293-70, 9 grades of ductile iron are provided.

Castings of these cast irons are used in auto and diesel engineering for crankshafts, cylinder covers; in heavy engineering - for parts of rolling mills; in forging and pressing equipment - for traverses of presses, rolling rolls; in the chemical and oil industry - for pump casings, valves, etc. They are also used for parts operating in bearings and other friction units at high and high pressures (up to 1200 MPa).

Ductile iron. Malleable cast irons are obtained by special graphitizing annealing (languishing) of white hypoeutectic cast irons containing from 2.27 to 3.2% C.

A significant drawback of the process of obtaining malleable iron is the duration of annealing, which is 70 - 80 hours. Various measures are used to accelerate it (modification with aluminum (less often with boron, bismuth), raising the temperature of the first stage (but not higher than 1080 ° C)).

At present, a method has been developed for accelerated annealing of malleable iron, which consists in the fact that castings from white iron are pre-quenched before graphitizing annealing, which helps to reduce the duration of annealing to 30 - 60 hours.

The production schedule for ductile iron is shown in Fig. 4.6.

Rice. 4.6. Ductile iron production schedules

To obtain ductile iron, you must:

- castings from low-carbon white iron, containing no more than 2.8% carbon, slowly heat for 20 - 25 hours in a neutral environment to a temperature of 950 - 1000 ° C and hold at this temperature for a long time (10 - 15 hours) (the first stage graphitization);

- then slowly cool to a temperature slightly below the eutectoid transformation (700 - 740 ° C, depending on the composition of cast iron and hold it for a long time (30 hours) at this temperature (the second stage of graphitization);

- carry out air cooling.

At the first stage of graphitization, ledeburite cementite and secondary cementite decompose with the formation of austenite and flaky graphite according to the reaction:

Fe 3 C ® Fe γ (C) + C

Cementite = austenite + graphite

When cooling from the first to the second stage of graphitization, the cooling rate should ensure the separation of secondary cementite from austenite and its decomposition into austenite and graphite according to the above formula.

At the second stage of graphitization, pearlite cementite decomposes into ferrite and graphite according to the reaction:

Fe 3 C ® Fe α (C) + C

Cementite = ferrite + graphite

The finished structure will be composed of ferrite and flake graphite.

The duration of the entire heat treatment is 70 - 80 hours.

If, at the second stage of graphitization, the exposure for the complete decomposition of pearlite cementite into ferrite and graphite is insufficient, then in this case ferrite-pearlitic ductile iron is obtained; if there is no holding at all, pearlitic malleable cast iron with a pearlite structure and flaky graphite is obtained.

It is desirable that the carbon content in ductile iron be low, since with an increase in the carbon content, the amount of free graphite after annealing of the cast iron increases and its properties deteriorate. However, reducing the carbon content raises the melting point, creates difficulties in casting, increases the cost of casting, etc.

To obtain pearlitic ductile iron, cupola white cast iron with a carbon content of up to 3.2% is sometimes used. Annealing is carried out in a decarburizing (oxidizing) environment, followed by cooling in air. This annealing provides significant carbon burnout.

Malleable cast irons are marked with letters KCH with numbers. The first two digits indicate the tensile strength in kg / mm 2, the second digits indicate the elongation in percent.

According to GOST 1215-59, malleable cast irons have the following grades:

- ferritic cast iron: KCh37-12, KCh35-10, KCh33-8, KCh30-6;

- ferrite-pearlite and pearlitic malleable cast irons: KCh45-6, KCh50-4, KCh56-4, KCh60-3, KCh63-2.

Ductile iron castings resist shock and vibration loads well, cut well, and have sufficient toughness.

Malleable cast iron is used in the automotive, tractor industry, agricultural engineering, car and machine tool building for high-strength parts that perceive alternating and shock loads, operating under conditions of increased wear. Its widespread use is due, first of all, to the good casting properties of the original white cast iron, which makes it possible to obtain thin-walled castings of complex shapes. Ferritic malleable cast irons are used for the manufacture of parts operated under high dynamic and static loads (gearbox craters, hubs, hooks, brackets) and for less critical ones (nuts, mufflers, flanges, couplings). Conveyor chain links and rollers, brake pads, etc. are made of malleable perlite cast iron.

Work order

1. Study the classification of cast irons, their structure, marking and methods of production.

2. Examine thin sections under a microscope and indicate which type of cast iron each sample belongs to.

3. Determine the conditions for obtaining the studied structure.

4. Establish the influence of each structural component on the properties of cast iron.

5. Etch thin sections and study the microstructure under a microscope, sketch, indicate the structural and phase components.

6. Set the difference in the properties of the considered structures.

7. Make a summary table of the considered structures, enter the obtained data in table. 4.2.

8. Make a progress report.

When drawing up a report, you must:

1) give a brief classification of cast irons;

2) define white, gray, ductile and ductile iron;

3) draw a part of the diagram Fe - Fe 3 C, which refers to the field of cast irons;

4) sketch all the examined structures of cast irons before and after etching, indicating the names of the structural components and the class of cast irons;

5) indicate the chemical composition of white cast irons and their position on the diagram;

6) describe the methods of obtaining, properties and scope of each type of cast iron; indicate the marking.

Data on the work done summarize in table. 4.2.

Table 4.2

Control questions

1. What are the advantages of cast irons over steel?

2. How are cast irons classified?

3. What are the characteristics of the structure and properties of cast iron?

4. How does the shape of graphite affect the properties of cast irons?

5. How much carbon does cast iron contain?

6. What types of carbon can be found in cast irons?

7. In which cast irons all carbon is chemically bound?

8. In which cast irons all or part of the carbon is in the form of graphite?

9. Methods of obtaining, properties and use of white cast irons.

10. How do you get white cast iron?

11. How much graphite is in white cast iron?

12. What elements contribute to whitening?

13. What elements contribute to graphitization?

14. What is the structure of hypoeutectic white cast iron?

15. What is the structure of eutectic white cast iron?

16. What is the structure of hypereutectic white cast iron?

17. What is Ledeburite?

18. What determines the strength of gray cast iron?

19. How do you get gray cast iron?

20. What is the structure of the metal base of gray cast irons?

21. Is malleable iron good forging?

22. How is ductile iron obtained?

23. What processes are going on at the first stage of graphitization (production of ductile iron)?

24. What processes are going on at the second stage of graphitization (production of ductile iron)?

25. What is the shape of graphite in ductile irons?

26. The structure of ductile iron:

27. How do you get ductile iron?

28. The structure of ductile iron:

29. What is the shape of graphite in ductile irons?

30. What is modification and for what purpose is it used?

31. What is the shape of graphite in gray cast irons?

32. Structure of gray cast iron

33. Marking of gray, ductile and ductile irons.

34. What does the number in the grade of cast iron SCH15 mean?

35. What does the number in the VCh60 cast iron brand mean?

36. What does the number 30 mean in the cast iron grade KCh 30-6?

37. What does the number 6 mean in the cast iron grade KCh 30-6?

The letter A in the middle of the brand designation indicates the presence of nitrogen specially introduced into the steel.

The letter A at the beginning of the brand designation indicates that this is an automatic steel intended for the manufacture of parts for mass production on automatic machines (AI2, A30, A40G - sulphurous; ACI4, AS40, AS35G2 - lead-containing; A35E, A40XV - sulphurous; AC20, AC40G - calcium-containing). The numbers indicate the average carbon content in hundredths of a percent.

Not to be confused with hardenability , which is characterized by the maximum value of the hardness acquired by the steel as a result of hardening. The hardenability depends mainly on the carbon content (see Fig. 6 of laboratory work No. 8).

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