Thermal processing of cast iron.

Gray, high-strength and forging cast iron belong to the materials in which the entire carbon or its part is in the form of graphite. The breakdown of these castoes is gray, matte. The structure differences in their structure: the structure of the metal base and selection of graphite. They differ from each other only the form of graphite selection.

In gray cast iron, graphite is highlighted in the form of plates (streaks, scales); in high strength - in the form of balls; in dagging - in the form of flakes (Fig. 4.2).

Plate graphite.In the usual gray cast iron, graphite is formed in the form of petals; Such graphite is called a lamellar. In fig. 4.2, and The structure of ordinary ferritic cast iron with graphite streaks is shown; The spatial form of such graphite inclusions is shown in Fig. 4.3, and (Seen the intersection of the plate inclusions with the plane of the grind).

Ball-shaped graphite. In modern so-called high-strength cast irons, splashed with a slight amount of magnesium (or cerium), graphite acquires the shape of the ball. In fig. 4.2, b. The microstructure of gray cast iron with spherical graphite is shown, and in fig. 4.3, b. - Photo of spherical graphite inclusion in the electron microscope.

Flake graphite.If you get white cast iron when casting, and then, using the instability of cementite, to decompose it using annealing, the forming graphite acquires a compact, almost equaporyable, but not rounded shape. Such graphite is called flakes, or carbon annealing. The microstructure of cast iron with flakes is shown in Fig. 4.2, in. In practice, cast iron with flourishing graphite is called dyeing cast iron.

a B C D

Fig. 4.2. Form graphite in cast iron:

and - lamellar (ordinary gray cast iron), × 100; b. - spherical (high-strength cast iron), × 200; in - flue-shaped (macaque cast iron), × 100; g. - vermicular, × 100

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

and - lamellar; b. - Schrovoid

Vermicular graphite - in the form of wist-shaped streaks (Fig. 4.2, g.).

Thus, cast iron is called:

- with lamellar graphite with ordinary gray cast iron;

- with a heart-shaped graphite - gray vermicular cast iron;

- cast iron with spherical graphite - high-strength cast iron;

- cast iron with flourishing graphite - damp cast iron.

According to the structure of the metal base, all cast iron is classified:

1) for ferritic - with the structure of ferrite and graphite (the amount of associated carbon with the connection \u003d 0.025%);

2) ferrite-pearlite - with the structure of ferrite, perlite and graphite (the amount with the connection \u003d from 0.025 to 0.8%);

3) pearlite - with a perlit and graphite structure (the amount with the connection \u003d 0.8%).

From here, it is possible to conclude that the metal basis in this group of iron is similar to the structure of eutectoid and dashing steel and iron and differs only by the presence of graphite inclusions (carbon in free state), which predetermining the specific properties of cast iron.

a B C

Fig. 4.4. Microstructure of gray cast iron:

and - pearlite, × 200; b. - ferrito-pearlite, × 100; in - ferritic, × 100

The structure of the pearline cast iron consists of a perlite with the inclusions of graphite (Fig. 4.4, and - graphite in the form of a strey; Typically for gray cast iron). Perlite contains 0.8% C, therefore, this amount of carbon in gray perlite cast iron is in the associated state (i.e., in the form of Fe 3 C), the rest is in a free form, that is, in the form of graphite.

Ferrito-pearlic cast iron (Fig. 4.4, b.) It consists of ferrite and perlite + inclusion of spindle-shaped graphite. In this cast iron, the number of connected carbon is less than 0.8% C.

In ferritic cast iron (Fig. 4.4, in) The metal base is ferrite, and the entire carbon existing in the alloy is present in the form of graphite (in the form of spindle-shaped graphite).

In the schemes of structures (Table 4.1), the classification of cast iron on the structure of the metallic base and the shape of graphite is described above.

Gray cast iron.Gray cast iron, like white, is obtained directly when casting (when crystallization from a liquid melt). Since the formation of graphite from a liquid is a slow process (the work of the formation of the embryo is large: they require significant diffusion of carbon atoms and the removal of iron atoms from the edge of graphite crystallization), then it is possible only in a narrow temperature range. Consequently, the cooling of the gray cast iron is slow, and cementite, released from a liquid or solid solution, being an unstable chemical compound, especially at high temperatures, decomposes with the formation of graphite:

Fe 3 C ® Fe Γ (C) + C cgra at temperatures above 727 ° C

Fe 3 C ® FE α (C) + s c at temperatures below 727 ° C (below the PSK line).

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

Graphite in the gray cast iron is highlighted in the form of plates. Plastic inclusions of graphite in gray cast iron can be considered as cracks, cuts that create large stress concentrations in a metallic basis. Therefore, the properties of these castoffs are very different from the properties of steel.

To determine the presence of graphite and the shape of its inclusions, a rapid microclide is investigated using a metallographic microscope. Graphite looks like a dark phase on a light background of polished metal base, then the microchelf is etched (3-5% HNO 3 solution in alcohol) and set the structure of the metal base.

According to the degree of graphitization, several types of gray castoffs are distinguished: pearlit, pearly ferrite and ferrite cast iron. If the amount of associated carbon will be greater than 1%, such cast iron is called half. Its structure consists of laburita, perlite and graphite.

Table 4.1.

Schemes of the structures of the cast iron

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

All items entered into cast iron are divided:

1) on elements that prevent graphitization (Mn, Cr, W, MO, S, O 2, etc.), which contribute to the preparation of carbon in a coherent state in the form of doped cementite and other carbides and prevent the decay of it at elevated temperatures;

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

The impurities Mn, Si, S, P, present in cast iron, mainly affect the graphitization process, and therefore, the structure and properties of the cast iron.

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

Fig. 4.5. Effect of cooling rates and total silicon content

and carbon in cast iron on its structure:

I - white cast iron; II - gray pearlic cast iron; III - gray ferrite cast iron

Therefore, in order to avoid the bone of the cast iron, the details of the thin section are cast from cast iron with an elevated content of graphically forming elements (Si, Ni, C). For casting parts of large cross section, cast iron can be applied with a lower content of these elements.

The magnitude and form of excluding graphite inclusions also depends on the presence of crystallization centers in liquid cast iron.

Crystallization centers can be the smallest particles of Al 2 O 3 oxides, Cao, SiO 2, MGO, and others. Impact on the graphitization process using the formation of additional crystallization centers is called modifying, and the elements themselves are called modifiers. Modifiers are introduced into liquid cast iron before its casting.

Gray cast iron has low mechanical properties, since the plates of graphite cut the metal base.

Depending on the strength of the metal base and the amount of graphite, gray cast irons may have a tensile strength of approximately 100 to 400 MPa with a practically zero value of relative elongation. On compression, gray cast irons work much better than the stretching, since, with compressive loads, the cutting effect of graphite plates is insignificant.

According to GOST 1412-70, it distinguishes 11 grades of gray cast iron: the sch00 (not tested); Sch12-28; Sch15-52; Sch18-36; CC21-40; Sch24-44; Sch28-48; Sch32-52; Cc6-56; Sch40-60; Sch-44-64.

The first digit shows the tensile strength limness, and the second is the tensile strength of bending in kg / mm 2.

Mark Country MC12-28 is characterized by a ferritic metallic basis.

Marks of cast iron sc15-52, sch18-36 - ferrito-pearlite metal base.

Castings of these brands are used for inappropriate parts with low loads (construction columns, foundation plates, brackets, flywheels, gear wheels).

The remaining brands have a pearlite metal base with a reduced content of carbon and silicon. Castings with a pearlite basis are used for responsible details working for wear at high pressures (machine tools, pistons, cylinders, parts of compressor, turbine and metallurgical equipment). The gray cast iron of these stamps is necessarily modified by silicocalciem or ferrosilicia, which contains about 2% calcium, or other additives in order to prevent primary crystallization over the metastable diagram.

High-strength cast iron. High-strength cast iron is obtained by modifying the liquid melt magnesium or cerium. Magnesium and cerium are introduced in relatively small quantities: 0.1 - 0.2% by weight of liquid cast iron, subjected to modification. Magnesium and cerium contribute to the formation of globular graphite graphite (Fig. 4.2, b., 4.3, b.).

Frame graphite can be formed in the primary crystallization process, as well as in the process of annealing white modified cast iron. Of course, the most desirable formation of spherical graphite directly during primary crystallization, since in this case there is no high-temperature annealing. In addition, graphite education in the primary crystallization structure sharply reduces alloy shrink. And this in turn significantly simplifies casting technology.

High-strength cast iron letters of HF and subsequent figures are labeled.

The first two figures of the brand show the average value of the tensile strength in kg / mm 2, the second is a relative lengthening in percent. For example, the cast iron of the VCh60-2 brand has a tensile strength σ \u003d 600MP; Relative elongation δ \u003d 2%.

According to GOST 7293-70, 9 grades of high-strength cast iron are provided.

Castings of these castors are used in auto and diesel stations for crankshers, covers of cylinders; In severe engineering - for the details of rolling mills; In forge-free press equipment - for traverse presses, rolling rolls; In the chemical and oil industry - for pumps for pumps, valves, etc. Also, they are also used for parts operating in bearings and other friction nodes at elevated and high pressures (up to 1200 MPa).

Dake cast iron. Dyeing cast irons are obtained by a special graphitizing annealing (taking) of white do-buttectic cast iron containing from 2.27 to 3.2% C.

A significant disadvantage of the process of obtaining forging cast iron is an annealing duration of 70 - 80 hours. For its acceleration, various measures are used (aluminum modification (less often boring bismuth), an increase in the temperature of the first stage (but not higher than 1080 ° C)).

Currently, a method of accelerated annealing forging cast iron has been developed, which consists in the fact that the castings from white cast iron in front of the graphitizing annealing are pre-hardened, which helps to reduce the duration of annealing to 30 - 60 hours.

The graph of the forging cast iron is shown in Fig. 4.6.

Fig. 4.6. Graphs for getting forged cast iron

For the forging cast iron, it is necessary:

- Castings from low-carbon white cast iron containing no more than 2.8% carbon, slowly heat for 20-5 hours in a neutral medium to a temperature of 950 - 1000 ° C and at this temperature for a long time (10 - 15 hours) to withstand (the first stage graphitization);

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

- keep cooling in air.

At the first stage of graphitization, cementitis Larburita and secondary cement are decaying with the formation of austenite and flake graphite by reaction:

Fe 3 C ® FE Γ (C) + C

Cement \u003d Austenit + graphite

When cooled from the first to the second stage of graphitization, the cooling rate should ensure the release of secondary cementitis from austenite and its decay to austenite and graphite according to the formula above.

At the second stage of graphitization, the cementitis perlite is disintegrated by ferrite and reaction graphite:

Fe 3 C ® FE α (C) + C

Cement \u003d ferrite + graphite

The structure after final processing will consist of ferrite and flake graphite.

The duration of all thermal processing is 70 - 80 hours.

If, with the second stage of graphitization, the excerpt for the complete decay of cementitis perlite to ferrite and graphite will be insufficient, then in this case ferrito-pearl-pearly cast iron is obtained; If the excerpts are not at all, a pearl-duck cast iron is obtained with a perlit structure and flake graphite.

It is desirable that the carbon content in the forging cast iron is low, since the amount of free graphite increases with increasing carbon content after the annealing of cast iron and its properties deteriorate. However, a decrease in carbon content increases the melting point, creates difficulties in casting, increases the cost of casting, etc.

To obtain pearlny macaw cast iron, a combustible white cast iron is used with a content of up to 3.2% carbon. Annealing is produced in a decarburing (oxidative) medium, followed by air cooling. Such annealing provides significant carbon burning.

Matching cast iron marked with digits with numbers. The first two digits indicate the tensile strength in kg / mm 2, the second digits are a relative lengthening in percent.

According to GOST 1215-59, damp cast iron has the following stamps:

- ferrite cast iron: kch37-12, kch35-10, kch33-8, kch30-6;

- Ferrito-pearlit and pearlized macaque cast iron: KCh45-6, KCh50-4, KCh56-4, KCh60-3, KCh63-2.

Movie cast iron castings are well resisting shocks and vibration loads, processed with cutting, have sufficient viscosity.

Dake cast iron is used in the automotive, tractor industry, agricultural engineering, carbonate, machine-tooling for parts of high strength, perceive alternate and shock loads operating under conditions of increased wear. Its widely applied is primarily due to the good casting properties of the original white cast iron, which allows to obtain thin-walled castings of a complex shape. Ferritic forging cast iron go to the manufacture of parts operated at high dynamic and static loads (gearbox craters, hubs, hooks, brackets) and for less responsible (nuts, silencers, flanges, couplings). From the pearl ductile cast iron, links and rollers of the conveyor chains, brake pads, etc. are made.

Procedure for performing work

1. Examine the classification of iron, their structure, labeling and methods of obtaining.

2. Explore the screams under the microscope and specify, to which type of castunov belongs to each sample.

3. Determine the conditions for obtaining the structure being studied.

4. Install the effect of each structural component on the properties of the cast iron.

5. Prove the grinds and examine the microstructure under the microscope, sketch, indicate the structural and phase components.

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

7. Make a consolidated table of the considered structures obtained data to bring in Table. 4.2.

8. Make a report on the work done.

When drawing up a report, it is necessary:

1) lead a brief classification of cast iron;

2) give the definition of white, gray, high-strength and dye cast iron;

3) draw part of the Fe - Fe 3 C diagram, which refers to the area of \u200b\u200bthe castoffs;

4) to draw all the viewed structures of cast iron before and after etching, indicating the names of the structural components and the class of cast iron;

5) specify the chemical composition of white cast iron and their position in the chart;

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

Data on the work done to reduce in Table. 4.2.

Table 4.2.

test questions

1. What are the advantages of cast iron before steel?

2. How are cast iron classified?

3. What is characterized by the structure and properties of the cast iron?

4. How does the form of graphite affect the properties of the castoffs?

5. How many carbon contains cast iron?

6. In which species can be carbon in cast iron?

7. In which castles the entire carbon is in a chemically related state?

8. In which castles the whole carbon or part of it is in the form of graphite?

9. Methods for obtaining, properties and use of white cast iron.

10. How do white cast iron get?

11. How many graphite in white cast iron?

12. What elements contribute to beat?

13. What elements contribute to graphitization?

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

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

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

17. What is iceburtie?

18. What determines the strength of the gray cast iron?

19. How do Gray cast iron get?

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

21. Is there a carpet of cast iron well?

22. How do missing cast iron get?

23. What processes go to the first stage of graphitization (receiving forging cast iron)?

24. What processes go at the second stage of graphitization (receiving forged cast iron)?

25. What is the form of graphite in forged cast iron?

26. Structure of forging cast iron:

27. How do high-strength cast iron get?

28. Structure of high strength cast iron:

29. What is the form of graphite in high-strength cast iron?

30. What is modifying and for what purpose does it apply?

31. What is the form of graphite in gray cast iron?

32. Structure of gray cast iron

33. Marking of gray, high-strength and tailing casts.

34. What do the figure in the cast iron MCC15?

35. What does the figure in the brand of the cast iron RF60?

36. What does the number 30 in the cast iron brand KC 30-6?

37. What does the number 6 indicate in the cast iron brand KCh 30-6?

The letter and in the middle of the march designation indicates the presence of nitrogen specifically entered into steel.

The letter A at the beginning of the march notation indicates that it is automatic steel, intended for the manufacture of parts of mass production on machine machines (AI2, A30, A40G - sulfur; ACI4, AC40, AC35G2 - Twin-containing; A35E, A40HB - sulfur-separable; ATS20, AC40G - calcium-containing). Figures indicate the average carbon content in hundredths of interest.

Should not be confused with hardening , which is characterized by the maximum value of hardness acquired by steel as a result of quenching. Hardening depends mainly on the carbon content (see Fig. 6 laboratory work number 8).

Similar information.

Among the most common types of cast iron are gray and white. What is each of them?

What is a gray cast iron?

The appropriate type of cast iron refers to the most common in the field of mechanical engineering. This metal is characterized by the presence of a plate-form graphite in the graffitivity. Its content in gray cast iron can be different. The more, the more dark it becomes metal on the break, as well as the softer cast iron. Castings from the type of metal under consideration can be produced any thickness.

The main features of gray cast iron:

1. minimal relative elongation - as a rule, not exceeding 0.5%;
2. low shock viscosity;
3. low plasticity.

In the gray cast iron there is a small percentage of associated carbon - no more than 0.5%. The remaining part of the carbon is represented in the form of graphite - that is, in a free state. Gray cast iron can be produced on pearlit, ferritic, as well as mixed - ferrito-pearlite-based. In the considered metal, as a rule, there is a significant percentage of silicon.

Gray cast iron is fairly easy to handle via cutting tools. This metal is used when the products are optimal from the point of view of compression resistance. For example, various reference elements, batteries, water pipes. The use of gray cast iron and mechanical engineering is common - most often in the manufacture of parts for which shock loads are not characteristic. For example, enclosures for machine tools.

What is a white cast iron?

This type of cast iron is characterized by the presence of carbon, which is almost entirely represented in the structure of the metal in the associated state. The metal under consideration is solid and at the same time quite fragile. It is resistant to corrosion, deterioration, temperature effects. White cast iron is quite difficult to handle through manual tools. At the break, this metal has a light shade, a radiant structure.

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

The percentage of silicon in white cast iron is significantly less than in gray. In the considered metal, there can also be a higher concentration of manganese and phosphorus (we note that, in many respects, their presence is predetermined by the chemical composition of ore from which cast iron is paid). Actually, an increase in the amount of silicon in the metal is accompanied by a reduction in the volume of associated carbon in its structure.

Comparison

The main difference between the gray cast iron from the white is that in the first there is a small percentage of associated carbon, in the second - on the contrary, there is mainly associated carbon. This feature predetermines the difference between the considered metals in the aspect:

• hardness;
• colors on a break;
• depreciation resistance;
• fragility;
• manual tools;
• applications;
• percentage of associated and free carbon;
• percentage of silicon, manganese, phosphorus.

More clearly examine what the difference between the gray and white cast iron lies in the specified aspects, a small table will help us.

Table

Gray cast iron	White cast iron
Less hard	More hard
Darker	Brighter
Less resistant to wear	More resistant to wear
Less fragile	More fragile
Good handling manual tool	Not too well to handle a hand tool
Actively applied in various fields of industry	Used mainly in the facilitation of steel, gray cast iron
Has a large percentage of free carbon - in the form of graphite	Includes mainly associated carbon
Characterized by a large percentage of silicon, smaller - manganese, phosphorus	Characterized by a smaller percentage of silicon, large - manganese, phosphorus

Iron alloys with carbon (\u003e 2.14% C) are called cast iron. The presence of eutectic in the structure of the cast iron 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. Cementitis gives a life-specific light brilliance, so cast iron in which the entire carbon is in the form of cementite, is called white. Graphite gives a gray cast iron gray. Depending on the form of graphite and the conditions of its formation, the following groups of cast iron distinguish: gray, high-strength with spherical graphite and dusting.

Gray cast iron. The gray cast iron (technical) is essentially an alloy FE - Si - C containing as inevitable impurities Mn, P and S. In the structure of gray cast irons, most or all carbon is in the form of graphite. The characteristic feature of the structure of gray iron, determining many of its properties, is that graphite has in the field of view of the microclife form of plates. The most widespread use was obtained by deetectoid cast iron containing 2.4 - 3.8% C. The higher the content in the cast iron of carbon, 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 foundry properties (good liquid process) carbon should be at least 2.4%.

The gray cast iron is marked with letters with gray and ch - cast iron (GOST 1412 - 70). After the letters follow the numbers. The first digits indicate the average tensile strength limit, and the second is the average strength limit when testing for bending. The tensile strength is used to estimate the plasticity of the cast iron, since the relative elongation in all gray castoffs is almost equal to zero.

White and bleached cast iron. White cast iron due to the presence of cementite in it has a high hardness, fragile and practically does not give up with cutting, therefore it has limited use. Twobled chopped castings, in which the surface layers have the structure of white (or halve) cast iron, and the core - gray cast iron. There may be a transition layer between these zones. Beyond some depth (12 - 30 mm) is a consequence of rapid cooling of the surface resulting from casting cast iron into metal forms (Kokil) or in a sandy form. High surface hardness (HB 400-500) causes good resistance against wear, especially abrasive, bleached cast iron rolling rolls, wheels, mills for mills, etc. In this case, cast iron is used with a reduced silicon content that is inclined To whitening. Its approximate composition: 2.8-3.6% C; 0.5-0.8% Si; 0.4-0.6% MP. Different with different cooling rates in cross section and obtaining different structures, the casting has large internal stresses that can lead to crack formation. To remove the castings, the castings are subjected to heat treatment, i.e. they are heated at 500-550 C.

Theme lesson: Cast iron

Type of lesson: Lesson learning a new material.

View of the lesson: Lecture.

Equipment lesson: Tutorial, computer, multimedia projector.

Objectives lesson:

• training: examine the classification of iron and influence on the quality and properties of carbon and impurities;
• developing: develop the ability to allocate the main thing and write it in the form of an abstract, to be able to assimilate their knowledge; develop creative thinking, attention;
• educational: Briefing the culture of communication, a sense of collectivism and empathizing the success and failures of comrades, the ability to work in the team, to form technical literacy of students.

At the lesson, students are formed by major and professional competencies that meet the requirements of GEF SPO in the specialty 23.02.03 "Maintenance and repair of road transport":

1. General competencies:

• the ability to understand the essence and social significance of their future profession, to exercise sustainable interest (OK 1);
• the ability to organize its own activity, choose typical methods and methods for performing professional tasks, evaluate their effectiveness and quality (OK 2);
• the ability to search and use the information necessary to effectively perform professional tasks, professional and personal development (OK 4);

2. Professional competencies:

• the ability to develop measures for the integrated use of raw materials, to replace scarce materials and finding production waste disposal methods (PC 1.3).

During the classes

1. Organizational moment -3 min.

Greeting the teacher, checking readiness for the lesson, the organization of attention.

2. Actualization of reference knowledge - 5 min.

During the front conversation, students recall the basic concepts of the previous topic, the following questions are set:

• Tell us about the classification of steels.
• What constant impurities are contained in steels?
• What is called cast iron?

3. Formulation of the theme and lesson purposes.

Topic: Cast iron.

Purpose: At the end of the lesson, students should know the classification of castors and marking according to GOST, be able to decipher the brand of cast iron.

4. Explanation of a new material.

5. Fastening a new material.

Questions for consolidation

1. Tell us about the classification of cast iron.
2. For the manufacture of which parts use white and gray cast iron?
3. What parts are made from high-strength and mackegrounds?
4. How are high-strength and macked cast iron design?

6. Homework.

A summary of lectures, draw up a table "Castings"

1. Classification of castunov
2. White cast iron. Its structure, properties, application
3. Gray cast iron, its structure, properties, GOST and application marking
4. Matchy cast iron
5. High-strength cast iron
6. Antifriction cast iron, labeling, and application

1. Classification of castunov

Cast iron is a multicomponent iron alloy with carbon, manganese, silicon, phosphorus and gray. Cast iron also contains minor amounts of hydrogen, nitrogen and oxygen. In alloyed cast iron, chromium, nickel, vanadium, tungsten and titanium can be chrome, the number of which depends on the composition of the regulated ores.

Depending on the appointment, the cast iron paid in the domain furnaces is divided into three main types: anterior, which goes to the redistribution into steel; foundry designed to obtain castings from cast iron in mechanical engineering; Domain ferroalloys used for steel deoxidation in steelmaking production.

In the structure of cast iron, it is possible to divide into the following groups:

• gray - cast iron, in which carbon is in the form of graphite.
• white - cast iron, in which carbon is in the form of cementite and perlite.

For the purpose of cast iron, it can be classified as follows:

• Dove
• Heat-resistant
• High-strength
• Antifriction.

2. White cast iron. Its structure, properties, application

This name of the cast iron was obtained by the form of a break, which has a matte-white color.

White cast iron (almoral) is rarely used in the national economy as structural materials, since due to the large cement content is very fragile and solid, they are hardly molded and processed by the tool. Of these, the details of hydromachins, sandstones and other structures operating under conditions of increased abrasive wear are made. White cast iron is used to make parts of increased fatigue strength: crankshaft and distributional shafts, valve saddles, oil pump gear, vase disc brake calipers, and others. To increase wear resistance, white cast iron is doped with chrome, vanadium, molybdenum and other carbido-forming elements. White cast iron marking is not installed.

A variety of white cast iron is bleached cast iron. Surface layers of products from such iron have a structure of white (or halm) cast iron, and the core is gray cast iron. Blocked on some depth (12 ... 30 mm) is obtained by rapidly cooling the surface (for example, casting cast iron into metal or sandy forms).

For the removal of structural stresses, which can lead to the formation of cracks, the castings are injected with heating. High wear resistance of bleached cast iron is due to surface hardness. From the bleached cast iron produce rolling rolls of leafy mills, wheels, balls for mills, etc.

3. Gray cast iron, its structure, properties, marking according to GOST and application

The structure of the gray (foundry) cast iron consists of a metal base with graphite plate shape, inserted into this base. Such a structure is formed directly when crystallizing the cast iron in the casting in accordance with the state diagram of the FE-C system (stable). Moreover, the more carbon and silicon in the alloy and the lower the speed of its cooling, the higher the probability of crystallization on this diagram to form a graphite eutectic. With low carbon and silicon content, cast iron modify in small doses of certain elements (for example, aluminum, calcium, cerium).

Metal modification is an introduction to metal melts of modifiers, i.e. substances, small quantities of which (usually no higher than tenths%) contribute to the creation of additional artificial crystallization centers, and therefore, the formation of structural components in a crushed or rounded form, which improves the mechanical properties of the metal.

To characterize the structure of the gray cast iron, it is necessary to determine the dimensions, shape, distribution of graphite, as well as the structure of the metal base. In the usual gray cast iron with slow cooling during crystallization, graphite is very weakly branched. It looks like a socket with a small number of curved petals.

The metal base of gray cast iron is formed from austenite during eutectoid decay and may be pearlit, ferritic and ferrite-pearlite. The formation of perlite takes place easily, at a relatively short period of time.

The mechanical properties of gray castoffs depend on the properties of the metal base and, mainly on the quantity, form and size of graphite inclusions. The pearlite base provides the highest values \u200b\u200bof strength and wear resistance.

The gras of gray cast iron according to GOST 1412 - 85 consist of the letters "SCH" and numbers corresponding to the minimum limit of tensile strength. Cast iron sch15 - ferritic; Sch25, sch30, sch35 - ferrite-pearlic cast iron, starting with sch40-pearlic cast iron.

Gray cast irons have high casting qualities (liquid process, low shrinkage, a slight tart of metal to the form and others) are well processed and resisted wear, however, due to low strength and plastic properties are mainly used for non-relevant parts. In machine tool, gray cast iron is the main structural material (stannes of machines, tables and upper sleds, columns, carriages, etc.); In the automotive industry from ferrite-pearlit castments, crankresses, covers, brake drums, etc., and from pearlite cast iron are blocks of cylinders, sleeves, flywheels, etc. In construction, gray cast iron is used mainly for the manufacture of parts operating in compression (shoes , columns), as well as sanitary parts (heating radiators, pipes). A significant amount of cast iron is spent for the manufacture of tubing, of which the metro tunnel is constructed.

4. Dake cast iron

Dyeing cast iron is called because it can be treated with pressure, although the cast irons do not bite, and the parts made of cast iron are obtained only by casting due to the fact that the marsh cast iron has a higher ductility compared to gray.

Muffin cast iron with a flake form of graphite is obtained from white do-buttectic cast iron, exposing them to a special graphitizing annealing. Graphitizing annealing of white cast iron is based on cementite metastability and is usually usually from two stages (drawing)

Drawing. Scheme of annealing of white cast iron into a dust

The first stage (930 ... 1050 ° C) is selected by the duration so that the entire cementite, located in the structure of the casting, collapsed on the austenite and flake graphite. The graphics process is facilitated by modifying (for example, aluminum and boron). The cast iron, thus obtained, is called modified.

In the second stage of the graphitizing annealing at an eutectoid transformation temperature, a metallic base of a forging cast iron is formed. Depending on the cooling modes, the forging cast iron can have pearlite (continuous cooling), ferritic (very slow cooling in the range of 700 ... 760 ° C) or ferrith-pearlite (reduction of the duration of the second annealing stage) metal bases. To obtain a pearlite base in a modified forging, it is recommended to increase the content of manganese, chromium and some other elements that increase cementis resistance to decay to ferrite and plate graphite in the eutectoid temperature range.

Dovety cast irons with a pearlite metal base have high hardness and durability in combination with a small plasticity. Matchy ferritic cast iron is characterized by high plasticity and relatively low strength.

Dake cast iron according to GOST 1215 - 79 is marked with the letters "KCH" and two numbers: the first indicates the time resistance when tensile; Second - relative elongation (%).

Matching cast iron goes to make parts of increased strength and viscosity: Carter, gearboxes, gearboxes, springs, etc.

5. High-strength cast iron

High-strength cast iron - cast iron with spherical graphite is obtained by modifying liquid magnesium additives from the mass of the processable portion of cast iron, cerium, yttrium and some other elements. In this case, before entering modifiers, it is necessary to reduce the content of sulfur.

To avoid formation in high-strength icefields of iceburit, they are subject to graphitizing annealing. The duration of such annealing due to the increased content of graphitizing elements (carbon, silicon) is significantly shorter than when annealing white cast iron.

The structure of high strength cast iron consists of a metal base (ferrite, perlite) and inclusions of graphite spherical shape. A ball-shaped graphite having a minimum surface with a given volume, significantly less weakens the metal base than plate graphite, and is not an active voltage concentrator.

The high-strength cast iron brands according to GOST 7293-85 consist of the letters "HF" and numbers, the first indicate the magnitude of the time resistance, the second - the relative elongation (%). The standard provides for the following brands of cast iron: HF35-22; HF40-15; Hp45-10; Hp50-7; HF60-3; HF70-2; Hp80-2; ВЧ100-2.

High-strength cast irons have good casting and consumer properties (cutting machinability, ability to extinguish vibration, high wear resistance, etc.) properties. They are used for massive castings instead of steel cast and forged parts - cylinders, gears, crankshaft and distribution shafts, etc.

6. Antifriction cast iron, marking, and application

Antifriction cast iron are gray and high-strength cast iron special marks.

Antifriction gray cast iron is pearline cast iron AHS-1 and ACS-2 and pearly ferrite cast iron AHS-3. These cast iron have a low coefficient of friction, depending on the ratio of ferrite and perlite based, as well as on the number and shape of graphite. In pearlite cast iron, high wear resistance is provided by a metal base consisting of thin perlite and evenly distributed phosphoric eutectic in the presence of isolated discharge graphite.

Antifriction gray cast iron is used for the manufacture of bearings of sliding, bushings and other parts, working with friction on metal, more often in the presence of lubricant material. Details working in a pair with hardened or normalized steel shafts are made from Castows AHS-1 and ACS-2, and for work in a pair with thermally untreated shafts, cast iron AHS-3 is used.

Antifriction high-strength (with spherical graphite) cast iron is made with a pearlite structure - ACV-1 and ferrithic-pearlite (50% perlite) - ACV-2. Cast iron ACV-1 is used to work in friction nodes with elevated circumferential speeds in a pair with hardened or normalized shaft.

The main advantage of antifriction cast iron compared to babbitis and antifriction bronze - low cost, and the main drawback is poor old worker, which requires accurate interface of rubbing surfaces.

The marking of iron is carried out by combining letters and numbers.

The following notation was adopted:

Prime: ACS1; ACS2; ACS3; ACV1; ACV2; ACHC1; ACH2.

The letters "ACS" denote anti-friction gray cast iron; "ACV" - antifriction high-strength cast iron; "ACH" - anti-friction macaque cast iron.

Bibliography

1. Kolesnik P. A. Materials science in road transport: textbook for stud. Higher. studies. Institutions / P. A. Kolesnik, V. S. Cleanitsa. - 2nd ed., Ched. - M.: Publishing Center "Academy", 2007. - 320 p.
2. Rogacheva L.V. Materials science. - M.: Kolos- Press, 2002.- 136 p.: Il. (Textbooks and studies. Benefits for secondary vocational education).
3. Stukanov V.A. Materials Science: Tutorial - M.: ID "Forum": Infra - M, 2008. - 368 p.: Il. - (Professional education)
4. GOST 1412-85 cast iron with lamellar graphite for castings. Brands
5. GOST 1585-85 cast iron antifriction for castings. Brands
6. GOST 7769-82 Cast iron alloyed for castings with special properties. Brands

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White cast iron is used in mechanical engineering much less often than gray, due to its greater fragility and high hardness, as a result of which it cannot be mechanically handled by cutting tools.

White cast iron is used in mechanical engineering much less often than gray, due to its large fragility and high hardness, as a result of which it is not amenable to mechanically handling cutting tools.

White cast iron mainly goes on the alteration into steel. It contains from 2 2 to 4% carbon, which is in a chemically related state. White cast iron is characterized by high hardness and fragility. The casting properties of this cast iron are low.

White cast iron contains carbon in the form of cementite and has a white radiant view of a break. Such cast iron is characterized by high hardness, durability, high wear resistance and fragility. It is poorly handling cutting, so it is almost not used. Usually gray cast iron with graphite inclusions and a bleached surface, i.e. Cast iron, the surface layers of which have the structure of white cast iron to increase wear resistance, and the core is the structure of gray cast iron. Gray cast iron has the best technological and good physical and mechanical properties and is the main material for different castings. The structure of the metallic base of such cast iron can be ferritic, pearlit or pearly ferrite, and graphite shape is a plate. Floating cast iron gray, due to the release of graphite in a metal basis.

White cast iron compared to the gray cast iron has worse cast properties, very solid and difficult to cut.

White cast iron for carpet cast iron is often mowed in two ovens: first in Vagranka, then in electrollobal furnaces, where cast iron is brought to a certain chemical composition and overheating.

White cast iron consists of pearlitis and cementite. Due to the large amount of cementite, white cast irons have a very high hardness (HB 450 - 600), but very low workability. The cutting speed of parts from white cast iron (the bleached cast iron is most often used, obtained from gray cast iron by hardening it) carbide tools do not exceed 3 - 10 m / min.

White cast iron used for the production of pearlite white ductile cast iron, contains an increased amount of carbon, so only the Vagranc is used for its smelting.

Determination of the minimum height of the riser. | Typical maritime system for casting from a ductile cast iron. | The value of the angle A, degrees.

White cast iron has a large shrinkage (up to 2%), poorly fills the form and also differs fragility. In order for the shrinkage in solidification of the metal more calmly and did not form cracks in the casting, the metal is recommended to lead through one feeder. The overall dimensions and the mass of castings allow it to do, as small castings are usually obtained from forging cast iron, the mass of which does not exceed 100 kg.

White cast iron used for the production of armored tiles (type of melting No. 115 and 226), have relatively low impairedness, but the use of them for the manufacture of plates - with a smooth or wavy surface (a large area of \u200b\u200bcontact with grinding bodies and grinding material) may be quite justified.