Difference between gray and white cast iron

Physical and mechanical properties

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

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

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

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

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

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

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

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

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

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

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

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

Notes

See also

Links

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

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

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

white cast iron- cast iron, in which all carbon is chemically bound in cementite; Received its name from its matte white fracture. White cast iron has high hardness and brittleness, and is practically impossible to process. cutting tool. White cast iron widely... ...

White cast iron Encyclopedic Dictionary of Metallurgy

WHITE CAST IRON- (named for the type of fracture that has a dull white color) cast iron in which all the carbon is in the form of cementite. The structure of white cast iron at normal temperature consists of cementite and perlite (Fig. B 7). White cast iron has high hardness and... Metallurgical Dictionary - WHITE, about color, suit, paint: colorless, opposite to black. | In a comparative sense, light, pale. White wine, white beer, honey, plums; white face, white bread, so called to distinguish it from red (wine, honey), black (beer, plums, bread) ... Dictionary Dahl

Gray, ductile and ductile cast 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 and matte. Their structure is distinguished: the structure of the metal base and graphite precipitation. They differ from each other only in the form of graphite deposits.

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

Flake graphite. In ordinary gray cast iron, the graphite forms in the form of petals; such graphite is called lamellar. In Fig. 4.2, A shows the structure of ordinary ferritic cast iron with graphite veins; the spatial appearance of such graphite inclusions is shown in Fig. 4.3, A(the intersection of lamellar inclusions with the polished section plane is visible).

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

Flaky graphite. If you obtain white cast iron during casting, and then, using the instability of cementite, decompose it by annealing, then the resulting graphite acquires a compact, almost equiaxed, but not rounded shape. This type of graphite is called flake or annealed carbon. The microstructure of cast iron with flake graphite is shown in Fig. 4.2, V. In practice, cast iron with flake graphite is called malleable cast iron.

a b c d

Rice. 4.2. Form of graphite in cast iron:

A– plate (regular gray cast iron), × 100; b– spherical (high-strength cast iron), × 200; V– flake (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 helminth-like veins (Fig. 4.2, G).

Thus, cast irons are called:

– with lamellar graphite and ordinary gray cast iron;

– with worm-shaped graphite – gray vermicular cast iron;

– cast iron with nodular graphite – high-strength cast iron;

– cast iron with flake graphite – malleable cast iron.

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

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

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

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

From this 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 (free carbon), which determine 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 (Fig. 4.4, A- graphite in the form of veins; typical for gray cast iron). Perlite contains 0.8% C, therefore, this amount of carbon in gray pearlitic cast iron is in a bound state (i.e. in the form of Fe 3 C), the rest is in free form, i.e. in the form of graphite.

Ferrite-pearlite cast iron (Fig. 4.4, b) consists of ferrite and pearlite + inclusions of spindle-shaped 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 the carbon present in the alloy is present in the form of graphite (in the photograph as spindle-shaped graphite).

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 iron. Gray cast iron, like white cast iron, is 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 nucleus formation is large: significant diffusion of carbon atoms and removal of iron atoms from the graphite crystallization front are required), it is possible only in a narrow temperature range. Consequently, the cooling of gray cast iron is slow, and the cementite released from the liquid or solid solution is unstable chemical compound, especially when high temperatures ah, decomposes to form graphite:

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

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

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

The graphite in gray cast iron is released in the form of plates. Lamellar graphite inclusions in gray cast iron can be considered as cracks and cuts that create large 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 unetched microsection is examined using a metallographic microscope. Graphite appears as a dark phase on a light background of a polished metal base, then the microsection is etched (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 iron are distinguished: pearlitic, pearlitic-ferritic and ferritic cast iron. If the amount of fixed carbon is more than 1%, such cast iron is called half-cast iron. Its structure consists of ledeburite, pearlite and graphite.

Table 4.1

Diagrams of cast iron structures

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

All elements introduced into cast iron are divided:

1) on 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 free carbon 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 cast iron.

To determine what structure should be expected depending on the total carbon and silicon content, as well as depending on the cooling rate (casting wall thickness), use a 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 iron; II – gray pearlitic cast iron; III – gray ferritic cast iron

Consequently, 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 casting large-section parts, cast iron with a lower content of these elements can be used.

The size and shape of the released graphite inclusions also depends on the presence of crystallization centers in the liquid cast 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 liquid iron before it is cast.

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

Depending on the strength of the metal base and the amount of graphite, gray cast irons can have a tensile strength of approximately 100 to 400 MPa with almost zero elongation. Gray cast irons perform much better in compression 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 bending 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 iron of these grades is used for low-critical parts with light loads (building columns, foundation slabs, brackets, flywheels, gears).

The remaining brands have a pearlite metal base with a reduced carbon and silicon content. Cast irons with a pearlite base are used for critical parts that experience wear at high pressures (machine machine beds, pistons, cylinders, parts of 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.

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

Nodular graphite can be formed during primary crystallization, as well as during the annealing process of white modified cast iron. Of course, the most desirable is the formation of spherical 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, significantly simplifies the casting technology.

High-strength cast irons are marked with the letters HF and subsequent numbers.

The first two digits of the brand indicate the average 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 = 600 MPa; relative elongation δ = 2%.

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

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

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

A significant drawback of the process of producing malleable cast iron is the duration of annealing, which is 70–80 hours. To speed it up, various measures are used (modification with aluminum (less often boron, bismuth), increasing the temperature of the first stage (but not higher than 1080°C)).

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

The schedule for obtaining malleable cast iron is shown in Fig. 4.6.

Rice. 4.6. Schedules for obtaining malleable cast irons

To obtain malleable cast iron you need:

– castings from low-carbon white cast 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 maintain at this temperature for a long time (10 – 15 hours) (first stage graphitization);

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

– conduct cooling in air.

During the first stage of graphitization, ledeburite cementite and secondary cementite decompose to form austenite and flake 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.

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

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

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

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

To obtain pearlitic malleable cast iron, cupola white cast iron containing up to 3.2% carbon is sometimes used. Annealing is carried out in a decarburizing (oxidizing) environment followed by cooling in air. Such annealing ensures significant carbon burnout.

Malleable cast irons are marked with the 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 iron has the following grades:

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

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

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

Malleable cast iron is used in the automotive, tractor industries, agricultural engineering, car and machine tool industries for high-strength parts that can withstand alternating and shock loads and operate 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 used under high dynamic and static loads (gearbox craters, hubs, hooks, brackets) and for less critical ones (nuts, mufflers, flanges, couplings). Links and rollers of conveyor chains, brake pads, etc. are made from pearlitic malleable cast iron.

Work order

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

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

3. Determine the conditions for obtaining the structure under study.

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

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

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

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

8. Write a progress report.

When preparing a report you must:

1) give a brief classification of cast irons;

2) define white, gray, high-strength and malleable cast iron;

3) draw part of the Fe – Fe 3 C diagram, which relates to the cast iron region;

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 chemical composition white cast irons and their position on the diagram;

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

Summarize the data on the work done in the table. 4.2.

Table 4.2

Security questions

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

2. How are cast irons classified?

3. How are the structure and properties of cast iron characterized?

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

5. How much carbon does cast iron contain?

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

7. In which cast irons is all the carbon in a chemically bound state?

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

9. Methods of production, properties and use of white cast iron.

10. How is white cast iron obtained?

11. How much graphite is in white cast iron?

12. What elements contribute to bleaching?

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 is gray cast iron produced?

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

21. Is malleable cast iron good for forging?

22. How is malleable cast iron obtained?

23. What processes take place at the first stage of graphitization (production of malleable cast iron)?

24. What processes take place at the second stage of graphitization (production of malleable cast iron)?

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

26. Structure of malleable cast iron:

27. How is high-strength cast iron produced?

28. Structure of ductile iron:

29. What is the form of graphite in ductile iron?

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

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

32. Structure of gray cast iron

33. Marking of gray, high-strength and malleable cast irons.

34. What do the numbers in the SCh15 cast iron grade mean?

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

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

37. What does the number 6 mean in the grade of cast iron 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 Automatic steel, intended for the manufacture of mass-produced parts on automatic machines (AI2, A30, A40G - sulfur; ACI4, AS40, AS35G2 - lead-containing; A35E, A40ХВ - sulfur-sulfur; 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 hardness acquired by steel as a result of hardening. Hardenability depends mainly on the carbon content (see Fig. 6 laboratory work № 8).

Related information.

White cast irons: composition, properties, scope.

The carbon is in the form of cementite Fe 3 C. The fracture will be white if broken. In the structure of hypoeutectic cast iron HB 550, along with pearlite and secondary cementite, there is brittle eutectic (ledeburite), the amount of which reaches 100% in eutectic cast iron. The structure of hypereutectic cast iron consists of eutectic (Ep) and primary cementite, which is released during crystallization from the liquid in the form of large plates. High hardness, difficult to cut. Ch. property: high wear resistance. Cast iron is brittle. Rarely used in mechanical engineering. It is used in the manufacture of millstones in mills, rolling rolls in rolling machines, and fences are made from this cast iron. If the casting is small (up to 10 kg), then white cast iron is formed during rapid cooling.

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

Gray cast iron.

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

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

Depending on the strength, gray cast iron is divided into 10 brands (GOST 1412).

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

Gray cast irons contain carbon - 3,2…3,5 % ; silicon – 1,9…2,5 % ; manganese – 0,5…0,8 % ; phosphorus – 0,1…0,3 % ; sulfur – < 0,12 % .

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

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

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

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

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

High-strength nodular cast iron.

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

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

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

High-strength cast irons have a high yield strength,

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

,

with pearlite base.

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

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

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

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

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

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

Malleable iron

Produced by annealing white hypoeutectic cast iron.

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

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

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

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

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

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

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

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

Forks are made from pearlitic cast iron, characterized by high strength and sufficient ductility. cardan shafts, links and rollers of conveyor chains, brake pads.

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

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

It was first mastered in China back in the 10th century, after which it became widespread in other countries of the world. The basis of cast iron is an alloy of iron with carbon and other components. Distinctive feature is that cast iron contains more than 2% carbon in the form of cementite, which is not found in other metals. A bright representative This 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 a white color when fractured and a characteristic metallic luster. The structure of white cast iron is fine-grained.

Properties

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

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

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

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

Compound

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

Species

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

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

Application

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

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

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

The alloy is also used for the manufacture of slabs with ribbed or smooth surface, and are also actively used for gray cast iron.

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

Bleached cast irons

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

Alloying elements of the alloy

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

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

Alloying elements can be added to the alloy:

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

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

• in a cupola;
• in electric melting furnaces.

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

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

Types of annealing

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

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

Additional information

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

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

Iron-carbon alloy (>2.14 % C) are 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 both in the form of cementite and graphite. Cementite gives the fracture a specific light shine. Therefore, cast iron, in which all the carbon is in the form of cementite, is called white. Graphite imparts fracture to cast iron gray that's why cast iron is called gray. Depending on the form of graphite and the conditions of its formation, the following cast irons are distinguished: gray, high-strength and malleable (see Fig. 101 and 102).

GRAY AND WHITE CAST IRON

Gray cast iron (commercial) is essentially a Fe-Si-C alloy containing Mn, P and S as permanent impurities. In the structure of gray cast iron, most or all of the carbon is in the form of graphite. Feature The structure of gray cast iron, which determines many of its properties, is that graphite has the shape of plates in the field of view of a microsection (see Fig. 88). Most wide application hypoeutectic cast irons containing 2.4-3.8% C were obtained. The higher the carbon content in cast iron, the more graphite is formed and the lower its mechanical properties. At the same time, to ensure high casting properties (good fluidity) there must be at least 2.4 % WITH.

A section of the Fe-Si-C ternary phase diagram 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 there is peritectic (F+

Rice. 99.

F - liquid phase; A austenite; G*graphite

F-6-ferrite-? A), eutectic (F-*A + G) and eutectoid (A -? F + G) transformations do not occur at a constant temperature, but in a certain temperature range.

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 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, 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 the content

Rice. 100.

A - influence of C in Si; nor the structure of cast iron: b - the influence of the cooling rate (casting thickness) and the sum of C + SI on the structure of cast iron; I - white cast iron; //-V - gray cast iron

Rice. 101.

A- white cast iron; b - pearlitic gray cast iron: V- ferritic-pearlite gray cast iron; G- ferritic gray cast iron

The concentration of silicon and carbon in cast iron is shown in Fig. 100, A. For a given carbon content, the more silicon there is in cast iron, the more complete graphitization occurs. The more carbon in cast iron, the less silicon is required to obtain a given structure.

Depending on the carbon content bound in cementite, there are:

• 1. White cast iron (Fig. 100, A,/), in which all the carbon is in the form of cementite Fe 3 C. The structure of such cast iron is pearlite, ledeburite and cementite (Fig. 100, a, I and 101, A).
• 2. Half ch>tun (Fig. 100, A, //), most of the carbon (>0.8%) is in the form of Fe 3 C. The structure of such cast iron is pearlite, ledeburite and lamellar graphite C
• 3. Pearlitic gray cast iron (Fig. 100, a, III) the structure of cast iron (Fig. 101, b) is pearlite and lamellar graphite. In this cast iron, 0.7-0.8 °b C is in the form of Fe 3 C, which is part of perlite.
• 4. Ferritic-pearlitic (Fig. 100, a, /V) gray cast iron. The structure of such cast iron (Fig. 101, V) - pearlite, ferrite and lamellar graphite (for compositions, see Fig. 100, a, III). In this cast iron, depending on the degree of decomposition of eutectoid cementite, there is from 0.7 to 0.1% C in a bound state.
• 5. Ferritic gray cast iron (Fig. 100, a, V). Structure (Fig. 101, G)- ferrite and lamellar graphite. In this case, all carbon is in the form of graphite.

At a given carbon and silicon content, graphitization proceeds more completely, the slower the cooling. In production 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 graphitization occurs (Fig. 100, b).

Consequently, the silicon content must be increased in a casting with a small cross-section that cools quickly, or in cast iron with a lower carbon content. In thick sections of castings that cool more slowly, graphitization occurs more completely and the silicon content may be lower. The amount of manganese in cast iron does not exceed 1.25-1.4 %. Manganese prevents graphitization, i.e., it complicates the separation of graphite and increases the ability of cast iron to bleach - the appearance, especially in the surface layers, of the structure of white or half-cast iron. Sulfur is harmful impurity, worsening 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 (FeS, MnS) or their solid solutions (Fe, Mn) S.

The mechanical properties of cast iron are determined by its structure, mainly the graphite component. Cast iron can be thought of as steel infused with graphite, which acts as a notch that weakens the metal backing of the structure. In this case, the mechanical properties will depend on the number, size and nature of the distribution of graphite inclusions.

The fewer graphite inclusions, the smaller they are and the greater the degree of their isolation, the higher the strength of cast iron. Cast iron o a large number 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 tear resistance, tensile strength and especially the ductility of cast iron. The relative tensile elongation of gray cast iron, regardless of the properties of the metal base, is practically zero (-"0.5%). Graphite inclusions have little effect on the reduction in compressive strength 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 occurs at an angle of 45°. The breaking load in compression, depending on the quality of cast iron and its structure, is 3-5 times greater than in tension. Therefore, cast iron is recommended to be used primarily for products working in compression.

Graphite plates reduce strength in bending less significantly than in tension, since part of the product experiences compressive stress. The flexural strength is intermediate between the tensile and compressive strengths. The hardness of cast iron is 143-255 HB.

Graphite, breaking the continuity of the metal base, makes cast iron insensitive to all kinds of stress concentrators (surface defects, cuts, recesses, etc.). As a result, gray cast iron has approximately the same structural strength in castings of a simple shape or with a smooth surface and in complex shapes with notches or a poorly machined surface. Graphite increases the wear resistance and anti-friction properties of cast iron due to its own “lubricating” effect and increasing film strength lubricant. It is very important that graphite improves machinability by making chips brittle.

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 S - gray and Ch - cast iron (GOST 1412-85). The letters are followed by numbers indicating the minimum value of tensile strength 10" 1 MPa (kgf/mm 2).

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

Ferritic and ferritic-pearlite cast irons(SCh 10, SCh 15, SCh 18) have a tensile strength of 100-180 MPa (10-18 kgf/mm 2), a bending strength of 280-320 MPa (28-32 MPa). Their approximate composition: 3.5-3.7 % WITH; 2.0-2.6% Si; 0.5--0.8% Mi;

SC 15). These cast irons are used for low-critical parts that experience light loads when working with a casting wall thickness of 10-30 mm. Thus, SCh 10 cast iron is used for building columns, foundation slabs, and cast iron SCh 15 and SCh 18 - for cast light-loaded parts of agricultural machines, machine tools, cars and tractors, fittings, etc.

Pearlitic cast irons(SCh 21, SCh 24, SCh 25, SCh 30, SCh 35) are used for critical castings (beds of powerful machines and mechanisms, pistons, cylinders, parts exposed to wear in conditions high pressures, compressors, fittings, 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 fine-plate perlite (sorbitol) with small swirling graphite inclusions. Perlite includes the so-called steely And modified cast irons

When smelting steel cast irons SCh 24, SCh 25, add 20-30 % steel scrap; cast irons have a reduced carbon content, which ensures a more dispersed pearlite base with fewer graphite inclusions. Approximate composition: 3.2-3.4 % WITH; 1,4-2,2 % Si; 0.7-

1,0 % MP; % P;

Modified cast irons (SCh 30, SCh 35) are obtained by adding special additives-modifiers (graphite, 75% ferrosilicon, silico-calcium in an amount of 0.3-0.8) to liquid cast iron before casting. % etc.). The modification is used to obtain a pearlite metal base interspersed with a small number of isolated medium-sized graphite plates in cast iron castings with different wall thicknesses.

Low-carbon cast iron is subjected to modification, containing a relatively small amount of silicon and an increased amount of manganese and having the structure of half-cast iron without introducing a modifier, i.e. ledeburite, perlite and graphite. Approximate chemical composition of cast iron: 2.2-3.2% C; 1.0-2.9% Si; 0.2-1.1% MP;

To relieve casting stress and stabilize dimensions, cast iron castings are annealed at 500-600 °C. Depending on the shape and size of the casting, the exposure at the annealing temperature is 2-10 hours. Cooling after annealing is slow, along with the furnace. After such treatment, the mechanical properties change little, and internal stresses are reduced by 80-90%. Sometimes, to relieve stress in cast iron castings, natural aging of cast iron is used - keeping them in a warehouse for 6-10 months; This exposure reduces stress by 40-50%.

Anti-friction cast irons used for the manufacture of sliding bearings, bushings and other parts that operate under friction with metal, often in the presence of a lubricant. These cast irons must provide low friction (low coefficient of friction), i.e. antifriction. The antifriction properties of cast iron are determined by the ratio of pearlite and ferrite in the base, as well as the amount and form of graphite. Anti-friction cast irons are manufactured in the following grades: :

ASF-1 (3.2-3.6% C; 1.3-2.0 % Si; 0.6-1.2 % MP; 0.15-0.4% R; % Cg; 1.5-2.0% Cu); AChS-2 (3.2-3.8% C; 1.4-2.2% Si; 0.3-1% Mn; 0.15-0.4 % R; % Ti; 0.2-0.5% Cu) and ASF-3 (3.2-3.8 % WITH; 1.7-2.6% Si; 0.3-0.7 % MP; 0.15-0.4% P; 0.2-0.5% Cu;

Parts working in tandem with hardened or normalized steel shafts are made from pearlitic gray cast iron AChS-1 and AChS-2; pearlite-ferritic cast iron AChS-3 is used to work in tandem with thermally untreated shafts.

Pearlitic cast iron, containing an increased amount of phosphorus (0.3-0.5%), is used for the manufacture of piston rings. High wear resistance of the rings is ensured by a metal base consisting of thin pearlite and uniformly distributed phosphide eutectic in the presence of isolated flake graphite deposits.

• Graphite crystallizes in the form of rather complex shapes (see Fig. 88, b, o), but their cross-section with a microsection plane gives the appearance of plates.
• 2 In white cast iron, the formation of eutectic (Fe + FeS) and the dissolution of sulfur in FeaC is possible.
• The greater the thickness of the casting walls, the lower the mechanical properties. 149
• A - anti-friction, C - cast iron, C - gray.