Cast iron and its types. Cast iron (white, gray, high-strength, malleable)

An alloy of iron and carbon is called cast iron. The presence of eutectic in the structure of cast iron (see Fig. 87) determines its use exclusively as a casting alloy. Carbon in cast iron can be in the form of cementite or graphite, or 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).

1. GRAY AND WHITE CAST IRON

Gray cast iron (technical) is essentially an alloy containing and as permanent impurities. In the structure of gray cast irons, most or all of the carbon is in the form of graphite. Feature The structure of gray cast iron, which determines many of its properties, lies in the fact that graphite has the shape of plates in the field of view of a microsection (see Fig. 88). Most wide application received hypoeutectic cast irons containing . 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 .

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

Rice. 99. Phase diagram of liquid faea; A - austenite; G - graphite

A section of the ternary phase diagram for constant silicon content is shown in Fig. 99. In contrast to the stable diagram (see Fig. 87), in the system peritectic eutectic and eutectoid transformations 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. How more content silicon, the wider the eutectic temperature range.

Cooling cast iron under 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

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

Rice. 101. Structure of cast iron, a - white cast iron; b - pearlitic gray cast iron; c - ferritic-pearlite gray cast iron; ferritic gray cast iron

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

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

2. Half cast iron (Fig. Most of the carbon is in the form of The structure of such cast iron is pearlite, ledeburite and flake graphite.

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% C is in the form of perlite.

4. Ferritic-pearlitic (Fig. 100, a, IV) gray cast iron. The structure of such cast iron (Fig. 101, c) is 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, from 0.7 to 0.1% is in a bound state.

5. Ferritic gray cast iron (Fig. 100, a, V). Structure (Fig. 101, d) - 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. Manganese prevents graphitization, i.e., it makes it difficult to separate out 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 a harmful impurity that impairs the mechanical and casting properties of cast iron. Therefore, its content is limited to 0.1-0.2%. In gray cast iron, sulfur forms sulfides or their solid solutions

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

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 with 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. Graphite inclusions have little effect on reducing the 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 takes the form of a cut at an angle of 45°. The breaking load in compression, depending on the quality of cast iron and its structure, is 3-5 times greater than in tension. Therefore, cast iron is recommended to be used primarily for products working under 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. Cast iron hardness

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 simple shape or with a flat surface and complex shape with cuts or a poorly processed surface. Graphite increases the wear resistance and anti-friction properties of cast iron due to its own “lubricating” effect and increasing the strength of the lubricant film. 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 C - gray and Ch - cast iron. The letters are followed by numbers indicating the minimum tensile strength value

Gray cast irons can be divided into: the following groups.

Ferrite and ferrite-pearlite cast irons have a temporary resistance to bending strength Their approximate composition: The structure of cast irons is pearlite, ferrite and graphite, often in the form of large precipitates

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, cast iron is used for building columns, foundation slabs, and cast iron is used for cast light-loaded parts of agricultural machines, machine tools, cars and tractors, fittings, etc.

Pearlitic cast irons are used for critical castings (beds of powerful machines and mechanisms, pistons, cylinders, parts exposed to wear under high pressure conditions, 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. Pearlitic cast irons include the so-called steel and modified cast irons.

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

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

Modification is carried out on low-carbon cast iron, which contains relatively little large number silicon and increased amount manganese and having, without the introduction of a modifier, the structure of half-cast iron, i.e. ledeburite, pearlite and graphite. Exemplary chemical composition cast iron:

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 holding time at the annealing temperature is Cooling after annealing is slow, along with the furnace. After such treatment, the mechanical properties change little, and internal stresses are reduced by 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%.

Antifriction cast irons are 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:

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

Pearlitic cast iron containing an increased amount of phosphorus is used for the manufacture 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 deposits of lamellar graphite.

Carbon in cast iron can be contained in the form of cementite (Fe3C) or graphite. Cementite is light in color, very hard and difficult to machine. Graphite, on the contrary, is dark in color and quite soft. Depending on which form of carbon predominates in the structure, they distinguish: white, gray, malleable and high-strength cast iron. Cast irons contain permanent impurities (Si, Mn, S, P), and in some cases also alloying elements (Cr, Ni, V, Al, etc.).

White cast iron- a type of cast iron in which carbon in a bound state is in the form of cementite, in fracture it has white and metallic shine. In the structure of such cast iron there are no visible inclusions of graphite and only a small part of it (0.03-0.30%) is detected by subtle methods of chemical analysis or visually at high magnifications. White iron castings are wear-resistant, relatively heat-resistant and corrosion-resistant. The strength of white cast iron decreases, and the hardness increases with increasing carbon content.

White cast iron is very hard, almost impossible to machine, and therefore is not used for making parts, but is used for conversion into steel and for making parts from malleable cast iron. This type of cast iron is also called pig iron.

Gray cast iron– an alloy of iron, silicon (from 1.2-3.5%) and carbon, also containing permanent impurities of Mn, P, S. In the structure of such cast irons, most or all of the carbon is in the form of plate-shaped graphite. The fracture of such cast iron is gray in color due to the presence of graphite. A separate variety (group of grades) of gray cast iron is high-strength cast iron with globular (spherical) graphite, which is achieved by modifying it with magnesium (Mg), cerium (Ce) or other elements.

Gray cast iron is characterized by high casting properties (low crystallization temperature, fluidity in the liquid state, low shrinkage) and serves as the main material for casting. It is widely used in mechanical engineering for casting machine beds and mechanisms, pistons, and cylinders.

High fragility inherent Gray cast iron, due to the presence of graphite in its structure, makes it impossible to use it for parts that work mainly in tension or bending; Cast irons are used only for “compression” work.

Gray cast iron is marked with the letters SCh, after which the guaranteed tensile strength value in kg/mm² is indicated, for example SCh30. High-strength cast irons are marked with the letters HF , after which the strength and, through a dash, the relative elongation in percent are indicated, for example VC60-2.

Malleable iron– the conventional name for soft and viscous cast iron, obtained from white cast iron by casting and further heat treatment. Long annealing is used, as a result of which cementite decomposes with the formation of graphite, that is, the process of graphitization, and therefore such annealing is called graphitizing.

Malleable cast iron, like gray cast iron, consists of a steel base and contains carbon in the form of graphite, but the graphite inclusions in ductile cast iron are different from those in ordinary gray cast iron. The difference is that the graphite inclusions in malleable cast iron are arranged in the form of flakes, which are obtained during annealing, and are isolated from each other, as a result of which the metal base is less separated, and the cast iron has some viscosity and ductility. Because of its flaky shape and the production method (annealing), the graphite in malleable iron is often called annealing carbon. Malleable cast iron got its name due to its increased ductility and viscosity (although it is not subject to pressure treatment).

Malleable cast iron has increased tensile strength and high impact resistance. Parts of complex shapes are made from malleable cast iron: car rear axle housings, brake pads, tees, angles, etc.

Malleable cast iron is marked with two letters and two numbers, for example KCH 370-12. The letters KCH mean malleable cast iron, the first number is the tensile strength (in MPa), the second number is the relative elongation (in percent), characterizing the ductility of cast iron.

Ductile iron– cast iron having graphite inclusions of spheroidal shape. Spheroidal graphite has a lower surface-to-volume ratio, which determines the greatest continuity of the metal base, and therefore the strength of cast iron.

High-strength cast iron is most often used for the manufacture of critical products in mechanical engineering, as well as for the production of high-strength pipes (water supply, drainage, gas and oil pipelines). Products and pipes made from high-strength cast iron are distinguished by high strength, durability, and high performance properties.

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

What is gray cast iron?

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

Main features of gray cast iron:

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

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

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

What is white cast iron?

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

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

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

Comparison

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

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

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

Table

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

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

Gray cast iron. Gray cast iron (commercial) is essentially an alloy of Fe - Si - C, containing Mn, P and S as inevitable impurities. In the structure of gray cast iron, most or all of the carbon is in the form of graphite. A characteristic feature of 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. The most widely used are hypoeutectoid cast irons containing 2.4 - 3.8% C. The higher the carbon content in cast iron, the more graphite is formed and the lower its mechanical properties. In this regard, the amount of carbon in cast iron usually does not exceed 3.8%. At the same time, to ensure high casting properties (good fluidity), carbon must be at least 2.4%.

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

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

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

Inclusions of graphite make the chips brittle, so cast iron is well processed by cutting. Thanks to the lubricating effect of graphite, cast iron has good anti-friction properties. Cast iron has high damping properties and dampens vibrations and resonant vibrations well. Marked gray cast iron with the letters SC and numbers characterizing the value of tensile strength during tensile tests. Nr, SCH10 contains (3.5...3.7)% C, (2.2...2.6)% Si, (0.5...0.8)% Mn, P<0,3% и S<0,15%, d В =100МПа, твёрдость <190НВ. SCH35 d V =350MPa, hardness<275НВ.

Gray cast iron - This is cast iron. Gray cast iron comes into production in the form of castings. Gray cast iron is a cheap construction material. It has good casting properties, is easy to cut, resists wear, and has the ability to dissipate vibrations under vibration and variable loads. The property of damping vibrations is called damping capacity. The damping capacity of cast iron is 2-4 times higher than that of steel. High damping strength and wear resistance have led to the use of cast iron for the manufacture of frames of various equipment, crankshafts and camshafts of tractor and automobile engines, etc. The following grades of gray cast iron are produced (the numerical values ​​of hardness NV are indicated in brackets): SCh 10 (143-29), SCh 15(163-229), SCh 20(170-241), SCh 25(180-250), SCh 30(181-255), SCh 35(197-269), SCh 40(207-285), SCh 45 (229-289).

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

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

Modified gray cast iron has a uniform structure over the cross-section of the casting and a smaller swirling form of graphite. Modifiers - ferrosilicon, silicoaluminum, silicocalcium, etc. - are added in an amount of 0.1 -0.3% by weight of cast iron directly into the ladle during its filling.

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

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