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 SCH 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, difficult to process 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.
Structure and properties of cast irons.
Iron-carbon alloys containing more than 2.14% carbon are called cast iron. In mechanical engineering, cast iron is one of the main casting materials, which is explained primarily by its good casting and strength properties. It is not subject to pressure treatment. The main factor determining the properties, and, consequently, the scope of application of cast iron, is its structure, which can be varied.
Based on their structure, cast iron is divided into white, gray, malleable and high-strength.
9.1. White cast iron.
White cast iron is called cast iron in which all the carbon is in a chemically bound state in the form of cementite Fe 3 C, which gives the cast iron a shiny white color.
Phase transformations in these cast irons proceed according to the metastable diagram Fe - Fe 3 C (see Fig. 23). According to their structure, white cast irons are divided into:
a) hypoeutectic, containing from 2.14 to 4.3 C. They consist of pearlite, ledeburite and secondary cementite, released from austenite grains in the temperature range from 1147° (EC line) to 727° (SK line). Secondary cementite merges with ledeburite cementite and may not be visible on a microsection as an independent structural component (Fig. 51a);
b) eutectic, containing 4.3% C. It consists of eutectic - ledeburite, which is a mechanical mixture of cementite and pearlite (Fig. 51, b);
B) hypereutectic, containing from 4.3% to 6.67 % C. They consist of primary cementite, released in the form of large plates and ledeburite (Fig. 51, c).
Rice. 51. Structure of white cast iron: a) hypoeutectic b) eutectic c) hypereutectic
The microstructure of white cast iron contains a lot of cementite, so it is very hard and brittle, but resists wear well. It is almost impossible to process by cutting (with the exception of abrasive), so white cast iron is not directly used in mechanical engineering; it is rarely used, only for the manufacture of parts operating under conditions of increased abrasive wear (parts of hydraulic machines, sand blowers, etc.). Being the main product of blast furnace smelting, this cast iron is used in metallurgy for conversion into steel (pig iron). White cast iron is also used in small quantities to produce malleable cast iron.
9.2. Gray cast iron.
Gray cast iron is called cast iron in which carbon is in the form of graphite, in the form of slightly curved plates or flakes, or branched rosettes with lamellar petals. Due to the large amount of graphite in the structure, such cast iron has a gray color when fractured.
Silicon promotes the graphitization process, reduces shrinkage, silicon is part of ferrite, forming a substitutional solid solution with α-iron.
Manganese increases the tendency of cast iron to retain cementite, and therefore increases the hardness of cast iron.
Sulfur is a harmful impurity in cast iron; it increases their hardness and brittleness 5-6 times more than Mn and significantly impairs casting properties.
Phosphorus in small quantities in cast iron is a useful impurity (unlike steels), it improves the casting properties of gray cast iron, since phosphorus forms the eutectic Fe+Fe2P, which melts at a temperature of 983°C, which is valuable for the production of thin-walled blast. . Chemical composition of gray cast iron: 2.5...4% C; 1.0…4.8% Si; 0.5...0.7% Mn; up to 0.12% S; 0.2…0.5% P.
Based on the structure of the metal base, gray cast irons are mainly divided into the following groups;
1. Perlite. The structure is P + PG (lamellar graphite), the metal base is P, and the amount of bound carbon (Fe 3 C) is equal to the eutectoid concentration of 0.8% (Fig. 52, a).
2. Ferrite-pearlite. The structure is F + P + PG, their metal base consists of F + P, and the amount of Fe 3 C is less than the eutectoid concentration (Fig. 52, b).
3. Ferritic. Structure F + PG. Their basis consists of F, and Fe 3 C = 0 (Fig. 52, c).
Fig. 52. Structure of gray cast iron: a) pearlitic b) ferritic-pearlitic c) ferritic
The mechanical properties of cast iron depend on the properties of the metal base, the number and size of graphite inclusions. When designing machine parts, it should be taken into account that gray cast iron works better in compression than in tension. They are little sensitive to cuts under cyclic loading, absorb vibrations well during vibrations, and have high anti-friction properties due to the lubricity of graphite. Gray cast irons are easy to cut, cheap and easy to manufacture. Along with these positive properties, they have relatively low strength and extremely low ductility.
The grade of gray cast iron consists of the letters SCH (gray cast iron) and a number indicating a 10-fold reduced value (in megapascals) of tensile strength (Table 7).
The strength of cast iron depends significantly on the thickness of the casting wall. The σ value indicated in the brand corresponds to castings with a wall thickness of 15 mm. As the wall thickness increases from 15 to 150 mm, the strength and hardness of cast iron decreases by almost half.
Graphite, while worsening mechanical properties, at the same time imparts a number of valuable properties to cast iron. It crushes chips during cutting, has a softening effect and, therefore, increases the wear resistance of cast iron and gives them damping ability. In addition, flake graphite ensures low sensitivity of cast iron to surface defects. Thanks to this, the fatigue resistance of cast iron and steel parts is comparable.
According to GOST 1412-85, castings are made from gray cast iron of the following grades: SCh10, SCh15, SCh18, SCh20, SCh25, SCh30, SCh35. The numbers in the brand designation correspond to the minimum tensile strength (σ in, kgf/mm 2). Cast iron SCh10 is ferritic, and starting from SCh25 and more - pearlitic, intermediate - ferritic-pearlitic.
Ferritic cast irons are used to make mainly non-critical parts, which are mainly subject to the requirements of good machinability rather than strength, for example, plates, weights, troughs, covers, casings, etc.
In the automotive industry, ferritic-pearlite cast iron is used to make crankcases, brake drums, covers, pistons, piston rings, large pulleys, gears, etc.
Perlite - cylinder blocks, liners, flywheels, etc. In machine tool industry, gray cast iron is the main structural material (machine beds, tables and upper slides, spindle heads, columns, carriages, etc.), wear-resistant ones include bleached gray cast iron (0H ), having a thin surface layer with the structure of white cast iron. used for the manufacture of castings of rolling rolls, carriage wheels, etc.
Malleable cast irons.
The name “malleable cast iron” is conditional, since products from it, like from any other cast iron, are made not by forging, but by casting. This cast iron received the name “malleable” due to its higher plastic properties compared to gray cast iron.
The principle diagram of the technology for producing parts from malleable cast iron consists of two operations. First, parts are produced by casting from white hypoeutectic cast iron (recommended chemical composition of the alloy poured into molds: 2.4...2.9% C; 1.0...1.6% Si; 0.3...1. 0% Mn; ≤ 0.1% S; ≤ 0.2% P, then the resulting castings are subjected to special graphitizing annealing (simmering). Annealing usually consists of two stages (Fig. 53).
First, white cast iron castings (usually packed in boxes with sand) are slowly heated over 20...25 hours to a temperature of 950...1050°C. And they are kept at the same temperature for a long time (for 10...15 hours). During this period, the first stage of graphitization occurs, i.e. decomposition of cementite, which is part of ledeburite (A + Fe 3 C), and the establishment of a stable equilibrium of austenite + graphite.
As a result of the decomposition of cementite, flake-like graphite (annealing carbon) is formed.
The metal base of cast iron is formed in the second stage of annealing during the eutectoid transformation. In the case of continuous cooling of the casting (in air) in the eutectoid temperature region (727°C), the austenite disintegrates into pearlite and the graphitization process does not have time to cover the pearlite cementite. Cast iron adopts the structure: lamellar pearlite + flake graphite (CG). It has high hardness, strength and low ductility (HB 235...305, σ in = 650... 680 MPa, δ = 3.0...15%) . To increase ductility while maintaining sufficiently high strength, a short (2...4 hours) isothermal holding of cast iron or slow cooling at temperatures of 690...650°C is carried out. This is the second stage of annealing, which in this case is annealing onto granular pearlite.
Rice. 53. Annealing schedule for white cast iron for malleable
In mechanical engineering, ferritic malleable cast iron is widely used, characterized by high ductility (δ = 10...12%) and relatively low strength (σ in = 370...300 MPa). The ferrite base of cast iron is formed by very slowly passing through the range of 760...720°C or during isothermal exposure at 720...700°C. Here austenite and cementite, including pearlite cementite, if pearlite has had time to rejoice, breaks down into ferrite + flake graphite. The flake form of graphite is the main reason for the higher strength and ductility of malleable cast iron compared to gray cast iron (see Table 7).
The duration of annealing in general is 48...96 hours (the duration of stage II is approximately 1.5 times longer than stage I). To reduce the duration of annealing into the melt before pouring it into molds, aluminum (less commonly boron, bismuth, etc.) is introduced (modified), which creates additional artificial centers of graphite formation. According to GOST 1215-79, the following grades of malleable cast iron are produced: KCh30-8, KCh35 -10, KCh37-12, KCh45-7, KCh50-5, KCh55-4, KCh60-3, KCh65-3, KCh70-2, KCh80-1.5.
tensile strength (σ in, kgf/mm 2); numbers after the dash - relative elongation (δ, % )
Malleable cast irons are used for parts operating under shock vibration loads (hubs, brake pads, crankshafts, hooks, gear housings, etc.).
The main disadvantage of obtaining CP is the long annealing of castings and the limitation of their wall thickness (up to 50 mm). In passive parts, as a result of slow cooling during crystallization, lamellar graphite appears (instead of flake-like), which reduces the strength and ductility of cast iron.
Table 7. Mechanical properties of cast irons.
Gray cast iron (GOST 1412 - 85)
| SCH 10 | - | - | -190 | F | |
| SCh 15 | - | - | 163-210 | F | |
| SCH 25 | - | - | 180-245 | F+P | |
| SCH 35 | - | - | 220-275 | P |
High-strength cast irons (GOST 7293 - 85)
| HF 35 | 140-170 | F | |||
| HF 45 | 140-225 | F+P | |||
| HF 60 | 192-227 | F+P | |||
| HF 80 | 248-351 | P | |||
| HF 100 | 270-360 | B |
Malleable cast iron (GOST 1215 – 79
| CC 30 – 6 | - | 100-163 | F+up to 10%P | ||
| CC 35 – 8 | - | 100-163 | |||
| KCH37 – 12 | - | 110-163 | |||
| KCH45 – 7 | - | 150-207 | |||
| CC 60 - 3 | - | 200-269 | P+up to 20%F | ||
| CC 80-1.5 | - | 1,5 | 270-320 |
9.4. High-strength cast irons.
High-strength cast iron is obtained by modification (microalloying of liquid cast iron with magnesium (0.1...0.5%) or cerium (0.2...0.3%). Moreover, under the influence of magnesium, graphite during the crystallization process takes on a non-platelike, a spherical shape. The microstructure of modified cast iron on a ferritic and pearlite basis is shown in Fig. 54, a, b.
Rice. 54. Structure of high-strength cast iron: a) ferritic b) pearlitic
The main reason for the high mechanical properties of high-strength cast iron (Table 7) is the spherical shape of graphite. Nodular graphite, which has a minimum surface area for a given volume, weakens the metal base of cast iron significantly less than flake graphite. Unlike the latter, it is not an active stress concentrator.
According to GOST 7293-85, castings are made from high-strength cast iron of the following grades: VC35, VC40, VC45, VC50, VC60, VC70, VC80, VC100 (the numbers in the designation correspond to the minimum tensile strength σ in, kgf/mm 2)
High-strength cast iron has high mechanical characteristics and good casting and technological properties. It is used as a new material and as a substitute for steel, ductile and gray cast iron with flake graphite. Compared to steel, it has greater wear resistance, better anti-friction and anti-corrosion properties, better machinability. Due to the lower density of the casting, it is 8...10% lighter than steel. High-strength cast iron, unlike ductile iron, can be used to cast parts of any cross-section, weight and size.
Areas of application: in machine tool industry - calipers, tool holders, heavy faceplates, spindles, levers, etc.; for rolling and press-forging equipment - rolling rolls, beds of rolling mills and forging hammers, chabots, press cross-beams; for other types of equipment - drums of excavator hoists, crankshafts, etc.
9.5. Alloy cast irons.
Requirements for alloy cast irons for castings with increased heat resistance, corrosion resistance, wear resistance or heat resistance are regulated by GOST 7769-82. The grades of alloyed cast irons and their properties are given in Table. 8.
Alloy cast irons are heat treated to provide the required properties and structure.
An important property of alloy cast irons is wear resistance.
Cast irons in accordance with GOST 1585-85 are used as antifriction ones. They are intended for the manufacture of parts operating in friction units with lubrication. The standard defines the brands of antifriction cast irons, their chemical composition, characteristics, purpose, shape, size and distribution of graphite, pearlite dispersion, the nature of the distribution of phosphide eutectic, hardness and maximum operating conditions of parts made from these cast irons. They are based on iron, constant components, %: 2.2-4.3 C; 0.5-4.0 Si; 0.3-12.5 Mn. Allowed impurities, %: 0.1-1 R; 0.03-0.2 S.
The brands of antifriction cast irons, their characteristics and values are presented in table. 9.
Table 8.
Grades and properties of alloy cast iron (GOST 7769-82)
| Cast iron grade | Properties |
| CH1, CH2, CH3 | Cast irons, which have increased corrosion resistance in gas, air and alkaline environments under conditions of friction and wear, are heat-resistant in air, can withstand temperatures from 500 to 700˚. intended for the manufacture of metallurgical production parts, glass mold molds, chemical equipment parts, etc. |
| ChH3T, ChH9N5, ChH22, ChH16M2, ChH28D2 | Cast irons with increased resistance to abrasive wear and abrasion |
| ChH22S | This cast iron is characterized by increased corrosion resistance at temperatures of 1000˚C |
| ChS13, ChS15, ChS17, ChS15MA, ChS17M3 | Resistant to concentrated and diluted acids, alkali solutions, salts |
| ChG6S3Sh, ChG7X4 | Cast irons with high resistance to abrasive environments |
| ChG8D3 | Non-magnetic wear-resistant cast iron |
| ChNHT, ChNHMD, ChN2H, ChNMSh | Cast irons with high mechanical properties, resist wear and corrosion well |
| ChN15D3Sh, ChN19H3Sh, ChN11G7Sh, ChN20D2Sh, ChN15D7 | Cast irons with high mechanical properties, high corrosion and erosion resistance in alkalis, weak acid solutions, and sea water. Cast iron ChN20D2Sh can be plastically deformed in a cold state |
Table 9.
Brands of antifriction cast irons, their properties and purpose
(GOST 1585-85)
| Cast iron grade | Properties and purpose |
| ASF-1 | Pearlitic cast iron alloyed with chromium (0.2-0.5%) and copper (0.8-1.6%); designed for the manufacture of parts working in tandem with a hardened or normalized shaft |
| ASF-2 | Pearlitic cast iron alloyed with chromium (0.2-0.5%), nickel (0.2-0.5%), titanium (0.03-0.1%) and copper (0.2-0.5% ); purpose - the same as cast iron grade ASCH-1 |
| ASF-3 | Pearlitic-ferritic cast iron alloyed with titanium (0.03-0.1%) and copper (0.2-0.5%); parts made of such cast iron can work in pairs with both a “raw” and a heat-treated shaft |
| ASF-4 | Pearlitic cast iron alloyed with antimony (0.04-0.4%); used for the manufacture of parts working in tandem with a hardened or normalized shaft |
| ASF-5 | Austenitic cast iron alloyed with manganese (7.5-12.5%) and aluminum (0.4-0.8%); This cast iron is used to make parts that work in particularly loaded friction units paired with a hardened or normalized shaft |
| ASF-6 | Pearlitic porous cast iron alloyed with lead (0.5-1.0%) and phosphorus (0.5-1.0%); recommended for the production of parts operating in friction units with temperatures up to 300 ˚ C paired with a “raw” shaft |
| AChV-1 | Pearlitic nodular cast iron; parts made of such cast iron can operate in friction units with increased peripheral speeds paired with a hardened or normalized shaft |
| AChV-2 | Pearlite-ferritic cast iron with nodular graphite; parts made from this cast iron work well under friction conditions with increased peripheral speeds when paired with a “raw” shaft |
| ABC-1 | Pearlitic cast iron with flake graphite, alloyed with copper (1.0-1.5%); designed for the manufacture of parts working in tandem with a heat-treated shaft |
| ABC-2 | Ferritic-pearlitic cast iron with flake graphite; parts made of this cast iron work in tandem with a “raw” shaft |
The letters in the designations of cast iron grades mean: ACh - antifriction cast iron, C - gray cast iron with flake graphite, B - high-strength cast iron with nodular graphite, K - malleable cast iron with flake graphite. The hardness of castings made of antifriction cast iron (from 100 to 290 HB) depends on the element content and heat treatment conditions.
Limit operating modes of parts made of these cast irons in friction units: specific pressure (50 - 300) 10 4 Pa (5-300 kgf/cm2), peripheral speed 0.3-10 m/s.
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:
- minimum relative elongation - as a rule, not exceeding 0.5%;
- low impact strength;
- low plasticity.
Gray cast iron contains a small percentage of 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 supporting 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 effects. 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 a higher concentration of manganese and phosphorus (note that their presence is largely determined by the chemical composition of the ore from which the cast 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 determines the difference between the metals under consideration in terms of:
- 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 industries | 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 |
Introduction
Cast iron is an alloy of iron with carbon and other elements containing more than 2.14% C.
In metallurgical production, cast iron is smelted in blast furnaces. The resulting cast irons are divided into: pig iron, special cast iron (ferroalloys) and foundry cast iron. Pipe and special cast irons are used for subsequent processing into steel. Foundry cast iron (about 20% of all cast iron produced) is sent to machine-building plants for use in the manufacture of cast parts (castings).
Unalloyed structural cast iron for the production of castings in mechanical engineering has the following chemical composition,%: 2.0 - 4.5 C; 1.0 - 3.5 Si; 0.5-- 1.0 MP; impurity content: no more than 0.3% S; no more than 0.15% S.
The widespread use of cast iron in industry is due to the optimal combination of various properties: technological (casting, machinability), operational (mechanical and special) and technical and economic indicators.
White and gray cast iron
The main structural component of white cast iron is brittle and hard cementite. Therefore, white cast irons have high hardness and brittleness. Because of these properties, they are rarely used in technology and are not used at all in construction. White cast iron is converted into steel and gray cast iron. Three types of white cast iron are smelted in blast furnaces: foundry coke iron, pigment coke iron and ferroalloys.
Foundry coke iron. (GOST 4832--72) contains from 3.5 to 4.6% carbon and is used for the production of gray cast iron.
Pig iron is used for steel smelting and casting production.
Ferroalloys are used as additives in steel smelting. They contain increased amounts of manganese and silicon. Thus, one of the types of ferroalloys - mirror cast iron contains 10-25% manganese, ferromanganese - 70-80% manganese, and ferrosilicon - 9-12% silicon.
Gray cast iron. Gray cast iron received this name from the gray color of the fracture, in contrast to the silver color of the fracture of white cast iron. The gray color of the fracture is given by carbon, which is part of gray cast iron in a free state in the form of graphite. Graphite is formed in gray cast irons 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.
The structure of gray cast iron consists of a metal base and unrelated graphite inclusions. The mechanical properties of gray cast iron depend on the structure of the metal base, the amount of carbon and the configuration of graphite inclusions.
The metal base in gray cast iron consists of one ferrite, or one perlite, or a mixture of both. The most durable, but at the same time, the least ductile, is pearlite-based cast iron.
Ferritic cast iron has the highest ductility with the lowest strength. The structure of the metal base depends on the heat treatment mode or on the amount of silicon. As the amount of introduced silicon increases, the degree of graphitization increases. With the introduction of about 5% silicon in the structure of gray cast iron, cementite is completely absent in the metal base consists of only ferrite. Gray cast iron is smelted on all three metal bases.
Graphite inclusions in cast iron are not associated with the metal base. Therefore, as the carbon content increases, the volume of graphite inclusions increases, which reduces their strength. This is due to the relatively low carbon content (from 3.5 to 4.5%) in coke pig irons used for the production of gray cast iron castings.
The configuration of graphite inclusions significantly affects the mechanical properties of gray cast irons. Cast irons with lamellar graphite inclusions have the worst properties, those with globular (spherical) or flake-like inclusions have the best properties, and cast irons with point graphite inclusions have average properties. The configuration of graphite inclusion depends on the method of producing gray cast iron.
The industry produces gray, high-strength, alloy and malleable cast irons.
Gray cast iron with lamellar graphite (GOST 1412--79) is produced in grades from SCh 10 to SCh 45. In the brands, the letters indicate the name of the cast iron, the numbers indicate the tensile strength of the cast iron, N/mm 2. Graphitization in gray cast iron is achieved by introducing from 1 to 2.9% silicon into their composition. In this case, lamellar graphite inclusions are formed.
To obtain higher mechanical properties, gray cast iron is modified. 0.3-0.8% modifiers are introduced into the molten cast iron, for which ferrosilicon or silicocalcium is used, containing 70-65% silicon and 30-35% calcium. With this modification, graphite is distributed in the form of point inclusions;
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. Such cast irons, with a high tensile strength of up to 12 MPa, also have a relatively high elongation of up to 17%. High-strength cast irons are produced in grades from VCh 38-17 to VCh 120-2. The letters mean the name of the cast iron, the first two numbers are the tensile strength of the cast iron, kgf/mm 2, the second are the relative tensile elongation,%.
Alloyed cast irons are produced by introducing a small amount of alloying additives into gray cast iron: chromium, nickel, copper, titanium, which improve the mechanical properties of the metal base of cast iron and contribute to obtaining a favorable form of graphite.
Malleable cast iron (GOST 1215--79) is a type of gray cast iron obtained by long-term (up to 80 hours) keeping white cast iron at high temperatures. This heat treatment is called simmering. 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. Such cast irons are the most ductile of all types of cast iron. The relative elongation of ferritic ductile cast iron is up to 12% with a tensile strength of 3.7 MPa, and of ferritic-pearlite cast iron is 5% with a strength of up to 5 MPa. Malleable cast irons are produced in grades from KChZO-6 to KCh 50-5. The brand decoding is the same as for high-strength cast iron.
All types of cast iron have good casting properties and also resist corrosion well. Gray cast iron is used to make elements of building structures, including such important ones as supporting parts of reinforced concrete beams, trusses, shoes for columns, and tubing for subway tunnels.
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 numbers indicate the average tensile strength, and the second numbers indicate the average bending strength. 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.
