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Micrograph of grey cast ironGray iron, or grey cast iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite.[1] It is the most common cast iron and the most widely used cast material based on weight.[2]
It is used for housings where the stiffness of the component is more important than its tensile strength, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors.[3]
Its former widespread use[clarify] on brakes in freight trains has been greatly reduced in the European Union over concerns regarding noise pollution.[4][5][6][7] Deutsche Bahn for example had replaced grey iron brakes on 53,000 of its freight cars (85% of their fleet) with newer, quieter models by 2019—in part to comply with a law that came into force in December 2020.[8][9][10]
Structure
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A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon by weight. Graphite may occupy 6 to 10% of the volume of grey iron. Silicon is important for making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides; at 3% silicon almost no carbon is held in chemical form as iron carbide. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. A moderate cooling rate forms a more pearlitic matrix, while a fast cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed.[1][11] Rapid cooling partly or completely suppresses graphitization and leads to the formation of cementite, which is called white iron.[12]
The graphite takes on the shape of a three-dimensional flake. In two dimensions, as a polished surface, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches at which stresses concentrate and it therefore behaves in a brittle manner.[12][13] The presence of graphite flakes makes the grey iron easily machinable as they tend to crack easily across the graphite flakes. Grey iron also has very good damping capacity and hence it is often used as the base for machine tool mountings.
Classifications
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In the United States, the most commonly used classification for gray iron is ASTM International standard A48.[2] This orders gray iron into classes which correspond with its minimum tensile strength in thousands of pounds per square inch (ksi); e.g. class 20 gray iron has a minimum tensile strength of 20,000 psi (140 MPa). Class 20 has a high carbon equivalent and a ferrite matrix. Higher strength gray irons, up to class 40, have lower carbon equivalents and a pearlite matrix. Gray iron above class 40 requires alloying to provide solid solution strengthening, and heat treating is used to modify the matrix. Class 80 is the highest class available, but it is extremely brittle.[12] ASTM A247 is also commonly used to describe the graphite structure. Other ASTM standards that deal with gray iron include ASTM A126, ASTM A278, and ASTM A319.[2]
In the automotive industry, the SAE International (SAE) standard SAE J431 is used to designate grades instead of classes. These grades are a measure of the tensile strength-to-Brinell hardness ratio.[2] The variation of the tensile modulus of elasticity of the various grades is a reflection of the percentage of graphite in the material as such material has neither strength nor stiffness and the space occupied by graphite acts like a void, thereby creating a spongy material.
Properties of ASTM A48 classes of gray iron[14] Class TensileAdvantages and disadvantages
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Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips. It also has good galling and wear resistance because the graphite flakes self-lubricate. The graphite also gives gray iron an excellent damping capacity because it absorbs the energy and converts it into heat.[3] Grey iron cannot be worked (forged, extruded, rolled etc.) even at temperature.
Relative damping capacity of various metals[15] Materials Damping capacity† Gray iron (high carbon equivalent) 100–500 Gray iron (low carbon equivalent) 20–100 Ductile iron 5–20 Malleable iron 8–15 White iron 2–4 Steel 4 Aluminum 0.47 †Natural log of the ratio of successive amplitudesGray iron also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increased fluidity when casting.[12] Gray iron is generally considered easy to weld.[16] Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact and shock resistance is almost non-existent.[16]
See also
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Notes
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References
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Further reading
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Cast iron material grade comparison and equivalent grades for grey iron, malleable iron and nodular iron.
Cast Iron Type
UK
Int'l
USA
USA
Germany
Germany
France
Italy
Sweden
Japan
Russia
new BS
ISO
UNS
AISI / SAE / ASTM
W.-Nr.
DIN
AFNOR
UNI
SIS
JIS
GOST
GREY IRON
(FLAKE
GRAPHITE)
Grade 100
Grade 100
F11401
A48, class 20 B
0.601
GG 10
Ft 10 D
G 10
01 10-00
FC 100
Sc 10
Grade 150
Grade 150
F11701
A48, class 25 B
0.6015
GG 15
Ft 15 D
G 15
01 15-00
FC 150
Sc 15
Grade 180
Grade 200
Grade 200
F12101
A48, class 30 B
0.602
GG 20
Ft 20 D
G 20
01 20-00
FC 200
Sc 20
Grade 220
F12401
A48, class 35 B
.
Grade 250
Grade 250
F12801
A48, class 40 B
0.6025
GG 25
Ft 25 D
G 25
01 25-00
FC 250
Sc 25
Grade 300
Grade 300
F13101
A48, class 45 B
0.603
GG 30
Ft 30 D
G 30
01 30-00
FC 300
Sc 30
Grade 350
Grade 350
F13801
A48, class 55 B
0.6035
GG 35
Ft 35 D
G 35
01 35-00
FC 350
Sc 35
F14101
A48, class 60 B
0.604
GG 40
Ft 40 D
01 40-00
Sc 40
MALLEABLE IRON
BLACKHEART
B30-06
B30-06
B32-10
B32-10
F20000
A602, grade M3210
FCMB 310
B35-12
F22200
A47M, grade 22010
GTS-35-10
FCMB 340
MALLEABLE IRON
PEARLITIC
P45-06
P45-06
F23131
A220M, grade 310M6
FCMP 440
P50-05
P50-05
F23530
A220M, grade 340M5
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FCMP 490
P55-04
P55-04
F24130
A220M, grade 410M4
FCMP540
P60-03
P60-03
F24830
A220M, grade 480M3
FCMP 590
P65-02
P65-02
F25530
A220M, grade 550M2
P70-02
P70-02
F26230
A220M, grade 620M1
FCMP 690
MALLEABLE IRON
WHITEHEART
W 35-04
W 35-04
GTW-35-04
FCMW 330
W 38-12
W 38-12
MB 380-12
W 40-05
W 40-05
GTW-40-05
FCMW 370
W 45-07
W 45-07
GTW-45-07
MB 450-7
FCMWP 440
NODULAR IRON
GRAPHITE
350/22L40
350/22
370/17
0.7043
GGG 40.3
FGS 370-17
GSO 42-15
0717-15
VC 42-12
400/18L20
400/18
400-18
F32800
A536, 60-40-18
0.704
GGG 40
FGS 400-12
GS 400-12
0717-02
FCD 400
420/12
VC 42-12
450/10
450-10
500/7
500-7
F33100
A536, 65-45-12
0.705
GGG 50
FGS 500-7
GS 500-7
0727-02
FCD 500
VC 50-2
600/3
600-3
F33800
A536, 80-55-06
0.706
GGG 60
FGS 600-3
GS 600-3
0732-03
FCD 600
VC 60-2
700/2
700-2
F34800
A536, 100-70-03
0.707
GGG 70
FGS 700-2
GS 700-2
0737-01
FCD 700
VC 70-2
800/2
800-2
F36200
A536, 120-90-02
0.708
GGG 80
FGS 800-2
GS 800-2
VC 80-2
900/2
900-2
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