Gray Cast Iron, also known as gray iron, is a popular type of iron used in castings. The composition of gray cast iron is 2.5%'4% carbon, 1%'3% silicon, and some additions of manganese ranging from 0.1% to 1.2%. Gray iron contains graphitic microstructures, giving the iron it's gray color. This iron is easily cast but it cannot be forged or worked mechanically, either hot or cold. While the tensile strength and impact resistance of gray iron is weaker than most other metals, the compressive strength is equivalent to low-carbon and medium-carbon steel. For most applications, gray cast iron typically has acceptable ductility, tensile strength, yield strength, and impact resistance.
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Hardness (Vickers): 161-321
Tensile Strength: 7 tons per square inch on average
Tensile Strength, Ultimate: - psi (115-700 MPa)
Tensile Strength, Yield: - psi (65.5-420 MPa)
Compressive Yield Strength: - psi (572- MPa)
Melting Point: - degrees Celsius
There are many unique benefits of using gray cast iron. One of the most notable is gray iron's enhanced lubrication. The graphite flakes in gray iron allow for better lubrication creating less wear on the cast iron. This makes gray cast iron a great choice for parts like brake rotors. Gray cast iron also has the ability to dampen vibrations making it an excellent choice for housing applications or industrial applications such as machinery bases. Additionally, gray iron has high thermal conductivity allowing it to move heat more easily through the metal.
Gray cast iron can withstand thermal cycling, meaning the iron can go through high and low temperature changes without strain. Thermal cycling can create stress and premature failure in other forms of metal casting, however gray cast iron has continuously proved its ability to withstand thermal cycling. Gray iron is very resistant when it comes to oxidation. Gray cast iron iron develops a protective film or scale on the surface, making it more resistant to corrosion than wrought iron or mild steel.
One of gray cast iron's biggest pros is also its biggest con. The graphite flakes that give gray iron its added lubrication and resistance to wear, also allows areas of weakness in the metal where fractures can begin. These fractures can cause splits and breakage. Fracturing is why gray iron has low tensile and impact strength.
Gray cast iron has many applications and is often used across every industry such as automotive, construction, and manufacturing.
Examples of gray cast iron applications:
Gray 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 'in part to comply with a law that came into force in December .[8][9][10]
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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.
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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[
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Class Tensile[
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Grade Brinell hardness t/h' Description G 120'187 135 Ferritic-pearlitic G 170'229 135 Pearlitic-ferritic G 187'241 150 Pearlitic G 207'255 165 Pearlitic G 217'269 175 Pearlitic 't/h = tensile strength/hardness[
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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[
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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]
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