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Posted on Oct 28, 2020
CI casting is actually cast iron. And cast iron belongs to a specialized group of iron and carbon alloys. The special carbon content is over 2%. This means it is practically derived from a relatively low melting point.
The components of the alloy affect its color when cracking: white cast iron contains carbide impurities that create ways for the alloy to crack and flow through. And gray cast iron may possess graphite flakes that simulate the cracks. This initiates countless new cracks when the material breaks, and ductile iron is spherical graphite " nodules ”that inhibit further fracture development.
Carbon (C) in an amount of 1.8 to 4 wt.%. And silicon (Si) 1-3 wt.% These are the main alloying ingredients of cast iron. Only the specific Iron alloys and a minimal amount of carbon are defined as steel. Cast iron is brittle, except for malleable cast iron. Due to the relatively low melting point, this relates to good fluidity. And this relates to excellent machinability, deformation resistance, and wear resistance.
Cast irons have become an engineered material with a wide range of applications and are used in pipes, machinery, and automotive parts. Typical applications are cylinder heads, engine blocks, and gearboxes. It is resistant to oxidation damage as well so this adds to its usefulness.
The earliest cast-iron artifacts date back to the 5th century BC and were discovered by archaeologists in what is now Jiangsu, China. Researchers think cast iron use began long ago in early mainland China. They believe it was used to create products for farms, homes and fighting.
In the 15th century, cast iron was used for cannons in Burgundy, France, and England during the Reformation. The quantities of cast iron used for the guns required large-scale production.
The first cast iron bridge was built in 1770 by Abraham Darby III and is known as The Iron Bridge in Shropshire, England. Cast iron was also used for the erection of many buildings. The wide use of it in an architectural setting began to be very prevalent.
Cast iron is created mainly from the molten pig iron or by re-melting pig iron. This is frequently, also with substantial quantities of iron, steel, limestone, carbon (coke), and taking various steps to remove undesirable contaminants. When the application is considered, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively.
Cast iron is made from pig iron, and this is the byproduct of melting iron ore from the blast furnace.
Phosphorus and sulfur can satisfy by being burnt out of the molten iron, but this also eliminates the carbon, which must be replaced. If wanted, other elements are then supplied to the melt before the final form is produced by casting. Cast iron is sometimes melted in a unique type of blast furnace and it’s called a cupola.
However, in modern applications, it is more often melted in electric induction furnaces or electric arc furnaces. When the melt is finished, the molten cast iron is poured into a holding furnace or ladle.
The properties of cast iron are altered by adding various alloying elements or alloys. Besides carbon, silicon is the most important alloy as it displaces carbon from solution. The low percentage of silicon allows the carbon to remain in solution, forming iron carbide and producing white cast iron.
A high percentage of silicon displaces carbon from solution, creating graphite and producing gray cast iron. Other alloying elements, manganese, chromium, molybdenum, titanium and vanadium, counteract silicon and promote carbon retention and the formation of these carbides. Nickel and copper increase strength and machinability, but do not change the amount of graphite produced.
Carbon in the form of graphite makes the iron softer, reduces shrinkage, lowers the strength and reduces density. Sulfur, if mostly present as an impurity forms iron sulfide which prevents the formation of graphite and increases hardness.
The problem with sulfur is that it creates molten cast iron is sticky, which causes defects. To counteract the sulfur effects, manganese is added as both form manganese sulfides instead of iron sulfide. Manganese sulfide is lighter than molten, so it tends to flow from the melt into the slag.
The amount of manganese required for sulfur neutralization is 1.7 x sulfur content + 0.3%. If more manganese than this is added, manganese carbide is formed which increases hardness and cooling, except for gray cast iron where up to 1% manganese increases strength and density.
:: Read more : Introduction to Cast Iron Castings
Nickel is one of the most common alloying elements as it refines the structure of perlite and graphite, improves ductility and equalizes hardness differences between the section thicknesses.
Chromium is added in small amounts to reduce free graphite, make it cool, and because it is a strong carbide stabilizer; nickel is often added together. The slight ingredients can be added as a substitute for 0.5% chromium.
Copper is added in the ladle or in the furnace in an amount of 0.5-2.5% to reduce the cold, refine the graphite and increase the fluidity.
Molybdenum is added in an amount of 0.3 to 1% to increase the coolness and to improve the graphite and pearlite structure; it is often added in combination with nickel, copper and chromium to produce high strength iron.
Titanium when introduced as a degasser and deoxidizer, but it also increases fluidity. 0.15–0.5% vanadium is added to cast iron to stabilize cementite, increase hardness, and increase wear and heat resistance. 0.1–0.3% zirconium helps form graphite, deoxygenates and increases fluidity.
In malleable iron melts, bismuth is added on a scale of 0.002–0.01% to increase the amount of added silicon. Boron is added to the white iron to aid the production of malleable cast iron; it also reduces the coarse effect of bismuth.
Gray cast iron is characterized by a graphite microstructure which causes cracks in the material to appear gray. It is the most widely used cast iron and the most used casting material by weight. Most cast iron has a chemical composition of 2.5–4.0% carbon, 1–3% silicon and the rest is iron.
Gray cast iron has lower tensile strength and impact resistance than steel, but its compressive strength is comparable to low and medium carbon steel. These mechanical properties are controlled by the size and shape of the graphite flakes present in the microstructure and can be characterized according to the guidelines given by ASTM.
So overall, CI casting is really just cast iron. This cast iron is just a part of the specialized group of iron and carbon alloys. Only pay attention to the carbon content if it is over 2%. And remember what that means is just has a very low melting point.
The ingredients of the alloy make a change of the color when cracking: white cast iron contains carbide impurities that allow the cracks to move right through. Gray cast iron has graphite flakes and this makes it just reflect the crack and starts many new cracks when the material breaks, and ductile iron is spherical graphite " nodules ”that inhibit further fracture development. This is all about CI casting!
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