While the hardness and strength of steel almost always increase as carbon content rises, in the case of gray cast iron the strongest, hardest grades have less carbon than some of the lower-strength, less expensive grades. Gray iron is usually cast in sand molds, and allowed to cool normally in the mold.
There are four basic types of cast iron: ductile, malleable, white and grey. There are significant differences in the composition and use of white and grey cast iron. Grey Cast Iron is one of the type of Cast iron.
Table of comparative qualities of cast irons
| Name | Nominal composition [% by weight] | Hardness [Brinell scale] |
|---|
| Grey cast iron (ASTM A48) | C 3.4, Si 1.8, Mn 0.5 | 260 |
| White cast iron | C 3.4, Si 0.7, Mn 0.6 | 450 |
| Malleable iron (ASTM A47) | C 2.5, Si 1.0, Mn 0.55 | 130 |
| Ductile or nodular iron | C 3.4, P 0.1, Mn 0.4, Ni 1.0, Mg 0.06 | 170 |
Cast iron pans are popular, especially for searing, and are generally safe to use. But they can leach iron, which is a strong pro-oxidant. Those genetically at risk for iron overload should learn more about cast iron safety. Our evidence-based analysis features 22 unique references to scientific papers.
Cast iron products range in price from something much cheaper than a new set of more modern pots and pans to a much more expensive cost per piece. The price is dependent on the brand, but cheaper does not always mean less effective.
The strength of both cast iron and steel is also controversial, as some think steel is stronger than cast iron and others think that iron and steel are same thing, but the truth is that cast iron has a more compressive strength, and steel is more tensile. Steel is an alloy or iron, and cast iron is a hard grey metal.
Because of the electric range set to high and thermal shock, cast iron pans can easily crack. The crack may appear as a curved line in the beginning, but if the temperature is not lowered, your cast iron pan may crack in two pieces.
Most cast irons have a chemical composition of 2.5–4.0% carbon, 1–3% silicon, and the remainder iron. Grey cast iron has less tensile strength and shock resistance than steel, but its compressive strength is comparable to low- and medium-carbon steel.
A minimum amount of silicon is necessary to improve fluidity of the melt and to produce a fluid slag, but of equal importance is its effect on as-cast hardness. Increased levels of silicon, in the range of 1 to 1.5%, have been found to increase the amount of martensite and the resulting hardness.
It is the most common cast iron and the most widely used cast material based on weight. 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.
If more than this amount of manganese is added, then manganese carbide forms, which increases hardness and chilling, except in grey iron, where up to 1% of manganese increases strength and density.
Silicon is important for making gray iron as 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.
Grey cast iron is characterized by its graphitic microstructure. Most cast irons have a chemical composition of 2.5 to 4.0% carbon, 1 to 3% silicon, and the remainder is iron.
Definition of steadite. : a eutectic of iron phosphide Fe3P and iron that occurs as a microconstituent of high-phosphorus cast iron.
Chilled iron castings are produced by casting the molten metal against a metal chill, resulting in a surface virtually free from graphitic carbon. In the production of chilled iron, the composition is selected so that only the surfaces cast against the chill will be free from graphitic carbon.
0.15–0.5% vanadium is added to cast iron to stabilize cementite, increase hardness, and increase resistance to wear and heat. 0.1–0.3% zirconium helps to form graphite, deoxidize, and increase fluidity. In malleable iron melts, bismuth is added, on the scale of 0.002–0.01%, to increase how much silicon can be added.
Silicon element results in presence of free graphite in C.I.
Grey cast iron
It is the most commonly used cast iron and the most widely used cast material based on weight. Most cast irons have a chemical composition of 2.5–4.0% carbon, 1–3% silicon, and the remainder iron.Carbon controls the hardness of the martensite. Increasing the carbon content increases the hardness of steels up to about 0.6wt%. Carbon also increases the hardenability of steels by retarding the formation of pearlite and ferrite. However, the effect is too small be be commonly used for control of hardenability.
It increases tensile strength, hardness, and resistance to wear and abrasion. Alloying elements improve its physical properties of steel. Hardness,tensile strength,yeild strength etc etc are improved due to its higher hardenability due to the addition of alloying elements.
Cast iron is made from pig iron, which is the product of melting iron ore in a blast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron, often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants.
A few common mechanical properties for cast iron include:
- Hardness – material's resistance to abrasion and indentation.
- Toughness – material's ability to absorb energy.
- Ductility – material's ability to deform without fracture.
- Elasticity – material's ability to return to its original dimensions after it has been deformed.
Alloying elements increase the strength of cast irons through their effect on the matrix. Common alloy elements include manganese, copper, nickel, molybdenum, and chromium, elements added primarily to control the matrix microstructure, having only a small effect upon the solidification microstructure.
Sulfur (S)
Sulfur is normally regarded as an impurity and has an adverse effect on impact properties when a steel is high in sulphur and low in manganese. Sulphur improves machinability but lowers transverse ductility and notched impact toughness and has little effects on the longitudinal mechanical properties.Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite. Most steel contains less than 0.35 percent carbon.
Phosphorus prevents the sticking of light-gage sheets when it is used as an alloy in steel. It strengthens low carbon steel to a degree, increases resistance to corrosion and improves machinability in free-cutting steels.
Nitrogen (N): Nitrogen has the effect of increasing the Austenite stability of stainless steels and is, as in the case of Nickel, an Austenite forming element. Yield strength is greatly improved when nitrogen is added to stainless steels as is resistance to pitting corrrosion.
Aluminum (Al) is used for deoxidizing and grain refining in steels. It is a strong deoxidizer. It is also used as nitride former and as an alloying agent. Its ability to scavenge nitrogen (N) from steel makes it a useful addition in drawing quality steels, especially for automotive applications.
Sulfur is characterized as a non-metal because it is consistent with the 3 physical properties listed for nonmetals. It is a poor conductor of heat and electricity because the electrons are not free to move. Sulfur or sulphur is a chemical element with symbol S and atomic number 16.
Chromium (Cr): Chromium is added to steel to increase resistance to oxidation. This resistance increases as more chromium is added. Nickel (Ni): Nickel is added in large amounts, over about 8%, to high Chromium stainless steels to form the most important class of corrosion and heat resisting steels.
While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium.
Molybdenum adds corrosion resistance and high temperature strength. Molybdenum primarily increases the corrosion resistance of stainless steels (see Grades and Properties). They are used in applications that are more corrosive, such as chemical processing plants or in marine applications.
