Cast Iron | In detail
Cast irons are a family of ferrous alloys with a wide range of properties, and as their name implies, they are intended to be cast into the desired shape instead of being worked in the solid state. They are distinguished by their elevated carbon content.
What is cast iron?
Cast iron is a group of iron-carbon alloys with a carbon content greater than 2%. It is one of the oldest types of iron alloys and has a long history of use in various applications due to its desirable properties. The key features of cast iron include its excellent castability, good machinability, and specific metallurgical structure.
What is cast iron made from?
Cast irons consist of these phases: ferrite, pearlite, cementite (Fe3C), and, in some types, graphite. Unlike steels, which usually contain less than about 1% carbon, cast irons normally contain 2% to 4% carbon and 1% to 3% silicon. Other alloying elements, such as manganese, sulfur, and phosphorus, may also be present to control or vary certain properties.
Note that:
- Cementite (Fe3C) is a metastable compound, and under some circumstances it can be made to dissociate or decompose to form 𝛼-ferrite and graphite, according to the reaction:
Fe3C (cementite) →3Fe (𝛼) + C (graphite)
What are the main properties of cast iron?
Cast iron typically contains a higher carbon content than other types of iron and steel. This high carbon content contributes to its hardness and brittleness.The iron–carbon phase diagram reveals that alloys within this composition range become completely liquid at temperatures between approximately 1150°C and 1300°C (2100°F and 2350°F), which is considerably lower than for steels. Thus, they are easily melted and amenable to casting.
Cast irons make excellent casting alloys since they are easily melted, are very fluid in the liquid state, and do not form undesirable surface films when poured. Cast irons solidify with slight to moderate shrinkage during casting and cooling. This makes it suitable for a wide range of applications, from cookware to engine components.Cast iron exhibits high wear resistance, making it suitable for applications where durability and resistance to abrasion are crucial, such as in engine components.
Cast iron has a high compressive strength, making it suitable for applications where the material needs to withstand heavy loads or pressure.
Cast iron is relatively easy to machine, allowing for the production of intricate shapes and precision components.Low impact resistance and ductility due to cementite and high carbon content.
Types of cast iron
There are several types of cast iron, each with its own composition, microstructure, application, and properties. Here are some common types of cast iron:
1. Gray cast iron
Gray cast iron forms when the carbon in the alloy exceeds the amount that can dissolve in the austenite, precipitating as graphite flakes.
Mechanically, gray iron is comparatively weak and brittle in tension, a consequence of its microstructure. The tips of graphite flakes are sharp and pointed, potentially serving as stress concentration points under external tensile stress. However, strength and ductility are much higher under compressive loads. Despite its weaknesses in tension, gray irons have desirable characteristics and are extensively used. They are particularly effective in damping vibrational energy.
When a piece of solidified gray iron fractures, the exposed surface appears gray due to the presence of graphite. This material is crucial in engineering due to its relatively low cost and useful properties, including excellent machinability at hardness levels that provide good wear resistance.
Gray cast iron Composition and Microstructure
Unalloyed gray cast irons usually contain 2.5% to 4% C and 1% to 3% Si. Since silicon is a graphite-stabilizing element in cast irons, a relatively high silicon content is used to promote the formation of graphite.
The solidification rate is also an important factor that determines the extent to which graphite forms. Moderate and slow cooling rates favor the formation of graphite. The solidification rate also affects the type of matrix formed in gray cast irons. Moderate cooling rates favor the formation of a pearlitic matrix, whereas slow cooling rates favor a ferritic matrix. To produce a fully ferritic matrix in an unalloyed gray iron, the iron is usually annealed to allow the carbon remaining in the matrix to deposit on the graphite flakes, leaving the matrix completely ferritic.
Gray cast iron applications
Gray cast iron (G2500) is used in small cylinder blocks, cylinder heads, clutch plates, pots and pans. Gray perlite cast iron (G3500) is used in cylinder blocks of trucks, tractors and heavy gearboxes. Gray perlite cast iron (G4000) is used in diesel engine castings.
2. White cast iron
White cast iron is so named because it decomposes to produce a "white" or shiny crystalline surface. To retain carbon in the form of iron carbide (Fe3C) in white cast iron, the carbon and silicon contents must remain relatively low and the solidification rate must be high.
As a result of the large amounts of cementitious phase, white iron is extremely hard but also very brittle, so much so that it cannot be physically shaped. Its use is limited to applications that require a very hard, wear-resistant surface, without a high degree of ductility.
White cast irons are often used for their excellent resistance to wear and corrosion. The large amount of iron carbides present in its structure is mainly responsible for its wear resistance. White cast iron is the raw material for malleable cast iron.
White cast iron composition and microstructure
White cast iron is formed when much of the carbon in molten cast iron forms iron carbide (Fe3C) instead of graphite upon solidification.
For cast irons with low silicon (0.5-1.5% Si and 2.5-3.0% C) and fast cooling rates, most of the carbon is present as cementite rather than graphite (needle-shaped area) in the eutectic matrix.
White cast iron applications
High Chromium White Iron (ASTM A532) is used in mining and earth-moving equipment components, wear-resistant parts in the cement and energy industries, and components of dredging pumps, while Ni-Hard (ASTM A532) is used in grinding balls, pump casings and impellers.
Abrasion Resistant White Iron (ASTM A532) is used in chute liners, bucket liners, wear plates and other applications where high wear resistance is required.
3. Malleable cast iron
Malleable cast iron is a type of cast iron that has been
heat-treated to impart ductility and malleability to the material.
Malleable cast irons are important engineering materials
since they have the desirable properties of castability, machinability,
moderate strength, toughness, corrosion resistance for certain applications,
and uniformity since all castings are heat-treated.
Malleable cast iron Composition and Microstructure
Malleable cast irons are first cast as white cast irons that
contain large amounts of iron carbides and no graphite. The chemical
compositions of malleable cast irons are therefore restricted to compositions
that form white cast irons. The carbon and silicon contents of malleable irons
are in the 2% to 2.6% C and 1.1% to 1.6% Si ranges.
To produce a malleable iron structure, cold white iron castings are heated in a malleablizing furnace to dissociate the iron carbide of the white iron to graphite iron. The graphite in the malleable cast iron is in the form of irregular nodular aggregates called (temper carbon).
1. Graphitization: In this stage the white iron
castings are heated above the eutectoid temperature, usually about 940°C
(1720°F), and held for about 3 to 20 h depending on the composition, structure,
and size of the casting. In this stage, the iron carbide of the white iron is
transformed to temper carbon (graphite) and austenite.
2. Cooling: In this stage, the austenite of
the iron can be transformed to three basic types of matrixes: ferrite,
pearlite, or martensite.
- Ferritic
malleable iron. To produce a ferrite matrix, the casting, after the first
stage heating, is fast-cooled
to 740°C to 760°C (1360°F to 1400°F) and
then slowly cooled at a rate of
about 3°C to 11°C (5°F to 20°F) per hour. During
cooling, the austenite is transformed to ferrite and graphite,
with the graphite depositing on existing particles of temper carbon.
- Pearlitic
malleable iron. To produce this iron, the castings are slowly cooled
to about 870°C (1600°F) and are air-cooled. The
rapid cooling in this case transforms the austenite to pearlite;
as a result, pearlitic malleable iron is formed, which consists of
temper carbon nodules in a pearlite matrix.
- Tempered martensitic malleable iron. This type of malleable iron is produced by cooling the castings in the furnace to a quenching temperature of 845°C to 870°C (1550°F to 1600°F), holding for 15 to 30 min to allow them to homogenize, and quenching in agitated oil to develop a martensitic matrix. Finally, the castings are tempered at a temperature between 590°C and 725°C (1100°F to 1340°F) to develop the specified mechanical properties. The final microstructure is thus temper carbon nodules in a tempered martensitic matrix.
Malleable cast iron applications
Cast iron (32510) is used in hand tools, the production of pipe fittings, and the manufacture of components for the railway industry, such as brackets, supports, and connecting elements.
Martensitic cast iron (M7002) is used in mining and construction equipment, cutting tools, and some railway components, such as wear plates on railway tracks or components in braking systems, can benefit from the wear resistance and strength of martensitic.
4. Ductile (Nodular) cast iron
Ductile iron, also known as nodular cast iron or spheroidal graphite iron, has a composition and microstructure that gives it improved ductility compared to other types of cast iron.
Ductile iron has good fluidity and castability, excellent machinability, and good wear resistance. In addition, ductile cast iron has a number of properties similar to those of steel such as high strength, toughness, ductility, hot workability, and hardenability. For example, ferritic ductile irons have tensile strengths between 380 and 480 MPa (55,000 and 70,000 psi) and ductility (as percent elongation) from 10% to 20%.
Ductile cast iron Composition and Microstructure
The exceptional engineering properties of ductile iron
are due to the spherical nodules of graphite in its internal structure.
The composition of unalloyed ductile iron is similar to that
of gray iron with respect to carbon and silicon contents. The carbon
content of unalloyed ductile iron ranges
from 3% to 4% C and the silicon content
from 1.8% to 2.8%. The sulfur and phosphorus levels of
high-quality ductile iron, however, must be kept very low
at 0.03% S maximum and 0.1% P maximum,
which are about 10 times lower than the maximum levels for gray cast iron.
Other impurity elements also must be kept low because they interfere with
the formation of graphite nodules in ductile cast iron.
The spherical nodules in ductile cast iron are formed during
the solidification of the molten iron because the sulfur and oxygen levels
in the iron are reduced to very low levels by adding magnesium to the
metal just before it is cast. Magnesium reacts with sulfur and oxygen so
that these elements cannot interfere with the formation of the sphere-like
nodules.
Ductile (Nodular) cast iron applications
Ferritic (60-40-18) cast iron is used
in Pressure castings, such as valve and pump bodies. Martensitic (120-90-02) cast
iron is used in Pinions, gears, rollers, and slides.
Cast iron applications in industry
Cast iron has a wide range of uses across various industries and applications. Some common uses of Cast iron include cookware, construction, engine blocks, railroad components, street furniture, water and sewer Pipes, industrial machinery, decorative castings, fireplaces, and cooking stoves.
Cookware:
Cast iron cookware, such as skillets, pans, and Dutch ovens,
is popular for its excellent heat retention and even distribution. It's
commonly used for frying, baking, and slow cooking.
Construction:
Cast iron has been used in construction for various
purposes, including pipes, gutters, and decorative elements. It was commonly
used in the 19th century for structural components in buildings.
Engine Blocks:
Cast iron is often used for manufacturing engine blocks in
the automotive industry. Its strength and heat resistance make it suitable for
withstanding the stresses and high temperatures generated in internal
combustion engines.
Railroad Components:
In the past, cast iron was extensively used in the construction of railroad
components such as bridges, track components, and wheels due to its strength
and durability.
Street Furniture:
Cast iron has been used for making street furniture, such as lamp posts,
park benches, and decorative fountains. Its ability to be molded into intricate
designs makes it suitable for ornamental purposes.
Cooking Stoves:
Historical cooking stoves were often made of cast iron due to its ability
to withstand high temperatures. Cast iron stoves provided a reliable and
durable means of heating and cooking in homes.
What is so special about a cast iron pan?
Cooking pans made of cast iron are favored for their durability, excellent
heat retention, and versatility. Cast iron pans has the ability to
retain and distribute heat evenly across its surface, can last for
generations, and work well on various heat sources. When properly seasoned,
they develop a natural non-stick surface, making them suitable for a range of
cooking methods. Additionally, cast iron can contribute to iron enrichment in
food and is more affordable than some high-end cookware materials. Despite
these advantages, proper care, including regular seasoning and avoiding harsh
cleaning methods, is essential for maintaining the longevity and performance of
cast iron pans.
Water and Sewer Pipes:
Cast iron pipes were commonly used for water distribution
and sewer systems. While newer materials have replaced cast iron in some modern
installations, older cities may still have cast iron pipes in use.
Industrial Machinery:
Cast iron is used in the construction of various industrial
machinery and equipment due to its strength and resistance to wear. It's often
found in components like gears, pulleys, and heavy machinery parts.
Decorative Castings:
Cast iron is widely used for decorative purposes, such as
ornate gates, fences, and architectural elements. Its ability to be cast into
intricate designs allows for the creation of aesthetically pleasing structures.
Fireplaces and Wood Stoves:
Cast iron's heat retention properties make it an ideal
material for constructing fireplaces, wood stoves, and fireplace inserts. It
can efficiently radiate heat and withstand the high temperatures generated by
burning wood.
