types of cast irons

8/10/2019 Types of Cast Irons

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TYPES OF CAST IRONSCast irons generally contain more than 2% C and a variety of alloying elements. These

aregenerally classified by a rather simple and archaic system. Classificationis done on the basisofthe appearance of their fracture surface, their microstructure and properties. There has been

two

class of cast irons historically, one having a gray fracture appearance and other having a whitefracture appearance, named asgray cast iron and white cast iron respectively. Those irons

having both gray and white appearance are called mottled iron. It is interesting to note that these

names still apply today. Over the years, other cast irons have been evolved which have their

name derived from their mechanical property, such as malleable iron and ductile iron. Morerecently compacted graphite iron and austempered ductile iron have been introduced. There are

four factors which lead to the different types of cast irons namely, the carbon content, the alloy,

the impurity content, the cooling rate and the heat treatment after casting. These parameters

control the composition as well as the form of parent matrix phase present.[2][3][14]

Cast irons can be broadly classified into these 5 categories.

1. Gray cast irons:

It is the most common type of cast iron found. It has a gray fracture surface due to highvolume of graphiteflakes. Carbon in graphite form is more stable than carbide form.

During cooling if it is subjected to a controlled cooling rate and adequate alloying

addition then carbon gets precipitated out as graphite flakes. It has high Si contentbecause it promotes the formation of graphite during solidification. Gray cast irons have

negligible ductility but are useful because they can be easily casted to complex shapes

and are inexpensive. These also have very low impact resistance.

2. White cast iron:Rapid solidification of gray iron results in white cast iron. It has white fracture surface.

Graphite flakes are not present in this type of cast irons rather; an iron carbide network is

present that gives a white fracture surface. The Si content is lower to minimize thegraphitizing effect. They are hard and have excellent abrasion resistance. But they also

have excessive brittleness and poor machinability. To enhance wear resistance generally

Mo, Cr, Ni are added to it.

3. Mottled iron:This type of cast iron is not intentionally produced and results from a transition between

gray and white cast irons. It is not necessarily a desirable material.

4. Malleable cast iron:It is produced by heat treatment of white cast iron in which the iron carbide network

decomposes or breaks down into temper carbon. This process is called malleablization

which involves two stages of annealing as the first stage of annealing and the second

stage of annealing. Because of the absence of hard and brittle carbide phase, iron

becomes malleable. Disadvantage of this type of cast iron is its limited section thicknessand prolonged annealing cycles.

5. Spheroidal graphite cast iron or ductile iron:

It is produced by adopting special alloy addition and proper cooling rates so that thecarbon can be converted to spherical forms which can be used in those fields where

carbon in flake form or temper form cant be used. The nodules are formed during

solidification and not during heat treatment. It can be of three types namely, ferritic,pearlitic/ferritic, martensitic. It has excellent mechanical properties which can be


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compared to steels.

There is a subclass of ductile iron named as Austempered ductile iron. It has the same

nodular or spherical graphite as in ductile iron but the matrix is a combination of bainiteand stabilized austenite. Austempering is necessary to get this type of cast iron structure.

Here graphite is present in compact form and shape of graphite is controlled by minor

alloying addition. Austemperd ductile irons have excellent mechanical properties suchhas tensile strength, ductility and wear resistance.[2][3][14]

Cast Irons and Alloy Cast IronCast iron is a cheap alloy. Ordinary cast iron is an alloy containing a total of up to 10% of the

elements carbon, silicon, manganese, sulphur and phosphorus; the balance being iron. Alloy castirons, contain also varying amounts of nickel, chromium, molybdenum, vanadium and copper.

GraphitizationCementite (Fe3C) is a metastable compound, and under some circumstances it can be made todissociate or decompose. Thus, the true equilibrium diagram for iron and carbon is not that presented

in Fe-Fe3c phase diagram but rather as shown in Figure 11.1. Figure 11.1 extends to 100 wt% carbon

such that graphite is the carbon-rich phase, instead of cementite at 6.7 wt% C. This tendency to formgraphite is regulated by:

1-Composition: Graphite formation is promoted by the presence of silicon and to less degree

phosphorus, nickel and copper. If silicon content is lower than 1 wt% graphitization may not takes

place.2-Cooling rate: Slower cooling rates during solidification favor graphitization. While higher rate ofcooling during solidification tends to favor the formation of cementite. This effect is illustrated by

casting a 'stepped bar' of cast iron of a suitable composition (Fig. 11.2). Here, the thin sections havecooled so quickly that solidification of cementite has occurred, as indicated by the white fracture and

high Brinell values. The thicker sections, having cooled more slowly, are graphitic and consequently

softer.3-Heattreatment 2


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FIGURE 11.1 The true equilibrium ironcarbon phase diagram with graphite instead of cementiteas a stable phase.

FIGURE 11.2 The effect of thickness of cross-section on the rate of cooling, and hence upon the

microstructure of a grey cast iron.

Types of Cast Iron

1-White Cast IronIf silicon content is lower and the cooling rates during solidification are higher, the resulting structurewill contain cementite, and the fracture surface will appears bright .Since white cast iron is extremely

hard and brittle is not used as such but they are made as the first step for conversion into themalleable iron.It is possible to form white cast iron structure on the surface layers of grey cat iron by chilling thesurface. This is called chilled iron and is used for making wear resistance surfaces for iron rolls and

ploughs

2-Grey Cast IronIf silicon content is higher and the cooling rates during solidification are lower, completegraphitization takes place and the 3


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resulting structure will contain graphite flakes only. Then it is called grey cast iron, the fracturesurface may be dull and grey.

The important engineering properties of grey cat iron are;1. High compressive strength.

2. Moderate tensile strength

3. Good wear resistance

4. High damping capacity

The shortcoming of grey iron is the brittleness du to the flake form of graphite which introduces

sharp notches at the edges. The most important applications of cast iron are machine beds, ingot

moulds, lamp spots and others.

3-Spheroidal-graphite (SG) Cast Iron

Also known as nodular cast iron or ductile cast iron. In SG cast iron the graphite flakes are replaced

by spherical particles of graphite as shown in Fig. 11.6, so that the sharp stress raisers are eliminated.

The formation of this spheroidal graphite is effected by adding small amounts of cerium or

magnesium to the molten iron just before casting.4-Compacted-graphite (CG) IronThe mechanical properties of this type is intermediate between those of ordinary grey flake-graphiteirons and those of SG iron. The graphite flakes produced are short and stubby and have roundededges as shown in figure 11.7. CG iron is produced when molten iron of near-eutectic composition is

treated with a single alloy containing appropriate amounts of magnesium, titanium and cerium. CG

has good resistance to scaling and 'growth' at high temperatures, so CG iron attractive as a heat-resisting material and it was developed originally for the manufacture of ingot moulds and vehiclebrake components.

5-Malleable Cast Irons

The names of the two original malleabilising processes, the Blackheart and the White heart, refer tothe color of a fractured section after heat treatment has been completed. Another process used for 4


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manufacturing pearlitic malleable iron. In all three processes the original casting is of white iron,which will be very brittle before heat-treatment.

a)Blackheart MalleableIron castings are manufactured from white iron of which the following composition is typical:2.5%Carbon, 1.0%Silicon, 0.4%Manganese, 0.08% Sulphur and 0.1% Phosphorus. White iron castings

placed in white-iron boxes filling sand or small gravels to reduce the distortions and oxidation. The stepsof heat treatment can be summarized as following:1. Heating the castings by slowly rate to between 850-950C and stabilized the temperature at this range

for long time depending on composition and geometry of casting. In this step, all eutectic cementitetransforms to graphite (temper carbon) .

2. Cooling the castings by slowly rate to the eutectoid temperature.

3. Cooling the castings by very slowly rate from the eutectoid temperature to 680C . At this stepaustenite transforms to ferrite and temper carbon instead of pearlite, and diffusion process led to

compact the temper carbon particles in rosettes form.

So the final structure of blackheart malleable cast iron consisting of rosettes of temper carbon in a

matrix of ferrite as shown in figure 11.8. Blackheart malleable castings find particular application inthe automobile industries.

b)White heart MalleableIron castings are manufactured from white iron of which the following composition is typical:Carbon 3.3%, Silicon 0.6%, Manganese 0.5%, Sulphur 0.25% and Phosphorus 0.1%. In this process

the castings are heated in contact with some oxidizing material, such as haematite ore, for between70 and 100 hours at a temperature of about 1000C. During annealing, the carbon at the surface of

the casting is oxidised by contact with the hematite ore, and lost as carbon dioxide. This causes morecarbon to diffuse outwards from the core, and, in turn, this is lost by oxidation. Thus, after treatment

a thin section may be completely ferritic, and on 5


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fracture present a steely white appearance; hence the name 'whiteheart'. Heavier sections will not becompletely decarburised, and so, whilst the outer layer is ferritic, the inner zone containing some

pearlite, and at the extreme core some nodules of temper carbon. Thin-sectioned components

requiring high ductility are often made in the form of white heart malleable castings. Examplesinclude fittings for gas, water, air and steam pipes; bicycle- and motorcycle-frame fittings, parts foragricultural machinery and parts for textile machinery.

c) Pearlitic MalleableThe raw material similar in composition to that of blackheart malleable iron. Iron is used it is first

malleabilised. It is then reheated to 900C so that carbon will dissolve in the austenite present at that

temperature. Subsequent treatment consists air-cooling to produce a pearlitic matrix. The finalstructure will consist of rosettes of temper carbon in a matrix of pearlite.

Alloy Cast IronsThe microstructural effects which alloying elements have on a cast iron are, in most cases, similar tothe effects these elements have on the structure of a steel.

1-Martensitic ironsMartesitic irons, which are very useful for resisting abrasion, usually contain 4-6% nickel and about1% chromium. Such an alloy is Ni-hard, is martensitic in the cast state, whereas alloys containingrather less nickel and chromium would need to be oil-quenched in order to obtain a martensitic

structure Figure11.9.

2-Austenitic ironsAustenitic irons usually contain between 10 and 30% nickel and up to 5% chromium. These are

corrosion-resistant, heat-resistant, non-magnetic alloys. Some of them are treated to producestructures containing spheroidal instead of flake graphite Fig.11.10.

Cast Irons and Alloy Cast IronCast iron is a cheap alloy. Ordinary cast iron is an alloy containing a total of up to 10% of theelements carbon, silicon, manganese, sulphur and phosphorus; the balance being iron. Alloy castirons, contain also varying amounts of nickel, chromium, molybdenum, vanadium and copper.

GraphitizationCementite (Fe3C) is a metastable compound, and under some circumstances it can be made todissociate or decompose. Thus, the true equilibrium diagram for iron and carbon is not that presentedin Fe-Fe3c phase diagram but rather as shown in Figure 11.1. Figure 11.1 extends to 100 wt% carbon

such that graphite is the carbon-rich phase, instead of cementite at 6.7 wt% C. This tendency to formgraphite is regulated by:

1-Composition: Graphite formation is promoted by the presence of silicon and to less degree

phosphorus, nickel and copper. If silicon content is lower than 1 wt% graphitization may not takes

place.2-Cooling rate: Slower cooling rates during solidification favor graphitization. While higher rate ofcooling during solidification tends to favor the formation of cementite. This effect is illustrated by

casting a 'stepped bar' of cast iron of a suitable composition (Fig. 11.2). Here, the thin sections have

cooled so quickly that solidification of cementite has occurred, as indicated by the white fracture andhigh Brinell values. The thicker sections, having cooled more slowly, are graphitic and consequentlysofter.

3-Heattreatment 2


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FIGURE 11.1 The true equilibrium ironcarbon phase diagram with graphite instead of cementiteas a stable phase.

FIGURE 11.2 The effect of thickness of cross-section on the rate of cooling, and hence upon the

microstructure of a grey cast iron.

Types of Cast Iron

1-White Cast IronIf silicon content is lower and the cooling rates during solidification are higher, the resulting structurewill contain cementite, and the fracture surface will appears bright .Since white cast iron is extremely

hard and brittle is not used as such but they are made as the first step for conversion into themalleable iron.It is possible to form white cast iron structure on the surface layers of grey cat iron by chilling thesurface. This is called chilled iron and is used for making wear resistance surfaces for iron rolls and

ploughs

2-Grey Cast IronIf silicon content is higher and the cooling rates during solidification are lower, completegraphitization takes place and the 3


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resulting structure will contain graphite flakes only. Then it is called grey cast iron, the fracturesurface may be dull and grey.

The important engineering properties of grey cat iron are;1. High compressive strength.

2. Moderate tensile strength

3. Good wear resistance

4. High damping capacity

The shortcoming of grey iron is the brittleness du to the flake form of graphite which introduces

sharp notches at the edges. The most important applications of cast iron are machine beds, ingot

moulds, lamp spots and others.

3-Spheroidal-graphite (SG) Cast Iron

Also known as nodular cast iron or ductile cast iron. In SG cast iron the graphite flakes are replaced

by spherical particles of graphite as shown in Fig. 11.6, so that the sharp stress raisers are eliminated.

The formation of this spheroidal graphite is effected by adding small amounts of cerium or

magnesium to the molten iron just before casting.4-Compacted-graphite (CG) IronThe mechanical properties of this type is intermediate between those of ordinary grey flake-graphiteirons and those of SG iron. The graphite flakes produced are short and stubby and have roundededges as shown in figure 11.7. CG iron is produced when molten iron of near-eutectic composition is

treated with a single alloy containing appropriate amounts of magnesium, titanium and cerium. CG

has good resistance to scaling and 'growth' at high temperatures, so CG iron attractive as a heat-resisting material and it was developed originally for the manufacture of ingot moulds and vehiclebrake components.

5-Malleable Cast Irons

The names of the two original malleabilising processes, the Blackheart and the White heart, refer tothe color of a fractured section after heat treatment has been completed. Another process used for 4

Iron is the fourth most abundant element on Earth and is one of the most widely disbursed

elements in the Earth's crust. In nature it is found in various compounds with oxygen, sulfur, or

more complicated ores such as carbonates and silicates (Table 1). Because iron is soabundant, combines readily with other elements (such as manganese to form steel)and requires relatively little energy to extract it from ore, it is one of the most attractive elements to use

for the prodCastiron is a generic term used to designate a family of metals with a wide variety of

properties. All cast irons contain more than2% carbon and an appreciable amount ofsilicon (usually 1-3%). The high carbonand silicon content means that they are

easily melted, have good fluidity in the liquidstate and have excellent pouring properties.The basic types ofdifferentiated by their microstructure asopposed to their chemical analysis because the various types overlap

The metallurgy of cast iron is more complexthan its economics and, in fact, is oneof the more complex metallurgical systems[Fig2]. Iron-carbon alloys with less than2% carbon are metastable; the true stablesystem being iron-graphite (Fe-C). Thegeneral termcast iron includes pig iron,gray iron, malleable iron, chilled iron,white iron, and nodular or ductile iron.If an iron alloy exceedsabout 2% carbon,the carbon does not have to nucleatefrom decomposition of austenite, butinstead, it can form directly from

themeltby a eutectic reaction. Note that cementite(Fe3C) can still nucleate at the eutecticmore readily than graphite, but onsufficientlyslow cooling, graphite itself is able

to form and grow.Consider the solidification of a 3% carboncast iron (Fig. 3). At a rapid coolingrate, dendrites of austenite form as the


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alloy cools below the liquidus and growuntil the eutectic temperature is reached.At the eutectic, graphite formation is suppressed,

but austenite and cementite precipitateto form ledeburite, a form ofeutectic that consists of spheres of austeniteembedded in cementite. Ledeburite

forms at the Fe-Fe3C eutectic (solid linenm). On further cooling, the cementite

grows as the austenite decreases in carboncontent (along the solid line no) At the

eutectic (point o), the remaining

austenite transforms to pearlite. At roomtemperature, the iron is hard and brittleand is called white iron because the surface

of a fractured piece of iron is whiteand (somewhat) lustrous.Upon slow cooling of a 3% carbon castiron, austenite forms from the melt, but

eutectic freezing is now slow enough so the cast iron are bestucts we require in everyday life (Fig. 1).


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manufacturing pearlitic malleable iron. In all three processes the original casting is of white iron,which will be very brittle before heat-treatment.

a)Blackheart MalleableIron castings are manufactured from white iron of which the following composition is typical:2.5%Carbon, 1.0%Silicon, 0.4%Manganese, 0.08% Sulphur and 0.1% Phosphorus. White iron castings

placed in white-iron boxes filling sand or small gravels to reduce the distortions and oxidation. The stepsof heat treatment can be summarized as following:1. Heating the castings by slowly rate to between 850-950C and stabilized the temperature at this range

for long time depending on composition and geometry of casting. In this step, all eutectic cementitetransforms to graphite (temper carbon) .

2. Cooling the castings by slowly rate to the eutectoid temperature.

3. Cooling the castings by very slowly rate from the eutectoid temperature to 680C . At this stepaustenite transforms to ferrite and temper carbon instead of pearlite, and diffusion process led to

compact the temper carbon particles in rosettes form.

So the final structure of blackheart malleable cast iron consisting of rosettes of temper carbon in a

matrix of ferrite as shown in figure 11.8. Blackheart malleable castings find particular application inthe automobile industries.

b)White heart MalleableIron castings are manufactured from white iron of which the following composition is typical:Carbon 3.3%, Silicon 0.6%, Manganese 0.5%, Sulphur 0.25% and Phosphorus 0.1%. In this process

the castings are heated in contact with some oxidizing material, such as haematite ore, for between70 and 100 hours at a temperature of about 1000C. During annealing, the carbon at the surface of

the casting is oxidised by contact with the hematite ore, and lost as carbon dioxide. This causes morecarbon to diffuse outwards from the core, and, in turn, this is lost by oxidation. Thus, after treatment

a thin section may be completely ferritic, and on 5


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fracture present a steely white appearance; hence the name 'whiteheart'. Heavier sections will not becompletely decarburised, and so, whilst the outer layer is ferritic, the inner zone containing some

pearlite, and at the extreme core some nodules of temper carbon. Thin-sectioned components

requiring high ductility are often made in the form of white heart malleable castings. Examplesinclude fittings for gas, water, air and steam pipes; bicycle- and motorcycle-frame fittings, parts foragricultural machinery and parts for textile machinery.

c) Pearlitic MalleableThe raw material similar in composition to that of blackheart malleable iron. Iron is used it is first

malleabilised. It is then reheated to 900C so that carbon will dissolve in the austenite present at that

temperature. Subsequent treatment consists air-cooling to produce a pearlitic matrix. The finalstructure will consist of rosettes of temper carbon in a matrix of pearlite.

Alloy Cast IronsThe microstructural effects which alloying elements have on a cast iron are, in most cases, similar tothe effects these elements have on the structure of a steel.

1-Martensitic ironsMartesitic irons, which are very useful for resisting abrasion, usually contain 4-6% nickel and about1% chromium. Such an alloy is Ni-hard, is martensitic in the cast state, whereas alloys containingrather less nickel and chromium would need to be oil-quenched in order to obtain a martensitic

structure Figure11.9.

2-Austenitic ironsAustenitic irons usually contain between 10 and 30% nickel and up to 5% chromium. These are

corrosion-resistant, heat-resistant, non-magnetic alloys. Some of them are treated to produce

structures containing spheroidal instead of flake graphite Fig.11.10.

types of cast irons

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