10. Cast iron. 

The bonding Fe3C or cementite doesn’t appear to be stable. The dissection velocity is so small that is 

appeared the cemenitite was apparently stable. We speak of metastable bonding.

The dissection relation Fe3C → 3Fe + C is ending and cannot be written as a equilibrium reaction. For 

reaching a reasonable dissection velocity a high temperature is needed. 

There are certain elements which accelerate the reaction: silicium, nickel. Other elements have an 

opposite effect: manganese and chromium. 

Metastability of cementite implies that also the iron-carbon diagram is metastable.

 The solubility of cementite and graphite.

 Iron, at a suitable chosen temperature brought into contact with cementite , will absorb this 

bonding till there is saturation.  The same is the case with iron in contact with graphite. The 

solubility of cementite, expressed in percents of carbon, will be different than that of graphite. A 

stable substance will have less inclination to go into solution than a metastable substance. The 

solubility of graphite at a certain temperature will be less that that of cementite. This holds for the 

solubility in austenite as that in ferrite. 

 The stable iron-carbon diagram.

 From the above their follows that the 3 solubility lines D’C’, E’S’ and P’Q’ for graphite are to the left 

of the corresponding solubility lines for cementite. The cementite line expires, so that the 3 

mentioned solubility lines form the border of 2-phase areas in which graphite occurs besides liquid, 

austenite and ferrite. The diagrams extends to 100%C. 

The 3-phase line austenite-liquid-graphite becomes higher than that of the 3-phase-line ferrite-

austenite-cementite. The upper line is followed. The liquid solidifies as a mixture of austenite and 

graphite. 

 

 

Iron-Iron Carbide Phase Diagram

Fig 

Figure shows the equilibrium diagram for combinations of carbon in a solid solution of iron. The diagram shows iron

and carbons combined to form Fe-Fe3C at the 6.67%C end of the diagram. The left side of the diagram is pure iron

combined with carbon, resulting in steel alloys. Three significant regions can be made relative to the steel portion of

the diagram. They are the eutectoid E, the hypoeutectoid A, and the hypereutectoid B. The right side of the pure

iron line is carbon in combination with various forms of iron called alpha iron (ferrite), gamma iron (austenite), and

delta iron. The black dots mark clickable sections of the diagram.

Allotropic changes take place when there is a change in crystal lattice structure. From 2802º-2552ºF the delta iron

has a body-centered cubic lattice structure. At 2552ºF, the lattice changes from a body-centered cubic to a face-

centered cubic lattice type. At 1400ºF, the curve shows a plateau but this does not signify an allotropic change. It is

called the Curie temperature, where the metal changes its magnetic properties.

Two very important phase changes take place at 0.83%C and at 4.3% C. At 0.83%C, the transformation is eutectoid,

called pearlite.

gamma (austenite) --> alpha + Fe3C (cementite) 

At 4.3% C and 2066ºF, the transformation is eutectic, called ledeburite.

L(liquid) --> gamma (austenite) + Fe3C (cementite)

Examples of Iron-Iron Carbide Phase Transformations on the T-T-T Diagram

 

Figure . Fe-Fe3C T-T-T Diagram,

The time-temperature transformation curves correspond to the start and finish of transformations which extend into

the range of temperatures where austenite transforms to pearlite. Above 550 C, austenite transforms completely to

pearlite. Below 550 C, both pearlite and bainite are formed and below 450 C, only bainite is formed. The horizontal

line C-D that runs between the two curves marks the beginning and end of isothermal transformations. The dashed

line that runs parallel to the solid line curves represents the time to transform half the austenite to pearlite. Below we

have listed some simple examples as an exercise at other temperatures that result in different phase transformations

and hence different microstructures.

Example

Fig . Time-Temperature Paths on Isothermal Transformation Diagram

 

a. (Red) The specimen is cooled rapidly to 433 K and left for 20 minutes. The cooling rate is too rapid for 

pearlite to form at higher temperatures; therefore, the steel remains in the austenitic phase until the Ms 

temperature is passed, where martensite begins to form. Since 433 K is the temperature at which half of 

the austenite transforms to martensite, the direct quench converts 50% of the structure to martensite. 

Holding at 433 K forms only a small quantity of additional martensite, so the structure can be assumed 

to be half martensite and half retained austenite.

b. (Green) The specimen is held at 523 K for 100 seconds, which is not long enough to form bainite.

Therefore, the second quench from 523 K to room temperature develops a martensitic structure.

c. (Blue) An isothermal hold at 573 K for 500 seconds produces a half-bainite and half-austenite structure.

Cooling quickly would result in a final structure of martensite and bainite.

d. (Orange) Austenite converts completely to fine pearlite after eight seconds at 873 K. This phase is stable

and will not be changed on holding for 100,000 seconds at 873 K. The final structure, when cooled, is fine

pearlite.

 

Cast irons typically contain 2-4 wt% of carbon with a high silicon concentrations and a greater concentration of

impurities than steels. The carbon equivalent (CE) of a cast iron helps to distinguish the grey irons which cool into a

microstructure containing graphite and and the white irons where the carbon is present mainly as cementite. The

carbon equivalent is defined as:

A high cooling rate and a low carbon equivalent favours the formation of white cast iron whereas a low cooling rate or

a high carbon equivalent promotes grey cast iron.

During solidification, the major proportion of the carbon precipitates in the form of graphite or cementite. When

solidification is just complete, the precipitated phase is embedded in a matrix of austenite which has an equilibrium

carbon concentration of about 2 wt%. On further cooling, the carbon concentration of the austenite decreases as

more cementite or graphite precipitates from solid solution. For conventional cast irons, the austenite then

decomposes into pearlite at the eutectoid temperature. However, in grey cast irons, if the cooling rate through the

eutectoid temperature is sufficiently slow, then a completely ferritic matrix is obtained with the excess carbon being

deposited on the already existing graphite.

White cast irons are hard and brittle; they cannot easily be machined.

The iron-carbon phase diagram showing the eutectic and eutectoid reactions.

Grey cast irons are softer with a microstructure of graphite in transformed-austenite and cementite matrix. The

graphite flakes, which are rosettes in three dimensions, have a low density and hence compensate for the freezing

contraction, thus giving good castings free from porosity.

The flakes of graphite have good damping characteristics and good machinability (because the graphite acts as a

chip-breaker and lubricates the cutting tools. In applications involving wear, the graphite is beneficial because it helps

retain lubricants. However, the flakes of graphite also are stress concentrators, leading to poor toughness. The

recommended applied tensile stress is therefore only a quarter of its actual ultimate tensile strength.

Sulphur in cast irons is known to favour the formation of graphite flakes. The graphite can be induced to precipitate in

a spheroidal shape by removing the sulphur from the melt using a small quantity of calcium carbide. This is followed

by a minute addition of magnesium or cerium, which poisons the preferred growth directions and hence leads to

isotropic growth resulting in spheroids of graphite. The calcuim treatment is necessary before the addition of

magnesium since the latter also has an affinity for both sulphur and oxygen, whereas its spheroidising ability depends

on its presence in solution in the liquid iron. The magnesium is frequently added as an alloy with iron and silicon (Fe-

Si-Mg) rather than as pure magnesium.

However, magnesium tends to encourage the precipitation of cementite, so silicon is also added (in the form of ferro-

silicon) to ensure the precipitation of carbon as graphite. The ferro-silicon is known as an inoculant.

Spheroidal graphite cast iron has excellent toughness and is used widely, for example in crankshafts.

The latest breakthrough in cast irons is where the matrix of spheroidal graphite cast iron is not pearlite, but bainite.

This results in a major improvement in toughness and strength. The bainite is obtained by isothermal transformation

of the austenite at temperatures below that at which pearlite forms.

 

White cast iron.

 Because cast iron besides iron and carbon also contains Si, Mn and P , it can never be considered a 

binary Fe-C diagram. The course of the solidification cannot be deduced from the iron-carbon 

diagram. 

White cast iron melts wholly according to the metastable diagram. The carbon content lies above 

2%. There originates ledeburite. Between 2 and 4.3% carbon there first originates austenite, at more 

than 4.3% carbon first cementite. Usually the carbon content lies at about 3%. The cast iron will 

between the beginning solidification point and the ledeburite point separate off austenite (if we use 

the binary diagram). At the three-phase-line the structure consists of {(4.3 -3)/(4.3 -2)} 100 =57% 

austenite with 2%C and 43% liquid with 4.3%C. The last phase solidifies into ledeburite in which 

{(6.7-4.3)/(6.7-2)}100 =51% austenite and 49% cementite is present. 

Directly after solidification the structure will consist of dendrites of austenite with between it 

ledeburite. 

At further cooling the carbon content of the austenite will become smaller by formation of 

cementite, the composition changes from E at 1130˚C to S at 723˚C.  Arrived at 723˚C the structure 

consists of {(6.7-3}/(6.7-0.8)) 100 =63% austenite with 0.8% C and 37% cementite. 

After passing the lowest 3-phase line the asuetnite goes into pearlite. If we neglect the formation of 

tertiar cementite after this point there are no changes. 

 

Grey ferritic cast iron.

At solidification of ferritic cast iron the stable diagram is completely followed (till cooling). For this is 

needed that the cementite dissection is promoted by suitable admixtures (e.g. Ni). (Does nt apply 

for binary Fe-C alloys). Solidification according to the stable diagram means that ferritic cast iron 

cosists exclusively of ferrite and graphite. A cast iron with 3% carbon, which solidifies according to 

the stable diagram, will consist of {(100-3)/100}100 =97% ferrite and 3% graphite.  This cast iron will 

be very soft by absence of cementite. 

 Gray pearlitic cast iron.

 The pearlitic cast iron consist of pearlite and graphite. At solidification first the stable diagram is 

followed and the eutecticum C’ originates. The presence of pearlite indicates that later when cooled 

the austenite will dissect according to the metastable 3-phase line.  Somewhere between 1135˚ and 

723˚ there is a transition from the stable to the metastable system. Assuming we may describe the 

structure using the iron-carbon diagram, we can calculate the composition of grey pearlitic cast iron 

with 3% C will be {(100-3)/(100-0.8)}100 =97.8% pearlite and 2.2% graphite. 

 

Grey cast iron, Fe-3.2C-2.5Si wt%, containing graphite flakes in a matrix which is pearlitic. The speckled 

white regions represent a phosphide eutectic. Etchant: Nital 2% 

 

Grey cast iron, Fe-3.2C-2.5Si wt%, containing graphite flakes in a matrix which is pearlitic. The lamellar 

structure of the pearlite can be resolved, appearing to consist of alternating layers of cementite and 

ferrite. The speckled white regions represent a phosphide eutectic. Etchant: Nital 2%

 In decreasing order of carbon content we have white, grey pearlitic and grey ferritic cast iron. There is 

no sharp transition between these sorts. 

 Shrinking and growth of cast iron.

 An alloy in general is more castable is the melting point is lower an the volume change during 

solidification and further cooling is smaller. Grey cast iron satisfies these conditions. A cast temperature 

of 1200-1350˚C is easily obtained. 

Because dissection of cementite into iron and graphite is paired with a volume increase, cast iron at 

solidification will shrink less than cast iron and the less if it is more ferritic. A ferritic cast iron with much 

carbon will expand even at solidification. The cooling after solidification is paired with graphite 

separation, so the shrink is relatively small. (this doesn’t occur at white cast iron). 

Gray cast iron containing cementite at use log use at high temperature will have a slow volume increase 

by dissecting of the cementite. This growth is halted if the cementite has disappeared wholly. Often the 

growth continues because the graphite disappears by oxidation and makes place for iron oxide. 

 The carbon factor

 The eutectic alloy in general is better castable than a non-eutectic of the same sort because: there is no 

solidification trajectory, the solidification point is low and the alloy stays thin and liquid till full 

solidification. The carbon factor is the ratio between the percentage present carbon and that of the 

corresponding eutecticum. At a eutectic cast iron the carbon factor is one, at a under-eutectic iron it is 

smaller and at above-eutectic larger.   In the iron-carbon diagram the eutectic carbon content is 4.3%. 

Because manganese and sulphur are to be neglected we following formula is used for the eutectic 

carbon content :

4.3 – (%Si + %P)/3

 

The graphite separation.

 The amount of graphite formed during cooling or solidification depends on the cooling velocity and the 

composition. Low cooling velocity (and this long time stay at high temperature) will encourage graphite 

separation and dissection of cementite. The cooling velocity is small when iron a piece with thick walls is 

cast in a dry, pre-heated sand form, at high cast temperature. Under these circumstances the form 

material has a bad heat conduction power and the melt has a large heat-content. 

The most important accelerators of graphite separation are the carbon and the silicium. Manganese has 

a delaying effect. Weichelt used round rods of 30, 20 , 10, 6 and 3 mm for different iron sorts. See the 

Maurer-Wechelt diagram below.

  

Fig. Weichelt diagram for thickness of 30 and 6 mm.

 

The form of graphite.

 As can be seen in the following figures the graphite has the form of lamellas,  because the structure of 

the graphite is hexagonal. The size of the graphite lamellas relates to their number: the more there are, 

the finer they are.

 

 

Fig. Grey ferritic cast iron. Structure composites: graphite, ferrite, and steadite (ternary Fe3C-Fe2P - - 

eutecticum).  

 

Fig. Ordinary cast iron.

 

Fig. Grey pearlitic cast iron. 

 

 

Spheroidal Graphite Cast Iron

The chemical composition of the cast iron is similar to that of the grey cast iron but with 0.05 wt% of magnesium. All

samples are etched using 2% nital.

Spheroidal graphite cast iron, Fe-3.2C-2.5Si-0.05Mg wt%, containing graphite nodules in a matrix which 

is pearlitic. One of the nodules is surrounded by ferrite, simply because the region around the nodule is 

decarburised as carbon deposits on to the graphite. Etchant: Nital 2%

 

Spheroidal graphite cast iron usually has a pearlitic matrix. However, annealing causes the carbon in the pearlite to

precipitate on to the existing graphite or to form further small graphite particles, leaving behind a ferritic matrix. This

gives the iron even greater ductility. All samples are etched using 2% nital.

Graphite nodules in a ferritic matrix.

 

 

 

Graphite nodules in a ferritic matrix. Some carbon deposited during tempering is also visible. Etchant: 

Nital 2%

  

Blackheart Cast Iron

Blackheart cast iron is produced by heating white cast iron at 900-950oC for many days before cooling slowly. This

results in a microstructure containing irregular though equiaxed nodules of graphite in a ferritic matrix. The term

"blackheart" comes from the fact that the fracture surface has a grey or black appearance due to the presence of

graphite at the surface. The purpose of the heat treatment is to increase the ductility of the cast iron. However, this

process is now outdated since spheroidal graphite can be produced directly on casting by inoculating with

magnesium or cerium. All samples are etched using 2% nital.

Blackheart cast iron. Blackheart cast iron. Etchant: Nital 2%

 

 

 

Mechanical properties

 White cast iron is due to its content of cementite very hard and brittle and can only be processed by 

grinding. It is used for its hardness in e.g. bullets. 

Grey cast iron contains graphite in the forms of lamellas which causes a change in mechanical properties 

compared with cast steel. The round mass which consists of pearlite and (or) ferrite , has a pull strength 

of 100 to 70 kg/mm2. This high values originate because the alloying elements like silicon and 

manganese strengthen the ferrite. The graphite, which takes 5 to 15 volume percentages has a higher 

value than expected: at pulling the lamellas will stand open, because of the little resistance which 

graphite has. When the load increase more lamellas will stand open, so that Hooke’s law doesn’t apply 

here. The elasticity modulus will become smaller if the load interval becomes larger. The tensions in the 

groundmass of the cast iron are larger than calculated from load and rod cross section because: a. the 

cross section consists of 5 a 15 % of graphite which takes no tensions, so that the rest is loaded higher. 

b. the tensile stress along the borders of the graphite lamellas will be larger tan in between the lamellas 

because of tension concentrations. See fig. below

 

 

Fig. Explanation of the mechanical properties of grey cast iron.

 

If the tension at the border of a graphite lamella surpasses the pull strength then the lamella cracks 

open. Because of this the tension in the remaining ground mass increases further and the crack expands 

over the whole surface. AT the pull test a low pull strength and low stretch is found. 

At the pressure test the lamellas are pressed closed and give resistance. The thumb rule that the 

hardness is trice the pull strength doesn’t hold for cast iron. Another consequence is that the elasticity 

modulus at pressure is larger than at pull, so that the bending strength is larger than the pull strength. 

The nodular cast iron is stronger, tougher and stiffer than the corresponding lamellar because the 

spheroid graphite influences the structure much less and causes tension concentration than the 

lamellar. See fig.

 

 

 

Fig. Nodular ferritic cast iron.

 

Internal tensions in cast pieces.

 In a cast piece there are internal tensions. The character of these tensions is determined by the way of 

casting, solidifying and cooling and by the form

At a cylindrical rod the solidification will start at the outside. The inner will shrink and at cooling shrink 

even further than the shell because the temperature is higher. As a consequence in the nucleus there 

will originate pull- and in the shell pressure tensions. If the cylinder is closed off at the outer side it will 

become shorter.

 

 

Fig. A cast piece for proving internal tensions. The side-rods solidify first, the middle piece later; the first 

will have pressure the last load. This is shown by increase of the distance between the marks. 

 

Glowing of cast iron

 The heat treatment of cast iron is mostly restricted to tension free glowing at about 500˚C. If also the 

processability must be improved there is glowed at a higher temperature. In both cases there is chance 

at growing which deteriorates the mechanical properties. 

The processability becomes larger if there is more grain, and by disappearance of cementite. Tension 

free glowing of white cast iron doesn’t make sense because there the ledeburite net doesn’t change 

much. Because the internal tensions mostly appear in the shell , the glowing occurs after the 

preprocessing. 

Hardening and tempering of cast iron is applied little. Samples of  white cast iron will crack at hardening 

by volume increase. Grey pearlite cast iron is  hardened in oil to avoid cracks. The present alloy elements 

silicon, manganese, phosphor, etc, delay the critical cooling velocity by obtaining hardening in oil by 

quenching. The tempering temperature depends on the desired properties.

 Forging and rolling of cast iron.

 Grey cast iron in the austenite area is forgeable because there all cementite is taken in the austenite; 

white cast iron difficult. The forgeability is not as good as that of steel because the cast lamellas give rise 

to cracks and because the silicon makes the austenite stiffer and less tough. At enduring forging the 

graphite lamellas are stretched in the direction of the forging or rolling. In this stadium the originated 

cracks can be welded closed. 

Nodular cast iron is better forgeable than ordinary. 

 Forgeable cast iron.

 Lack of toughness is a disadvantage for cast pieces. This can be undone by making cast iron that 

contains no graphite lamellas. This is the case at nodular cast iron. There are 2 methods of heat 

treatment to obtain the desired result. 

 The oldest process is researched by Reamur in 1716-1725, consist of heating of white cast iron in a 

weak oxidizing surrounding. The carbon is oxidized, but not the iron 

CO2 + C  2CO This CO gas can reduce iron ore according to:

CO + 3Fe2O3  CO2 + 2Fe3O4 

By diffusion  there originates a carbon stream from the middle to the surface.  If the process is long then 

the result is wholly ferritic iron without carbon. This method is called glow freshening; it is only 

applicable on cast pieces with small wall thickness. 

 The other process, the tempering:  white cat iron is heated in a neutral surroundings under such 

circumstances that the cementite is dissected. The carbon that hereby originates, forms no lamellas but 

nests of sphere-like separations of temper carbon. 

 Tempering

 The white cast iron to be tempered  will have to contain more silicon than the sort destined for anneal 

freshening. This content must be low enough to prevent separation of graphite during solidification. To 

obtain this the process must be split into 2 parts. The first part occurs at high temperature (900 to 950). 

White cast iron at this temperature consists of austenite and cementite , whereby the austenite is 

saturated with carbon in equilibrium with cementite. IF there form nuclei of temper carbon then the 

state changes, because these particles will be surrounded by austenite saturated with carbon in 

equilibrium with graphite. The solubilibty of graphite is smaller than that of cementite so that at the 

border layer between the cementite particles and the austenite the cementite dissolves. The carbon 

diffuses from this layer to that of austenite and temper carbon forms here. If the process has ended the 

structure consists of austenite and temper carbon. In the austenite there is ca 1.5% carbon and to 

remove this the temperature is gradually lowered, so that the solubility decreases till reaching the 3-

phase-equilibrium : austenite  ferrite + temper carbon. At this moment the carbon content has 

lowered to about 0.8%C. If the temperature is lowered even further then the austenite will dissect. If 

the cooling velocity is slow then ferrite will originate. If the process is not slwo enough to end the 

process then pearlite will originate. 

 

 Fig. Structure of completely tempered cast iron.

  

Wear-Resistant High-Chromium Cast Iron

This cast iron is used in circumstances where a very high wear resistance is desirable. For example, during the

violent crushing of rocks and minerals. It contains a combination of very strong carbide-forming alloying elements. Its

chemical composition is, therefore, Fe-2.6C-17Cr-2Mo-2Ni wt%.

The white phase is a chromium-rich 

carbide known as M7C3. The matrix 

consists of dendrites of austenite, some 

of which may have transformed into 

martensite. There may also be relatively 

small quantities of other alloy carbides.

The white phase is a chromium-rich 

carbide known as M7C3. The matrix 

consists of dendrites of austenite, some 

of which may have transformed into 

martensite. There may also be relatively 

small quantities of other alloy carbides.

 

 

 

 

Austempered Ductile Cast Iron

The chemical composition of the cast iron is Fe-3.52C-2.51Si-0.49Mn-0.15Mo-0.31Cu wt%. All samples are etched

using 2% nital. Colour micrographs are produced by first etching with 2% nital, followed by open air heat treatment of

the metallographic sample at 270oC for 3 h. This oxidises the sample and produces interference colours which are

phase dependent.

Ductile iron as-cast. Nodules of graphite, pearlite (dark islands) and ferrite (light background). Etchant: 

Nital 2%

 

Ductile iron as-cast. Nodules of graphite, pearlite (dark islands) and ferrite (light background). Etchant: 

Nital 2%

 

Austenitised 950°C, austempered 350°C for 64 min.

 

 

Austenitised at 950°C, austempered at 350°C for 64 min.