MECHANISM FOR SOLIDIFICATION OF GRAY CAST IRON &

CONDITION FOR OBTAINING DIFFERENT TYPES AND SIZES OF

GRAPHITE:

Gray iron refers to a broad class of ferrous casting alloys which are

defined as an iron having a chemical composition such that after

solidification, a large portion of its carbon is distributed throughout the

casting as free or graphite carbon in “flake form”

In other words, iron which is normally characterized by a micro

structure of flake graphite in a ferrous matrix. Gray irons are in

essence iron carbon silicon alloys containing small quantities of other

elements.

Gray iron always leaves a sooty surface when fractured.

For purpose of clarity and simplicity the chemical analysis of gray iron

can be broken down into 3 categories.

Major elements - the 3 major elements are iron, silicon, and carbon.

Both carbon and silicon influence the nature of the castings, so their

impact on the solidification is attributed for by introducing a factor

CARBON EQUIVALENCE, CE.

CE = % C + [% Si / 3]

If we take the effect of phosphorus in account,

CE = % C + [% Si + % P / 3]

Irons with a CE of 4.3 are considered to be of eutectic composition.

Most gray irons are hypoeutectic. Nearly all of the properties of gray

iron are closely related to CE value.

Minor elements - these are phosphorus and the two inter related

elements like manganese and sulphur.

Phosphorus increases the fluidity of iron, by forming a low melting

phosphide called steadite. At high levels it can promote shrinkage

porosity while very low levels can increase metal penetration into the

mold. Its concentration is 0.02 – 0.10 %.

Sulphur plays a significant role in nucleation of graphite in gray iron.

Sulphur levels should be in the range of 0.05 – 0.12 %.

Sulphur content should be balanced with the manganese to promote

the formation of manganese sulphides. This is achieved by

% Mn >= 1.7% S + 0.3%

Trace elements – trace elements like antimony, arsenic, bismuth,

boron, chromium, nickel can be present or can be added in the gray

iron to induce some properties.

All the elements normally present in gray iron exert some influence on

the microstructure of the iron. The effects of different elements on the

properties are discussed below:

Carbon: Carbon in gray iron is present from about 2.5 to 4.5 per cent

by weight. Carbon occurs in two different forms. Elemental carbon in

form of graphite flakes and combined carbon as Fe3C.

The degree of graphitization may be assessed by the following

relationship

% total carbon = % graphitic carbon + % combined carbon

If graphitization is complete the percentage of total carbon and

percentage of graphitic carbon is equal. If no graphitization has

occurred, the percentage of graphitic carbon is zero.

If about 0.5 to 0.8 % combined carbon exists in a gray iron, it generally

indicates that the microstructure is largely pearlitic since pearlite in

gray iron having about 0.2 % silicon forms from the austenite eutectoid

containing about 0.60 per cent carbon.

For sufficient graphitization to develop during solidification of a true

gray iron, certain minimum total carbon content necessary, which is

around 2.20 %, but this value depends on Silicon percentage in the

iron.

Silicon : silicon is present in gray iron about 1.0 to 3.50 per cent by

weight. Increasing silicon percentage shift the eutectic point of the iron

carbon diagram to the left.

The eutectic shift is described by the following relationship-

Eutectic Carbon % ( in Fe-C-Si) = 4.30 – 1/3 X %Si (in iron)

Microstructurally, silicon occurs dissolved in the ferrite of gray iron. As

such it hardens and strengthens the ferrite. Ferrite in pure iron will

measure 80 to 90 BHN, whereas 2% Silicon in a ferritic iron raises the

hardness to about 120 t 130 BHN.

Silicon promotes the graphitization. Low percentages are not sufficient

to cause graphitization during solidification, but it will cause nucleation

and graphitization in the solid state at high temperatures. Certain

silicon percentages will cause limited graphitization during

solidification, and a mottled iron, partly white and partly gray results.

Sulphur : sulphur which may be present up to about 0.25 percent, is

on of the important modifying elements present in gray irons. A low

sulphur iron-sillicon-carbon alloy, under 0.010% S will graphitize most

completely.

High sulphur percentage favors the retention of a completely pearlitic

microstructure in a gray iron. This effect is causes sulphur to be known

as an element restriciting graphitization (carbide stabilizing)

Manganese: when manganese is present, MnS or complex

manganese iron sulphides, are found, depending on the manganese

content. The manganese sulphides begins to precipitated early, and

continue to do so during the entire freezing process, and are therefore

randomly distributed.

The effect of Mn alone as an alloying element is to promote resistance

to graphitization. Therefore manganese above that is necessary to

react with the sulophur will assisit in retaining the pearlitic

microstructure.

For commercial gray irons in which a pearlitic microstructure is

desired, following rule offers a favorable combination of manganese

and sulphur percentages:

3 X % S + 0.35 = % Mn

Phosphorus: Phosphorus results in formation of steadite. The

percentage of steadite present in the final stricture may amount to ten

times the percentage of phosphorus. Excessive phosphorus content

raises the hardness and brittleness of gray iron because of the steadite

formed.

Because of the segregation the steadite usually adopts a cellular

pattern characteristic of the eutectic cell size developed during the

solidification process.

Because it forms a eutectic as it segregates, phosphorus is often

looked upon as increasing the tendency for a particular iron

composition to be eutectic type alloy. For this reason the Carbon

Equivalent is sometimes modified to include a factor fpr phosphorus as

follows:

CE = % C + [% Si + % P / 3]

SOLIDIFICATION OF GRAY CAST IRON:

Most gray irons are hypo eutectic in nature. The sequence of events

associated with the solidification of hypo eutectic irons can be studied

with simplified version of the iron- carbon- silicon ternary phase

diagram taken at 2% Si.

Figure 1: Simplified iron-carbon-silicon phase diagram at 2% Si

At temperatures above point 1 the iron is entirely molten.

As the temperature is decreased and the liquidus line is crossed,

primarily freezing begins with the formation of pro eutectic austenite

dendrites. These dendrites grow and new dendrites form as the

temperature drops through the primary freezing range, which is

marked by the points 1 and 2.

Dendrite size is governed by the CE of the iron and solidification rate.

Lower CE produces large dendrites because the temperature interval

between the liquidus and eutectic lines is greater for those irons than

those with a greater CE. As expected, rapid cooling promotes a fin

grain size.

During the formation of the austenite dendrites, carbon is rejected into

the remaining liquid. The carbon content of the liquid increase until it

reaches the eutectic composition of 4.3 %. Once this composition is

achieved, the liquid transform in two solids. This takes place between

points 2 and 3.

At eutectic temperature solidification of eutectic occurs by certain

amount of under cooling. The degree of under cooling determines the

mode of solidification of the eutectic. The nucleus which subsequently

grows to form eutectoid solid may be attached to the tip of the

austenite dendrite (endogenous solidification) or may form an

independent nuclei.

The type of solid formed depends on whether the solidification is

following the metastable or stable eutectic reaction. Iron carbide plus

austenite forms during the metastable reaction. Graphite plus

austenite forms during the stable reaction. When eutectic solidification

is complete, no liquid metal remains, and any further reaction takes

place in the solid state.

In the temperature interval between the eutectic and eutectoid

transformations, the high carbon austenite rejects the carbon, which

diffuses to the graphitic flakes. This allows the austenite to acquire the

composition needed for the eutectoid transformation, which under

equilibrium conditions takes place between points 4 and 5.

This transformation involves the decomposition of austenite into

pearlite or pearlite plus ferrite, depending on such factors as the

cooling rate and alloy content of the iron. In unalloyed gray irons, no

significant changes in micro structure occur below the eutectoid

transformation line.

At the end of eutectic solidification the low melting point contents of

the liquid namely p- constituents will form an envelope in the eutectic

and solidifies as a layer separating the neighboring eutectic grains. A

certain amount of under cooling is necessary fro nucleation to occur.

Increased cooling rate and nucleation affects the solidification

characteristics of the gray cast iron.

Condition for obtaining different types and sizes of graphite:

The following discussion is for ASTM A 247

Type A – this type of flakes are randomly distributed and oriented

throughout the iron matrix. This type of graphite is found in irons that

solidify with a minimum amount of under cooling, and is suitable if

mechanical properties are to be optimized.

Figure 2: uniform distribution, random orientation

CE is 3.4 – 4.0 %.

Type B – it is found in irons of near eutectic composition that solidify

with a grater amount of under cooling than that associated with type A.

rosettes containing fine graphite that are characteristics of type B,

precipitate at the start of eutectic solidification. The heat of fusion

associated with their formation increase the temperature of the

surrounding liquid, thus decreasing the under cooling and resulting in

the formation of type A graphite.

Figure 3: rosette grouping

CE is approximately 4.5 %.

Type C- superimposed flake sizes with random orientation. This is

obtained when hypo eutectoid iron are cooled at faster rate. CE is 4.3

%. This is formed when graphite precipitates in the primary freezing of

irons.

Figure 4: superimposed flake sizes, random orientation

It appears as coarse plates. It greatly reduces the mechanical

properties of the iron and produces a rough surface finish when

machined. It is however desired in application requiring a high degree

of surface finish.

Type D- this is formed when the amount of under cooling is high but it

is not sufficient to cause carbide formation.

Figure 5: inter dentritic segregation, random orientation

Both types are found in the inter dendritic regions. This type of

graphite is randomly distributed.

Type E – in this the graphite has inter dendritic segregation with

preferred orientation of graphite flakes. This is obtained with low CE

and with rapid cooling.

Figure 6: interdentritic segregation, preferred orientation

Large flakes are associated with irons having high CE and slow cooling

rates. The large flakes are suitable for applications requiring high

thermal capacity and thermal conductivity.

Strongly hypoeutectic irons and irons subjected to rapid solidification

generally exhibits small, short flakes. Small flakes because they disrupt

the matrix to lesser extent, are desired when maximum tensile

properties and a fine smooth surface finish is to be obtained.

INOCULATION AND ITS INFLUENCE ON THE STRUCTURE AND

PROPERTIES OF GRAY CAST IRON:

Inoculation is defined as the late addition of an element or elements to

the molten iron to produce changes in graphite distribution,

improvements in mechanical properties, and a reduction of chilling

tendency that are not explainable on basis of composition changes

with respect to silicon. It is recognized that 2 irons with the same

apparent composition can have dramatically different micro structures

and properties if one is inoculated and the other is not. Control

accomplished by the addition is defined as inoculation.

As the amount of inoculants is increased, a reduction in chill is

realized, until a point of diminishing returns is reached.

The purpose of inoculation is to increase the number of nuclei in

molten iron and decrease in the eutectic cell size. It is considered that

this increase in number of cells is accomplished by nucleation of

eutectic solidification and the graphite phase.

Due to inoculation graphite precipitation can start with minimum

amount of under cooling. When undercooling is minimized, there is a

corresponding reduction in the tendency to form eutectic carbide or

white iron, which is referred to as chill. Instead, a more uniform

microstructure consisting of small type A graphite flakes is produced.

These micro structural changes can result in improved machinability

and mechanical properties.

It is convenient to group inoculants into 4 performance categories.

Standard, Intermediate, High potency and Stabilizing.

The calcium bearing alloys fall into the standard category.

The calcium when mixed with barium falls in the intermediate group.

Strontium or calcium plus cerium alloys are the high potency

inoculants, which has the greatest tendency for chill reduction.

Stabilizing inoculants normally employ chromium as the major

element.

Performanc

e category

Si Al Ca Ba C

e

TRE Ti Mn Sr Others

Standard 46

-

50

0.5-

1.25

0.60

-

0.90

- - - - - - -

74

-

79

1.25

max

0.50

-1.0

- - - - - - -

74

-

79

0.75

-1.5

1-

1.5

- - - - - - -

Intermedi

ate

46

-

50

1.25

max

0.75

-

1.25

0.75

-

1.25

- - - 1.2

5

ma

x

- -

60

-

65

0.8-

1.5

1.5-

3

4-6 - - - 7-

12

- -

70

-

74

0.8-

1.5

0.8-

1.5

0.7-

1.3

- - - - - -

0.75

-

1.25

42

-

44

- 0.75

-

1.25

- - - 9-

1

1

- - -

50

-

55

- 5-7 - - - 9-

1

1

- - -

50

-

55

- 0.5-

1.5

- - - 9-

1

1

- - -

High 36

-

40

- - - 9-

1

1

10.

5-

15

- - - -

73

-

0.50

max

0.10

max

- - - - - 0.6

-

-

78 1.0

Stabilizin

g

6-

11

0.50

max

0.50

max

- - - - - - 48-52

chromiu

m

Table 1:Composition of ferro silicon inoculants for gray iorn

Ioculant Tensile

strength

Impact

Strength

B.H.N

Commercial low Al-

ferrous

0.3% Al, 70% Si

0.4% Al, 70% Si

18.8

17.9

21.7

20.2

222

220

Normal – commercial

Ferro- Si

(1.4% Al, 7% Ca)

20.1 29.1 21.9

Fe-Si 75%, Al 2.1% 20.5 27.1 224

Fe-Si 75%, Al 1% 16.5 22.8 205

Fe-Si 70%, Ca 2%,

Al0.3%

17.8 26.8 208

Fe-Si 75%, Ca 2%, Al-

0.3%

16.5 17 216

Fe-Si 60%, Al0.6%, Mg

2%

17.9 24.9 215

Fe-Si 80%, Al 0.4%,

Zn 2%

18.4 25 220

Table 2: Comparison of mechanical properties of various

inoculated Cast Irons

Type of

Iron

Shrinkage Fluidity

Gray Fe 3/16” ft/ft 3.4” inch-s/g

Inoculated

Gray Cst

Iron

1/4” ft/ft 20.5” inch-s/g

Table 3: Comparison of properties of regular Gray Cast Iron

and Inoculated Gray Cast Iron.

Stabilizing inoculants are designed to promote pearlite and at the

same time provide graphitization during solidification. They are useful

in producing high strength castings with a minimum of chill, and they

help to eliminate ferrite in heavy sections. Because these alloys an be

difficult to dissolve, the are not suggested for mould addition.

Figure 6: general classification of inoculants showing chill

reduction in iron with carbon equivalence of 4.0

Graphite or ferro - silicon based alloys can also be used as an

inoculants. The graphite used must be highly crystalline. But graphite

is rarely used by itself and most often mixed with crushed ferro silicon.

Careful addition and relatively higher temperatures are needed to

ensure its complete solution.

Property changes –

graphite has been found to promote extremely high eutectic cell

counts.

Because ferrosilicon dissolves readily, it helps to distribute the

reactive elements throughout the melt

The reactive elements in addition to reacting with iron, react

readily with sulphur and oxygen, their addition may therefore

lead to dross formation.

Inoculation methods:

Ladle inoculation is a common method for inoculation. In this

method, the alloy is added to the metal stream as it flows from the

transfer ladle in to the pouring ladle. A small heel of metal should be

allowed to accumulate in the bottom of ladle prior to inoculation. Thos

allows the inoculant to be mixed and evenly distributed.

Addition of alloys to bottom of an empty ladle may cause sintering and

reduction in inoculant effectiveness. Problems may also arise if the

alloy is added to a full ladle because the material can become

entrapped in the slag layer that forms on the surface.

By adding inoculant late in the process, the effect of time can be

greatly reduced. Stream and mold inoculation are two latest methods of

inoculation that ar believed to promote more uniform quality from casting to casting.

Stream inoculation requires that the alloy be added to the stream of metal flowing from

the pouring ladle in to the mold.

Figure 7: schematic showing the principle of stream inoculation

One of the electro pneumatic devices used to sense when the metal flow starts and stops

is shown in figure 7. This device ensures that that the alloy is dispensed in such a manner

that the last metal entering the mold is treated similarly to first metal.

The same inoculants used to treat iron in the ladle can be used for stream inoculation, but

less of a performance distinction is has been observed amongst them. An uniform and

consistent size is an important factor in stream inoculation. Too large a size can cause

plugging of the equipment and incomplete dissolution.

Mold inoculation involves placement of the alloy in the mold, such a sin pouring basins,

at the base of the sprue or in suitable chambers in the runner system. Inoculants fir this

method can be crushed material, powder bonded into a pellet, or pre-cast slugs or blocks.

As in stream inoculation, alloy dissolution rate is an important factor. The pre-cast and

bonded alloys are designed to dissolve at a controlled rate throughout the entire pouring

cycle.

There are several advantages of late inoculation over ladle inoculation. Fading is

virtually eliminated, and because the castings are inoculated to the same extent, there is

greater consistency in structure form casting to casting. It has also been observed that the

late inoculation is more successful in preventing carbide formation in thin sections, thus

eliminating heat treatment.

REFERENCES

1. ASTM Handbook. “Ferrous Casting Alloys”, pg 629-645, D.B.Craig,

M.J. Hornung and T.K.McCluhan,

2. “Principles of Metal Casting” Tata Mc Gram Hill Publications, 30th

reprint 2005, pg 575-611, , Philip .C. Rosenthal,

Richard .W.Heine, Carl. R. Loper

3. www.wikipedia.com/search=gray+cast+iron+metallurgy.htm ,

retrieved on 5-10-2010 at 3:30 pm

4. www.mifco.com/inoculation.htm retrieved on 5-10-2010 at 3:50

pm