Segregation in Casting

by

Ali Abdallah Ali

Section Three

Department of metallurgical and materials engineering

Faculty of petroleum and mining engineering

Suez Canal University

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Contents

1. Introduction ……………………………3

2. Microsegregation……………………….5

3. Macrosegregation………………………8

4. Dendritic segregation………………….12

5. Gravity segregation……………………14

6. Reference ……………………………...17

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Abstract

Segregation is one of the defects in the casting

process that have various shapes some are normal and some

are inverse, some occurs on microscopic scale and some on

macroscopic scale, some types result due to difference in

density and some due to difference in temperature, there are

a lot of types and shapes of segregation on which we will

spot some light.

Introduction

Segregation may be defined as any departure from

uniform distribution of the chemical elements in the alloy.

Because of the way in which the solutes in alloys partition

between the solid and the liquid during freezing, it follows

that all castings are segregated to some extent. [1]

During solidification of liquid metals and alloys, crystals

formation takes place. The resulting morphology has

certain characteristics peculiar to cast structures.

Morphology includes both macrostructure and

microstructure. [3]

Some variation in composition occurs on a microscopic

scale between dendrite arms, known as microsegregation. It

can usually be significantly reduced by a homogenizing

heat treatment because the distance, usually in the range

10-100 µm, over which diffusion has to take place to

redistribute the alloying elements, is sufficiently small. [1]

Macrosegregation cannot be removed. It occurs over

distances ranging from 1 cm to 1 m, and so cannot be

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removed by diffusion In general, therefore, whatever

macrosegregation occurs has to be lived with. [1]

5

Microsegregation

Intergranular segregation results from accumulation of

rejected solute between the growing crystals and its failure,

due to a physical barrier of solid, to diffuse or mix into the

main body of residual liquid. The final segregation pattern

thus follows the form of the grain or sub-structure. This is

the basis of the typical cored microstructure revealed by

etching contrast, and explains the frequent presence of non-

equilibrium phases in interdendritic regions. [2]

Figure 1 shows the relationship between composition and

dendritic microstructure in carbon chromium steel, in

which the dendrites are depleted and the interdendritic

spaces enriched in chromium. In this case the segregation

ratio

The mechanical properties of a cast alloy are naturally

sensitive to microsegregation, since strength, tensile

ductility; impact properties and fatigue resistance are all

affected by intercrystalline conditions which differ from the

matrix. Apart from functional properties, the

microsegregation of alloying elements and impurities can

affect strength and ductility in the solidus region and can

thus govern susceptibility to hot tearing. [2]

6

(a)

(b)

Figure 1: Cored microstructure in carbon–chromium steel, showing correlation between dendrite

morphology and composition. (a) Structure, (b) contour map of chromium content as established by microprobe analysis [2]

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Where microsegregation results in the appearance of a new

liquid interdendritic phase, there are a number of

consequences that may be important: [1]

1. The presence of a eutectic phase reduces the

problem for fluid flow through the dendrite mesh.

2. The alloy may now be susceptible to hot tearing,

especially if there is only a very few percent of the

liquid phase.

3. A low-melting-point phase may limit the

temperature at which the material can be heat

treated.

4. A low-melting-point phase may limit the

temperature, at which an alloy can be worked, since

it may be weakened, disintegrating during working

because of the presence of liquid in its structure. [1]

8

Macrosegregation

Segregation on a macroscopic scale is produced by

various mechanisms depending upon the mode of freezing.

However, the basic factor is the accumulation of rejected

solute by transport over relatively long distances through

the casting. In the simplest case, usually termed normal

segregation, the final parts of the casting to freeze contain

high concentrations of solute elements, whilst the term

inverse segregation is used to describe the opposite

condition. [2]

Macrosegregation occurs during solidification due to

relative movement or flow of segregated liquid and solid.

There are numerous causes of fluid flow and solid

movement in casting processes. One reason for this

movement of segregated liquid may be density differences

of the metal due to temperature or variations in

composition. [4]

The hot liquid metal becomes cooler close to the chill

surfaces and its density increase causing downward

movement. Liquid being enriched by rejected solutes with

higher density compared to the bulk composition will flow

downward and the opposite will happen when low mass

elements enrich the liquid. [4]

During ingot casting, the most common macrosegregations

are the positive, negative and channel segregations.

Positive segregation means that the concentration of

alloying element exceeds the average bulk concentration.

�egative segregation is instead a local lack of alloying

element. [4]

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The positive segregation is often found at the top and is the

result of segregated liquid flow toward the top and the

negative zone with more pure material at the bottom of the

ingot is explained by sedimentation of equiaxed crystals

formed in the bulk liquid. [4]

Figure 2 shows a plot over the segregation ratio, C/C0 of

carbon and sulphur in the rectangular ingot. As to be

expected the macrosegregations follow the well known

behavior. [4]

In the figure, the concentration along three horizontal lines,

representing three height levels, from surface to centre are

shown to the left, the centerline segregation is shown in the

middle and the position of each drill sample are shown to

the right. [4]

A sulphur print of the corresponding surface is shown in

figure 3. In this ingot, an increase of the segregation ratio is

seen toward the top. At the bottom of the ingot a somewhat

increasing negative segregation is found. [4]

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Figure 2: Segregation ratio of S and C. Sample location is shown in the right figure.

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Figure 3: Sulphur print of vertical cross-section of rectangular ingot.

12

Dendritic segregation

It is Inhomogeneous distribution of alloying elements

through the arms of dendrites. [5]

Figure 4 shows how microsegregation can lead to a form of

macrosegregatio. [1]

As freezing occurs in the dendrites, the general flow of

liquid carries the progressively concentrating segregate

towards the roots of the dendrites. [1]

Figure 4: Normal dendritic segregation (usually misleadingly called inverse segregation) arising as a result

of the combined actions of solute rejection and shrinkage during solidification in a temperature gradient.

For the case of dendritic growth against the wall of the

mould, however, the temperature gradient will ensure that

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all the flow is in the direction towards the wall,

concentrating the segregation here. Thus the presence of a

temperature gradient is necessary for a significant build-up

of segregation. [1]

Dendritic segregation is observable but is not normally

severe in sand castings because the relatively low

temperature gradients allow freezing to occur rather evenly

over the cross-section of the casting; little directional

freezing exists to concentrate segregates in the direction of

heat flow.

It will by now be clear that this type of segregation is in

fact the usual type of segregation to be expected in

dendritic solidification. [1]

14

Gravity segregation

Gravity plays an important role in the formation of

segregation. Settling or flotation of liquid or solid phases

having a different composition, and therefore a different

density than the bulk liquid, will produce gravity

segregation. [6]

Gravity segregation is mainly encountered in heavy

sections, where solid phases can be suspended in the liquid

for some time. [2]

Positive segregation have more solute than the average for

the ingot and it have two types the A segregates and the V

segregates

The 'A' type segregates in a steel ingot are formed in this

way (Figure 5) also called Freckles refers to the streaks

oriented almost vertically in an A-pattern at the upper and

outer regions of the ingots and it is a positive segregation.[6]

The 'V' type segregates also called channel centerline and it

is located in the center of the ingot. [6]

They are characterized by a sharply delineated edge on the

opposite side to that shown by the A segregates. [1]

It seems that they form at a late stage in the freezing of the

ingot, when the segregated pool of liquid floating at the top

of the ingot is drawn downwards to feed the solidification

shrinkage in the centre and lower parts of the ingot. [1]

15

Figure 5: Development of segregation in a killed steel ingot (a) during solidification and (b) in the final ingot.

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On sectioning the ingot transversely, and etching to reveal

the pattern of segregation, the A and V segregates appear as

a fairly even distribution of clearly defined spots, having a

diameter in the range of 2-10 mm. probably depending on

the size and shape of the ingot, they may be concentrated at

mid-radial to central positions in zones, or evenly spread. [1]

Negative segregation is distributed in a cone at the base of

the ingot. [6]

Negative segregation has less solute than the average for

the ingot and it increasing by increasing the width of the

ingot. [1]

Although few ingots are cast in modern steelworks, large

steel castings continue to be made in steel foundries. Such

castings are characterized by the presence of channel

segregates, in turn causing extensive and troublesome

macrosegregation. Channel segregates can be controlled

by:

1. Decreasing the time available for their formation by

increasing the rate of solidification.

2. Adjusting the chemical composition of the alloy to

give a solute-rich liquid that has more nearly neutral

buoyancy at the temperature within the freezing zone.

[1]

17

Reference

1. Castings 2nd edition by John Campbell OBE FREng

Professor of Casting Technology, University of

Birmingham, UK.

2. Foundry Technology 2nd edition by Peter Beeley

BMet, PhD, DMet, CEng, FIM, FIBF Life Fellow and

formerly Senior Lecturer in Metallurgy, University of

Leeds.

3. Segregation in cast products by A GHOSH

Department of Materials and Metallurgical

Engineering, Indian Institute of Technology, Kanpur

208 016, India.

4. Slag inclusion formation during solidification of Steel

alloys and in cast iron by Sofia Adolfi Licentiate

Thesis Materials Processing Department of Material

Science and Engineering School of Industrial

Engineering and Management Royal Institute of

Technology SE-10044 Stockholm, Sweden.

5. http://www.engnetglobal.com/tips/glossary.aspx?word

=Dendritic+Segregation

6. Science and Engineering of Casting Solidification By

Doru Michael Stefanescu